Copyright 2005, Offshore Technology Conference

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AbstractReview of single and double hull tanker designs has been

conducted to identify the relevant aspects of the hull structurein the context of conversion into future FPSO systems. Typicaldesigns have been selected and structural analyzes proceduresfor verification of the hull girder strength and the primary andsecondary structures have been reviewed regarding yielding,buckling and fatigue strength. The typical defects and the maindegradation modes based on the return of experience withtankers are also identified and a set of hull design parametersare described in order to evaluate the hulls candidates toconversion.

The paper provides guidelines to assist the selection oftrading tankers for conversion into FPSO. Based on a set ofhull design parameters, different hull configurations areevaluated and the respective advantages and disadvantagesassessed in order to identify the best candidates forconversion.

Reserve of strength and corrosion margins have to beanalyzed for each hull and compared with the FPSO projectrequirements, including field design life, environmentalconditions and corrosion rates. Lists of typical defects andhazards based on the return of experience with tankers are alsodiscussed. The paper also discuss the different designs basedon the several parameters analyzed in order to provideguidelines to assess the hull structure condition, estimate therepair work at conversion and the inspection effort along theFPSO life.

FPSO conversion projects based on existing tradingtankers are still alternatives to new constructions fordevelopments in areas like West Africa, Brazil and South EastAsia. Nevertheless there are few tankers built before 1985 stillavailable for conversion. Consequently single hull tankersbuilt after mid 80s and double hulls built after early 90sbecame natural candidates to conversion into FPSO.Nevertheless, the decision making process regarding the

selection of such hulls faces the higher cost of refurbishmentof single hulls during the conversion and the high cost ofpurchase of more recent double hulls.

Abbreviations

FPSO Floating Production, Storage and Offloadingsystem

FSO Floating, Storage and Offloading systemHTS High Tensile SteelIMR Inspection, Maintenance and RepairMARPOL International Convention for the Prevention

of Pollution from ShipsMIC Microbial Induced CorrosionSBT Segregated Ballast TankSWBM Still Water Bending MomentSWSF Still Water Shear ForceVLCC Very Large Crude Oil CarrierVWBM Vertical Wave Bending MomentVWSF Vertical Wave Shear Force

IntroductionAlthough we see a shift towards new-build FPSO’s, in

particular for developments in harsh environment conditions,conversion seems to remain the basis for several projects inareas where benign environmental conditions are predominant,such as West Africa, Southeast Asia and Brazil.

Therefore, the possibility of fast track schedules to have anearly first oil date is also a important parameter in the decisionprocess for selection of the floating unit type of hull:converted or new-build.

Data concerning VLCC tankers built between 1973 and2004 have been reviewed in order to identify the main types ofoil tanker still operating and that may be available for FPSOconversion projects. In-service VLCC tankers can be dividedin two main groups: single hull and double hull tankers. Singlehull designs have been in general delivered until 1995. Despitefirst projects early 90’s, double hull tankers have been builtafter 1995, following MARPOL resolutions 13F and 13G.

The various aspects concerning the hull designcharacteristics and review of the ship’s historical information,including operation conditions, maintenance procedures andrecords of survey and inspections are reviewed based onBureau Veritas and the industry return of experience on oiltankers. The aim is to identify the main parameters to beconsidered during the hull selection process and therefurbishment work required to fit the structure for conversion.

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Selection of Trading Tankers for FPSO Conversion ProjectsP. Biasotto, V. Bonniol, P. Cambos; Bureau Veritas

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The return of experience from previous FPSO conversionprojects is also considered and the several steps necessary toachieve the hull structural assessment are described in thepaper, incorporating the lessons learned from these projects.

The CandidatesStatistic data of in-service tankers built between 1973-2004

shows that around 56% are double hulls, 43% single hulls and1% corresponding to other designs, such as double sidetankers and double bottom tankers (figure 1).

Probably the main characteristic of single hull designs builtafter 1985 is the extensive use of HTS. As there are fewtankers built between 1973-85 still available for conversioninto floating units (figure 2), single hull and double hulltankers built after 1985 and 1995, respectively, are naturalcandidates to conversion into FPSO. The decision makingprocess to select the hull will balance between the expectedhigher cost for refurbishment of a single hull at conversion andthe higher cost to purchase a double hull tanker. As shown intable 1, in 2004 the average cost to purchase a tanker builtbetween 1985-95 was in average 60 m USD against 85.5 mUSD for a vessel built between 1996-2000 (figure 3).

