Introductiontothesubseasector oftheoilandgasindustry
1.1Introductionandhistory
Theterm“subsea”insimplelanguagereferstounderwaterlocationssuchasseas andoceanswhichareknownasasectoroftheoffshoreoilandgasindustry.Manyoiland gasresourcesarelocatedunderthewaterinoffshoreareasaroundtheworld.Americans begantheexplorationforoilandgasinthePacificOceanmorethan70yearsago.Infact, nocompanytooktherisktodrilloutsidethebordersofthelandbefore1947.Theoffshoreoilandgasindustrystartedin1947whenthefirstoilwellwascompletedbyKerrMcGeeinjust4.6mofwaterintheGulfofMexico.Atthetime,almosteveryone believedthattherewasnooilintheGulforMexicoandthatdrillingintheGulfwould leadonlytofindingsaltdomes.Littleequipmentandfewfacilitieswerespeciallydesigned atthattimeforoffshoreoilexploration,drilling,andproduction.Bytheendof1949, 11oilandnaturalgasfieldshadbeenfoundintheGulfofMexicowith44exploratory wellsestablishedaccordingtothereportoftheNationalOceanIndustriesAssociation. Theoffshoresectorofoilandgasexpandedafterwardandtheexplorationandproduction
ofoilandgasmovedtodeeperwaterdepths.In1961thefirstsubseawellwascompleted byShellcompany.Thefirstdiver-lesssubseacompletionwasperformedin1967.The firstNorthSeasubseadevelopmentprojectwasnamedEkofiskwhichwaslocatedata depthof220ft(approximately67m)ofthesea.Onehundredandfortyoperationalsubseawellsexistedworldwideby1978.ShellCoulombmadethedeepestsubseatiebackata depthof7300ft(approximately2225m)in2004.Bythen,thenumberofoperatingsubseawellshadreached2404worldwide.AjointventureofBritishPetroleumandExxon MobilstartedproductionoftheThunderHorsedeep-wateroilandgasfieldlocated 240kmoffthewestcoastofNewOrleansatadepthofapproximately1900m.The OrmenLangeprojectoperatedbyhydro,aNorwegiancompany,wasthelongesttieback atapproximately160km.
Nowadays,theventureofexplorationandproductionofoilandgasismovingto deep-waterdepthsbetween1and4km.Subseadevelopmentisgenerallydividedinto twocategories:shallowanddeep-waterdepths.Thereisnocleardefinitionofdeepwater but,ingeneral,depthsthatareoutofreachofadiverarecalleddeepwater.Basedonthis definition,depthsintherangeof300mandmorecanbeconsidereddeepmaybe 20yearsago.Thedefinitionofdeep-waterdevelopmentischangingnowsinceoilexplorationventureismovingmoreandmoretodeeperwaterdepths.Thereforethedefinition ofdeep-waterdepthcouldbechangednowtodepthsofmorethan1km.
Manyfixedplatformswereusedinthebeginningforthedevelopmentofoffshore fieldsinshallowwaterdepths.Fixedplatformsareattachedtotheseafloorbylegsmade eitherfromsteelorconcretethatrelyontheirownweighttobefixedtothefloor.These platformscanbedesignedtobeusedforlong-term(30+years)oilandgasproductionand aresuitableforinstallationonthefieldatamaximumof300mdepthasillustratedin Fig. 1.1.Atension-legplatformTLPorextendedtensionlegplatformisafloatingstructure
Fig.1.1 Offshorefielddevelopment/productionsolutions. (Courtesy:MODEC/Springer.)
thatismoored(connectedtotheseabedbytendons).ATLPisasuitablefielddevelopmentplatformforwaterdepthsbetween300and1500m.Sparplatformsarefloating platformsusedfordeep-waterdepthsofmorethan1.5km;theseplatformsareanchored totheseabed.Therobustdesignofthesparplatformmakesthisoptionresistanttowind, waves,andcurrents.Asparplatformcontainsalargediameterverticalcylindersupporting adeckasillustratedin Fig.1.2.Semisubmersibleplatformsarebuoyantandcontainmultileggedfloatingstructureswithalargedeck.Offshoreplatformsaresteelstructureswith legsdirectlyconnectedtotheseafloorandareonlysuitableforthedevelopmentofa singleoilwell.
