suppression system stabilizes long pipeline-riser liquid flows - offshore

5/3/2015 SuppressionsystemstabilizeslongpipelineriserliquidflowsOffshore

http://www.offshoremag.com/articles/print/volume60/issue10/news/suppressionsystemstabilizeslongpipelineriserliquidflows.html 1/9

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Suppressionsystemstabilizeslongpipelineriserliquidflows10/01/2000

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Producingnewfieldswithsubseatreesandtiebackofthesenewfieldstoexistingplatformsoffersoperatorscostsavingsovernewplatforminstallation.Oneoftheproblemsfacingthesetiebacksisthedevelopmentofliquidslugs,eitherintheflowlineoratthebaseoftheriser.

Theseslugsoftenoverwhelmthefirststageseparationsystemonthehostplatform,interrupting

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production.ShellGlobalSolutionshasdevelopedasystem(nowlicensedbyDrilQuip)toeliminatethisproblemandstabilizetheflow.Thisimprovesprocessoperation,decreasesstartuptimesforproduction,andincreasestheaverageproductionvolume.

Whenproductionmovesfromthesubseatree,downtheflowlinetoahostplatform,itistransportedasamultiphaseflowcontainingwater,liquidhydrocarbons,andgas.Mostflowlinescarryproductionseveralmilestoaplatformorproductionfacilityinshallowwater.Theselinesmoveovertheseafloorwheretherearedipsandlowpoints.Liquidsmaysettleintheselowpoints,especiallyduringshutdown.

LengthyslugsAstheflowmoves,thiscollectionofliquidscangrowsothatitfillstheinternaldiameter(ID)oftheflowlineoveracertainarea.Astheslugoffluidmovesthroughtheline,twothingsoccur:

Theliquidspushforward,collectingmoreliquid,andbroadeningtheslugGasbuildsupbehindtheslugunabletogetaroundit.

Asproductionmovesupthecontinentalshelf,gravitydrawstheliquidsdown,exacerbatingthiseffect.Bythetimethisflowreachesthehostplatform,theliquidslugorsectionofflowthatisallliquid,canbeuptoamilelong.Thisiscalledatransientslugbecauseitdevelopsgraduallyasproductiontravelsalongtheflowline.

Anumberoffactorsaffecthowbigtheseslugsareandhowoftentheydevelop,butslowerflowratesorinterruptioninflowcanencouragetheirdevelopment.

Evenastratifiedflowwithliquidsonthebottomandgasonthetopcanrunintotroubleasitreachesthehostplatform.Atthebaseoftheplatformwheretheflowlinetiesintotheriser,thereisanangleorelbowmovingthehorizontalflowlinetoverticalsoitwillrunupthesideofthejacketorplatformtothetopsidesprocessingequipment.

Thistransitionleadstoliquidscollectingoutoftheflowandpoolingatthebaseoftheriser.Thisslugcontinuestogrowuntilthegasbehindithasbuiltupsufficientpressuretopushtheliquidsuptheriser.Thistypeofslugflowiscalledsevereslugging.Itischaracterizedbylongperiodsofproductionstarvationfollowedbylargeliquidandgassurges.

InterruptingproductionSpaceandweightonoffshoreplatformsareverylimited.Whenaseparationsystemandprocessingequipmentaredesigned,theyarebuiltlargeenoughtohandletheanticipatedproductionwithalittleexcesscapacity.Theproductionequipmentistailoredtotheproductionmixthefieldsproduce.Thesesystemsareadequate,butnolargerthantheyhavetobebecauseofthepremiumonspaceand



weight.

Whenaslugmovesintothefirststageseparationequipmentithastwonegativeaffectsontheequipment.

Theseparatorquicklyfillswithliquidstriggeringanalarmandalertingthecrewthattheliquidsareabouttofloodthevessel.Theliquidshavetoeitherbechokedbackatthispointordrainedoff.Oncetheliquidspassthroughtheseparatorthereisasurgeofgas.Thisoverwhelmsthecompressionequipmentandcausesgastoflare,orevencauseshighsuctionpressureshutdownatthecompressors.

Thiscycleofsluggingcanoccurmorethanonceanhour.Duringproductionstartup,whentheliquidsandgasfirststarttomove,theseliquidslugsandgassurgesmayresultinalargeproductiondeferment.Duringnormaloperation,suchinterruptionsinproductionareveryinefficientanddonotmakeforoptimumuseoftheprocessingequipment.Inaddition,overallproductionisreducedbecauseoftheneedtochoketheflowandshutinproductiontohandlethesemassiveamountsofliquidsandgas.

ExistingplatformsAsmoreexistingplatformsarebeingusedashubstotiebacknewfields,theprocessingequipmentisreliedupontohandleaflowratedifferentthanwhatitwasdesignedfor,ortohandleflowsfrommorethanonepipeline.