Oil Tankers – Average Sale Price (m USD)Year

1973-85 1986-95 1996-00 2001-04

2000 11.8 43.0 73.8 -

2001 12.6 35.2 79.2 82.5

2002 11.0 21.5 59.5 75.0

2003 13.0 32.8 52.0 65.4

2004 15.0 60.0 85.5 110.1

AverageSale Price 12.3 36.3 66.1 88.3

Shipsoperating 29 185 116 118

table 1: oil tanker average sale price (source: CW Kellock& Co. Ltd.)Three main categories of oil tankers are here defined and

their main characteristics are further described here after.Return of experience with converted FPSO’s shows that foreach of them the structure degradation during the tanker tradewill determine the FPSO design life. The main parameters aredetail design standard, fabrication standards, operationconditions, maintenance procedures and trading in harshenvironment.

Single Hulls (mid 70’s - 1985)Conversion of single hull tankers built before 1985 have

been a worthwhile alternative for FPSO projects. Such vesselsare relatively cheaper and their primary and secondarystructures use to be stiffer than in hulls built after 1985, wherehigh tensile steel has been extensively used.

A number of different designs can be identified,comprising American, French, German, Swedish and Japanesestandards among others. The industry has a good return ofexperience with oil tankers and an extensive list of areas prone

to defects due to stress concentration and corrosion is given[1], [2] and [3]. Nevertheless, it is well known that occurrenceof corrosion and other defects might vary according to theshipyard design, fabrication standards, workmanship and theoperation and maintenance procedures adopted during thetrade period.

There are a number of differences and particularitiesinherent to each shipyard design, but one of the mostimportant characteristics is the primary structure arrangement.Generally two main types of structural arrangement arenoticed:• Longitudinal ring stiffener system comprising deep

girders within center and side tanks, supporting thetransverse bulkheads horizontally stiffened. There are nohorizontal stringers.

• Horizontal system comprising in general four stringerswithin center and side tanks, supporting the transversebulkheads vertically stiffened. In general a longitudinalring is provided in way of the center line girder.

Fatigue was clearly not the main concern in the design ofconnections of primary and secondary elements for such typeof vessels, but in some way, it was apparently compensated forthe stiffer structural arrangement, larger corrosion margins andless use of HTS.During this period, oil tankers were built worldwide indifferent shipyards, some of them having poor fabricationquality.

Single Hulls (1985 - 1995)The structure design of such ships is optimized by means

of finite element calculations and the extensive use of HTS toreduce the weight of steel. The hull structure design has usedST355, ST315 and ST235 steel types in different manners, inparticular in the side shell plates, stiffeners and web frames.Some of them are built only using HTS in the cargo area.

The use of HTS, in particular in the side shell area aroundthe neutral axis, was found to be one of the main causes ofproblems associated to this type of vessel. Fatigue cracks ofside shell longitudinal connections to transverse primarystructures are probably the most typical defect of such designs,as fatigue strength of side shell longitudinals and corrosionmargins are affected by the less stiffer structural panels atbottom and side shell areas.

As a general rule, hulls built with ST355 should bespecially regarded as such vessels have an extensive record ofdefects and likely have been reinforced with additionalbrackets after construction and before the second ClassRenewal Survey.

Another important parameter is the use of asymmetricprofiles, i.e. angle stiffeners in the side shell and bottompanels. The use of angles at those locations having highprobability of failure should be specially considered whenevaluating hulls candidate to conversion, in particular whenassociated with use of steel ST315 and ST355.

Double Hulls (1996 - towards)Despite double hulls are here considered from 1996, a

number of design alternatives have been proposed in the early90’s. But the double hull milestone is 1998/99, when new

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builds experimented a sharp increasing and typical Japaneseand Korean shipyards have been consolidated as the mainreferences on double hull designs.

In the same way as for single hulls, the industry [4] haspublished guidelines listing the areas prone to defects due tostress concentration and corrosion. But again the occurrence ofcorrosion and defects might vary according to the shipyarddesign, fabrication standards, workmanship and the operationand maintenance procedures adopted during the trade period.Nevertheless, such designs profit of improvements on fatigueassessment methodologies, design of connection details andfabrication standards over the past 10 years.

