MARINE PIPELINE PROTECTION WITH FLEXIBLE MATTRESS

8/12/2019 MARINE PIPELINE PROTECTION WITH FLEXIBLE MATTRESS

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MARINEPIPELINEPROTECTIONWITHFLEXIBLEMATTRESS

Alan D .CrowhurstMaccaferriGabions,Inc.Williamsport,Md . USA

AbstractDevelopmentofoilandgasfieldsoffshorepresentsnewproblemsinthedesignofthecivilengineeringworkssinceinmanycaseslittleisknownaboutprevailingconditionsonth ebedoftheseaovertheconsiderablelengthsinvolved. Movementofpipelinesafterconstruction,is,therefore,no tuncommonandmayresultinthefailureoftheline. Consequentlossescanbeconsiderable.Pipelinesarealsovulnerabletomechanicaldamagebyanchorsandbyfishingactivities,th elattergivingrisetopoliticalproblemswhereth epipelinesar elaidacrossexistingfishinggrounds. Togiveprotectiontooilan dgaspipelinesaswellastomarineout-fallsandcablesamethodofprotectionhasbeendevelopedusingthewellknownconceptofrockfilledwiremeshorgabions.Toadaptthismethodto useinunderwaterworksanother wellprovenmaterial,sandmasticasphalt,hasbeencombinedwithth egabionmattressestogivetherequiredqualitiesofstrength,durability,flexibilitytogetherwithsufficientweighttorestrainmovementinthepipeline.

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24 4OASTALENGINEERING 982INTRODUCTIONAlthoughpipelineshavebeenbuiltinthetidalzoneandforshortdistancesoffshoreformanyyears,earlierworkhasbeenlargelyinthesewagedisposalandcoolingwateroutfallsectors. Inworksofthesetypesfailures,althoughserious,aregenerallycapableofrepairbyconventionalmeans.Offshorepipelinesforoilandgastransmission havenowbeenlaidinmuchdeeperwaterandfo rmanymilesatconsiderabledepths. Inaddition,inareassuchasth eNorthSea,workcan onlybecarriedoutfo rafe wmonthsperyearandfailuresthereforebecomeextremelyseriouseven beforeproductionlossesar ecalculated.Seabedconditionsoverthelengthsofthesenewpipelinesareby nomeansuniformorconsistent,no rissurveyinformationascompleteasthedesignerswouldlike. Overundulatingterrainpipelineslaidonth esurfacemay wellbridgeacrossmanyofth evalleys.Strongcurrentscanleadtomajormovementsofpipelineswhileinservice,unlesssuitablyanchored,andcanalsogiverisetovibrationswhichmayshortenth elifeofth epipes. Similarprotectionmethodsusedtoanchorpipelinesmayalsobeemployedtoprotectagainstmechanicaldamagebytrawlsorbydragginganchors. Sincepipelinesmaycrossvaluablefishinggroundsitisnecessarytoensurethattheseprotectivemeasuresdono tcauseadditionaldamagetothetrawlsandnets.Althoughmorefamiliarasariverdefensesystemth euseofwire-enclosedriprap,gabions*hasbeenadaptedtomarinestructuresofmanytypes. Themainchangeisth eadditionofaPV Csleevetoth egalvanizedwiremeshtogiveresistancetocorrosioninsaltwater. Gabionshavebeenpre-filledan dusedforoutfallprotection,jettiesininshorewatersandvariouscoastaldefenseprojectsworldwide.Amethodofprotectionandanchoragehasbeendevelopedknownasth eSarmacmattresswhichcombinesthisancientandwellprovengabionmethodofconstructionwithsandmasticasphaltgrouttogiveadenseyetflexiblemattresscapableofbeingplacedoverarelativelysmalldiameterpipewithoutfailure. Th eunitsca nbemadewithsufficientlengthoneithersideofth epipelinesoastogiveconsiderablepro-tectionagainstscoursincetheyfolddowntoaccommodatechangesinbedlevelsoverth eservicelifeofth epipeline.Considerablecarehastobetakeninthedesignofth esandmasticasphaltmixsinceth erequiredflexibilitymayhavetobemaintainedoveralargetemperaturerangebetweenthatatth eassemblyyardandthatatlayingdepth.