Whiletheseslugscanbeminimizedbyslowingproductionrates,operatorsarelookingatalternativesthatwouldallowformaximumproductionrateswithouttheinterruptionofslugs.JimKubasta,ProjectManagerwithDrilQuipsaidtherearetwoapproachescurrentlybeingused.

Injectinggasatthebaseoftheriser:Thisprovidesartificialliftfortheliquids,movingthemsteadilythroughthesystem.Thiscanalleviatetheproblemofsevereslugging,butdoesnothelpwithtransientslugginginwhichtheliquidcolumnisalreadyformedbeforeitreachestheriserbase.Theintelligentchoke:Thisisasystemthatopensandcloseschokestoincreaseorreduceproductionflow,Kubastasaid,deebasedonthelevelsofliquidsintheprimaryseparatorvessel.Whilethiscankeepliquidsfromoverwhelmingthesystem,itdoesnothelpwiththegassurgeormaintainoptimumflowrates.

Slugsuppression



Asproductionmovesintodeeperwaterflowlineswillbecomelonger,IDswillbecomelarger,andthepotentialforslugswillincrease.Anticipatingthisproblem,GertHaandrikman,multiphaseflowteammemberwithShellGlobalSolutionssaidhiscompanystartedworkintheearly1990sonasystemtocontrolsurges.

Atthetime,somefieldsintheNorthSeawereexpectedtoreachastageofmaturitywheretheywouldproduceunderlowerpressureswithlessgasandmorewater.Thisaddedtothesurgeproblems.WhatShellGlobalSolutionscameupwithisasystemtomonitorandadjusttheflowofgasandliquidstotheprimaryseparatorfromtheriser.

ThisSlugSuppressionSystem,orS3asitiscalled,takesliquidandgasproductionintoaminiseparatorthatacceptsflowfromtheriserbeforeitreachestheprimaryseparator.Sensorsmonitorthepressureandliquidlevelintheminiseparator,aswellasthegasandliquidflowratesandfeedthisinformationtoacomputer.

Basedonacontinuouslyadaptedoptimumflowrateandmix,thecontrolsoftwareactivatesvalvesthatcanopenorclosefast,comparedtothoseofthefirststageseparator.

Bymonitoringthepressureandliquidlevelsintheminiseparator,thecontrolunitcandeterminewhenliquidsaregatheringatthebaseoftheriser.Onceitsensesthis,thesystemopensthevalveregulatinggasflow.Thedropinpressuredrawstheliquidsuptheriser,avertingtheslug.

Withatransientslug,thereisnowaytopreemptslugdevelopment.Thegoalofthesystemistomovethefluidsthroughquickly,bybleedingoffthegaspressureinfrontofit.Theliquidsarethenmovedintothemainseparatortankataspeedsuchthattheydonotfloodthetank.

Asthefluidleveldecreases,thesystemknowsagassurgeiscoming.Oncethefluidismovedthroughtheminiseparator,thegasbehinditiscontainedandthepressurebledoffgraduallyintothemainseparator.Theobjectistoanticipatethechangesinflowcompositionandcompensatebyopeningandclosingthesevalves.

Bybleedingthecompressedgasfromtheminiseparatortothemainseparatorslowly,thesystemavoidsoverwhelmingtheprocessingequipmentandminimizesthedisturbancesintheflowline.Inasense,theminiseparatorhandlesthesurgesandlullsinflow,todampenvariationsinproductionlevelsgoingtotheprimaryseparatorforthetransientslugsandpreventtheformationofaslugattheriserbase.

Continuousproduction

Byeliminatingtheproblemofsevereslugsandcompensatingfortransientslugs,theS3systemisabletoprovidereducedstartuptimesbecausethesurgesassociatedwithstartupareminimized.

Oncetheproductionstreamisonline,theS3controlsproductionbyeliminatingpeaksandvalleyscausedbysurgesthatoverwhelmequipmentandwhichwouldotherwisecauseinterruptionsin



Cantarellproductionoutputup60%followinginjectionstartup|PreviousArticle

NextArticle|Managingdrillfloorpractices,equipmentimportantinquick

drillingThaiwells

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production.Overall,thisresultsingreaterproductionvolumes.Laterinthelifeofafield,asmore

fluidsareproducedandpressuresarelower,theS3canoptimizethisproductionaswell.

Themaximumliquidoutputofthesystemisspecifiedbythesoftwarebasedonthesizeoftheprimaryseparator,Haandrikmansaid.Thisishelpfullaterinthelifeoftheplatformwhenitmightbehandlingproductionfromadifferentfieldorverydifferentproductionfromthesamefieldafterthefieldmatures.

Installation

TheS3canbeinstalledontoplatformsalreadyinservice,accordingtoLarryReimertDrilQuip'sCoChairman.Itisdesignedtouseaspoolpiecebetweentheriserterminationandtheprimaryseparator.Thisspoolpieceinstallationcanbeaccomplishedduringaroutineshutdown.

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suppression system stabilizes long pipeline-riser liquid flows - offshore

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