DERBYSHIRELoss of a Bulk Carrier.docx
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DERBYSHIRELoss of a Bulk Carrier.docx
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DERBYSHIRELossofaBulkCarrier
CaseStudyII
DERBYSHIRE-LossofaBulkCarrier
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VesselParticulars
SummaryofStructuralFailure
Background
DetailedDescriptionofStructuralFailure(Proposed)
EndResult
Acknowledgements
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VesselParticulars:
Length:
281.94Meters
Beam:
44.20Meters
Depth:
24.90Meters
Constructiondate:
1976
Location:
UnitedKingdom
Classification:
LloydsRegister
VesselType:
Oil/Bulk/Ore(OBO)Carrier
Arrangement:
Accommodationsandmachineryaft.Ninebulkcargoholdsseparatedbydoubleskinnedbulkheads.
ArrangementandConstructionDetails:
Lastofsixsistershipsdesignedin1969intheUKandbuiltfrom1970-76.Thehullwasdoubleskinnedanddoublebottomequipped.Eachcargoholdwascoveredby2hatchcovers(portandstarboard).Atthetimeoftheloss,shehadbeeninserviceonly4years,andwasconsideredawell-maintainedship,andwaspilotedbyanexperiencedcaptain.
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SummaryofStructuralFailure
ThispaperdiscussestwopossibleexplanationsforthelossoftheMVDERBYSHIREduringaseverestorm,bothofwhichpostulatefounderingduetostructuralfailureofcriticalcomponents.Onescenarioassumesthebreakupoftheshipduetofatiguefailureoflongitudinalstructuralmembersintheaftportionoftheship,whiletheotherassumesfounderingresultingfromcollapseofholdcoversundersealoading.Thetwolossscenarioswerecomparedinthelightoftheresultsofrecenttechnicalinvestigationsandwreckagesurveys.Inthepastdecadethistopichasproducedintensedebateinthemaritimecommunity,sotheauthorsconsiderthetopicworthyofreview.Thereadershouldbearinmindthecomplexityofshipsystemsandthehighlyirregularenvironmentinwhichtheyoperate,andrealizethatthelossofashipisusuallytheresultofmanyfactors,includingenvironmental,structural,andoperational.
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Background
MVDERBYSHIREwasaBritishOil/Bulk/Ore,(OBO),carriertransportingorefromCanadatoJapanwhenshewaslostduringthetyphoonORCHIDonthe9thor10thofSeptember1980.Shewentdownwithall44onboardwithoutanydistresssignal.DERBYSHIREisthelargestBritishbulkcarriereverlostandhasbeentheobjectofseveralinvestigationsanddiscussionsregardingbulkcarriersafety.Thislossisjustoneinalonglistofbulkcarrierlossesfromthe1970stothemid-1980s.Everyyear10to20bulkcarrierlossesoccur,wherestructuralfailuremightbethecause,(Ref.2).Thesharedopinionamongmarineengineersisthatthisisanunacceptablenumberoflosses,andthatitisnecessarytoimprovethedesigncriteriaoftheclassificationsocieties.
Becauseofthepoorhistoryofbulkcarriersafety,agreatdealofenergyhasbeenputintotheinvestigationoftheDERBYSHIREcasualty.Itwashopedthatifitcouldberecovered,thecompletepictureofthecatastrophewouldgivevaluableinsightintopotentialstructuralweaknessesandinadequateshipdesignprocedures.
HistoryofInvestigation
Sincethereinitiallywasnoevidencethatstructuralfailurecausedtheloss,theUKGovernmentdidnotholdaformalinvestigationintothelosstheDERBYSHIRE.In1982,eighteenmonthsaftertheloss,hersistership,TYNEBRIDGE,experiencedseverebrittlefractures.Thesecracksinitiatedatframe65andpropagatedintothedeck.Afterthisfracture,theDERBYSHIREFamilyAssociation(DFA)startedinvestigatingframe65cracksontheothersisterships.TheyhypothesizedthatthismightbethecauseofthelosstheDERBYSHIRE.In1986anothersistership,KOWLOONBRIDGE,brokeatframe65aftergrounding.TheseeventscausedacloserlookintothelossoftheDERBYSHIRE.
In1994theDFAhadraisedenoughfundstoconductasurveyofthewreckage.ThisfirstsurveyhadthegoaloflocatingtheDERBYSHIREandinvestigatingthecauseoftheloss.ThesurveywascompletedinMay1994.Theresultsofthesurveywereinconclusive,buttheylocatedanobjectassumedtobethestern,separatedfromtherestofthewreckbyapproximately600meters.
Thesurveyledtofurtherinvestigation.ThenewgoalwastoexaminethelossoftheDERBYSHIREtolearnmoreaboutstructuralweaknessesandtoincreasethesafetyofshipsforthefuture.Thirteenlossscenarioswerelistedafteraninvestigationoftheserviceexperiencefortheclassandgeneralcasualtydataforships,specificallyforbulkcarriers,(Ref.1).Alotofeffortwasputintotheanalysisofeachlossscenariowiththeresultsthatsomewereexcludedwhileotherswereconfirmedasmoreprobable.Thisremainderofthispaperdiscussesthetwofailuremodes/lossscenariosthathavebeenofferedthemostattention:
FatigueFailureatFrame65andHatchCoverCollapse.
