GAS TANKERS
GAS TANKERS
Familiarisation Level
01.
INTRODUCTION
1 Introduction
This course is intended for officers and key ratings that have not
previously served on board liquefied gas tankers as part of the regular
complement. It covers mandatory minimum training requirements
prescribed by Regulation V/1, paragraph 1.2 of the International
Convention on Standards of Training, Certification and Watchkeeping for
Seafarers, STCW-95 and it includes basic safety and pollution-prevention
precautions and procedures, layouts of different types of liquefied gas
tankers, types of cargo, their hazards and their handling equipment,
general operational sequence and liquefied gas tanker terminology.
1.1 The course
The background for and the purpose of the course as being:
- The STCW-95 Convention contains mandatory minimum requirements
for training and qualification of masters, officers and ratings of liquefied
gas tankers.
- This training is divided into two parts:
· Level 1: liquefied gas tanker familiarization – a basic safety-training
course for officers and ratings on board.· Level 2: advanced training in
liquefied gas tanker operations for masters, officers and others who are to
have immediate responsibilities for cargo handling and cargo equipment.
- This course covers the requirements for level 1 training required by
Regulation V/1, paragraph 1.2 of the International Convention on
Standards of Training, Certification and Watchkeeping for Seafarers,
STCW-95
1.2 Development of Liquefied gas Shipping
Learning Objectives
Lists important stages in the transport of liquefied gas by ships, such as:
· gas shipping began in the late 1920s
· the earliest ships were designed to carry liquefied gas in pressure
vessels at ambient temperature
· the first cargoes on the market were butane and propane
· development of refrigeration techniques and meta s suitable for low
temperature made it possible to carry liquefied gas at temperatures lower
than ambient
· defines terminology and explains abbreviations commonly used aboard
gas tankers and on gas terminals
In the late 1920th transportation of liquefied gases in bulk started. In the
very beginning it was transportation of propane and butane in fully
pressurised tanks. Around 1959, semi-pressurized ships entered the
market and liquefied gas was now transported under lower pressure,
which was made possible by lowering the temperature. By 1963, fully
refrigerated ships for LPG, LNG and certain chemical gases wore in
service, carrying cargo at atmospheric pressure.
Liquefied gas is divided into different groups based on boiling point,
chemical bindings, toxicity and flammability. The different groups of gases
have led to different types of gas carriers and cargo containment system
for gas carriers.
The sea transport of liquefied gases in bulk is internationally regulated with regard to safety through standards established by the International
Maritime Organization (IMO) and these standards are set out in the IMO's
Gas Carrier Codes, which cover design, construction and other safety
measures for ships carrying liquefied gases in bulk.
1.3 Terminology
BOILING: This is the action, which takes place when a liquid changes its
state from a liquid into a gas or vapour. The heat required to bring this
change of state about is called Latent Heat.
BOILING TEMPERATURE: This is the temperature at which a liquid boils.
As the boiling temperature rises with an increase in pressure (see
saturated vapour pressure), the boiling temperatures are usually given for
atmospheric pressure. At this pressure, water boils at + 100oC. butane at
- ½oC., ammonia at -33oC. and propane at -43oC.
CONDENSATION: This is evaporation in reverse. If a vapour becomes
supersaturated, condensation takes place and heat is surrendered. For
example, in a seawater-cooled condenser, a compressor has raised the
pressure of the vapour to such an extent that at seawater temperature, it
is supersaturated. Condensation takes place, and the latent heat released
heats up the water passing through the condenser tubes; the heated
seawater passing overboard into the sea, to be replaced continuously by
fresh cool water. The resulting condensate will be somewhat warmer than
the seawater coolant.
EVAPORATION: This is the process of converting a liquid into a vapour,
and it requires latent heat to do this. If a liquid (say liquid propane) in a
closed container at 10oC. Has a saturated vapour pressure of 5
atmospheres, and the vapour in the space above the liquid is allowed to
escape, the pressure in the container will fall. As soon as this happens,
the vapour in the space above the liquid will be undersaturated and
evaporation will take place (or the liquid boil). Heat will be used up in the
boiling process and the temperature of the liquid will fall. The "boil off" will
largely replace the vapour which has been allowed to escape until such
time as the pressure in the container corresponds to the saturated vapour
pressure of the liquid at the new lower temperature. Continuous
withdrawal of vapour means continuous evaporation, which in turn means
continuous loss of heat (cooling).
FILLING OF CARGO TANKS: The correct maximum volume of liquid to load
in a cargo tank is such a quantity that after allowance for the product to
warm up and expand to a temperature the saturated vapour pressure of
which would lift the safety valves, 2 per cent. of the space would remain.
A tank so filled is described as Full. A tank filled above this level is
described as Overfull. A tank completely filled with liquid is described as
one hundred per cent.
FLASHOVER: Firefighting on board ships differs from firefighting ashore in
that allowance has to be made for the fact that the metal with which a
ship is constructed, conducts heat to a far greater extent than normal
shore building materials. The result is that a fire on board ship tends to
spread horizontally as well as vertically.
If the temperature of combustible material in a compartment adjacent to
one where a fierce fire is burning, is raised to above its ignition
temperature (q.v.), that material will ignite spontaneously (auto ignition),
so spreading the fire from one compartment into another, through a
bulkhead, without a spark or flame being directly involved. Such a means
of a fire spreading is termed a flash-over.
GAS/VAPOUR: Gas is a substance which has the property of indefinite
expansion. In the context of this book, it is above its critical temperature
and cannot be condensed into a liquid. If the temperature of a gas is
reduced to below its critical temperature, it then becomes a vapour, and
can be condensed into a liquid. Gases are frequently referred to as
incondensibles.
Flammable or Explosive Mixture: Petroleum as a liquid does not burn. At
ordinary temperatures, it gives off vapour, which when mixed within
certain proportions with air, will burn. The lowest proportion of petroleum
vapour in air mixture, which will burn, is termed lower explosive limit
(L.E.L.) and the strongest mixture that will burn is termed upper explosive
limit (U.E.L.). The flammable mixtures between the lower and upper
explosive limits are called the explosive range. A mixture of vapour in air
weaker than the L.E.L. is described as too lean or over-lean whilst a
mixture of vapour in air stronger than the U.E.L. is described as too rich
or over-rich. Mixtures outside the explosive range will not burn, the words
explosive and flammable within this context being virtually synonymous.
Flash Point: This is the lowest temperature at which a flammable mixture
of air and vapour will burn when exposed to a naked flame.
Ignition Temperature: This is the temperature at which a flammable
mixture of vapour and air will ignite spontaneously (without being exposed
to a naked flame). The operation of a diesel engine depends upon this
effect.
GAS LAWS
Avogadro's Hypothesis: Equal volumes of different gases at the same
pressure and temperature contain the same number of molecules.
Boyle's Law: The volume of a given mass of gas varies inversely with the
pressure provided that the temperature remains constant:
P=
Charles's Law: The volume of a given mass of gas varies directly with the
absolute temperature provided the pressure remains constant:
Volume =
or density =
Clerk Maxwell's Kinetic Theory: A gas may be imagined as a vast number
of molecules moving in all directions at irregular velocities, colliding with
one another and with the walls of the containing vessel. The path of a
molecule is zigzag in three dimensions and the mean free path is defined
as the average length between collisions, the denser the gas, the shorter
will be the mean free path.
On the assumption that the molecules are microscopic spheres, it can be
shown that the pressure and absolute temperature of a gas are
proportional to the mean kinetic energy of translation of the molecules
bombarding the walls of the vessel containing the gas. Thus, at the same
temperature the average kinetic energy of translation of the molecules of
any gas are the same whatever its mass-a "large" molecule having low
velocity and a "light" molecule having high velocity.
This theory correlates Avogadro's Hypothesis, Boyle's Law, Charles's Law
and Gay Lussac's Law.
Dalton's Law of Partial Pressures: The pressure of a mixture of gases is
the sum of the pressures each would exert if it alone were to occupy the
containing vessel.
Gay Lussac's Law: The density of a gas at standard pressure and
temperature is proportional to its molecular weight. This is a corollary of
Avogadro's Hypothesis.
Joule's Law: When a perfect gas expands without doing external work and
without taking in or giving out heat and therefore without changing its
stock of internal energy, its temperature does not change.
HEAT
Latent Heat: This is the heat used up in changing the state of a substance
without changing its temperature. In the case of changing the state of a
substance from a solid into a liquid (melting), it is called the latent heat of
fusion, and in the case of heat changing the state of a liquid into a gas or
vapour (boiling), it is called the latent heat of vaporisation. It takes 80
calories to change 1 gramme of ice into water and about 539 calories to
change 1 gramme of water into steam at standard atmospheric pressure.
The value of latent heat of vaporisation varies with temperature and
pressure (see critical temperature).
Sensible Heat: This is the heat used in raising the temperature of a
substance without changing its state. 1 calorie is used to raise the
temperature of 1 gramme of water 1oC.
HEEL: This is the small quantity of liquid remaining after discharge which
it is impossible to pump out, but which is used to assist in keeping the
cargo tank cold during the ballast (unloaded) passage, and is usually
carried over to the next loading. When it is know that the vessel will be
changing grades or gas freeing, every effort should be made to reduce
this heel to the absolute minimum.
LIQUID CARRY OVER: This occurs when vapour moves swiftly over the
surface of a liquid and droplets of liquid become entrained with the vapour
and are carried over with it.
It is the entrained droplets of lubricating oil that are recovered in the
lubricating oil separator trap of the compressor, and entrained liquid
droplets which cause wet suction on a compressor.
MOLE: This is the quantity of gas the weight of which is equal to its
molecular weight in pounds or grammes. Thus a mole of hydrogen would
be 2, a mole of oxygen 32 etc. This is fairly closely related to Avogadro's
Hypothesis, a mole having the same volume for all products at the same
pressure and temperature.
PRESSURE
Absolute Pressure: This is the pressure above a vacuum. Thus a pressure
of 7 p.s.i. absolute, is really a suction pressure of 7.7 p.s.i. at atmospheric
pressure (atmospheric pressure equals 14.7 p.s.i.).
Gauge Pressure: This is the pressure above one atmosphere and is the
usual method of measuring pressures and vacuums. Absolute pressure is
therefore equal to gauge pressure plus one atmosphere.
Atmospheric Pressure: This is the pressure exerted at sea level. This
pressure varies from place to place and from time to time. The standard
atmospheric pressure is 1012.5 millibars, corresponding to 29.90 inches
or 760 millimetres of mercury.
SPAN GAS: This is a laboratory-measured mixture of gases used for the
purpose of calibrating gas detectors. In gas tankers, the mixture is usually
30 per cent. L.E.L. of the product mixed with pure nitrogen.
STRATIFICATION: This is the layering effect of two gases or vapours with
dissimilar densities, the lighter vapour floating above the heavier.
TEMPERATURE
Absolute Temperature: As a result of studying Charles's Law, it seemed
that the volume of a gas would reduce to nothing at about -273oC. (or
absolute zero). (Physicists have never been able to reach this
temperature.) It therefore follows that absolute temperature equals
temperature + 273oC.
Adiabatic Changes in Temperature: When a gas (or vapour) is
compressed, its temperature rises. When it expands, its temperature falls.
This is the adiabatic process and compression ignition (diesel) engines rely
upon this property for their operation.
Critical Temperature: This is the temperature above which it is not
possible to liquefy a gas. Saturated vapour pressure rises with an increase
in temperature. At the same time, the density of a liquid falls with an
increase in its temperature. Therefore, there must come a time when so
many atmospheres of pressure are required to liquefy the vapour that the
density of the compressed vapour and the liquid are the same. When this
state is achieved, there is virtually no difference between the liquid and
vapour phases and they freely change into each other. The value of latent
heat is reduced to zero and with any increase in temperature, no amount
of increasing the pressure will bring about liquefaction, and the vapour is
then described as a gas. Associated with the critical temperature is the
critical pressure.
VAPORISATION: This is the action of converting a liquid into a vapour.
Batch Vaporisation: This is the method of evaporation whereby vapour is
withdrawn from the top of a tank, causing the liquid in the tank to boil,
with a consequent drop in temperature. With a mixture of products such
as butane and propane, the more volatile element tends to evaporate
first, so that the proportions comprising the mixture will change and after
a time one is left with almost pure butane. This process of altering a
mixture in a tank due to the volatile constituent evaporating first is called
"weathering". However, batch vaporisation is the simplest method and
because, in L.P.G. tankers, the vapour which has been withdrawn is
condensed into a liquid and returned to the tank, there is no tendency to
alter the constituents of the mixture, so this is used as a method of
refrigeration.
Flash Vaporisation: This is the method whereby liquid is withdrawn from
the bottom of the tank and evaporated in a vaporising unit. In this
method, the constituents of a mixture remain fairly constant, as does the
temperature of the product in the tank.
VAPOUR: This is the term used for a "gas" below its critical temperature
and therefore capable of being liquefied.
Saturated Vapour Pressure (S.V.P.) All liquids tend to evaporate under
normal conditions, but if kept in a closed container, evaporation will only
take place until the atmosphere in the container becomes saturated. In
the case of water, the following experiment can be carried out. Into the
top of a barometer some water is introduced. Due to the evaporation of
the water that has been introduced, the level of the mercury will fall. If
sufficient water is introduced, the level will virtually stop falling because
the space above the mercury will be saturated with water vapour, and a
little water will show on top of the mercury. The fall in the mercury level
converted into pressure would indicate the absolute S.V.P. at that
temperature. By rising the temperature, more water will evaporate and
the level of the mercury fall further. The new level, converted into
pressure, will indicate the new S.V.P. at the new temperature. At 100oC,
the level of the barometer will register zero. The absolute vapour pressure
of water at 100oC. is therefore one atmosphere (1.0125 bar). It therefore
follows that under atmospheric conditions, a liquid will, apart from minor
evaporation, keep its state until with the addition of heat, and its absolute
S.V.P. reaches one atmosphere. From then on, all the extra heat will be
used to assist evaporation and the temperature will not rise. In other
words, the liquid boils. If the boiling action takes place in a closed
container, e.g., a boiler, as the temperature rises, so the pressure
increases. That is, the boiling temperature of the water rises as the
pressure increases. The pressure in the boiler is an indication of the water
temperature and vice versa.
If a thermometer and pressure gauge were fitted to a container holding,
say, propane, the temperature and pressure would be directly related to
each other, the pressure rising as the temperature rose and vice versa.
A sudden release of pressure would result in continuous evaporation, this
using up latent heat so cooling the liquid until the temperature of the
liquid reached that appropriate to the S.V.P. of the product at the new
pressure. This means that if warm propane escaped onto the deck, it
would immediately evaporate and refrigerate itself down to approximately
–43oC.
Supersaturated Vapour: If the vapour pressure in a container is rapidly
increased, condensation will take place, but until the process of
condensation has been completed, the vapour will be supersaturated.
Undersaturated Vapour: This is super-saturation in reverse.
Superheated Vapour: In the absence of liquid to continue the evaporating
process and so keep the vapour saturated, the vapour temperature can be
raised to well above the temperature corresponding to that at which the
vapour would be saturated at the pressure concerned. Any superheated
vapour would have no tendency to condense. This property is used
particularly with steam. The saturated steam coming from the boilers is
heated further in the superheater to prevent condensation taking place in
the engine.
VAPOUR RETURN LINE: This is a balancing pipeline between the ship when
loading (or discharging) and the shore tank, so that the vapour trapped in
the space above the incoming liquid, and therefore being compressed, is
returned to the shore tank from which the product is being discharged.
WET SUCTION: This occurs when liquid droplets are carried over into the
compressor suction, and get sucked into the compressor. It can only take
place if the vapour at the compressor suction is at or near saturation.
On the compression stroke, the adiabatic increase in temperature is used
up evaporating the liquid droplets which have been sucked into the
cylinder, resulting in a dramatic drop in the discharge temperature. The
temperature of the cylinder head falls and in extreme cases can become
covered with ice.
Wet suction frequently causes damage to the compressor suction and
discharge valves, and in extreme cases, where too much unevaporated
liquid collects in the cylinder, can cause the cylinder head to be shattered.
ZERO GAS: This is pure nitrogen used to calibrate the zero reading of gas
detectors.
02.
PROPERTIES AND
HAZARDS OF
LIQUEFIED GAS
2 PROPERTIES AND HAZARDS OF LIQUEFIED GAS
2.1
TYPES OF GAS CARRIERS
IMO divides liquefied gases into the following groups:
•
•
•
•
•
•
LPG - Liquefied Petroleum Gas
LNG - Liquefied Natural Gas
LEG - Liquefied Ethylene Gas
NH3 - Ammonia
Cl2 - Chlorine
Chemical gases
The IMO gas carrier code define liquefied gases as gases with vapour
pressure higher than 2,8 bar with temperature of 37,8oC.
IMO gas code chapter 19 defines which products that are liquefied gases
and have to be transported with gas carriers. Some products have vapour
pressure less than 2,8 bar at 37,8oC, but are defined as liquefied gases
and have to be transported according to chapter 19 in IMO gas code.
Propylene oxide and ethylene oxides are defined as liquefied gases.
Ethylene oxide has a vapour pressure of 2,7 bar at 37,8oC. To control
temperature on ethylene oxide we must utilise indirect cargo cooling
plants.
Products not calculated as condensed gas, but still must be transported on
gas carriers, are specified in IMO’s gas code and IMO’s chemical code. The
reason for transportation of non-condensed gases on gas carriers is that
the products must have temperature control during transport because
reactions from too high temperature can occur.
Condensed gases are transported on gas carriers either by atmospheric
pressure (fully cooled) less than 0,7 bars, intermediate pressure
(temperature controlled) 0,5 bars to 11 bars, or by full pressure
(surrounding temperature) larger than 11 bars. It is the strength and
construction of the cargo tank that is conclusive to what over pressure the
gas can be transported.
2.1.1
LPG
LPG - Liquefied Petroleum Gas is a definition of gases produced by wet gas
or raw oil. The LPG gases are taken out of the raw oil during refining, or
from natural gas separation. LPG gases are defined as propane, butane
and a mixture of these. Large atmospheric pressure gas carriers carry
most of the LPG transported at sea. However, some LPG is transported
with intermediate pressure gas carriers. Fully pressurised gas carriers
mainly handle coastal trade. LPG can be cooled with water, and most LPG
carriers have direct cargo cooling plants that condenses the gas against
water.
The sea transport of LPG is mainly from The Persian Gulf to Japan and
Korea. It is also from the north- west Europe to USA, and from the
western Mediterranean to USA and Northwest Europe.
LPG is utilised for energy purposes and in the petro-chemical industry
2.1.2
LNG
LNG - Liquefied Natural Gas is a gas that is naturally in the earth. Mainly
LNG contains Methane, but also contains Ethane, Propane, and Butane
etc. About 95% of all LNG are transported in pipelines from the gas fields
to shore, for example, gas pipes from the oil fields in the North Sea and
down to Italy and Spain. Gas carriers transport the remaining 5%. When
LNG is transported on gas carriers, the ROB and boil off from the cargo is
utilised as fuel for propulsion of the vessel. Cargo cooling plants for large
LNG carriers are very large and expensive, and they will use a lot of
energy. Small LNG carriers have cargo-cooling plants, and can also be
utilised for LPG transportation.
The sea transport of LNG is from the Persian Gulf and Indonesia to Japan,
Korea and from the Mediterranean to Northwest Europe and the East
Coast of USA and from Alaska to the Far East.
LNG is used for energy purposes and in the petro-chemical industry.
2.1.3
NGL
NGL - Natural Gas Liquid or wet gas is dissolved gas that exists in raw oil.
The gas separates by refining raw oil. The composition of wet gas varies
from oil field to oil filed. The wet gas consists of Ethane, LPG, Pentane and
heavier fractions of hydrocarbons or a mixture of these. Atmospheric
pressure gas carriers and semi-pressurised gas carriers carry the most of
the wet gas.
Ethane can only be transported by semi-pressurised gas carriers, which
have direct cascade cooling plants and are allowed to carry cargo down to
–104oC. This is because Ethane has a boiling point at atmospheric
pressure of –89oC. This will create too high condense pressure if using
water as cooling medium. The cargo is condensed against Freon R22 or
another cooling medium with boiling point at atmospheric pressure lower
than –20oC.
Wet gas is transported from the Persian Gulf to the East, Europe to USA
and some within Europe. There is also some transport of wet gas in the
Caribbean to South America.
NGL is utilised for energy purposes and in the petro-chemical industry.
Methane CH4
Ethan C2H6
LNG
NGL
LPG
Propane C3H8
Butane C4H10
Pentane and heavier
fractions of HC
Water, carbon dioxide, nitrogen and other
non-hydrocarbon containment
2.1.4
COMPOSITION OF NATURAL GAS
2.1.5
LEG
LEG - Liquefied Ethylene Gas. This gas is not a natural product, but is
produced by cracked wet gas, such as, Ethane, Propane, and Butane or
from Naphtha. Ethylene has a boiling point at atmospheric pressure of 103,8oC, and therefore has been transported in gas carriers equipped with
cargo compartment that can bear such a low temperature. Cascade
plants are used to condense Ethylene. As critical temperature of Ethylene
is 9,7oC one cannot utilise water to condense Ethylene. The definition of
Ethylene tankers is LPG/LEG carrier.
Ethylene is very flammable and has a flammable limit from 2,5% to 34%
by volume mixed with air. There are stringent demands regarding the
oxygen content in Ethylene. The volume of ethylene must be less than 2%
in the gas mixture to keep the mixture below the LEL “lower explosion
limit”. Normally, there are demands for less than 0,2% oxygen in the gas
mixture in order to prevent pollution of the cargo.
Ethylene is utilised as raw material for plastic and synthetic fibres.
Ethylene is transported from the Persian Gulf to the East, the
Mediterranean to the East and Europe, the Caribbean to South America.
There is also transport of Ethylene between the countries Malaysia,
Indonesia and Korea
2.1.6
AMMONIA NH3
The next gas we will focus on is Ammonia, which is produced by
combustion of hydrogen and nitrogen under large pressure. Ammonia is a
poisonous and irritating gas, it has TLV of 25 ppm and the odour threshold
is on 20 ppm. It responds to water and there are special rules for vessels
that transport Ammonia. We can locate the rules in the IMO Gas Code,
chapters 14, 17 and 19.
When ammonia gas is mixed with water, a decreased pressure is formed
by 1 volume part water absorbing 200 volume parts ammonia vapour. A
decreased tank pressure will occur if there is water in the tank when
commence loading ammonia and the tank hatch is closed. With an open
hatch, we can replace the volume, originally taken up by the ammonia
gas, with air.
One must not mix ammonia with alloys: copper, aluminium, zinc, nor
galvanised surfaces. Inert gas that contains carbon dioxide must not be
used to purge ammonia, as these results in a carbamate formation with
the ammonia. Ammonium carbamate is a powder and can blockage lines,
valves and other equipment.
The boiling point for ammonia at atmospheric pressure is –33oC, and
must be transported at a temperature colder than –20oC. One can cool
ammonia with all types of cargo cooling plants. Ammonia is transported
with atmospheric pressure gas carriers or semi-pressurised gas carriers.
Gas carriers carrying Ammonia must be constructed and certified in
accordance with IMO’s IGC code for transportation of liquefied gases. The
definition for ammonia tanker is LPG/NH, carrier.
Ammonia is utilised as raw material for the fertiliser industry, plastic,
explosives, colours and detergents.
There is a lot of transportation from the Black Sea to USA, from USA to
South Africa and from Venezuela to Chile.
2.1.7
CHLORINE CI2
Chlorine is a very toxic gas that can be produced by the dissolution of
sodium chloride in electrolysis. Because of the toxicity of Chlorine it is
therefore transported in small quantities, and must not be transported in a
larger quantity than 1200m3. The gas carrier carrying chlorine must be
type 1G with independent type C tanks. That means the cargo tank must,
at the least, lie B/5 “Breadth/5” up to 11,5 meter from the ships side. To
transport Chlorine, the requirements of IMO IGC code, chapters 14, 17
and 19 must be fulfilled. Cooling of Chlorine requires indirect cargo
cooling plants.
The difference of Chlorine and other gases transported is that Chlorine is
not flammable.
Chlorine is utilised in producing chemicals and as bleaching agent in the
cellulose industry.
2.1.8
CHEMICAL GASES
The chemical gases mentioned here is the gases produced chemically and
are defined in IMO’s rules as condensed gases. Because of the gases’
boiling point at atmospheric pressure and special requirements for
temperature control, these gases must be carried on gas carriers as
specified by the IMO gas code. Condensed gases are liquids with a vapour
pressure above 2,8 bars at 37,8oC. Chemical gases that are mostly
transported are Ethylene, Propylene, butadiene and VCM. Chemical gases
that have to be transported by gas carriers are those mentioned in
chapter 19 in IMO IGC code. There are, at all times, stringent demands for
low oxygen content in the cargo tank atmosphere, often below 0,2% by
volume. This involves that we have to use nitrogen to purge out air from
the cargo compartment before loading those products.
In addition, even though the vapour pressure does not exceed 2,8 bars at
37,8oC such as, ethylene oxide and propylene oxide or a mixture of these,
they are still in the IMO gas code as condensed gases. Gas carriers that
are allowed to transport ethylene oxide or propylene oxide must be
specially certified for this. Ethylene oxide and propylene oxide have a
boiling point at atmospheric pressure of respectively 11oC and 34oC and
are therefore difficult to transport on tankers without indirect cargo
cooling plants. Ethylene oxide and propylene oxide cannot be exposed to
high temperature and can therefore not be compressed in a direct cargo
cooling plant. Ethylene oxide must be transported on gas tanker type 1G.
Chemical gases like propylene, butadiene and VCM are transported with
medium-sized atmospheric pressure tankers from 12000 m3 to 56000 m3.
Semi-pressurised gas carriers are also used in chemical gas trade and
then in smaller quantity as from 2500 m3 to 15000 m3.
Chemical gases are transported all over the world, and especially to the
Far East where there is a large growth in the petro-chemical industry.
Chemical gases are mainly utilised in the petro-chemical industry and
rubber production.