Astheoilfieldsgetdeeper,oilfielddevelopmentischangingandtheuseofshipscalled FloatingProductionStorageandOffloading(FPSO)vesselsisbecomingmorepopular. AnFPSOreceivestheproducedhydrocarbonfromthesubseaunitsandprocesses,stores, andfinallytransportsit.Unlikeajacketplatformwhichisconnectedtoasinglewell,a FPSOcanbeconnectedtoseveralwells.Infact,anFPSOmaybeusedbyoffshoreoiland gasoperatorcompaniesfordifferentpurposessuchastheproductionandprocessingof
Fig.1.2 Sparplatform. (Courtesy:OffshoreTechnology.)
petroleumandtostorethefinishedproducts. Fig.1.1 comparesdifferentfielddevelopmentsolutionsbasedonthedepthofthefield.
Themainaimofsubseadevelopmentistoselecttheoptimumwayintermsofsafety andeconomicalgaintoextracttheoilandgasfromsubseaoilfields.Subseadevelopment iscomprisedofdifferentactivitiessuchasexploration,drilling,completion,andproduction.Subseatechnologyisdesignedtomeetveryspecialanduniquechallengesandthus requiresspecializedengineeringtasks.
Theconceptofsubseaoilfielddevelopmentwasimplementedintheearly1970sby installingsubseawellheads,Christmastrees,productioncomponents,andfacilitiessuchas manifolds,jumpers,andpipelineontheseabed.Moredetailedinformationaboutsubsea productionsystemsisprovidedinthenextsectionofthischapter.Thefollowingparametersshouldbetakenintoaccountforsubseadevelopment:
- Waterdepth:shallow,deep,orintermediate;
- Standaloneortieback:asubseatiebackisdefinedasanengineeringsolutionthatconnectsanewoilfieldtoanexistingproductionfieldandisapopulartrendinsubsea development;
- Verticalorhorizontaltreewhichaffectstheweightandfigureofthetree;
- Hydraulicandchemicalsupplyunits;
- Subseaprocessingequipmentandcomponents;
- Fielddevelopmentarchitectureorconfigurationtoprovideareliable,safe,andcosteffectivearrangement;
- Artificialliftmethod.
1.1.1Benefitsofsubseaengineering
Akeybenefitofsubseaprocessingisthatitimprovesoilandgasproductionandrecovery fromthereservoir.Theotheradvantageofsubseaprocessingisthatitsavescostand increaseseaseofproductionbysavingspaceonthetopsidefacilities,whetherplatform orFPSO,andreducesthetransferoflargevolumesofuntreatedhydrocarbonfromdeep subseadepthstothesurface.Asanexample,theseparationofthreephasesofoil,gas,and waterproducedfromthereservoircanbedonesubseawithoutanyneedtotransferallof thesephasesallthewaytothetopside.Inaddition,undesirableby-productsofoilandgas suchascarbondioxide(CO2)andhydrogensulfide(H2S)mayberemovedfromthereservoirfluid(typicallyoilandgas)ontheseafloorandthereisnorequirementtotransfer themtothetopsidefacilitiesthroughaflowline.Aflowline,alsocalledapipeline,is definedasasegmentofpipingthattransfersthefluidcomingoutfromthewellhead tothetopsidefacilitiesfromthemanifolds.Animportantpointisthataflowlineisconnectedtothetopsidefacilitiesindirectlythroughrisers(see Fig.1.3).Ariserisdefinedasa segmentofpipethatconnectsanFPSOoraplatformtoasubseasystemsuchasaflowline sothispipe(theriser)isextendedbetweentheseabedandthesurface.Thereforeboth
risersandflowlines(pipelines)arepartsofatransportationsystemtobringtheproduced hydrocarbontothetopsidefacilities.Riserstypicallyrangeinsizefrom300 to1200 indiameterandcanbeeitherflexibleorrigid.
Subseadevelopmentcansolvetheflowassuranceproblemstoalargeextent. Multiphaseflow,hydrate,andwaxformationareoperationalproblemsassociatedwith flowassuranceandpipingsystems.Multiphaseflowcanhavemanydifferentforms:one ofthemostsevereformsiscalledslugflow.Slugflowreferstoaconditioninwhichsubsea flowlineorrisersectionscompletelyfillupwithliquidandholdupthegasflow.Slugging hasmanydifferentadverseconsequencessuchashighloadsandpressuredropsinthepipingsystem,corrosion,anderosion.