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DetailedDescriptionofStructualFailure(Proposed)
FatigueFailureatFrame65
Beforethepossiblestructuraldefectsarediscussed,thegeneralarrangementfortheregionaroundframe65mustbedescribed.Frame65isthetransversewatertightbulkheadthatseparatesHold9fromthepumproom.ThelongitudinalstructuremakesadrasticchangeatFrame65fromamainlongitudinalgirdertoalongitudinalbulkheadseparatingthepumproomfromthesloptanks(seesketchofgeneralarrangementbelow).
Thepossibilityoffailureatframe65wasrealizedaftertheKOWLOONBRIDGEranagroundandbrokeintothreepieces.Onebreakoccurredatthebowwhereitwasrestingonthereefandthesecondnearframe65.Othersistershipsbeganreportingvisiblecrackinginthedeckaboveframe65.Theseincidentscausedtheindustrytoexaminethestructuraldesignforweaknesses.TheexaminationrevealedthatthemethodusedtotransitionfromthelongitudinalgirdertothelongitudinalbulkheadcouldhaveresultedinthecatastrophicfailureoftheMVDERBYSHIRE.
FURNESSBRIDGE,thefirstshipoftheclass,usedanaccepteddesignmethodforthelongitudinaltransitionatframe65bycontinuingthegirderthroughBulkhead65andthentaperingandfinallyweldingittothelongitudinalbulkheadinthepumproom(thedrawingbelowshowsthegeneralstructuraldesign).
Inthelaterships,however,thedesignwaschangedtoaccommodateofcofferdamsaddedtoisolatethesloptanksfromHold9andthepumproom.ThenewdesignendedthegirderatBulkhead65andstartedthelongitudinalbulkheadontheaftsideofthebulkhead(seedrawingbelowfordetails).
Ideally,thelongitudinalgirderandbulkheadwouldbeinlineatthetransversebulkhead,butiftheyaremisaligned,thefatiguelifeofthestructurecouldbereduced.Astheshipcyclesfromhoggingandsaggingmomentsthemisalignmentcausesthetransversebulkheadtobedistortedforeandaft.Thefluctuatingdistortionswouldresultinhighlocalstressesthatcouldleadtocrackingduetotheshorteningoffatiguelife.Theexistingsistershipswerefoundtohavemisalignmentsbetweenthegirderandbulkhead.Ofcourse,thealignmentfortheDERBYSHIREisunknown.Althoughthemagnitudeofthemisalignmentisunderdebate,itcouldhavebeenasmuchas45mm(seedrawingbelowforabasicrepresentation).
Becauseofthehighprofilenatureoftheaccident,eventuallyenoughmoneywasgatheredtoconductasurveyofthewreck.Thesurveyrevealedthattheshipwasintwomainsections,butthesternsectionwasonly600metersfromtheforwardsection.Sincetheshipdescended4000meterstoreachthebottom,itseemsunlikelythatthebreak-upoccurredatthesurface.Therefore,failureintheregionofBulkhead65wasnotthelikelycauseofthefounderingoftheDERBYSHIRE.
HatchCoverCollapse
Thehatchcoverdesignisanotherfrequentlydiscussedfailureargument.In1966theInternationalConventionofLoadLines,(ICLL),categorizedanewfreeboardclassB-60,wherethefreeboardrequirementsoftheordinaryBclassmaybereducedby60cm.Theonlyrequirementisthattheshipsurvivefloodingofonecompartmentwithoutlossofseakeeping.ThenewB-60classresultedinadecreaseoffreeboardinthemajorityofnewlybuiltbulkcarriers.Thesmallerfreeboardcausedanincreaseofwetteddeckoccurrencesandpressureheadexperiencedbydeckplatingandhatchcovers.Thedesignpressurewassetto1.75tonne/m2atthesameconference,butitcanbequestionedifthispressureishighenough.
DescriptionofNo1HatchCovers
Therearetwocoversoneachhatchwithlength14.68mandbeam11.0m(CoverNo.1)
ThestiffenersareTbeamswith635mmwebanflangesof280mmx25mm.Theyaretaperedattheendwithadepthof483mm.
Thetransversecentergirderis920mmdeepwithasmall75mmx25mmflangeandthesidegirdershavewebsof560mmandflangesof100mmx25mm.
Allwebsandplatethicknessesare10.5mm.
ThestressesinthehatchcoverstiffenersmettherequirementssetbyICLLin1966accordingtoRef.1.Therequirements,takenfromRef.3,page3,arelistedbelow:
Hatchcoversintheforwardquarter-length(0.25L)tobedesignedforauniformpressureof1.75tonne/m2.
LevelofstressnottoexceedUltimateTensileStrength(UTS)/4.25.(whichequals0.40sigmayformildsteel)
Limitingplatethicknessformildsteelisb/100or6mm,wherebisspacingbetweenstiffeners.
Todeterminethemaximumloadarule-designedhatchcovercancarry,wesetplasticcollapseastheultimatestate.TheTstiffenerhasaplastics
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