Ship Construction, Seventh Edition

363 Pages·2012·5.213 MB·English

by  George J Bruce, David J Eyres

1
Basic design of the ship
ChapterOutline
Preparation ofthedesign 3
Information providedbydesign 4
Purchase ofanewvessel 6
Ship contracts 7
Further reading 8
Some usefulwebsites 9
Thekeyrequirementofanewshipisthatitcantrade profitably,soeconomicsisof
prime importance in designing a merchant ship. An owner requires a ship that will
givethebestpossiblereturnsfortheowner’sinitialinvestmentandrunningcosts.The
final design should be arrived at taking into account not only present economic
considerations, but also those likely to develop within the life of the ship. This is
especiallythecaseforsometrades,forexampleLNG,wheretheshipisexpectedto
workthesamerouteforitsworkinglife.Designforoperationistheresult.Forother
ships,includingbulkcarriers,thefirstcostoftheshipisthemajorfactorfortheowner
and the ship may be designed for ease of production. Resale value is also often
a major consideration, leading todesignfor maintenance.
With the aid of computers it is possible to make a study of a large number of
varying design parameters and to arriveat a ship design that is not only technically
feasiblebut,moreimportantly,isthemosteconomicallyefficient.Ideallythedesign
will take into considerationfirst cost,operating cost, andfuture maintenance.
Preparation of the design
The initial design of a ship generally proceeds through three stages: concept;
preliminary;andcontractdesign.Theprocessofinitialdesignisoftenillustratedby
thedesignspiral(Figure1.1),whichindicatesthatgiventheobjectivesofthedesign,
thedesignerworkstowardsthebestsolutionadjustingandbalancingtheinterrelated
parameters asthe designergoes.
A concept design should, from the objectives, provide sufficient information for
abasictechno-economicassessmentofthealternativestobemade.Economiccriteria
that may be derived for commercial ship designs and used to measure their profit-
ability are net present value, discounted cash flow, or required freight rate.
ShipConstruction.DOI:10.1016/B978-0-08-097239-8.00001-5
Copyright(cid:1)2012ElsevierLtd.Allrightsreserved.
4 ShipConstruction
Vessel
objectives
Proportions Cost Concept
estimate design
Preliminary
design
Lines
Stability
Contract
design
Hydrostatics Capacities
Weight
Freeboard
estimate
and subdivision
Powering
General
arrangements
Structure
Figure1.1 Designspiral.
Preliminary design refines and analyzes the agreed concept design, fills out the
arrangementsandstructure,andaimstooptimizeserviceperformance.Atthisstage
the builder should have sufficient information to tender. Contract design details the
final arrangements and systems agreed with the owner and satisfies the building
contractconditions.
Thedesign ofthe shipisnotcomplete atthisstage,ratherfor themajor effortin
resourcesithasonlyjuststarted.Post-contractdesignrequiresconfirmationthatthe
ship will meet all operational requirements, including safety requirements from
regulators. It also entails in particular design for production where the structure,
outfit,andsystemsareplannedindetailtoachieveacost-andtime-effectivebuilding
cycle. Production of the ship must also be given consideration in the earlier design
stages,particularlywhereitplacesconstraintsonthedesignorcanaffectcosts.The
post-contractdesignwillalsoideallyconsiderthefuturemaintainabilityoftheshipin
the arrangement ofequipment andservices.
Information provided by design
Whenthepreliminarydesignhasbeenselectedthefollowinginformationisavailable:
l Dimensions
l Displacement
l Stability
Basicdesignoftheship 5
l Propulsivecharacteristicsandhullform
l Preliminarygeneralarrangement
l Principalstructuraldetails.
Each item of information may be considered in more detail, together with any
restraints placed on these items by the ship’s service or other factors outside the
designer’s control.
