Guide for PlanninG GaS TrialS for

lnG VeSSelS

ConTenTS

• IntroductIon• PurPoseofGastrIals• delIverycondItIonofthevessel• onboardtestInGofcarGohandlInGequIPment• coldtestInGwIthln2

• Pre-GastrIalsPreParatIon• GastrIals• Glossary

Guide for PlanninG GaS TrialS for lnG VeSSelS

introduction

theterm‘Gastrials’isusedtoencompassthoseteststhatareundertakenduringaperiodwhenaquantityof lnG is taken onboard for testing shipboardfacilities at, or near to, the design temperature.Gastrialsareusuallycarriedoutpriortothevesselentering service in order to confirm the correctfunction, integrity and capacity of the completecargo system, including containment, insulation,cargo handling equipment, pipelines and valvesandtheequipmenttohandleboil-offgas(boG).

this document is intended to provide members ofsIGtto some guidance on the requirements fortesting the systems for handling the lnG cargo,anditsconsequentialboG,usinglnGloadedontoa vessel specifically for that purpose. the scope isconsidered applicable for established membrane,spherical tank and sPb designs. however, it doesnotextendtonovelorprototypedesigns,whichmayrequire specificadditional testing. thisdocument isnotintendedtobeusedasanoperationalprocedure,butmerelyasanaidetopreparingaspecificationforgastrials.

theguidanceaimstocoverthoseitemstestedpriortothehandoverofavesselandalsothetestingcarriedout post delivery, when a first full cargo has beenloaded.

any figuresprovided in the textareonly indicativeandreferenceshouldbemadetotheshipbuilderandmanufacturer’s information for figuresapplicable toaspecificvessel.

the underlyingassumption in this document is thattheGastrialsareconductedbytheshipbuilderpriortodeliveryofthevessel.thisisthecommonpractice

forlnGships,butbynomeansexclusive(note,thenormalpracticeinthelPGindustryappearstobeforthetrialstobeconductedatthefirst loadportafterdelivery).

‘Gas trialsGuide for lnGvessels’, sname1977is an informative reference often cited in lnGshipbuildingcontracts.

Purpose of Gas Trials

Gastrialsareperformedprimarilyforconfirmationof the correct operation of the lnG and gashandlingsystems,whichcannototherwisebefactorytested,and forverificationof thecorrectoperationof theequipmentandsystems thatwillbeused forcontainment and custody transfer measurement ofthelnGcargo.

function testing should usually include the gasmanagement system, the monitoring control andalarmsystemsforthelnGtanksandliquidpumps,valveoperation,thegascompressors,heatexchangers,gascombustionunits (Gcu)and reliquefactionplant, ifprovided.whereapplicable,dualortrifuelenginesshouldusuallyalsobeincludedinthetestprogrammeintheirrespectivegasburningmodes.

boG generated during gas trials may be usedeitheras fuelburnt inboilersormulti-fuel engines,sent to theGcu,orreliquefiedandreturned to thecargotanks,asapplicable.thevapourmayalsobereturnedashorewhilethevesselisalongsidethelnGterminalifthisfacilityisavailable.

disposalofexcessboGmaybedealtwiththroughsteam dumping or thermal oxidising in a Gcu.ventingofnaturalgastoatmosphere,otherthantotesttheventsystemoperation,shouldnotbeincludedaspartofGastrialsprocedureand it is suggestedthatitshouldonlytakeplaceifunavoidablethroughequipmentfailureoremergencycondition.

correct function of the entire system shouldbe confirmed as being in compliance with therequirements of the classification society, theflag administration, the particular ship-buildingspecificationforthevesselandanyothercriteriathatmayapply.

Itisusualthattheperformancetestingofcargopumpsandcompressorsiscarriedoutatthemanufacturer’sfacilitypriortodeliverytotheshipyard,toensurethatthe equipmentmeets the specification requirementsand to facilitate accuracy of the recording ofcalibrated measurements. the operational testingof this equipmentat full capacityduringGas trialsis therefore not usually intended to prove that theperformancemeetsthecontractualrequirements.

similarly the calibration and testing of cargotemperature, level sensor and pressure measuringdevicesusedforcustodytransferpurposes,isusuallycarriedoutattheequipmentmanufacturer’sfacilityinthepresenceofathirdpartysurveyor,priortotheirinstallationonthevessel.

delivery Condition of the Vessel

a major consideration in the planning of the GastrialsmaybetheconditioninwhichthevesselistobedeliveredtotheowneraftercompletionofGastrials.