Differently of the second category of single hull tankerdesigns, ST355 is almost no longer used and typical steeldistribution combines ST315 at bottom and upper deck areaswith ST235 around the neutral axis areas. The latest one within general a percentage not less than 30% of the total crosssection.

Double hull designs have the number of structuralconnections increased up to 15% due to the inner bottom andside longitudinal bulkhead stiffener connections to transversefloors and web frames, where despite the easier access toinspection might increase the inspection effort of critical jointsalong the FPSO service.

Double hull tankers have an optimized hull design, ingeneral less than 5% margin in comparison with the classrequirements, in particular at lower areas of the transversesection.

Asymmetric profiles as angle stiffeners are again one ofthe most important aspects regarding fatigue strength for sideshell longitudinal end connections. The use of angles, inparticular at those locations having high probability of failureshould be specially considered when evaluating hullscandidate to conversion.

Vessels built after 1997 have similar structuralarrangement, one of the most significant differences betweensuch designs is probably the position of the cross-ties, fittedeither within the center cargo tanks or within the side cargotanks. It is noticed that for the first one (figure 4.a), the sideshell and the lower hopper structures use to have higherdeflections. It means that special attention should be paid toside and inner shell longitudinal stiffeners connections,horizontal girders and hopper connections with the innerbottom and inner hull structures.

For designs where the cross-ties are fitted within the sidecargo tanks (figure 4.b), side shell structure deflections aredecreased and stress levels are expected to be reduced. On theother hand, longitudinal bulkheads are expected to havedeflections increased, in particular at alternate loadingconditions where center tanks are full filled and side tanks areempty. Therefore, special attention should be paid to the innerlongitudinal bulkhead connections and in way of horizontalstringers.

Return of Experience on FPSO’sThere are approximately 97 FPSO’s and 74 FSO’s in

operation or available worldwide [5]. Over 60% areconversion projects.

Return of experience shows that some have experimentedstructural anomalies and defects, independently of they are

newbuild or converted units. Despite they may be presented ina different degree of intensity, corrosion wastage and defectsdue to fatigue are the major threats for both types of hull.

Deterioration processes are initiated during the tanker tradeperiod (approximately 45 FPSO’s operating were built before1985, 4 between 1985-95 and 1 after 1995) and will be presentalong the FPSO’s service life.

A number of FPSO’s have been converted on the basis ofthe Classification Society rules requirements, unless additionalrequirements were specified by operators. It means thatinspections at conversion have been carried out within thescope of Class Renewal Survey based on a 5-year cycle, wheresubstantial corrosion of structural components is verified inaccordance with applied rules. Surveys are carried out in otherto identify anomalies and defects, and necessary repairs areperformed in order to bring the structure to as-builtconfiguration.

The problem with such approach is that the durability ofthe hull structure is not properly assessed against the requiredservice life for the FPSO, in addition safety margin of criticalcomponents is not evaluated. As a consequence, these unitsmight arrive at the first and second class cycle havingsubstantial corrosion of the structural components of thestructure, increasing the risk of buckling and fatigue.Consequences may be divers, comprising low and majorsevents in case proper mitigation actions are not taken:• Reduction of storage capacity,• non planned shutdowns for repairs,• dry-docking for major repairs,• Structural failure• Leak of oil, consequently risk of pollution and

explosion,Most common anomalies found in converted FPSO’s are

the following:• Excessive pitting of horizontal structures• Substantial corrosion (grooving)• Knife edging• Cracks due to fatigueCracks due to fatigue result from a combination of several

parameters. The trading history of the vessel, site specificconditions (for harsh environments) and high stress rangesexperienced during the loading and offloading cycle in theFPSO service are the main reasons. However, poor detaildesigns, low standard of workmanship and corrosion ofwelded joints accelerating the occurrence and increasingsignificantly the number of defects, where some of the mostcommon are the following:• transverse brackets at transverse ordinary frames• end of horizontal stringers• end of cross ties in way of the side shell and

longitudinal bulkhead web frames• end of longitudinal stiffeners in way of bulkhead

penetrations and ordinary web frames.The damage cumulated during the trading history of the

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vessel uses to be the most significant parcel of the totalcumulated damage comprising tanker and FPSO phases, inparticular for ships operating in severe environment such asNorth Sea and North Atlantic areas. Cumulated fatiguedamage due to wave cycles for an FPSO operating in benignenvironments such as West Africa and Brazil (Campos Basin)are in general less significant when compared with a harsherenvironment. For this one the fatigue damage due to sea loadmay be in the same order of that one observed during thetanker trade period.