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MARINEPIPELINEPROTECTION4 5Methodsofplacingar ealsocriticalinviewofth ehighcostofemployingdivers. Fo rarecentprojectinth eMediterraneanspecialliftingframesweredevelopedbyth esubcontractor,Ing.SartiGiuseppeSpAofRavenna. Thisenabledplacingtobecarriedou tindeepwaterbyremotecontrolmethodsusingaremotecontrolvesselguidedbytelevisioncameras. TodateSarmacmattresseshavebeenplacedsatisfactorilyatdepthsofupto60 0metres.MATERIALS METHODSWiththeintroductionofth eSarmacmattressintoth eoffshorepipe-lineprotectioninventorytherehasbeenaninterestingcombinationofancientandverymodernmethodsofconstruction.Gabionsar eofconsiderableantiquityhavingoriginsinmilitaryusesseveralcenturiesag oand,itisthought,inhydraulicworksinChina,much,muchearlier. Ofcoursetheseearlygabionswereno tmadewithmetalbutusedwickerworkfilledwithstones,soilandturf. Fromtheseoriginsgabionsweredevelopedusingsteelwireinth eformofcylindersofmesh,filledwithrock,usedforemergencyriverprotect-ion.ateri twasrealizedthati twouldbepossibletomanufactureth egabionsinrectangularformthusenablingmuchlargerandmoreuniformstructurestobebuilt.Afurtherdevelopmentcameinth euseofwoven meshwithadoubletwistforaddedsecurity. Fromtheserectangularformsweredevelopedmattresseswhichweresimplygabionswiththicknessesrelativelysmallcomparedwiththeirlengthand width. Galvanized wiremayhavealimitedlifeexpectancyinhighlycorrosiveconditionsandth eaddit-ionofaPVCsleeveextrudedontogalvanizedwireenableth emeshtobeusedincoastalan doffshorelocations.Foruseinth etidalzoneinvestigationshavebeencarriedou tonth euseofPVCcoatedwovenwiremeshmattressesusedascontinuousrevet-mentsbyanumberofresearchersinCanada,Australiaandelsewhere,( 2 ) ,withgenerallyfavorableresults. Inonecasedesignruleshavebeenformulatedgivingequationsforeventualfailurebydownslopesliding,bucklinganduplift( 3 ) .Fo rslopingrevetmentswireenclosedriprapormattressesar eno tnormallylaidatinclinationsgreaterthanIVto2 Hor2 6wheredown-rushwaveslimitthestonesize. ThishasbeendescribedbyStephensonwhohaslistedcomparisonswithroundedrock,angularrockandalsoconcreteunits(4).Proprietaryconcreteblocksystemshavebeentestedinth ewavezonealongwithother low-cost systemsincludinggabionsandmattressesinacomprehensivestudybytheU.S.ArmyCorpsofEngineers( 5 ) .


