Chapter 22
Liquefied Gases
A liquefied gas is a gaseous substance at ambient temperature and pressure,
that is liquefied by pressurisation or refrigeration and sometimes a combination
of the two. Virtually all liquefied gases are hydrocarbons and flammable in
nature. Liquefaction compresses the gas into volumes suited to international
carriage. The principal gas cargoes are LNG (see Chapter 23), LPG and
a variety of petrochemical gases, and all have their specific hazards. By
regulation, all liquefied gases when transported in bulk must be carried on a
gas carrier, as defined by the IMO. The IMO’s Gas Codes (see Sections 22.2
and 22.3) provide a list of safety precautions and design features required for
each product.
22.1
LPG
The term LPG (liquefied petroleum gas) covers butane or propane or a mixture
of the two. The main use for these products varies from country to country,
but sizeable volumes are used as power station or refinery fuels. However,
LPG is also sought after as a bottled cooking gas and it can form a feedstock
at chemical plants. It is also used as an aerosol propellant and is added to
gasoline as a vapour pressure enhancer. LPG may be carried in either a
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pressurised or refrigerated state. Occasionally, it may be carried in a special
type of carrier known as a semi-pressurised ship. When fully refrigerated,
butane is carried at minus 5°C (–5°C) and propane at minus 42°C (–42°C).
This low carriage temperature for propane introduces the need for special low
temperature (LT) steels.
22.2
Chemical and Other Gases
Ammonia is one of the most common chemical gases and it is carried
worldwide in large volumes, mainly for agricultural purposes. It has particularly
toxic qualities and requires great care during handling and carriage.
Another important liquefied gas is ethylene. Very sophisticated ships are
available for this product as carriage temperatures are minus 104°C (–104°C)
and onboard systems require a high degree of expertise. Within this group, a
subset of highly specialised ships is able to carry multigrades simultaneously.
The recent exploitation of shale gas has brought an increase in LPG and
ethane production, as by-products. Ethane may be used as an alternative to
naphtha or LPG as a feedstock for the chemical industry. Liquefied ethane,
with a temperature close to minus 89°C (–89°C) at atmospheric pressure,
has typically been shipped in small ethylene/ethane carriers, the design
characteristics of the cargo containment system for the ships in this trade being
in the region of 27,500 to 35,000 m3. The economies of scale needed for a
profitable global trade typically require ethane to be shipped in large volumes.
The first of the new class of dedicated very large ethane carriers (VLECs) were
delivered at the end of 2016 and these have a capacity of 87,000 m3. These
ships have a reinforced membrane cargo containment system similar to that
fitted to LNG carriers.
Significant in the design and operation of all gas carriers is that methane
(the main constituent of LNG) vapour is lighter than air, while LPG vapours
are heavier than air. For this reason, the current gas carrier regulations allow
only methane to be used as a propulsion fuel, with any minor gas seepage in
engine spaces being naturally ventilated. With the adoption in January 2017
by the IMO of the International Code of Safety for Ships using Gases or other
Low-flashpoint Fuels (IGF Code) (Reference 44), it may, in future, be possible
to use ethane and LPG as propulsion fuel.
The principal hydrocarbon gases such as butane, propane and methane are
non-toxic in nature. A comparison of the relative hazards of oils and gases is
provided in Table 22.1.
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Gases
Oils
Hazard
LNG
LPG
Gasoline
Fuel Oil
No
No
Yes
Yes
No
Yes
Yes
No
No
Toxic
Carcinogenic
No
Asphyxiant
Yes (in confined
spaces)
Yes (in confined
spaces)
Others
Low temperature
Moderately low
temperature
Eye irritant,
narcotic, nausea
2 to 10
1 to 6
Eye irritant,
narcotic, nausea
Flammability limits
in air (%)
5 to 15
Storage pressure
Atmospheric
Often pressurised Atmospheric
Atmospheric
Behaviour if spilt
Evaporates
forming a visible
‘cloud’ that
disperses readily
and is nonexplosive, unless
contained
Evaporates
forming an
explosive vapour
cloud
Forms a
flammable pool;
environmental
clean-up is
required
Forms a
flammable pool
which if ignited
would burn with
explosive force;
environmental
clean-up may be
required
Not applicable
Table 22.1: Comparative hazards of some liquefied gases and oils.