If contractual arrangements require the vessel to bedelivered in a gas-free condition then any liquidremainingonboardoncompletionofGastrialswillneedtobedischargedashore,necessitatingasecondvisittotheselectedlnGterminal.thereafter,cargotankwarm-up,inertingandaerationwillhavetobeundertaken.

Ifcontractualandcommercialarrangementsallowfordeliveryofthevesselwiththecargotanksin‘gassedup’conditionand/orwithresiduallnGremainingonboard,thenasecondvisittotheterminalfordischargeofresidualliquidmaynotberequired.howeverthe

‘gassed-up’conditionwillusuallyprecludethereturnofthevesseltotheshipyardforanyremainingworkstobeundertakenandcertainposttrialoptions,suchastheinternalinspectionofthecargotanks,willnotbepossible.

should an owner require the cargo tanks to besubjected toacomplete thermalcycle, forexamplecooling the tanks until the secondarymembrane isreducedto-100°cinallregions,thiswillusuallyresultin an extendedperiodalongside the lnG terminaland a significant increase in the quantity of lnGrequiredfortheGastrials.Itisrecommendedthat,inthesecases,therequirementsarediscussedwiththeshipyardatanearlystageanddetailsincludedinthecontractualarrangementsforvesseldelivery.

Prior to Gas Trials

before Gas trials can be carried out, it is to beexpected that the shipyard’s confirmation that thecargocontainmentandcargohandlingsystemsarecompletedisrequired.

toinclude:• completionofthecargocontainmentsystem• onboardtestingofcargoequipment• coldtesting(withln2)

Completion of the Cargo Containment System

Irrespectiveofthetypeofcontainmentsystemselected,theinspectionandtestingofthecargotankcomponentswillusuallybeundertakeninfabricationshopsandonboardthevesselthroughoutitsconstruction.

cargo tanks, on completion of construction, willusuallyhavebeenstrengthandleaktested,atambienttemperature,inaccordancewithclassificationsocietyandflagadministrationrequirements.

tightnesstestsareusuallycarriedoutonmembranetanksaftertherepairofanyleakagefoundduringthe

appropriateambienttemperaturetestingprogrammefor the particular tank design. a tightness testof the secondary barrier of the membrane tanksystemsisusuallycarriedoutaftercompletionoftheconstruction of the containment system.a pressuredifferential between the primary and secondaryinsulation spaces is usually established by use ofvacuumpumpstocreateavacuuminthesecondaryspace. the variation of pressure difference againsttimeisrecordedanda‘vacuumdecaycurve’plottedto summarise the results. thedataobtainedduringthis test provides a reference for comparison withperiodictestingatintervalsduringtheservicelifeofthevessel.

Internalcleaningandfinalinspectionofcargotanks,toensureremovalofallequipmentanddebris,isthencarriedout;afterwhichthetanklidsareimmediatelysecured topreventunauthorisedentryandpossiblecontamination.

as far as practical, it is suggested that an internalinspectionofthecargolinesiscarriedouttoensurethatthereisnodebrisleftinside.

for thosesystemswithgluedsecondarybarriers, ieGttmarkIIIandcs-1,thereispresentlyanadditionalrequirement under Imo resolution a.997(25) toconductfurthersecondarybarriertightnesstestsaftertheinitialcooldowninaccordancewiththeapprovedproceduresofthesystemmanufacturer.

onboard Testing of Cargo Handling equipment

Priortotestinganyitemsofequipmentitissuggestedthatallrelevantalarmsandsafetydevicesshouldbeconfirmedasoperational.

It is also recommended that, as a minimum, thefollowing itemsare included in theonboard testingprogramme:

• confirmation of cargo pump and spraypumpinstallation,includingcheckstoensurecorrect pump rotation on start up. whereanemergencypumpisprovided, thechecksmadeshouldincludeconfirmationofcolumnfoot valve operation when the pump isdeployed.adummypumpmaybeusedforthispurpose.

