Double Hull Tankers

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DOUBLE HULL TANKERS


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A double hullis a shiphulldesign andconstruction method where the bottom

and sides of the ship have two completelayers of watertight hull surface:

one outer layer forming the normal hull ofthe ship, and a second inner hull which issome distance inboard, typically by a fewfeet, which forms a redundant barrier to

seawater in case the outer hull is damagedand leaks.
http://en.wikipedia.org/wiki/Shiphttp://en.wikipedia.org/wiki/Hull_(watercraft)http://en.wikipedia.org/wiki/Hull_(watercraft)http://en.wikipedia.org/wiki/Ship


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The space between the two hulls isalways used for storage of fuel or

ballast water.Double hulls' ability to prevent or reduce oilspills led to their being standardized for othertypes of ships including oil tankersby the

nternational !onvention for the "revention of"ollution from #hipsor $A%"& !onvention.

After the ())on *aldez oil spilldisaster,when that ship grounded on +ligh %eefoutside the port of *aldez, Alaska, the #-overnment reuired all new oil tankers

built for use between # ports to be
http://en.wikipedia.org/wiki/Oil_tankershttp://en.wikipedia.org/wiki/International_Convention_for_the_Prevention_of_Pollution_from_Shipshttp://en.wikipedia.org/wiki/International_Convention_for_the_Prevention_of_Pollution_from_Shipshttp://en.wikipedia.org/wiki/Exxon_Valdez_oil_spillhttp://en.wikipedia.org/wiki/Bligh_Reefhttp://en.wikipedia.org/wiki/Valdezhttp://en.wikipedia.org/wiki/Alaskahttp://en.wikipedia.org/wiki/Alaskahttp://en.wikipedia.org/wiki/Valdezhttp://en.wikipedia.org/wiki/Bligh_Reefhttp://en.wikipedia.org/wiki/Exxon_Valdez_oil_spillhttp://en.wikipedia.org/wiki/International_Convention_for_the_Prevention_of_Pollution_from_Shipshttp://en.wikipedia.org/wiki/International_Convention_for_the_Prevention_of_Pollution_from_Shipshttp://en.wikipedia.org/wiki/Oil_tankers


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/urthermore, a double0hulled

tanker does not needlongitudinal bulkheads forlongitudinal strength, as the

inner hull already provides this.(liminating longitudinalbulkheads would result in much

wider tanks, signi1cantlyincreasing the free surface e2ect

. 3owever, this problem is easily
http://en.wikipedia.org/wiki/Free_surface_effecthttp://en.wikipedia.org/wiki/Free_surface_effect


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Double Hulls5 #ince 6778, the nited#tates and nternational regulations havereuired all new ships to be designed with

a double hull. A double hull is essentially ahull within a hull. The cargo is carriedinside the inner hull. The space between

the inner and outer hull varies by ship sizefrom 9 to 68 feet or more. f an accidentshould occur i.e., grounding, stranding,

collision or striking a submerged ob;ect


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t has been some eleven years since

the nited #tates> &il "ollution Act&"A 78< and subseuently regulation6?/ of Anne) 6 of the nternational

!onvention for the "revention of"ollution from #hips e2ectivelymandated double hulls for new build

oil tankers of over @,888 deadweighttonnes as a means of preventing orreducing oil pollution in the event of a

grounding or collision resulting in


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d l


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Arrangement and layout?.6..6 The %ules cover typical double hulltankers of greater than or eual to 6@8m in

length and with arrangements as follows:a< engine room and deck house located aftof the cargo tank region, and

b< in addition to the inner skin twolongitudinal oil0tight bulkheads with nocentreline longitudinal bulkhead, or

c< in addition to the inner skin onecentreline longitudinal oil0tight bulkhead.


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#hip #tability diagram showing entre ofgra"!ty-


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External envronment?.6.9.6 To cover worldwide trading operations and also todeal with the uncertainty in the future trading pattern ofthe ship and the corresponding wave conditions that will

be encountered, a severe wave environment is used forthe design assessment. The rule reuirements are basedon a ship trading in the =orth Atlantic wave environmentfor its entire design life.

The e2ects of wind and current on the structure areconsidered to be negligible and hence are not e)plicitlyincluded.The %ules do not include the e2ects of ice.