1. The dimensions of most ships are primarily influenced by the cargo-carrying
capacityofthevessel.Inthecaseofthepassengervessel,dimensionsareinfluenced
by the height and length of superstructure containing the accommodation. Length,
wherenotspecifiedasamaximum,shouldbeaminimumconsistentwiththerequired
speedandhullform.Increaseoflengthproduceshigherlongitudinalbendingstresses
requiring additional strengthening and a greater displacement for the same cargo
weight.Breadthmaybesuchastoprovideadequatetransversestability.Aminimum
depthiscontrolledbythedraftplusstatutoryfreeboard,butanincreaseindepthwill
result in a reduction of the longitudinal bending stresses, providing an increase in
strength, or allowing a reduction in scantlings (i.e. plate thickness/size of stiffening
members etc.). Increased depth is therefore preferred to increased length. Draft is
oftenlimitedbyareaofoperation,butifitcanbeincreasedtogiveagreaterdepththis
can bean advantage.
Manyvesselsarerequiredtomakepassagesthroughvariouscanalsandstraitsand
pass under bridges within enclosed waters, and this will place a limitation on their
dimensions.Forexample,locksinthePanamaCanalandStLawrenceSeawaylimit
length,breadth,anddraft.Atthetimeofwriting,theMalaccaStraitsmainshipping
channelisabout 25metersdeepandtheSuez Canalcouldaccommodateshipswith
abeamofupto75metersandmaximumdraftof16metres.Amaximumairdrafton
container ships of around 40 meters is very close to clear the heights of the Gerard
Desmond Bridge, Long Beach, California and Bayonne Bridge, New York. Newer
bridgesovertheSuezCanalat65metersandovertheBosporusat62metersprovide
greater clearance.
2.Displacementismadeupoflightweightplusdeadweight.Thelightweightisthe
weightofvesselasbuiltandreadyforsea.Deadweightisthedifferencebetweenthe
lightweightandloadeddisplacement,i.e.itistheweightofcargoplusweightsoffuel,
stores,waterballast,freshwater,crewandpassengers,andbaggage.Whencarrying
high-density cargoes (e.g. ore) it is desirable to keep the lightweight as small as
possible, consistent with adequate strength. Since only cargo weight of the total
deadweightisearningcapital,otheritemsshouldbekepttoaminimumaslongasthe
vesselfulfills its commitments.
3. In determining the dimensions, statical stability is kept in mind in order to
ensurethatthisissufficientinallpossibleconditionsofloading.Beamanddepthare
the main influences. Statutory freeboard and sheer are important together with the
weight distributionin arranging thevessel’s layout.
4.Adequatepropulsiveperformancewillensurethatthevesselattainstherequired
speeds. The hull form is such that economically it offers a minimum resistance to
motion so that a minimum power with economically lightest machinery is installed
withoutlosing the specified cargo capacity.
6 ShipConstruction
Aservicespeedistheaveragespeedatseawithnormalservicepowerandloading
under averageweatherconditions. Atrialspeed istheaveragespeedobtainedusing
the maximum power over a measured course in calm weather with a clean hull and
specifiedloadcondition.Thisspeedmaybeaknotorsomorethantheservicespeed.
Unless a hull form similar to that of a known performance vessel is used,
a computer-generated hull form and its predicted propulsive performance can be
determined.Thepropulsiveperformancecanbeconfirmedbysubsequenttanktesting
ofa modelhull, which may suggest further beneficialmodifications.
The owner may specify the type and make of main propulsion machinery instal-
lation with which their operating personnel are familiar.
5. Thegeneral arrangement is prepared in cooperation with the owner, allowing
for standards of accommodation particular to that company, also specific cargo and
stowage requirements. Efficient working of the vessel must be kept in mind
throughoutandcompliancewiththeregulationsofthevariousauthoritiesinvolvedon
trade routes must also be taken into account. Some consultation with shipboard
employees’ representative organizations may also be necessary in the final accom-
modationarrangements.
