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UK P&I CLUB

Gas mattersA focus on some of the issues surrounding

gas tanker fleets in the P&I world

UK P&I CLUBIS MANAGEDBY THOMASMILLER

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IntroductionThe renewed interest in gas, which started in the 1990sdue to its excellent environmental credentials, has seenan increase in the order book for LNG carriers – LNGcarriers being the leviathans of the gas carrier fleet.Yet, while attracting great interest, the gas trade stillemploys relatively few ships in comparison to oiltankers, and hence its inner workings are little knownexcept to a specialist group of companies andmariners.

Considering the fleet of gas carriers of over 1,000m3

capacity, the total of over 1,500 ships can be dividedinto 5 major types according to the following table:

record is acknowledged as an industry leader. As anillustration of the robustness of gas carriers, when theGaz Fountain was hit by rockets in the first Gulf War,despite penetration of the containment system withhuge jet fires, the fires were successfully extinguishedand the ship, together with most cargo, salved.

The relative safety of the gas carrier is due to a numberof features. One such, almost unique to the class, isthat cargo tanks are always kept under positivepressure (sometimes just a small overpressure) andthis prevents air entering the cargo system. (Of coursespecial procedures apply when stemmed for drydock).

By contrast, the world oil tanker fleet for a similar sizerange is over 12,000 ships!

Given the relative paucity of knowledge on gas tankersin comparison to oil tankers, the purpose of this articleis to describe the gas carrier genre, its particularitieswithin each type and its comparison with other tankers.The aim is to provide basic knowledge about gascarriers and an overview of their strengths andweaknesses, both from design and operationalviewpoints.

The article on page 11 describes the liquefied naturalgas (LNG) carrier in more detail. The introduction of atanker designed to carry compressed natural gas(CNG) is anticipated in the near future. A number ofdesigns have been produced but, due to the relativelylow deadweight and high cost of these ships, the firstcommercial application of this technology cannot bepredicted.

The gas carrier is often portrayed in the media as apotential floating bomb, but accident statistics do notbear this out. Indeed, the sealed nature of liquefied gascargoes, in tanks completely segregated from oxygenor air, virtually excludes any possibility of a tankexplosion. However, the image of the unsafe shiplingers, with some administrations and port statecontrol organisations tending to target such ships forspecial inspection whenever they enter harbour. Thetruth is that serious accidents related to gas carriercargoes have been few, and the gas carrier’s safety

This means that only liquid cargo or vapour can bepresent and, accordingly, a flammable atmospherecannot exist in the cargo system. Moreover all large gascarriers utilise a closed loading system with no ventingto atmosphere, and a vapour return pipeline to theshore is often fitted and used where required. Theoxygen-free nature of the cargo system and the veryserious limitation of cargo escape to atmospherecombine to make for a very safe design concept.

The liquefied gasesFirst let us consider some definitions in the gas trade.According to the IMO, a liquefied gas is a gaseoussubstance at ambient temperature and pressure, butliquefied by pressurisation or refrigeration – sometimesa combination of both. Virtually all liquefied gases arehydrocarbons and flammable in nature. Liquefactionitself packages the gas into volumes well suited tointernational carriage – freight rates for a gas in itsnon-liquefied form would be normally far too costly. Theprincipal gas cargoes are LNG, LPG and a variety ofpetrochemical gases. All have their specific hazards.LNG is liquefied natural gas and is methane naturallyoccurring within the earth, or in association with oilfields. It is carried in its liquefied form at its boiling pointof -162°C. Depending on the standard of production atthe loading port, the quality of LNG can vary but itusually contains fractions of some heavier ends such asethane (up to 5%) and traces of propane.

The second main cargo type is LPG (liquefiedpetroleum gas). This grade covers both butane and

The gas carrier fleet

Pressurised Semi-pressurised Ethylene Fully refrigerated LNG carriersLPG carriers LPG carriers carriers LPG carriers

Ship numbers 673 313 140 261 372

Total capacity (m3) 1,812,823 2,849,355 1,234,029 10,725,479 29,059,620

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propane, or a mix of the two. The main use for theseproducts varies from country to country but sizeablevolumes go as power station or refinery fuels. HoweverLPG is also sought after as a bottled cooking gas and itcan form a feedstock at chemical plants. It is also usedas an aerosol propellant (with the demise of CFCs)and is added to gasoline as a vapour pressureenhancer. Whereas methane is always carried cold,both types of LPG may be carried in either thepressurised or refrigerated state. Occasionally theymay be carried in a special type of carrier known as thesemi-pressurised ship. When fully refrigerated, butaneis carried at -5°C, with propane at -42°C, this lattertemperature already introducing the need for specialsteels.

Ammonia is one of the most common chemical gasesand is carried worldwide in large volumes, mainly foragricultural purposes. It does however haveparticularly toxic qualities and requires great careduring handling and carriage. By regulation, allliquefied gases when carried in bulk must be carried ona gas carrier, as defined by the IMO. IMO’s Gas Codes(see next section – Design of gas carriers) provide a listof safety precautions and design features required foreach product.