• commissioningandinstallationchecksofthevapourcompressors, includingconfirmationofcorrectrotationandafunctionaltestonthesurge control systemswith the unit runningonair.

• cargo valve operation, including timing ofemergencyshutdownvalvesandemergencyshut down systems. checks should includeall means of emergency shut down (esd)activation.

• demonstration of the fitting of all the cargopipeline portable bends and spools pieces,withoutrequiringundueforce.

• cargo tank relief valves, insulation spacerelief valves, hold space relief valves andpipeline relief valves should have beentestedandpressuresettingsconfirmedinthemanufacturers’ facilitiesprior to installationon a vessel. It is recommended that cargotankreliefvalves,thatweretestedandsealedat the manufacturers’ factory, are checkedafter installation on a vessel to confirmtheircorrectopening/closingpressuresandoperation1.

Cold Testing (with ln2)

Prior toanycold testing takingplace thedewpointin cargo piping system, cargo tanks and insulationspaces should be reduced to a level below that at

1‘anIntroductiontothedesignandmaintenanceofcargosystemPressurereliefvalvesonboardGascarriers’sIGtto1998

whichcondensationislikelytooccur.toachievethis,whereprovided,botha vessel’sn2plantsand inertgas/dryairplantsarerequiredtobeoperational.

an n2 purge of cargo tank insulation spaces iscarriedout toreduceboth theo2contentanddewpoint in those spaces. the duration and details ofthis operation are dependent on the type of cargocontainment system, but typically the o2 contentin then2purged spaceswill be reduced to3%byvolumeandthedewpointwillbereducedtoatleastminus40°c.

dryair,producedbytheinertgasplant, isusedtoblowthroughthecargopipingsystemandtopurgethecargotanksofhumidair,reducingthedewpointto minus 20°c or below. where applicable, holdspacesarealsopurgedwithdryair.

Priortoliquidn2beingtakenonboardavessel,thelnGsprayline,fromtheliquidmanifoldtothecargomachineryroomisfurtherdriedandpurgedbytheuseofn2generatedonboard.

should cool down of cargo tank(s) be requiredduringthecoldtesting,thenn2takenonboardmaybevaporisedintheship’slnGvaporiserandusedtoinert selectedcargo tank(s).coolingof theselectedtank(s)may be carried out usingn2supplied fromshore,receivedonthevesselattheliquidmanifold,anddirectedtotheselectedcargotankviathesprayheader. liquid cargo, vapour and spray lines arecooleddownbyintroductionofcoldn2gastoachievethe target temperature, typically minus 100°c,and thereafter theyarepressurised toconfirm theirintegrity. freedom of movement of the pipelines inthewayoftheslidingsupportsshouldbeconfirmedandpipelinedeflectionsmeasuredtocheckthat thedesigncriteriaaremet.

whereapplicablethecorrectoperationofcofferdamheatingsystemsshouldbeconfirmed.

If cargo tank(s) havebeenpreviously cooleddownthesetanksareusuallywarmedbyrecirculationusingthehighdutycompressorandthegasheater.

throughout these operations displaced humid air,dry air orn2 gas is usually vented to atmospherefromtheforward-mostventmast.

Pre-Gas Trials Preparation

ItissuggestedthatGastrialsarenotcommenceduntilallfunctionaltestingofgasandassociatedequipmenthas been satisfactorily completed both at ambienttemperatureandatln2orlnGtemperature.thegasequipment system should have been tested and befullyoperationalpriortothestartofgastrials.

All fire-fighting and safety equipment, as required by applicable regulatory authority rules, must be onboard, appropriately located, proven by testing to work correctly and ready for use. Due attention should be paid to both the number of persons on the vessel during trials over and above the normal complement, and also to any statutory requirement for safety drills.

thecargosysteminstrumentationandcontrolsshouldbe confirmedasbeing fullyoperationaland readyforuse.

drip pans and trays should be appropriatelypositioned.