The fatigue damage caused by loading and unloadingcycles during FPSO operations can be also estimated andcumulated with the damage due to wave cycles, but in generalthe number of cycles associated to the shuttle tanker arrival islow, consequently the cumulated damage is in generalnegligible.

On the other hand, the effect of the loading and unloadingcycle due to differential head on the oil tight bulkheads andside shell panels may lead to high stress ranges, specially inpoor detail designs. Crack propagation analysis shows that theloading sequence may accelerate the growth of cracks initiatedduring the tanker trade period [6].

Hull SelectionEvaluation of VLCC’s candidates for conversion into

FPSO’s may include a significant number of parametersconcerning the characteristics of these vessels. Obviously thefirst step is to evaluate their adequacy regarding the basicrequirements of the FPSO conversion project, which areassociated with the field development. Storage capacity andthe cargo tanks arrangement (number of tanks, subdivision,etc) are some of these parameters.

Certainly one of the most important and difficult aspects tobe evaluated is the effective service life of the unit, whichshall meet the required service life established based on thefield estimated life.

Once several candidates may be available, attention shouldbe paid to the strength verification of the overall hull and itsstructural components. Structure safety factors should beassessed taking into account the hull condition at the end ofthe FPSO service life.

Engineering assessment of the overall and local hullstructure strength against yielding and buckling criteria hasbeen quite successful for the majority of the FPSO projects.However, experience shows that an accurate assessment of themain deterioration modes of these components may not be aneasy task, in particular concerning corrosion and fatiguedeterioration.

Fatigue is related with cycle loads (due to wave and cargoloading-offloading), the structure stiffness, structural detaildesign, shipyard fabrication standards, workmanship andcorrosion rates. The hull condition is linked to operation andmaintenance aspects in such way that the condition of aspecific vessel might not be the same as for a similar shipoperated by different owners and in different trades.Therefore, the FPSO design life will be directly affected bythe fatigue damage and corrosion wastage cumulated duringthe previous tanker phase.

Ideally, design review and assessment of the hull conditionshould be supported by a detailed inspection of the entire hull

structure before select the vessel to be converted. A hot spotmap of the hull structure, derived from a detailed hull structureassessment based on finite element analysis, is also useful toevaluate the different hulls, however it may not be availableduring the selction process. Therefore, the various aspectsconcerning the hull design characteristics may need bereviewed in a more qualitative way.

The hull design review may comprise a preliminaryanalysis of the existing as-built drawings of the vessel. Basedon the return of experience, a number of parameters can bedefined and a checklist elaborated for a quick review of thecandidate vessels.

For long term projects, where the unit should stay on siteduring 10, 15 or even 25 years, this would not be enough toensure the integrity of the unit along the intended service life.A complete hull structure assessment should be available inthe early stage of the project and preferable before therefurbishment program is defined.

As a minimum scope, the tanker selection process shouldinclude for each vessel the review of the following data:• As-built drawings: look for detail designs prone to

fatigue, percentage of HTS, corrosion additions, etc.• UTM Reports and survey and inspection records to

identify corroded areas and typical defects: look fortypical defects due to fatigue, high stressed areas andbuckling. Check for substantial corroded areas.

• repair specifications in order to evaluate:- extension of scantling renewal due to substantialcorrosion along the tanker service

- mitigation of defects, including repairs, modificationsand strengthening undertaken on board

• Operation and maintenance practices (type of oil,temperature, washing, corrosion protection, etc)

• Review of tanker trading history: tankers operating insevere weather are prone to have fatigue relatedproblems

Details DesignCritical areas within the tank structure can be defined as

locations that, by reason of stress concentration, alignment ordiscontinuity, need particular attention for what regards theconstruction, the design and the survey.

In general these locations can be divided in two maingroups:• connections of the longitudinal ordinary stiffeners with

transverse primary supporting members (transversebulkheads and web frames)

• connections of primary supporting members

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Ordinary stiffener connection with transverse supportingstructures

It is through these connections that the loads aretransmitted from the secondary to the primary structuremembers.

These details are subjected to high cyclic loading throughthe ship’s life and they constitute one of the most subjected tofatigue potential problem areas. Repair of such details mayaffect the tanker refurbishment schedule, as the work requiredto repair, renew and strengthen these details is much moresignificant than the required weigh of steel.