6. Almost all vessels will be built to the requirements of a classification society
such as Lloyd’s Register. The standard of classification specified will determine the
structuralscantlingsandthesewillbetakenoutbytheshipbuilder.Thedetermination
of the minimum hull structural scantlings can be carried out by means of computer
programs made available to the shipyard by the classification society. Owners may
specify thicknesses and material requirements in excess of those required by the
classificationsocietiesandspecialstructuralfeaturespeculiartothetradeorowner’s
fleet may be asked for.
Purchase of a new vessel
In recent years the practice of owners commissioning ‘one-off’ designs for cargo
ships from consultant naval architects, shipyards, or their own technical staff has
increasinglygivenwaytothe selectionofanappropriate ‘stock design’tosuit their
particular needs. To determine which stock design, the shipowner must undertake
adetailedprojectanalysisinvolvingconsiderationoftheproposedmarket,route,port
facilities, competition, political and labor factors, and cash flow projections. Also
takenintoaccountwillbethechoiceofshipbuilder,whererelevantfactorssuchasthe
provision of government subsidies or grants or supplier credit can be important as
wellastheprice,dateofdelivery,andtheyard’sreputation.Moststockdesignsoffer
some features that can be modified, such as outfit, cargo handling equipment, or
alternatemanufactureof mainengine,for which the owner willhaveto payextra.
Purchase of a passenger vessel will still follow earlier procedures for a ‘one-off’
design,butthereareshipyardsconcentratingonthistypeofconstructionandtheowner
may be drawn to them for this reason. A nonstandard cargo ship of any form and
anumberofspecialistshipswillalsorequirea‘one-off’design.Havingdecidedonthe
basic ship requirements, based on the intended trade, after an appropriate project
Basicdesignoftheship 7
analysis the larger shipowners may employ their own technical staff to prepare the
tenderspecificationandsubmitthistoshipbuilderswhowishtotenderforthebuilding
of the ship. The final building specification and design is prepared by the successful
tendering shipbuilder in cooperation with the owner’s technical staff. The latter may
overseeconstructionofthevesselandapprovethebuilder’sdrawingsandcalculations.
Othershipownersmayretainafirmofconsultantsorapproachafirmwhomayassist
withpreliminarydesignstudiesandwillpreparethetenderspecificationsandinsome
cases call tenders on behalf of the owner. Often the consultants will also assist the
ownersinevaluatingthetendersandoverseetheconstructionontheirbehalf.
Ship contracts
The successful tendering shipbuilder will prepare a building specification for
approvalbytheownerortheowner’srepresentativethatwillformanintegralpartof
the contract between the two parties and thus have legal status. This technical
specification will normally include the followinginformation:
l Briefdescriptionandessentialqualitiesandcharacteristicsoftheship
l Principaldimensions
l Deadweight,cargoandtankcapacities,etc.
l Speedandpowerrequirements
l Stabilityrequirements
l Qualityandstandardofworkmanship
l Surveyandcertificates
l Accommodationdetails
l Trialconditions
l Equipmentandfittings
l Machinery details, including the electrical installation, will normally be produced as
aseparatesectionofthespecification.
Most shipbuilding contracts are based on one of a number of standard forms of
contract that have been established to obtain some uniformity in the contract rela-
tionshipbetweenbuildersandpurchasers.Thereareanumberof‘standard’contract
forms, all very similar in structure and content. Four of the most common standard
forms ofcontracthave been established by:
1. CESA—CommunityofEuropeanShipyardsAssociations
2. MARADMaritimeAdministration,USA
3. SAJ—ShipbuildersAssociationofJapan
4. Norwegian Shipbuilding Contract—Norwegian Shipbuilders Association and Norwegian
ShipownersAssociation.