A specialist sector within the trade is the ethylenemarket, moving around one million tonnes by seaannually, and very sophisticated ships are available forthis carriage. Temperatures here are down to -104°Cand onboard systems require perhaps the highestdegree of expertise within what is already a highlyspecialised and automated industry. Within this groupa sub-set of highly specialised ships is able to carrymulti-grades simultaneously.

Significant in the design and operation of gas carriers isthat methane vapour is lighter than air while LPGvapours are heavier than air. For this reason the gascarrier regulations allow only methane to be used as apropulsion fuel – any minor gas seepage in enginespaces being naturally ventilated. The principalhydrocarbon gases such as butane, propane andmethane are non-toxic in nature and a comparison ofthe relative hazards from oils and gases is provided inthe table below.

Design of gas carriersThe regulations for the design and construction of gascarriers stem from practical ship designs codified bythe International Maritime Organization (IMO). This wasa seminal piece of work and drew upon the knowledgeof many experts in the field – people who had alreadybeen designing and building such ships. This workresulted in several rules and a number ofrecommendations. However all new ships (from June1986) are built to the International Code for theConstruction and Equipment of Ships CarryingLiquefied Gases in Bulk (the IGC Code). This codealso defines cargo properties and documentation,provided to the ship (the Certificate of Fitness for theCarriage of Liquefied Gases in Bulk), shows the cargogrades the ship can carry. In particular this takes intoaccount temperature limitations imposed by themetallurgical properties of the materials making up thecontainment and piping systems. It also takes intoaccount the reactions between various gases and theelements of construction not only on tanks but alsorelated to pipeline and valve fittings.

When the IGC Code was produced an intermediate

Comparative hazards of some liquefied gases and oils

GASES OILS

HAZARD LNG LPG GASOLINE FUEL OIL

Toxic No No Yes Yes

Carcinogenic No No Yes Yes

Asphyxiant Yes (in confined spaces) Yes (in confined spaces) No No

Others Low temperature Moderately low Eye irritant, narcotic, Eye irritant, narcotic,temperature nausea nausea

Flammability limits 5-15 2-10 1-6 Not applicablein air (%)

Storage pressure Atmospheric Often pressurised Atmospheric Atmospheric

Behaviour if spilt Evaporates forming a Evaporates forming an Forms a flammable Forms a flammablevisible ‘cloud’ that explosive vapour cloud pool which if ignited pool, environmentaldisperses readily and is would burn with clean-up is requirednon-explosive, unless explosive force,contained environmental clean-up

may be required

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code was also developed by the IMO – the Code forthe Construction and Equipment of Ships CarryingLiquefied Gases in Bulk (the GC Code). This coversships built between 1977 and 1986.

As alluded to above, gas carriers were in existencebefore IMO codification and ships built before 1977are defined as ‘existing ships’ within the meaning of therules. To cover these ships a voluntary code wasdevised, again by the IMO – the Code for ExistingShips Carrying Liquefied Gases in Bulk (the Existing

Ship Code). Despite its voluntary status, virtually allships remaining in the fleet of this age – and because oflongevity programmes there are still quite a number –have certification in accordance with the Existing ShipCode as otherwise international charteringopportunities would be severely restricted.

Cargo carriage in the pressurised fleet comprisesdouble cargo containment – hull and tank. All other gascarriers are built with a double hull structure and thedistance of the inner hull from the outer is defined in the

3,200 m3 coastal LPG carrier with cylindrical tanks

78,000 m3 LPG carrier with Type-A tanks

16,650 m3 semi pressurised LPG carrier

135,000 m3 LNG carrier with membrane tanks

137,000 m3 LNG carrier with Type-B tanks (Kvaerner Moss system)

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pressure vessels without the need for any extrametallurgical consideration appropriate to coldertemperatures. Design pressures are usually forpropane (about 20 bar) as this form of LPG gives thehighest vapour pressure at ambient temperature. Asdescribed above, ship design comprises outer hull andan inner hold containing the pressure vessels. Theserest in saddles built into the ship’s structure. Doublebottoms and other spaces act as water ballast tanksand if problems are to develop with age then the ballasttanks are prime candidates. These ships are the mostnumerous class, comprising approximately 40% of thefleet. They are nevertheless relatively simple in designyet strong of construction.

Cargo operations that accompany such ships includecargo transfer by flexible hose and in certain areas,such as China, ship-to-ship transfer operations fromlarger refrigerated ships operating internationally arecommonplace.

Records show that several ships in this class have beenlost at sea because of collision or grounding, butpenetration of the cargo system has never been proven.In one case, a ship sank off Italy and several years laterrefloated naturally, to the surprise of all, as the cargohad slowly vaporised adding back lost buoyancy.

Pressurised LPG carrier with cylindrical tanks

Semi-pressurised LPG carrier

gas codes. This spacing introduces a vital safetyfeature to mitigate the consequences of collision andgrounding. Investigation of a number of actual collisionsat the time the gas codes were developed drewconclusions on appropriate hull separations whichwere then incorporated in the codes. Collisions dooccur within the class and, to date, the codes’recommendations have stood the test of time, with nopenetrations of cargo containment having beenreported from this cause. The double hull conceptincludes the bottom areas as a protection againstgrounding and, again, the designer’s foresight hasproven of great value in several serious groundingincidents, saving the crew and surrounding populationsfrom the consequences of a ruptured containmentsystem.