Gas Trials

thefollowingactivitiesareintendedtocoverthemainstagesoftheGastrials

Prior to arrival at lnG Terminal

insulation Space n2 filling (for membrane ships). Prior toputting thecargo tanks into service it isnecessary to replace ambient air in the barrier

spaceswithdryn2.thisoperationcanoftenbecarried out concurrentlywith cargo tank dryingandinerting.

equator ring n2 Purge (for spherical tank vessels)Prior to putting the cargo tanks into service,the equator ring purging system must becommissioned.

Cargo Tank dryingcargo tanksandpipelinesarepurgedwithdryairproducedbytheinertgasgeneratorandthedewpointreducedinthesespacestolowerthanminus20°c.

Cargo Tank inertingbefore the introductionofcargovapour into thesystem,thecargotanksandpipelinesareinertedtofurtherreducethedewpointandreducingtheo2contenttoalevelthatisinsufficienttosupportcombustion.atypicaltargeto2contentisalevelthatisbelow2%byvolume.

In order to mimic normal operation, the tankinertingiscarriedoutusingshipboardequipment.however,dependentupontherequirementsoftheselectedlnGterminal,itmaybenecessarytousen2gasfortheinertingofcargotanksthataretobe‘gassedup’alongside.

arrival at lnG Terminal

Testing of emergency Shut down SystemstheextentoftestingthatcanbecarriedoutduringGas trials will usually depend upon the systeminstalled at the lnG terminal. where primaryandbackupship/shorecommunicationsystemsare installed at the terminal, then testing of thecorresponding vessel’s systems is recommended.once the vessel is securelymoored at the lnGberth, details of the pending gas trials arediscussedatthepre-loadingmeeting.

the ship–shore link functions should be tested.these will include the voice communicationchannelsandthedatatransferchannels,includingmooring tension monitoring, in addition to theesdalarmandfunctionchannels.

Cargo Tank ‘Gassing-up’lnG is usually supplied by the terminal to theshipboard lnG vaporiser through the spraylineand thegeneratedvapour isdirected to thecargo tanks. the inert gas displaced from thecargo tanksmaybedirectedashore, accordingtotheterminal’srequirements.typicallythetargetforgassingupisa98%hydrocarboncontent ineach applicable tank. Gassing-up may only becarriedout in thosetanks thatarescheduledforcoolingandloadingwhilethevesselisatthelnGterminal.

Cargo Tank Cool-downlnGsupplied to thesprayheader isdirected tothespraynozzlesofthecargotankstobecooled.boG, generated from tank cooling, is usuallydischarged to the terminal through the vapourheader via the vessel’s gas compressors. tanksarecooledtoreducethetemperatureofthetankwallanditsinternalstructureatacontrolledratetoensurethatthevolumeofboGiskeptwithintherequiredlimits.forImotype‘b’tanks(sphericaltanksandIhI-sPb),controlof thecoolingrateiscriticalinordertominimisethermalstressesinthetankstructure.tanktemperaturesaremonitoredtoensurecoolingiscarriedoutinaccordancewiththedesignrequirements.

lnG loadingwhenthecargotankshaveattainedtherequiredtemperature,theamountoflnGrequiredfortheentireGastrialsprocesscanbe loadedinto thecooledtanks.displacedboGisdischargedtotheterminalthroughthevapourheaderandmanifold,viathevessel’sgascompressors,asnecessary.

depart lnG Terminal, at Sea

oncelnGloadingiscompleteandthepostloadingcustodytransferformalitieshavebeencompletedthe vessel can usually depart from the terminal.afterthevesselhasdepartedthelnGterminaltheboGfromtheloadedcargotanksiseitherusedasfuelgas,handledbythereliquefactionplant(ifinstalledonthevessel)orthermallyoxidisedinagascombustionunit(Gcu),dependinguponthereadinessofequipmenttohandletheboG.

Cargo Tank Gassing-upusingthespraypumpinoneofthetankscontaininglnG,liquidissuppliedtothelnGvaporiser.thegasgeneratedbythevaporiserisdirectedtothoseremainingcargotanks thatwerenotgassedup,cooledorloadedatthelnGterminal.displacedinert gas from these remaining tanks is usuallyvented through the forward-most ventmastuntilahydrocarboncontentisdetectedintheinertgasstream.

