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Bottom Structure
KEEL
is located at the center line of the bottom structure and are the backbone of the ship.
Contributes substantially to the longitudinal strength and distributes local loading
caused by docking.
Commonest form is flat plate keel which is fitted in ocean-going vessels. Bar keel is used in small vessels (e.g. trawlers, tugs).
When double is fitted, flat plate type is always used.
Duck keel is provided in some double bottoms of some vessel. It carries the double-
bottom piping.
Single Bottom Structure
theverticalplateopenfloorsarefittedateveryframespaceandarestiffenedattheir
upperedge.
Acenterlinegirderisfittedandonesidegirderisfittedeachofthecenterlinewhenbeamislessthan10m.
Whenthebeamisbetween10and17m,twosidegirdersmaybefittedandadditional
continuousorintercostalstiffenersarefittediftheaspectratiow/lofbottomshellpanel
isgreaterthan4
Awoodceilingmaybefittedacrossthefloorsbutthisdoesnotconstituteaninner
bottomofferinganyprotectioniftheouterbottomisdamaged.
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Double Bottom
composedofouterandinnerwatertightbottomplatingtoprovidecompletewatertight
integrity
Thedoublebottomspaceisutilizedtocarryoilfuelandfreshwateraswellas
providingtherequiredballastcapacity.
Theminimumdepthofthedoublebottomdependsonthesizeofthevesselandis
determinedbytherulerequirementsofclassificationsocieties.
Theincreaseintheinnerbottomheightisalwaysmadebyagradualtaperinthe
longitudinaldirectiontoavoidsuddendiscontinuityinthestructure
Oneortwosidegirders,(continuousorintercostal),arefitteddependingonthewidth
Thesidegirders,andplatefloorshavelighteningholestoreducethestructuralweight,
andmanholestoprovideaccess.
Doublebottominthewayofmachineryspacesthatareadjacenttotheaftpeakare
requiredtobetransverselyframed.
Innerbottomplating
Innerbottomplatingmaybeslopedatthesidetoformabilgefordrainage
Themiddlestrakeoftheinnerbottomistheupperflangeofthecenter-linedocking
girderandthusheavierthanotherstrakes.
Ifgrabsareusedfordischargingcargoes,theplatethicknessisfurtherincreasedora
doubleceilingisused.
Floors
Floorsattheendsofthebottomtankspacesaremadewatertightoroiltightbyclosing
anyholesorweldingcollarsaroundanymembersthatpassthroughthefloors.
Solidsplatefloorsarefittedtostrengththebottomtransverselyandsupporttheinner
bottom
Theyruntransverselyfromthecontinuouscentergirdertothebilge.
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Thespacingofthesolidplatefloorvariesaccordingtotheloadssupportedandlocal
stresses.
Manholesareprovidedforaccessthroughtanksandlighteningholesarecutineach
solidplatefloor.
Transverselyframeddoublebottom
Transversesolidplatefloors,andbracketedfloorswithtransverseframes,provides
theprinciplesupportfortheinnerbottomandbottomshellplating.
Solidplatefloorsarefittedateveryframespaceintheengineroomandinthe
poundingregion
Oneintercostalsidegirderisprovidedportandstarboardwhere1014m
Thesidegirdersextendasfarforwardandaftaspossible.
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WritedownthenamesofShipStructuralcomponentsinshipbottom.ORbottomstructure
1.
Keel
1)
Flatplatekeel
2) Barkeel
3) Ductkeel
2. Singlebottomstructure
1. Openfloor
2. Garboardstrake
3. Transverseplatefloors
4. Intercostalsidegirderorside
keelsons
5.
Centergirder
3. Doublebottomstructure(transverselyorlongitudinally)
1. Centergirder
2. Intercostalsidegirder
3. Solidfloor
4. Tanktoporinnerbottom
5. Bottomlongitudinals
6. Bracketfloors
7. Flatplatekeel
8. Marginplate
4.
Bilgekeel
Fore end StructureMajorcomponentsare
1. Stem
2. Bulbousbow
3. Chainlocker
4. Hawsepipes
5. BowsteeringArrangement
6. Bowthruster
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2.Bulbousbows
3.Chainlocker
Chainlocker,whosedimensionsaredeterminedinrelationtothelengthandsizeof
cable,isarrangedfwdofcollisionbulkhead.
Afalsebottommaybeformedbyperforatedplatesonbearersarrangedataheight
abovethefloorofthelocker.
Itwillhelptocleanupthemud.
On many ships, soft nose stem, which is a solid
round bar is fitted from keel to the waterline region.
In the event of collision, it will buckle under load
keeping the impact damage to a minimum.
Vessel operating at higher speeds and those with
high CB are fitted with bulbous bow to reduce the
vessels resistance.
It involves a greater degree of plate curvature
Floors are fitted at every frame space and a centerbulkhead is fitted if the bulb is large.
Shell plating has an increased thickness to cover a
area likely to damage by the anchors and chains.
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4.Hawsepipe
5.Bowsteeringunits
Double-endedferriesareprovidedwitharudderateitherendthatislockedinposition
whenitisattheforeendofthevesselunderway.
6.Bowthrusters
Formaneuveringinconfinedwatersatlowspeedsand/orwhenveryaccurate
positioningisrequired,lateralbowthrustersareprovided.