So a principal feature of gas carrier design is doublecontainment and an internal hold. The cargo tanks,more generally referred to as the ‘cargo containmentsystem’, are installed in the hold, often as a completelyseparate entity from the ship; ie. not part of the ship’sstructure or its strength members. Herein lies adistinctive difference between gas carriers and theirsisters, the oil tankers and chemical carriers.

Cargo tanks may be of the independent self-supportingtype or of a membrane design. The self-supportingtanks 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 (ie. shaped to fit the hold).

Type-B comprises tanks where fatigue life and crackpropagation analyses have shown improvedcharacteristics. Such tanks are usually spherical butoccasionally may be of prismatic types. Type-C tanksare the pure pressure vessels, often spherical orcylindrical, but sometimes bi-lobe in shape to minimisebroken stowage.

The fitting of one system in preference to another tendstowards particular trades. For example, Type-C tanksare suited to small volume carriage. They are thereforefound most often on coastal or regional craft. The largeinternational LPG carrier will normally be fitted withType-A Tanks. Type-B tanks and tanks followingmembrane principles are found mainly within theLNG fleet.

The pressurised fleetThe first diagram, on the previous page, and thephotograph above show a small fully pressurisedcarrier. Regional and coastal cargoes are often carriedin such craft with the cargo fully pressurised at ambienttemperature. Accordingly, the tanks are built as pure

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The semi-pressurised fleetIn these ships, sometimes referred to as ‘semi-refrigerated’, the cargo is carried in pressure vesselsusually bi-lobe in cross-section, designed for operatingpressures of up to 7 bars. The tanks are constructed ofspecial grade steel suitable for the cargo carriagetemperature. The tanks are insulated to minimise heatinput to the cargo. The cargo boils off causinggeneration of vapour, which is reliquefied byrefrigeration and returned to the cargo tanks. Therequired cargo temperature and pressure is maintainedby the reliquefaction plant.

These ships are usually larger than the fully pressurisedtypes and have cargo capacities up to about 20,000m3.As with the fully pressurised ship, the cargo tanks are ofpressure vessel construction and similarly located wellinboard of the ship’s side and also protected by doublebottom ballast tanks. This arrangement again results ina very robust and inherently buoyant ship.

The ethylene fleetEthylene, one of the chemical gases, is the premierbuilding block of the petrochemicals industry. It is usedin the production of polyethylene, ethylene dichloride,ethanol, styrene, glycols and many other products.Storage is usually as a fully refrigerated liquid at-104°C.

Ships designed for ethylene carriage also fall into thesemi-pressurised class. They are relatively few innumber but are among the most sophisticated shipsafloat. In the more advanced designs they have theability to carry several grades. Typically this range canextend to ethane, LPG, ammonia, propylene butadieneand vinyl chloride monomer (VCM), all featuring on theircertificate of fitness. To aid in this process severalindependent cargo systems co-exist onboard to avoidcross contamination of the cargoes, especially for thereliquefaction process.

The ships range in size from about 2,000m3 to15,000m3 although several larger ships now trade inethylene. Ship design usually includes independentcargo tanks (Type-C), and these may be cylindrical orbi-lobe in shape constructed from stainless steel. Aninert gas generator is provided to produce dry inert gasor dry air. The generator is used for inerting and for thedehydration of the cargo system as well as the inter-barrier spaces during voyage. For these condensationoccurs on cold surfaces with unwanted build-ups ofice. Deck tanks are normally provided for changeoverof cargoes.

The hazards associated with the cargoes involved are

obvious from temperature, toxic and flammableconcerns. Accordingly, the safety of all such craft iscritical with good management and serious personneltraining remaining paramount.

The fully refrigerated fleetThese are generally large ships, up to about100,000m3 cargo capacity, those above 70,000m3

being designated as VLGCs. Many in the intermediaterange (say 30,000m3 to 60,000m3) are suitable forcarrying the full range of hydrocarbon liquid gas frombutane to propylene and may be equipped to carrychemical liquid gases such as ammonia. Cargoes arecarried at near ambient pressure and at temperaturesdown to -48ºC. Reliquefaction plants are fitted, withsubstantial reserve plant capacity provided. The cargotanks do not have to withstand high pressures and aretherefore generally of the free standing prismatic type.The tanks are robustly stiffened internally andconstructed of special low temperature resistant steel.

All ships have substantial double bottom spaces andsome have side ballast tanks. In all cases the tanks areprotectively located inboard. The ship’s structuresurrounding or adjacent to the cargo tanks is also ofspecial grade steel, in order to form a secondary barrierto safely contain any cold cargo should it leak from thecargo tanks.

All cargo tanks, whether they be of the pressure vesseltype or rectangular, are provided with safety reliefvalves amply sized to relieve boil-off in the absence ofreliquefaction and even in conditions of surroundingfire.

The LNG fleetAlthough there are a few exceptions, the principal shipsin the LNG fleet range from 75,000m3 to 265,000m3

capacity. The cargo tanks are thermally insulated andthe cargo carried at atmospheric pressure. Cargotanks may be free standing spherical, of the membrane

Fully refrigerated LPG carrier

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type, or alternatively, prismatic in design. In the case ofmembrane tanks, the cargo is contained within thinwalled tanks of invar or stainless steel. The tanks areanchored in appropriate locations to the inner hull andthe cargo load is transmitted to the inner hull throughthe intervening thermal insulation.