22.2.1 The Liquefied Gas Fleet
Courtesy of Clarksons Platou LNG
Shipbroking
The fleet of liquefied gas carriers of over 1,000 m3 capacity can be divided into
6 major types:
Of which
SemiEthylene
Pressurised refrigerated
and Ethane
LPG carriers LPG carriers
carriers
(inc Ethylene)
comprise:
Ship
numbers
Total
capacity
million (m3)
Fully
LNG
LNG
refrigerated
carriers FSRUs
LPG carriers
690
347
158
324
432
23
2.199
3.22
1.4
20.64
62.7
3.5
Table 22.2: The gas carrier f leet (end 2015).
Chapter 23 describes the liquefied natural gas (LNG) carrier in more detail.
The introduction of a tanker designed to carry compressed natural gas (CNG)
is anticipated in the near future. A number of designs have been produced
but, due to the relatively low deadweight and high cost of these ships, the first
commercial application of this technology cannot be predicted.
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While the gas carrier is often portrayed negatively in the media as a potential
floating bomb, accident statistics do not bear this out. The sealed nature of
liquefied gas cargoes, in tanks completely segregated from oxygen or air,
virtually excludes any possibility of a tank explosion. However, the image
of the unsafe ship lingers, and some administrations and port state control
organisations tend to target gas ships for special inspection whenever they
enter harbour.
However, serious accidents related to gas carrier cargoes have been few and
the gas carrier’s safety record is acknowledged as an industry leader. As an
illustration of the robustness of gas carriers, when the ‘Gaz Fountain’ was hit
by rockets during the Iran/Iraq War in the 1980s, despite penetration of the
containment system with huge jet fires, the fires were successfully extinguished
and the ship, together with most of the cargo, was salved.
The relative safety of the gas carrier is due to a number of features. One such,
almost unique to the class, is that cargo tanks are always kept under positive
pressure (sometimes just a small overpressure) and this prevents air entering
the cargo system. This means that only liquid cargo or vapour can be present,
so a flammable atmosphere cannot exist in the cargo system. In addition, all
large gas carriers use a closed loading system with no venting to atmosphere,
and a vapour return pipeline to the shore is often fitted and used where
required.
22.3
Regulation of Gas Carrier Design
The regulations for the design and construction of gas carriers stem from
practical ship designs codified by the IMO. However, all new ships (from June
1986) are built to the International Code for the Construction and Equipment
of Ships Carrying Liquefied Gases in Bulk (the IGC Code) (Reference 45).
This code also defines cargo properties and documentation provided to the
ship (the Certificate of Fitness for the Carriage of Liquefied Gases in Bulk) and
shows the cargo grades the ship can carry. In particular, this takes into account
temperature limitations imposed by the metallurgical properties of the materials
making up the containment and piping systems. It also considers the reactions
between various gases and the elements of construction, not only on tanks
but also related to pipeline and valve fittings. The IGC Code has recently been
revised and the revised code, adopted by the IMO in 2014, applies to all new
ships built (having their keel laid) after 1st July 2016.
When the IGC Code was produced, an intermediate code was also developed
by the IMO – the Code for the Construction and Equipment of Ships Carrying
Liquefied Gases in Bulk (the GC Code) (Reference 46). This covers ships built
between 1977 and 1986.
Gas carriers were in existence before IMO codification and ships built
before 1977 are defined as ‘existing ships’ within the meaning of the rules.
To cover these ships, a voluntary code was devised, again by the IMO – the
Code for Existing Ships Carrying Liquefied Gases in Bulk (the EGC Code)
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(Reference 47). Despite its voluntary status, virtually all ships remaining in the
fleet that are of this age, and because of longevity programmes there are still
quite a number, have certification in accordance with the EGC Code.