Theyconsistofcontrollablepitchimpellerfittedinanathwartshipwatertighttunnel.
Toreducetheincreasedhullresistancebecauseofthetunnel,flush-mounted,butterfly
action,hydraulicallyoperateddoorsarefitted.
Deck, hatches and superstructures
Decks
Decksatdifferencelevelsinshipservevariousfunctions
Watertightdecksarefittedtomaintaintheintegrityofthemainwatertighthull
Freeboarddeckistheuppermostdeck
Strengthdeckformstheupperflangeofthemainhullgirder
Weatherdeckarecamberedtofacilitatethedrainageofthewater
Deckplating
Asthegreatestlongitudinalbendingstresseswilloccurovertheamidships,the
greatestdeckplatethicknessismaintainedover40%ofthelengthamidshipsandittapersto
aminimumthicknesspermittedattheendsoftheships.
Alllargeopeningsarewell-roundedtoavoidstress,withinsertedplatesfitted,unless
thecornersareparabolicorellipticalwiththemajoraxisforeandaft,localstress
concentrationsbeingreducedifthelatteristypeofcorneriscut.
Itprovidesaneasyleadforthecablefromthe
windlasstotheanchors.
Tubularhawsepipesarefabricatedandcastings
areweldedattheshellanddecktopreventchafin
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Deckmaybeframedtransverselyorlongitudinally
Withinthefwd7.5%oftheLOA,theforecastleandweatherdecktransversesare
closelyspacedandthelongitudinalscantlingsincreased.
Hatches
Hatch coaming
The construction and means of closing hatches in
watertight decks are contained within the ConditionsAssignment of Freeboard of the Load Line Conventio
Folding hatch cover
Single pull hatch cover
Direct pull hatch cover
Rolling hatch cover
Piggy-back hatch cover
Heights of coamings and cover closing arrangements depend on the hatch positi
Position 1: On the exposed freeboard deck, raised quarter deck or superstructur
decks within 25% of the ships length form fwd
Hatches at position 1have coaming at least 600mm high
Position 2: On exposed superstructure decks abaft the fwd 25% of LOA
Hatches at position 2 have at least 450 mm high coamings.
Coamings of 600mm or more are stiffened by a horizontal bulb flat at the upper
Coaming s of less than 600 mm are stiffened by a cope at the upper edge
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Bulwarks
Superstructure
Superstructuresmaybedefinedasthoseerectionsabovethefreeboarddeckthat
extendtotheshipssideoralmosttotheside.Deckhousesarethoseerectionsondeckthat
arewellwithinthelineoftheshipsside.
Thesestructuresarenon-continuous.Attheseiscontinuitieslargestressesmayarise
andadditionalstrengtheningwillberequired.
Longsuperstructuresexceeding15%oftheshipslengthandextendingwithin50%of
thevesselslengthamidshipsreceivespecialconsiderationastheycontributetothe
longitudinalstrengthoftheship
Aft-end StructureMajorcomponentsinaft-endare
1. Sternframe
2. Rudder
3. Steeringgear
4. Sterntube
5. ShaftbossingorAbracket
6. Propellers
Bulwarks fitted on weather decks are provided as protection for
personnel
They are of light scantlings.
Free ports are cut in bulwarks forming wells on decks to drain wate
It should be at least 1m high on the exposed freeboard and
superstructure decks
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1.Stern
2.Sternframe
o Theformofsternframeisinfluencedbythesternprofileandruddertype.
o Thesternframemaybecast,forgedorfabricatedfromsteelplateandsections.
o Topreventseriousvibrationattheafterendtheremustbeadequateclearancesbetweenthe
propellerandsternframe.
o Sternpostisofastreamlineform.
3.Rudders
o Ruddersonpresent-dayshipsaresemi-balanced.
o Balancedrudderswithlargeareafwdofaxis(25-30%)andunbalancedrudderswiththefull
areaaftoftheaxisarealsofitted.
o Theobjectofbalanceistoachieveareductionintorquebutthefullybalancedrudderwillat
lowanglestendtodrivethegear.
Construction:
o
Modernruddersareofstreamlinedform.
o Onesideplateispreparedandtheverticalandhorizontalwebsareweldedandtheotherpla
calledtheclosingplate,istheweldedtotheinternalframing.
o Adrainholewithaplugandaliftingholeisprovided
o Interiorsurfacesaresuitablycoated.
Basicallythreekindsofsterntypesexist
a.ellipticalorelevatedstern
b.cruiserstern
c.transomstern
Ellipticalsternisfoundinancientships
Cruisersternpresentsmorepleasantprofileandis
hydrodynamicallyefficientbutthetransomsternoffersagreaterdec
areaaft
Construction
Cruiserstern:itsubjectedtolargeslammingforces.Solidfloorsare
fittedineveryframeandaheavycenter-linegirderisfittedrightaft.
Sternplatingisstiffenedbycantframesorwebsand/orhorizontalstringers.
Cruiserstern:Cantframeisnotrequired.Deepfloorsandacenter-li
girderisfittedatthelowerregionofthestern
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4.Rudderpintles:Pintlesinthegudgeonshaveataperontheradiusanditsbearinglengthexceedsthediameter.