All LNG carriers have a watertight inner hull and mosttank designs are required to have a secondarycontainment capable of safely holding any leakage fora period of 15 days. Because of the simplicity andreliability of stress analysis of the sphericalcontainment designs, a full secondary barrier is notrequired but splash barriers and insulated drip traysprotect the inner hull from any leakage that might occurin operation.

Crew training and numbersAs they did for oil tankers and chemical carriers, theIMO has laid down a series of training standards forgas carrier crews which come in addition to normalcertification. These dangerous cargo endorsementsare spelt out in the STCW Convention.

Courses are divided into the basic course for juniorofficers and the advanced course for senior officers.IMO rules require a certain amount of onboard gasexperience, especially at senior ranks, before taking on

LNG carrier with Type-B tanks (Kvaerner Moss system)

LNG carrier with membrane tanks

the responsible role or before progressing to the nextrank. This can introduce checks and balances (say) inthe case of a master from the bulk ore trades wanting toconvert to the gas trade.

The only way, without previous gas experience, toachieve this switch is to have the candidate completethe requisite course and sail as a supernumerary,understudying the rank for a specified period on a gascarrier. This can be costly for seafarer and companyalike. Accordingly, as the switch can be difficult tomanage, especially at senior ranks, currentrequirements tend to maintain a close-knit cadre of‘gas men or women’ well experienced in the trade.

In addition to the official certification for hazardouscargo endorsements, a number of colleges operatespecial courses for gas cargo handling. In the UK aleader in the field is the Warsash Maritime Centre.While this situation provides for a well-trained andhighly knowledgeable environment the continuedgrowth in the fleet currently strains manpowerresources and training schedules and it is possible thatshort cuts may be taken.

While the small gas carriers normally operate atminimum crew levels, on the larger carriers it is normalto find increased crewing levels over and above theminimum required by the ship’s manning certificate. Forexample, it is almost universal to carry a cargo engineeronboard a large gas carrier. An electrician is a usualaddition and the deck officer complement may well beincreased.

Gas carriers and port operationsAs gas carriers have grown in size, so too has aconcern over in-port safety. Indeed, the same concernsapplied with the growth in tanker sizes when theVLCC came to the drawing board. The solutions aresimilar; however, in the case of the gas carrier, a higherdegree of automation and instrumentation is oftenapparent controlling the interface between ship andshore.

Terminals are also protected by careful risk analysis atthe time of construction so helping to ensure that thelocation and size of maximum credible spill scenariosare identified, and that suitable precautions includingappropriate safety distances are established betweenoperational areas and local populations.

Regarding shipping operations, risk analysis oftenidentifies the cargo manifold as the area likely toproduce the maximum credible spill. This should becontrolled by a number of measures. Primarily, as for all

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large oil tankers, gas carriers should be held firmly inposition whilst handling cargo, and mooringmanagement should be of a high calibre. Mooringropes should be well managed throughout loading anddischarging. Safe mooring is often the subject ofcomputerised mooring analysis, especially for newships arriving at new ports, thus helping to ensure asensible mooring array suited to the harshestconditions. An accident in the UK highlighted theconsequences of a lack of such procedures when, in1993, a 60,000m3 LPG carrier broke out from her berthin storm conditions. This was the subject of an officialMCA/HSE inquiry concluding that prior mooringanalysis was vital to safe operations.

The safe mooring principles attached to gas carriersare similar to those recommended for oil tankers (theyare itemised in Mooring Equipment Guidelines, seeReferences, inside back page).

The need for such ships to be held firmly in positionduring cargo handling is due in part to the use ofloading arms (hard arms – see photos) for cargotransfer. Such equipment is of limited reach incomparison to hoses, yet it provides the ultimate inrobustness. It also provides simplicity in the connectionat the cargo manifold.

Hard arms at cargo manifold, including vapour return line

Hard arm connection to manifold, showing double ball valvesafety release

The use of loading arms for the large gas carrier is nowquite common and, if not a national requirement, iscertainly an industry recommendation. The alternativeuse of hoses is fraught with concerns over hose careand maintenance, and their proper layout and supportduring operations to prevent kinking and abrasion.Further, accident statistics show that hoses haveinferior qualities in comparison to the hard arms.

Perhaps the worst case of hose failure occurred in1985 when a large LPG carrier was loading atPajaritos, Mexico. Here, the hose burst and, in a shorttime, the resulting gas cloud ignited. The consequentfire and explosion impinged directly on three other shipsin harbour and resulted in four deaths. It was one ofthose accidents which has led directly to a muchincreased use of loading arms internationally. The jettywas out of action for approximately six months.Fortunately the berth was in an industrial area andcollateral damage to areas outside the refinery waslimited.

As ships have grown in size the installation of vapourreturn lines interconnecting ship and shore vapoursystems has become more common. Indeed, in the

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ChecklistThe following checklist, made availablefrom SIGTTO*, may be used as guidancein a casualty situation involving a disabledgas carrier.

● What cargo is onboard?

● Is specialist advice available in respectof the cargo and its properties?