Anotherformofflowproblemiscalledhydrate.Thisreferstosolidslikeicethatcan forminthenaturalgasathighpressureandlowtemperature.Formationofhydrateisvery likelytooccurduringthemultiphaseflowmovementofthethreephasesofoil,gas,and waterorthetwo-phaseoilandgasflowregime.However,hydratecanbeformedina single-phasegasafterseparationfromtheothertwophasesofwaterandoil.Thusagas dehydrationunitcouldbeusedontheplatformofanFPSOtoseparatethesaturated waterfromthegastopreventhydrateformationthroughtheinjectionofmethanolor glycolintothegasservice.Thereforeseparationofmultiphaseflowontheseafloor cansignificantlyreducetheriskofhydrateformationinthegasservice. Fig.1.4 illustrates hydrateformationinathree-phaseflowinanonstraightpipingsystem.
Crudeoilcancontainwaxorparaffinwaxesthatcanbecomedepositedinthepiping systemespeciallyatlowtemperatureandreducethepipingflowcapacityorblockthe piping.Injectionofawaxinhibitorintothehydrocarboninthesubseacanmitigate theriskofpipingblockageandflowreductionduetowaxaccumulation.Thusitcan beconcludedthatsubseadevelopmentandprocessingcanreduceflowassuranceproblemstoalargeextent.Thereforesubseadevelopmentcanalterthestrategyforthedevelopmentofmarginalfields.Marginalfieldsrefertofieldsthatarenoteconomicalto
Fig.1.3 Flowlineandrisertransportationsystem.
developandproducesincetheycannotprovideacceptablerevenue.Manyfieldsonce consideredmarginalcannowbeconsideredeconomicalduetosubseadevelopment.
Fig.1.5 illustratesasubseaseparationtechnologythatleadstotheseparationofoil, gas,andwaterorjusttwophasesofoilandgas.Subsea-producedoilandgasaretransportedtothetopsideplatformseparatelythroughoilandgasflowlines.
Themainchallengesindesigningasubseasystemarethatofdealingwithhighpressureanddeepwater.Inconclusion,thebenefitsofsubseadevelopmentandengineering aresummarizedasfollows:
- Subseaengineeringandprocessingmayeliminatetheneedforhavingaplatformora ship;
- Ifaplatformorshipisrequired,alessexpensiveonemaybeused;
Fig.1.4 Hydrateformationinthree-phasefluidinanonstraightpiping. (Courtesy:Elsevier.)
Fig.1.5 Subseaseparationandtransportation. (Courtesy:Springer.)
- Oilandgasfieldsdiscoveredindeeperandmoreremoteareasaremoreeconomicalto developandproduce;
- Theoilrecoveryfactorandproductionratecanbeincreased;
- Flowassuranceimprovement;
- Potentialcostsavingandimprovementofprojecteconomicaspects.
1.2Subseaproductionsystems
Subseaproductionsystemsarelocatedontheseafloorratherthanonthesurface. Asubseaproductionsystemcontainsthewholeprocessandallthefacilitiesusedfordrilling,wellcompletion,fielddevelopment,andproduction.Atypicalsubseaproduction systemincludessubseacompletedwells,subseawellheadsandproductiontrees,subsea manifolds,subseatie-instoconnectthepipingtoanotherpipingsystemorafacility includingpipelineendsandinlinestructures,aflowlineandsubseaequipmentsuchas aboostingpump,compressors,achemicalandhydraulicinjectionsystem,anumbilical andaproductioncontrolsystem. Fig.1.6 illustratesatypicalsubseaproductionsystem.
Someofthesubseaproductioncomponentscanbecategorizedas“subseaprocessing” components.Processingreferstothetreatmentoftheproducedfluidfromthereservoir. Apetroleumreservoiroroilandgasreservoirisasubsurfacepoolofhydrocarbonscontainedinporousorfracturedrockformations.Subseaprocessinginvolvesdifferent
Fig.1.6 Typicalsubseaproductionsystem. (Courtesy:Elsevier.)
activitiessuchasseparation,pumping,compression,filtering,etc.whichareexplained laterinthischapter.
Someofthemainsubseaproductioncomponentsandstructuresarehighlightedin Fig.1.7.Duetothehighpressureandsometimeshightemperature,aswellastheharsh subseaenvironmentandtheexternalpressurefromtheseawatercolumn,subseacomponentsandequipmentareexposedtohigh,complex,andcriticalloadcases.