5.Rudderstocks
Itmaybeofcastorforgedsteelanditsmeterisdeterminedinaccordancewiththetorque
andanybearingmomentitistowithstand.
6.Rudderbearing:carriestherudderweight
7.Ruddertrunk:rudderstocksarecarriedinruddertrunkanditslowerendisnotwatertight
8.Steeringgear:Steeringgearareusuallyelectro-hydraulictype.
9.Sterntube
Twoformsexist:water-lubricatedbearingswiththeafterendopentotheseaandmetal
bearingsurfaceslubricatedbyoilwithbothendsclosed.
10.ShaftbossingandAbrackets Multi-screwvesselshavepropellersshaftsthatleavethelineofshellatsomedistancefw
ofthestern.
Theysupporttheshaftoverhang.
Abracketsfittedinlieuofbossingtoreducetheresistance.
11.Propellers
Theboreofthepropellerbossistaperedtofitthetailshaftandthepropellermaybekeye
ontothisshaft.
ThebladesofCPParemountedseparatedonthebossinwhichthepitchcanbechange
Shroudedpropellersareenclosedinafixednozzletoincreasethethrust.Thenozzlehas
reducingdiameteraft.
Electricpoddedpropellersarefittedinlargecontainerandpassengershipstoreducenoi
andvibrationandimprovemaneuverability.
Bulkheads
Verticalpartitioninashiparrangedtransverselyorforeandaftarereferredtoas
bulkheads.
Theyaresubjectedtohosetested
Functions
1. Thebulkheadsdividetheshipintoanumberofwatertightcompartmentsandsupportthe
externalandstructuraltransverseloading.
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2. Theyserveasahullstrengthmemberandsupportdeck,andbottomgirders,and
longitudinals
3. Theyresistanytendencyfortransversedeformationoftheship.
4. Themainhullbulkheadsarewatertighttocontainanyfloodingifacompartmentis
breached.
5. Bulkheadsareeffectiveforcontainingfires.
6. Thestrengthofthewatertightbulkheadismaintainedtothestrengthdeck.
Spacing(location) of watertight bulkheads
Theminimumnumberoftransversebulkheadsisthreewhentheshiphasanaft
engineroomandfourwhentheengineroomisnotattheaftend.
Forcollisionbulkhead,aheavybulkheadisspecifiedandlocatedsothatitisnotsofar
forwardastobedamagedonimpact.(minimumdistanceis10mfromforeend)
Theaftpeakbulkheadenclosesthesterntubesinawatertightcompartment,preventinganyemergencyleakage.
Itislocatedwellaftsothatthepeakwhenfloodedwouldnotcauseexcessivetrimbythe
stern.
Machinerybulkheadsprovideaself-containedcompartmentforboilerandengines,
preventingdamagetothesevitalcomponentsoftheshipfromflooding.
Eachofthemainbulkheadmayextendtotheuppermostcontinuousdeck.
ForBulkcarriers,reducedfreeboardnecessitatesfurtherconsiderationintothespacingothewatertightbulkheads.
Forapassengership,thespacingofthewatertightbulkheadsmustcomplywithSOALS.
Construction of bulkhead
Twotypesexist:plainandcorrugated
Plaintype:theplatingofaflattransversebulkheadisgenerallyweldedinhorizontalstrak
withthethicknessincreaseswithdepth.
Verticalstiffeningisprovidedintheformofweldedinvertedanglebarsoroffsetbulbplate
Verticalstiffenersmaybesupportedbyhorizontalstringerstoreducethescantling
Theboundariesaredoublecontinuouslyfilledweldeddirectlytotheshell,decks,andtank
top.
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Corrugatedtype
Theyaredesignedtoprovidesufficientrigiditytotheplatebulkheadinorderthat
conventionalstiffenersmaybedispensedwith
Thesebulkheadsarenormallyfittedwithtopandbottomspool.
Ifpossiblethepassageofpipingandventilationtrunksthroughwatertightbulkheadsis
avoided.
Ifpipespassthrough,pipesareflanged
Ifaventilationtrunkpassesthrough,awatertightshutterisprovided.
Water tight door
Toprovideaccessbetweencompartmentsoneithersideofawatertightbulkhead,
watertightdoorsarefitted.
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Theopeningisframedandreinforcediftheverticalstiffenersarecutinwayoftheopenin
Thewatertightdoorsareeitheroftheverticalorhorizontalslidingtype.
Theyaresubjectedhosetest.
Deep tanks
Theywerefittedadjacenttothemachineryspacesamidshipstoprovideballastcapacity,
improvingthedraftwithlittletrimwhentheshipwaslight.
Theywerealsousedforcarryinggeneralcargoesandalsoutilizedtocarryspecialistliqu
cargoes.
Mostshipsnowarefittedwithdeeptanksforwardtoimprovethetriminthelightcondition
Construction
Bulkheadsthatformtheboundariesofadeeptankdifferfromholdbulkheadsinthatthey
areregularlysubjectedtopressurefromaheadofliquid.
Thegreaterplatingthicknessofthetankboundarybulkheadsincreaseswithtankdepth,andwithincreasingstiffenerspacing.
Toprovidethegreaterrigiditytheverticalstiffenersareofheavierscantlingsandmore
closelyspaced.