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24 6 COASTALENGINEERING 982

FIGURE 2

GABIONEFORESSEMBLY ASSEMBLYFABION

ENOOrHEIKALIR EONNECTING IMtARllTlONO tASEAND

BE S7BEKHEO-OVT IP fitlTION N L

USEHEHUKAlimcmriCHED-OUTTO iA .THE RTITION Mt

FIGUREM TTRESSYPE IONS


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MARINEPIPELINEPROTECTION4 7Aton elocation,Ninilchik,inAlaskath egabionswerefilledbymeansoffilterclothbagswhichwerethemselvesfilledwithacombinationofbeachgravel,cobblesandconcreterubble. HoweverboththisstructureandasecondatKotzebuealsoinAlaskahadarockriprapto eprotection whichwasdisplacedduringth etestperiod. Ingeneralthisisno tagoodpracticesinceth eriprapca nbethrownagainstthemeshcausingdamage. HoweverthereportonthesetrialsandathirdatOak Harbor,Washingtonwasveryfavorable.Gabionsand mattresseshavealsobeenusedfo rmanyyearstoformgroynesasamethodofcounteringbeacherosioninth eUnitedKingdom,Italyandelsewhere( 3 ) . Sandmasticasphaltgroutedriprapgroyneshavealsobeenusedwithlimitedsuccessinsomelocationsbutgivingrisetoproblemswhenextendedbeyondthelo wwatermark( 6 ) ( 7 ) Thesematerialshavebeencombinedinrevetmentsandgroynesforcoastaldefensesandhavebeensuggestedforanumberofrelatedusessuchasoffshorebreakwatersorheadlands.Theuseofpre-filledgabionsforunderwaterstructureshadbeendevelopedovermanyyearsfromth eoriginaluseofcylindricalgabionstofillbreachesinriverdefensesan dscourholesaroundbridgepiersandinsimilaremergencysituations.Itwassoonrealizedthatmodernrectangulargabionscouldbeusedinasimilarfashionandgabionshavesubsequentlybeenusedpre-filledtoformmanytypesofstructures(Fig.4). ExamplesincludedalargelandingstageorjettyatPulaLangkawiinMalaysia,seawallsinHelsingforsHarbor,inFinland(Fig.5).AstructurewasbuiltsomefiveorsixmetreshightoprovideaquayfordhowsatRuweisinQatar,the.gabionshavingbeenpre-filledoutinthedesertandtruckedtothesite.Thesestructuresemployedcranestoplacetheunitseitherfromdrylandorpontoonsorbargesbutinrecentyearsplacingha sevenbeencarriedoutusinghelicopters.Howeverforoffshorestructuresofsomesizeothermethodswereadoptedwithgabionswiredtogetherintoth eformofaraftconstructedabovelow watermarkandsubsequentlyfloatedintopositionusingbuoyancysupportsofseveraltypes. Severaloutfallstructureshandlingcoolingwaterfrom nuclearpowerstationsweresoprotected.Whilemethodsofplacementforstandardgabionswerebeingdevelopedsimilarattentionw a , sbeingpaidtoenableRenoorRevetMattressest obepre-filledforplacingunderwaterinriverandcanalprojects.Althoughliftingframessimilartothoseusedforplacingfilledgabionswereemployed,othermethodswerealsoinitiated. Thesein-cludedlaunchingfrom pontoonswhichatfirstweresimplefixeddecks(Fig.6) .


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24 8 COASTALENGINEERING 982

FIGURE4SIMPLELIFTINGMETHOD

FIGURE5PLACINGGABIONSBYCRANE

Firm anchorageby M S bars or t imberstakes

FIGURE 6PLACINGPREFILLEDMATTRESSES FROMASIMPLEP O N T O O N


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MARINEPIPELINEPROTECTION 2409

Pontoonswerethendevelopedwithtiltingdecksupon whichmattressescouldbebuiltandthenlaunched*theinshoreendsbeingsecurelyanchoredtoth ebank.Thisformofbankprotectionwasalsousedinareaswhereimpermeabil-it ywasnecessary. Fo rthispurpose,ofcourse,gabion mattresseswereunsuitableifusedalone.Th eadditionofasand masticasphaltgroutprovidedth enecessaryimpermeabilitywhilstretainingtheessentialflexibilityoftherevetments.Th eus eofpre-filledandgroutedmattressesfo runderwaterapplicat-ionswhereconstructioninth edry wasno tfeasibleemployedpontoonsasdevelopedforregularmattresseswiththegroutingoperationactuallycarriedou twithth emattressesalreadyinpositionondeck.Whenusedwithagroutthethicknessemployedcouldbereducedfo rsomestructures.Pre-filledmattressescouldalsobeplacedbycranewhengroutedwithsandmasticasphaltsincethestonefillwasheldinplacebyth egroutandno tpermittedt o - falltothebottomofeachcellaswouldhappenifanordinarymattresswassolifted( 8 ) ( 9 ) ( 10 ) . ForoffshoreworksgabionandRenoMattresseshadbeenusedfrequentlysincemethodsofplacing wererelativelyexpensiverequiringtheuseofdiversin manycases.HoweverprotectionwasgiventothecoolingwateroutfallatanuclearpowerstationonthesoutherncoastofEnglandusingaraftofMaccaferriGabionsfloatedintopositionan dthenloweredaroundth emouthofth eoutfallwhichwasbuiltintunneloutfromthepowerstat-ion,butthismethodwasno tapplicabletomosttypesofundwaterstructure.