● Are all parties involved aware of cargoproperties?

● Is the cargo containment systemintact?

● Is the ship venting gas?

● Is the ship likely to vent gas?

● What will be the vented gas and whatare its dispersal characteristics?

● Is a gas dispersion modelling toolavailable?

● Is the ship damaged?

● Does damage compromise the ship’smanoeuvring ability?

● What activities and services areplanned to restore a seaworthycondition?

● Is ship-to-ship transfer equipmentavailable if required?

● When is it expected the ship will beseaworthy again?

● Is prevailing shelter (and otherdangers) suitable for the intendedrepairs?

● What contingency plans are required?

● Who will control the operation?

● How will the ship operator and port orpublic authorities co-operate?

● Will customs and immigrationprocedures need facilitation forequipment and advisers?

* Society of International Gas Tanker and TerminalOperators – see inside back page

LNG industry it is required, with the vapour return beingan integral part of the loading or discharging system. Inthe LPG trades, vapour returns are also common, butare only opened in critical situations such as whereonboard reliquefaction equipment is unable to copewith the loading rate and boil-off.

A feature common to both ship and shore is that bothhave emergency shutdown systems. It is now commonto interconnect such systems so that, for example, anemergency on the ship will stop shore-based loadingpumps. One such problem may be the automaticdetection of the ship moving beyond the safe workingenvelope for the loading arms. A further refinement atsome larger terminals is to have the loading arms fittedwith emergency release devices, so saving the loadingarms from fracture (see top photo, previous page).

Given good moorings and well-designed loading arms,the most likely sources of leakage are identified andcontrolled.

Hazards to shore workers andcrewmembers at refitWhile the gas carrier accident record is very good fornormal operations, and exemplary with respect to cargooperations and containment, the same cannot be saidfor the risks it faces in drydock. Statistics show that thegas carrier in drydock presents a serious risk topersonnel, particularly with respect to adequateventilation through proper inerting and gas-freeingbefore repairs begin. Most often the risk relates tominor leakage from a cargo tank into the insulationsurrounding refrigerated LPG tanks. A massiveexplosion occurred on the Nyhammer at a Koreanshipyard in 1993 for this very reason, whereconsiderable loss of life occurred. Although the shipwas repaired, it was a massive job.

Hard arm quick connect/disconnect coupler (QCDC)

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BackgroundIt was as far back as 1959 that the Methane Pioneercarried the first experimental LNG cargo and, in 1964,British Gas at Canvey Island received the inauguralcargo from Arzew on the Methane Princess. Togetherwith the Methane Progress these two ships formed thecore of the Algeria to UK project, and the project-basednature of LNG shipping was set to continue until the endof the 20th century. LNG carriers only existed wherethere were projects, with ships built specifically foremployment within the projects. The projects werebased on huge joint ventures between cargo buyers,cargo sellers and shippers, all in themselves largecompanies prepared to do long-term business together.

The projects were self-contained and operated withoutmuch need for outside help. They supplied gas using apurpose-built fleet operating like clockwork on a CIFbasis. Due to commercial constraints, the need forprecisely scheduled deliveries and limited shore tankcapacities, spot loadings were not feasible and it is onlyin recent years that some projects now accept LNGcarriers as cross-traders, operating more like theirtramping cousins – the oil tankers. Doubtless the trendto spot trading will continue. However, the co-operativenature of LNG’s beginnings has led to severaloperational features unique to the ships. In particularthere is the acceptance that LNG carriers burn LNGcargo as a propulsive fuel. They also retain cargoonboard after discharge (the ‘heel’) as an aid to keepingthe ship cooled down and ready to load on arrival at theload port. Thus matters that would be anathema tonormal international trades are accepted as normalpractice for LNG.

Again, looking back to the early days, there was alsogreat interest in this new fuel in the USA and France.Receiving terminals sprouted. However, gas pricingdifficulties in the USA saw an end to early Americaninterest while Gaz de France consolidated rather thanexpanded. Indeed, the American pricing problems, andthe failure of an early US-built shipboard Conchcontainment system on newbuildings, blanketed anyspectacular progress in the Atlantic basin until theregeneration of interest initiated by the Trinidad projectin 1999.

At that time, the stifling of European interest was alsodue to the discovery of natural gas in the North Sea, soquantities to replace town gas were available insufficient volume on the doorstep without the need forimports. This being so, the first LNG project fromAlgeria to UK eventually faltered, with the receiving

terminal at Canvey Island switching to other interests.The stagnation of LNG in the 70s and 80s applied theworld over, with the singular exception of imports toJapan and Korea. Here interest in LNG’s potential as anenvironmentally-friendly fuel stayed vibrant; as it doestoday.

LNG projects are massive multi-billion dollarinvestments. Major projects in the Far East includedBrunei to Japan, Indonesia to Japan, Malaysia to Japanand Australia to Japan, comprising some 90% of theLNG trade of the day. Consequently, the Japanesedefined much of what is seen best today in way ofsafety standards and procedures. It is worthy of note,however, that some early safety standards andpractices are being questioned today in the light ofexperience in a more mature industry.