1.2.1WellheadandChristmastrees
Awellheadtransferstheloadsfromthecasinginthewellandfromthecompletedwellto theseabed.Italsoprovidesapressurebarrierandaccesstothewellboreandproduction tubing.Asubseawellheadprovidesstructuralandpressureintegrityforthewellcasing andtubing.Inshort,asubseawellheadcansupportcasing,atubinghanger,aproduction tree,andotherdrillingandcompletioncomponents.Subseawellheadsaredesignedbased onAPI17D/ISO13628-4standardsrequirements.Differentconsiderationsshouldbe consideredforwellheadselectionanddesignsuchastheloadstransferredbythetubing hanger,annularinjectionrequirementssuchasgasliftorwaterinjection,specialmaterial selectionforsubseawellheadsandinstrumentation.Definitionsforthecasingandtubing hangerwillbeprovidedlaterinthischapter.
Asubseawellincludesdownholewellcompletioncomponentsaswellassubseatrees. Wellcompletionreferstoaprocessofmakingawellreadyforproductionandinjection intothewellafterdrilling.Infact,adrilledwellshouldbeclosedwithapiping,fitting, andvalveassemblyknownasaChristmastree(see Fig.1.8)togatherthereservoirfluid fromthewellheadanddirecttheproducedhydrocarbontotheproductionfacilities.
Fig.1.7 Subseaproductioncomponentsandstructures. (Courtesy:Springer.)
ThusaChristmastree(orX-masstree)istheinterfacebetweenthecompletedwelland theproductionfacilitiesandstructures,i.e.,aconnectionbetweenthewellheadandthe flowlineormanifold.ItshouldbenotedthatthefunctionofaChristmastreeismorethan aconnectionbetweenthecompletedorfinishedwellandproductionfacilities.Other functionsofasubseaChristmastreearesummarizedasfollows:
- AsubseaChristmastreeisapressure-containingbarrierbetweenthewellandthesubseaenvironment;
- Itprovidescontroloftheproducedfluidthroughachokevalve;
- Itallowsinjectionofwaterorgastothewellforimprovinghydrocarbonproduction alsocalled“advancedoilrecovery”process;
- Allowsoilwellintervention,i.e.,maintenance,repair,andreplacementtasksonthe wellandshuttingdownthewell;
- Provideshydraulicinterfacefordownholehydraulicallyoperatedcomponentssuchas subsurfacesafetyvalves;
- Provideselectricalinterfacefordownholeequipmentandinstrumentationsthatwork withelectricity;
- Providesaccesstotheannulusforcontrollingthewellandothertaskssuchasinjection tothewell.
Subseawellscanbesplitintotwocategories:satelliteandclusteredwells.Asatellitewellis asinglewellthatsharesaminimumnumberoffacilitieswithotherwells.Whenseveral wellsareconnectedwithcommonsharingfacilities,theyareconsideredclusteredwells. ItisimportanttonotethatChristmastreescanbeinstalledonland—thesearecalled dryorsurfacetrees—landedontheseabed;theseareknownaswetorsubseatrees.
Fig.1.9 illustratesadryorsurfaceChristmastree.Christmastreescanbeverticalor
Fig.1.8 Subsea(wet)Christmastree.
horizontal(see Fig.1.8)withthepossibilityofaccesstothewellborefromthetoporside, respectively.Thedifferencebetweenahorizontalandverticaltreeisweight,size,and configuration.Thesizeofthewellisequaltothenominalboreorinternaldiameter ofthewellhead,normallymeasuredininches.Asanexample,18 ¾00 isverycommon sizeforawell.Traditionally,16 ¾00 wellboresareusedforproducingthehydrocarbons fromthereservoir.ThepressureratingofthetreecouldbeoneoftheAPI6A/API17D standardpressureclassessuchas5000psi,10,000psi,15,000psi,or20,000psi.ThetemperatureratingofaChristmastreeisbasedonthetemperatureclassesgiveninAPI6A/ 17Dintherangeof 60°Cto121°C.Thesetemperatureclassesaredefinedin Chapter2 ofthisbook.ItisimportanttoconsiderthatthetemperatureratingforaChristmastree canbeoutsidethetemperaturerangesgivenintheabove-mentionedstandards.