Cargo Tank Cool-downtheremainingtanksarenowcooledbysprayinglnG into them. the boG generated is eitherusedasfuelgas,reliquefiedordisposedofintheGcu.Only in case of an equipment failure that prevents handling of the gas or, in an emergency, should venting through the forward-most vent mast be performed.tanktemperaturesshouldbemonitoredtoensurethecool-downiscarriedoutin accordance with design requirements. In thefinal stages of the tank cooling, the liquid linesshouldbecooledinpreparationforthefollowingcargopumptestingsequence.

Cargo Pump Testing (Includingmaincargopumps,sprayandfuelgasandemergencycargopumpswhereprovided).

Itisrecommendedthateachpumpineverytankistestedsequentially,thelnGnecessaryforthese

testsbeingtransferredfromtanktotank.whereanemergencycargopumpisprovided,thegastrialstesting programme should incorporate a test ofthispump.whereemergencytanktotanktransferis designed to be performed by pressurising ofthecargotank,thiscargotransferisnotnormallyperformedduringthegastrials,exceptinthecaseoftestingaprototypetank.

Ifitisarequirementoftheshipbuildingspecificationthatanycargopumpcanberunusingthevessel’semergencygeneratoras theelectric source, it isrecommendedthatthiscapabilitybedemonstratedduringtheGastrials.

Boil-off Gas Management System testingofdualfuelburningorreliquefactionplantoperationandGcuoperationisrecommended,ifapplicable.thetestingshouldbeforadurationsufficienttodemonstratesuccessfulcontrolofcargotankpressuresoverarangeofoperations.

theinterfacesbetweenthecargomachineryplantand the boG management system should bedemonstratedascompliantwiththeshipbuildingspecification.

dual fuel BurningIt is recommended that dual fuel burning testsshould ordinarily include the following asapplicable:• Gasburningeither inboilerswith thesteam

dump, or inmain engines or in aGcu, tohandleexcessboil-off.

• Gasburning in themainpropulsionsystem,including the operation of the plant at theequivalentof100%gasburningatmcr,withthefuelgasrequirementsbeingmetbyforcedvaporisation. should the gas managementsystem be designed to allow for solely gasburning at all ranges of load requirements,thisshouldalsobetested.

• Gasburningoperationtestscaninclude:• n2purgesequenceconfirmation• burnerfuelmodechangeoversequence• burnerfuelincrease/decreasesequence• maximumturndownratio(wheredefined)• fueloilbackupsequenceconfirmation• dual fuel change over sequence

confirmation

reliquefaction Plantwhereareliquefactionplantisinstalledonthevessel,sufficientlnGshouldbeloadedtotesttheentiresysteminnormaloperation,thegasusuallybeinggeneratedbysprayinglnGintothecargotanks.

when duplicated equipment, such as boGcompressors, n2 companders, n2 boostercompressors and lnG transfer pumps areprovided,thetrialsshouldbeprogrammedtoensurebothsetsofequipmentaretested.

It is recommended that the following shouldusuallyalsobetested:• operation of the plant in the stand-by

mode• operationoftheplantinthefreeflowor

idlemode• operation of the vent gas heater (if

installed)• validation of warm start and cold start

sequences• operation of the Gcu for a period of

sufficientdurationatasuitablethroughputcapacity to demonstrate successfuloperation of the tank pressure controlsystem with or without reliquefactionequipmentoperating.

Blackout Testthepurposeofsucha test is todemonstrate thecapability of the system to be restarted safely;

consideringboGtreatmentrecoverybyburningand/orreliquefying.

unmanned engine room operationIt is recommended that an unmanned engineroom (ums) operation test, at full sea speedunder normal operating condition (ie burningboil-offgas)beperformedifpossibleforaperiodofatleast4hourstosatisfytherequirementsforumsoraccunotation.

Crew familiarisationIt is considered essential that a period beallocated,underthesupervisionoftheshipbuilder,sufficienttoallowthevessel’screwtobecomefullyfamiliarisedwith the safeoperationofall cargohandlingequipmentandsystems.

