Hull Thickness Measurements - The SHIPS Project

Making hull thickness measurements on submarine A7 ... Thus any method used to measure hull plate thickness should ideally give minimal disturbance to this ... Home History BuildingtheA7 TheA7inservice ThelossoftheA7 ThecrewoftheA7 OtherAclass FindingA7 WhywasA7lost? WreckSite Sitelocation Thesiteplan Marinebiology Imagegallery Project Management Geophysics Research Virtualreality(VR) Projectteam Methods Hullthickness Outreach Engineeringdrawings ePublicationwithSR4 Report Resources Abouttheproject Downloads DivingPlan WreckSignificance Bibliography Project7:HullThicknessMeasurements Previous|Next Introduction ThemostsignificantthreattothesurvivaloftheA7submarineiscorrosion;thesubmarinehasbeenimmersedinseawaterformorethan100yearssothestrongsteelhullisrustingaway,slowlyturningintostructurallymuchweakercorrosionproducts. OneoftherequirementsfortheA7Projectwasthatmeasurementsbemadeoftheremainingthicknessofmetalinthehullplates. Thiswouldprovideinformationabouttheconditionofthehullstructureandgivessomeindicationofhowlongthesubmarinewillremainintactontheseabed. Thethicknessofmetalremaininginthehullplates(strakes)weremeasuredbydiversatanumberofpointsonthehullusingaCygnusInstrumentsDIVEMk2underwaterdigitalultrasonicthicknessgauge. Theultrasonicthickness(UT)measurementswerecomparedtotheoriginalthicknessoftheplateswhentheshipwasconstructedtogiveanideaofthedegreeofcorrosionofthehullandanestimateoftherateofcorrosion. Subsequentmeasurementsmadeatalaterdatecanbeusedtoimprovethisestimateoftherateofcorrosion. MakinghullthicknessmeasurementsonsubmarineA7 CorrosionofIronandSteel Ironandsteelcorrodeinseawaterformingahardscaleorconcretionlayeronthemetalsurfacewhichismadeupofironcorrosionproducts,theremainsofmarineorganismsplusseabedmaterialiftheitemisclosetothebottom. Thecompositionoftheconcretioncanvaryconsiderablyascanitshardnessanditsabilitytoadheretotheunderlyingmetal. Theconcretionformsabarrieroverthemetalwhichslowstherateofdecaybyreducingtheamountofdissolvedoxygenreachingthemetalsurface,astheconcretionformsaprotectivelayeroverthebaremetaltheremovalofthislayermayincreaselocalisedcorrosionatthatpoint. Thusanymethodusedtomeasurehullplatethicknessshouldideallygiveminimaldisturbancetothislayerofconcretion,andshouldincludestepstominimisethedisturbanceifithastooccur. Theunderlyingmetalslowlycorrodesandisreplacedbylayersofcorrosionproducts;thecorrosionisunevenacrossthemetalplatewhichgivesthemetalaroughorpittedsurface. Eventually,thecorrodingmetalplatecanbecomesothinthatholesformwithintheplateleavingalace-likewebofmetalheldtogetherbycorrosionproducts,finallyallofthemetalcorrodesawayleavingjustthelayersofstructurallyweakcorrosionproducts.  Atsomepointinthedecayprocessthestructuremaybecomesoweakthatitcannotsupportitsownweightoranyforcesactingonitthroughtheeffectsoftidesandstorms. Atthispointthestructurecollapses,sheddingsomeoftheouterconcretionlayersintheprocessandexposingwhatremainsofthebaremetaltoseawater,furtheracceleratingthecorrosion.ForadetaileddiscussionaboutironcorrosionseePearson,1987,ConservationofMarineArchaeologicalObjects,p212. TestingtheCygnusDIVEgaugeunderwateronawreckoffPlymouth TheCygnusInstrumentsDIVEMk.2ultrasonicthicknessgauge