2.2
CARGO PROPERTIES
2.2.1
States of matter
Most substances can exist in either the solid, liquid or vapour state. In
changing from solid to liquid (fusion) or from liquid to vapour
(vaporisation), heat must be given to the substance. Similarly in changing
from vapour to liquid (condensation) or from liquid to solid (solidification),
the substance must give up heat. The heat given to or given up by the
substance in changing state is called latent heat. For a given mass of
the substance, the latent heats of fusion and solidification are the same.
Similarly, latent heats of vaporisation and of condensation are the same,
although different from the latent heat of fusion or solidification. Fusion
or solidification occurs at a specific temperature for the substance and this
temperature is virtually independent of the pressure. Vaporisation or
condensation of a pure substance, however, occurs at a temperature
which varies widely dependent upon the pressure exerted on the
substance. The latent heat of vaporisation also varies with pressure.
Figure 2.1 illustrates these temperature/heat relationships as a substance
is heated or cooled through its three states; the temperatures of fusion or
solidification (A) and of vaporisation or condensation (B) are all well
defined. For liquefied cases, we are not concerned with the solid state
since this can only occur at temperatures well below those at which the
liquefied gas is carried. Temperatures, pressures and latent heats of
vaporisation, however, are of fundamental importance. This data may be
presented in graphical form such as Figure 2.2 which gives curves for
vapour pressure, liquid density, saturated vapour density and latent heat
of vaporisation against temperature for methane. Similar graphical
presentation of these properties are available for all the principal liquefied
gases carried by sea and some of these presentations are reproduced in
the Data Sheets of Appendix 1 of the ICS Tanker Safety Guide (Liquefied
Gas).
Figure 2.1 Temperature/heat energy relationship for the various states of matter
It is convenient here, against the background of the preceding,
paragraphs, to consider what happens when a liquefied gas is spilled.
Firstly, consider the escape from its containment of a fully refrigerated
liquid. The liquid is already at or near atmospheric pressure but, on
escape, it is inevitably brought immediately into contact with objects such
as structures, the ground or the sea, which are at ambient temperature.
The temperature difference between the cold liquid and the objects it
contacts provides an immediate transfer of latent heat to the liquid,
resulting in rapid evolution of vapour. The abstraction of heat from
contacted solid objects cools them, reducing the temperature difference
and stabilising the rate of evaporation to a lower level than initially until
the liquid is completely evaporated. In the case of spillage on to water,
the convection in the upper layers of the water may largely maintain the
initial temperature difference and evaporation may continue at the higher
initial rate. Spillage from a pressurised container is initially different in
that the liquid on escape is at a temperature not greatly different from
ambient temperature but the liquid is released from its containment
pressure down to ambient pressure.
Figure 2.2 Vapour pressure (P), liquid density (у’), saturated vapour density (у’’)
and heat of vaporisation (r) for methane.
Extremely rapid vaporisation ensues, the necessary latent heat being
taken primarily from the liquid itself which rapidly cools to its temperature
of vaporisation at atmospheric pressure. This is called flash evaporation
and, depending upon the change in pressure as the liquid escapes from its
containment, a large proportion of the liquid may flash off in this way.
The considerable volume of vapour produced within the escaping liquid
causes the liquid to fragment into small droplets. Depending upon the
change in pressure as the liquid escapes, these droplets will be ejected
with a considerable velocity. These droplets take heat from the
surrounding air and condense the water vapour in the air to form a white
visible cloud and vaporise to gas in this process. Thereafter any liquid
which remains will evaporate in the same way as for spilled fully
refrigerated liquid until the spillage is wholly vaporised. Apart from the
hazards introduced by the generation of vapour which will become
flammable as it is diluted with the surrounding air, the rapid cooling
imposed upon contacted objects will cause cold burns on human tissue
and may convert metallic structure to a brittle state.
2.2.2
Saturated vapour pressure
Vapour in the space above a liquid is not static since liquid molecules near
the surface are constantly leaving to enter the vapour phase and vapour
molecules are returning to the liquid phase. The space is said to be
unsaturated with vapour at a particular temperature if the space can
accept more vapour from the liquid at that temperature. A saturated
vapour at any temperature is a vapour in equilibrium with its liquid at that
temperature. In that condition the space cannot accept any further
vapour from the liquid, although a continuous exchange of molecule,
between vapour and liquid takes place.
The pressure exerted by a saturated vapour at a particular temperature is
called the saturated vapour pressure of that substance at that
temperature. Various methods exist for measurement of saturated
vapour pressures and one is illustrated in Figure 2.3. This apparatus
consists of a barometer tube (C) which is filled with mercury, inverted and
immersed in a mercury reservoir (A). The space above the mercury is a
vacuum (B) though not perfect because of the presence of mercury
vapour in that space. The height of mercury (X) is a measure of
atmospheric pressure. A small amount of the liquid under test is
introduced into the mercury barometer and rises to the vacuum space
where it immediately vaporises and exerts a vapour pressure. This
vapour pressure pushes the mercury down in the barometer tube to a new
level (Y). The saturated vapour pressure exerted by a test liquid is the
difference between the heights of the mercury column X and Y, usually
expressed in mm of mercury.
If the mercury column containing the small amount of liquid under test is
now suitably heated, then the mercury level will fall indicating that the
saturated vapour pressure has increased with increasing temperature. It
is possible by this means to determine the saturated vapour pressure for
the liquid under test at various temperatures.
Whereas evaporation is a surface phenomenon where the faster moving
molecules escape from the surface of the liquid, boiling takes place in the
body of the liquid when the vapour pressure is equal to the pressure in the
liquid. By varying the pressure above the liquid it is possible to boil the
liquid at different temperatures. Decreasing the pressure above the liquid
lowers the boiling point and increasing the pressure raises the boiling
point. The curve marked P in Figure 2.4 illustrates the variation in
saturated vapour pressure with temperature for propane. It will be
noticed that an increase in the temperature of the liquid causes a nonlinear increase in the saturated vapour pressure. Also shown on Figure
2.4 are the variations of propane liquid densities and saturated vapour
densities with temperature.
Figure 2.3 Barometer methods for measuring saturated vapour pressure (SVP)
Figure 2.4 Saturated vapour pressure (P), density of saturated vapour ( V ") and density
of liquid ( P') for propane
Different liquefied gases exert different vapour pressures as can be seen
from Figures 2.5 and 2.6. The vertical axis in these two figures gives the
saturated vapour pressure on a logarithmic scale which changes the shape
of the curves from that of P in Figure 2.4. Figure 2.5 shows that for the
hydrocarbon gases, smaller molecules exert greater vapour pressures
than large ones. In general the chemical gases shown in Figure 2.6 exert
much lower saturated vapour pressures than the small hydrocarbon
molecules. The point of intersection of these curves with the horizontal
axis indicates the atmospheric boiling point of the liquid (the temperature
at which the saturated vapour pressure is equal to atmospheric pressure).
This is the temperature at which these cargoes would be transported in a
fully refrigerated containment system.
Figure 2.5 Pressure/temperature relationships for saturated and unsaturated liquefied
hydrocarbon gases
Figure 2.6 Pressure/temperature relationships for liquefied chemical gases
Whereas the bar is now the most frequently used unit in the gas industry
for the measurement of pressure, other units such as kgf/cm2,
atmospheres or millimetres of mercury are frequently encountered. The
conversion factors for these units of pressure are given in Table 2.6.
All gauges used for the measurement of pressure measure pressure
difference. Gauge pressure is therefore the pressure difference between
the pressure to which the gauge is connected and the pressure
surrounding the gauge. The absolute value of the pressure being
measured is obtained by adding the external pressure to the gauge
pressure.
Vapour pressures, though they may be often determined by means of a
pressure gauge, are a fundamental characteristic of the liquid and are
essentially absolute pressures. Tank design pressures and relief valve
settings, however, like pressure gauge indications, are physically the
differences between internal and external pressure and thus are gauge
pressures. For consistency throughout this book all such pressures are
given in bars but to avoid confusion the unit is denoted as "barg" where a
gauge pressure is intended.
•
A liquefied gas has been defined in terms of its vapour
pressure as being a substance whose vapour pressure at
37.8o C is equal to or greater than 2.8 bar absolute (IMO
definition).
2.2.3
Liquid and vapour densities
The density of a liquid is defined as the mass per unit volume and is
commonly measured in kilograms per decimetre cubed (kg/dm3).
Alternatively, liquid density may be quoted in kg/litre or in kg/m3. The
variation with temperature of the density of a liquefied gas in equilibrium
with its vapour is shown for propane in curve y' of Figure 2.4. As can be
seen, the liquid density decreases markedly with increasing temperature.
This is due to the comparatively large coefficient of volume expansion of
liquefied gases. All the liquefied gases, with the exception of chlorine,
have liquid relative densities less than one. This means that in the event
of a spillage onto water these liquids would float prior to evaporation.
Table 2.7 Conversion factors for units of pressure
The variation of the density of the saturated vapour of liquefied propane
with temperature is given by curve y" of Figure 2.4. The density of vapour
is commonly quoted in units of kilograms per cubic metre (kg/m'). The
density of the saturated vapour increases with increasing temperature.
This is because the vapour is in contact with its liquid and as the
temperature rises more liquid transfers into the vapour phase in order to
provide the increase in vapour pressure. This results in a considerable
increase in mass per unit volume of the vapour space. All the liquefied
gases produce vapours which have a relative vapour density greater than
one with the exceptions of methane (at temperatures greater than –
100oC). Vapours released to the atmosphere and which are denser than
air tend to seek lower ground and do not disperse readily.
2.2.4
Flammability and explosion
Combustion is a chemical reaction, initiated by a source of ignition, in
which a flammable vapour combines with oxygen in suitable proportions
to produce carbon dioxide, water vapour and heat. Under ideal conditions
the reaction for propane can be written as follows:
C3 H8
+
502
+
Heat
propane
oxygen
Combustion
Ζ
carbon
dioxide
3CO2
+
4H2O
water
vapour
Under certain circumstances when, for example, the oxygen supply to the
source of fuel is restricted, carbon monoxide or carbon can also be
produced.
The three requirements for combustion to take place are fuel, oxygen and
ignition. The proportions of flammable vapour to oxygen or to air must be
within the flammable limits.
The gases produced by combustion are heated by the combustion
reaction. In open, unconfined spaces the consequent expansion of these
gases is unrestricted and the combustion reaction may proceed smoothly
without undue overpressures developing. If the free expansion of the hot
gases is restricted in any way, pressures will rise and the speed of flame
travel will increase, depending upon the degree of confinement
encountered. Increased flame speed in turn gives rise to more rapid
increase in pressure with the result that damaging overpressures may be
produced and, even in the open, if the confinement resulting from
surrounding pipework, plant and buildings is sufficient, the combustion
can take on the nature of an explosion. In severely confined conditions,
as within a building or ship's tank where the expanding gases cannot be
adequately relieved, the internal pressure and its rate of increase may be
such as to disrupt the containment. Here, the resultant explosion is not
so much directly due to high combustion rates and flame speed as to the
violent expulsion of the contained high pressure upon containment
rupture.
The boiling liquid expanding vapour explosion (BLEVE) is a phenomenon
associated with the sudden and catastrophic failure of the pressurised
containment of flammable liquids in the presence of a surrounding fire.
Such incidents have occurred with damaged rail tank car or road tank
vehicle pressure vessels subject to intense heat from surrounding fire.
This heat has increased the internal pressure and, particularly at that part
of the vessel not wetted by liquid product, the vessel's structure is
weakened to the point of failure. The sudden release of the vessel's
contents to atmosphere and the immediate ignition of the resultant rapidly
expanding vapour cloud have produced destructive overpressures and
heat radiation. There have been no instances of this kind, nor are they
likely to occur, with the pressure cargo tanks on liquefied gas tankers
where, by requirement, pressure relief valves are sized to cope with
surrounding fire, tanks are provided with water sprays and general design
greatly minimises the possibilities of a surrounding fire occurring.
The term flammable range gives a measure of the proportions of
flammable vapour to air necessary for combustion to be possible. The
flammable range is the range between the minimum and maximum
concentrations of vapour (per cent by volume) in air, which form a
flammable mixture. These terms are usually abbreviated to LFL (lower
flammable limit) and UFL (upper flammable limit). This concept is
illustrated for propane in Figure 2.9.
All the liquefied gases, with the exception of chlorine, are flammable but
the values of the flammable range are variable and depend on the
particular vapour. These are listed in Table 2.9. The flammable range of a
particular vapour is broadened in the presence of oxygen in excess of that
normally in air; the lower flammable limit is not much affected whereas
the upper flammable limit is considerably raised. All flammable vapours
exhibit this property and as a result oxygen should not normally be
introduced into an atmosphere where flammable vapours exist. The
oxygen cylinders associated with oxyacetylene burners and oxygen
resuscitators should only he introduced into hazardous areas under strictly
controlled conditions.
The flash point of a liquid is the lowest temperature at which that liquid
will evolve sufficient vapour to form a flammable mixture with air. High
vapour pressure liquids such as liquefied gases have extremely low flash
points, as seen from Table 2.8. However, although liquefied gases are
never carried at temperatures below their flash point, the vapour spaces
above such cargoes are non-flammable since they are virtually 100 per
cent rich with cargo vapour and are thus far above the upper flammable
limit.
Figure 2.8 Flammable range for propane
Table 2.9 Ignition properties for liquefied gases
Table 2.20 Flammability range in air/oxygen for various liquefied gases
The auto-ignition temperature of a substance is the temperature to
which its vapour in air must be heated for it to ignite spontaneously. The
auto-ignition temperature is not related to the vapour pressure or to the
flash point of the substance and, since most ignition sources in practice
are external flames or sparks, it is the flash point rather than the autoignition characteristics of a substance which is generally used for the
flammability classification of hazardous materials. Nevertheless, in terms
of the ignition of escaping vapour by steam pipes or other hot surfaces,
the auto-ignition temperature of vapours of liquefied gases are worthy of
note and are also listed in Table 2.9.
Should a liquefied gas be spilled in an open space, the liquid will rapidly
evaporate to produce a vapour cloud which will be gradually dispersed
downwind. The vapour cloud or plume would be flammable only over part
of its downwind travel. The situation is illustrated in general terms in
Figure 2.11. The region B immediately adjacent to the spill area A would
be non-flammable because it is over-rich, i.e. it contains too low a
percentage of oxygen to be flammable. Region D would also be nonflammable because it is too lean, i.e. it contains too little vapour to be
flammable. The flammable zone would be between these two regions as
indicated by C.
Figure 2.11 Flammable vapour zones emanating from a liquefied gas spill
2.2.5
Saturated hydrocarbons
The saturated hydrocarbons methane, ethane, propane and butane are all
colourless and odourless liquids under normal conditions of carriage.
They are all flammable gases and will burn in air and/or oxygen to
produce carbon dioxide and water vapour. As they are chemically nonreactive they do not present chemical compatibility problems with
materials commonly used in handling. In the presence of moisture,
however, the saturated hydrocarbons may form hydrates.
Sulphur compounds such as mercaptans are often added as odourisers
prior to sale to aid in the detection of these vapours. This process is
referred to as "stenching".
2.2.6
Unsaturated hydrocarbons
The unsaturated hydrocarbons ethylene, propylene, butylene, butadiene
and isoprene are colourless liquids with a faint, sweetish characteristic
odour. They are, like the saturated hydrocarbons, all flammable in air
and/or oxygen, producing carbon dioxide and water vapour. They are
chemically more reactive than the saturated hydrocarbons and may react
dangerously with chlorine. Ethylene, propylene and butylene do not
present chemical compatibility problems with materials of construction,
whereas butadiene and isoprene, each having two pairs of double bonds,
are by far the most chemically reactive within this family group. They
may react with air to form peroxides which are unstable and tend to
induce polymerisation. Butadiene is incompatible in the chemical sense
with copper, silver, mercury, magnesium and aluminium. Butadiene
streams often contain traces of acetylene, which can react to form
explosive acetylides with brass and copper.
Water is soluble in butadiene, particularly at elevated temperatures and
Figure 2.12 illustrates this effect. The figures quoted are for the purpose
of illustration only. On cooling water-saturated butadiene the solubility of
the water decreases and water will separate out as droplets, which will
settle as a layer in the bottom of the tank. For instance, on cooling watersaturated butadiene from + 15oC to + 5oC approximately 100 ppm of free
water would separate out. On this basis, for a 1,000 3m tank, 100 3dm of
free water would require to be drained from the bottom of the tank. On
further cooling to below 0 oC this layer of water would increase in depth
and freeze.
Figure 2.12 The solubility of water in butadiene
Chemical gases
2.2.7
The chemical gases commonly transported in liquefied gas carriers are
ammonia, vinyl chloride monomer, ethylene oxide, propylene oxide and
chlorine. Since these gases do not belong to one particular family their
chemical properties vary.
Liquid ammonia is a colourless alkaline liquid with a pungent odour. The
vapours of ammonia are flammable and burn with a yellow flame forming
water vapour and nitrogen, however, the vapour in air requires a high
concentration (16-25 per cent) to be flammable, has a high ignition
energy requirement (600 times that for propane) and burns with low
combustion energy. For these reasons the IMO Codes, while requiring full
attention to the avoidance of ignition sources, do not require flammable
gas detection in the hold or interbarrier spaces of carrying ships.
Nevertheless, ammonia must always be regarded as a flammable cargo.
Ammonia is also toxic and highly reactive. It can form explosive
compounds with mercury, chlorine, iodine, bromine, calcium, silver oxide
and silver hypochlorite. Ammonia vapour is extremely soluble in water
and will be absorbed rapidly and exothermically to produce a strongly
alkaline solution of ammonium hydroxide. One volume of water will absorb
approximately 200 volumes of ammonia vapour. For this reason it is
extremely undesirable to introduce water into a tank containing ammonia
vapour as this can result in a vacuum condition rapidly developing within
the tank.
Since ammonia is alkaline, ammonia vapour/air mixtures may cause
stress corrosion. Because of its highly reactive nature copper alloys,
aluminium alloys, galvanised surfaces, polyvinyl chloride, polyesters and
viton rubbers are unsuitable for ammonia service. Mild steel, stainless
steel, neoprene rubber and polythene are, however, suitable.
Vinyl chloride monomer (VCM) is a colourless liquid with a
characteristic sweet odour. It is highly reactive, though not with water,
and may polymerise in the presence of oxygen, heat and light. Its
vapours are both toxic and flammable. Aluminium alloys, copper, silver,
mercury and magnesium are unsuitable for vinyl chloride service. Steels
are, however, chemically compatible.
Ethylene oxide and propylene oxide are colourless liquids with an
ether-like odour. They are flammable, toxic and highly reactive. Both
polymerise, ethylene oxide more readily than propylene oxide, particularly
in the presence of air or impurities. Both gases may react dangerously
with ammonia. Cast iron, mercury, aluminium alloys, copper and alloys of
copper, silver and its alloys, magnesium and some stainless steels are
unsuitable for the handling of ethylene oxide. Mild steel and certain other
stainless steels are suitable as materials of construction for both ethylene
and propylene oxides.
Chlorine is a yellow liquid, which evolves a green vapour. It has a
pungent and irritating odour. It is highly toxic but is non-flammable
though it should be noted that chlorine can support combustion of other
flammable materials in much the same way as oxygen. It is soluble in
water forming a highly corrosive acid solution and can form dangerous
reactions with all the other liquefied gases. In the moist condition,
because of its corrosivity, it is difficult to contain. Dry chlorine is
compatible with mild steel, stainless steel, monel and copper. Chlorine is
very soluble in caustic soda solution, which can be used to absorb chlorine
vapour.
2.2.8
Toxicity
Toxicity is the ability of a substance to cause damage to living tissue,
impairment of central nervous system, illness or, in extreme cases, death
when ingested, inhaled or absorbed through the skin. Exposure to toxic
substances may result in one or more of the following effects.
i)
i)
Irritation of the lungs and throat, of the eyes and
sometimes of the skin. Where irritation occurs at comparatively low
levels of exposure, it may serve as a warning which must always be
obeyed. However, this cannot be relied upon since some substances
have other toxic effects before causing appreciable irritation.
ii)
ii)
Narcosis, which results in interference with or
inhibition of normal responses and control. Sensations are blunted,
movements become clumsy and reasoning is distorted. Prolonged
and deep exposure to a narcotic may result in anaesthesia (loss of
consciousness). While a victim removed from narcotic exposure will
generally fully recover, the danger is that while under the influence
he will not respond to normal stimuli and be oblivious of danger.
iii)
iii)
Short or long term or even permanent damage to
the body tissue or nervous system. With some chemicals this may
occur at low levels of concentration if exposure is prolonged and
frequent.
2.2.9
Threshold Limit Values (TLV)
As a guide to permissible vapour concentrations for prolonged exposure,
such as might occur in plant operation, various governmental authorities
publish systems of Threshold Limit Value (TLV) for the toxic substances
most handled by industry. The most comprehensive and widely quoted
system is that published by the American Conference of Governmental
and Industrial Hygienists (ACGIH). The recommended TLVs are updated
annually in the light of experience and increased knowledge.
The ACGIH system contains the following three categories of TLV in order
adequately to describe the airborne concentrations to which it is believed
that personnel may be exposed over a working life without adverse
effects. TLV systems promulgated by advisory bodies in other countries
are generally similar in structure.
A)
A)
TLV-TWA. Time weighted average concentration
for an 8 hour day or 40 hour week throughout working life.
B)
B)
TLV-STEL. Short term exposure limit in terms of
the maximum concentration allowable for a period of up to 15
minutes duration provided there are no more than 4 such excursions
per day and at least 60 minutes between excursions.
C)
C)
TLV-C. The ceiling concentration, which should
not be exceeded even instantaneously. While most substances that
are quoted are allocated a TLV-TWA and a TLV-STEL, only those
which are predominantly fast-acting are given a TLV-C.
TLV are usually given in ppm (parts of vapour per million parts of
contaminated air by volume) but may be quoted in mg/r& (milligrams of
substance per cubic metre of air). Where a TLV is referred to but without
the indications TWA, STEL or C, it is the TLV-TWA which is meant.
However, TLV should not be regarded as sharp dividing lines between safe
and hazardous concentrations and it must always be best practice to keep
concentrations to a minimum regardless of the published TLV. TLVs are
not fixed permanently but are subject to revision. The latest revision of
these values should always be consulted. TLV presently quoted by ACGIH
for some of the liquefied gases are given in Table 9.1 by way of illustration
but it must be appreciated that the application of TLV to a specific work
situation is a specialist matter.
2.3
METHODS OF LIQUEFACTION
2.3.1
Evaporation
A liquid change to gas is called
evaporation. This may happen by
evaporation or boiling. To achieve
G as
evaporation, heat of evaporation is
needed. Some liquids evaporate very
quickly, such as gasoline and ether.
Other liquid substances evaporate very
slowly, such as in crude oil.
Evaporation is vapour formed out of
the liquid surface and occurs at all
F luid
temperatures.
This is explained by some of the
liquid’s surface molecules being sent
into the air, which is strongest at high
temperatures, dry air and fresh wind. The specific temperature calls the
amount of heat needed for one kilo of liquid with fixed temperature to
form into one kilo of steam with the same temperature”. The heat from
evaporation is set free when the steam forms to liquid again, or
condenses.
The heat necessary to evaporate one kilo of a certain liquid is called
“specific heat of evaporation”, abbreviated as (r). The unit for specific
heat of evaporation is J/kg.
2.3.2
Boiling
Boiling is steam formed internally in the liquid. The boiling occurs at a
certain temperature, called “the boiling point”. Water is heated in normal
atmospheric pressure (1 atm), in an open container. In common, some
parts of air are always dissolved. The rise in temperature is read from a
thermometer placed in the liquid’s surface. When the temperature has
reached 100oC, steam bubbles will form inside the liquid substance,
especially in the bottom of the container. With continuous heat supply, the
bubbling will rise like a stream towards the surface and further up into the
air. The water is boiling.
The formation of bubbling steam can be explained as follows:
During the heating, the water molecule’s kinetic energy increases,
consequently the molecules demand more space. During the boiling, as
long as there is water in the container, the temperature will be 100oC.
The boiling point is dependent upon the pressure. If the steam or the
atmospheric pressure increases above liquid substance, the boiling point
will also rise. If the surface temperature is just below the boiling
temperature, then the water steam will evaporate on the surface. The
evaporation point and the boiling point will be the same accordingly.
The pressure from the surrounding liquid is the total amount of pressure
above the liquid, Pa, plus the static liquid pressure.
pa
P = Pa + (ρ x g x h )
P = pressure in Pascal (100 000 Pa +
1 bar)
Pa = barometer pressure
ρ=
h
the liquid density in kg/m3
g = force of gravity acceleration
(9,81m/s2)
p
p = pa + (ρ x g x h)
h =
liquid column in meter.
When reducing the pressure above the liquid, the boiling point will also be
reduced. A practical use of this characteristic is the production of fresh
water on board (fresh water generator).
2.3.3
Condensation
Condensation is the opposite of evaporation. If a gas is to be changed to
liquid at the same temperature, we must remove the heat of evaporation
from the gas. A gas can be condensed at all temperatures below the
critical temperature. By cooling a gas, the molecule speed decreases
hence the kinetic speed. The internal energy decreases, as well as, the
molecule units and liquid forms.
2.3.4
Distillation
Distillation is a transferring of liquid to vapour, hence the following
condensing of vapour to liquid. Substances, which were dissolved in the
liquid, will remain as solid substance. With distillation it is possible to
separate what has been dissolved from the substance, which was being
dissolved. When a mixture of two liquids with different boiling point is
heated, will the most volatile liquid evaporate first while the remaining
becomes richer on the less volatile? On board, for instance, seawater is
distillated by use of an evaporator.
2.3.5
Saturated, Unsaturated or Superheated Steam
Let us imagine boiling water, releasing vapour from a container, leading
the steam into a cylinder that is equipped with a tightening piston, a
manometer and two valves. The steam flows through the cylinder and
passes the valves, whereon the valves are closing. There now is a limited
and fixed volume of steam in the cylinder. Around this cylinder a heating
element is fitted. Vapour from the container is constantly sent through
this heating element to ensure that the temperature is maintained
constant.