Cold Spot inspection of Cargo lines and Tank Boundaries. during tank-to-tank liquid transfer a coldspot inspection of the cargo line insulation isrecommended; while the cargo tanks are inthe cold condition, a hold space inspection ofindependenttypecargotankinsulationsurfacesshould be performed. (note: a full cold spotinspectionof independentandmembranetankboundaries should be carried out after thefirst loading, when the cargo tanks are fullyloaded)

return to lnG Terminal (thisand the following itemmaybe consideredoptional depending upon the contractualarrangements for the condition of the vessel atdeliverytoowners)

unloading remaining lnGfollowing return to the lnG terminal andberthing and testing of the esd systems, theremainingpumpablelnGshouldbedischargedashore to the terminal. It is recommended

that, if possible, and subject to the terminal’sacceptance,theautomaticunloadingsequencebetested.

depart lnG Terminal and at Sea

Cargo Tank Warm-upthe remaining un-pumpable lnG is usuallydisposed of by vaporisation, ensured by therecirculationofheatedvapourusingthehighdutycompressor and the warm-up heater(s). excessvapour isusuallyeitherusedas fuelorhandledby theGcu. following the removal of any un-pumpableliquidthecargotankwarm-upprocesscan be continued until the tank temperaturesreach the required level (typicallyaroundminus20°c on the equator profile of spherical tanksand+5°conthesecondarybarrierofmembranetanks).tanktemperaturesshouldbemonitoredtoensurewarm-upiscarriedoutinaccordancewiththedesignrequirements.

Cargo Tank inertingafter the tanks have been warmed to nearambienttemperaturesthelnGvapourshouldbedisplacedbytheintroductionofinertgasintothecargotanksandpipelines.Inertingshouldusuallythenbecontinueduntil thehydrocarboncontentisreducedtobelow2%byvolume.thedisplacedcargo vapour is usually either used as fuel ordisposedofinagascombustionunitforaslongas safely combustible. thereafter, the displacedmixturecanbeventedthroughtheforward-mostventmast.

Cargo Tank aeration dry air, produced by the inert gas plant, isintroduced into the top of the cargo tanks.displaced inert gas is discharged from the tankbottom,throughthefillinglineandliquidheaderto the forward-most vent mast. normally, tank

aerationshouldthencontinueuntil,inthevapourdischarge stream from the vent-mast and in thecargotanks,theo2contentapproaches21%byvolume, thehydrocarboncontent isbelow0.2%by volume, the co2 content is below 0.5% byvolumeandthecocontentisbelow25ppm.

Throughout the gas trials the following should be monitored.

• the insulation space pressures andtemperatures

• the cargo tankpressures, temperaturesandlevels

• n2flowratestoinsulationspaces• the gas concentrations in the insulation

spaces• the cofferdam and inner hull temperatures,

with cofferdam heating system ready forservice.

note:dependingonthecontainmentsystemdesign,thedetailsofactualitemstobemeasuredmayvary.

duringeachofthetestingphasesofthetrials,datarelevant to the equipment in operation should berecorded at regular intervals. where equipmentoperationisdesignedforautomaticcontrol,thismodeshouldbetestedandoperatingdatarecorded.

Quantity of lnG to be taken onboardInordertotestthepumpingarrangementsinallcargotanks, the minimum quantity of lnG to be takenonboard should be based on the amount requiredto allow for pump start-up and transfer of liquid,sequentially fromtank to tank,untilallpumpshavebeensoakedandoperated.

thecalculationofquantitytoloadwilldependuponasignificantnumberoffactorsand,forthisreason,examplequantitiesarenotincludedinthisguidancedocument.

amongthefactorsrequiringconsiderationare:• thenumber,shapeanddimensionsofcargo

tanksonvessel.• the number of tanks to be ‘gassed-up’ and

cooleddownatthelnGterminal.• the requirements for ‘gassing-up’and cool-

downoftheremainingtanksatsea.• theamountofun-pumpableliquidremaining

ineachtankaftertransferbythemaincargopumps and the spray/stripping pumps.dependent upon the containment systemdesign, trimand listmayhavean effect onthisfigure.

• theamountofliquidthatwillboiloffduringalltherequisitetrials.