Angleofplatform Incl inaisonduplanNeigungd erEbene

FIGURE 6


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241 COASTALENGINEERING 982

SANDMASTICASPHALTSandmasticasphalthasbeenusedincreasinglyingabionandmattressstructuresofvarioustypesrangingfromcanallinings,powerstationoutfalls,protectionofth eupstreamfaceofearthdamsan dforspill-ways. Whenusedforcanalliningsth ecombinationofth etw oproducts,gabionandsandmasticasphalt,enabledconsiderablereductionsinthicknesstobemadeinsomecases. Thedesignothesandmasticasphaltmix variesaccordingtoth erequireddesigncharacteristics. Informationonsuchapplicationsiswelldocumented(11).

WEIGHTOFBITUMINOUSMASTICTYPE THICKNESSINCH SURFACEGROUTING COMPLETEPENETRATIONLBS/SQ.FT. LBS./SQ.FT.

RevetMattress

0 '-4 14-15 22-2 60 '-6 16-18 26-280 '-9 20-2 5 28-3 0

Gabions 1 '-0 27-30 37-401 '-6 3 0-40 50-603 '-0 50-70 80-12 0

TABLE1EIGHTSOFGROUTSARMACMATTRESSES

Fromtheseoriginswasdevelopedth eSarmacmattresswhichcombinedth estrengthandflexibilityofth eRenoorRevetMattressformofconstruc-tionwithth eadditionofth eweightandimpermeabilitygivenbytheadditionofth esandmasticasphaltgroutan dwhileretainingth enecessaryflexibilityofth emattressconcept.Whenusedforhydraulicstructuressuchasriverandcoastaldefenses,damsan dcanalsthedesignofthemasticasphaltmixwassubjecttoratherlessvigorousconstraintsthanwhenappliedtoth eSarmactech-nique. Iti sonlynecessarythatth easphaltcanpenetrateandflowthroughth egabionsorripraptoth edesigndepthandremaininplacewithoutfurthermovementshouldambienttemperaturesincreasefromthoseexperiencedduringth econstructionperiod. Th erangeoftemperatureduring whichsuchgroutingtakesplacean dtowhichth ecompleted worksar eexposedisusuallywelldocumentedandth erangeno tto ogreattomakeadditivesnecessary. Whenmasticgroutedmattressesar eplacedfrompontoonsorbycranetheirthicknessesarerelativelysmallandflexibilityremainsgood.