The following parameters are considered the mostimportant ones regarding fatigue strength of such connections:• the location and the number of brackets (on one or both

sides of the transverse primary member),• the shape (soft toes or not) and the size of the brackets,• the longitudinal stiffener profile (symmetrical or not).• the misalignment of the webs of longitudinal ordinary

stiffeners• the use of HTS in the side shell plate, stiffeners and

web frames• the details of slots and collar platesThe different types of ordinary stiffener connections with

the transverse web stiffeners, and how the change of type ofconnection may increase the fatigue strength of the detail byreducing the stress concentration factor is illustrated in figure5.

The position of the center of torsion of angle profiles(shifted off the profile) induces additional local stress in theprofile, differently T profiles have their center of torsion inway of web plate (plane of symmetry). In general T profilesshould be used in areas subject to high local pressure, i.e.upper zones of side shell, longitudinal bulkheads and innerhull.

The type of scallop is another important parameterregarding fatigue of longitudinal stiffener end connections(figure 6). The main aspects are the following:

Scallop with or without collar plate:The classic connection is the scallop connection, a

relatively large cut-out of the primary member leaving onlyone side welding possible for the web of the secondarystiffener against the primary member.

To provide welding on other side, a collar plate may befitted. This collar plate is not in the plan of the web of theprimary member, which could lead to possible problems ofcracks. Nevertheless, this is less critical than the profiled slotform the fabrication point of view.

Profiled slot:The method is to cut out of the transverse member a

section, which is infinitesimally larger than the web plate ofthe secondary stiffener and a larger cut–out for the flange ofthe longitudinal. This gives the possibility to weld from theboth side the connection between the web of the stiffener andthe plate of primary member allowing a good transmission ofthe stresses.

The risk of cracks with bending mode is limited with thisdesign but it remains a probability of cracks due to shearmode. Nevertheless, tolerances should be strict in such waythat production and construction are delicate processes.

Connections of primary supporting membersThe most critical types of joint are the welded angles and

cruciform joints that are subjected to high magnitudes oftensile stresses.

The following parameters are considered the mostimportant parameters regarding fatigue strength of suchconnections:Angle connection (figure 7):

The most general type of welded joint is the angleconnection found mainly in the following structures:

• double bottom in way of transverse bulkheads withlower stool,

• the double bottom in way of hopper tanks,• the lower part of transverse bulkheads in way of the

lower stool (if any),• the lower part of inner side in way of hopper tanks.

Cruciform joint (figure 8)The cruciform connection is a particular case of angle

connection. Indeed, the angle between the plates is now a rightangle. The cruciform connections may be found in:• the double bottom in way of transverse bulkheads

without lower stool,• the double bottom in way of the inner side when there

are no hopper tanks,• the double bottom in way of longitudinal bulkheads.

Return of Experience on Oil Tankers

Corrosion protection systemIn general in service single hull tankers are found having

moderate corrosion rate and thickness diminution. Howeverthere are some cases where excessive pitting in the bottom andtop tank in cargo tank plating have been reported due tomicrobial attack in areas where coating protection is notprovided.

Cargo tanks of typical single hull tankers may not havebeen provided with corrosion protection systems, i.e. coatingand anodes. Residual water from oil cargo causes groovingand pitting corrosion in the upper surface of horizontalstructures like stringers and bottom plating at the aft end of thecargo tank.

The normal corrosion rate in cargo tanks uncoated areas ofdouble hull tankers is expected to be moderate based on theprevious experience with SBT tankers, where horizontalstructures like stringers and bottom plating would be areas tobe subjected to special attention. However, cases ofaccelerated corrosion rate in cargo tanks have been reported.In addition, there has also been an increase in the pittingcorrosion rate in cargo tank bottom plating.

Double hull designs can be found having cargo tanks partlypainted with epoxy coating in the under deck (3 meter below)

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and eventually the tank top plating (inner bottom), varyingdepending on owner requirements. But areas where coatingfails may have accelerated corrosion, in particular in the deckhead areas subject to increased deflections and stress levelsdue to the use of HTS.

Accelerated corrosion in cargo tanks may be due tomicrobial attack from bacteria in the cargo oil. Cargotemperatures in double hull tankers can be found up to 20°Chigher than in single hulls due to the insulation provided bythe inner hull. These higher temperatures would provide thenecessary conditions to the microbes remain active longer andproduce corrosive acidic compounds increasing the risk ofMIC.