The piston is pressed inwards, and now the manometer should show a rise
in pressure. But, the manometer shows an unchanged pressure regardless
how much the volume is reduced. What’s happening is, the further the
piston is pressed inwards, some parts of the steam is condensed more
using less volume. The vapour from the heating element removes the
condensed heat, which is liberated during the condensation process.
We find that the amount of steam, which is possible to contain per volume
unit, remains constant when the steam’s temperature is equal to the
condensation point at the set pressure. The room cannot absorb more
vapour, it is saturated with steam and called “saturated”. If the piston is
pressed outwards, the pressure will still show constant. The conclusion is:
•
With temperature equal to the condensation point
by set pressure, steam is saturated.
Steam above boiling water is saturated.
•
•
Saturated steam with a set temperature has a set
pressure. This is called saturation pressure.
With constant temperature saturated steam cannot
•
be compressed.
•
This also concerns vapour as saturated steam of other gases. Using the
same cylinder arrangement as before.
The cylinder contains saturated steam, no water. The piston is drawn
outward. When no water exists over the piston no new steam will be
supplied underneath. The manometer will now show reduced (falling)
pressure as the steam expands. When saturated steam expands without
supplying new steam, it is called unsaturated steam. The room has
capacity to collect more steam.
Unsaturated steam contains lower pressure than saturated steam at the
same temperature. The unsaturated steam in the cylinder can be made
saturated again in two ways. Either by pushing the piston inward to the
originated position, or let the unsaturated steam be sufficiently cooled
down. When the temperature is reduced, the saturation pressure will
reduce. Unsaturated steam will, in other words, have a too high
temperature to be saturated with the temperature it originally had.
Therefore, this often is referred to as superheated steam.
2.4
HAZARDS FROM LIQUEFIED GAS
This section deals with the properties common to all or most bulk liquefied
gas cargoes. These cargoes are normally carried as boiling liquids and, as
a consequence, readily give off vapour.
The common potential hazards and precautions are highlighted in the
following sections.
2.4.1
Flammability
Almost all cargo vapours are flammable. When ignition occurs, it is not
the liquid which burns but the evolved vapour. Different cargoes evolve
different quantities of vapour, depending on their composition and
temperature.
Flammable vapour can be ignited and will burn when mixed with air in
certain proportions. If the ratio of vapour to air is either below or above
specific limits the mixture will not burn. The limits are known as the lower
and upper flammable limits, and are different for each cargo.
Combustion of vapour/air mixture results in a very considerable expansion
of gases which, if constricted in an enclosed space, can raise pressure
rapidly to the point of explosive rupture.
2.4.2
Toxicity
Some cargoes are toxic and can cause a temporary or permanent health
hazard, such as irritation, tissue damage or impairment of faculties. Such
hazards may result from skin or open-wound contact, inhalation or
ingestion.
Contact with cargo liquid or vapour should be avoided. Protective clothing
should be worn as necessary and breathing apparatus should be worn if
there is a danger of inhaling toxic vapour. The toxic gas detection
equipment provided should be used as necessary and should be properly
maintained.
2.4.3
Asphyxia
Asphyxia occurs when the blood cannot take a sufficient supply of oxygen
to the brain. A person affected may experience headache, dizziness and
inability to concentrate, followed by loss of consciousness. In sufficient
concentrations any vapour may cause asphyxiation, whether toxic or not.
Asphyxiation can be avoided by the use of vapour and oxygen detection
equipment and breathing apparatus as necessary.
2.4.4
Anaesthesia
Inhaling certain vapours (e.g ethylene oxide) may cause loss of
consciousness due to effects upon the nervous system. The unconscious
person may react to sensory stimuli, but can only be roused with great
difficulty.
Anaesthetic vapour hazards can be avoided by the use of cargo vapour
detection equipment and breathing apparatus as necessary.
2.4.5
Frostbite
Many cargoes are either shipped at low temperatures or are at low
temperatures during some stage of cargo operations. Direct contact with
cold liquid or vapour or uninsulated pipes and equipment can cause cold
burns or frostbite. Inhalation of cold vapour can permanently damage
certain organs (e.g. lungs).
Ice or frost may build up on uninsulated equipment under certain ambient
conditions and this may act as insulation. Under some conditions,
however, little or no frost will form and in such cases contact can be
particularly injurious.
Appropriate protective clothing should be worn to avoid frostbite, taking
special care with drip trays on deck which may contain cargo liquid.
2.4.6
Comparison of hazards in liquefied gas carriage and in
the transport of normal petroleum
While the carriage of liquefied gases incurs its own special hazards, some
of its features are less hazardous than those of the heavier petroleum.
The following is a brief summary.
Hazards peculiar to carriage of liquefied gases:
(a)
Cold from leaks and spillages can affect the strength and ductility of
ship's structural steel.
(b)
Contact by personnel with the liquids, or escaping gases, or with
cold pipework can produce frost burns.
(c)
Rupture of a pressure system containing LPG could release a
massive evolution of vapour.
Features of liquefied gas carriage resulting in a reduction of hazard
compared with normal tanker operation:
(i)
Loading or ballasting do not eject gases to atmosphere in vicinity of
decks and superstructures. Gas-freeing is rarely performed and
does not usually produce gas on deck.
(ii)
Liquefied gas compartments are never flammable throughout the
cargo cycle. Static electricity and other in-tank ignition sources are
therefore no hazard.
(iii)
There is no requirement for tank cleaning and its associated
hazards.
03.
Cargo Containment
Systems
3 CARGO CONTAINMENT SYSTEMS
3.1
INDEPENDENT TANKS
These types of tanks are completely self-supporting and do not form part
of the ship’s hull and do not contribute to the hull strength. Depending
mainly on design pressure, there are three different types of independent
tanks for gas carriers, Types A, B and C
3.1.1
Independent tanks, type A (MARVS < 0. 7 bar)
Independent tanks of type A are prismatic and supported on insulationbearing blocks and located by anti-roll chocks and anti-flotation chocks.
The tanks are normally divided along their centreline by a liquid-tight
bulkhead; by this feature, together with the chamfered upper part of the
tank, the free liquid surface is reduced and the stability is increased.
When these cargo tanks are designed to carry LPG (at -50oC), the tank is
constructed of fine-grained low-carbon manganese steel.
The Conch design has been developed for carriage of LNG (at-163oC).
The material for these cargo tanks has to be either 9% nickel steel or
aluminium.
Figure 3.1 Prismatic self-supporting Type A tank for a fully refrigerated LPG carrier
3.1.2
Independent tanks, type B (MARVS < 0. 7 bar)
Independent tanks of type B are normally spherical and welded to a
vertical cylindrical skirt, which is the only connection to the ship's hull.
The hold space in this design is normally filled with dry inert gas but may
be ventilated with air provided that inerting of the spaces can be achieved
in the event of the vapour detection system detecting cargo leakage. A
protective steel dome covers the primary barrier above deck level, and
insulation is applied to the outside of the primary barrier surface. This
containment system has been used for carriage of LNG. The material of
construction is either 9% nickel steel or aluminium.
Figure 3.2 Self-supporting spherical Type B tank
3.1.3
Independent tanks, type C (MAR VS< 0. 7 bar)
Independent tanks of type C are cylindrical pressure tanks mounted
horizontally on two or more cradle-shaped foundations. The tanks may be
fitted on, below or partly below deck and be both longitudinally and
transversely located. To improve the poor utilization of the hull volume,
lobe-type tanks are commonly used at the forward end of the ship. This
containment system is used for LPG and LEG. The material, if used for
the construction of tanks designed to carry ethylene, is 5% nickel steel.
Figure 3.3 Type C tanks as found on fully pressurised gas carriers
Figure 3.4 Type C tanks as utilised on semi-pressured/fully refrigerated gas carriers
3.2
MEMBRANE TANKS (MARVS NORMALLY < 0.25 BAR)
Membrane tanks are not self-supporting tanks; they consist of a thin layer
(membrane), normally not exceeding 1 mm thick, supported through
insulation by the adjacent hull structure. The membrane is designed in
such a way that thermal and other expansion or contraction is
compensated for, and there is no undue stressing of it. The membrane
design has been developed for carriage of LNG. The material of
construction is lnvar steel (36% nickel steel) or 9% nickel steel.
3.3
BAR)
SEMI-MEMBRANE TANKS (MARVS NORMALLY < 0.25
Semi-membrane tanks are not self-supporting; they consist of a layer
which is supported through insulation by the adjacent hull structure. The
rounded parts of the layer are designed to accommodate thermal
expansion and contraction, and other types thereof. The semi-membrane
design has been developed for carriage of LNG, and the material of
construction is 9% nickel steel or aluminium.
3.4
INTEGRAL TANKS (MA RVS NORMALLY< 0.25 BAR)
Integral tanks form a structural part of the ship's hull and are influenced
by the same loads which stress the adjacent hull structure, and in the
same manner. This form of cargo containment is not normally allowed if
the cargo temperature is below -1 0 OC. Today, this containment system
is partly used on some LPG ships dedicated to the carriage of butane.
3.5
INTERNAL INSULATION TANKS
Thermal insulation must be fitted to refrigerated cargo tanks for the
following reasons:
a) a) To minimise heat flow into cargo tanks and thus reduce boil-off.
b) b) To protect the general ship structure around the cargo tanks
from the effects of low temperature
For use aboard gas carriers insulation materials should process the
following characteristics:
i)
i)
Low thermal conductivity.
ii)
ii)
Non-flammable or self-extinguishing.
iii)
iii)
Ability to bear loads.
iv)
iv)
Ability to withstand mechanical damage.
v)
v)
Light weight.
vi)
vi)
vapour
Material should not be affected by cargo liquid or
The material’s vapour-sealing properties to prevent ingress of water or
water vapour is very important. Not only can ingress of moisture result in
loss of insulation efficiency but progressive condensation and freezing can
cause extensive damage to the insulation. Humidity conditions must
therefore be kept as low as possible in hold spaces.
04.
The Liquefied Gas
Tanker
4 THE LIQUEFIED GAS TANKER
4.1
GAS TANKER TYPES
Gas carriers can be grouped into six different categories according to the
cargo carried and the carriage condition, i.e.
(a) Fully pressurised ships
(b) Semi-refrigerated/semi-pressurised ships
(c) Semi-pressurised/fully refrigerated ships
(d) Fully refrigerated LPG ships
(e) Ethylene ships
(f) LNG ships
Ship types (a), (b) and (c) are most suitable for the shipment of smallersize cargoes of LPG and chemical gases on short-sea and near-sea routes
whereas ship type (d) is used extensively for the carriage of large-size
cargoes of LPG and ammonia on the deep-sea routes.
4.1.1
(a) Fully pressurised ships
These ships are the simplest of all gas carriers in terms of containment
systems and cargo-handling equipment and carry their cargoes at ambient
temperature. Type C tanks - pressure vessels fabricated in carbon steel
with a typical design pressure of 17.5 barg, corresponding to the vapour
pressure of propane at 45oC, must be used. Ships with higher design
pressures are in service: 18 barg is quite common - a few ships can
accept up to 20 barg. No thermal insulation or reliquefaction plant is
necessary and cargo can be discharged using either pumps or
compressors.
Because of their design pressure the tanks are extremely heavy. As a
result, fully pressurised ships tend to be small with maximum cargo
capacities of about 4,000 m3 and they are used to carry primarily LPG and
ammonia. Ballast is carried in double bottoms and in top wing tanks.
Because these ships utilise Type C containment systems, no secondary
barrier is required and the hold space may be ventilated with air. Figure
3.3 shows a section through a typical fully pressurised ship.
4.1.2
(b) Semi-refrigerated ships
These ships are similar to fully pressurised ships in that they incorporate
Type C tanks - in this case pressure vessels designed typically for a
maximum working pressure of 5-7 barg. The ships range in size up to
7,500 m3 and are primarily used to carry LPG. Compared to fully
pressurised ships, a reduction in tank thickness is possible due to the
reduced pressure, but at the cost of the addition of refrigeration plant and
tank insulation. Tanks on these ships are constructed of steels capable of
withstanding temperatures as low as –10oC. They can be cylindrical,
conical or spherical in shape.
4.1.3
(c) Semi-pressurised/fully refrigerated ships
Constructed in the size range 1,500 to 30,000 m3, this type of gas carrier
has evolved as the optimum means of transporting the wide variety of
gases, from LPG and VCM to propylene and butadiene, found in the busy
coastal gas trades around the Mediterranean and Northern Europe. Like
the previous two types of ship, SP/FR gas tankers use Type C pressure
vessel tanks and therefore do not require a secondary barrier. The tanks
are made either from low temperature steels to provide for carriage
temperatures of –48oC which is suitable for most LPG and chemical gas
cargoes or from special alloyed steels or aluminium to allow the carriage
of ethylene at –104oC (see also ethylene ships). The SP/FR ship's flexible
cargo handling system is designed to be able to load from, or discharge
to, both pressurised and refrigerated storage facilities. A typical SP/FR
ship section is shown in Figure 3.4.
4.1.4
(d) Fully refrigerated LPG ships
Fully refrigerated (FR) ships carry their cargoes at approximately
atmospheric pressure and are generally designed to transport large
quantities of LPG and ammonia. Four different cargo containment
systems have been used in FR ships: independent tanks with double hull,
independent tanks with single side shell but double bottom and hopper
tanks, integral tanks and semi-membrane tanks, both these latter having
a double hull. The most widely used arrangement is the independent tank
with single side shell with the tank itself a Type A prismatic free-standing
unit capable of withstanding a maximum working pressure of 0.7 barg
(Figure 3.1). The tanks are constructed of low-temperature steels to
permit carriage temperatures as low as –48oC. FR ships range in size
from 10,000 to 100,000 m3.
A typical fully refrigerated LPG carrier would have up to six cargo tanks,
each tank fitted with transverse wash plates, and a centre line longitudinal
bulkhead to improve stability. The tanks are usually supported on wooden
chocks and are keyed to the hull to allow expansion and contraction as
well as prevent tank movement under static and dynamic loads. The
tanks are also provided with anti-flotation chocks. Because of the lowtemperature carriage conditions, thermal insulation and reliquefaction
plant must be fitted.
The FR gas carrier is limited with respect to operational flexibility.
However, cargo heaters and booster pumps are often used to allow
discharge into pressurised storage facilities.
Where Type A tanks are fitted, a complete secondary barrier is required.
The hold spaces must be inerted when carrying flammable cargoes.
Ballast is carried in double bottoms and in top-side tanks or, when fitted,
side ballast tanks.
4.1.5
(e) Ethylene ships
Ethylene ships tend to be built for specific trades and have capacities
ranging from 1,000 to 30,000 m3. This gas is normally carried fully
refrigerated at its atmospheric pressure boiling point of –104oC. If Type C
pressure vessel tanks are used, no secondary barrier is required; Type B
tanks require a partial secondary barrier; Type A tanks require a full
secondary barrier and because of the cargo carriage temperature of –
104oC the hull cannot be used as a secondary barrier, so in this case a
separate secondary barrier must be fitted. Thermal insulation and a high
capacity reliquefaction plant are fitted on this type of vessel.
As mentioned, many ethylene carriers can also carry LPG cargoes thus
increasing their versatility. Ballast is carried in the double bottom and
wing ballast tanks and a complete double hull are required for all cargoes
carried below –55oC whether the tanks are of Type A, B or C.
4.1.6
(f) LNG ships
LNG carriers are specialised vessels built to transport large volumes of
LNG at its atmospheric pressure boiling point of –163oC. These ships are
now typically of between 120,000 and 130,000 M3 capacity and are
normally dedicated to a specific project where they will remain for their
entire contract life, which may be between 20-25 years. Apart from a few
notable exceptions built during the early years of LNG commercial
transportation these ships are of three types: (1) Gaz Transport
membrane (Figure 4.1), (2) Technigaz membrane (Figure 4.2) and (3)
Kvaerner Moss spherical independent Type B (Figure 4.3).
All LNG ships have double hulls throughout their cargo length, which
provides adequate space for ballast; the membranes have a full secondary
barrier, the spheres a drip-pan type protection. Another characteristic
common to all is that they burn the cargo boil-off as fuel (permitted with
methane cargo, being lighter than air at ambient temperature - but not
propane or butane which are heavier than air gases).
Figure 4.1 Gaz Transport membrane containment systems as utilised on larger-sized LNG
carriers
Hold spaces around the cargo tanks are continuously inerted except in the
case of spherical Type B containment where hold spaces may be filled with
dry air provided that there are adequate means for inerting such spaces in
the event of cargo leakage being detected. Continuous gas monitoring of
all hold spaces is required.
Figure 4.2 (a) Technigaz membrane containment system as utilised on larger LNG
carriers
To date, reliquefaction plants have not been fitted to LNG vessels
because, being a much colder cargo than LPG, the necessary equipment is
much more costly and it has been more economic to burn the boil-off gas
in steam turbine propulsion plants. However, due to the rising cost of oil
fuel and increasing value accredited to LNG, future designs of LNG ships
tend towards the provision of greater tank insulation (to reduce boil-off), a
reliquefaction plant and diesel engined main propulsion.
Figure 4.2 (b) Detail of the Technigaz membrane’s barrier and insulation
construction
4.2
LAYOUT OF A GENERAL GAS CARRIER
Liquefied Gas tankers are designed, built and operated so they obey rules
and regulations. These rules and regulations are defined by the
Governments of the countries in which the ships are registered, and are
based on rules made Internationally and recommended by IMO. The rules
are strictly applied, and ships are inspected throughout the world to make
sure that they conform. These rules protect you, the ship, the
environment and the cargo, and help to make the operation safe.
It is not permitted for a cargo pump-room to be placed below the upper
deck; nor may cargo pipework be run beneath deck level; therefore
deepwell or submersible pumps must be used for cargo discharge. Cargo
pipework to tanks beneath deck level must be taken through a cargo tank
dome, which penetrates the deck.
Where a gas tanker is fitted with a reliquefaction plant, this plant is
housed in a compressor house on deck. Contiguous to this compressor
house is an electric motor room, which contains the motors for driving the
compressors of the reliquefaction plant and booster pumps when fitted. A
gastight bulkhead must separate the electric motor room and compressor
room.
The IMO codes detail the requirements for mechanical ventilation of these
rooms. Positive pressure ventilation must be provided for the electric
motor room with negative pressure ventilation for the cargo compressor
area, thus ensuring a positive pressure differential between the rooms.
An airlock entrance to the electric motor room from the weather deck,
with two gastight doors at least 1.5 metres apart, prevents loss of this
pressure differential on entry into the motor room. To ensure that both
doors are not opened simultaneously they must be self-closing with
audible and visual alarms on both sides of the airlock. In addition, loss of
over-pressure in the motor room should trip the electric motors within.
The importance of these protective systems is fundamental to the safety
of the gas tanker. Another safety feature associated with the
motor/compressor room area concerns sealing of the driving shafts
penetrating the gastight bulkhead between the compressor and motor
room.
The cargo tanks cannot be used for ballast purposes on gas carriers and
therefore separate ballast tanks are required.
The cargo containment and handling systems must be completely
separate from accommodation spaces, machinery spaces, etc., with
cofferdam separation or other means of gastight segregation between the
cargo area and the engine room, fuel tanks and chain lockers. The IMO
codes also give specific recommendations for positioning of doors leading
from accommodation spaces into cargo areas. In addition, air intakes for
accommodation and engine spaces must be sited at a minimum distance
from ventilation outlets associated with gas dangerous areas. All air
intakes into accommodation and service spaces should be fitted with
closing devices.
Gas tankers are fitted with a fixed water spray system for fire protection
purposes. This covers cargo tank domes, cargo tank areas above deck,
manifold areas, the front of the accommodation area, boundaries of
control rooms facing the cargo area, etc. Minimum water flow rates of 10
litre/m2 per minute for horizontal surfaces and 4 litre/m2 per minute for
vertical surfaces should be achieved. In addition to this fixed water spray
system, all gas tankers must be fitted with a fixed dry powder installation
capable of fighting local cargo area fires. At least two hand hose lines
must be provided to cover the deck area. The dry powder installation is
activated by nitrogen, which is stored in pressure vessels adjacent to the
powder containers.
4.3
SURVIVAL CAPABILITY AND TANK LOCATION
The IMO codes divide gas carriers into four categories, ship types IG, IIG,
IIPG and IIIG, which reflect the hazard rating of the cargoes to be carried.
For example, Type IG ships, where the cargo tanks are located at the
greatest distance from the side shell (and may also be restricted in
capacity), must be used for cargoes representing the greatest hazard, eg.,
chlorine. Ship types IIG/IIPG and IIIG can carry cargoes which represent
progressively decreasing environmental hazards and therefore
progressively less stringent constructional requirements in respect of
damage survival capability in the event of collision or grounding.
A fully refrigerated ship, say with Type A tanks, designed for LPG must
comply with the requirements for tank location and survival capability of a
category IIG ship whereas a semi-refrigerated ship with Type C tanks
carrying LPG can comply with the requirements either of a IIG or a IIPG
ship. For the latter case the Type C pressure vessels must be designed
for a design vapour pressure of at least 7 barg, and a design temperature
of not lower than –55oC. The IIPG category takes into account the fact
that the pressure vessel design provides increased survival capability
when the ship is damaged by collision or grounding.
The IMO codes and classification rules should be referred to for the
detailed construction requirements for each category of ship.
05.
Cargo Equipment and
Instrumentation
5 CARGO EQUIPMENT AND INSTRUMENTATION
“Norgas Challenger”
5.1
TANKS, PIPING AND VALVES
The loading lines and pipes mentioned here refer to gas carrier’s cargo
handling system. This involves liquid lines, vapour lines, condensate return
lines, lines to vent mast, pipes inside the cargo tank and seawater pipes to
the cargo cooling plant.
All loading lines on gas carrier: liquid lines, gas lines and lines to vent mast
have the same requirements as pressure vessels regarding of temperature
and pressure they are meant to handle. All welding on pipes exceeding 75
mm in diameter and 10 mm wall thickness or more must be X-rayed and
classed by the class company. The same regulation do we have on flanges
and spool pieces also.
All loading lines outside the cargo tank must be produced by material with
melting point no less than 925oC. The loading lines on gas carriers are
mostly produced of stainless steel, but low temperature nickel steel is also
in use. All loading lines with an outside diameter of 25 mm or more must
be flanged or welded. Otherwise, lines with an outside diameter less than
25 mm can be connected with treads.
Loading lines designed for cargo with low temperature, less than –10oC
must be insulated from the ship hull. This to prevent the ship hull to be
cooled down to below design temperature. The hull has to be protected
against cold cargo spill under spool pieces and valves on all liquid lines.
This is done with wood planks or plywood. To prevent cold cargo spill on
the hull plates, a drip tray must be placed under the manifold flanges.
All lines that are thermally insulated from the hull must be electrically
bonded to the hull with steel wire or steel bands. On each flange on lines
and pipes where gaskets is used, there must be electrical bonding with
steel wire or steel band from flange to flange.
On all cargo lines where it can be liquid it is required with safety valve.
Vapour from the safety valve outlet must go back to the cargo tank or to
the vent mast. If the return goes to vent mast the pipe must be equipped
with a liquid collector to prevent liquid to the vent mast. The safety valve’s
set point is dependent upon the pressure for which the line is designed. The
safety valves must be tested and sealed by the ship Class Company.
No cargo pipework is allowed beneath deck level on gas carriers; therefore,
all pipework connections to tanks beneath deck level must be taken
through the cargo tank domes, which penetrate the deck. Vapour relief
valves are also fitted on the tank domes; these relieve to vent stacks
whose height and safe distances from accommodation spaces etc. are
specified in the IMO Codes.
Figure 5.1 Cargo tank dome piping arrangement
5.1.1
Cargo valves
Isolating valves for gas carriers must be provided in accordance with the
IMO requirements. Where cargo tanks have a MARVS greater than 0.7
barg (Type C cargo tanks), all main and liquid vapour connections (except
relief valve connections) should normally be fitted with a double valve
arrangement comprising a manually operated globe valve with a remotely
operated isolation valve in series with this manual valve. For Types A and
B cargo tanks with the MARVS less than 0.7 barg the IMO Codes allow
shut-off valves for liquid and vapour connections which can be remotely
actuated but which must also be capable of local manual operation.
Remotely operated emergency shutdown valves are provided at the liquid
and vapour crossovers for all gas carriers. Figure 5.1 shows the cargo tank
dome piping and valving arrangement for a typical semi-refrigerated
vessel.
At several locations around the ship, e.g. bridge front, gangway,
compressor room and cargo control room, emergency control stations,
pneumatic vent valves or electric push buttons are provided which, when
operated, close remotely actuated valves and stop cargo pumps and
compressors where appropriate - effectively creating a "dead ship" as far as
cargo-handling is concerned. Emergency shut down (ESD) is also required
to be automatic upon loss of electric or control power, valve actuator power
or fire at tank domes or manifold where fusible elements are suitably
situated to actuate the ESD signal system. Individual tank filling valves are
required to be automatically closed upon the actuation of an overfill sensor
in the tank to which they are connected. ESD valves may be either
pneumatically or hydraulically operated but in either case must be "fail
safe", i.e. close automatically upon loss of actuating power.
The possibility of surge pressure generation when the ship's ESD system is
actuated during loading is a vital consideration. The situation varies from
terminal to terminal and is a function of the loading rate, the length of the
pipeline at the terminal, the rate of closure of the valve and the valve
characteristic itself. The phenomenon of surge pressure generation is
complex and its effects can be extreme, such as the rupture of hoses or
hard arm joints. Precautions may therefore be necessary to avoid the
possibility of damage. Terminals may need to check ships' ESD valve
closure rates and adjust loading rates accordingly or place on board a
means whereby the ship may actuate the terminal ESD system and so halt
the flow of cargo before the ship's ESD valves start to close. NOTE:
Consultation between the ship and shore must always take place in order to
establish the parameters relevant to surge pressure generation and to
agree upon a safe loading rate.