Anultrasonicthickness(UT)gaugecanbeusedtomeasurethicknessofmetalandmanyothersolidmaterials. TheUTgaugesendsshortpulsesofveryhighfrequencysoundwavesfromahand-heldprobeincontactwiththehullplatingandmeasuresthetimetakenforeachsoundpulsetotravelthroughtheplate,reflectoffthebackwallofthematerialthenreturntotheprobe. Ifthespeedofsoundinthematerialbeingtestedisknownthenitispossibletocalculatethethicknessofthematerialbymultiplyingthespeedofsoundbyhalfthetotaltraveltime. TheinstrumentchosenforthisprojectwastheDIVEMk2underwaterultrasonicdigitalthicknessgaugemadebyCygnusInstrumentsLtd. Thisinstrumentisrobust,smallandlightweightsocanbewornonthediver’swrist. Thegaugeisautomaticinoperationsotherearenocontrolstoadjustbeforemakingameasurement. Thisisabigadvantagewhentimeonsiteisshortandalsomakestheinstrumentveryeasytouse. ThegaugecanbefittedwithdifferenttypesofprobesoweundertookexperimentstodeterminetheeffectivenessofeachtypeforUCHwork. Thedisplayshowntotheoperatoriseasilyunderstoodbutprovidesessentialfeedbackaboutthequalityoftheultrasonicsignalsbeingreceivedandthemeasurementsthathavebeenmade. Anoptionalupgradetothebasicgaugeistheabilitytostoreupto5000thicknessmeasurementsinitsinternalmemory,arrangedingroupsselectedbytheoperator. Themeasurementsareautomaticallyloggedwhenastablemeasurementismadesotheoperationishands-free. OncethemeasurementshavebeenmadeandthegaugeisbackonthesurfacetheloggedmeasurementdatacanbeuploadedintoacomputerfordisplayandannotationusingtheCygnusInstrumentsCygLinksoftwaresuppliedwiththegauge. OnceanyannotationshavebeenaddedtheCygLinksoftwarecanbeusedtoproduceaPDFreportonthemeasurementsorthemeasurementscanbeexportedtoaspreadsheet. TheCygnusDIVEMk2usedonthisprojectisthenextgenerationoftheinstrumentthatwasusedbyWessexArchaeologyforhullthicknessmeasurementsonsubmarinesHollandNo.5andHMS/MA1.ACygnusDIVEgaugewastobeloanedtotheprojectbyMODSalvageandMarineOperations;insteadsponsorshipwasobtainedthroughProMareandCygnusInstrumentstopurchaseaninstrumentfortheproject. Asmallareaofconcretionremovedtoshowthebrightbutunevenandpittedmetalunderneath Method Althoughtheuseofultrasonicthickness(UT)gaugesiscommonplacefortheinspectionshiphulls,pipelinesandoffshoreinstallations,theiruseisrelativelynewinthefieldofunderwaterculturalheritage. Assuch,fewexperimentshavebeendoneintotheuseoftheseinstrumentsonsubmergedshipwrecksandrecommendationsfortheirusearerare. Oneofthemostrecentpaperstoproviderecommendationswasthe2012reportbyWessexArchaeology‘UltrasonicThicknessMeasurementMethodologyDevelopmentandTestingHollandNo.5andHMS/MA1’whichusedmanyofthepreviouspapersforinspiration,sotheWessexreportandanassociatedpaperwereusedasastartingpointforfurtherwork. Aseriesofexperimentswereundertakenbytheprojectteampriortoandduringfieldworktodeterminethemostreliablewaytorecordplatethicknessmeasurements. Theexperimentsshowedthatthebestresultswouldbeobtainedusingthe5MHztwincrystalprobe(CygnusTypeT5B,yellowband)sothisiswhatwasusedontheA7hull. Theinitialaimwastorecord measurementsateightlocationsonthehull. Additionalmeasurementswereplannedtobemadeonnon-structuralelementstoprovidecomplementarydatafromthissitetothatcollectedfortheFirstWorldWarSubmarineProjectrunbyEnglishHeritage. TheprocedureformakingUTmeasurementsisgivenbelow: 1. Justbeforediving,20mmdiameterballsofepoxyputtytobeusedforfillingholesintheconcretionweremixedtogetherthenplacedinaplasticbag 2. Onsite,thesurfaceofthemetaltomeasurewaspreparedbyremovingthesurfaceconcretionwithachiseltoleaveacleanbutpittedsurfacewithanarealargeenoughtoadmittheUTprobe 3. TheUTgaugewassettorecordthenextsetofmeasurementsinanewlogginggroup 4. Theprobewasplacedonthecleanedareaandaminimumofthreemeasurementsweremadeandloggedateachlocation 5. Finallythecavitymadeintheconcretionwasfilledwithepoxyputty Results Plateandframeconstructiondiagramshowingthearrangementoflaps(NMM) Theintentionwastorecordeightmeasurementpointsonthehull;howeverthreeofthepointswerenotmeasuredduetolackofavailabletimeonsite.TheDIVEgaugewasveryeasytouseasitcouldbewornonthediver’swristleavingthehandsfreetooperatedivingequipmentduringdescentandascenttoandfromthewreck. Onceonsitethedisplaywasbright,clearandeasilyunderstoodinlowlight,poorvisibilityandundertheeffectsofnarcosis.Theconcretiononthepressurehullofthesubmarinewasquitesoftwithatexturelikebiscuitsocouldberemovedeasilywithachisel. Undertheconcretionwasathin,blackgraphitisedlayerwhichwaswashedorrubbedawayleavingbrightandshinymetalthatwasunevenintextureandseverelypitted. AlthoughtheDIVEgaugewasveryeasytouseunderwatertherewereafewdifficultiesinmakingthemeasurements. Ifthedivermakingthemeasurementsleftinsufficienttimebetweenfinishingonemeasurementandstartinganothertheautomaticloggingcapabilityinthegaugewouldnottriggeranothermeasurement. Thiswasanoperatorerrorratherthanaproblemwiththeinstrumentsochangingtheoperatingproceduresoonremediedthisproblemonceithadbeenidentified. Theunevenandpittedsurfaceofthesteelhullmadeitdifficulttoholdtheprobesufficientlystillforameasurementtobemadeastheprobehadtogiveareadingthatvariednomorethan0.2mmfor2seconds. Theprobeneededtobepressedtothehullquitefirmlywhichwasfoundtobedifficultforthenormallyneutrallybuoyantdiveraspressingdownontheprobejustpushedthemoffthehull. Thesubmarinehullshapemademattersworseasbeingsmooththerewaslittletoholdontoandthecurveofthecircularhulllimitedmeasurementstotheupperarea. Initiallywehadplannedtomeasurethehullthicknessonthecentrelineofthesubmarinejustbelowseabedlevelbutthiswasfoundtobeimpossible. Thedivercouldnotholdtheprobesufficientlystillwhilelyingupsidedownonthehullandavoidingdamagetotheseafansfoundgrowingthere. Workingfromtheseabedwasalsonotpossibleastheoverlyingsiltwassosoftthediverwouldsimplysinkintoitbeforeameasurementcouldbemade,alsoreducingvisibilitytonilinstantly. ThecircularsteelframesusedtoconstructtheAclassboatswere3½in.x3in.(89x76mm)7.8lb(3/16in.,5mm)anglespaced18in.(458mm)apart. Eightstrakeswereusedtoplateovertheframeswith4in.(102mm)lapswheretwostrakesjoined. Theupper,lowerandsideplateswere20lbs(1/2in.,12.7mm)whiletheotherswerethinnerat17½lb(7/16in.,11.1mm). Fig.63isacrosssectionoftheboatatframe25showingacircularframeoverlaidwithasectionthrougheachstrake. Thearrangementforoverlappingeachstrakeisalsoshown. UltrasonicthicknessmeasurementpointsonthehulloftheA7 Measurementsweremadeatfiveoftheeightpoints: Point Location Value Measurements(mm) Notes UT1 Foredeckstarboard 5.1 3.4,5.4,5.1 Poorquality UT2 Foredeckport 8.9 8.0,8.9 Traceshowsfirstvaluewasshort. Onstrakeoverlap UT3 Conningtowerfairing None   Plannedbutnotcompleted UT4 Stern,belowmudline None   Plannedbutnotcompleted UT5 Conningtower,forward 3.7 4.4,3.8,3.6,3.8,3.7,4.0 Somemeasurementsnoisy UT6 Torpedoloadinghatch None   Plannedbutnotcompleted UT7 Aftdeckport 9.6 10.1,9.2,9.6,9.6 First&secondvalueshaveindistinctleadingedge UT8 Aftdeckstarboard 9.7 8.8,9.7,9.8 Firsttraceisnoisy PointUT1 PointUT1wasonthestarboardside,1mforwardoftheconningtower,onthesecondstrakejustbelowthelapwhereitjoinstheupperstrake,whichwasoriginally11.1mmthick. Itwasdifficulttomakemeasurementsatthispointandthosethatweremadewerepoorquality;anumberofmeasurementswereattemptedontwoseparateoccasions. Onemeasurementof3.4mmwasrecordedwiththeothersat5.4mmand5.1mmbutallwerenoisysowerelessreliable.  ThelocationisclosetotheholeH1sothehullplatingmaybeparticularlythinorpittedinthisarea. PointUT2 Themeasurementwasinadvertentlymadeontheoverlapbetweentheupperstrake(12.7mm)andthesecondstrake(11.1mm)andonlydiscoveredaftertheconcretionhadbeenremoved. Theremainingplatingthicknesswasrecordedtobe8.9mm. Thismeasurementisassumedtobefromjusttheupperstrakeoriginally12.7mmthick. Atthepointwherethemeasurementwasmadethesecondstrakeoverlapstheupperstrakeandprovidesabarriertocorrosionfortheundersideoftheupperstrake,soitislikelythatcorrosionoftheupperstrakewillbelessatthispoint. PointUT5 TheconningtowerinsubmarineA1wasintheformofatruncatedconeof15lbs(3/8in.,9.5mm)steelplatingandA7isthoughttohaveaconningtowermadewiththesamethicknessofsteel.  