Verticalstiffenersizesmaybereducedbyfittinghorizontalgirders
Cofferdamsarefittedbetweendeeptanksespeciallywhendifferentliquidcargosare
carried
Toreducethefreesurfaceeffect,acenter-linebulkhead,whichmaybeintactorperforatemaybefittedwherethetanksextendfromsidetosideoftheship
Bothswedgedandcorrugatedplatingmaybeusedintheconstructionofdeeptanks
Topside tanks Theyareusedforwaterballastorsometimesforthecarriageoflightgrains.
Theyarestiffenedinternallybylongitudinalframingsupportedbytransverses.
Shaft tunnel Theyareusedtoenclosedthepropellersshaftsinawatertighttunnelbetweentheaftend
ofthemachineryspaceandtheaftpeakbulkhead
Construction
Thethicknessofthetunnelplatingisdeterminedinthesamemannerasthatforthe
watertightbulkhead.
Thetopofthetunnelmaybeflatorwellrounded.
Stoolsarebuilttosupporttheshaftbearings
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Awalkwayisinstalledononesideoftheshafttopermitinspection
Oncompletiontheshafttunnelissubjectedtohosetest.
Pillars
Functions
Theprimaryfunctionistocarrytheloadofthedecksandweightsuponthedecksverticaldowntotheshipsbottomstructurewheretheseloadsaresupportedbytheupward
buoyantforces.Pillarsincargoholdsfulfillthefunctionandareincompressionwithout
bracketconnectionsattheirends.
Thesecondoneistotietogetherthestructureinaverticaldirection.Pillarsinthe
machineryspacesservethisfunctionandareheavilybracketedattheirendstopermit
tensileloadings.
Holdpillarsmaybeusedasmainstructuralelementstoreducethescantlingofthedeck
transversesanddeckgirdersbyreducingtheeffectivespansoftheseheavystructural
members.
Spacing of the pillars
Sincepillarsinholdswillinterferewiththestowagearrangements,widelyspacedpillarso
largesectionareused.
Commonarrangementistwo-rowsystem,withpillarsatthehatchcornersmid-lengthof
hatchsupportingdeckgirdersadjacenttothehatchsides.
Asthedeckgirdersizeistosomeextentdependentonthesupportedspan,whereonlya
mid-hatchlengthpillarisfittedthegirderscantlingswillgreaterthanthatwheretwohatch
cornerpillarsarefitted.
Pillarscanbeeliminatedifaclearspaceistobeprovided,butthedeckgirderwillbelargeandmaybesupportedbywebsatthebulkhead.
Construction
Holdpillarsareprimarilysubjectedtocompression
Idealsectionisthetubular.Hollowrectangularandoctagonalsectionsarealsoused.
Pillarshaveabearingfitanditisimportantthattheloadsattheheadandheelofthepilla
shouldbewelldistributed.
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Machineryspacepillarsarefabricatedfromangles,channels,orrolledsteeljoistsandare
heavilybracketed.
Ship Building Quality SteelThe following steels are predominantly used in shipbuilding
1.
Rolled plain steel (for plates and profiles, foundation, etc.)2. Rolled special steel (high-tensile steel, low-temperature steel for LNG tanker,
corrosion resistance for product tanker, non-magnetic steel for compass area)
3. Cast steel (rudder, stern, stem)
4. Forged steel (for anchors, chains, etc.)
IACSgradedthesteelsintofivedifferentqualities(GradeA,B,C,D,E)withGradeAbein
anordinarymildsteeltoLRrequirement.GradeBisbetterqualityandusedwhenthickerplates
arerequiredinthemorecriticalregions.GradeC,D,Epossessincreasingnotch-tough
characteristics,withGradeCbeingABSrequirements.
GradesA,B,D,DS,CS,andEaremostextensivelyused.
The Composition
Themicrostructureofshipbuildingsteelsconsistsofiron-carbide(cementite)dispersedin
matrixofferrite.
Steelforhullstructureismildsteel(0.15%-.23%C)withhighMncontent.
BothSandParekepttoaminimum(
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Steels Production WeldabilityRimmedSteels
Producedwithoutdeoxidizers PoorqualityNotusedashullsteelsinthicknessover13mm
Semi-killedSteels
Partiallyoxidized BetterqualityUsedashullsteels
KilledSteels Producedwithstrongdeoxidizingagents
Bestquality
Special Shipbuilding steels
1.High Tensile Steel
Theyareemployedinthemorehighlystressedregionsoflargertankers,containerships
andbulkcarriers.Theyallowreductionsinthicknessbutleadtolargerdeflections.
TheycontainAl,NbandVasadditives.
2. Corrosion-resistance Steel
Usedincargotankscontaininghighlycorrosivecargoes.
3.Abrasion-resistance Steel
Usedincargoholdsofbulkcarriers
4.Steels for lowtemperature
Usedwhereextraordinarycoolingeffectsexist(e.g.inrefrigerationshipsandLNGcarrier
Heat Treatment of Carbon Steel
1. Annealing
Steelisheatedslowlyto850-950.Candthencooledataveryslowrate.
Thiswillrelieveanyinternalstresses,softenthesteelandbringthesteeltoacondition
suitableforanotherheattreatment.
2.
Normalizing
Steelisheatedasincaseofannealingbutcooledinair.
Becauseoffastercooling,aharderstrongersteel(thanannealingproduces)withrefined
grainsizeisobtained.