The types of isolation valve normally found on gas tankers are ball, globe,
gate or butterfly valves. These valves are usually fitted with pneumatic or,
occasionally, hydraulic actuators. Ball valves for LNG and Ethylene service
are provided with some means of internal pressure relief; usually, a hole is
drilled between the ball cavity and downstream side of the valve. Valves
must be of the fire safe type.
Strainers are normally provided at the manifold connections for loading/
discharging. It is important not to bypass these strainers and to ensure
they are frequently checked and cleaned. The strainers are installed to
protect cargo handling plant and equipment from damage by foreign
objects. Many strainers are designed for one-way flow only.
5.2
PRESSURE-RELIEF AND VACUUM PROTECTION SYSTEM
The IMO Codes require at least two pressure relief valves of equal capacity
to be fitted to any cargo tank of greater than 20 M3 capacity. Below this
capacity one is sufficient. The types of valves normally fitted are either
spring-loaded or pilot-operated relief valves. Pilot-operated relief valves
may be found on Types A, B and C tanks while spring-loaded relief valves
are usually only used on Type C tanks. The use of pilot-operated relief
valves on Type A tanks ensures accurate operation at the low pressure
conditions prevailing while their use on Type C tanks, for example, allows
variable relief settings to be achieved using the same valve. Changing the
pilot spring may do this. Figure 5.2 shows a typical pilot-operated relief
valve of this type. Other types of pilot valve are available for adjustment of
“set pressure” and “blow down pressure”.
Figure 5.2 Pilot operated relief valve
Adjustable setting of pilot operated relief valves, where provided, are used
mainly in two different roles. Firstly, they may be used to provide a higher
set pressure (but not exceeding the MARVS) than normal during cargo
handling ('harbour" setting). Secondly, on Type C tanks, they permit an
acceptable means of reducing the MARVS to comply with USCG regulations,
which impose more stringent safety factors in pressure tank design than do
the IMO Code requirements.
Whenever such valves are used for more than one pressure setting a
proper record must be kept of any changes in the pilot valve springs, the
pilot assembly cap always being resealed after such changes.
Cargo tank relief valves relieve into one or more vent stacks. Vent stack
drains should be provided, and regularly checked, to ensure no
accumulation of rainwater, etc., in the stack. Accumulation of liquid has
the effect of altering the relief valve setting due to the resulting increased
backpressure.
The IMO Codes require all pipelines or components which may be isolated
when full of liquid to be provided with relief valves to allow for thermal
expansion of the liquid. These valves can relieve either into the cargo
tanks themselves or, alternatively, they may be taken to a vent stack via
liquid collecting pots with, in some cases, level switch protection and a
liquid vaporising source.
5.3
PUMPS AND UNLOADING SYSTEM
Cargo pumps fitted aboard refrigerated gas tankers are normally of the
centrifugal type, either deepwell or submerged, operating alone or in series
with a deck-mounted booster pump where cargo heating is required on
discharge to pressurise storage from a refrigerated vessel (see 5.3). Some
fully pressurised ships discharge cargo by pressurising tanks and require
booster pumps to assist in the transfer of cargo ashore.
5.3.1
Pump performance curves
An understanding of the significance of a pump performance curve is
important when considering the work done by cargo pumps. Figure 5.3
shows typical set of performance curves for a multi-stage deepwell pump.
Figure 5.3 Pump performance curves for a typical deepwell pump
Curve A
Curve A shows the capacity, given in terms of volumetric flow rate
(normally m3/hr), of the pump as a function of the head developed by the
pump, given in terms of metres liquid column (mlc). Adopting these
parameters, the capacity/head curve is the same irrespective of the fluid
being pumped. Taking the capacity curve shown in Figure 5.3, the pump
will deliver 100 m3/hr with a head of 115 mlc across the pump. To convert
this head into a differential pressure reading the specific gravity of the
cargo being pumped must be known. For example, at a head of 105 mlc
the differential pressure across the pump when pumping ammonia at –33oC
with a specific gravity of 0.68 would be 105 x 0.68 = 71.4 m H2O =
71.4/10.2 = 7 bar.
(Note:-the factor 10.2 in the foregoing equation denotes the height, in
metres, of a water column maintained solely by atmospheric pressure)
Curve B
Curve B shows the Net Positive Suction Head (NPSH) requirement for the
pump in question as a function of pump capacity. The NPSH requirement
at any flow rate through the pump is the positive head of fluid required at
the pump suction over and above the cargo SVP to prevent cavitation at
the pump impeller. For example, at a capacity of 100 m3/hr for the pump
performance shown in Figure 5.3 the NPSH requirement for the pump is 0.5
mlc. This would mean that with a flow rate of 100 M3/hr a minimum head
of cargo equivalent to 0.5 metres would be required at the pump suction to
prevent cavitation. An over-pressure of 0.03 bar in the cargo tank would
be equivalent to 0.5 metres head when pumping ammonia at –33oC. NPSH
considerations are particularly significant when pumping liquefied gases
because the fluid being pumped is always essentially at its boiling point. It
must be remembered that if cavitation is allowed to occur within a deepwell
pump, for example, not only will damage occur to the pump impeller but
also the shaft bearings themselves will be starved of cargo for cooling and
lubrication and bearing damage will quickly result.
Curve C
Curve C shows the power absorbed as a function of pump capacity. This
curve is normally given for water (SG = 1) and can be converted for any
fluid by multiplying by the appropriate specific gravity. In this respect, of
the cargoes normally transported in gas carriers, VCM has the highest
specific gravity (0.97 at its atmospheric pressure boiling point). In cases
where cargo pump motors have been sized on the basis of LPG/NH3
cargoes, it will therefore be necessary to reduce discharge rates when
pumping VCM in order to avoid overloading the motor.
5.3.2
Running pumps in parallel and in series
When gas carriers discharge, cargo tank pumps are usually run in parallel
but where a refrigerated ship discharges to pressurised storage, cargo tank
pumps are run in series with booster pumps. When pumps are run in
parallel their individual performance curves can be combined to give, for
example, a capacity/head curve for two, three or four pumps together.
Taking the pump characterised by Figure 5.3, the capacity/head curve for
running two pumps in parallel can be easily plotted by doubling the flow
rate available at the appropriate head for a single pump, as shown in Figure
5.4.
Figure 5.4 Running centrifugal pumps in parallel –
combined pump characteristics
Figure 5.5 Running centrifugal pumps in series –
combined pump characteristics
Similarly, when running three pumps in parallel the flow rate at the
appropriate developed head can be obtained by multiplying the flow rate at
the same head for a single pump by three. Thus, a series of curves can be
built up from the curve of a single pump.
When pumps are run in series, again the individual performance curves can
be combined to give the appropriate curve for the series configuration.
Figure 5.5 shows how this can be done using, for example, two pumps
characterised by Figure 5.3 in series. This time for each value of flow rate,
the appropriate head developed by the pump is doubled to give the head
developed by two pumps in series.
The cargo flow rates achieved by any pump or combination of pumps will
depend upon the backpressure encountered due to static head (difference
in liquid levels of receiving tank and tank being discharged) and the
resistance to flow in the connecting pipeline. To determine the flow rate in
any particular circumstance the pipeline flow characteristic must be
superimposed upon the pumping characteristic. But suffice it here to note
that because of the way back pressure rises steeply with increasing flow
rate, pumps in series or in parallel will provide flow rates much less than
may be initially imagined from the augmentation of mlc or volumetric flow
capacity respectively of the series or parallel combination. The minimum
necessary pumping power should be used in order to reduce heat input to
the cargo and the rise in saturated vapour pressure of the delivered cargo.
5.3.3
Deepwell pumps
Deepwell pumps are the most common type of cargo pump for LPG
carriers. Figure 4.6 shows a typical deepwell pump assembly. The pump is
operated electrically or hydraulically by a motor, which is flange-mounted
outside the tank. The drive shaft is guided in carbon bearings inside the
discharge tube and these bearings are in turn lubricated and cooled by the
cargo flow up the discharge tube.
Figure 5.6 Typical deepwell pump assembly
The impeller assembly is mounted at the bottom of the cargo tank and will
frequently comprise two or three impeller stages together with a first stage
inducer; this latter is an axial flow impeller used to minimise the NPSH
requirement of the pump. The shaft sealing arrangement consists of a
double mechanical seal with an oil flush. The accurate installation and
alignment of the motor coupling, thrust bearing and mechanical oil seal is
important.
5.3.4
Submerged pumps
This type of pump is used on all LNG carriers, and on many of the larger
fully refrigerated LPG carriers. The pump assembly and electric motor are
close coupled and installed in the bottom of the cargo tank; power is
supplied to the pump motor through copper or stainless steel sheathed
cables, which pass through a gastight seal in the tank dome and terminate
in a flameproof junction box. Submerged pumps and their motors are
cooled and lubricated by the cargo and are therefore susceptible to loss of
flow rate damage. Figure 5.7 shows a typical submerged pump/motor
assembly.
Figure 5.7 Typical submersible pump/motor assembly
5.3.5
Booster pumps
Booster pumps are also of the centrifugal type and may be either vertical
in-line pumps deck-mounted in the appropriate discharge line and driven
by an "increased safety" electric motor or, alternatively, horizontal pumps
installed on deck or in the cargo compressor room driven through a gastight bulkhead by an electric motor installed in the electric motor room.
Figures 5.8 and 5.9 show examples of these types of pump. The particular
pumps shown are fitted with a double mechanical seal, which is methanolflushed and pressurised between the seals.
Figure 5.8 Vertical in-line booster pump
Figure 5.9 Horizontal booster pump
5.3.6
Methanol injection to cargo pumps
The formation of ice or hydrates may occur in ships carrying refrigerated or
semi-refrigerated LPG products or they may be transferred from shore
during loading operations. Such formations may enter cargo pumps, block
lubricating passages, unbalance the impeller or seize bearings. To prevent
such damage it is common practice to inject methanol, or an alternative
freezing point depressant, into the cargo pump to facilitate de-icing.
Because of the danger of methanol contamination to certain LPG cargoes,
injection quantities should be strictly controlled. Cargo filters in the loading
lines should remove hydrates from the shore. A small quantity of methanol
is often injected into cargo pumps, especially submerged pumps, to ensure
that any ice formed from moisture in the pump during initial cool down will
be freed prior to starting the pump.
5.4
CARGO HEATERS AND CARGO VAPORIZERS
A cargo heater is used to heat the cargo when discharging to an ambient
shore tank. A cargo heater is also used when loading a fully pressurised gas
carrier with cargo with temperature less than –10oC. Seawater or oil is used
to heat the cargo in the cargo heater. It is of importance to remember that
the cargo heater is full of water and have good flow out with water before
letting cold cargo into the heater. Fully pressurised gas carriers are carriers
that are designed to transport condensed gases at ambient temperature,
and they normally don’t have cargo cooling plant.
5.4.1
Heat exchanger
Heat exchangers are utilised in several different parts of cargo handling on
gas carriers, as heat exchangers (cargo heater), condensers for cargo
cooling plant, vapour risers, super heaters and oil coolers for compressors.
In most of the heat exchangers seawater is used as the medium on gas
carriers, which the products are cooled or heated against.
The heat exchangers that are used for cargo handling must be designed
and tested to tolerate the products the gas carrier is certified for. Heat
exchangers that are used for cargo handling are considered as pressure
vessels, and IMO requires one safety valve if the pressure vessel is less
than 20 m3 and two safety valves if it is above 20 m3. All heat exchangers
that are used for cargo handling must be pressure tested and certified by
the gas carriers Class Company.
Heat exchangers where water is used as the medium and are utilised for
heating have little or no effect with water temperature less than 10oC.
Seawater became ice at about 0oC and starts to free out salt at about 50oC.
So with operating temperatures with a larger variation than from 10oC to
45oC, one ought to use another cooling medium than seawater. Some
terminals do not accept water as medium in heat exchangers, therefore
one must either heat the cargo on route at sea or the gas carrier must
have heat exchangers that do not use water as medium.
It is of importance to ensure that the water out of a heat exchanger is
never below 5oC. These prevent the water in the heat exchanger from
freezing and eventually damage the heat exchanger.
5.4.2
Tube heat exchangers
Tube heat exchangers are produced with tube bundles either as
straightened pipes or u-formed pipes placed into a chamber. The pipes in
the tube bundle have an inside diameter on 10 to 20 millimetres. There is a
cover installed on each end of the chamber to clean the pipes more easily
and maintain these. It is, at all times, important to ensure that the velocity
of the liquid that is being pumped through the heat exchanger is not too
high, to prevent cavity damage in the tube bundle or the end covers.
Drawing of tube heater
The tube bundle is made of stainless steel, carbon steel, copper-nickel
alloy, brass-brass alloy or titan.
Which choice of material one decides to choose, depends on the product
one will operate and the costs associated with the investment and
maintenance.
In tube heat exchangers, where seawater is used as medium, the product
to be heated goes in the tube bundle. This prevents remaining seawater
from freezing or prevents remnants of salt deposits inside the tubes. Tube
heat exchangers must at regular intervals be cleaned to prevent particles
from settling inside the tubes in the tube bundle or in the end covers. One
must closely check for cavity damage when cleaning the heat exchanger.
Ensure that the gasket is produced in a quality that tolerates the products
and temperature one operates it with. Also, ensure that the gasket is
correctly placed.
5.4.3
Plate heat exchangers
Plate heat exchangers are more utilised in cold storage plants on shore, for
example in the fish industry and the meat industry. Plate heat exchangers
are built with thin plates with double liquid channels. The plates are
installed with the flat side toward each other. The cooling medium and
product are pumped each way in the channels to achieve the best possible
cooling or heating. Water or oil is used as the cooling medium and is
dependent upon the temperature of the product that is to be cooled or
heated. Plate heat exchangers are also used as condensers on newer cargo
cooling plants aboard gas tankers.
Drawing of plate heater
Plate heat exchangers must be cleaned at regular intervals to prevent the
channels from clogging with salt deposits or particles from the medium or
the product.
5.5
RELIQUEFACTION SYSTEMS AND CONTROL OF BOIL-OFF
With the exception of fully pressurised gas carriers, means must be provided
to control cargo vapour pressure in the tanks both during loading and on
passage. In the case of LPG and chemical gas tankers some form of
reliquefaction plant is fitted; this plant is specifically designed to perform the
following essential functions:
(1) To cool down the cargo tanks and associated pipe work before loading;
(2) To reliquefy the cargo vapour generated by flash evaporation, liquid
displacement and boil-off during loading when there is no vapour
return line to shore;
(3) To maintain or reduce cargo temperature and pressure within the
prescribed design limits of the cargo system on passage.
There are two main types of liquefaction plant:
(a) Direct cycles - where the evaporated or displaced cargo vapour is
compressed, condensed and returned to the tank. This is the most
commonly used system, but may not be employed for certain gases. (See
IMO Codes, Chapter 17).
(b) Indirect cycles - where an external refrigeration system is employed
to condense the cargo vapour without it being compressed. This cycle is
relatively uncommon, as it requires, for efficiency, a very cold refrigerant
and large surfaces.
There are three main types of direct cycle:
i)
i)
Single-stage direct cycle
A simplified flow sheet of a single compression stage reliquefaction cycle is
shown in Figures 5.11 (a) and (b). Where suction pressures are relatively
high as in the carriage of semi-refrigerated products, then this cycle is
suitable. Boil-off vapours from the cargo tank are drawn off by the
compressor and compressed. The compression process increases pressure
and temperature of the vapour allowing it to be condensed against
seawater in the condenser. The condensed liquid is then flashed back to
the tank via a float-controlled expansion valve. The returned liquid/vapour
mixture to the cargo tank may either be distributed by a spray rail at the
top of the cargo tank, or taken to the bottom of the tank to discourage
revaporisation, depending on whether the tank is empty or full respectively
Figure 5.11 (a) Single stage direct reliquefaction cycle
Figure 5.11 (b) Single-stage direct
reliquefaction cycle Mollier diagram
ii)
ii)
Two-stage direct cycle
A simplified flow sheet showing a two-stage direct cycle is shown in Figures
5.12(a) and (b). The two-stage cycle with inter-stage cooling is used
where suction pressures are low and, as a result, compression ratios high
(assuming sea water condensing) compared to the single-stage cycle.
Two-stage compression with cooling between the stages is therefore
sometimes necessary to limit compressor discharge temperatures, which
increase significantly with increasing compression ratio.
Figure 5.12 (a) Two-stage direct reliquefaction cycle with interstage cooling
Figure 5.12 (b) Two-stage direct reliquefaction cycle Mollier diagram
The vapour from the first stage discharge is taken to an intercooler where
its superheat is removed. The cooling medium is cargo liquid "flashed
down" to intercooler pressure from the sea water-cooled condenser/
receiver. The remaining parts of the cycle are similar to the single-stage
cycle.
(iii) Cascade direct cycle
The cascade system uses a refrigerant such as R22 to condense cargo
vapours; a simplified flow sheet is shown in Figure 4.13. The single-stage
compression of cargo vapour is identical to the single-stage direct cycle,
but the cargo condenser is cooled using R22 instead of seawater. The
cargo, in condensing, evaporates the liquid R22 and the R22 vapours are
then taken through a conventional R22 closed refrigeration cycle
condensing against seawater - hence the term cascade. The cascade cycle
is used for fully refrigerated cargoes and plant capacities are not so
affected by seawater temperature changes, as are other reliquefaction
cycles.
Figure 5.13 Simplified cascade reliquefaction cycle (sea water)
5.6
CARGO COMPRESSORS
Compressors are used as vapour pumps in all modern cargo cooling plants,
either to compress or pump cargo vapour. Compressors are also used to
compress or pump cooling medium as Freon vapour on indirect cargo
cooling plant and cascade plant. The compressors in the cargo cooling
plants are produced either as piston, screw or centrifugal type. We will now
look at the different types of compressors and starting with piston
compressors.
5.6.1
Piston compressors
Piston compressors used directly against cargo are of oil free type. Oil free
compressors are used to prevent pollution of oil into the cargo, and thereby
contamination of the cargo. All cargoes we are cooling demand a high rate
of purity. Consequently, it cannot be mixed with oil or be polluted by other
products. With an oil free piston compressor, we mean that the cylinder
liners are not lubricated or cooled with oil.
Piston compressors that are used against Freon normally have oil
lubrication of cylinder liners. Piston compressors are either built with
cylinders in line, v-form or w-form. Compressors with cylinders in line are
built with two or three cylinders either single-acting or double-acting. Vform compressors are built with two, four, six, eight or twelve cylinders and
are single acting.
5.6.2
Double-acting compressors
Double-acting compressors are normally oil free and compress the vapour
above and under the piston. The vapour is compressed on top of the piston
when the piston goes up and vapour is sucked into the cylinder below the
piston. The vapour is compressed below the piston when the piston goes
down and is sucked into the cylinder above the piston. This indicates that
each cylinder has two suction valves and two pressure valves. The pistons
are equipped with compression grooves and are not equipped with piston
rings.
There is no oil lubrication of the piston itself, but there is oil in the
crankcase on the compressor. It is of importance that the sealing device
between the cylinder liner and crankcase is intact. In the first stage, the oil
pressure in the crank is checked and compared to the suction pressure and
the cargo tank pressure. Check the user manual for the cargo compressors
and the marginal values for the pressure difference with oil and suction.
This type of compressor is used as cargo compressor onboard gas carriers.
It is important to change the oil in the crank when changing cargo. This is
to prevent pollution to the next cargo from the previous cargo. Small
amounts of leakage between the cylinder and crank will at all times occur,
so the oil in the crank contains some of the product that is cooled.
Double action compressor
5.6.3
Single-acting compressors
Single-acting compressors compress and suck the gas on one side of the
piston and then normally above the piston. A suction valve and pressure
valve is then installed in the top of the cylinder. The cylinder top is springloaded as a safety precaution against liquid “knock”. The compressors are
built with the cylinders in pairs: two, four, six, eight and twelve, then often
as v-form or w-form. Single-acting compressors are used both as Freon
and cargo compressors on gas tankers.
Piston compressors are operated by electric motor with direct transmission
or strap transmission with a constant number of revolutions. The number of
revolutions is between 750 to 1750 rpm. Unloading of the compressor
occurs by hydraulic lifting of the suction valves. The drawback of piston
compressors is that they are vulnerable when the cylinder liner is filled with
liquid and they also have relatively low capacity for cooling.
Onboard many gas tankers; there is a liquid receiver on the vapour line
between the cargo tank and the cargo compressor, which prevents the
liquid from being carried with into the compressor. The liquid receiver is
equipped with a level alarm to control the liquid level.
Single-action compressor
5.6.4
Screw compressors
Screw compressors are either oil free or oil lubricated. The type used on
the cargo side must be of oil free type for the same reason as the piston
compressors.
Screw compressor
The principle for screw compressors are two rotating screws, the screw that
operates has convex threads and the operated screw has concave threads
which rotates them in different directions. Vapour is screwed through the
threads and with rotation on the screws; the confined gas volume
decreases successively resulting in compression. Please also refer to “cargo
cooling process” for more information.
The advantage with screw compressors is that they wear few parts and
have low weight in proportion to cooling capacity. Oil free screw
compressors are operated by electric motors with a constant number of
revolutions and have a gear transmission for the compressor, which has
approx. 12000 rpm. The high speed prevents leakage between the pressure
and suction side. Screw compressors with oil injection in the rotor house
have a lower number of revolutions, about 3500 rpm. One can also use
electric motors with direct shaft transmission.
5.6.5
Oil free screw compressors
Screw compressors for use with liquefied gas cargoes can be either dry oilfree or oil-flooded machines. In the dry machines the screw rotors do not
make physical contact but are held in mesh and driven by external gearing.
Due to the leakage effects through the clearances between the rotors, high
speeds are necessary to maintain good efficiency (typically 12,000 rpm).
Figure 5.16 shows a diagram of a typical rotor set with the common
combination of four and six lobes.
The lobes intermesh and gas is
compressed in the chambers 1, 2, 3, which are reduced in size as the
rotors turn. The compressor casing carries the suction and discharge ports.
The oil-flooded machine relies on oil injection into the rotors and this
eliminates the need for timing gears, the drive being transmitted from one
rotor to the other with the injected oil acting as lubricant and coolant.
Because of the oil sealing between the rotors, gas leakage is much less and
therefore oil-flooded machines can run at lower speeds (3,000 rpm). An oil
separator on the discharge side of the machines removes oil from the
compressed gas. Capacity control of screw compressors can be achieved in
a number of ways, the most common being to use a sliding valve, which
effectively reduces the working length of the rotors. This is more efficient
than suction throttling. Screw compressors consume more power than
reciprocating compressors.
Figure 5.16 Typical rotor set of dry oil-free screw compressor
5.6.6
Compressor suction liquid separator
It is necessary to protect cargo vapour compressors against the possibility
of liquid being drawn into the compressor. Such a situation can seriously
damage compressors since liquid is essentially incompressible. It is normal
practice, therefore, to install a liquid separator on the compressor suction
line from the cargo tanks, the purpose of this vessel being to reduce the
vapour velocity and, as a result, allow any entrained liquid to be removed
from the vapour stream. This separator vessel is fitted with high-level
sensors, which set off an alarm and trip the compressor.
5.6.7
Purge gas condenser
Many reliquefaction plants are fitted with a shell and tube heat exchanger
mounted above the cargo condenser. The purpose of this heat exchanger
is to condense any cargo vapours which, mixed with incondensible gases
such as nitrogen, have failed to condense at the pressure and temperature
existing in the main condenser. For example, commercial propane that
may have two per cent ethane in the liquid phase, will have perhaps 14 per
cent ethane in the vapour phase, ethane being the more volatile
component. This can cause difficulties in a conventional sea water-cooled
condenser.
Figure 5.17 shows a typical purge gas condenser system. The gases not
condensed in the main condenser are displaced into the shell of the purge
condenser. Here they are subjected to the same pressure as exists in the
main condenser but to a condensing temperature equivalent to the outlet
temperature from the expansion valve since the whole or part of this liquid
passes through the tube side of the purge condenser. This lower
condensing temperature allows cargo vapours to be condensed with any
incondensible gases being purged from the top of the purge gas condenser
by a pressure control system.
Figure 5.17 Typical purge gas condenser system
5.6.8
LNG boil-off and vapour handling systems
LNG ships use steam turbine-driven axial flow compressors to handle boiloff vapours produced during cool down, loading, loaded and ballast
passages. Normally, a low-duty compressor handles the boil-off whilst on
passage; a high duty compressor handles vapours produced during cool
down and loading, returning these vapours to shore.
Whilst on passage the low-duty compressor collects the boil-off from a
common header connected to each cargo tank, passes it through a steam
heater to the poop front, whence it enters a specially designed double duct
trunking system leading to the boiler fronts or diesel engine dual fuel
systems. This trunking is continuously monitored for leakage and has
automatic shut down protection in the event of system malfunction or
leakage.
The compressors are provided with surge controls and other protective
devices.
5.7
INERT GAS SYSTEM
On gas carriers inert gas is used for different purposes, some are
requirements other is to maintain the ships hull and spaces:
Have neutral atmosphere in hold and inter barrier
•
spaces
Elimination of cargo vapour from the cargo tank
•
when gas freeing
Eliminating oxygen from the cargo tank before
•
loading
Drying up hold spaces or inter barrier spaces to
•
achieve a neutral atmosphere and to prevent corrosion in the spaces
Placing a neutral vapour above the cargo if required
•
•
When carrying flammable cargo on fully refrigerated gas carriers there is a
requirement to have a neutral atmosphere in the hold space or inter barrier
space either with dry inert gas or nitrogen. If the gas carrier does not have
an inert gas plant or nitrogen plant, it must have a storage vessel with inert
gas or nitrogen with capacity of 30 days and nights consumption. The
definition of consumption here is the leakage in the vents and manhole. If
the cargo is not flammable we can have dry air, inert gas or nitrogen in the
spaces.
If the cargo is Ammonia, one must not use inert gas that contains
carbon dioxide, only dry air or nitrogen, because carbon dioxide reacts
chemically with Ammonia. It is always beneficial to keep spaces around the
cargo tanks dry.
