DNV RPF101 腐蝕管道標準 DNV RPF101 CORRODED PIPELINES 21

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DNVRPF101腐蝕管道標準DNVRPF101CORRODEDPIPELINES2

RecommendedPracticeRP-F1015.AssessmentofComplexShapedDefects(PartA)5.1RequirementsThismethodmustonlybeappliedtodefectssubjectedtointernalpressureloadingonly.Theminimuminformationrequiredcomprises:1)Alengthanddepthprofileforthecomplexshape.Thelengthmustbetheaxiallengthalongtheaxisofthepipe.Thedefectdepth,atagivenaxiallengthalongthedefect,shouldbethemaximumdeptharoundthecircumferenceforthataxiallength(i.e.ariverbottomprofileofthedefect).2)Thelengthoftheprofilemustincludeallmaterialbetweenthestartandendofthecomplexshapeddefect.5.2AllowableCorrodedPipePressureEstimateThepartialsafetyfactorsforacomplexshapeddefecthavenotbeenderivedfromanexplicitprobabilisticcalibration.Thepartialsafetyfactorsforasingledefectsubjecttointernalpressureloadinghavebeenused.Theallowablecorrodedpipepressureofacomplexshapeddefectcanbeestimatedusingthefollowingprocedure:Guidancenote:Theprincipleunderlyingthecomplexshapeddefectmethodistodeterminewhetherthedefectbehavesasasingleirregular‘patch’,orwhetherlocal‘pits’withinthepatchdominatethefailure.Potentialinteractionbetweenthepitshasalsotobeassessed.Aprogressivedepthanalysesisperformed.Thecorrosiondefectisdividedintoanumberofincrementsbasedondepth.Ateachdepthincrementthecorrosiondefectismodelledbyanidealised‘patch’containinganumberofidealised‘pits’.The‘patch’isthemateriallossshallowerthanthegivenincrementdepth.The‘pits’aredefinedbytheareaswhicharedeeperthantheincrementdepth,seeFigure15andFigure16.Theallowablecorrodedpipepressureofthe‘pits’withinthe‘patch’isestimatedbyconsideringanequivalentpipeofreducedwallthickness.Thecapacity(failurepressure)oftheequivalentpipeisequaltothecapacityofthe‘patch’.Theidealised‘pits’intheequivalentpipeareassessedusingtheinteractingdefectmethod(seeSection4).Theestimatedallowablecorrodedpipepressureatagivendepthincrcment,istheminimumoftheallowablecorrodedpipepressureofthe‘patch’,theidealised‘pits’,andtheallowablecorrodedpipepressureofthetotalcorrodedareabasedonitstotallengthandaveragedepth.Theprocedureisrepeatedforalldepthincrementsinordertodeterminetheminimumpredictedallowablecorrodedpipepressure.Thisistheallowablecorrodedpipepressureofthecomplexshapeddefect.STEP1Calculatetheaveragedepth(do,)ofthecomplexshapeddefectasfollows:Ad=-m‘rota,STEP2Calculatetheallowablecorrodedpipepressureofthetotalprofile(p,,,ol),usingd,,,,andl,,,,,inthesingledefectequation:21SMTS(~-~,(d,,lt)*)Proro~=Ymwhere:Ify,(d,,/t)*1thenproral=0Fullycorrelateddepthmeasurements:Uncorrelateddepthmeasurements:Guidancenote:NotethatE,,andydarefunctionsofStD[d,,JI].ThedifferencesbetweencorrelatedanduncorrelatedmeasurementsarediscussedinAppendixC.Assuminguncorrelateddepthmeasurementswill,ingeneral,resultinasignificantlyhigherallowablecorrodedpipepressure(becauseStD[d,,lf]islessthanStD[dlf]).Incaseswheretheconditionsarenotknownitisrecommendedtoassumefullycorrelateddepthmeasurements.STEP3Dividethemaximumdefectdepthintoincrements,andperformthebelowcalculationsforalldepthincrements(4)(seeFigure15).Eachsubdivisionoftheprofileseparatestheprofileintoanidealised‘patch’portion,shallowerthanthedepthsubdivision(i.e.themaximumdepthofthe‘patch’is4),andinto‘pits’whicharedeeperthanthesubdivision(seeFigure16).Therecommendednumberofincrementsisbetween10and50.STEP4Calculatetheaveragedepthofanidealised‘patch’asfollows(seeFigure16):RecommendedPracticeRP-FlOIApril1999STEP5Calculatetheallowablecorrodedpipepressureoftheidealised‘patch’andthepredictedfailurepressure(capacity)oftheidealised‘patch’@cap,w,c,J,usingI,,,,,anddWchinthesingledefectequation:Calculatealsoforuseinstep7:where:Q,,,=,I’Tg1+0.31--Fulbcorrelateddepthmeasurements:Uncorrelateddepthmeasurements:Guidancenote:NotethatE,andydarefunctionsofStD[d,,l/~].ThedifferencesbetweencorrelatedanduncorrelatedmeasurementsarediscussedinAppendixC.Assuminguncorrelateddepthmeasurementswill,ingeneral,resultinasignificantlyhigherallowablecorrodedpipepressure(becauseStD[dpatch/t]islessthanStD[d/t]).Incaseswheretheconditionsarenotknownitisrecommendedtoassumefullycorrelateddepthmeasurements.STEP6Foreachoftheidealised‘pits’,calculatethearealossinthenominalthicknesscylinder,asshowninFigure16,forthecurrentdepthinterval,andestimatetheaveragedepthofeachoftheidealised‘pits’from:STEP7Estimatetheeffectivethicknessofan‘equivalent’pipewiththesamefailurepressureasthe‘patch’,@cup.patch),ascalculatedinSTEP5(seeFigure15).STEP8Theaveragedepthofeach‘pit’iscorrectedfortheeffectivethickness(t,)using:STEP9Calculatethecorrodedpipepressureofallindividualidealised‘pits’@,,p2....pN)asisolateddefects,usingthe‘corrected’averagedepth(d,!),andthelongitudinallengthoftheeachidealisedpit(I,)inthesingledefectequation:where:(d,,/re)*=(d,,1t),,+,StD[dIt]Ifyd(d,,/re)*LIthenpi=0Guidancenote:STEPS10to12estimatetheallowablecorrodedpipepressuresofallcombinationsofadjacentdefects.Theallowablecorrodedpipepressureofthecombineddefectnm(i.e.definedbysingledefectntosingledefectrn,wheren=I...Nandm=n...N)isdenotedp,,.STEP10Calculatethecombinedlengthofallcombinationsofadjacentdefects(seeFigure13andFigure14).Fordefectsntomthetotallengthisgivenby:STEP11Calculatetheeffectivedepthofthecombineddefectformedfromallofindividualidealised‘pits’fromntom,asfollows(seeFigure13):18RecommendedPracticeRP-FlOlApril1999STEP12CalculatetheallowablecorrodedpipeSTEP14RepeattheSTEP’S4to13forthenextpressureofthecombineddefectfromntomintervalofdepthincrement(4)untilthe(p,,)(seeFigure14),usingIn,,t,andde,,,maximumdepthofcorrosionprofilehasbeeninthesingledefectequation:reached.21.Sms(1-~d(de,,‘re)*)Pnm=Ymn,m=STEP15CalculatetheallowablepipepressureaccordingtothesingledefectequationinSection3.2usingthemaximumdefectdepthwhere:andthetotallengthofthedefect.Ifyd(d,,,It,)*21thenPnm=0NotethatcdandydarehnctionsofStD[de,,,,,/t].Fullycorrelateddepthmeasurements:Uncorrelateddepthmeasurements:Guidancenote:ThedifferencesbetweencorrelatedanduncorrelatedmeasurementsarediscussedinAppendixC.Assuminguncorrelateddepthmeasurementswill,ingeneral,resultinasignificantlyhigherallowablecorrodedpipepressure(becauseStD[d,,,,,lt]islessthanStD[dlr]).Incaseswheretheconditionsarenotknownitisrecommendedtoassumefullycorrelateddepthmeasurements.STEP13Theallowablecorrodedpipepressureforthecurrentdepthincrementistakenastheminimumofalltheallowablecorrodedpipepressuresfromabove:STEP16Theallowablecorrodedpipepressureofthecomplexshapeddefect(p,,,)shouldbetakenastheminimumofthatfromallofthedepthintervals,butnotlessthantheallowablepressureforasingledefectcalculatedinSTEP15.