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5/3/2015 SuppressionsystemstabilizeslongpipelineriserliquidflowsOffshore
http://www.offshoremag.com/articles/print/volume60/issue10/news/suppressionsystemstabilizeslongpipelineriserliquidflows.html 1/9
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Producingnewfieldswithsubseatreesandtiebackofthesenewfieldstoexistingplatformsoffersoperatorscostsavingsovernewplatforminstallation.Oneoftheproblemsfacingthesetiebacksisthedevelopmentofliquidslugs,eitherintheflowlineoratthebaseoftheriser.
Theseslugsoftenoverwhelmthefirststageseparationsystemonthehostplatform,interrupting
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5/3/2015 SuppressionsystemstabilizeslongpipelineriserliquidflowsOffshore
http://www.offshoremag.com/articles/print/volume60/issue10/news/suppressionsystemstabilizeslongpipelineriserliquidflows.html 2/9
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
5/3/2015 SuppressionsystemstabilizeslongpipelineriserliquidflowsOffshore
http://www.offshoremag.com/articles/print/volume60/issue10/news/suppressionsystemstabilizeslongpipelineriserliquidflows.html 3/9
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
5/3/2015 SuppressionsystemstabilizeslongpipelineriserliquidflowsOffshore
http://www.offshoremag.com/articles/print/volume60/issue10/news/suppressionsystemstabilizeslongpipelineriserliquidflows.html 4/9
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
5/3/2015 SuppressionsystemstabilizeslongpipelineriserliquidflowsOffshore
http://www.offshoremag.com/articles/print/volume60/issue10/news/suppressionsystemstabilizeslongpipelineriserliquidflows.html 5/9
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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