Gas carriers use various forms of inert gas and these are listed below:
•
•
Inert gas from combustion-type generators
•
•
•
Nitrogen from shipboard production systems, and
•
Pure nitrogen taken from the shore (either by road
tanker or barge)
Unlike oil tanker inert gas systems, which have their design and operation
established by extensive regulations and guidelines, the fitting of inert gas
systems to gas carriers is subject to limited advice in the Gas Codes,
special consideration by administrations and the particular demands of the
trade. In general, for gas carriers, the production of combustion generated
inert gas will be covered in new building specifications at about one per
cent oxygen
LNG ships were once provided with storage facilities for liquid nitrogen but
newer designs include a nitrogen generation plant. However, up to now,
the quantity of nitrogen produced on board has not been of sufficient
volume for tank-inerting operations. It is fitted mainly for interbarrier
space inerting. Where cargo tank inerting is required on LNG ships,
nitrogen from the shore, or combustion-generated inert gas is used.
Most ships, barring only the smallest pressurised gas carriers, have the
capability of generating their own inert gas. Furthermore, all LNG ships
have the capability of producing nitrogen for hold space and interbarrier
space inertion - this is a necessary specification, as the carbon dioxide in
inert gas would freeze when in close proximity to the cargo. The methods
of producing the inert gases, as listed at the beginning of this section, are
covered below.
5.7.1
Inert gas generators
The Gas Codes require continuous oxygen monitoring in the inert gas
stream and the oxygen content should normally be no more than about one
per cent. High oxygen content can trigger an alarm; however, the
generator is not normally shut down on this alarm but the gas is diverted to
atmosphere via a vent riser.
The main advantages of the on board inert gas generator are as follows:
•
•
The cost of inert gas is less than the purchase of liquid nitrogen
The inert gas plant capacity is available either at sea or in port
The disadvantages of the combustion-type generator centre on the quality
of gas produced. Combustion must always be carefully adjusted to avoid
the production of toxic carbon monoxide and soot. Also, even under good
operating conditions, the volume of oxygen in the inert gas may be
unsuitable for use with the chemical gases. Accordingly, given that an
oxygen-critical gas is to be loaded, as a preliminary operation, pure
nitrogen must be taken from the shore.
Inert gas produced by the careful combustion of diesel or gas oil, results in
a reduced oxygen content in the products of combustion. In the inert gas
generator, the resulting gases are further treated to give an inert gas of
acceptable standard. Apart from plant operation, the final quality of the
inert gas also depends on the fuel used and generally fuel of low sulphur
content is preferred. In this regard, experience often dictates that gas oil
should be used in preference to marine diesel oil but bunker prices also
have a bearing on the final choice.
The quality of the inert gas produced, however, is very dependent on the
conditions under which the generator is operated and, in this respect, the
manufacturer's guidance should be closely followed. A particular point to
watch is that poorly maintained plant can produce significant Quantities of
carbon monoxide or soot such that, even after aerating, carbon monoxide
levels in a tank may be unacceptable.
The mode of operation is shown in Figure 5.18. Here it will be seen that the
inert gas generator has three main parts. These are as follows:
•
•
•
A combustion chamber with scrubbing and cooling (the
generator)
A refrigerated drier - cooled normally by R22, and
An absorption drier
Figure 5.18 Flow diagram of an inert gas generator
5.7.2
Combustion chamber
Combustion-type generators must be located outside the cargo area and
are usually installed in the ship's engine room. It is usual to find the inert
gas main permanently piped into the cargo holds and temporary
connections are provided between the inert gas main and the cargo system
for tank inerting operations. When not in use, these must be disconnected
and blanks fitted. Two non-return valves (or equivalent) are fitted in the
inert gas main to prevent any back-flow of cargo vapours. When not being
used for high capacity tank inerting operations the inert gas plant is used
from time to time to top up hold and interbarrier spaces.
Within the combustion chamber, the burner is designed to ensure good
combustion so producing a minimum of oxygen residue in the inert gas.
Operationally, however, there is a fine balance to be achieved in generator
adjustment as minimising oxygen output tends to increase the production
of carbon monoxide: and further adjustment can result in the
overproduction of soot. The combustion chamber itself is water-jacketed.
After combustion, the inert gas enters the washing section of the generator
at a very high temperature and is cooled and scrubbed by spraying with
seawater. This is also carried out for the removal of soluble acid gases
such as sulphur dioxide and the oxides of nitrogen. The inert gas is then
filtered to remove solid particles. The gas leaves the generator at
approximately five degrees Centigrade above sea water temperature and
by this time it should be essentially free from sulphur oxides formed by
burning the sulphur present in the fuel - but it is saturated with water
vapour. Accordingly, it is then further cooled and dried (as covered below)
and delivered to the cargo tanks.
5.7.3
The refrigerated drier
In the refrigerated drier, the inert gas is cooled to approximately four
degrees Centigrade, resulting in the condensation of much of the water
vapour. Figure 5.19 shows the content of water vapour in saturated inert
gas as a function of temperature. From this diagram, the reduction in
water vapour content can be seen as the temperature is reduced.
Figure 5.20 Saturated water content of inert gas
5.7.4
The absorption drier
The absorption drier consists of two vessels filled with activated alumina or
silica gel. One vessel is on drying duty while the other is being
regenerated. Typically, the cycle time is six hours.
Drying in the absorption drier reduces the dew point of the inert gas to 400C or below. A layer of molecular sieves can be added to the bottom of
the drying tower to improve the dew point. In order to ensure stable
combustion in the generator, the pressure in the drying system must be
kept constant and this is achieved by means of a pressure control valve as
shown in Figure 5.20.
5.7.5
Nitrogen production on ships
The most common system utilised for the production of nitrogen on ships is
an air separation process. This system works by separating air into its
component gases by passing compressed air over hollow fibre membranes.
The membranes divide the air into two streams - one is essentially nitrogen
and the other contains oxygen, carbon dioxide plus some trace gases. This
system can produce nitrogen of about 95 to 97 per cent purity. The
capacity of these systems depends on the number of membrane modules
fitted and is dependant on inlet air pressure, temperature and the required
nitrogen purity. Figure 5.21 shows one such system.
Figure 4.21 The membrane system for producing nitrogen
5.7.6
Pure nitrogen from the shore
The quality of inert gas produced by shipboard systems is usually
inadequate for oxygen-critical cargoes - see strict in-tank oxygen
requirements in Table 2.3(b). Bearing in mind the components in the inert
gas, this may create restrictions on use if tanks have been previously gasfreed for inspection; and this is often necessary when a change in grades is
involved. Under these circumstances, and prior to loading, it is normal for
shipmasters to arrange for cargo tanks to be inerted with pure nitrogen,
taken from the shore. Road tanker or barge usually delivers this. As
deliveries are in liquid form, where immediate inerting is required, a
nitrogen vaporiser is needed.
5.8
INSTRUMENTATION
A commonly used definition of area safety classification for electrical
equipment in shore installations is as follows:
Zone 0: An area with a flammable mixture continuously present.
Zone 1: An area where flammable mixtures are likely to occur during
normal operations.
Zone 2: An area where flammable mixtures are unlikely to occur during
normal operations.
The electrical installations of all gas carriers are subject to the
requirements of the Flag Administration, the Classification Society and of
IMO. Zones and spaces are classified as either "gas-safe" or "gasdangerous" depending on the risk of cargo vapour being present. For
example, accommodation and machinery spaces are "gas-safe", while
compressor rooms, cargo tank areas and holds, etc. are "gas-dangerous".
In gas-dangerous spaces, only electrical equipment of an approved
standard may be used; this applies to both fixed and portable electrical
equipment. There are several types of certified safe electrical equipment
found on gas carriers.
5.8.1
Intrinsically safe (i.s.) equipment
Intrinsically safe equipment is defined as an electrical circuit of connected
apparatus and wiring in which no spark or thermal effect under normal
operation or specified fault conditions is capable of causing ignition of a
given explosive mixture. Limitation of such energy may be achieved by
placing a barrier in the electrical supply in the 'safe' area as shown in
Figure 5.22. Zener barriers are frequently used and in the circuit shown
the voltage is limited by the Zener diodes, and the maximum current flow
to the hazardous area is restricted by the resistors.
The use of
intrinsically safe systems are normally limited to instrumentation and
control circuitry in hazardous areas. Because of the very low energy
levels to which they are restricted, intrinsically safe systems cannot be
used in power circuits.
Figure 5.22 Intrinsic safety using zener barriers
5.8.2
Flameproof equipment
A flameproof enclosure is one which can withstand the pressure developed
during an internal ignition of a flammable mixture and whose design is
such that any products of the explosion occurring within the enclosure
would be cooled below ignition temperature before reaching the
surrounding atmosphere.
Therefore, the gap between flanged joints through which the hot gases
are allowed to escape (flame path) is very critical and great care must be
taken in assembly and maintenance to ensure that this flame path is
maintained; no bolts must be omitted or tightened incorrectly; the gap
must not be reduced by painting, corrosion or other obstructions.
5.8.3
Pressurised or purged equipment
This is a technique to ensure that an enclosure remains essentially gasfree either by pressurisation or by purging. In the case of pressurisation
an overpressure of 0.5 mbar relative to the surrounding atmosphere must
be maintained by leakage compensation while in the case of purged
enclosure, a continuous supply of purging gas must be provided to the
enclosure. Air or inert gas can be used.
5.8.4
Liquid level
Both the IMO Codes and Classification Society Rules require every cargo
tank to be fitted with at least one liquid level gauge; specific types of
gauging system are required for certain cargoes as defined in Chapter XIX
of the IMO Code.
The IMO classification for gauging systems is as follows:
(a)
Indirect systems - weighing or pipe flow meters.
(b) Closed devices which do not penetrate the cargo tank - ultrasonic
devices or radioisotope sources.
(c)
Closed devices which penetrate the cargo tank - float gauges,
bubble tube indicators, etc.
(d) Restricted devices which penetrate the tank but which release small
volumes of liquid or vapour when in use, such as fixed or slip, tube
gauges. When not in use, the restricted device should be kept completely
closed.
The most common types of level gauging on conventional gas carriers are
those described in (c) and (d) above.
5.8.5
Float gauges
The float gauge is widely used in all tanker work and consists of a float
attached by a tape to an indicating device which can be arranged for local
and remote readout. Figure 5.23 shows a typical float gauge which is
normally installed in a tubular well or with guide wires, with a gate valve
for isolation so that the float can be serviced in a safe atmosphere. The
float must be lifted from the liquid level when not in use; if left down, the
fluctuation in level at sea will damage the tape-tensioning device. Float
gauges cannot normally register a liquid level of less than four inches in
depth.
Figure 5.23 Typical float gauge
5.8.6
Nitrogen bubbler gauges
This system measures the pressure necessary to displace liquid from a
small bore tube mounted vertically in a tank.
Enough nitrogen is
introduced into the tube to displace the liquid and just begin to bubble at
the bottom. The pressure necessary to do this is measured and is a
function of the liquid level and the liquid density. For cargoes of known
density, level readout is obtained directly. By installing two such tubes
alongside each other and with lower extremities a known vertical distance
apart, the density of the cargo can also be determined. Figure 5.24 shows
the principle of the bubbler gauge.
Figure 5.24 Bubbler-type level gauge
5.8.7
Differential pressure gauges
This device operates on differential pressure between liquid and vapour.
The signal lines for the instrument are normally purged with inert gas.
This type of gauge can only be used on ships when the tank is all above
deck, thus it is more generally found in use ashore. Figure 5.25 shows
the principle of the differential pressure gauge.
Figure 5.25 Differential pressure level gauge
5.8.8
Capacitance gauges
This type of gauge measures the change in electrical capacitance between
two probes as cargo liquid rather than vapour takes up the space between
them. Figure 5.26 illustrates the device where the two probes enclosed
within an open protective tube extend throughout the depth of the tank
and provide a continuous indication of liquid content at all levels. For
single preset level indication, as for a high level alarm or overfill shut-off,
a short probe sensor may be fitted horizontally precisely at the level
required. The electrical circuits are, of course, intrinsically safe and the
devices, having no moving parts, are very reliable but must be kept free
of dirt, rust and water/ice since such contaminants will cause inaccuracy.
Figure 5.26 Electrical capacitance level gauge
5.8.9
Radar gauges
Another type tank gauging equipment is that designed to operate in the
principle of radar. Such equipment works at very high frequencies –
approximately 11 gigahertz. Radar type liquid level gauges have now been
specially developed for liquefied gases and their usage on gas tankers.
The equipment provides measurements adequate to meet industry
requirements.
5.8.10
Level alarm and automatic shutdown systems
With the exception of Type C tanks whose capacity is less than 200 m3,
every cargo tank must be fitted with an independent high-level sensor
giving an audible and visual alarm. Float, capacitance or ultrasonic
sensors may be used for this purpose. This high level alarm or other
independent sensor is required automatically to stop the flow of cargo to
the tank. During cargo loading there is a danger of generating significant
surge pressure if the valve stopping the flow closes too quickly against a
high loading rate and when no other tank is open to the flow.
5.8.11
Pressure and temperature monitoring
The IMO Codes call for pressure monitoring throughout the cargo system
including cargo tanks, pump discharge lines, liquid and vapour crossovers,
etc. In addition, pressure switches are fitted to various components to
protect personnel and equipment by operating alarms and/or shutdown
systems.
The IMO Codes also require at least two devices for indicating cargo
temperatures, one placed at the bottom of the cargo tank and the second
near the top of the tank, below the highest allowable liquid level. It is
necessary to be aware of the lowest temperatures to which the cargo
tanks steel can be exposed and these values should be marked on the
temperature gauges.
Where cargo is carried in Type A tanks below –55oC, the IMO Codes call
for temperature-indicating devices within the insulation or on the hull
structure adjacent to the cargo containment systems. The devices should
be set to provide adequate warning prior to the lowest temperature for
the hull steel being approached.
The Codes also call for temperature devices to be fitted to certain tanks in
order to monitor the cargo system during cool down and warm-up
operations to avoid undue thermal stresses being set up.
5.8.12
Gas detection systems
The provision of efficient gas detection systems on board gas tankers is of
great importance. The IMO Codes call for every gas carrier to have a
fixed gas detection system with audible and visual alarms on the
navigating bridges in the cargo control room and at the gas detector
readout location. Detector heads must be provided in the following:
(a)
Cargo compressor room.
(h)
Electric motor rooms.
(c)
Cargo control rooms unless classified as gas-safe.
(d) Enclosed spaces such as hold spaces and interbarrier spaces
excepting hold spaces containing Type C cargo tanks.
(c)
Air locks.
(f)
Vent hoods and gas to E.R. supply ducts (LNG ships only).
The detector heads should be sited with due regard to the density of the
vapours of the cargo being carried, i.e. heavier than air vapours at low
level and lighter-than-air vapours at high level. The sensing units from
the gas detection system are normally located in the cargo control room,
if fitted, or the wheelhouse. Provision should be made for regular testing
of the installation; span gas of a certified mixture for calibration purposes
should be readily available and permanently piped if possible.
Sampling and analysing from each detector head is done continuously and
sequentially; the Codes call for sampling intervals from any one space
generally not exceeding 30 minutes. Alarms should be activated when the
vapour concentration reaches 30 per cent LFL.
In addition to the fixed gas detection system, every vessel must have at
least two sets of portable gas detection equipment, together with means
for measuring oxygen levels in inert atmospheres.
It is of fundamental importance that all personnel on gas carriers are
familiar with gas detection equipment and its operating principles.
Manufacturer's instructions should always be fully read and understood.
06.
Tank Environmental
Control
6 TANK ENVIRONMENTAL CONTROL
6.1
METHODS OF CONTROL
Before any cargo operations are carried out it is essential that cargo tanks
be thoroughly inspected for cleanliness; that all loose objects are
removed; and that all fittings are properly secured. In addition, any free
water must be removed. Once this inspection has been completed, the
cargo tank should be securely closed and air-drying operations may start.
Figure 6.0 Sequence of operations
6.2
WARMING UP
Drying the cargo handling system in any refrigerated ship is a necessary
precursor to loading. This means that water vapour and free water must
all be removed from the system. If this is not done, the residual moisture
can cause problems with icing and hydrate formation within the cargo
system. (The reasons are clear when it is appreciated that the quantity of
water condensed when cooling down a 1,000M3 tank containing air at
atmospheric pressure, 30oC and 100% humidity to 0oC would be 25
litres.)
Whatever method is adopted for drying, care must be taken to achieve the
correct dew point temperature. Malfunction of valves and pumps due to
ice or hydrate formation can often result from an inadequately dried
system. While the addition of antifreeze may be possible to allow freezing
point depression at deep-well pump suctions, such a procedure must not
substitute for thorough drying. (Antifreeze is only used on cargoes down
to –48oC; propanol is used as a de-icer down to –108oC but below this
temperature, for cargoes such as LNG, no de-icer is effective.)
Tank atmosphere drying can be accomplished in several ways. These are
described below.
6.2.1
Drying using inert gas from the shore
Drying may be carried out as part of the inerting procedure when taking
inert gas from the shore. This method has the advantage of providing the
dual functions of lowering the moisture content in tank atmospheres to
the required dew point and, at the same time, lowering the oxygen
content. A disadvantage of this and the following method is that more
inert gas is used than if it is simply a question of reducing the oxygen
content to a particular value.
6.2.2
Drying using inert gas from ship’s plant
Drying can also be accomplished at the same time as the inerting
operation when using the ship's inert gas generator but satisfactory water
vapour removal is dependent on the specification of the inert gas system.
Here, the generator must be of suitable capacity and the inert gas of
suitable quality - but the necessary specifications are not always a design
feature of this equipment. The ship's inert gas generator is sometimes
provided with both a refrigerated dryer and an adsorption drier which,
taken together, can reduce dew points at atmospheric pressure to –45oC
or below.
6.2.3
On board air-drying systems
An alternative to drying with inert gas is by means of an air-drier fitted on
board. The principle of operation is shown in Figure 6.1. In this method,
air is drawn from the cargo tank by a compressor or provided by the on
board inert gas blower (without combustion) and passed through a
refrigerated drier. The drier is normally cooled by R22 refrigerant. Here
the air is cooled and the water vapour is condensed out and drained off.
The air leaving the drier is, therefore, saturated at a lower dew point. A
silica gel after-drier fitted downstream can achieve further reduction of the
dew point. Thereafter, the air may be warmed back to ambient conditions
by means of an air heater and returned to the cargo tank. This process is
continued for all ship tanks (and pipelines) until the dew point of the intank atmosphere is appropriate to carriage conditions.
Figure 6.1 Air Drying – operational cycle
6.3
INERTING
Inerting cargo tanks, cargo machinery and pipelines is undertaken
primarily to ensure a non-flammable condition during subsequent gassingup with cargo. For this purpose, oxygen concentration must be reduced
from 21 per cent to a maximum of five per cent by volume although lower
values are often preferred.
However, another reason for inerting is that for some of the more reactive
chemical gases, such as vinyl chloride or butadiene, levels of oxygen as
low as 0.1 per cent may be required to avoid a chemical reaction with the
incoming vapour. Such low oxygen levels can usually only be achieved by
nitrogen inerting provided from the shore
There are two procedures, which can be used for inerting cargo tanks:
displacement or dilution. These procedures are discussed below.
6.3.1
Inerting by displacement
Inerting by displacement, also known as piston purge, relies on
stratification of the cargo tank atmosphere based on the difference in
vapour densities between the gas entering the tank and the gas already in
the tank. The heavier gas is introduced beneath the lighter gas at a low
velocity to minimise turbulence. If good stratification can be achieved,
with little mixing at the interface, then just one tank volume of the
incoming inert gas is sufficient to change the atmosphere. In practice
mixing occurs and it is necessary to use more than one tank-volume of
inert gas. This amount may vary by up to four times the tank volume,
depending on the relative densities of the gases together with tank and
pipeline configurations. There is little density difference between air and
inert gas; inert gas from a combustion generator is slightly heavier than
air while nitrogen is slightly lighter. These small density differences make
inerting by displacement difficult to achieve and usually the process
becomes part displacement and part dilution (discussed below).
Combustion-generated inert gas is usually introduced through the liquid
loading line with the effluent being exhausted through the vapour line into
the vent header.
Figure 6.2 Inerting cargo tanks by displacement method
Figure 6.2 shows the inerting of a cargo tank by the displacement method.
The symbols used in this and the cargo handling diagrams, which follow,
are identified at the beginning of this book.
Inerting by displacement is an economical procedure as it uses the least
amount of inert gas and takes the shortest time. However, it is only
practical when mixing with the initial tank vapour can be limited. If the
tank shape and the position pipe-entries are suitable for the displacement
method, then results will be improved by inerting more than one tank at a
time. This should be done with the tanks aligned in parallel. The sharing
of the inert gas generator output between tanks reduces gas inlet speeds,
so limiting vapour mixing at the interface. At the same time the total
inert gas flow increases due to the lower overall flow resistance. Tanks
being inerted in this way should be monitored to ensure equal sharing of
the inert gas flow.
6.3.2
Inerting by dilution
When inerting a tank by the dilution method, the incoming inert gas
mixes, through turbulence, with the gas already in the tank. The dilution
method can be carried out in several different ways and these are
described below:1.1.1.1 6.3.2.1 Dilution by repeated pressurisation
In the case of Type “C” tanks, inerting by dilution can be achieved through
a process of repeated pressurisation. In this case, inert gas is pressurised
into the tank using a cargo compressor. This is followed by release of the
compressed gases to atmosphere. Each repetition brings the tank nearer
and nearer to the oxygen concentration of the inert gas. Thus, for
example, to bring the tank contents to a level of five per cent oxygen
within a reasonable number of repetitions, inert gas quality of better than
five per cent oxygen is required.
It has been found that quicker results will be achieved by more numerous
repetitions, each at low pressurisation, than by fewer repetitions at higher
pressurisation.
1.1.1.2 6.3.2.2 Dilution by repeated vacuum
Type “C” tanks are usually capable of operating under considerable
vacuum and, depending on tank design, vacuum-breaking valves are set
to permit vacuums in the range from 30 per cent up to 70 per cent.
Inerting by successive dilutions may be carried out by repeatedly drawing
a vacuum on the tank. This is achieved by using the cargo compressor
and then, breaking the vacuum with inert gas. If, for instance, a 50 per
cent vacuum can be drawn, then, on each vacuum cycle, half the oxygen
content of the tank is removed. Of course, the oxygen content of the
inert gas will replace some of the withdrawn oxygen.
Of all the dilution processes, this method can be the most economical as
only the minimum quantity of inert gas is used to achieve the desired
inerting level. The overall time taken, however, may be longer than with
the pressurisation method because of reduced compressor capacity when
working on vacuum and a slow rate of vacuum breaking due to limited
output of the inert gas generator.
1.1.1.3 6.3.2.3 Continuous dilution
Inerting by dilution can be carried out as a continuous process. Indeed,
this is the only diluting process available for Type 'A' tanks that have very
small over-pressure or vacuum capabilities. For a true dilution process,
(as opposed to one aiming at displacement) it is relatively unimportant
where the inert gas inlet or the tank efflux is located, provided that good
mixing is achieved. Accordingly, it is usually found satisfactory to
introduce the inert gas at high speed through the vapour connections and
to discharge the gas mixture via the bottom loading lines.
When using the continuous dilution method on ships with Type 'C' tanks,
increased inert gas flow (and thereby better mixing and reduced overall
time) may be achieved by maintaining the tank under vacuum. This is
accomplished by drawing the vented gas through the cargo vapour
compressor. Under these circumstances care should be taken to ensure
good quality inert gas under the increased flow conditions.
6.3.3
Inert gas - general considerations
It can be seen from the preceding paragraphs that inert gas can be used
in different ways to achieve inerted cargo tanks. No one method can be
identified as the best since the choice will vary with ship design and gas
density differences. Generally, each individual ship should establish its
favoured procedure from experience. As already indicated, the
displacement method of inerting is the best but its efficiency depends
upon good stratification between the inert gas and the air or vapours to be
expelled. Unless the inert gas entry arrangements and the gas density
differences are appropriate to stratification, it may be better to opt for a
dilution method. This requires fast and turbulent entry of the inert gas,
upon which the efficiency of dilution depends.
Whichever method is used, it is important to monitor the oxygen
concentration in each tank from time to time, from suitable locations,
using the vapour sampling connections provided.
In this way, the
progress of inerting can be assessed and, eventually, assurance can be
given that the whole cargo system is adequately inerted.
While the above discussion on inerting has centered on using an inert gas
generator, the same principles apply to the use of nitrogen. The use of
nitrogen may be required when preparing tanks for the carriage of
chemical gases such as vinyl chloride, ethylene or butadiene. Because of
the high cost of nitrogen, the chosen inerting method should be consistent
with minimum nitrogen consumption.
6.3.4
Inerting prior to loading ammonia
Modern practice demands that ships' tanks be inerted with nitrogen prior
to loading ammonia. This is so, even though ammonia vapour is not
readily ignited.
Inert gas from a combustion-type generator must never be used when
preparing tanks for ammonia. This is because ammonia reacts with the
carbon dioxide in inert gas to produce carbamates. Accordingly, it is
necessary for nitrogen to be taken from the shore as shipboard nitrogen
generators are of small capacity.
The need for inerting a ship's tanks prior to loading ammonia is further
underscored by a particular hazard associated with spray loading. Liquid
ammonia should never be sprayed into a tank containing air, as there is a
risk of creating a static charge, which could cause ignition.
6.4
GASSING-UP
Neither nitrogen nor carbon dioxide, the main constituents of inert gas,
can be condensed by a ship's reliquefaction plant. This is because, at
cargo temperatures, each is above its critical temperature and is,
therefore, incondensible. Accordingly, removal of inert gas from the cargo
tank is necessary. This is achieved by gassing-up, using vapour from the
cargo to be loaded at ambient temperature and venting the incondensibles
to atmosphere so that subsequently the reliquefaction plant can operate
efficiently.