TheCESAstandardformofcontractwasdevelopedbythepredecessororganization,
the Association of Western European Shipyards (AWES).The contract form can be
downloaded from the CESAwebsite. The sectionsof the contractare:
1. Subjectofcontract(vesseldetails,etc.)
2. Inspectionandapproval
8 ShipConstruction
3. Modifications
4. Trials
5. Guarantee(speed,capacity,fuelconsumption)
6. Deliveryofvessel
7. Price
8. Property (rights to specifications, plans, etc. and to vessel during construction and on
delivery)
9. Insurance
10. Defaultbythepurchaser
11. Defaultbythecontractor
12. Guarantee(afterdelivery)
13. Contractexpenses
14. Patents
15. Interpretation,referencetoexpertandarbitration
16. Conditionforthecontracttobecomeeffective
17. Legaldomicile(ofpurchaserandcontractor)
18. Assignment(transferofrights)
19. Limitationofliability
20. Addressesforcorrespondence.
Irrespectiveofthesourceoftheowner’sfundsforpurchasingtheship,paymenttothe
shipbuilder is usually made as progress payments that are stipulated in the contract
underitem7above.Atypicalpaymentschedulemayhavebeenfiveequalpayments
spread over the contract period, butin recent years payment arrangements advanta-
geous to the purchaser and intended to attract buyers to the shipyard have delayed
ahigherpercentageofpaymentuntildeliveryoftheship.Thepaymentschedulemay
be asfollows:
l 10%onsigningcontract
l 10%onarrivalofmaterialsonsite
l 10%onkeellaying
l 20%onlaunching
l 50%ondelivery.
Because many cargo ships are of a standard design, and built in series, and modi-
fication can be very disruptive to the shipyard building program, item 3 in the
standard form of contract where modifications are called for at a late date by the
owner can have a dramatic effect on costs and delivery date given the detail now
introducedatanearlystageofthefabricationprocess.Manyshipyardswillrefuseto
accept modifications once a design is agreed and detailed work and purchasing
commences. Item 3 also covers the costs and delays of compulsory modifications
resulting from amendment of laws, rules, and regulations of the flag state and
classification society.
Further reading
Rawson,Tupper:BasicShipTheory.ed5,vol2.Chapter15:Shipdesign,2001,Butterworth
Heinemann.
Basicdesignoftheship 9
WatsonDGM: Practical Ship Design,2002,Elsevier.
Some useful websites
www.cesa.eu Community ofEuropean ShipyardsAssociations.
www.sajn.or.jp/eShipbuildersAssociationofJapan;provideslinkstomembershipyardsites.
2
Ship dimensions, form, size, or
category
ChapterOutline
Oil tankers 13
Bulk carriers 13
Container ships 15
IMO oiltankercategories 15
Panama canallimits 15
Suez canallimits 16
Some usefulwebsites 16
Thehullformofashipmaybedefinedbyanumberofdimensionsandtermsthatare
oftenreferredtoduringandafterbuildingthevessel.Anexplanationoftheprincipal
terms isgivenbelow:
After Perpendicular (AP): A perpendicular drawn to thewaterline at the point where the
aftersideoftherudderpostmeetsthesummerloadline.Wherenorudderpostisfitteditis
takenasthecenterlineoftherudderstock.
ForwardPerpendicular(FP):Aperpendiculardrawntothewaterlineatthepointwherethe
fore-sideofthestemmeetsthesummerloadline.
Length Between Perpendiculars (LBP): The length between the forward and aft perpen-
dicularsmeasuredalongthesummerloadline.
Amidships:Apointmidwaybetweentheafterandforwardperpendiculars.
LengthOverall(LOA):Lengthofvesseltakenoverallextremities.
Lloyd’sLength:UsedforobtainingscantlingsifthevesselisclassedwithLloyd’sRegister.
Itisthesameaslengthbetweenperpendicularsexceptthatitmustnotbelessthan96%and
neednotbemorethan97%oftheextremelengthonthesummerloadline.Iftheshiphasan
unusualstemorsternarrangementthelengthisgivenspecialconsideration.
RegisterLength:Thelengthofshipmeasuredfromthefore-sideoftheheadofthestemto
theaftsideoftheheadofthesternpostor,inthecaseofashipnothavingasternpost,tothe
fore-sideoftherudderstock.Iftheshipdoesnothaveasternpostorarudderstock,theafter
terminalistakentotheaftermostpartofthetransomorsternoftheship.Thislengthisthe
officiallengthintheregisterofshipsmaintainedbytheflagstateandappearsonofficial
documents relating to ownership and other matters concerning the business of the ship.