The %ules assume that the structural assessment of hullstrength members is valid for the following designtemperatures:

0 C !nternal envronment "#argo and $ater ballast


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!nternal envronment "#argo and $ater ballasttan%s&A speci1c gravity #-< of 6.8@, or a higher value ifspeci1ed by the designer, is to be used for oil cargoes for

the strength assessment of cargo tank structures.

/or the fatigue assessment of cargo tank structures, arepresentative mean cargo density throughout the ship>s

life is to be used. The representative mean density is tobe taken as 8.7 tonnesm? or the cargo density from thehomogeneous full load condition at the full load designdraught Tfull, if this is higher.

A #- of 6.8@ is to be used for water ballast.

The %ules are based on the following design temperaturesfor the cargo:

a< ma)imum cargo temperature is E8 C!C

S l d


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Stru#tural #onstru#ton and ns'e#tonThe structural reuirements included in the %ules aredeveloped with the assumption that construction andrepair will follow acceptable shipbuilding and repair

standards and tolerances. The %ules may reuire thatadditional attention is paid during construction and repairof critical areas of the structure. Tank strength andtightness testing are to be carried out as a part of the

veri1cation scheme.

The %ules de1ne the renewal criteria for the individualstructural items. The structural reuirements included aredeveloped on the assumption that the structure will besub;ect to periodical survey in accordance with individual!lassi1cation #ociety %ules and %egulations. All structuralelements are to be arranged to allow aess for!ns#et!on. It is assumed that close-up ns'e#ton o(

the #rt#al areas $ll be #arred out on a regular

Th hi > t t i d i d


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The ship>s structure is designedsuch that:

it has inherent redundancy.

The ship>s structure works in a

hierarchical manner and, as such,failure of structural elements lower

down in the hierarchy should notresult in immediate conseuentialfailure of elements higher up in

the hierarchy.

Overve$ o( the stru#tural desgn 'ro#ess


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Overve$ o( the stru#tural desgn 'ro#essThe strength and acceptable safety of the hulland the structural elements is veri1ed through

the application of the following %ulereuirements:

a


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Ghen double hulls were 1rst mandatedthere were a number of risk related

concerns e)pressed.The more signi1cant areas of concern witheach of these items considered further

below< included:F maintenanceHF operationsH

F constructionHF salvageHF designHF stabilityH andF ventilation and access


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*antenan#e

/ailure to maintain the integrity of protective

coatings and cathodic protection in ballast tanks inparticular has led to leakage, pollution and

sometimes 1re.

!A%-& TA=I !lose up access can also be diBcult ifadeuate staging is not provided, as most Jcherry

pickersK will not pass through cargo tank accessopenings and the alternative of partial Looding ofthe tank together with use of a raft is often less thansatisfactory.


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O'eratons

Double hull tankers have two distinct operationaldisadvantages in terms of stability over single hulltankers.

/irst, for a given depth of ship, adding a double

bottom raises the ship>s centre of gravity andthereby reduces the ship>s reserves of stability.

#econd, free surface e2ects in cargo and ballasttanks during cargo operations may cause double

hull tankers to lose stability and su2er an angle ofloll, particularly if the design does not incorporatea longitudinal centre line bulkhead subdividingthe cargo space.

D bl h ll k i l


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Double hull tankers in generalgive improved cargo out turns

over single hull ships. Thesmoother internal tank surfaces

coupled with pump suctionsrecessed into wells in thedouble bottom make cargodischarge and tank washingmuch easier, giving an overall

reduction in cargo residue


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+onstru#ton

The con1ned spaces of double hull ballast tanks,whether sides or bottom, are far morerestrictive to work in than the comparativelyspacious ballast tanks of the single hull tanker

The double sides and double bottom form

natural three0dimensional rigid building blocks,less susceptible to deformation than thepredominantly two0dimensional components ofthe single hull ship.


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Salvage

f a double hull tanker should run aground andrupture the outside shell, the available damagestatistics suggest that the inner hull will, in mostcases, not be breached

Damage to an '' shaped double bottom ballast

tank on the other hand would cause Looding onone side resulting in a considerable list shouldthe ship not come to rest on supporting ground,but remain free0Loating.


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Desgn

Double hull tankers operate with global stresslevels some ?8M higher than those with singlehulls because of the uniform distribution of cargoand ballast over the length of the ship.

The conseuence is most likely to be smallfatigue fractures in early years of service,especially in larger double hull tankers, unlessgreat care is e)ercised in the design detail andsupervision of workmanship during construction.