• If gas burning tests are to run concurrentlywith pumping trials, the liquid amount willhave to be increased accordingly to ensuresufficientliquidremainsforthefinalstagesofthetransferoperation.

• formulti-fueldieselenginesthequalityofthefuel gasduringgasburning tests shouldbemaintainedwithin the limitsprovidedby theenginemanufacturer.as,duringthecourseofgas trials, theboGmethanenumberdrops,it is important to factor this in to the trialsprogramme.forexample,itmaybedecidedtoperformthegasburningtestsprior tothepumpingtrials.

• IftanksaretobeemptiedandgasfreedaftergastrialsthensufficientquantitymustremainonboardatthetimeofreturntolnGterminaltoallowforstart-upofthecargopump.

Itisnotnormallyconsiderednecessarytofullyloadanycargotankforthepurposeofconfirmingstresslevelsorthetestingofoverfillalarmsandshutdowns,as such alarms and shut downs should be fullytestedtothesatisfactionoftheclassificationsocietysurveyor, during the loading of the first cargo, byslowlyfillingthetanksuntilthealarmsandtripsareactivated.

Post Gas Trials inspection

aftercompletionofGastrialsavisualinspectionofanumberofitemsisrecommended,includingbutnotlimited to: filters, cargo compressors, the inert gasgeneratorandthen2generators.

Testing at first loading

whilstloadingthefirstcargo,allhighandhigh-highlevel alarms, overfill alarms and automatic shut-downs should be fully tested to the satisfaction ofthevessel’sclassificationsocietysurveyor.themostrigorous method of testing will usually involve thecarefultransferoflnGfromtanktotankinordertotestthesystemsatoverfilllevels.however,alternativemethodsmaybeagreedwithclass.

afullcoldspotinspectionofthecargotankinsulation/innerhullshouldusuallybecarriedoutaftertemperatureshavestabilisedintheinsulationsystem.thecompartmentstobeenteredandthenatureofinspectionwillusuallydependuponthecontainmentsystemdesign.

Boil-off rate Measurement and reliquefaction Plant Testing at design Condition

during theearlystagesof thevessel’sservice lifeanumber of boGmeasurement voyages are usuallycompletedtoconfirmthatthecargoboil-offratemeetsthe contractual requirements and the containmentsystemdesignspecifications.

reliquefactionplantcapacityforoperationatdesignconditions will usually require monitoring over anumberofloadedvoyagestocollectsufficientdatainordertoconfirmthattheequipmentsizingandcontrolsuccessfullyhandlesnormalboGvolume,withoutthenecessityofoperatingtheGcu.apossiblemethodformeasuringboGvolumemightbetouseonecargotankasabuffertankforreceivingallreliquefiedlnG.boil-offratemeasurementscouldthenbeperformedon the remainingcargo tanks.followinganumber

ofreadings,anothercargotankisdesignatedasthebuffertank.Insodoing,theboil-offfromeachcargotankcouldbedetermined.

Testing at or after refit

during routine refits classification societyrequirements should include the inspection andtesting of a number of cargo containment systemitems.

the items for survey should include tank mountedcargo valves and relief valves for the cargo tank,insulationspaceandliquidpipelines.

verification of the operation of the high level andoverfillalarmsandshutdowns,followingtheloadingoffirstpost-refitcargo,shouldbecarriedouttothesatisfactionoftheclassificationsocietysurveyor.

It is presently an IGc code requirement that thesecondary barrier should be capable of beingperiodically checked for its effectiveness by meansofavacuumdecaytestoranothersuitablemethod.thetestprocedureforthecargocontainmentsystemwill be dependent upon the actual system design.suchtestingshouldusuallybecarriedoutatperiodsnot exceeding five years, following licensors’procedures.

Timing of Gas Trials with respect to delivery of the Vesselasnotedintheintroduction,thecommonpracticeistocompleteGastrialspriortothedeliveryofanlnGvessel.however,conductingGastrialspostdelivery,asforlPGships,shouldnotnecessarilyberejectedoutofhand.thisoptionmayhavetobeconsideredif, for instance, the shipyard is unable to arrange,withasuitableterminal,fortheprovisionofGastrialservicesinatimelymanner.suchcircumstancesmightperhapsariseinaperiodofhighgasdemandwheretheberthissimplynotavailableforthetimeneeded

andcouldbepresentedasaforce-majeuredelaytodeliveryofthevessel.