Similarly, on changing grade, without any intervening inerting, it may first
be necessary to remove the vapour of the previous cargo with vapour of
the cargo to be loaded. The basic principles discussed previously in
respect of inerting methods apply equally to this type of gassing-up.
6.4.1
Gassing-up at sea using liquid from deck storage tanks
Gassing-up at sears a procedure normally only available to fully
refrigerated, or semi-pressurised ships. Such carriers are often equipped
with deck tanks, which may have a compatible cargo in storage. In this
case, either vapour or liquid can be taken from the deck tanks into the
cargo tanks.
Liquid can be taken directly from deck storage through the tank sprays
(with the exception of ammonia). This is done at a carefully controlled
rate to avoid cold liquid striking warm tank surfaces. In this case, vapour
mixing occurs in the cargo tanks and the mixed vapours can be taken into
other tanks (when purging in series) or exhausted to the vent riser.
Alternatively, liquid from the deck storage tanks can be vaporised in the
cargo vaporiser and introduced gradually into the top or bottom of the
cargo tank, depending on vapour density, to displace the existing inert
gas or vapour to other tanks or to the vent riser.
Only when the concentration of cargo vapour in the tanks has reached
approximately 90 per cent (or as specified by the compressor
manufacturer) should the compressor be started and cool-down of the
system begin.
6.4.2
Gassing-up alongside
Gassing-up operations, which take place alongside, are undertaken using
cargo supplied from the shore. At certain terminals, facilities exist to
allow the operation to be carried out alongside but these terminals are in
a minority. This is because the venting of hydrocarbon vapours alongside
a jetty may present a hazard and is, therefore, prohibited by most
terminals and port authorities.
Thus, well before a ship arrives in port with tanks inerted, the following
points must be considered by the shipmaster:•
Is venting allowed alongside? If so, what is permissible?
•
Is a vapour return facility to a flare available?
•
Is liquid or is vapour provided from the terminal for gassing-up?
•
Will only one tank be gassed-up and cooled down initially from the
shore?
•
How much liquid must be taken on board to gas-up and cool-down
the remaining tanks?
•
Where can the full gassing-up operation be carried out?
Before
commencing gassing-up operations alongside, the terminal
will normally sample tank atmospheres to check that the oxygen is less
than five per cent for LPG cargoes (some terminals require as low as 0.5
per cent) or the much lower concentrations required for chemical gases
such as vinyl chloride.
Where no venting to atmosphere is permitted, a vapour return facility
must be provided and used throughout the gassing-up operation. In this
case, either the ship's cargo compressors or a jetty vapour blower can be
used to handle the efflux. Some terminals, while prohibiting the venting
of cargo vapours, permit the efflux to atmosphere of inert gas. Thus, if a
displacement method of gassing-up is used the need for vapour return to
shore may be postponed until cargo vapours are detected at the vent
riser. This point may be considerably postponed if tanks are gassed-up
one after the other in series.
Figure 6.3(a) Gassing up cargo tanks using liquid from shore
Figure 6.3(b) Gassing-up cargo tanks using vapour from shore
6.5
COOLING DOWN
6.5.1
Cool-down - refrigerated ship
Cooling down is necessary to avoid excessive tank pressures (due to flash
evaporation) during bulk loading. Cool-down consists of spraying cargo
liquid into a tank at a slow rate. The lower the cargo carriage
temperature, the more important the cool down procedure becomes.
Before loading a refrigerated cargo, ship's tanks must be cooled down
slowly in order to minimise thermal stresses. The rate, at which a cargo
tank can be cooled, without creating high thermal stress, depends on the
design of the containment system and is typically 10oC per hour.
Reference should always be made to the ship's operating manual to
determine the allowable cool-down rate.
The normal cool-down procedure takes the following form. Cargo liquid
from shore (or from deck storage) is gradually introduced into the tanks
either through spray lines, if fitted for this purpose, or via the cargo
loading lines. The vapours produced by rapid evaporation may be taken
ashore or handled in the ship's reliquefaction plant. Additional liquid is
then introduced at a rate depending upon tank pressures and
temperatures. If the vapour boil-off is being handled in the ship's
reliquefaction plant, difficulties may be experienced with incondensibles,
such as nitrogen, remaining from the inert gas. A close watch should be
kept on compressor discharge temperatures and the incondensible gases
should be vented from the top of the condenser as required.
Figure 6.4 Cargo tank cool-down using liquid from shore: vapour returned to shores
As the cargo containment system cools down, the thermal contraction of
the tank combined with the drop in temperature around it tend to cause a
pressure drop in the hold and interbarrier spaces. Normally, pressure
control systems supplying air or inert gas will maintain these spaces at
suitable pressures but a watch should be kept on appropriate instruments
as the cool-down proceeds.
Cool-down should continue until boil-off eases and liquid begins to form in
the bottom of the cargo tanks. This can be seen from temperature
sensors. At this stage, for fully refrigerated ammonia for example, the
pool of liquid formed will be at approximately –34oC while the top of the
tank may still be at –14oC. This gives a temperature difference of 20oC.
The actual temperature difference depends on the size of the cargo tank
and the spray nozzles positions.
Difficulties that may occur during cool-down can result from inadequate
gassing-up (too much inert gas remaining) or from inadequate drying. In
this latter case, ice or hydrates may form and ice-up valves and pump
shafts. In such cases, antifreeze can be added, provided the cargo is not
put off specification, or the addition will not damage the electrical
insulation of a submerged cargo pump. Throughout the cool down,
deepwell pump shafts should be turned frequently by hand to prevent the
pumps from freezing up.
Once the cargo tanks have been cooled down, cargo pipelines and
equipment should be cooled down. Figure 6.4 shows the pipeline
arrangement for tank cool-down using liquid supplied from the shore.
6.5.2
Cool-down - semi-pressurised ships
Most semi-pressurised ships have cargo tanks constructed of steels
suitable for the minimum temperature of fully refrigerated cargoes.
However, care must be taken to avoid subjecting the steel to lower
temperatures. it is necessary to maintain a pressure within the cargo tank
at least equal to the saturated vapour pressure corresponding to the
minimum allowable steel temperature. This can be done by passing the
liquid through the cargo vaporiser and introducing vapour into the tank
with the cargo compressor. Alternatively, vapour can be provided from
the shore.
07.
Safety Precautions
and Measures
7 SAFETY PRECAUTIONS AND MEASURES
7.1
TANK ATMOSPHERE EVALUATION
7.1.1
The need for gas testing
The atmosphere in enclosed spaces must be tested for oxygen and
hydrocarbon content in the following circumstances:
•
Prior to entry by personnel (with or without protective
•
equipment)
•
•
During gas-freeing, inerting and gassing-up operations
•
•
As a quality control before changing cargoes, and
•
To establish a gas-free condition prior to dry-dock or ship
•
repair yard
The atmosphere in a cargo tank is rarely, if ever, homogeneous. With the
exception of ammonia and methane, most cargo vapours at ambient
temperatures are denser than air. This can result in layering within the
cargo tank. In addition, internal structures can hold local pockets of gas.
Thus, whenever possible, samples should be drawn from several positions
within the tank.
Atmospheres, which are inert or deficient in oxygen, cannot be checked
for flammable vapours with a combustible gas indicator. Therefore,
oxygen concentrations should be checked first, followed by checks for
flammable and then toxic substances. All electrical instruments used
should be approved as intrinsically safe.
7.1.2
Oxygen analysers
Several different types of oxygen analyser are available. A common type
of analyser is illustrated in Figures 7.2(a) and (b). In this example,
oxygen diffuses through the teflon membrane into a potassium chloride
solution and activates the chemical cell. When the switch is closed,
current flows round the circuit and deflects the ammeter needle. The
more oxygen absorbed by the solution, the greater the current and the
needle deflection indicates the percentage of oxygen in the atmosphere
being sampled.
The instrument described above operates without batteries and is
relatively insensitive. Other types of analysers include the polarographic
and paramagnetic-type instruments. These are much more sensitive and
require batteries.
It should be noted that batteries should never be changed in a gas
dangerous zone.
Such instruments have dual scales, each having a separate function. For
example:Scale 1 - oxygen deficiency in air - zero to 25 per cent oxygen
by volume;
Scale 2 - oxygen in nitrogen - zero to 1 per cent oxygen by
volume.
Figure 7.2(a) Oxygen indicator – circuit diagram
A schematic diagram of the polarographic cell used in some oxygen
analysers is shown in Figure 7.2(c). In this cell, the current is controlled
by the electrochemical reaction of oxygen at the cathode (the permeable
membrane). The life of the cell is approximately six months when
continuously operated in air.
Figure 7.2(b)
These instruments should be regularly spanned (calibrated) with fresh air
(21 per cent oxygen) and test-nitrogen (a virtual zero per cent oxygen
content). Liquid contamination, pressure or temperature effects may
result in drifting of instrument response.
Figure 7.2(c) A polarographic cell
7.1.3
Combustible gas indicators
Catalytic instruments
The basic electric circuit (Wheatstone Bridge) of the combustible gas
indicator is shown in Figure 7.3(a). The gas to be measured is aspirated
over the sensor filament, which is heated by the bridge current. Even
though the gas sample may be below the lower flammable limit, it will
burn catalytically on the filament surface. In so doing, it will raise the
temperature of the filament, increase its electrical resistance and
unbalance the bridge. The resultant imbalance registers on the meter,
which indicates the hydrocarbon content in the air.
Such instruments are designed principally to indicate flammability but are
also used to detect the presence of small concentrations of gases in air.
Figure 7.3(a) Combustible gas indicator – circuit diagram
The meter scale commonly reads from zero per cent to 100 per cent of the
lower flammable limit (LFL). On instruments having a dual range, a
second scale indicates zero to 1 0 per cent of the LFL. Instruments of this
type contain batteries, which must be checked prior to use, and it is a
recommended practice to check the instrument using a calibration gas at
frequent intervals. When calibrating the instrument, the meter reading
should fall within the range indicated on the calibration graph which is
provided by the manufacturers - see Figure 7.3(b).
Figure 7.3(b) Combustible gas indicator - calibration
In the example shown in Figure 7.3(b), a meter reading of between 68
and 92 per cent of LFL for a calibration gas containing three per cent
methane in air indicates that the detector filament is in good order. These
values are only given for illustration and reference must always be made
to the graphs, which accompany each calibration kit.
Tank spaces being sampled which have an atmosphere above the
flammable range will produce a low or even zero reading on this type of
meter. However, as the sample is initially drawn into the meter, the
meter needle will give a momentary strong deflection before returning to
its steady low or zero reading. This momentary deflection must always be
watched for, since it gives warning that the following steady reading will
be misleading and that the gas being sampled is above the lower
flammable limit.
Some instruments may have sensor filaments whose catalytic action may
be spoilt by the presence of other gases such as halogenated
hydrocarbons (halon) sometimes used for fire extinguishing. Whenever
opportunity arises, instruments should be checked against each other and
any doubt resolved by a calibration kit. It should be noted that the
batteries fitted within such instruments should only be changed in gassafe areas.
Non-catalytic heated filament gas indicators
Since the action of the catalytic gas indicator depends upon combustion
with air, it cannot be used for inerted atmospheres because of oxygen
deficiency. Instruments suitable for such use, while operating on a similar
Wheatstone Bridge principle, contain a filament sensitive to variations in
heat conductivity of the sample, which varies with its hydrocarbon
content. Such meters usually register over the range 0 to 25 per cent
hydrocarbon vapour by volume and are useful for monitoring inerting
operations.
Multipoint flammable gas monitors
The catalytic and heated filament flammable gas indicators are widely
used as portable, hand-aspirated instruments. They are intrinsically safe.
Their main purpose is for testing cargo tanks, void spaces and other
enclosed spaces and this is most often carried out during gas freeing
operations and before entry by personnel.
The catalytic instrument is also used in multi-point form for continuous
monitoring of air-filled or air-ventilated spaces such as compressor rooms,
motor rooms, machinery spaces and cargo holds. In multi-point form, the
indicator is installed on ships' bridges or in cargo control rooms. These
instruments draw samples sequentially from points in the various spaces
monitored. The indications may be automatically recorded and individual
alarms are provided when a low percentage of the Lower Flammable Limit
is detected.
Figure 7.4 Infra-red gas analyser
Where void spaces are inerted continuously with nitrogen, the catalytic
type will not function and an infra-red analyser is often provided as the
central multi-point instrument. Figure 7.4 illustrates the principle of a
typical infra-red analyser. This instrument employs the property of
hydrocarbon gas to absorb infra-red radiation. Two similar nickel/chrome
emitters within the instrument beam provide infra-red radiation to two
separate channels, one through the sample cell and one through a
reference cell free of hydrocarbon. The two channels are alternately
blocked by a semi-circular beam chopper driven by an electric motor. The
transmitted radiation from both channels passes to a detector cell in which
the gas is heated by the received radiation. The resultant rise in pressure
is detected by the sensitive membrane of a condenser microphone. As a
result of the chopping of the two beams and the absorptive effect of any
hydrocarbon in the sample cell, the output of the microphone is an
alternating current signal, directly related to the hydrocarbon content of
the sample. This signal is amplified and recorded and, when gas is
detected, actuates the alarm for the point being sampled.
7.1.4
Toxicity detectors
Toxic gas detectors usually operate on the principle of absorption of the
toxic gas in a chemical tube, which results in a colour change. A common
type of toxic gas detector is illustrated in Figure 7.5. Immediately prior to
use, the ends are broken from a sealed glass tube. This is inserted into
the bellows unit and a sample aspirated through it. The reaction between
the gas being sampled and the chemical contained in the tube causes a
colour change. Usually, readings are taken from the length of the colour
stain against an indicator scale marked on the tube. These are most often
expressed in parts per million (ppm). Some tubes, however, require the
colour change to be matched against a control provided with the
instruction manual. As tubes may have a specific shelf life, they are datestamped and are accompanied by an instruction leaflet, which lists any
different gases, which may interfere with the accuracy of the indication.
Figure 7.5 Toxic gas indicator
When using this type of instrument, it is important to aspirate the bulb
correctly if reliable results are to be obtained. Normally, the bellows are
compressed and the unbroken tube inserted. By this means the
instrument is checked for leaks prior to breaking the tube. If found to be
faulty, it should be replaced.
This type of instrument can also be used to good effect during gassing-up
operations when changing from one cargo to another. By using tubes
suitable to detect trace amounts of the previous cargo, a careful
estimation can be made regarding a suitable cut-off point for the
operation.
7.1.5
ENTRY INTO ENCLOSED SPACES
Precautions for tank entry
Because of the danger of hazardous atmospheres, an enclosed space
should only be entered when it is essential to do so. At such times a
permit to work should be issued and this should be specific as to date,
time and space concerned and list the precautions to be taken.
Alternatively, for ship tank entry purposes, the Maritime Safety Card
should be completed.
The Maritime Safety Card gives an appropriate procedure for entering
enclosed spaces on ships.
Particular hazards atmospheres can include:•
•
•
•
Amounts of hydrocarbon gas
Trace amounts of toxic gas
The intrusion of inert gas, and
Oxygen deficiency (often caused by the rusting process in
unventilated tanks)
The table below lists those spaces on a gas carrier which are either
enclosed or which may be considered gas-dangerous for entry.
Enclosed Spaces on Gas Carriers Include
Enclosed Spaces in Cargo Area Enclosed Spaces Elsewhere
Enclosed Spaces Entered
Routinely
Cargo tanks
Void spaces
Compressor rooms
Hold spaces
Bunker tanks
Interbarrier spaces
Cofferdams
Duct keels
Ballast tanks
Spaces containing cargo pipes
Spaces adjacent to cargo
spaces having unsafe
atmospheres
Note: Even if a space is already considered gas-free and fit for entry, where it is immediately adjacent to a
tank having a dangerous and pressurised atmosphere, the space should always be entered with caution and
only after suitable checks have been made.
7.1.6
Procedures
For those special cases where tank entry is required, every ship and
terminal should have procedures for safe entry and these should be
written into operating manuals. Manuals should be clear on questions of
area responsibility; shore tanks should not be entered without the
terminal manager's permission and the ship's tanks should not be entered
without the shipmaster's permission. As far as the terminal operating
manual is concerned, such procedures should give advice on terminal
operations and the requirements expected from their own, or contracted,
personnel when they are visiting or inspecting ships. Terminal managers
should take this matter most seriously, as accidents to shore personnel
when entering enclosed spaces on ships are not uncommon.
Generally, entry into enclosed spaces should only be permitted when a
responsible officer has declared the atmosphere gas-free and fit for entry.
Only in very exceptional circumstances should tank entry be allowed when
the tank atmosphere is unsafe - and then, only with full protective
equipment and breathing apparatus.
7.1.7
Rescue from enclosed spaces
Experience has shown that the rescue of persons from within an enclosed
space can be extremely hazardous and especially so in cases of oxygen
deficiency. These risks are heightened where access to a compartment
can only be achieved with difficulty. In such circumstances, it is vital that
rescuers always pay strict attention to the correct procedures and the use
of proper equipment and do not rush into ill-considered action. Many
fatalities have resulted from failure to comply with these basic rules.
For training purposes, full-scale exercises in non-hazardous atmospheres
have been found extremely beneficial. Exercises involving weighted
dummies, with rescuers wearing protective equipment and breathing
apparatus, are essential if rescue teams are to be properly prepared for a
real emergency. Ship’s personnel often conduct such simulations. They
can also involve terminal employees and shore based emergency services
such as the fire brigade.
7.2
FIRE PREVENTION
7.2.1
Fire fighting in general
There are two conventions in particular that deals with safety at sea. One
is the “International Convention on Load Line, 1996, that was adopted at
an IMO conference in 1996. The other is the “International Convention for
the Safety of Life at Sea”.
The Safety convention is a comprehensive convention that intervenes in
many areas regarding safety of human life at sea. It starts with the
construction of the ship to maintain an as high level of safety as possible
due to divisions, stability of the machinery and electrical installations.
There are detailed rules for fire, protection, fire discovery and fire
extinguishing and of life saving equipment.
7.2.2
Management tasks & tactics – fire emergency
preparedness
Fire Emergency preparedness onboard is comprised of the
following:
• Sufficient and adequate equipment.
• Organisation and management.
• Training and practice.
Organization and management are essential factors, which deserves a
great deal of attention. The leader of the fire fighting must, in any case,
consider the situation and depending on a number of circumstances
execute adequate initiatives. The leader of the fire fighting should be able
to take care of his/her responsibilities in the best possible way. Essential
to this, training and practice must be fulfilled.
7.2.3
Fire onboard - Management’s duty:
A fire burst onboard represents a threatening and critical situation. To
prevent disaster, a quick and determined effort from the whole crew on
board is needed.
For most of the people, fire is an unfamiliar event and it is therefore
natural that such a threatening occurrence can lead to unpremeditated
actions and panicky contributions to the situation.
When this happens, it is the management’s first duty to, as soon as
possible, activate the different teams in accordance with the fire
instruction plan. Fire resistance arrangements onboard the specific vessel
should be utilised to the fullest extent.
If a fire should occur, the management will be confronted with a lot of
problems that all seem to be equal in importance. It is important to
prioritise when dispersing the tasks. This means that those tasks that
seem to be most important must be delegated to the most competent unit
or team in the emergency squad. The squad will have to do their best to
solve the problems in a satisfactory way. In many cases, the first
decisions must be made based on few and uncertain pieces of information
about the situation. Any hesitation from the management about which
approach to use, will promote the feeling of fear and insecurity among the
crew.
Since the crew has been trained in relevant practical skills, the
management must also be prepared and trained for the problems they are
expected to solve. The ship’s fire instructions must be considered as a
tool. The benefit and effect that this tool will give depends on how the
management decides to utilise it.
There is nothing that can really replace the valuable experiences you will
get by managing extinguishing operations in real fire situations onboard.
As this, of course, is practically impossible to accomplish as part of a
training programme, other methods have to be tried out. Typically the
standby crew (e.g. fire brigade, first aid teams, civil defence) will need to
make quick decisions and judgements of the situation.
This type of responsibility requires special
training. Imagine a situation and try to
picture the conditions and based on that try
to find out how you can, as best as possible,
use the resources you have available. This is
one way to manage a situation. However,
you have to be aware that in a real
situation, the approach to the problem
cannot be changed to fit your own
perception.
By using similar methods onboard, consider
imagined fire situations and at leisure find
out how to handle the situations, so that the
management of the ship can prepare their
fire fighting duties. Even though you have
worked through a lot of imagined situations, and one day there is a fire,
there will never be a situation similar in detail to one of the imagined
situations. On the other hand there will most likely be a situation similar
to something you had been through before. In any case you will be better
prepared, at least mentally, to manage the situation.
7.2.4
Plans of Action
The more people know the main guidelines for fire fighting situations
onboard each particular ship, the better the chance for a successful
response. Therefore it is of urgent importance that the management
group (The Captain, The Chief Engineer and The Chief Officer) is fully
aware of the existing plans. When considering these imagined situations
where you find the best solutions, several points of views will improve the
plans.
The management group together should work out the plans for the actions
for different kinds of fire situations. Therefore, the managers will be
informed about the plans, which will make it easier for them to manage
accordingly.
In hectic situations, as a fire, it will be easier to change an existing plan
rather than making a new plan from scratch. The plan will be easier to
execute, if more people know about its contents.
If training is arranged according to appointed plans, the crew will get
familiar with the plans in addition to variations in training. Realistic and
well-planned training exercises are good practice, as well as, it is
interesting and instructive. Successful fire fighting is a result of good
planning, good leadership and a well-trained standby crew.
7.2.5
Tactics
By tactics we really mean line of action. It is a calculated way to act out a
plan of action where we want to use the crew available, in such a way that
maximises the effect achieved.
The intention with tactics is to reach the goal you have set. You have to
be aware of what you want, what is the result you aim for. In a fire
situation, it should be easy to conclude that you want to extinguish the
fire, as soon as possible, with as little mess as possible, without any risks
to the fire fighters.
7.2.6
Select an Action
When planning a line of action, choose tactics, try to clarify the situation
first (reconnoitre). The more details you know about the situation, the
easier it is to evaluate the situation. In a critical situation, decisions have
to be made quickly. The next step in the planning process will be the
evaluation of the situation. Based on the information known, you have to
try to determine how the fire will grow. Here it is important to prioritise,
as there could be parts of the fire that has to be stopped no matter what.
Meanwhile, other things have to be held off, as long as possible. There are
may be some parts that can be temporarily disregarded.
With the evaluation of the situation as a basis the disposals of resources
are being made. The extents of the contribution depends on how
important the effort is, how demanding the work to be done is, and how
quickly it has to be effectuated. You should always be prepared to change
tactics if unforeseen difficulties occur. Well-prepared tactics considers all
known factors whether there are only a few, or many and detailed at any
stage.
7.2.7
Extinguishing Tactics
Extinguishing tactics make use of resources available so that maximum
effect in an action is achieved. It also makes a sufficient effort at the right
place at the critical moment. Offensive tactics is a well-known expression;
it means that you will use all resources in the fight to win back the terrain
and to get the situation under control.
Defence tactics are when you use the whole force to last as long as
possible to prevent being forced to back out, avoid loss of terrain, try to
hold the position, as long as possible, while waiting for backup. In the
following, you will find some situations listed where you will have to
consider the influence these situations have on the actions to be taken.
7.2.8
Fire preparedness
Fire preparedness is the capability the crew has to fight a fire with the
help of the equipment available on board. To manage a fire situation,
preparedness promotional efforts are done. Fire preparedness is the result
of a number of arrangements and different efforts, for example fire
protection organisation, strategic placing of equipment, instructions,
maintenance of equipment, training, exercise. Remember the
preparedness is not stronger than the weakest link.
Practical (technical) exercises are meant as a test to see if the crew has
the necessary skills. The exercises are also designed to train in the skill of
being prepared. Tactical exercises will reveal the management’s capability
to evaluate situations and delegate the right effort at the right time. The
practical and technical skills together will contribute to an effective force.
It is therefore very important that realistic and varying exercises are
exercised on board. The technical will cover the quality of the “tool” at
disposal, while the tactical will cover what capability one has to utilise the
strength at his disposal.
7.2.9
Alarm instructions
Central part of fire preparedness on board is the safety plan part on the
fire-fighting organisation. The ship’s alarm instructions provide the
emergency plan if there is a need for a united and systematic effort of the
crew. Main features in the emergency plan should include special
distribution of the crew, duties when fire fighting, plus another special
distribution, if preparations for abandon the ship become a reality. All
emergency plans organise the crew into practical teams or units, plus
instruct of the duties that everyone has when the organisation is active.
Emphasise the importance of knowing the alarm instructions well, on
board your specific ship. There can also be other situations that can be
covered by the preparedness organisation, for example man-over-board,
tank accident, and personal injury and helicopter preparedness.
7.2.10
Extinguishing of fire
The faster the extinguishing activity is effectuated, the greater the chance
of a successful result. In choosing an extinguishing method, quencher
remedy and capacity, the goal must be total elimination. One must also
consider the amount of damage the extinguishing agent will cause to the
area. However, put out the fire before causing any larger damage.
In some parts of the vessel, one can choose between permanently
installed extinguishing equipment and manual efforts. On parts of the
ship, a manual effort is the only alternative. Permanent equipment should
be used in an area where the fire risk is large and has a large risk of
spreading.
Any manual combating involves a large risk for the extinguishing force.
The decision about what to utilise in a specific situation must be well
substantiated.
7.2.11
Fixed fire fighting plans & fire fighting remedy
Manual call point plant
Fixed fire detection’s plants, discovery and alarm equipment should be
installed on vessels that are regulated by SOLAS. Approval type for these
detection’s plants takes place according to a determined procedure by
posting the plant’s documentation. This documentation should contain
user instructions, procedures for routine testing on board, fault location
procedures, power supply information, connection of detector loop, alarm
organs, fan failure, door magnet, assembly work, function description,
accordingly all requirements in accordance to the documentation claim.
The plant is tested to determine if it fulfils the regulations required. The
manual call point plant should at all times be according to the regulations
in force. Some of the criteria follow:
•
•
•
•
power to supply
It should give optical and acoustic alarm at fire.