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MARINEPIPELINEPROTECTION41 1Withth enewpossibilitiespresentedbyth econstructionofoffshorepipelinesofconsiderablelengthandsize,th etechniquesdescribedinth eprecedingparagraphs werecombinedtogiveamaterialwithth erequirecharacteristicsand,asimportant,th eabilitytobeplacedeconomicallyatconsiderabledepths.Atth edesignstageofsuchpipelinesi thasoftenbeennecessarytoplacerelianceonclimaticdataandtidalinformationfromavailablesourceswhich wasnotalwayscomprehensiveenoughforth epurpose.Inparticular,althoughinformationmightwellbeapplicableforsur-faceconditions,thisdidno tapplyt othoseexistingonth eseabedwherestrongcurrentscouldbemet. Thesecouldresultinscouringaroundandunderpipelineslaidonth esurface.Asaresultofsuchscouringactionpipelinesmay wellbecomeentirelyunsupportedandthiscanevenoccurwhereinitialconstructionwasasburiedpipelines. Th eresultingeffectsarisingfrom unsupportedlengthsonfatiguelifehavebeenassessedinanumberofstudies(l).Experiencegainedinthedesignandconstructionofpipelineshasresultedinth edevelopmentofamethodofbothanchoringandprotect-ingsuchpipelines.Th emethodwasalsocapableofuseinth econstr-uctionofsupportsunderth epipeswhichcouldthusreduceth elengthofunsupportedspansandcouldreducevibratoryeffects.Sincetheseabedinth evicinityofthepipelinescouldvaryconsider-ablyitwasadvantageoustodevelopth emattressusedforitsprotect-io nwithcapabilitytofoldinservicet oadjusttothechanginglevels.Flexibilitywasalsonecessarytotakeupthecurveofth epipewhichcouldbeassmallas5 0 0mmdiameter.Considerableweightwasalsocalledforinordertofunctionasanchorsandthereforeth ecombinationofsandmasticasphaltandgabionsormattressesalreadyfrequentlyusedincombinationinhydraulicstruct-uresofseveraltypes,wasalogicaldevelopment.Th eS a r t n a cmattresswasdevelopedtoaddressseveralofth eproblemsexperiencedinth erapidconstructionofthemassivenetofoilandgaspipelinesthatnowcrossvastexpansesofseasandoceans.RenoorRevetMattressesareusedasth ebasisofadense,flexibleunit,usingstonefillinth eusualwaybutwithstonesizeselectedtopermitth emostsuitablepenetrationbyth esandmasticasphaltandtogivetherequiredfinished weight. Choiceofsingleordoublethick-nessesismadeaccordingtositeconditions.Thesandasphaltmixisdesignedtogiveth erequiredflexibilityatth eambienttemperaturesinwhichitistobeplaced,butatth esametimemustno tbesosoftastodeformorevenflowduringmanufactureortransporttosite. Iti sequallyimportantthatitshouldnotbecometo ohardthuslosingflexibilityinserviceandriskingfailurebybrittleness. Sincepipelinesar ecurrentlybeingplacedincondit-ionsasdiverseastheNorthSeaandth eIndianOcean,mixdesignsmustbeselectedwithcareforeachapplication.


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PENETRATION R. &B .SOFTENINGPOINT DUCTILITY

~VOLATILITYAFTER5HOURS

AT3 2 2 F8 0/1 0 0Pen . 3 1 4- . 3 4 4 12 5-1 4 0F 3 9 , 3 7Inch 0 . 5 %

60/70 . 2 3 4- . 27 5 115-1 2 9 F H 0 . 2 %4 0/5 0Pen .157-.196 12 4-1 36 F 0 . 2 %

BITUMENSPECIFICATIONS

Havingselectedth ecorrectmixdesignandthickness,overalldimens-ionsar edesignedtosuitth econditionsofuse,i.e.formattressesusedasprotectionagainstscour,asanchorswhereth epipelinei s liabletomovelaterally,asdeadweightstoconstrainth epipelinewhereitisnotlaidcontinuouslyonth eoceanbedandasprotectionagainstmechanicaldamage.Fo rplacingunderwaterindepthsofupto60 0metresormoreth eus eofdiverstolocatean dplacetheSarmacunitsmay beimpracticableorveryexpensiveandmethodshavebeendevelopedbywhichth eunitscanbelocatedinpositionan dreleasedbyremotecontrolvesselswhilstbeingmonitoredonclosecircuittelevision.Fo rsuchunderwaterstructurestemperaturerangesareofteneven morerestricted. Placinggenerallytakesplaceduringth ewarmermonthsalthoughworkwasrecentlycarriedoutonaprojectinCumbriainNorth WestEnglandduringaverycoldperiodfromNovember'81toJan-uary'8 2whenth etemperaturesrarelyroseaboveSelsiusandmuchofth egroutingwascarriedou tatmuchlowertemperatures.Thisrequiredthatth etemperatureatth emixerwasatorverynearth emaximumforth ematerialinorderthatreasonableflowscouldbeobtainedinth everycoldmattresses.Earlierwork hadinfactbeencarriedou tbyth eFrenchinAntarticamanyyearsag oan dthoseexperienceshavebeendocumented(12).Howeverinitialstudieswereincorporatedinprojectssitedinlessintemperateareasandforth efirstfullscaleapplicationthistypeofconstructionwasusedatpointsalongth egaspipelinebeingbuiltfromAlgeriatoItalywhichhas170kilometresofitstotallengthof2500kilometresonth ebedofth eMediterranean. Thisproject,whichwasinitiallyproposedin1971,wasauthorizedin19 7 7followingnegotiationsbetweentheAlgerianauthoritySONATRACHandth eItalianENIcompany. Designworkwascarriedou twithth ecooperationofbothcompanies.