22.4
Design of Gas Carriers
Cargo carriage in the pressurised fleet comprises double cargo containment.
All other gas carriers are built with a double-hull structure and the distance
of the inner hull from the outer is defined in the Gas Codes. This spacing
introduces a vital safety feature to mitigate the consequences of collision and
grounding.
A principal feature of gas carrier design is therefore double containment and
an internal hold. The cargo tanks, more generally referred to as the cargo
containment system, are installed in the hold, often as a completely separate
entity from the ship, ie not part of the ship’s structure or its strength members.
This is a distinctive difference between gas carriers and oil tankers or chemical
carriers.
(a) 3,200 m3 coastal LPG carrier with cylindrical tanks
(b) 16,650 m3 semi-pressurised LPG carrier
(c) 78,000 m3 LPG carrier with Type-A tanks
Figure 22.1: Gas carrier types.
Cargo tanks may be of the independent self-supporting type or of a membrane
design. Self-supporting tanks are defined in the IGC Code as being of Type-A,
Type-B or Type-C. Type-A containment comprises box-shaped or prismatic
tanks
to fit
the with
hold).
Type-Btanks
comprises tanks where fatigue life and
(a)(ie shaped
135,000 m3 LNG
carrier
membrane
crack propagation analyses have shown improved characteristics. Such tanks
are usually spherical but occasionally may be of prismatic types. Type-C tanks
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(b) 137,000 m3 LNG carrier with Type-B tanks (Kvaerner Moss system)
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are pure pressure vessels, often spherical or cylindrical, but sometimes bi-lobe
in shape to minimise broken stowage.
The fitting of one system in preference to another tends towards particular
trades. For example, Type-C tanks are suited to small volume carriage. They are
therefore found most often on coastal or regional craft. Large international LPG
carriers will normally be fitted with Type-A tanks.
Type-B tanks and tanks following membrane principles are found mainly within
the LNG fleet and will be discussed in Chapter 23.
22.4.1 The Pressurised Fleet
Diagram (a) in Figure 22.1 shows a small fully pressurised LPG carrier.
Regional and coastal cargoes are often carried in such craft, with the cargo
fully pressurised at ambient temperature. The tanks are built as pure pressure
vessels without the need for any extra metallurgical consideration appropriate
to colder temperatures. Design pressures are usually for propane (about
20 bar) as this form of LPG has the highest vapour pressure at ambient
temperature. The ship design comprises the outer hull and an inner hold
containing the pressure vessels. These rest in saddles built into the ship’s
structure. Double bottoms and other spaces act as water ballast tanks and,
if problems are to develop with age, the ballast tanks are prime candidates.
These ships are the most numerous class, comprising approximately 40% of
the fleet. They are relatively simple in design but strong in construction. Cargo
operations include cargo transfer by flexible hose and, in certain areas such as
China, ship to ship transfer operations from larger refrigerated ships operating
internationally are commonplace.
Figure 22.2: Pressurised LPG carrier with cylindrical tanks.
22.4.2 The Semi-pressurised Fleet
In these ships, sometimes referred to as semi-refrigerated, the cargo is carried
in pressure vessels that are usually bi-lobe in cross section, designed for
operating pressures of up to 7 bars. The tanks are constructed of special grade
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steel suitable for the cargo carriage temperature and the tanks are insulated
to minimise heat input to the cargo. The cargo boils off causing generation
of vapour, which is reliquefied by refrigeration and returned to the cargo
tanks. The required cargo temperature and pressure are maintained by the
reliquefaction plant.
These ships are usually larger than the fully pressurised types and have cargo
capacities of up to about 20,000 m3. As with the fully pressurised ship, the
cargo tanks are of pressure vessel construction and similarly located well
inboard of the ship’s side while protected by double-bottom ballast tanks. This
arrangement results in a very robust and inherently buoyant ship.
Figure 22.3: Semi-pressurised LPG carrier.