It should indicate where fire breaks out.
It allows for fault warning.
The central unit automatically goes over to reserve
upon voltage failure.
•
Positive indication on the panel by interruption of
functions.
•
Otherwise according to the approval companies, it is important to notice
that the plant should have two independent power sources. If one “falls
out” the other will operate the plant with full power. However, please refer
to the regulations regarding complete approval.
Safety plan
The fire control draft or as called on board; the “safety plan” illustrates
the safety installations and equipment on board. The draft shows the
vessel sidewise and a sketch of each deck top wise.
It indicate zones with isolated bulkheads and fire doors, manual call point
plants with detectors, alarm buttons and alarm bells, the fixed main
extinguishing plant and where on board these can be remote controlled.
Valves to stop engines, machinery, and from where one can remote
operate these are also indicated.
It indicates where the ventilation plant with fans, ducts and damper is and
from where one can stop the plant. All portable extinguishing equipment,
protection equipment and utility equipment appear on the draft, and
where on the vessel the equipment is kept. It also displays all decks,
rooms, and all emergency exits.
Symbols for marking equipment are utilised to make the draft well
arranged. Also, on the draft is a list with an explanation of the different
symbol. Colouring is often utilised to keep the symbols apart. This draft is
available for all on board. To effectively utilise the different fire technical
installations, thorough knowledge of the individual plants is required, plus
how to use them.
The gangway during the port stay should keep a copy of the safety plan. If
anything occurs during the stay and local help is required, the local fire
department can quickly approach the plan, and from an early stage, have
knowledge of the preparedness plan.
All are advised to thoroughly study the “safety plan” in detail.
Fire pumps
A fire pump in the engine room is connected to the fire pipeline network.
In addition, there is a separate fixed emergency fire pump installed in a
distance from the engine room. One can either operate the emergency fire
pump by its own diesel engine; it can be hydraulically driven or electrically
driven by power from the emergency power unit.
Oil, for at least 12 hours of running power, is kept nearby the emergency
pump, in addition to oil for the fuel tank itself in case it a should be filled
at any time. Fire pumps, which are able to produce more pressure than
the pipeline network is designed for, are at all times equipped with a
safety valve. All centrifugal pumps, for instance, are supplied with nonreturn valves.
Fire pipeline network
The fire pipeline network branches all over the vessel and has a number of
hydrants - hose connections with valves. The pipeline network is divided
into sections with a cross over,
arranged in a way that if damage
occurs on a part of the system,
the damaged part is shut off
without shutting off the entire
pipeline network. Properly study
the pipeline network on board to
understand how the network is
divided, plus where the shut-off
valves are placed. If parts of the
network are damaged, it is
possible to bypass the damaged
part by help of hoses from hydrant to hydrant.
Hydrants are placed such that two water jets at the same time can reach
any part of the vessel, one jet from a hose length, the other from two
hose lengths. On the main line of the tank area there should be one shutoff valve for each 40 metres. This is, of course, fitted to the size and type
of the vessel.
7.2.12
Main fire extinguishing plants (For gas and chemical
carriers)
Dry chemical system
Powder is elected as extinguishing remedy on the tank deck of gas carriers
and chemical tankers. A number of minor stationary powder aggregates can be
placed on deck or a powder central unit with pipes forward to a number of
powder monitors and hose stations on deck. One or several powder
containers are placed with a capacity calculated for the specific vessel with
accompanying pressure bottles in the powder central unit. The plant can
be released from each powder post by opening the valve of the releasing
bottle. The gas is lead into tubes to the releasing mechanism of the
pressure bottles in the powder central unit. It opens the valve of the
powder tube that proceeds to the powder post being released. Several
posts can be utilised at the same time, but each post must be triggered in
the same way.
Stationary dry powder systems are normally delivered with powder
(NaHCO3 – Natrium hydrogen carbonate or KHCO3 - calcium hydrogen
carbonate) for extinguishing fire in class B or E. That is all types of liquid
like: petrol, alcohol, acetone, oil, painting etc., and different types of
gases like methanol, methane, butane, propane etc.
Dry powder systems utilise N2 (Nitrogen) or CO2 (carbon dioxide) as
propellant gas. The gas is kept in pressure cylinders. A gas pressure
regulator reduces N2 –gas or CO2 – gas (200kg/cm2) to 20 kg/cm2 before
it goes via the riser in to the powder aggregate. The riser’s gas taps are
very important, as the powder together with the propellant gas must be
able to “float” as a liquid through the pipe system and the powder jet. The
stationary powder post (monitor) should have a capacity of at least
10kg/second. Manual equipment, “hand hoses”, should have a capacity of
at least 3,5kg/second, but not too large for one man to operate. The
length of a hand hose should not exceed 33 m. It is very important that
the hose is pulled out to its full length before setting the pressure. The
extension should be at a minimum of 10 metres for both stationary and
hand based equipment. The plant’s powder capacity should be of the size
that utilises all posts. The delivery of powder should progress at a
minimum of 45 seconds.
Below is an example of this with the following data: 4 stationary and 4
hand stations:
Stationary: (4 pcs. x min.10kg./s x min. in 45s)
kg.
Hand based: (4 pcs. x min.3,5kg./s x min. in 45s)
kg.
Minimum powder capacity:
= 2430 kg.
= 1800
=
630
7.2.13
Technical description
The powder type NaHCO3 and KHCO3 has an extinguishing effect based on
a reaction inhibitor along with some cooling of the fuel surface and the gas
face. Powder is not electrically conductive in dry conditions. To avoid
humidity in the powder, a water-repellent material is added usually
silicon.
Dry chemical systems consist of a mechanical part that includes a powder
aggregate with valves, release mechanism, pipe system and jets.
Everyone must memorise maintenance routines and test routines, based
on the plant on the specific vessel. (This is part of the fire drill onboard).
Powder central incl.
remote release.
Powder monitors incl.
remote release to the central
Seagull
Powder line
Schematic powder plant
Water - spray system
7.2.14
(Gas and chemical carriers)
In addition, certain ship types should be equipped with a “water-spray
system”, as an object for a cooling, fire preventive and crew protective
effect. We refer here to the IGC-code, chapter 11, point 11.3.1, what
areas the plant should cover. The plant onboard the specific ship is
designed according to this.
The system should have the capacity to cover the designated area with at
least 10 ltr./m2 pr minute on horizontal surfaces, and 4 ltr./m2 pr minute
on vertical surfaces.
If parts of the line are damaged, shut-off valves must exist on the main
line so that the line can still be utilised. This is operable by shutting off the
line to the damaged area. The alternative is that the system is devisable
into several sections that can be operated independent of each other.
The delivery pumps should have such a capacity that they can deliver
simultaneously with full capacity to the whole plant. The plant should
contain a material that is resistant to corrosion.
There has to be a possibility of remote start of the water delivery pumps,
plus remote control of the plants shut valves from a place outside the
cargo area.
We recommend studying the plant on your vessel, how it is operated,
where the remote control is, plus the inclusion of this in the fire drill
executed onboard.
7.2.15
Foam in general
A system consisting of gas or air bubbles bound in a water coating
(membrane), is called foam. Constant foam is when the wall/membrane
consists of a constant material, such as pumice stone, gas concrete and
foam rubber are examples of constant foam. When the wall has a coating,
we are talking about floating foam, such as soapsuds. Different types of
floating foams are used for fire extinguishing. On new gas and chemical
carriers we also find foam utilised for fire extinguishing.
Producing foam
In order to produce foam that will extinguish fire, you need: water, a
frothing material that dissolves in water in anatomised condition, and a
non-flammable gas mixed with the solvent. The foam is shaped when
gas/air is mixed into the foam/frothing liquid and into the water by help of
mechanical equipment. The result is mechanic foam.
Mechanical foam
Different types of pumps, sprinklers and foam pipes are used. The foam
liquid is dissolved (or emulsified) in the water. After this, the air is mixed
in by mechanical means. Normal equipment produces bubbles, which have
a diameter of 0,1mm to 1,5mm.
Extinguish effect
Foam has a suffocating effect and acts as a cooling extinguishing agent.
The suffocating or the cooling effect can be more or less the dominating
effect, but depends on what material is burning and what sort of foam is
used. By extinguishing a burning liquid with a surface temperature higher
than +100o C, the cooling effect is the dominating force. This is caused by
evaporation of the liquid that penetrates into the surface’s layer of the
burning material as the foam collapses. By extinguishing fire when the
temperature in the surface is below +100oC, the extinguishing effect is
connected with the heat-insulating foam and, above all, a differentiation
effect. When the foam cover has spread outward across the liquid’s
surface, the heat rays from other, still burning parts of the liquid surface,
is not able to penetrate through the area covered with foam. Therefore,
combustible gases are no longer formed, evaporation ceases and the fire
die out.
7.2.16
Foam plant
Foam is chosen as the main extinguishing agent for the tank area. A foam
plant consists of a foam central unit with a foam tank, foam pump that is
also connected to an emergency generator, distribution manifold, foam
jets, automatic valves, and a pipe system connected to fixed monitors on
the tank deck. The capacity of the plant should be big enough that the
whole tank area could be covered with foam. If the vessel has an inert gas
plant, the foam capacity must have a volume that can deliver foam for a
minimum of 20 minutes. The demand is at a minimum of 30 minutes if the
ship is not equipped with inert gas plant.
The main foam line from the foam central unit to the monitors should
contain shut-off valves within determined requirements, in order to bind
the line in case of damage. The foam line going to each monitor has a
delivery valve installed to supply foam. The valve can also be used to
regulate the amount of foam supplied in order to achieve the right mixture
condition between foam and water.
A foam jet pipe is attached to the monitors. Study the plant installed on
your vessel, and understand how this plan is operated. This equipment
(the foam plant) is mandatory for oil tankers.
Mobile foam equipment is also available on many ships, gas and chemical
carriers also. This consists of a fire hose with a foam nozzle unit, small
foam containers (20 litre), a foam ejector, a small hose for the
transmission of foam from a foam container to a foam hosepipe, and
protection equipment. This equipment is prepared for use with fire hoses
and a foam nozzle unit connected to the fire line. A foam ejector with a
tap for supplying foam liquid is installed between the fire hose and foam
nozzle unit. Water pressure is established, foam liquid is sucked (ejector
function) from the foam container via hose connection between the foam
container and ejector.
7.3
POLLUTION PREVENTION
It is the responsibility of the master or those in charge of transfer
operations involving cargo or bunkers to know the applicable pollution
prevention regulations and to ensure that they are not violated. Exercises
should be held to train personnel in accordance with the Shipboard Oil
Pollution Emergency Response Plan, and recorded.
There is a danger of violating pollution prevention regulations if ballast
taken on in polluted waters is discharged in another port. If ballast has to
be taken on in polluted areas, it may be necessary to exchange it for clean
ballast when in deep water on passage. Some terminals have specific
requirements in this respect, and the master should ensure that they are
observed.
7.3.1
Pollution in general and its effect on Ecology
Note that pollution is usually related to human activity. Phenomena, such
as radiation due to natural radioactivity in the earth, volcano eruptions
and the like, are not usually considered as pollution. They exist, however,
in areas where the environment is burdened. This is nature’s own way to
balance and renew itself.
Any pollution has a main source and a receiver. The main receivers are
air, sea, and soil. The most effective way of spreading pollution is through
air. But eventually the pollution always falls to the ground and into the
sea. The earth is most resistant to pollution as a receiver, but the
problems appear because this pollution almost without restrictions has
free flow to pollute sea and waters. Compare the human body with its own
immune system to the environmental system (Eco-system), and you will
find that all basic “building blocks” are linked together in some way or
another with the same influence and with the same purpose. Every part is
equally important in obtaining the ability to function as a whole unit.
7.3.2
Definition of pollution:
Substances and materials spread through air - sea - and soil that cause
damage and malfunction due to human activity.
Many factors contribute to pollution, such as the chemical, physiological or
biological characteristics. Life on earth is dependent on solar energy.
Plants turn solar energy, water and carbon into plant tissues. This is called
the first tropic level. The herbivores (vegetable-only eating animals)
cannot exploit solar light directly in their growth or tissue change.
Herbivores use the plants to produce tissue. This is called the second
tropic level. The energy loss caused by transmission from the first level to
the second level is calculated to be at approximately 90%. An even
greater loss appears at the next level, which is the third tropic level. This
level includes the humans and the animals, which survive by eating
animal meat. The demolishing link in this process is the carrion eaters and
small organisms, which demolish dead plants and animal materials into
simple organic and inorganic compounds, which the plants need to grow.
An Ecology System appears as a result of developing and adapting to each
other as a species in nature throughout millions of years. Accurate balance
and stability is obtained and smoothly functioning. This system is an
everlasting process and is continuous throughout time and space. An
Ecology System can endure huge changes and variations in nature, but
faced with artificial factors and synthetic substances spread by human
actions, important parts (areas) in this process can be demolished. The
reason is simply that no natural mechanism exists to keep the process
active and in balance. In numerous cases, these unwanted non-natural
substances are spread throughout the nature process creating disharmony
and malfunctions both geographically and ecologically.
7.3.3
Pollution of air and sea and the influence of ship trade
Burning sulphurous fossil fuel forms sulphur-dioxides and compounds of
this gas. The gas responds to air and transforms into sulphur acid.
Nitrogen oxides are also formed by combustion of fossil fuel, and release
nitrogen mono oxides, which again transforms into nitric acid and nitrogen
oxides.
Carbon mono oxides formed by uncompleted consumption of organic
material can further react to air and transform into carbon dioxide.
Further, a number of gases are released with the gas freeing of cargo
tanks and cooling plant. These are CFC – gases (chlorous fluor carbons).
Carbon dioxide and CFC - gases function as a glass roof in a hothouse, the
heat radiation from the sun is easily received and is harder to let go. This
is the hothouse effect in a nutshell.
Sulphur and nitrogen oxides in outlets (pollution) cause huge destruction
of soil and sea. The consequences of this are recognised in areas where
the forest is dead and fishing lakes are empty.
7.3.4
OPA90 The American “Oil Pollution Act of 1990”.
In USA, the accidents involving “The Exxon Valdez” and “Mega Borg”
were in focus and were well covered by the media and press, which
influenced public opinion. This resulted in the OPA90. The media
distributed pictures of the rich animal life and the magnificent coastline in
Alaska covered with oil and showing the suffering of dying seals and
seabirds. This presentation made a strong impression, which made the
U.S. Congress realise that the existing International Conventions had to
be reviewed and bettered, in order to protect and take care of the
American interests. American lawyers developed the OPA90 and the
Congress supported the proposed Act.
7.3.5
The Main Items In OPA90:
1. 1. The threat of unlimited responsibility.
2. 2. Demand of double hull.
3. 3. Direct access to the means in P & I - Companies, in case of
indemnity due to accidents.
4. 4. Higher graded demands meant for the crew regarding narcotics
and alcohol testing.
5. 5. Use of pilot in sensitive waters.
Entering American waters OPA requires drill (training) according to OPA90
regulations.
The drill (training) should be logged and reported due to the ship
owners/operators policy.
OPA90 regulations are in force for all kind of ships.
7.3.6
MARPOL 73/78
-MARPOL-73, which is The International Convention about preventing
Marine Pollution.
The Convention consists of 20 articles, 2 Protocols and 5 Enclosures:
The 5 Enclosures are as follows:
Enclosure I
- Oils
Enclosure II - Chemicals
Enclosure III - Damaging elements in wrapped form, barrels, tanks,
containers and so on.
Enclosure IV - Sewage
Enclosure V
- Garbage
7.4
PERSONAL SAFETY EQUIPMENT
The minimum requirements for lifesaving equipment on board all ships are
laid down by national and international regulations. All equipment should
be inspected regularly and kept ready for immediate use in a clearly
marked and accessible place. Practical demonstrations, training and drills
should be regularly undertaken so that personnel become experienced in
use of all safety equipment and know the location of each item.
7.4.1
Breathing apparatus
As previously indicated, it is always preferable to achieve a gas-free
condition in a tank or enclosed space prior to entry by personnel. Where
this is not possible, entry into tanks should only be permitted in
exceptional circumstances and when there is no practical alternative, in
which case, breathing apparatus (and if necessary, protective clothing)
must be worn. There are four types of respiratory protection:•
Short duration breathing apparatus
•
Fresh air respirators
•
Compressed air breathing apparatus
•
Canister filter respirators
Each type is described in the following sections:
7.4.1.1 Short-duration breathing apparatus
Short-duration breathing apparatus consists of a small compressed air
cylinder and a polythene hood, which may be rapidly placed over the
head. Their duration is limited to about 15 minutes of comparatively nonexertive effort and the sets must be used only for emergency escape
purposes. Depending on the cargoes specified on the ship's Certificate of
Fitness, short-duration breathing apparatus may be provided in
accommodation spaces for each crewmember. Such equipment may also
be supplied for inspections of gas-free enclosed spaces, as an aid in case a
hazardous atmosphere is encountered, although, in cases of known
danger, it is recommended that compressed air breathing apparatus be
worn.
7.4.1.2 Fresh air respirators
Fresh air respirators consist of a helmet or facemask linked by a flexible
hose (maximum length 40 metres) through which air is supplied by a
manual bellows or rotary blower. The equipment is simple to operate and
maintain and its operational duration is limited only by the stamina of the
bellows or blower operators. However, movement of the user is limited by
the weight and length of hose and great care must be taken to ensure
that the hose does not become trapped or kinked.
Users of such equipment should always wear a safety line for
communication and rescue.
While this respirator has been largely superseded by the self-contained or
airline compressed air breathing apparatus, it will be found on many ships
as a backup to that equipment.
7.4.1.3 Compressed air breathing apparatus
Compressed air breathing apparatus may be adapted into two forms. It
may be the self contained type (SCBA) or the airline version (ALBA).
In the self-contained (SCBA) version, the wearer carries air for breathing
in a compressed air cylinder at an initial pressure of up to 300 bars. The
pressure is reduced at the outlet to about 5 bars and fed to the facemask
through a demand valve. This provides a slight positive pressure within
the mask. The working duration of the equipment depends upon the
capacity of the air cylinder and respiratory demand. A pressure gauge
and an alarm are provided to warn of low air supply pressure.
A typical set, providing approximately 30 minutes operation with physical
exertion, may weigh about 13 kg and the bulk of the cylinder on the back
of the wearer imposes some restriction on manoeuvrability in confined
spaces. When properly adjusted, the SCBA is simple and automatic in
operation. However, maintenance requires care and skill. To ensure
serviceability, all such breathing sets should be checked monthly and used
during exercises. This should be done using special exercise air cylinders
in order to keep the operational cylinders always fully charged or,
alternatively, an air compressor may be used for immediate refilling.
Although demand valves are designed to maintain a slight positive
pressure within the facemask, it should not be assumed that this feature
would prevent a contaminated atmosphere leaking into an ill-fitting mask.
It is essential that, before entry into a dangerous space, the air tightness
of the mask on the wearer's face be thoroughly checked in accordance
with the manufacturer's instructions. Tests have shown that it is virtually
impossible to ensure continued leak tightness in operational conditions on
a bearded face.
Most compressed air breathing sets may be used in the airline version
(ALBA) whereby the compressed air cylinder and a pressure reducing
valve are placed outside the contaminated atmosphere and connected to
the face mask and demand valve by a trailed air hose. At the expense of
decreased range and the need for extra care in guiding the trailing air
hose, the wearer is relieved of the bulk of the air cylinder. Also,
operational duration may be extended by the use of larger air cylinders or
special cylinder changeover arrangements.
7.4.1.4 Canister filter respirators versus SCBA
Canister filter respirators consist of a mask, which has a replaceable
canister filter attached. In this type of equipment, the normal breathing
of the wearer draws in contaminated air and toxic elements are filtered
out. They are simple to operate and maintain, can be put on quickly and
are used as personal protection for emergency escape purposes on ships
certified for carrying toxic cargoes. They are, however, only suitable for
relatively low concentrations of toxic gas. Once used, there is no simple
means of assessing the remaining capacity of the filter. Filter materials
are specific to a limited range of gases and, of course, the respirator gives
no protection in atmospheres of reduced oxygen content. For these
reasons, the requirements of the Gas Codes for emergency escape
protection is now almost exclusively met by lightweight self-contained
breathing apparatus.
Canister filter respirators are not suitable for use in atmospheres
where the oxygen content is insufficient to support life.
7.4.2
Training
Good training is essential in the use of this life-saving appliance. Specially
marked cylinders should be used for training to ensure that in an
emergency, only fully charged units are used. Cylinder pressures should
be regularly checked and low-pressure cylinders should be recharged
promptly.
7.4.3
Protective clothing
In addition to breathing apparatus, full protective clothing should be worn
when entering an area where contact with cargo is a possibility. Types of
protective clothing vary from those providing protection against liquid
splashes to a full positive pressure gas-tight suit which will normally
incorporate helmet, gloves and boots. Such clothing should also be
resistant to low temperatures and solvents.
It is particularly important to wear full protective clothing when entering
an enclosed space which has contained toxic gas such as ammonia,
chlorine, ethylene oxide, propylene oxide, vinyl chloride or butadiene.
For certain cargoes, the Gas Codes require the use of suitable eye
protection.
7.4.4
Fire fighter equipment
The requirement onboard oil tankers, as well as onboard gas tankers less
than 5000 m3, are 4 sets of fire fighter equipment. Onboard gas carriers
of more than 5000 m3, a minimum of 5 sets of fire fighter equipment is
required. Each set consists of:
•
One breathing apparatus (BA) with an air capacity
of minimum 1200 litres.
Protection suit including boots and gloves.
•
•
Fire resistance safety line with belt.
•
Safety lamp.
•
Fireman’s axe.
•
The equipment is specified in SOLAS, chapter 11-2, rule 17. National, and
classification companies requirements may come in addition. This is of
course considered for each vessel and the equipment is at all times in
accordance to existing requirement and rules.
7.4.5
Fire stations
The fire stations are marked on the safety plan, and also the content of all
required equipment at the stations. In addition to mentioned fire fighting
equipment, the content must include personal protective equipment, fire
hoses, jet nozzles that can switched from jet to fog dispersement, keys to
hose coupling and an extra fire axe.
Other equipment included is an electrical drill with 5/8” drill steel together
with an extension cord. It is smart to obtain a smaller drill steel to drill a
pilot hole, if this is a matter of necessity. A portable oxyacetylene torch
that renders it possible to make a quick carving of a manhole or other
openings to ease access is also included. This equipment is marked on the
safety plan, where it is placed onboard and at the right number according
to type and size of vessel.
Everyone is encouraged to know the seriousness of exercises onboard,
being prepared in a realistic and objective way can be, as a matter of fact,
very interesting and informative.
Anxiety is relieved because confidence leads to safety.
7.5
ACCOMMODATION
Regulations require that superstructures are designed with certain
portholes fixed shut and openings positioned to minimise the possibility of
vapour entry. These design features should not be modified in any way.
All doors, portholes and other openings to gas-safe spaces should be kept
closed during cargo operations. Doors should be clearly marked if they
have to be kept permanently closed in port, but in no circumstances
should they be locked.
Mechanical ventilation should be stopped and air conditioning units
operated on closed cycle or stopped if there is any possibility of vapour
being drawn into the accommodation.
08.
Ship/Shore Interface
8 SHIP/SHORE INTERFACE
8.1
SUPERVISION AND CONTROL
Within the gas trade, the ship/shore interface plays a vital part in
operations. It is an area where differing standards and safety cultures may
coexist.
There is no major difference between the general operation of a liquefied
gas tanker and the operation of any other type of ship. However, in view of
the hazardous cargo transported by a liquefied gas tanker, the crew must
be trained to be extra vigilant and to consider at all times the potential risk
under which the ship, its crew and its cargo are placed.
Close co-operation between ship and shore personnel is essential for the
safe handling of a ship transferring cargo in a terminal. If the operation is
well prepared and if open channels of communication are maintained
between ship and terminal, there is a good chance that the transfer will be
carried out smoothly and that any unexpected incident will be tackled
promptly before it can develop into something more serious.
With respect to the equipment fitted on jetties, the ship/shore interface
covers:Moorings
Fenders
Breasting dolphins
Hard arms and hoses
Ship/shore gangways
Emergency shut-down arrangements
Ship/shore links, and
Fire-fighting equipment capability
Liquefied gases are loaded and discharged at many terminals around the
world by a wide variety of ship types and sizes. Operations range from the
very large self-contained LNG projects to smaller LPG terminals handling
many different products.
8.1.1
The terminal
During the design of a new marine terminal, minimum and maximum ship
size is established. Furthermore, the jetty and its equipment are designed
accordingly. Farther offshore, the port approaches and river channel are
surveyed. Once a terminal is ready for service, the relevant information
needed by visiting ships should be advised to the port authority, ship’s
agents, pilots and ship owners’ associations.
8.1.2
The ship
Gas carriers are normally built in such a way that there is maximum
compatibility with a range of terminals. Terminal personnel prior to
acceptance of any nomination should always confirm compatibility of any
particular ship and terminal from a technical viewpoint. Confirmation
should include items such as mooring studies, manifold configurations and
ESD link (Emergency Shut Down) compatibility.
8.1.3
Communications
Communications should start before the intended voyage and continue until
the arrival of the ship alongside: they must also include the period of cargo
operations and continue until the ship departs. All communications should
be carried out in a common language so that misunderstanding cannot
develop. Usually, apart from some coastal trades, this will be English.
8.1.4
Prior to arrival
As a ship approaches a port, direct contact should be established between
ship and shore as soon as possible. Modern communications will readily
allow the terminal to update the ship on its requirements for the envisaged
transfer operation. Additionally, port requirements, berthing arrangements
and the facilities available can also be advised. Similarly, the shipmaster
may inform the terminal of the cargo arrival temperatures and pressures,
stores and bunker requirements and personnel joining or leaving.
For the planning of ship cargo operations, the shipmaster should be advised
by the terminal of all port and terminal requirements relevant to gas
carriers.
8.1.5
Alongside the jetty
As for the earlier parts of a ship's voyage described in the foregoing
paragraphs, reliable and effective communications are a necessity once
the ship is alongside.