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Stablty

The free surface e2ect is the degradation in transversestability which occurs when there are slack surfaces

Ghen combined with the e2ect of the double bottom

ballast tanks that e2ectively raise the centre of gravityof the cargo, there is a conseuential large reduction inintact stability

much more care needs to be taken in distributing thecargo on board a double hull than single hull tanker

task is helped by the use of on board computers


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,entlaton and a##ess

(ase of access for close up structuralinspection is an issue for all oiltankers, especially in the case of thecomparatively large single hulltanker cargo and ballast tanks. %afts,

remotely controlled vehicles, both inand out of water, ladder access andstaging are all used with varying

degrees of success.


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DO+U*ENTAT!ON- .LANS ANDDATA RE/U!RE*ENTS

Loadng n(ormaton

oading guidance information containingsuBcient information to enable the

master of the ship to maintain the shipwithin the stipulated operationallimitations is to be provided onboard theship. The loading guidance information isto include an approved loading manualand loading computer system

.lans to be su''led onboard the sh'


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.lans to be su''led onboard the sh'&ne copy of the following plans indicating thenew0building and renewal thickness for each

structural item:a< main scantling plansb< one copy of the 1nal approved loading manualc< one copy of the 1nal loading instrument test

conditionsd< detailed construction planse< weldingf< details of the e)tent and location of higher

tensile steel together with details of thespeci1cation and mechanical properties, and anyrecommendations for welding, working andtreatment of these steels < details and information on use of s ecial

Hull $eght .arametr# Estmaton


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Hull $eght .arametr# EstmatonA well known way of estimating is to use the ratiofrom one or more reference ships and multiply by

known uantities for the new design.3ere is an e)ample where, for a reference ship,we know that the ratio between lightship weightand the

cubic numberLghtsh' $eght- L0Length bet$een'er'end#ulars- L''Beam- B

Heght- DRato number- % 1L02L..3B3D

Lghtsh' $eght- L0 1K3L..3B3D

This ratio number can be used to calculate the lightshipweight for a ship with other main dimensions

%evised phase0out schedule for single0hull tankers enters


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%evised phase out schedule for single hull tankers entersinto force

A revised schedule for the phasing out of oil tankers and anew regulation banning the carriage of heavy grade oil insingle0hull oil tankers enter into force on @ April 88@. Themeasures were adopted in December 88? as

amendments to Anne) of the $A%"& !onventionN,following the =ovember 88 sinking of the oil tanker.restgeo2 the #panish coast.A revised regulation 6?- of $A%"& Anne) bringsforward the phase0out schedule for e)isting single0hull

tankers that was 1rst established in 677 and wassubseuently revised in 886 following the Er%aincident.t speci1es that tankers of single hull construction shouldbe phased out or converted to a Odouble hullO according

to a schedule based on their year of delivery. The double

Sngle4hull ol tan%er 'hase4out


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Sngle4hull ol tan%er 'hase4outnder the phase0out schedule, O!ategory6O single0hull oil tankers will not be able to

trade after @ April 88@, for shipsdelivered on or before @ April 67E orearlier< or after their anniversary date in

88@ for ships delivered after @ April67E


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!ategory oil tankers, which have some level ofprotection from protectively located segregatedballast tank reuirements will be phased out

according to their age up to 868. The year 868is also a 1nal cut o2 date for !ategory ? oiltankers which are generally smaller oil tankers.!ategory oil tankers commonly known as

$A%"& tankers< include oil tankers of 8,888tonnes deadweight and above carrying crude oil,fuel oil, heavy diesel oil or lubricating oil as cargo,and oil tankers of ?8,888 tonnes deadweight and

above carrying other oils, which comply with theprotectively located segregated ballast tankreuirements. !ategory ? oil tankers are oiltankers of @,888 tonnes deadweight and abovebut less than the tonna e s eci1ed for !ate or 6

Revised phase-out schedule:


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+ategory o( ol tan%er Date or year (or 'hase4out

Category 10 oil tankers of8,888 tonnes deadweight andabove carrying crude oil, fuel oil,heavy diesel oil or lubricating oilas cargo, and of ?8,888 tonnesdeadweight and above carryingother oils, which do not complywith the reuirements for

protectively located segregatedballast tanks commonly knownas "re0$A%"& tankers

Double Hull Tankers

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