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MARINEPIPELINEPROTECTION41 3Acrossth eSicilyCanalunderwaterconstruction wascarriedoutbyaconsortiumofItalianandAlgeriancompaniesunderth etitleTMPC.Howevertechnicaldirectionwasth eresponsibilityofSNAMPROGETTI.Th edepthsatwhichpipeswerelaidinthissectorexceeded50 0metresandth epipelinewasdividedintothreeseparatelinesforsafetyreasons.Th eSarmacunitsdesignedforthispartofth eprojecthadoveralldimensionsof4metresby3metres. Eachunitcomprisedtwoseparatethicknessesof20 0mm eachgivinganoverallthicknessof4 0 0 m mjoinedtogetherwithsteelcables. Doubletwistmeshmattresseswereusedtogiveflexibilityreinforcedwithsteelwireropesforliftingpurposes.Th eunitswerewrappedinanon wovenpolyesterfilterclothofweight0.55kilogrammespersquaremetre. Polyesterwasrequiredasotherfabricsar eunabletowithstandth etemperaturesatw h i c h ,thesandmasticasphaltisplaced.Th etotalweightoftheunitswas1 2, 0 0 0kgs.or1 , 0 0 0kgs.persquaremetreandthespecificgravity25 0 0kgs.percubicmetre.Manufacturetookplaceatth eSNAMPROGETTIyardinTrapani,Italybyth econtractor,Ing.SartiGuiseppe Co.andthentransportedth eshortdistancetothedocks. Placingwasmadefromth eship,SaipemRagno2 ,formostofthework.Th eapplicationforwhichth eSarmacunitswererequiredInthispro-jectwasthatofanchoringthepipes,byvirtueoftheirweight,totheseabedacrossareaswherethebedwasirregular.AlateruseofSarmacmattressesofsimilarsizeandconstructionbutwithoverallthicknessesof5 0 0mmisbySaipeminaBritishpetroleumfieldintheNorthSea. Inthiscasetheapplicationi stoprotectonepipelinewherei ti scrossedbynew workandthetotalweightperunitand,ofcourse,thespecificgravityhasbeendesignedtobesome-whatlower.Whenth erequirementisforthickermattressesofth etypesusedforSarmacunits,typically4 0or50cmsthick,thecompositionofthemasticmixismuchmoredifficulttodesignsinceth ecompletedmattressmusthaveexcellentflexibilityi fiti stoflexaroundsmalldiameterpipesindeepwater.Inadditiontheunitsmaybemanufacturedinanareaoffairlyhighambienttemperaturesforplacingincolddeepseas. Ifth erequiredflexibilityisno tmaintained,th eunitcouldbecomebrittleandnottakeupth erequiredcurvatureinservice.Toestablishdesigncriteriaearlyexperimentalworkwascarriedou tinth eUnitedKingdombyRiverandSeaGabions,Ltd.butatanearlystageth eworkwastakenoverbyOfficineMaccaferriSpAinItalywheretherehadbeenextensiveworkwithsandmasticasphaltforawidevarietyofhydraulicworks.