While alongside and transferring cargo, various means of communication
need to be agreed. Decisions must be made on the use of portable radios
or telephones. These tools usually form the basis of good communications
under normal operating conditions. However, emergency means of
communication must also be developed and this will normally take the
form of an established terminal operating procedure.
In many terminals, the actuation of emergency shut-down (ESD) valves is
interlinked between ship and shore. This communication channel requires
a suitable system having plugs and sockets fitted on ship and jetty. Both
ship and shore need to be properly outfitted. Such methods of
communication are recommended so that a controlled emergency
shutdown can always be accomplished. This will always ensure that either
the ship or shore emergency shutdown valve, whichever is nearest to the
operational cargo pump, is closed first.
8.2
DISCUSSIONS PRIOR TO CARGO TRANSFER
Before the start of any cargo transfer operation, the intended cargo
handling procedures must be thoroughly discussed at a meeting held
between the responsible personnel from the ship and the terminal.
The purpose of the meeting is primarily to draw up a suitable cargo plan
and to check on safety issues. Furthermore, the meeting has the benefit
of making both sides familiar with the essential characteristics of ship and
shore cargo handling systems. At the meeting, the envisaged operational
and safety procedures and requirements should be covered. Finally, any
limitations to be observed during the transfer should be noted in writing.
Written agreements should include a cargo handling plan (including
transfer rates), communication procedures, emergency signals,
emergency shutdown procedure and the tank venting system to be used.
The content of the meeting will depend on a wide variety of circumstances
but the following broad outline forms the normal basis for such meetings.
(i)
The names and roles of terminal and ship personnel who will
be responsible for cargo transfer operations should be
noted.
(ii) The terminal representative should check that pre-arrival
instructions to the ship on cargo, cargo disposition and
cargo arrival temperature have been carried out. They also
check that all necessary ship equipment inspections and
tests have been performed.
(iii) Similarly, the ship's officers should satisfy themselves that
the relevant terminal equipment is satisfactory and that
appropriate inspection checks have been carried out.
(iv) The terminal representatives and, where necessary,
customs and independent surveyors should be informed of
the cargo tank data, such as:•
•
•
•
•
•
•
•
•
•
•
•
Temperatures
Liquid heel or arrival dip
Pressures
Composition of tank vapour, and
Cargo tank quantities
Total quantity of cargo on board
(v) The ship and terminal should then discuss and agree in
writing the quantity and types of cargo to be loaded or
discharged and in what order. The anticipated transfer
rates and, for discharge, the receiving tank allocations
should also be agreed.
The cargo transfer operation should be planned and confirmed
in writing in order to assure full mutual understanding. The
items to be addressed should include:•
•
•
•
•
•
The order of loading or discharging
The total quantities of cargo to be transferred
The sequence of discharging and receiving tanks
The intended transfer rates
The transfer temperatures and pressures to be
expected, and
The use of vapour return line
(vi) To reconfirm earlier pre-charter advice, the previous three
cargoes carried by the ship and the relevant dates should
be noted in order to identify and assess any possible cargo
contamination problems, particularly after ammonia.
(vii) The appropriate Cargo Information Data Sheets should be
provided and should be posted in prominent places on
board the ship and within the terminal.
8.3
SHIP/SHORE SAFETY CHECK LIST
When a ship is alongside, no cargo operations or inerting should
commence until the ship and the terminal have completed the
international Ship/Shore Safety Check List and it has been confirmed that
such operations can be safely carried out. It is normal practice that this
checklist is presented to the ship by the terminal.
Recommendations on the Safe Transport of Dangerous Cargoes
and Related Activities in Port Areas were revised by IMO in 1995.
They refer to a comprehensive Ship/Shore Safety Check List
covering the handling of bulk liquid dangerous substances with a
special section for liquefied gases. It also includes guidelines for
its completion.
8.4
OPERATIONAL CONSIDERATIONS
8.4.1
Berthing and mooring
Berthing
Port and terminal authorities should establish berthing and unberthing
criteria for safe operations, including limiting wind, wave, current and tide
conditions. Requirements for the number and size of tugs must also be
set.
Mooring
Mooring line configurations should be agreed as suitable. The initial
mooring of the ship to the terminal and the subsequent tending of
moorings is most important if the ship is to be safely held alongside and
damage to transfer facilities and jetty prevented.
8.4.2
Connection and disconnection of cargo hoses and hard
arms
Terminal equipment, such as hoses and hard arms, are designed to
connect with the ship's manifold. Irrespective of the type of equipment
being used, there are certain operational procedures to be considered.
No flanges should be disconnected or blanks
removed until it is confirmed that line connections are
liquid-free and depressurised and, where possible,
inerted with nitrogen or other suitable inert gas.
Care must be taken to avoid air or contaminants
entering cargo pipelines.
The manifold area of a gas carrier is a zone where
flammable vapours may be present. Therefore, care
must be taken to ensure that ignition sources are
eliminated from this area.
8.4.3
Cargo tank atmospheres
Prior to any cargo transfer, the oxygen content in the ship's cargo tank
vapours should be carefully checked. As stated elsewhere in this book, at
these times the oxygen content should never exceed five per cent and is
commonly required to be not more than two per cent by volume in tanks
containing vapour only. Lower oxygen contents may be required for cargo
quality purposes.
For example, products such as butadiene and vinyl chloride, which can
react with oxygen to form unstable compounds, require maximum oxygen
concentrations of 0.2 per cent by volume and 0.1 per cent by volume,
respectively.
8.4.4
Cargo handling procedures
Cargo handling is described in Chapter Seven but procedural aspects of
these operations, directly relevant to the ship/shore interface, are
considered here.
All operations carried out alongside should be under the continuous
supervision of experienced ship and shore personnel. These personnel
should be familiar with the details, hazards and characteristics of the
cargoes being handled and capable of ensuring that such operations can
be safely and efficiently completed. Facilities for instant and reliable
communications (such as separate telephone, portable radio or VHF)
between the ship and the shore control should be provided at all times
during cargo operations.
Before commencing operations, maximum cargo transfer rates have to be
agreed. This should be done in accordance with vapour return
specification, ship or shore reliquefaction capacity and emergency shutdown requirements. Inevitably, some of these considerations may be
based on best practical estimates. Accordingly, during operations, a strict
watch should be maintained on flow rates, tank pressures and
temperatures. By means of ship/shore communications, adjustments to
initial agreements can be made as appropriate.
If cargo transfer operations need to be stopped, this should be carried out
under previously agreed controlled conditions with proper communication.
If the situation demands an emergency shut-down, the agreed procedure
should be followed, bearing in mind the dangers of excessive surge
pressures. It is particularly important to maintain appropriate
communication in emergency conditions and, if the responsible person
becomes over-occupied in controlling operations, the communication task
should be delegated to another officer.
8.4.5
Gangways and ship security
It is the duty of both the ship and the terminal to ensure that adequate
and safe ship/shore access is provided. Where possible, the manifold
areas should be roped off to limit the access of personnel to that area.
The gangway should be located away from the immediate vicinity of the
manifold and, ideally, should be positioned about midway between the
cargo manifold and the accommodation. As appropriate, it should be
rigged with a strong safety net beneath. Both on the terminal and on
board ship it is good practice to provide a lifebuoy at the gangway
entrances. Proper illumination of the gangway and its approaches should
be provided during darkness.
A notice warning against unauthorised personnel should be posted at the
gangway and provision should be made for all ship visitors to be met and
escorted to the accommodation.
8.4.6
Bunkering
In general, on gas carriers, bunkering operations by barge will not take
place during cargo operations as this is usually disallowed by terminal
regulations. This avoids a bunker craft with possible ignition sources
being allowed alongside the gas carrier.
Bunkering from the shore can be carried out during cargo operations so
long as shipside scuppers can be closed quickly. In case of cargo leakage
open scuppers on gas carriers are an important feature to allow cold
liquids to escape quickly so reducing the risk of metal embrittlement and
the possibility of small pool-fires on a ship's deck.
Oil tanker practice is to operate with scuppers closed and, in general, this
standard is also applied to bunkering operations. It is therefore essential
for gas carrier port operations to be properly considered in this respect
and either suitable operational procedures must be in place or bunker tank
openings and air pipes should be well bunded so that bunkering from
ashore can take place during liquid cargo handling.
8.4.7
Work permits
While a ship is alongside, only under exceptional and well-controlled
circumstances should any hot work (including the use of power tools) be
undertaken, either on board or within the vicinity of the ship. In the
unlikely event that such work must be carried out, the most stringent
safety precautions and procedures should be drawn up and rigidly adhered
to.
To cover these and similar circumstances, a Permit to Work system should
be in place. In the event that hot or cold work becomes necessary when a
ship is alongside, a Work Permit should be agreed between the ship, the
terminal and, where necessary, the port authority. The Work Permit
should cover a limited period and the terms and conditions for which it is
issued should be rigidly enforced.
8.5
FIRE-FIGHTING AND SAFETY
When a ship is alongside a terminal jetty, it is important that a joint
emergency plan be available. The preparation of such a plan is the
responsibility of each terminal. The details of the plan should consider the
appropriate actions to be taken in all envisaged emergencies. This should
include communication with local emergency services and the port
authority. A summary of the essential elements within the plan should be
made available to ships' personnel and an appropriate method of
providing this information is by inclusion of suitable data in the Terminal
Information and Regulation booklet.
Whilst a ship is alongside the terminal, fire-fighting equipment, both on
board and on shore, should be correctly positioned and ready for
immediate use. Although the requirements of a particular emergency
situation will vary, fixed and portable fire fighting equipment should
always be stationed to cover the ship and jetty manifold area. As
described in the Ship/Shore Safety Check List Guidelines, fire hoses
should be laid out with nozzles attached; hoses from fixed dry powder
units should be laid out; and portable fire extinguishers readied for
immediate action. The international ship/shore fire connection should also
be made available for use at short notice.
Water spray systems should be tested on a regular basis. Where water
sprays are designed to operate automatically, in the event of fire, the
functioning of the automatic devices should be included in the test.
The ship's fire fighting and safety plan should be placed in a container
near the gangway. This plan should provide the most up-to-date
information. It is good practice to include a copy of the ship's Crew List in
the container
09.
EMERGENCY
OPERATIONS
9 EMERGENCY OPERATIONS
9.1
ORGANIZATIONAL STRUCTURE
An emergency can occur at any time and in any situation. Effective action
is only possible if pre-planned and practical procedures have been
developed and are frequently exercised.
When cargo is being transferred, the ship and shore become a combined
operational unit and it is during this operation that the greatest overall risk
arises. In this respect, the cargo connection is probably the most
vulnerable area.
The objective of an emergency plan to cover cargo transfer operations
should be to make maximum use of the resources of the ship, the terminal
and local authority services. The plan should be directed at achieving the
following aims:Rescuing and treating casualties
Safeguarding others
Minimising damage to property and the environment, and
Bringing the incident under control
9.2
ALARMS
Each gas ship and terminal should have fire-fighting plans and muster lists
prominently displayed. These should be carefully read and understood by
all personnel. As a general guide, when a liquid gas fire occurs, the correct
procedure to adopt is as follows:Raise the alarm
Assess the fire's source and extent, and if personnel are at risk
Implement the emergency plan
Stop the spread of the fire by isolating the source of fuel
Cool surfaces under radiation or flame impingement with water,
and
Extinguish the fire with appropriate equipment or, if this is not
possible or desirable, control the spread of the fire as above
9.2.1
Raising the alarm and initial action
Fundamental to emergency procedures is how to report and how the alarm
should be given to all concerned. These procedures should be developed
independently for the terminal, the ship and the ship/shore system.
Procedures should warn that a seemingly minor incident may quickly
escalate to one of a more serious nature. Much is gained by immediately
reporting any abnormal occurrence, thereby permitting early consideration
of whether a general alarm is desirable.
In the case of incidents on a ship or on a jetty while a ship is alongside, the
manpower and facilities immediately available on the ship will generally
make it appropriate that the ship takes first autonomous action by initiating
cargo transfer ESD by the agreed safe means, alerting the terminal to
provide assistance as quickly as possible and immediately putting into
action the ship's own emergency procedure.
9.3
EMERGENCY PROCEDURES
Effective emergency response requires an emergency organisation round
which detailed procedures may be developed. The international character
of ocean shipping and its universally similar command structures lend
themselves to the development of a standard approach in ships' emergency
planning. For gas carriers this broad uniformity can be extended further to
the development of incident planning.
Such standardisation is of
advantage since ships' personnel generally do not continuously serve on
the same ship. It is also of advantage in the handling of incidents in port in
that terminal emergency planning can be more effective if there is
knowledge of the procedures a ship is likely to follow.
Outlined below is a suggested emergency organisational structure for gas
carriers in port, which has received wide acceptance. As shown, the basic
structure consists of four elements:
(i) Emergency Command Centre. In port the Emergency
Command Centre should be established in the Cargo Control
Room. It should be manned by the senior officer in control of the
emergency, supported by another officer and a crewmember
acting as a messenger. Communication should be maintained
with the three other elements (see below) and with the terminal
emergency control room by portable radio or telephone.
(ii) Emergency Party. The Emergency Party is a pre-designated
group. It is the first team sent to the scene and reports to the
Emergency Command Centre on the extent of the incident. The
Party recommends the action to be taken and the assistance
required. The Party is under the control of a senior officer and
comprises officers and other suitable personnel trained to deal
with rescue or fire-fighting.
(iii) Back-up Emergency Party. The Back-up Emergency Party
stands by to assist the Emergency Party at the direction of the
Emergency Command Centre. The Back-up Party should be led
by an officer and comprises selected personnel.
(iv) Engineers Group. Some engineering personnel may form part of
either emergency party. However, the Engineers Group is
normally under the leadership of the chief engineer and has prime
responsibility for dealing with an emergency in the main
machinery spaces. Additionally, the Group provides emergency
engineering assistance as directed by the Emergency Command
Centre.
9.3.1
Incident plans
In developing plans for dealing with incidents, the following
scenarios should be considered:
Checks for missing or trapped personnel
Collision
Grounding
Water leakage into a hold or interbarrier space
Cargo containment leakage
Cargo connection rupture, pipeline fracture or cargo spillage
Lifting of a cargo system relief valve
Fire in non-cargo areas
Fire following leakage of cargo
Fire in a compressor or motor room
9.3.2
Emergency shut-down (ESD) - ship/shore link
In any serious incident associated with cargo transfer, on shore or on ship,
it is essential to shutdown cargo flow by stopping pumps and to close ESD
valves. All gas carriers and all large terminals have a system for the rapid
emergency shutdown of cargo transfer.
Where gas carriers and terminals are dedicated to each other, as in most
LNG projects, terminal and ship ESD systems are linked during cargo
transfer and act in combination.
In general trading of other liquefied gases, the ship and shore ESD systems
are not always linked and consideration must be given to avoiding
escalation of an incident by creating disruptive surge pressures at the
ship/shore cargo connection by the over-rapid closure of ESD valves
against cargo flow. It is preferable that in loading a ship, the terminal ESD
is actuated and completes its shutdown before the ship's ESD valves close.
Similarly, it is preferable during a ship discharge that the ship completes its
ESD before the terminal's ESD valves close.
It is a growing practice for loading terminals to present the ship with a
pendant by means of which the ship may actuate the terminal's ESD.
Similarly, some receiving terminals encourage discharging ships to provide
the jetty with a pendant by means of which the ship's ESD may be actuated
from the shore. In any case it is desirable that the maximum cargo flow
rate be limited to that which will not cause excessive surge pressure should
ESD valves downstream of the cargo connection be closed, at their known
rate of closure, against the cargo flow.
While the above procedures and pendant-controls may be suitable in some
circumstances, they cannot always be relied upon, especially in an
emergency when personnel may activate the system incorrectly.
To
overcome this difficulty, it is recommended that ship and shore systems be
fitted with a linked system. This must be engineered to ensure the
appropriate procedure is followed, no matter which party initiates the shutdown.
9.4
FIRST-AID-TREATMENT
9.4.1
In general
What is health? In short, it is when the physical is in balance with the nonphysical, and the harmonisation here has a natural function. The result is
good health. To maintain this, knowledge about harmonisation is the vital
factor in health. Health is different for each one of us based on individual
tendencies and external/internal influences that mark (or chooses to mark)
our life.
All crewmembers that sign on a vessel should have been through a medical
check in order to have a regular status of his/her health condition. Life at
sea is a special place to work, it is important that the general health
condition at all times is good. What can be done to maintain a good general
health condition on board? The answer is built into the safety and
protection of personnel on board. You can also take care of one another in
a good manor by being aware of the risks that may have direct and
external effect on health, regarding the special cargoes carried onboard
your vessel.
9.4.2
The body
The doctrine of how the body is built is called anatomy. The doctrine of the
body’s function is called physiology. This will be roughly illustrated to
achieve a synopsis of how the “machine” functions.
9.4.3
The cell
This is the smallest, independent unit of the body and the basis for all living
organisms. All the processes in the body are caused by the chemical
reactions that take place in the cells. Cells in different tissue and organisms
co-operate in their duties. The cell has a water content of approximately
70% in addition to proteins, carbohydrates, fat and inorganic material. All
the cells have the same basic structure and a number of mutually basic
qualities. Simultaneously each part of the cell has its function. We all utilise
nutrients both to achieve energy and as “building stones”. In new cell
components, glucose (grape sugar) is the most important energy source. It
is important to have nutrient rich and varying diet.
9.4.4
Tissue
Cells that look alike remain lying to form tissue. All surfaces of the body are
covered with epithelial tissue (type of tissue that mainly covers all surfaces,
the cavity and channels of the body). Connective tissue and support tissue
forms the tissue network in the body and keeps tissue and organs together.
There is an innumerable of tissues, for example osseous tissue, muscular
tissue and nerve tissue. The cell co-operation is controlled by chemical
signals. These signals consist of two types, nerve signals and hormone
signals. These two systems co-operate for an appropriate reaction. This is
fully necessary for our survival. The hormone system controls the activity
of many internal organs; the nerve system controls muscles and glands.
Several organ systems co-operate to keep the composition of tissue fluid
constant. The blood renews this tissue fluid. The blood must circulate the
whole time. The duty of the lymph artery is to drain excess tissue fluid.
9.4.5
The respiratory organs
These absorb oxygen and partly carbon dioxide. Respiration is an exchange
of gases between the blood arteries and the air in the lungs. The blood
absorbs oxygen into the body’s cells and partly the excess carbon dioxide
that arises. The respiratory organs consist of the bronchia and the lungs.
Gas exchange between blood and air takes place in the lungs.
9.4.6
The skin
The skin forms an essential boundary to the surroundings, and is the body’s
largest “breathing organ”. The skin consists of different tissue with different
qualities and covers the body surface, like an almost impenetrable
protective film. The skin is an important sensory organ with large
adaptability.
9.4.7
The immune system
This system protects the body and consists of several parts. There is no
possibility of living a normal life without this defence, as its duty is to
render harmless infective agents or other strange material. In addition to
combating infection from outside, this defence system also fights against
any internal cell changes.
9.4.8
Thought, Action, Result, Feeling
Positive thoughts and attitudes together with a healthy diet form the basis
for good health. We can do a lot ourselves by choosing the right things, as
we are free to choose.
We now take a look at your work place, onboard a vessel, and the influence
this has on your health. We will also discuss what external influences can
be found in the atmosphere and the injuries/incidents that may occur on
board.
Onboard different types of vessels carrying different types of cargo, danger
to health from external influences are considered regarding the vessel’s
protective equipment and routines. This protective equipment is placed
practically and can be utilised, as necessary. Familiarise yourself with the
equipment onboard your vessel and use it!
With a sudden injury or illness on board, medical advice and guidance can
be gathered from Radio Medico – the radio medical service for vessels at
sea. It is important to have all the important information when help is
needed for a serious condition onboard, such as:
•
Age
•
Sex
•
Weight
•
Duration of the illness
•
Extent of the injury
•
Symptoms
•
Patient's comments (complaints)
•
Clinical findings (sign of a specific illness)
•
How the injury happened
•
Character of the pain (grumbling, stabbing, squeezing)
•
Whereabouts of the pain
•
Face colour, limpness, drowsiness, temperature, pulse,
breathing trouble, nausea, blood, mucus, urination, etc.
All of the above is important.
There is a “hospital” onboard containing ordered equipment for treatment
and medication. The ship medical directions regarding the ship’s hospital
deal with the maintenance, supply, inspection, etc.
It is important to know how to protect oneself against harmful skin contact,
skin absorption and respiratory absorption of dangerous gases in the
atmosphere surrounding us, such as entering tanks and closed spaces.
Help given in the first minutes of an emergency situation is crucial. All
must endeavour to have respectable first aid skills.
9.4.9
First aid
First aid is used with sudden unconsciousness, stopped breathing
and lack of air.
(Call for help, but do not abandon the patient, immediately start helping.)
A
Air: Try to free the airflow, lie the patient on a flat surface, bend the
head backwards, remove any dentures, vomit, etc.
B
Breathing: If the patient is not breathing, start resuscitation with 3-5
breaths/insufflations. Use the “Pocket Mask” as an option. Hold the head
curved backward, check the pulse on the neck. If pulse is felt, continue
with 12 respiration’s per minute
C
Circulation: With deadly paleness and no pulse, give 2-3 powerful
knocks over the heart. If this has no effect, start external
heart compression once per second.
9.4.10
ABC
The method stands for air, breathing, and circulation.
The priority of first-aid training and practice is of great importance. The
better you are at first aid in an emergency; the chance of a good outcome
is greater.
9.4.11
Heart problems
Heart problems can be suspected if sudden, strong pain behind the
breastbone is experienced. For cardiac arrest, use the ABC.
9.4.12
Shock injuries
Description of shock is acute circular failure. This may be caused by
reduced blood volume from bleeding, shock by drop of blood pressure or
reduced pump functions from a cardiac infarction. If a big incident occurs,
shock must be calculated. The symptoms are fast pulse, coldness, pail and
difficulty in breathing. Supply oxygen, warm blankets and fluids.
9.4.13
Head injuries
All knocks against the head must be taken seriously. The symptoms are
headache, nausea and dizziness. Flat bed rest for 2-3 days. Limited fluid
intake and be sure to supervise.
9.4.14
Poisoning and etch injuries
Refer to the IMO’s book “Medical First Aid and Guide for use in accidents
involving dangerous goods”. This refers to the data sheets on the different
cargo onboard. (This is illustrated later on in this part). Poisoning and etch
injuries appear in connection with cargo contact, as air absorption,
swallowing or skin absorption (skin contact). The symptoms are pink
coloured skin, smell of almonds on the breath, headache, dizziness, nausea
and vomiting. Remember that in connection with cargo contact, the
emergency squad should efficiently use protective equipment, gloves etc.
Supply oxygen and follow the instructions on the data sheet for the cargo in
question.
9.4.15
Fire injuries
In fire injuries, ensure a stabile lateral position for the patient, if possible.
Supply oxygen and fluid. With fire injuries, quick help is double the
help. Quickly cool for at least 20 minutes. Estimate the extent of the
injury. The patient mustn’t freeze. Provide warm blankets and abundant
fluid. The patient should rest, be under supervision, and have their pulse
checked. Check the medical box for proper use of medication and
bandages.
9.4.16
Frost injuries
Localised frost injuries on the skin’s top layer begins with a prickling
feeling, then ascends to white spots on the skin. Careless handling of
pipeline and cranes onboard vessels, which carry strongly cooled gases, can
lead to localised frost injuries. Important: Frozen hands and feet must
not be warmed up actively with warm water. Cover frozen skin parts with
a soft woollen garment. Do not massage or rub. It helps a lot to warm up
frozen skin with warm skin
9.4.17
Bone, joint & soft part injuries
A lot of injuries are sprains, fracture and soft part injuries. Use the ICE
method, as the proper first aid, in such injuries.
ICE means ice,
compression and bandage, and elevation.
I – stands for ice. Ice the injury in order to lower the injured spot’s
temperature. By doing so, the bleeding is reduced in the underlying tissue.
Swelling and pain will also be reduced.
C - stands for compression bandage or compression.
If cooling the injury
is not sufficient, compression around the injured spot is recommended in
order to counter the pressure from haemorrhage and reduce swelling and
pain. Confer with the patient regarding the tightness of the bandage.
E – stands for elevation and rest.
To decrease the blood pressure and
reduce the seepage of blood on and around the injured place, raise an
injured arm or foot to approximately heart height and rest for 1-2 days.
9.4.18
Intake of poison materials
Poisonous materials can be taken in by inhaling (gas, dust), skin
penetration, skin absorption (gas and liquid) and swallowing (gas and
fluid). If any of this occurs, different reactions will occur depending on the
kind of material, how much, etc. Refer to the material’s data sheet
regarding treatment. Blood is most important, since it is the higher brain
centre that is first affected from lack of oxygen.
A poisonous material emerges quickly to the brain cells and deprives them
of oxygen. This may cause unconsciousness, at worst death. By inhaling
small concentrations, we are exposed to localised effects (nasal, throat,
and lung) or poisonous gas absorption into the blood.
Through skin penetration, gases and fluids are quickly absorbed into the
blood and the effects depend on the characteristic of the material, the
velocity of the penetration and poisonous elements. If material is
swallowed, this is easily absorbed by the mucous membrane in the mouth.
9.4.19
The eyes
The eyes are very exposed to any spill or contact to cargo. There is
normally irritation, burns and tears from harmful exposure. It is of utmost
importance with a very fast first aid and abundant rinsing with water.
With all injuries and illness it is of the utmost importance to administer first
aid and contact competent medical help if any doubt of the outcome exists.
Enclosed is a data sheet for Propane, which illustrates the layout and the
content of information. There are such sheets for all types of dangerous
cargo, which are made readily available and visible onboard.
The data sheets tell us about the cargo’s character, the emergency
procedure for a cargo fire or cargo spill. There is also information about
health hazards, fire, explosion, chemical data, reaction data, physical data
and the condition of the material in freight. Information regarding the
quality of material is required with the freight of the material.
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