-p,,,,isnotallowedtoexceedp,,,.RecommendedPracticeRP-F10119April19996.PartB-AllowableStressApproach6.1IntroductionTheapproachgiveninPartBisbasedontheASD(AllowableStressDesign)format.Thefailurepressure(capacity)ofthepipelinewiththecorrosiondefectiscalculated.andthisfailurepressureismultipliedbyasinglesafetyfactorbasedontheoriginaldesignfactor.Whenassessingcorrosiondefects,dueconsiderationshouldbegiventothemeasurementuncertaintyofthedefectdimensionsandthepipelinegeometry.Intheequationsthatfollow,theultimatetensilestrength(UTS)isquoted.Iftheultimatetensilestrengthisnotknownthenthespecifiedminimumultimatetensilestrengthshouldbeused(i.e.substituteSMTSforUTS).ThemeasuredUTScanbeobtainedfromtheresultsofstandardtensiletestsonrepresentativepipespecimens,orfrommillcertificates.GuidancenoteIncaseofhighoperatingtemperatureareductioninthematerialtensilestrengthshouldbeconsidered.Thereductionishighlymaterialdependentandshouldpreferablybebasedondetailedknowledgeoftheactualmaterial.Inlackofanymaterialinformationalinearreductionof10%from50Cupto200’~shouldbeusedforlinepipematerial.6.2TotalUsageFactorTheusagefactortobeappliedindeterminingthesafeworkingpressurehastwocomponents:F,=0.9(ModellingFactor)F,=OperationalUsageFactorwhichisintroducedtoensureasafemarginbetweentheoperatingpressureandthefailurepressureofthecorrosiondefect(andisnormallytakenasequaltotheDesignFactor)TheTotalUsageFactor(F)tobeappliedtodeterminethesafeworkingpressureshouldbecalculatedfrom:F=FIFl20RecommendedPracticeRP-FlO1April19997.3SafeWorkingPressureEstimate-Internal7.AssessmentofaSingleDefect(PartB)PressureandCombinedCompressiveLoading7.1RequirementsForacorrosiondefecttobeassessedasasingledefect(seeFigure6),thedefectmustbeanisolated,defect.Adjacentdefectscaninteracttoproduceafailurepressurethatislowerthanthefailurepiessureofeitheroftheisolateddefects(iftheywheretreatedassingledefects).Forthecasewhereinteractionoccurs,thesingledefectequationisnolongervalidandtheproceduregiveninSection8mustbeapplied.Figure8showsthekeydimensionsfordefectinteraction.Adefectcanbetreatedasanisolateddefectifanyofthefollowingconditionsaresatisfied:1)Thecircumferentialangularspacingbetweenadjacentdefects,?(degrees)2)Theaxialspacingbetweenadjacentdefects,s:s2.0fi7.2SafeWorkingPressureEstimate-InternalPressureOnlyThevalidationofthemethodforassessingcorrosiondefectssubjecttointernalpressureandlongitudinalcompressivestressesisnotascpmprehensiveasthevalidationofthemethodforassessingcorrosiondefectsunderinternalpressureloadingonly.Amethodforassessingasingledefectsubjecttotensilelongitudinaland/orbendingstressesisgiveninAppendixD.Thesafeworkingpressureofasinglecorrosiondefectsubjecttointernalpressureandlongitudinalcompressivestressescanbeestimatedusingthefollowingprocedure:STEP1Determinethelongitudinalstress,atthelocationofthecorrosiondefect,fromexternalloads,asforinstanceaxial,bendingandtemperatureloadsonthepipe.Calculatethenominallongitudinalelasticstressesinthepipeatthelocationofthecorrosiondefect,basedonthenominalpipewallthickness:ThesafeworkingpressureofasingledefectsubjecttoThecombinednominallongitudinalstressesis:internalpressureloadingonlyisgivenbythefollowingequation:Dl.