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2414OASTALENGINEERING 982ThisworkproceededjointlywithIng.SartiGuiseppewithconsiderableassistancefromDr.V.CastagnettiofIndustriaItalianaPetroliofGenova. Muchofthisworkwascarriedou tsimultaneouslywithth edevelopmentofsystemsforuseinth ewideningoftheSuezCanal( 1 3 )( 1 4 ) ( 1 5 ) .Th eexperimentalworkno tonlyconcentratedonth edesignofthemixtoachieveflexibilitybu talsooncompletedthicknesses,densityandapplicationrateswhichwerethemselvesvariationsonthosealreadycalculatedandusedinhydraulicworks,seeTable1 .Th eSarmacmattressconceptalsonecessitatedth edevelopmentofmethodstosimplifyliftingfromth epointofmanufactureandtrans-portationinlandandbyseatothejobsite.Thesewere,ofcourse,relatively minorproblemssincetherewasalreadyafundofknowledgebasedonprevioususesofpre-filledand groutedmattressesan dgabions. Howeverplacingunderwaterhad pre-viouslyinvolvedth euseofdiverswhichwasnotimpracticablewhenonlyafe wunitsweretobeplacedinshallowseas.Forgasandoilpipelinesthedepthsatwhichprotection wastobecarriedou tweresuchthateffectivedivingtimepershiftwouldbeverylimitedandcostsaccordinglyvery high. Itwasthereforenecessarytodevelopunderwaterplacingmethodsusingremotecontroltechniquesandaliftingframewasdevisedwhichcouldbeoperatedfromaremotecontrolvessel.Thishasworkedsatisfactorilyandhaspermittedrelativelyfastplacingtimesalthough,unfortunately,detaileddataremainsconfid-entialtoth ecompaniesconcerned.Asimplificationofth econstructiontechnique,omittingbothth efilterclothenvelopeandth eliftingcablesbuiltintoth ebodyofth eSarmacunits,hasbeenemployedforsmallscalepipelineprotectioninestuarialareasintheNorthAdriatic.heseunits,resemblingth estandardwireenclosedriprapmattressesusedforriverdefenses,weregroutedwithamixalsobeingusedtogroutaripraprevetmentseveralyearsoldsituatedafe wmilesfromthemixingplant. Sinceth eapplication wasclosetonormalwaterlevel,therangeoftemper-aturesunderwhichth egroutedmattresses weretobeemployedwasverylimitedandth emixdesign wasthereforenotformulatedspecificallyforthatproject.Itshouldbenotedthatfe wfilterfabricsar esuitableforuseasenvelopesforgroutedmattressessincethemeltingpointofmostmembranesisatorbelowth etemperatureatwhichth esandmasticisnormallypoured.Thefinalproduct,asusedinoffshorepipelinework,isverydifferenttoearlyunderwatergabionstructuressuchasthatfora


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MARINEPIPELINEPROTECTION41 5 nuclearpowerstationinth eUnitedKingdomwhereth egabionswereassembledas araftwithinFolkestoneHarboratalocationbetween highandlo wwatermarks. Theraft,supported bysteeldrumsan dfastenedtogetherbysteelrails,wasfloatedintopositionandsunkonto pofth eoutfallposition.Thistypeofsimpleconstructionha sthereforeevolvedintoadegreeofsophisticationno tforeseenatthattimebutstilloffersaconsiderablescopeforfurtherdevelopmentparticularlyininshoreareasandinth eimmediatevicinityofplatformswherecomplexpipelayoutsaretobefound.

CONCLUSIONS

Useofthesemethodsofconstructiontoprotectpartsofth egaspipelinefromAlgeriatoItalyandalsoasmallapplicationinth eMagnusProjectinth eNorthSe ahaveshownth eeffectivenessofthismethodofconstruction. Theexperiencegainedinearlierunderwatergabionworkan dinth edesignofsandmasticasphaltforhydraulicworkshasenabledchallengesposedbythenewapplicationstobemetthusmarryingoldan dverynewtechniquesinth emanufactureanduseoftheSarmacMattress.

ACKNOWLEDGEMENT

TheauthorwouldliketoexpressappreciationforassistancereceivedovermanyyearsfromDr.Ing.R.Agostini,OfficineMaccaferriS.p.Aologna.

TAGBristow,C .Eng.ofRiver SeaGabions,(London)Ltd.


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2416OASTALENGINEERING 982REFERENCES

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MARINE PIPELINE PROTECTION WITH FLEXIBLE MATTRESS

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