AsubseaChristmastreetypicallyconsistsofthefollowingcomponents:
1. AtreeconnectorthatconnectstheChristmastreetothewellhead;
2. TheChristmastreebody,alargeforgingandpressure-containingcomponentthat containstheproductionflowpathinthecaseofasinglebore,orproductionflow pathorannulusflowpathinthecaseofadualboredesignandselection;
3. TreevalvesthatmaybeintegralwiththeChristmastreeorbolted.Thesevalvesare locatedontheproductionboreinthecaseofsingleboretreeconfigurationorlocated onboththeproductionandannulusinthecaseofdualboreChristmastree configuration;
4. Controlcomponents,i.e.,actuatorsinstalledonvalvesintheChristmastree;
5. Achokeusedforregulatingandreducingthepressureoffluidcomingoutofthewell;
6. Thetreecapwhichisusedtoblindoffthewellhead.Thecaponthetreeshouldbe protectedagainstdroppingobjects;
Fig.1.9 SurfaceChristmastree.
7. Thetubinghangerwhichisplacedandsitinthewellheadandthetreeisconnectedto thewellheadaftercompletion.Thetubinghangertransferstheloadsfromthecasing totheseafloor.Inaddition,thetubinghangerprovidesstructuralsupportforthetubingandprovidesapressureandfluidbarrier. Fig.1.10 illustratesawellheadassembly includingatubinghanger.Differenttechnicalparametersshouldbeconsideredfor tubinghangerdesignandselectionsuchaspressurerating,numberofbores,andoperationalconsiderationsuchasloads.
8. Tubingisnotapartofthewellheadbutitisanimportantpartofthewellcompletion. Tubingcostcanbeashighas20%ofthewellheadcost.Failureoftubingcanbevery costlyandleadtolossofproductionaswellasinjuriesandlossofoperationlife.Load analysisanddeterminingthemechanicalstrengthofthetubingmaterialareessential aspectsoftubingdesign.Tubingisexposedtocombinationsofbothinternaland
Lower master valve
Tubing-head adapter
Tubing hanger
Tubing head
Production tubing
Casing bowl or spool
Casing hanger
Port for casing valve
Fig.1.10 Wellheadassemblyincludingtubinghanger. (Courtesy:Schlumberger.)
externalloadsthatcancauseburstingandcollapseofthetubing,respectively.The loadsappliedtothetubingarenotlimitedtointernal,external,andcombination loads.Otherfactorscreateloadssuchastheweightofthetubingitself,buoyancy, thermalloads,buckling,etc. Fig.1.11 showshowthecombinationeffectsofinternal andexternalpressuresaffecttubinglengthandfeatures.
9. Afoundationisapplicableforallsubseastructuresincludingthewellhead. Afoundationisatemplatethatwithstandstheweightandotherloadsofthewellhead. Theschematicsontheleftin Fig.1.11 showasituationwhereinternalpressureishigher thanexternalpressure.Thetubingisgettingshortenedandsqueezedintoaballoonshape. Theschematicontherightsideindicatesasituationinwhichtheexternalpressure exceedstheinternalpressure.Thisconditionincreasesthelengthofthetubingand reverseballooningoccurs.
Bucklingoftubingisgeneratedbycompressionforcesthatleadtochangesintubing shapeasillustratedin Fig.1.12
Fig.1.13 illustratestheinstallationofasubseatree.
1.2.2WellheadandChristmastreevalves
ChristmastreevalvesaretypicallythroughconduitslabgatevalveswithROVoractuator operation.ROVstandsforRemotelyOperatedVehiclewhichisexplainedindetailin Chapter9.Oneoftheadvantagesofthroughconduitgatevalvesislessrestrictionand Fig.1.11
Top suspended section
Top and second Suspended section
Second suspended section Third suspended section
Continuous contact section
Bottom suspended section
Third and bottom suspended section
flowdropcreationduringhydrocarbonproductionfromthewell.Slabgatevalvesare normallyopenforproductionfluidpassage.Moreinformationaboutgatevalvesisavailablein Chapter4.Theactuatorsofgatevalvesarelinearfail-safeclosedorspringreturn typeinalmostallcaseswhichareexplainedmoreindetailin Chapter9.
Fig.1.15 illustratesadualboreverticalsubseatreewithdifferentvalves.Dualbore wellheadconfigurationallowsaccesstobothtubingandthetubingandproductioncasing
Tubular string
Wellbore
(a)(b)(c) (d) (e) (f)
Fig.1.12 Tubingbuckling.
Fig.1.13 Installationofsubseatree. (Courtesy:Oceaneering.)
annulus.Anannulusisdefinedasanareabetweenthetubingandthecasing.Thecasingis alargediameterpipethatisassembledandinsertedintothenewlydrilledwelltoprotect thesideofthewellfromcollapse. Fig.1.14 illustratesthedifferentkindsofcasingand productiontubing.