Insuchasituation,itissuggestedthattheownershouldbe fully conversant with the contractual warrantyprovisionsand these shouldbe fullyacceptable. Inparticular,theresponsibilitiesandcourseofactiontobetakenintheeventthatamajordefectshouldcometolight,whichnecessitatesthevesselgoingforrepairbeforeentryintoservice,mustbeclarified.

It is suggested that the procedures adopted shouldfollow,ascloselyaspracticable,thosesetoutinthisdocument,whilenotingthatafullwarm-up,inertandgasfreeoperationisunlikelytobeconducted.

the timingof theGastrialsshouldnotbean issuefor the classification society, but should remaina contractual issue between the owner andshipyard.nevertheless,theflagadministrationandclassificationsocietyarelikelytoneedtobesatisfiedthatthesystemsaretestedandprovenbeforetheycanissue thecertificateof fitnessandclasscertificaterespectively.such requirementswillusually include,whereappropriate, theneed for secondarybarriertestingonmembraneshipspostgastrials.

whilst the decision to use this option has to beevaluated on a case by case basis, it is suggestedthatitisprobablynotappropriateforafirst-in-seriesvessel,particularlyiftherearenewdesignconceptsto prove. the concept however is not withoutadvantagesfortheowner,particularlysincehisstaffwould, effectively, run the trials, rather thenmerelyactingasobserversofa shipyard teamconductingthetrials.

GloSSarY

aeration: theintroductionoffreshairintoatankwiththeobjectofremovingtheinertgasesandincreasingoxygencontentto~21%byvolume.

Boil-off Gas (BoG): the vapour produced abovethesurfaceofaboiling liquidcargo.theboilingiscausedbyheatingressintothecargotankoradropinpressureinthetank.

Classification Society: an organisation that setstechnicalrules,confirmthatdesignsandcalculationsmeet theserules,surveyshipsandstructuresduringtheprocessofconstructionandcommissioning,andperiodicallysurveyvesselstoensurethattheycontinuetomeet the rules (e.g. lloyd’sregisterofshipping,americanbureauofshipping,detnorskeveritas)

Compander:arotarymachineincorporating,inonedrivesystem,compressorstagesandturbo-expanderstages.

Cold Spot:anareaindicatedbyfrostingontheoutersurfaceofpipelineinsulationorcargotankinsulation,thereby showing the reduced thermal insulationpropertiesinthatparticulararea.

Cool-down:theoperationtoreducethetemperatureofacargotanktoatemperatureatwhichitissafetocommencetheloadingoflnGintothetank.

emergency Shut down System:a system specificallydesignedtosafelyshutdowncargotransferoperations.

flag administration: the government of the statewhoseflagtheshipisentitledtofly.

Gassing-up:replacinganinertatmosphereinacargotankwithnaturalgasvapourderivedfromlnG.

Gaztransport & Technigaz (GTT): licensors of anumber of membrane cargo containment systemswhicharedesignedforinstallationinlnGvessels.

iGC Code:theInternationalcodefortheconstructionandequipmentofshipscarryingliquefiedGasesinbulk.

inerting:eitherofthefollowingtwoprocesses:(i) the introduction of inert gas into an aerated

cargotankwiththeobjectofattaininganinertcondition suited to commencement of a safegassing-upoperation.

(ii) the introduction of inert gas into cargo tankto displace the cargo vapour, with the objectof attaining an inert atmosphere suited tocommencementofasafeaerationoperation.

liquefied natural Gas (lnG): a colourless fluid intheliquidstatecomposedpredominatelyofmethaneandcontainingsmallquantitiesofc2,c3,n2orothercomponentsnormally found innaturalgas.seebsen1160

ln2 :liquefiednitrogen

n2 :nitrogengas

un-pumpable liquid:theliquidremainingincargotanksaftercompletionofstripping.

Warm-up:theoperationtoincreasethetemperatureofacargo tank toa temperatureatwhich inertingandaerationcanbesafelycommencedwithout theriskofcondensationforminginsidethetank.

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