AnotherimportantlengthmeasurementiswhatmightbereferredtoastheIMOLength.This
length is found in various international conventions such as the Load Line, Tonnage,
SOLAS and MARPOL conventions, and determines the application of requirements of
thoseconventionstoaship.Itisdefinedas96%ofthetotallengthonawaterlineat85%of
ShipConstruction.DOI:10.1016/B978-0-08-097239-8.00002-7
Copyright(cid:1)2012ElsevierLtd.Allrightsreserved.
12 ShipConstruction
theleastmoldeddepthmeasuredfromthetopofkeel,orthelengthfromthefore-sideof
stemtotheaxisofrudderstockonthatwaterline,ifthatisgreater.Inshipsdesignedwith
arakeofkeelthewaterlineonwhichthislengthismeasuredistakenparalleltothedesign
waterline.
Molded dimensions are often referred to; these are taken to the inside of plating on
a metal ship.
Base Line: A horizontal line drawn at the top of the keel plate. All vertical molded
dimensionsaremeasuredrelativetothisline.
MoldedBeam:Measuredatthemidshipsection,thisisthemaximummoldedbreadthofthe
ship.
MoldedDraft:Measuredfromthebaselinetothesummerloadlineatthemidshipsection.
MoldedDepth:Measuredfromthebaselinetotheheeloftheupperdeckbeamattheship’s
sideamidships.
ExtremeBeam:Themaximumbeamtakenoverallextremities.
ExtremeDraft:Takenfromthelowestpointofkeeltothesummerloadline.Draftmarks
representextremedrafts.
ExtremeDepth:Depthofvesselatship’ssidefromupperdecktolowestpointofkeel.
Half Breadth: Since a ship’s hull is symmetrical about the longitudinal centre line, often
onlythehalfbeamorhalfbreadthatanysectionisgiven.
Freeboard:Theverticaldistancemeasuredattheship’ssidebetweenthesummerloadline
(or service draft) and the freeboard deck. The freeboard deck is normally the uppermost
completedeckexposedtoweatherandseathathaspermanentmeansofclosingallopen-
ings,andbelowwhichallopeningsintheship’ssidehavewatertightclosings.
Sheer:Ariseintheheightofthedeck(curvatureorinastraightline)inthelongitudinal
direction.Measuredastheheightofdeckatsideatanypointabovetheheightofdeckatside
amidships.
Camber(orRoundofBeam):Curvatureofdecksinthetransversedirection.Measuredasthe
heightofdeckatcenterabovetheheightofdeckatside.Straightlinecamberisusedon
manylargeshipstosimplifyconstruction.
RiseofFloor(orDeadrise):Theriseofthebottomshellplatinglineabovethebaseline.
Thisriseismeasuredatthelineofmoldedbeam.Largecargoshipsoftenhavenoriseof
floor.
Half Siding of Keel: The horizontal flat portion of the bottom shell measured to port or
starboardoftheship’slongitudinalcenterline.Thisisausefuldimensiontoknowwhendry-
docking.
Tumblehome:The inward curvature of theside shell above thesummer load line.This is
unusualonmodernships.
Flare:Theoutwardcurvatureofthesideshellabovethewaterline.Itpromotesdrynessand
isthereforeassociatedwiththeforeendofship.
StemRake:Inclinationofthestemlinefromthevertical.
Keel Rake: Inclination of the keel line from the horizontal. Trawlers and tugs often have
keels raked aft to give greater depth aft where the propeller diameter is proportionately
larger in this type of vessel. Small craft occasionally have forward rake of keel to bring
propellersabovethelineofkeel.
Tween DeckHeight: Vertical distance between adjacent decksmeasured from the tops of
deckbeamsatship’sside.
ParallelMiddleBody:Thelengthoverwhichthemidshipsectionremainsconstantinarea
andshape.