Usingadualborewellismorereliablethansingleboreandithassomeadvantages.For example,thepressurebetweentheproductiontubingandcasingcanbecirculatedand bledoff.Themainboresizecanbestandardizedto500 butsmallersizessuchas300 or400 can beused.Theannulusborecanbestandardizedtoa200 size.Eachvalvelocatedonthedual boreChristmastreeisexplainedasfollows:
MastervalveslocatedatthebottomoftheChristmastreecontroltheflowofthe wellbore.Sincethefunctionofthemastervalveisimportant,acoupleofmastervalves aretypicallyinstalledinthewellhead;oneactsasabackupfortheotheroneincaseof leakageandforrequiredmaintenance.Asmallerboremastervalveislocatedontheannulustocontroltheflowintheannulus.Thevalvelocatedontheverticallinein Fig.1.15 abovethelowestmastervalveiscalledunderwatersafetyvalve(USV)incaseoflocation ofwellheadunderthewaterwhichishydraulicallyactuatedwithafail-safeclosefunction toprotecttheproductionfacilitiesifafailureordamageoccursinthewell.Thisvalveis connectedtoanemergencyshutdown(ESD)systemandisclosedrapidlywhennecessary.SwabvalvesinstalledonthetoppartoftheChristmastreeprovideaccesstothebore forinterventionsuchasmaintenanceorcleaningofthewell.Asmallerswabgatevalveis locatedontheannulusbore.WingvalvesarelocatedonthesideoftheChristmastree. Aproductionwingvalveisusedtoisolateandcontroltheproductionlineandakillwing
Condector Casing
valvelocatedontheoppositesideofthewellisusedforwelltreatmentorcontrol.Crossovervalvesallowcommunicationbetweentheannulusandproductionbore.Eachbore ofthewellindualborewellcompletionprovidesanindependentbarriertoflow.
1.2.3Manifolds
Subseamanifolds(see Fig.1.16),whicharearrangementsofpiping,valves,connections, structures,andafoundation,areusedinsubseaproductionsystemsinordertoreceive, combine,anddistributehydrocarbonfluidandsimplifythesubseasystembyreducing thequantityofpipingsuchaspipelinesandflowlines.Pipingandvalvesareresponsible fordirectingandcontrollinghydrocarbonflowandstructuresareusedtoprotectandsupportthepipingandvalveslocatedinsidethemanifold.Eachpipingheaderistypicallyconnectedtoaflowline.ThesizeofthepipingdependsontheChristmastreeandthesizeof theconnectedflowline.Foundationsprovideaninterfacebetweentheseabedandmanifold structure.Thevalvesonthemanifolddirectorstopthefluidandcanbeeithermanualor actuated.ManuallyoperatedvalvesareoperatedwithROV.Themaintypesofvalvesused inmanifoldsarethroughconduitgate(TCG),ballandaxialflow,ornozzlecheckvalves.In general,ballvalvesareselectedforheadersinsizesabove800 andTCGvalvesareselectedfor branchesorheadersin800 andbelow.Smallboremanuallyoperatedballvalvessuchas ½00 and ¾00 areusedforchemicalandhydraulicinjectionlinescomingfromthesubseadistributionunit(SDU).Axialflowornozzlecheckvalvesarenonreturnvalvesinstalledonthe
Tubing
Fig.1.15 Dualboreverticaltree.
chemicalinjectionlines.Thechoiceofanactuatorformanifoldvalvesmaybeeither hydraulicorelectrical.Theactuatorsmaybespringreturn,alsocalledsingleacting,with bothfail-safecloseorfail-safeopenfunctions;ordoubleactingwithafail-as-ismodeof operation.Doubleactingactuatorsarenormallyselectedforlargervalvesinsizessuchas 1600 or2200 andhigherpressureclassesduetothecompactdesignandhightorqueproduction.Mostmanifoldvalvesarenormallyopen.Moreinformationaboutactuatorsisavailablein Chapter10.Manifoldvalvesandactuatorsshouldbedesignedtowithstandhigh pressureandoperateunderthehydrostaticseawaterhead.Installingapressurebalance orcompensationsystemontheactuatorcanmakeitsuitableforuseindeep-waterdepths.