Higher temperatures mean that the humidity is higher,increasing the amount of water vapour in the air space abovethe ballast and cargo tanks. Therefore, coating in ballast tanksbottom shell remains continuously wet, having mud settled inparticular at aft locations due to the ship trim. Thisaccumulation of mud could also generate a higher risk of MIC.

Corrosion prevention systems are likely provided in ballasttanks. In order to prevent accelerated corrosion of the underdeck and the tank bottom areas, a number of operators requirepainting of these areas also in cargo tanks.

The total surface area to be coated and maintained indouble hulls can be up to three times larger when compared tosingle hull tankers. Consequently, the maintenance of coatingsystems is one of the most important aspects regarding the hullstructure condition.

The following main aspects should be considered in orderto evaluate the durability of the coating system of cargo andballast tanks:• Coating specification• Coating preparation and application• Workmanship during construction (grinding of sharp

edges for instance)Double hull tankers have structural flexing increased

compared to single hull designs which leads to higher crackingpotential in particular in the deck, inner shell and inner bottomareas.

Fatigue of longitudinal stiffeners connectionsConnections in way of transverse bulkheads are, in

general, fitted with double brackets, however thoseconnections in way of ordinary frames are often fitted withsingle brackets. In cases where HTS is extensively used,backing brackets might be necessary be fitted in way ofordinary frames along the hull length at conversion into FPSO.

As a general rule, the following parameters were found tobe the most significant ones regarding fatigue strength of sideshell longitudinal connections and increase fatigue life:• Double brackets reduce stress concentration factors.• Type of steel: ST235 construction leads to stiffer

structures around the neutral axis. In the contraryST315 and ST355 lead to less stiff structures.

• Symmetric profiles avoid lateral bending oflongitudinal stiffeners. On the other way angle profileslead to lateral bending.

• Shaped brackets and flat bars reduce stressconcentration factor. However fatigue life dropsquickly where flat bars are fitted.

Flexural (figure 9) and shear (figure 10) are the two mostcommon failure modes regarding fatigue cracks at such typeof connections.

The most common defects in single hull designs areattributed to the extensive use of HTS, in particular in the sideshell construction. Some tankers built between 1985-95 hadlongitudinal stiffener connections locally renewed andprovided with backing brackets around ten years service life inorder to increase fatigue strength.

In a general way, utilization of HTS in double hulldesigns results in the increasing of deflections and stresslevels, affecting negatively fatigue life of structuralconnections and the effective life time of coating systems.

Double hull tankers built early 90’s were subject to anumber of typical defects due to poor design details, speciallyat side shell longitudinal connections to primary transversestructures and connections of hopper to inner bottom and sidelongitudinal bulkhead. Significant improvements on designdetail and workmanship have been achieved and implementedin the designs built late 90’s.

It is remarkable that a number of double hull tankersoperating in severe environments like North Sea and NorthAtlantic have reported damages at side shell and bottomlongitudinal stiffeners in way of transverse primary structure.

Environmental LoadsDifferent to other Class societies, Bureau Veritas ([7], [8])

apply loads depending on the operational conditions of thefloating unit. There are variations in the overall severity of theclimate in a given area, additionally different vesseldimensions, shape and load distributions need be taken intoaccount. Therefore each project should be provided withcalculation of hydrodynamic loads and vessel motions in thefrequency domain, using the 3D diffraction-radiation methodand taking account of site water depth.

Site specific conditions need be taken into account in thehull structure assessment of the FPSO, including metoceandata of the site and the mooring conditions, in order toproperly define the sea loads on the structure. Such datainclude the wave directions, the wave spectrum and therelative headings.

The main wave load parameters derived from the directhydrodynamic models and used in the structural anlyzes arethe following:• Global Hull girder loads (Wave Bending Moment and

Wave Shear Force)• Relative wave elevation, that is indicative of variation

of pressure on side shell• Vessel accelerations

Hydrodynamic analysisThe outputs of the hydrodynamic analysis determine the

design load parameters to be applied to the structural model inorder to assess the scantlings of the structure. Thehydrodynamic model takes into account the unit hull forms,

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the lightweight distribution (including structure weight,topside weight, turret weight, etc.), the loading conditions andthe connections with the seabed. At least three loadingconditions are in general analyzed, comprising the full rangeof the FPSO loading conditions:• Minimum draught• Intermediate draught• Maximum draughtFPSO’s are generally moored by two different types of

systems that influence the calculation conditions of thehydrodynamic analysis:• Spread moored design: the unit is maintained in a

constant position independent of the sea and currentheading, while the environmental loads have apredominant direction. Generally, the mooring lines areconnected to the main deck at the fore and aft ends.