LNG as a fuelBecause the ships, terminals and commercial entitieswere all bound together in the same chain, advantagescould be seen in limiting ‘unnecessary’ shipboardequipment, such as reliquefaction plant, and allowingthe boil-off to be burnt as fuel. One way or another theship would need fuel, be it oil or gas and, if gas, it wasonly then a matter to quantify usage and to direct theappropriate cost to the appropriate project partner.Interestingly, this concept was recognised in the IMO’sGas Codes from the very earliest days, and with theappropriate safety equipment in place the regulationsallow methane to be burnt in ships’ boilers. This is notthe case for LPG, where reliquefaction equipment is afitment, but specifically because the LPGs are heavierthan air gases and use in engine rooms is therebydisallowed.

LNG qualityLNG is liquefied natural gas. It is sharply clear andcolourless. It comprises mainly methane but has apercentage of constituents such as ethane, butane andpropane together with nitrogen. It is produced fromeither gas wells or oil wells. In the case of the latter it isknown as associated gas. At the point of production thegas is processed to remove impurities and the degreeto which this is achieved depends on the facilitiesavailable. Typically this results in LNG with between80% and 95% methane content. The resulting LNGcan therefore vary in quality from loading terminal toloading terminal or from day-to-day.

Other physical qualities that can change significantlyare the specific gravity and the calorific value of the

Liquefied natural gas

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LNG, which depend on the characteristics of the gasfield. The specific gravity affects the deadweight ofcargo that can be carried in a given volume, and thecalorific value affects both the monetary value of thecargo and the energy obtained from the boil-off gasfuel.

These factors have significance in commercialarrangements and gas quality is checked for eachcargo, usually in a shore-based laboratory by means ofgas chromatography. LNG vapour is flammable in airand, in case of leakage, codes require an exclusionzone to allow natural dispersion and to limit the risk ofignition of a vapour cloud. Fire hazards are furtherlimited by always handling the product within oxygen-free systems. Unlike oil tankers under inert gas, or insome cases air, LNG carriers operate with the vapourspace at 100% methane. LNG vapour is non-toxic,although in sufficient concentration it can act as anasphyxiant.

Gas quality is also significant from a shipboardperspective. LNGs high in nitrogen, with anatmospheric boiling point of -196°C, naturally allownitrogen to boil-off preferentially at voyage start thuslowering the calorific value of the gas as a fuel. Towardsthe end of a ballast passage, when remaining ‘heel’ hasall but been consumed, the remaining liquids tend to behigh on the heavier components such as the LPGs. Thisraises the boiling point of the remaining cargo and has adetrimental effect on tank cooling capabilities inreadiness for the next cargo.

The good combustion qualities attributed to methanemake it a great attraction today as a fuel at electricpower stations. It is a ‘clean’ fuel. It burns producinglittle or no smoke and nitrous oxide and sulphur oxideemissions produce figures far better than can beachieved when burning normal liquids such as lowsulphur fuel oil. Natural gas has thus become attractiveto industry and governments striving to meetenvironmental targets set under various internationalprotocols such as the Rio Convention and the KyotoProtocol. The practice of firing marine boilers onmethane provides the further environmental advantageof lesser soot-blowing operations and much fewercarbon deposits.

Cargo handlingThe process of liquefaction is one of refrigeration and,once liquefied, the gas is stored at atmosphericpressure at its boiling point of -162°C. At loadingterminals any boil-off from shore tanks can bereliquefied and returned to storage. However, on shipsthis is almost certainly not the case. According to

design, it is onboard practice to burn boil-off gas (oftentogether with fuel oil) in the ship’s boilers to providepropulsion. In the general terms of seaborne trade this isan odd way to handle cargo and is reminiscent of oldtales of derring-do from the 19th century when a cargomight have been burnt for emergency purposes. It isnevertheless the way in which the LNG trade operates.Boil-off is burnt in the ship’s boilers to the extent that itevaporates from its mother liquid. Clearly cargo volumesat the discharge port do not match those loaded.

Accounting however is not overlooked and LNGcarriers are outfitted with sophisticated means of cargomeasurement. This equipment is commonly referred toas the ‘custody transfer system’ and is used inpreference to shore tank measurements. These systemsnormally have precise radar measurement of tank ullagewhile the tanks themselves are specially calibrated by aclassification society to a fine degree of accuracy. Thesystem automatically applies corrections for trim and listusing equipment self-levelled in drydock. The resultingcargo volumes, corrected for the expansion andcontraction of the tanks, are normally computedautomatically by the system.

Cargo tank design requires carriage at atmosphericpressure and there is little to spare in tank design forover or under pressures. Indeed, the extent to whichpressure build-up can be contained in a ship’s tanks isvery limited in the case of membrane cargo tanks,although less so for Type-B tanks. Normally this is not aproblem, as at sea the ship is burning boil-off as fuel or inport has its vapour header connected to the terminalvapour return system. Clearly, however, there are shortperiods between these operations when pressurecontainment is necessary. This can be managed. Sotaken together, shipboard operations efficiently carriedout succeed in averting all possible discharges toatmosphere, apart that is from minor escapes at pipeflanges, etc. Certainly this is part of the design criteriafor the class as it is recognised that methane is agreenhouse gas.