PointUT5wasonthefrontfaceoftheconningtower0.5mabovethecasing. Sixmeasurementsmadeatthispointgavearepeatablethicknessof3.7mm. PointsUT7andUT8 PointsUT7andUT8were2.2maftoftheconningtowerand0.8moffthecentrelineontheportandstarboardsides. Themeasurementsweremadeontheupperedgeofthesecondstrakewhichwasoriginally11.1mmthick. ThemeasurementforUT7(Port)was9.6mmandforUT8(Stbd.)itwas9.7mm. Thisvalueisconsiderablythickerthantheequivalentmeasurementsmadeatpointsforwardoftheconningtower. Itispossiblethatbothmeasurementsweremadedirectlyoveraframewhichmayreducetherateofcorrosionontheinnerfaceofthemetal. MaterialLoss Point Location Meas. (mm) Original(mm) Loss(mm) UT1 Foredeckstarboard 5.1 11.1 6.0 UT2 Foredeckport 8.9 12.7 3.8 UT5 Conningtower,forward 3.7 9.5 5.8 UT7 Aftdeckport 9.6 11.1 1.5 UT8 Aftdeckstarboard 9.7 11.1 1.4 Analysis ThemeasurementsatUT5ontheconningtowerwerebothreliableandrepeatableandshowalossof5.8mmfromthesteelplating. TheestimateofplatethicknesslostatUT1(foredeckstarboard)is6.0mm,thisisbasedonjusttwomeasurementssoislessreliable,butitdoesagreewiththebetterqualitymeasurementsfromtheconningtower.  ThelossofmetalatpointUT2(foredeckport)wasjust3.8mmbutthislowervalueisprobablycausedbytheprotectiontotheundersideofthisplatebytheoverlappingstrakeunderneath. ThemeasurementsforUT7andUT8areananomalyasifmeasuredthroughjustthehullplatealoneshowametallossofjust1.5mm. Inconclusion,themostreliablemeasurementforthelossofhullplatethicknessis5.8mmin100years,orarateof0.058mm/yearor58μm/year. Thisisalossof61%oftheoriginalconningtowerthicknessand52%oftheoriginalhullplatethickness.  NotethatthemeasurementsonlyincludetheexposedportionofthehullofA7. Theportionofthehullintheseabedislikelytobemuchbetterpreservedasitisburiedsomaysurviveforlongerwithintheseabedafterthevisibleparthascorrodedaway. ComparisonwithA1 WessexArchaeologyundertookasimilarultrasonichullthicknesssurveyonsubmarineA1in2012. ThisisusefulforcomparisonwiththeresultsfromtheA7UTsurveyastheboatssharethesamedesignforthepressurehullandA1sankforthefinaltimejust3yearsbeforeA7.  ThreehullthicknessmeasurementsweremadeintotalonthehullofA1butnocalculationofmetalthicknesslosswasincludedinthereport. ThelocationofthepointswherethemeasurementswereisvaguebutFigure1inthereporthintsthatthemeasurementsweremadeonthethinnersecondstrake(11.1mm)ratherthanthethickerupperstrake. MeasurementpointTL2locatedapproximately1maftofthebowgaveathicknessof5.6mmwhichequatestoalossof5.5mmofsteelfromthatstrake. PointTL5waslocatedinlinewiththefrontoftheconningtowerandreportedathicknessof5.7mm,whichisalossof5.4mminthickness. LittlecanbeconcludedfromafewmeasurementstakenfromuncertainlocationsonthehullofA1butitisstillinterestingtonotethatthesemeasurementsaresimilartothe5.8mmofsteelplatelostfromthehullofA7. Prediction Asimpleprojectionforwardintimewiththisrateoflossofmaterialgivesapredictionthattheconningtowerwillhavecorrodedcompletelyinjust63yearsandthepressurehullin87years. Howeverthesevaluesarelikelytobeover-estimatesasthecorrosionproductsareconsiderablyweakerthanthemetaltheyreplace. Thestrakesarelikelytocollapseundertheirownweightbeforeallofthemetalcorrodesandtheframessupportingthehullplatingwereoriginallyonly5mmthicksotheremaynowbelittlemetalremainingwithinashellofconcretion.  Bothofthesefactorswillshortenthepredictedsurvivaltimeforthehull. Thecorrodingsteelconningtowerholdsuptheheavygunmetalcap,hatchandperiscopesotheweightofthistoploadislikelytocollapsetheconningtowerbeforethesteelinthetowercorrodescompletely. Thehullofthesubmarineisalmostcompletelysealedatthemomentsowaterflowinsidethehullisminimalbutasthehullstrakesbecomethinnertheywillbeperforatedbysmallholeswhichenlargeovertime.  Asholesdevelopinthehullplatesanytidalcurrentswillstarttoflowthroughthehullwhichwillincreaseerosion,andpotentiallyincreaseoxygenlevelswithinthehullwhichwillthenincreasetherateofcorrosion. Inconclusion,aconservativeestimateforthesurvivalofthevisiblepartofhulloftheA7submarineis40to50years. Previous|Next ©SHIPSProject2015 Home-About