3. Quenching (or Hardening)
Steelisheatedasincaseofannealingandnormalizingbutcooledinwateroroil.
Thisproducesaveryhardstructurewithahighertensilestrength.
4. Tempering
Quenchedsteelisfurtherheatedupto680 .Candcooledfairlyrapidly.Thiswillrelievethe
severeinternalstressesproducedbytheoriginalhardeningprocessandtomakethematerial
lessbrittlebutretainthehighertensilestress.
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5. Stress relieving
Torelieveinternalstress,steelisheatedthatnostructuralchangesareoccurredandslow
cooled.
Aluminum AlloyThe advantages of Al alloy over the steel are
1.
Low density
2. High strength-to-weight ratio
3. Corrosion resistance in certain environment
4. Retention of toughness at low temperature
5. Non-magnetic (useful for naval ships)
A major disadvantage is higher initial cost and fabrication costs.
The alloys have relatively low melting points and tend to lose strength rapidly upon
exposure to elevated temperature. (must use appropriate insulation)
Al alloys are frequently used in superstructure, and for entire hull structure of some ferrie
and small boats.Non-heat-treatable Al-Mg alloys of 5XXX series are widely used for marine
structures.
AL alloys are anodic to steel and could lead to aluminum sacrificing. It is difficult to join tw
metals avoiding corrosion. Either bolted connections with washers or an explosively bonded
steel/aluminum transition piece can be used.
Al Alloy TestsAl alloy plate and sections are subjected to tensile tests and bar Al are subjected- to a
tensile test and dump test.
Dump test requires compression of the bar until its diameter is increased to 1.6 times of t
original without cracks.
Classes, Rules and Regulations and IACSInternational Association of Classification Societies is held by twelve leading classification
societies.
1 DNV GL 5 ABS
2 BV 6 KR
3 LR 7 RINA
4 CCS 8 RS
9 NK 10 CRS
11 PRS 12 IRS
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IACS is the only non-governmental organization with the observer status at IMO able to
develop rules. {It endeavors to ensure that IMO technical or procedural requirements are easily
applicable and less ambiguous.}
The government body of IACS is the council, which considers and adopts unified
requirement (URs) and unified interpretations (UIs) of international codes and conventions.
Typical examples of UR are
Minimum longitudinal strength standard
Fire protection of machinery spaces
Use of steel grades for various hull members
Rules and regulations of the ClassesThe classification society(CS) is responsible to assure that the ships and marine structures
presented to it comply with Rules and Regulations that it has established for design, constructio
and periodic surveys.
Classification itself does not judge the economic viability of a vessel, neither is the society
a position to judge whether a vessel is ultimately employed according to the stated intended
service for which it was created.
Classification controls strength and quality of materials and workmanship in connection
with the ship, when built under Class.
The Certificate of Class is issued upon completion of construction
The CS each publish rules and regulations concerning
1. strength and structural integrity of the ship
2. provision of the adequate equipment
3. reliability of the machinery
Ships are built in accordance with Rules and Regulations of a CS in a procedure involving:
1. Technical plan review
2. Surveys during construction
3.
Acceptance by the Classification Committee4.
Subsequent periodic surveys for maintenance of class
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Periodic surveys
To maintain the assigned class, the vessel has to be examined by the society
surveyors at regular periods. At annual surveys, the general conditions of the vessel (e.g. t
freeboard mark, closing appliances, watertight doors, structural fire protections, etc.) are
assessed.
In-water survey may be acceptable in lieu of docking if high-resistance paint has beeapplied. Special surveys are due at five-yearly intervals with the amount of inspections
increasing as the vessel ages.
Testing of Material
Stress and strain curve of mild steel
CS Tests for hull materials1. TensileTest
Aspecimenissubjectedtoanaxialpullandaminimumyieldstress,ultimatetensilestress
andelongationareobtained.
Astandardgaugelengthequalto5.65timesthesquarerootofthecross-sectionalarea,whi
isequivalentofagaugelengthof5timesthediameterisadoptedbytheCS.
Strength=ability to resist deformation
Hardness=ability to resist abrasion
Toughness=ability to absorb energy
Ductility=ability to be deformed before it fails
Ship structures are designed to work within the
elastic range and much lower than the ultimate
tensile strength to allow for safety factor
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2. ImpactTest
Charpy-VorCharpy-Unotchtestisspecifiedataspecifictesttemperaturetodeterminethe
toughness:abilitytowithstandfractureundershockloading.
Thespecimenisplacedonananvilandthependulumisallowedtoswingsothatthestriker
hitsthespecimenoppositethenotchandfracturesit.Energyabsorbedinfracturingthespecime
isautomaticallyrecordedbythemachine.
3. Hardnesstest
Itiscarriedoutbyindentingthesurfacewithanindenterhavingaspecificgeometryundera
specifiedloadandresultingimpressionisexamined.Softermaterialwillindentmore.
4. Fatiguetest
S-Ncurveisobtainedbyrotating-bendingtest.Forsteelfatiguelimitsrangefrom35%to60%
ofthetensilestrength.
5.
AlAlloyTests
AlalloyplateandsectionsaresubjectedtotensiletestsandbarAlissubjectedtoatensilet
anddumptest.Dumptestrequirescompressionofthebaruntilitsdiameterisincreasedto1.6
timesoftheoriginalwithoutcracks.