Boil-off gas (BOG) is limited by tank insulation andnewbuilding contracts specify the efficiency required.Usually this is stated in terms of a volume boil-off perday under set ambient conditions for sea and airtemperature. The guaranteed maximum figure forboil-off would normally be about 0.15% of cargo volumeper day.

While at sea, vapours bound for the boilers must beboosted to the engine room by a low-duty compressorvia a vapour heater. The heater raises the temperature ofthe boil-off to a level suited for combustion and to a pointwhere cryogenic materials are no longer required in

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construction. The boil-off then enters the engine roomsuitably warmed but first passes an automaticallycontrolled master gas valve before reaching an array ofcontrol and shutoff valves for direction to each burner.

As a safety feature, the gas pipeline through the engineroom is of annular construction, with the outer pipepurged and constantly checked for methane ingress. Inthis area, operational safety is paramount and sensorscause shutdown of the master gas valve in alarmconditions. A vital procedure in the case of a boilerflameout is to purge all gas from the boilers beforeattempting re-ignition. Without such care boilerexplosions are possible and occasional accidents of thistype have occurred.

Cargo careThe majority of LNG shippers and receivers have alegitimate concern over foreign bodies getting into tanksand pipelines. The main concern is the risk of valveblockage if (say) an old welding rod becomes lodged ina valve seat. Such occurrences are not unknown with a

ship discharging first cargoes after newbuilding orrecently having come from drydock.

Accordingly, and despite discharge time diseconomies,it is common practice to fit filters at the ship’s liquidmanifold connections to stop any such material fromentering the shore system. The ship normally suppliesfilters fitting neatly into the manifold piping.

In a similar vein, even small particulate matter cancause concerns. The carry-over of silica gel dust frominert gas driers is one such example. Another possiblecause of contamination is poor combustion at inert gasplants and ships tanks becoming coated with soot andcarbon deposits during gas freeing and gassing upoperations. Subsequently, the contaminants may bewashed into gas mains and, accordingly, cargoes maybe rejected if unfit. Tank cleanliness is vital and,especially after drydock, tanks must be thoroughlyvacuumed and dusted.

A cargo was once rejected in Japan when, resultingfrom a misoperation, steam was accidentally applied tothe main turbine with the ship secured alongside theberth. The ship broke out from the berth, but fortunatelythe loading arms had not been connected. This actionwas sufficient however for cargo receivers to reject theship, and the cargo could only be delivered after aspecialised ship-to-ship transfer operation had beenaccomplished. The ship-to-ship transfer of LNG hasonly ever been carried out on a few occasions and is anoperation requiring perfect weather, great care andspecialist equipment.

Another case of cargo rejection, this time resulting in adistressed sale, involved a shipment to Cove Point inthe USA, where the strict requirements which prevail onin-tank pressures on arrival at the berth were notadhered to. The ship had previously been ordered toreduce pressure for arrival. This is a difficult job toperform satisfactorily and, if it is to be successful, thepressure reduction operation must progress withdiligence throughout the loaded voyage by forcingadditional cargo evaporation to the boilers. This coolsthe cargo and hence reduces vapour space pressure.The process of drawing vapour from the vapour spaceat the last moment is ineffective, because the cargoitself is not in balance with that pressure and once gasburning stops the vapour space will return to its highequilibrium pressure. This process is known in thetrade as ‘cargo conditioning’.

Ship careA temperature of -162°C is astonishingly cold. Moststandard materials brought into contact with LNGMoss design

LNG carrier with Type-B tanks (Kvaerner Moss system)

Cou

rtes

y of

Mos

s M

ariti

me

14

become highly brittle and fracture. For this reasonpipelines and containment systems are built fromspecially chosen material that do not have thesedrawbacks. The preferred materials of construction arealuminium and stainless steel. However these materialsdo not commonly feature over the ship’s weatherdecks,tank weather covers or hull. These areas areconstructed from traditional carbon steel. Accordingly,every care is taken to ensure that LNG is not spilt. Aspill of LNG will cause irrevocable damage to the decksor hull normally necessitating emergency drydocking.Accidents of this nature have occurred, fortunatelynone reporting serious personal injury, but resulting,nevertheless, in extended periods off-hire.

LNG carriers are double-hulled ships speciallydesigned and insulated to prevent leakage and rupturein the event of accident such as grounding or collision.That aside, though sophisticated in control andexpensive in materials, they are simple in concept.Mostly they carry LNG in just four, five or six centrelinetanks. Only a few have certification and equipment forcross trading in LPG. The cargo boils on passage andis not reliquefied onboard – it is carried at atmosphericpressure. Although there are four current methods to

construct seaborne LNG tanks, only two are in majorityusage. There are the spherical tanks of Moss designand the membrane tanks from Gaz Transport orTechnigaz (two French companies, now amalgamatedas GTT). Each is contained within the double hull wherethe water ballast tanks reside. The world fleet dividesapproximately 50/50 between the two systems.

Regarding spherical tanks, a very limited number wereconstructed from 9% nickel steel, the majority areconstructed from aluminium. A disadvantage of thespherical system is that the tanks do not fit the contoursof a ship’s hull and the consequent ‘broken-stowage’ isa serious diseconomy. In general terms, for two LNGships of the same carrying capacity, a ship of Mossdesign will be about 10% longer. It will also have itsnavigating bridge set at a higher level to allow goodviewing for safe navigation. On the other hand thespherical tanks are simple in design and simple to installin comparison to the membrane system, with itscomplication of twin barriers and laminated-typeconstruction.