22.4.3 The Ethylene Fleet
Ethylene is the primary building block of the petrochemicals industry and is
used in the production of polyethylene, ethylene dichloride, ethanol, styrene,
glycols and many other products. Storage is usually as a fully refrigerated liquid
at minus 104°C (–104°C).
Ships designed for ethylene carriage also fall into the semi-pressurised class.
They are relatively few in number but are among the most sophisticated ships
afloat. In the more advanced designs, they can carry several grades. Typically,
this range can extend to ethane, LPG, ammonia, propylene butadiene and vinyl
chloride monomer (VCM), all featuring on their certificate of fitness. To aid in
this process, several independent cargo systems coexist on board to avoid
cross contamination of the cargoes, particularly for the reliquefaction process.
The ships range in size from about 2,000 m3 to 15,000 m3, although several
larger ships now trade in ethylene. Ship design usually includes independent
cargo tanks (Type-C) and these may be cylindrical or bi-lobe in shape
constructed from stainless steel. An inert gas generator is provided to produce
dry inert gas or dry air. The generator is used for inerting and for dehydration
of the cargo system and the interbarrier spaces during the voyage, when
condensation can occur on cold surfaces creating unwanted build-ups of ice.
Deck tanks are normally provided for changeover of cargoes.
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The hazards associated with the cargoes involved are from the temperature,
toxicity and flammability. The safety of ethane carriers is critical, requiring good
management and rigorous personnel training.
Ethane carriers may be seen as a subset of the ethylene fleet and ethane can
be carried by ships designed to carry ethylene. The new VLEC class (see
Section 22.2) are constructed with a reinforced membrane cargo containment
system, similar to that used in LNG carriers.
22.4.4 The Fully Refrigerated Fleet
Figure 22.4: Fully refrigerated LPG carrier.
These are generally large ships, up to about 85,000 m3 cargo capacity, with
those above 70,000 m3 designated as VLGCs. Many in the intermediate
range (30,000 m3 to 60,000 m3) are suitable for carrying the full range of
hydrocarbon liquid gas, from butane to propylene, and may be equipped
to carry chemical liquid gases such as ammonia. Cargoes are carried at
near ambient pressure and at temperatures down to minus 48°C (–48°C).
Reliquefaction plants are fitted (to manage boil-off) with a substantial reserve
plant capacity provided. The cargo tanks do not have to withstand high
pressures and are, therefore, generally of the freestanding prismatic type.
The tanks are robustly stiffened internally and constructed of special low
temperature resistant steel.
All ships have substantial double-bottom spaces and some have side ballast
tanks. In all cases, the tanks are protectively located inboard. The ship’s
structure surrounding or adjacent to the cargo tanks is also of special grade
steel and this forms a secondary barrier to safely contain any cold cargo should
it leak from the cargo tanks.
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All cargo tanks, whether of the pressure vessel type or rectangular, are
provided with safety relief valves amply sized to relieve boil-off in the absence
of reliquefaction and even in conditions of surrounding fire.
Figure 22.5: LNG carrier with membrane tanks.
22.5
Crew Training and Numbers
The IMO has laid down a series of training standards for gas carrier crews that
are additional to normal certification. These dangerous cargo endorsements
are detailed in the STCW Convention (Reference 48). Courses are divided into
the basic course for junior officers and the advanced course for senior officers.
IMO rules require a certain amount of onboard gas experience, particularly at
senior ranks, before taking on a responsible role or before progressing to the
next rank.
In addition to the official certification for hazardous cargo endorsements, a
number of colleges operate special courses for gas cargo handling. While
this provides for a well-trained and highly knowledgeable environment, the
continued growth in the fleet currently strains manpower resources and training
schedules. To mitigate these pressures, in addition to the STCW requirements,
the Society of International Gas Tanker and Terminal Operators (SIGTTO)
provides guidance on competency standards and experience levels for officers
serving on gas ships. While small gas carriers normally operate at minimum
crew levels, on larger carriers it is normal to find increased crewing levels over
and above the minimum required by the ship’s manning certificate.