Stress On a ShipThestressesashipexperiencedare
1. Stillwaterbendingstresses
2.
Wave-inducedbendingstresses
3. Rackingstresses
4. Torsionstresses
5. Pantingstresses
6. Poundingstresses
7. Bucklingstresses
8.
Otherlocalstresses
Localstresses
Transversestresses
Hoggingandsagging
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1.Stillwaterbendingstresses
Eventhoughtotalbuoyancyandweightareinbalanced,buttheyarenon-uniform,which
resultsinverticalshearforces.
Avesselwithexcessweightamidshipswillsag(deckincompressionandbottomshellin
tension). Avesselbendinginthereversemannerissaidtobehog.
Bendingstressis yI
M
Thegreaterthesecondmomentofarea[I],thelesserthebendingstresswillbe. varies
thedepthsquaredandsoasmallincreaseindepthofsectionwillreducethebendingstress.
Zy
whereZisthesectionmodulus.sectionmodulusisthequantitythroughwhichthedesignercan
controlthemaximumhullgirderstressandsafetyfactor.
Bendingstressaregreateroverthemiddleportionofthelengthandmaximumscantlingsmustbeprovidedover40%ofthelengthamidships
2.Wave-inducedBendingstress
3.RackingStress
4.Torsion
Asawavepassesbyavessel,theworsthoggingmoment
willoccurwhenthemidshipisonthecrestofawaveandt
bowandsternareinthetroughs.Theworstsaggingmome
willhappenwhenthebowandsternareontwocrests,with
themidshipinthetroughbetween.
Whetherforsaggingorhogging,theworstconditionwill
occurwhenthewavelengthisclosetothevessellength.
Whenashipisrolling,thedecktendstomovelaterallyrelativetothe
bottomstructureandtheshellononesidetomoveverticallyrelative
totheotherside.
Transversebulkheadsresistracking
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5.Pantingstress
6.Pounding
Pitchingoccursinthebottomoftheforeshipasaresultofheavingpitchingoftheship,
whichcanbereducedbyincreasingthebottom-platingthickness.Itismostsevereinalightly
ballastedcondition.
7.Bucklingstresses
Bucklingmayoccursinstructureundercompressionatastresslevelalotlowerthanthe
yieldstress.TheloadatwhichbucklingwilloccursisafunctionofthestructuresgeometryandthematerialsE.
8.otherlocalstresses
Atthediscontinuityofthehullgirderattheendofdeckhousestructures,athatchandoth
openingcornersandwhentherearesuddenbreaksinbulwarks,highlystressedoccurs .
WeldabilityOrdinarystrengthshipbuildingsteelsarereadilyweldable.
StainlesssteelsarereadilyweldablebyGMAW,GTAW,FCAWorSMAWusingfillerwireofcompatiblecomposition.
AlalloysusedinmarinecanbeweldedwithGMAWorGTAWorFSWbutGMAWisused
moreb/ohigherspeed.Whenjoiningthecommonmetalssuchasstainlesssteeltocarbonand
tocarbonsteel,galvaniccorrosionmustbeconsidered.
Alisnotweldabletosteelbyconventionalmethods.Anintermediatecompositeplate
material(lconsistingofAlandsteel),whichismanufacturedbyexplosionbondingtechniques.
Ashipheadingobliquely(45.)toawavewill
subjectedtorightingmomentsofopposite
directionatitsends,twistingthehullandputt
itintorsion.
Aheavytorsionboxgirderstructureincluding
upperdeckresistthetorsion
Itiscausedbythefluctuatingpressuresonthehull
attheendsandcausestheshellplatingtoworkin
andout.
Itisseverewhenthevesselisrunningintowaves
andpitchingheavily.
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Welding AdvantagesFortheshipbuilder
1. Weldinglendsitselftotheadoptionofprefabricationtechniques.
2. Easiertoobtainwatertightnessandoiltightness.
3. Jointsareproducedquickly
4.
Lessskilledlaborisrequired.
Fortheship-owners
1. Reducedhullsteelweight(MoreDeadweight)
2. Lessmaintenance
3. Smoothhullwhichresultsinreductionofskinfrictionresistance
Welding Processes used in Shipbuilding
Welding Processes generally used in Shipbuilding
1. Gas Welding
2. Shield metal arc welding(SMAW)
3. Gas Metal arc welding(GMAW)
4. Submerged arc welding(SAW)
5. Flux Core Arc welding(FCAW)
6. Stud welding(SW)
7. Tungsten inert gas welding(TIG) or(GTAW)
8. Metal inert gas welding(MIG)
9.
Plasma welding
10.Electro-slag welding(ES)
11.Electro-gas welding(EW)
12.Laser welding
13.Thermit welding
14.Fusion-stir welding(FSW)
1. Gas welding
Gas welding normally use oxyacetylene flame (3000.C).
It is slower than other fusion welding processes b/c process
temperature is lower than metal melting temperature.
The welding is obsolete nowadays in ship building.
2. SMAW
It is a process where heat is produced by an electric arc between a covered metal electrode and the work
Slag-shielded welding
Gas-shielded arc welding
ding
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3. GMAW
It is an automatic or semi-auto process in which a welding arc is formed between the work and bare
electrode, which is continuously feed from the spool.