=oA+OBSTEP1Calculatethefailurepressureofthecorrodedpipe(P,):STEP2Determinewhetherornotitisnecessarytoconsidertheeffectoftheexternalcompressive2tUTS(D-t)where:STEP2Calculatdthesafeworkingpressureofthecorrodedpipe(P,,,):longitudinalloadsonthefailurepressureofthesingledefect(seeFigure7).Itisnotnecessarytoincludetheexternalloadsiftheloadsarewithinthefollowinglimit:OL01where:Psw=FP,IftheaboveconditionissatisfiedthenSTEP4canbeneglected.Dueconsiderationshouldbegiventothemeasurementuncertaintyofthedefectdimensionsandthepipelinegeometry,whichisnotaccountedforintheequations.STEP3Calculatethefailurepressureofthesinglecorrosiondefectunderinternalpressureonly,usingthefollowingequation:RecommendedPracticeRP-FIOI21April1999where:STEP4Calculatethefailurepressureforalongitudinalbreak,includingthecorrectionfortheinfluenceofcompressivelongitudinalstress(Figure4):where:2tUTS(~-40,11+-UTSA.STEP5Detenninethefailurepressureofasinglecorrosiondefectsubjectedtointernalpressureloadingcombinedwithcompressivelongitudinalstresses:STEP6Calculatethesafeworkingpressureofthecorrodedpipe(P,,):RecommendedPracticeRP-FIOlApril19998.AssessmentofInteractingDefects(PartB)8.1RequirementsTheinteractionrulesarestrictlyvalidfordefectssubjecttoonlyinternalpressureloading.Therulesmaybeusedtodetermineifadjacentdefectsinteractunderotherloadingconditions,atthejudgementoftheuser.However,usingtheseinteractionrulesmaybenon-conservativeforotherloadingconditions.ThemethodsgiveninSection7forassessingcorrosiondefectsundercombinedloadsareonlyvalidforsingledefects.Theminimuminformationrequiredcomprises:1)Theangularpositionofeachdefectaroundcircumferenceofthepipe.2)Theaxialspacingbetweenadjacentdefects.3)Whetherthedefectsareinternalorexternal.4)Thelengthofeachindividualdefect.5)Thedepthofeachindividualdefect6)Thewidthofeachindividualdefect.8.2SafeWorkingPressureEstimateThesafeworkingpressurecanbeestimatedfromthefollowingprocedure:Guidancenote:Withinthecolonyofinteractingdefects,allsingledefects,andallcombinationsofadjacentdefects,areconsideredinordertodeterminetheminimumsafeworkingpressure.Combineddefectsareassessedwiththesingledefectequation,usingthetotallength(includingspacing)andtheeffectivedepth(calculatedthetotallengthandarectangularapproximationtothecorrodedareaofeachdefectwithinthecombineddefect).STEP1Forregionswherethereisbackgroundmetalloss(lessthan10%ofthewallthickness)thelocalpipewallthicknessanddefectdepthscanbeused(seeFigure9-CorrosionDepthAdjustmentforDefectswithBackgroundCorrosion.).STEP2Thecorrodedsectionofthepipelineshouldbedividedintosectionsofaminimumlengthof5.06,withaminimumoverlapof2.5aSTEPS3to12shouldberepeatedforeachsectionedlengthtoassessallpossibleinteractions.STEP3Constructaseriesofaxialprojectionlineswithacircumferentialangularspacingof:2=360-(degrees)izSTEP4Considereachprojectionlineinturn.Ifdefectsliewithin+Z,theyshouldbeprojectedontothecurrentprojectionline(seeFigure10).STEP5Wheredefectsoverlap,theyshouldbecombinedtoformacompositedefect.Thisisformedbytakingthecombinedlength,andthedepthofthedeepestdefect(seeFigure11).Ifthecompositedefectconsistsofanoverlappinginternalandexternaldefectthenthedepthofthecompositedefectisthesumofthemaximumdepthoftheinternalandexternaldefects(seeFigure12-ProjectionofOverlappingInternalandExternalDefectsontoaSingleProjectionLineandtheFormationofaCompositeDefect.).STEP6Calculatethefailureressures(PI,P2...PN)ofReachdefect,totheN’defect,treatingeachdefect,orcompositedefect,asasingledefect:where:GuidancenoteSTEPS7to9estimatethefailurepressuresofallcombinationsofadjacentdefects.Thefailurepressureofthecombineddefectnm(i.e.definedbysingledefectntosingledefectrn,wheren=1...Nandrn=n...N)isdenotedP,,.STEP7Calculatethecombinedlengthofallcombinationsofadjacentdefects(seeFigure13andFigure14).Fordefectsntomthetotallengthisgivenby:STEP8Calculatetheeffectivedepthofthecombineddefectformedfromalloftheinteractingdefectsfromntom,asfollows(seeFigure13):STEP9Calculatethefailurepressureofthecombineddefectfromntom(P,,)(seeFigure14),usingI,,,,,andd,,inthesingledefectequation:RecommendedPracticeRP-F10123April1999where:STEP10Thefailurepressureforthecurrentprojectionline,istakenastheminimumofthefailurepressuresofalloftheindividualdefects(P,toPN),andofallthecombinationsofindividualdefects(Pn,,J,onthecurrentprojectionline.STEP11Calculatethesafeworkingpressure(P,,,.)oftheinteractingdefectsonthecurrentprojectionline:STEP12Thesafeworkingpressureforthesectionofcorrodedpipeistakenastheminimumofthesafeworkingpressurescalculatedforeachoftheprojectionlinesaroundthecircumference.STEP13RepeatSteps3to12forthenextsectionofthecorrodedpipeline.RecommendedPracticeRP-F1019.AssessmentofaComplexShapedDefect(PartB)9.1RequirementsThismethodmustonlybeappliedtodefectssubjectedtointernalpressureloadingonly.Theminimuminformationrequiredcomprises:1)Alengthanddepthprofileforthecomplexshape.Thelengthmustbetheaxiallengthalongtheaxisofthepipe.Thedepth,atagivenaxiallengthalongthedefect,shouldbethemaximumdeptharoundthecircumferenceforthataxiallength(i.e.ariverbottomprofileofthedefect).2)Thelengthoftheprofilemustincludeallmaterialbetweenthestartandendofthecomplexshapeddefect.9.2SafeWorkingPressureEstimateThesafeworkingpressureofacomplexshapeddefectcanbeestimatedfromthefollowingprocedure:Guidancenote:Theprincipleunderlyingthecomplexshapeddefectmethodistodeterminewhetherthedefectbehavesasasingleirregular‘patch’,orwhetherlocal‘pits’withinthepatchdominatethefailure.Potentialinteractionbetweenpitsisalsotobeassessed.Aprogressivedepthanalysesisperformed.Thecorrosiondefectisdividedintoanumberofincrementsbasedondepth.Ateachdepthincrementthecorrosiondefectismodelledbyanidealised‘patch’containinganumberofidealised‘pits’.The‘patch’isthemateriallossshallowerthanthegivenincrementdepth.The‘pits’aredefinedbytheareaswhicharedeeperthantheincrementdepth,seeFigureI5andFigure16.Thefailurepressureofthe‘pits’withinthe‘patch’isestimatedbyconsideringanequivalentpipeofreducedwallthickness.Thefailurepressureoftheequivalentpipeisequaltothefailurepressureofthe‘patch’.Theidealised‘pits’intheequivalentpipeareassessedusingtheinteractingdefectmethod(seeSection8.Theestimatedfailurepressureatagivendepthincrement,istheminimumofthefailurepressureofthe‘patch’,theidealised‘pits’,andthefailurepressureofthetotalcorrodedareabasedonitstotallengthandaveragedepth.Theprocedureisrepeatedforalldepthincrementsinordertodeterminetheminimumpredictedfailurepressure.Thisisthefailurepressureofthecomplexshapeddefect.STEP1Calculatetheaveragedepth(dm)ofthecomplexshapeddefectasfollows:Ad=-meIlOlOlSTEP2Calculatethefailurepressureofthetotalprofile(P,loI),usingdmandinthesingledefectequation:where:STEP3Dividethemaximumdefectdepthintoincrements,andperformthebelowcalculationsforalldepthincrements(4)(seeFigure15).Eachsubdivisionoftheprofileseparatestheprofileintoanidealised‘patch’portion,shallowerthanthedepthsubdivision(i.e.themaximumdepthofthe‘patch’isd,),andinto‘pits’whicharedeeperthanthesubdivision(seeFigure16).Therecommendednumberofincrementsisbetween10and50.STEP4Calculatetheaveragedepthofanidealised‘patch’asfollows(seeFigure16):STEP5Calculatethefailurepressureoftheidealised‘patch’(Ppwlch),using/,or,,andd,,,,hinthesingledefectequation:where:STEP6Foreachoftheidealised‘pits’,calculatethearealossinthenominalthicknesscylinder,asshowninFigure16,forthecurrentdepthinterval,andestimatetheaveragedepthofeachoftheidealised‘pits’from:RecommendedPracticeRP-F101April1999STEP7Estimatetheeffectivethicknessofan‘equivalent’pipewiththesamefailurepressureasthe‘patch’,(Pporch),ascalculatedinSTEP5(seeFigure15).STEP8Theaveragedepthofeach‘pit’iscorrectedfortheeffectivethickness(t,)using:STEP9Calculatethefailurepressureofallindividualidealised‘pits’(PI,P2....PN)asisolateddefects,usingthe‘corrected’averagedepth(dei)andthelongitudinallengthoftheeachidealisedpit(1,)inthesingledefectequation:p=2t,UTS‘@-re)where:Guidancenote:STEPS10to12estimatethefailurepressuresofallcombinationsofadjacentdefects.Thefailurepressureofthecombineddefectnm(i.e.definedbysingledefectntosingledefectm,wheren=1...Nandm=n...N)isdenotedP,,,.STEP10Calculatethecombinedlengthofallcombinationsofadjacentdefects(seeFigure13andFigure14).Fordefectsntomthetotallengthisgivenby:STEPI1Calculatetheeffectivedepthofthecombineddefectformedfromallofindividualidealised‘pits’fromntom,asfollows(seeFigure13):STEP12Calculatethefailurepressureofthecombineddefectfromntom(Prim)(seeFigure14),usingI,,,,t,andd,,,,inthesingledefectequation:where:STEP13Thefailurepressureforthecurrentdepthincrementistakenastheminimumofallthefailurepressuresfromabove:STEP14RepeattheSTEP’S4to13forthenextintervalofdepthincrement(4)untilthemaximumdepthofcorrosionprofilehasbeenreached.STEP15CalculatethefailurepressureaccordingtothesingledefectequationinSection7.2,STEP1,usingthemaximumdefectdepthandthetotallengthofthedefect.STEP16Thefailurepressureofthecomplexshapeddefect(PJ)shouldbetakenastheminimumofthatfromallofthedepthintervals,butnotlessthanthefailurepressureforasingledefectcalculatedinSTEP15.STEP17Calculatethesafeworkingpressure(P,,,)ofthecomplexshapeddefect:P,,=FP,RecommendedPracticeRP-F101April199910.ReferencesI11ASMECodeForPressurePiping,B31LiquidPetroleumTransportationPipingSystems,ASMEB31.4-1989Edition.I21ASMECodeForPressurePiping,B31GasTransmissionAndDistributionPipingSystems,ASMEB31.8-1992Edition,IncorporatingASMEB31.8b-1994Addenda.I31Bjerrnray,O.H.,Fu,B.,Sigurdsson,G.,Cramer,E.H.,Ritchie,D.,(1999),IntroductionandbackgroundtoDNVRP-FlO1CorrodedPipelines,ISOPE1999,Brest,France.141Bjerrnray,O.H.,Sigurdsson,G.,Cramer,E.H.,Fu,B.,Ritchie,D.,(1999),BackgroundtoDNVRP-F101CorrodedPipelines,OMAE1999,Newfoundland,Canada.I51BritishStandardCodeofPracticeforPipelines,BS8010:1989,BritishStandardsInstitute,1989.I61BSISTANDARDS;GuidanceonMethodsforAssessingtheAcceptabilityofFlawsinFusionWeldedStructures,PD6493,BritishStandardsInstitute,August1991.I71CanadianStandardsAssociation(CSA)1994,OilandGasPipelineSystems,2662-94,Rexdale,Ontario.I81DNVOffshoreStandard0s-F101,SubmarinePipelineSystems,DetNorskeVeritas(plannedpublishedlate1999).I91DNVRulesforSubmarinePipelineSystems(DNV196),DetNorskeVeritas,1996,Norway.1101IGEITDIIEdition3:1993;SteelPipelinesforHighPressureGasTransmission,RecommendationsonTransmissionandDistributionPractice,InstituteofGasEngineers,Communication1530,1993.I111ISOIDIS13623PetroleumandNaturalGasIndustries-PipelineTransportationSystems,1997.I121MILNE,I.,AINSWORTH,R.A.,DOWLING,A.R.andSTEWART,A.T.;AssessmentoftheIntegrityofStructuresContainingDefects-Revision3,CentralEl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