22.6
Gas Carriers and Port Operations
As gas carriers have grown in size, so too has a concern over in-port safety. A
higher degree of automation and instrumentation is often apparent, controlling
the interface between ship and shore.
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Terminals are also protected by careful risk analysis at the time of construction,
helping to ensure that the location and size of maximum credible spill scenarios
are identified and that suitable precautions, including appropriate safety
distances, are established between operational areas and local populations.
Risk analysis of cargo transfer operations often identifies the cargo manifold
as the area likely to produce the maximum credible spill. This is controlled by a
number of measures. Primarily, as for all large oil tankers, gas carriers should be
held firmly in position while handling cargo, and mooring management should
be of a high calibre. Mooring ropes should be well managed throughout loading
and discharging. Safe mooring is often the subject of computerised mooring
analysis, particularly for new ships arriving at new ports, helping to ensure a
sensible mooring array suited to the harshest conditions. An accident in the
UK highlighted the consequences of a lack of such procedures when, in 1993,
a 60,000 m3 LPG carrier broke out from her berth in storm conditions. This
was the subject of an official MCA/HSE inquiry concluding that prior mooring
analysis was vital to safe operations. The safe mooring principles attached to
gas carriers are similar to those recommended for oil tankers (they are itemised
in Mooring Equipment Guidelines, 3rd Edition (Reference 49).
The need for such ships to be held firmly in position during cargo handling
is due in part to the use of marine loading arms (MLAs) (see Figure 22.6) for
cargo transfer. Such equipment is of limited reach in comparison to hoses, yet
it provides the ultimate in robustness and simplicity in connection at the cargo
manifold.
Figure 22.6: Hard arms at cargo manifold (on an LNG carrier).
The use of MLAs for large gas carriers is common and is an industry
recommendation. The use of hoses creates concerns over hose care and
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Chapter 22 Liquefied Gases
maintenance, and their proper layout and support during operations to prevent
kinking and abrasion. Further, accident statistics show that hoses have inferior
qualities in comparison to MLAs. Perhaps the worst case of hose failure
occurred in 1985 when a large LPG carrier was loading at Pajaritos, Mexico.
Here, the hose burst and, in a short time, the resulting gas cloud ignited. The
consequent fire and explosion impinged directly on three other ships in harbour
and resulted in four deaths.
Figure 22.7: MLA quick connect/disconnect coupler (QC/DC).
Figure 22.8: MLA connection to manifold, showing double ball valve safety release.
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As ships have grown in size, the installation of vapour return lines
interconnecting ship and shore vapour systems has become more common for
LPG carriers (and is an integral part of the LNG system – see Chapter 23).
A feature common to both ship and shore is an emergency shutdown system.
It is common to inter­connect such systems so that, for example, an emergency
on the ship will stop shore-based loading pumps or, conversely, an emergency
at an unloading terminal will stop the ship’s cargo pumps. It is common for
hard arms to be fitted with automatic detection and alarm systems to guard the
operating envelope and a further refinement at some larger LPG terminals is to
have the loading arms fitted with emergency release devices that allow the hard
arms to automatically release with minimal loss of product before they reach
the limits of the operating envelope. These devices are commonly referred to as
ERCs (emergency release couplings).
22.7
SIGTTO
Valuable assistance in the preparation of these chapters has come from the
Society of International Gas Tanker and Terminal Operators (SIGTTO).
SIGTTO is the leading trade body in this field and has 190 members (January
2017) covering nearly 95% of the world’s LNG fleet and 60% of the LPG fleet.
SIGTTO members also control most of the terminals that handle these products.
SIGTTO’s stated aim is to encourage the safe and responsible operation of
liquefied gas tankers and marine terminals handling liquefied gas; to develop
advice and guidance for best industry practice among its members and to
promote criteria for best practice to all who have responsibilities for, or an
interest in, the continuing safety of gas tankers and terminals.
SIGTTO operates from its London office at:
17 St Helen’s Place
London, EC3A 6DG.
Further details on activities and membership are available at www.sigtto.org
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