GMAW is the most important welding technology in shipbuilding.
4. SAW
Itis semi-auto or auto process where an arc is maintained between a continuous fed spool and a work ar
The welding zone is completely buried and shielded under a flux from a feed tube.
5.
SWAn arc is kept between stud and the work and heated for a time.
The stud is brought to the work by spring pressure.
It is used to attach studs, clips and hangers and insulation pins to structural members.
6. TIG or GTAW or TAGS
The arc is drawn between a water-cooled non-consumable tungsten electrode and the plate and shielded
by inert gas.
7. MIG or MAGS
Similar to TIG, but electrode is consumable metal wire.
8. Plasma Welding
Similar to TIG, but the tungsten electrode is within the body of the torch and the plasma arc is forcedthrough a fine-bore copper nozzle. Three modes can be achieved
Micro plasma
Medium current
Keyhole plasma
9. ES and EW
Are high-deposition rate process
Cause a greater degree of grain growth in HAZ and require local normalizing treatment afterwards.
ES
Used for heavy casting structure components such as stern frame.
Current passes into the weld pool through the wire.
Welding is achieved by resistance path heating through the flux.
EW
It is arc welding with features of gas-shield welding and ES.
Used for welding thick plates with square or vee edge preparations.
10.Laser Welding
1. Two type of laser sources: 1.CO2 and 2. Nd: YAG crystals
The laser beam focuses via mirrors on the surface of the work piece. After reaching the vaporization
temperature, a steam capillary is formed in the work. This allows deep, narrow welds with practically
parallel sides with small thermic loads and small brittle Heat Affected Zone.Used for sandwich panel structures.
11.Thermit welding
Used to join large steel sections such stern frame.
It is a fusion process heated by a mixture of powdered Al and iron oxide.
12.Friction stir welding
It is a solid state process that relies on the friction between two parts to generate heat to soften the met
and provide a joint.
Currently used only for welding Al alloy plates for high-speed crafts and honeycomb panels.
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Welding practice
Edge preparation (2015)
Forplatethicknesslargerthan5-6mm,multi-passweldsareneededtoachieveafull
penetrationweld.
Unlessapermanentbackingbaroronesidedweldingisused,abackrunisrequiredaftechoppingorgouging.
Tackweldsarelightrunofweldmetalusedtoholdpatesandsectioninplacepriorto
completionofthefilletoffullbuttweld.
Theseareshortlightrunsofweldmetalwhichmaybeweldedoverorcutoutduringthe
finalweldingofthejoint.
Filletweldsmaybecontinuousorintermittentinachainorstaggeredfashion.
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Weldingthickerplatesnecessitatebevelingtheedges,whichisdonebygasorplasmacuttingormechanicalmethods.
Platesofvaryingthicknessmaybebuttweldedasshowninthefollowingfigure.
Welding Distortion (14,12)
Distortionsarisesformexpansionofthemetalwhenheatedandcontractsuponcooling.
Thecausesofdistortionarecomplexandinvolveanyresidualstressesinthesteelplate.
Tocontainthein-planedistortionsuchasshrinkage,theplatesarecutover-sizedsothat
theeffectofshrinkageistobringthemtothecorrectshape.
Highdepositionelectrodes,automaticwelding,andhighweldingcurrentsreducedistortio
Theleastamountofweldmetal,depositedwithfewpasses,isdesirable.
OnDouble-Vjoints,weldalternatelyonbothsidesoftheplate.
Jointsthatmayhavethegreatestcontractiononcoolingshouldbeweldedfirst.
DistortionControl
1. Avoid over-welding
2. Edge preparation & fit up
3. Intermittent welding
4. Minimum no: of pass
5. Welding near neutral axis
6.
Balancing weld around neutral axis
7. Backstep Welding
8. Pre-bending
9. Back to back welding
10.Weld sequence (Joint)
11.Sequence weld (Scatter fillet welds)
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Welding FaultsVariousfaultsmaybeobservedinbuttandfilletwelds.Thesemaybeduetoanumberof
factors:baddesign,incorrectweldingprocedure,useofwrongmaterials,andbadworkmanship
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Welding Sequences
Non-destructive testing1. Dye penetrant
2.
Magnetic particle3. Visual examination
4.
Radiographic
5. Ultrasonic
6. Gauging
7. Sound or percussion test
8.
Pressure testing9. Penetrate fluorescent test
10.Sonic test
11.Eddy current test12.Thermography
Dyepenetrant
Itwillshowasurfaceflawafterthecastinghasbeenwashedafterapplyingthedye.
Itisoftenluminousandisrevealedunderanultraviolentlight.
Rarely used nowadays
AtTintersectionsitisnecessarytoweldthe
buttfirstfully,thengougeouttheendstorenewtheseamedgepreparationbeforeweldingthe
seam.
Generalpracticewhenweldingshellpanels
istostartbyweldingthecentralbuttsandthen
adjacentseams,workingoutwardsboth
transverselyandlongitudinally.
Thelowersideplatingseamsshouldnotbe
weldedbeforetheupperseams.
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Magnetic particle
It is carried out by magnetizing the casting and spreading a fluid of magnetic particles on t
surface. Any discontinuity such as a surface crack will show as the particles concentrate where
there is an alteration in the magnetic field.