Fig.1.17 illustratesthevalvearrangementinasix-slotsubseamanifoldthatincludes threepiecesoflargebore1600 ballvalvesontheheaders,anda71/1600 slabgatevalveon eachbranchconnectedonthebranchpipingin800 nominalpipesize:
Thematerialofthepipingandvalvesshouldbecompatiblewiththeproducedhydrocarbonaswellastheinjectedchemicals.Manifoldscancollecttheflowfromdifferent wellsorinjectwaterorgasintodifferentwellsforadvancedoilrecovery.Manifolds mayhavetwo,four,six,oreightslots.Thenumberofslotsisequaltothenumberof thewellsconnectedtothemanifold.Asanexample,six-slotmanifoldshavesixbranches thatcanbeconnectedtosixdifferentwellheads. Fig.1.18 illustratesafour-slotmanifold connectedtofourwellheadsthroughfourbranchlines. Fig.1.19 illustratesasix-slotmanifoldconnectedtothreewellheadsfromonesidethroughthreebranchpipingconnections,andthreemorewellheadsontheothersidethatarenotvisibleinthepicture.
1.2.4PLETandPLEM
Inadditiontotheirotherfunctions,subseamanifoldsmayalsoprovidesubstructuralsupportformorethanonepipe-to-pipeconnectionortie-in.Suchmanifoldsareknownas PLEM(PipelineEndManifold).ThereforeaPLEMisasubseastructure(asimple
Subsea valve
Subsea piping
Fig.1.16 Subseamanifolds.
Gate valve on the branches
Fig.1.17 Valvearrangementonasix-slotproductionmanifold.
Ball valves on the headers
4 wellheads connected to manifold branches
Fig.1.18 Afour-slotmanifoldschematic.
3 manifold branches (not visible)
3 manifold branches manifold headers
Fig.1.19 Asix-slotmanifold.
manifold)placedattheendofapipelinetoconnecttherigidpipelinewithanotherstructureoradditionalpiping.Asanexample,subseaflowlinescanbeconnecteddirectlytoa PLEM.Infact,pipe-to-pipeconnectionsinsubseadonothaveanysubstructurefor support,unlikeconnectingapipetoanotherpipeinsideastructurewhichisknown asatie-instructure.ThereforeaPLET(PipelineEndTerminal)isrequiredforasingle pipe-to-pipeconnection,e.g.,connectinganexportpipelineandaspool,orconnectinga flowlinetoamanifoldandriser.APLETisknownasapipelineendortie-instructure.In fact,pipelineorflowlinecannotbeconnecteddirectlytoamanifold.ThereforethepipelineisconnectedtoaPLETandapipespoolorjumperisinstalledbetweenthePLETand manifoldasillustratedin Fig.1.20. Fig.1.22 illustratesafour-slotmanifoldconnectedto twoflowlinesthroughtwoPLETs.Inaddition,aPLETcanbeusedtoconnectthe flowlinestotherisers.Theflowlinecanbeconnectedtothemanifoldindeepwaterwith differenttypesofconnectorsasfollows:
- API6AflangeswithSBXorSRXgaskets(moreinformationaboutAPI6Aflangesand gasketsisavailablein Chapter2);
- HubsandclampsaccordingtoAPI16A(moreinformationabouttheseconnectionsis availablein Chapter2);
- Proprietarymechanicalconnectorssuchasthecolletstyleconnectorsillustratedin Fig.1.21;
- Multiboreconnectors.
Apiggingloopontheleftsideofthemanifoldin Fig.1.22 providesthepossibilityofapig (pipingorpipelineinjectedgadget)runningfromthehostfacilitysuchasaplatform throughoneflowlineallthewaytothemanifoldheaderandreturningbackfromthe manifoldtoanotherflowlineandfinallyreturningtothehostfacility.Suchagadget mayberunthroughthepipingorpipelinefordifferentpurposessuchascleaningand inspection. Fig.1.23 illustratesapigusedforcleaningthepipingsystem.
Fig.1.21 Colletstyleconnector. (Courtesy:Cameron/Schlumberger.)
Fig.1.22 Afour-slotmanifoldandtheconnectiontothePLETs. (Courtesy:Subseapedia.)
Apiggingloopisnormallyrequiredforexportmanifoldsconnectedtoflowlinesand risers.Infieldmanifoldswhicharelocatedupstream(before)fromtheexportmanifolds areconnectedtodifferentwells.Let’sclarifyinfieldandexportmanifoldswithone example.