Tank designs are often a controlling factor in building anLNG carrier. Shipyards usually specialise in one type orthe other. Where a yard specialises in the Moss system,giant cranes are required to lift the tanks into the shipsand limits on crane outreach and construction toolingfacilities currently restrict such tanks to a diameter ofabout 40 metres.

Early LNG carriers had carrying capacities of about25,00m3. This swiftly rose to about 75,000m3 for theBrunei project and later ships settled on 125,000m3.For some years this remained the norm, giving a loadeddraught of about 11.5 metres, thus stretching the portfacilities of most discharge terminals to their limits.Since then, however, there have been someincremental increases in size, usually maintaining draftbut increasing beam, resulting in ship sizes now ofabout 145,000m3. That said, one of the newest in classis the Pioneer Knudsen, trading at only 1,100m3

capacity from a facility near Bergen to customers on theNorwegian west coast.

Large, modern LNG carriers have dimensionsapproximately as follows:

LNG carrier with membrane tanks

Membrane design (GTT)

Capacity (m3) 145,000 215,000 265,000

Length 295m 315m 345m

Beam 48m 50m 54m

Loaded draft 12m 12m 12m

LNG having a typical density of only 420kg/m3 allowsthe ships, even when fully laden, to ride with a high

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freeboard. They never appear very low in the water as afully laden oil tanker may do. Ballast drafts aremaintained close to laden drafts and, for a ship having aladen draft of 12 metres, a ballast draft of 11 metres islikely. This means that for manoeuvring in port in windyconditions the ships are always susceptible to beingblown to one side of the channel, and restrictions onport manoeuvring usually apply with extra tug powercommonly specified.

Another salient feature of the LNG class is thepropensity to fit steam turbine propulsion. This is ananachronism brought about by a reluctance to changeover the years, together with a fear that a system as yetuntried on LNG carriers may not find favour with theprincipal charterers – the Japanese. Most other shiptypes of this size have diesel engines and the engineersto run diesel equipment are plentiful and suitablytrained. On the other hand, engineers knowledgeablein steam matters are few and their training base is theship itself. This situation is changing though, with bothdiesel electric dual fuel systems and slow speed dieselsnow finding favour. With slow speed diesel propulsion,reliquefaction plants will be required onboard to handleboil-off gas, and all diesel systems will require back-upgas disposal facilities – also known as ‘gas combustionunits’ (GCUs) – for when either the reliquefactionplants or the duel fuel diesel engines are not available toprocess boil-off gas.

LNG ships are expensive to build. They comprise veryvaluable assets: generally far too good to let rust away.Shipowners and ship managers alike recognise thisand, together with inspection regimes, the overallquality of LNG tonnage is kept to a high standard. Agefor age, they are probably the best maintained ships inthe world. Of course some of these ships are now oldand only a few have ever been scrapped; some are over40 years old. This is very old for a large tanker tradingall its life in salt water, when 25 years is alreadyconsidered by many as a cut-off date.

On termination of their original projects we are nowseeing many of the older ships as surplus torequirements. Sometimes the project wishes tocontinue but only with new ships. So the older ships arelaid-off. In the past this would have been their deathknell but today this is not necessarily the case. The slowdevelopment of a spot market has allowed theshipowner to consider life extension programmes ofconsiderable cost; all this set against the value of a veryexpensive newbuilding. Today life extensionprogrammes are common with old ships makinghandsome profits in the spot market.

SIGTTOValuable assistance in the preparation of these articleshas come from the Society of International Gas Tankerand Terminal Operators (SIGTTO).

SIGTTO is the leading trade body in this field and hasover 120 members covering nearly 95% of the world’sLNG fleet and 60% of the LPG fleet. SIGTTOmembers also control most of the terminals that handlethese products.

The Society’s stated aim is to encourage the safe andresponsible operation of liquefied gas tankers andmarine terminals handling liquefied gas; to developadvice and guidance for best industry practice amongits members and to promote criteria for best practice toall who have responsibilities for, or an interest in, thecontinuing safety of gas tankers and terminals.

The Society operates from its London office at 17 St.Helens Place EC3. Further details on activities andmembership is available at www.sigtto.org

References

Liquefied Gas Handling Principles on Ships and inTerminals – SIGTTO

Safe Havens for Disabled Gas Carriers – 2003,SIGTTO

Mooring Equipment Guidelines – 2001, OCIMF

Ship-to-Ship Transfer Guide (Liquefied Gases) – 1995,SIGTTO

The International Code for the Construction andEquipment of Ships Carrying Liquefied Gases in Bulk,(IGC Code) – IMO

A Contingency Planning and Crew Response Guidefor Gas Carrier Damage at Sea and in PortApproaches – 1999, SIGTTO

The aforementioned publications are available fromWitherby & Company Ltd, London.

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UK P&I CLUBIS MANAGEDBY THOMASMILLER

For further information please contact:Loss Prevention Department, Thomas Miller P&I LtdTel: +44 20 7204 2307. Fax +44 20 7283 6517Email: Karl.Lumbers@thomasmiller.com