Visualexamination
Exceptsubsurfaceandinternaldefects,incorrectbeadshape,highspatter,undercutting,badstopandstartpoints,incorrectalignment,andsurfacecracksareallfaultsthatmaybe
observedbyvisualinspections.
Itisaroutineprocedureanditscostislow.
Radiographic
Itisdonebyrecordingchangesinradiationdensityemittedfromonesideofasubjectwh
itsanothersideisexposedtoradiation.
Itisusedtorecordtheconsistencyoftheweldmetal.(avoidwillshowupdarkershadow
ontheradiograph.)Itposespotentialhazardstotheoperatorandinspector.
Ultrasonic
Ultrasonicenergyissaferandusefulforlocatingfinecracksmissedbyradiography.
Anultrasonicimpulsefromacrystalpassesthroughthematerialandreflectedbacktothe
crystal.
Anydiscontinuitywilldeterminetheproportionofimpulsereflectedbacktothecrystal.
Thepatternofreflectionisrevealedonacathoderaytubeandanexperiencedoperator
recognizesflawfromthedisplay.
Classification society weld tests: destructive test CSspecifydestructivetestsareintendedtobeusedforinitialelectrodeandweldmateria
approval.Testsaremadeforconventionalelectrodes,deeppenetrationelectrodes,wire-gasan
wire-fluxcombinations,consumablesforelectro-slagandelectro-gaswelding,andconsumable
forone-sidedweldingwithtemporarybacking.Tensile,bendandimpacttestsarecarriedouton
thedepositedweldmetalandweldedplatespecimens.
Shell plating system Thebottomandsideshellplatingconsistsofaseriesofflatandcurvesteelplatesgenera
ofgreaterlengththanbreadthbuttweldedtogether.
Bottomshellplating
Throughoutthelengthoftheshipthewidthandthicknessofthekeelplateremainconsta
whereaflatplatekeelisfitted. Itsthicknessisneverlessthanthatoftheadjoiningbottomplatin
Strakesofbottomplatingtothebilgeshavetheirgreatestthicknessoverthe40%ofthe
LOAamidships.Thebottomplatingthentaperstoalesserthicknessattheendsoftheship.
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Sideshellplating
Thegreaterthicknessofthesideshellplatingismaintainedwithin40%ofthevessels
midshiplengthandthentaperstotherulethicknessattheends. Thethicknessmaybeincreasedinregionswherehighverticalshearstressesoccur.
Theupperstrakeoftheplatingadjacenttothestrengthdeckiscalledshearstrake.
Itisdressedsmoothandtheweldingofbulwarkstotheedgeofthesheerstrakeisnot
permittedwithintheamidships.
Allopeningsinthesideshellhaveroundedcorners,andopeningsforseainletetc.areke
clearoutofthebilge.Whenthisisnotpossible,theopeningsaremadeelliptical.
Transversframingsystem Itwillconsistofmainandholdframeswithbracketstopandbottom,andlightertweende
withbracketsatthetopsonly.Scantlingsofthemaintransverseframesareprimarilydependen
ontheirposition,spacinganddepth,andontherigidityoftheendconnections.
Webframes,i.e.built-upframesconsistingofplatewebandfaceflat,wherethewebis
considerablydeeperthantheconventionaltransverseframe,areoftenintroducedalongtheside
shell.
Webframesisintendedtoincreasetherigidityofthetransverseshipsection.
Webframeisrequirednotmorethanfivefamespacesapartat
1. Machineryspaces
2. FwdofthecollisionBHD
3. Inanydeeptank
4. Intweendecksabovedeeptanks
Webframeisrequiredateveryfourthframeapartinthetweendeckabovetheaftpeaktank.
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Longitudinalframingsystem
Ifthesideshellislongitudinallyframedoffsetbulbsectionswilloftenbeemployedwithth
greatersectionscantlingsatthelowersideshell.Directcontinuityofstrengthistobemaintaine
Transversewebswhicharespacednotmorethan3.8mapart,inshipsof100mlengthorless
arefittedtosupportthesidelongitudinals.Largershipsrequiredtohaveadoublehullare
longitudinallyframedatthesideswithtransversewebsarrangedinlinewiththefloorsinthe
doublebottomtoensurecontinuityoftransversestrength.
Bilge keel
Itsfunctionsare
1. tohelpdumptherollingmotionofthevess
2. toprotectthebilgeongrounding.
3. Toincreaselongitudinalstrengthatthebilg
Thegroundbarthicknessisatleastthatofthe
bilgeplateor14mm,andthematerialgradeisthsameasthatofthebilgeplate.
Connectionofthegroundbartotheshellisb
continuousfilletweldsandthebilgekeelis
connectedtothegroundbarbylightcontinuouso
staggeredintermittentweld.
Bilgekeelsaregraduallytaperedandfinishin
wayofaninternalstiffeningmember.
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1isfortheflatorhorizontal(piperolled)position. Fisforfillet2isforthehorizontalorvertical(pipe)position. Gisforgrooveweld
3isforverticalposition 4isforoverheadposition
5isforapipeinfixedhorizontalposition 6isforapipein45 Risforrestrictedposition
Throat thickness is leg length * cos(45) or .7 leg length
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