• Turret moored design: the unit is free to weathervaneand has a natural tendency to orientate in the directionof the most severe environment component. Themooring lines are connected to the turret, generallylocated in the fore part of the unit.

Wave load parametersThe hydrodynamic analysis results in the following

parameters valid for the offshore unit at the intended site andwith precision of each value along the length of the unit:• wave induced bending moment,• wave induced shear force,• total accelerations in all directions, at the center of

gravity of each compartment and at relevant positionsin topside areas,

• relative wave elevation,• sea pressure on the shell diagrams.The values of the parameters may be modified to include

safety margins or adjusted to rule values when applicable.Depending on the site, the results of the hydrodynamic studymay be higher or lower than the rule standard values.In the case when site-specific loads are considered, the resultsof the hydrodynamic calculation may be directly input in thestructural analysis, depending on the applied safety margins orclass rules, if the case.

Hull Girder CapacityThe hull girder total capacity needs to be checked along

the length of the unit.Maximum permissible still water bending moment

(SWBM) and shear force (SWSF) are derived from the wavebending moment (VWBM) and the wave shear force (VWSF)values obtained from the hydrodynamic analysis results.

Calculations show that in general SWBM and SWSF couldbe increased for benign environmental conditions, such as inAngola and in Nigeria for instance, but varying depending onthe ship characteristics.

The additional reserve of strength due to less severeenvironments should not be fully credited to the allowableSWBM and SWSF to assure the normal ratio between static

and dynamic loading applied to the hull structure.SWBM and SWSF will be driven by the maximum values

of VWBM and VWSF (including safety margins) and thecalculated total hull girder capacity, taking into considerationthe corrosion wastage along the tanker trading service.

It should be noted that the direct analysis of wave inducedshear force will result in a distribution slightly different to theone given by the typical rule distribution. Despite VWSFdistribution has two peak values at approximately L/3 from thehull aft and fore ends, the hull capacity needs be checkedalong the length of the unit and in particular at the position oftransverse bulkheads, where the total shear force has itsmaximum value. Oil tankers often have local reinforcementsin the longitudinal bulkhead and side shell plating, over one totwo web spacing, aft and fore of transverse bulkheads, toallow for peak values of the total shear force.

Considering the wave load parameter results for a unitmoored offshore Angola for instance, the site specific valuesfor VWBM and VWSF are approximatelly 0.3 and 0.5 of theNorth Atlantic reference values, leading to an increasing overthe minimum Rule value of 0.65. Nevertheless, the allowablevalues would still be increased up to 50% for SWBM and upto 35% for SWSF.

Hull Structure AssessmentAssessment of the FPSO hull structure should verify the

adequacy of the tanker hull with the project specification, itmeans the strength of global and local structures, consideringstorage capacity, topsides additional weight and specificenvironmental loads. Furthermore, the hull condition at theend of the unit intended service life should be also assessed toensure the hull structure integrity along this period.

The refurbishment work necessary to fit the hull forconversion is not the most expensive cost regarding the wholeconversion, however it plays an important role in theconversion schedule, as delays caused by the works in the hullmight delay the unit commissioning. Repair and strengtheningworks prior or at conversion will also affect the unit along itsservice life.

Due to time constraints, extensive structural analyses maynot be feasible before inspections carried out prior to therefurbishment work. However, always when possible it isrecommended to have first calculations and results of the hullstructure assessment available in an early stage. This would behelpful to define the scope of the inspections and consequentlyto specify the required steel renewals and modifications.

Hull structural assessment is a multi-step analysisprocedure, where global coarse mesh analysis are usedfollowed by local fine mesh analyses at critical locations,selected based on the coarse mesh results and on the return ofexperience.

The structural analysis is based on the design loadparameters given by the hydrodynamic analysis.

Differently of newbuild hulls, structural assessment ofconverted FPSO’s requires two phases to be considered:tanker and FPSO, as an FPSO presents specific characteristicswhen compared with trading tankers. The main characteristicsof the FPSO are the following:

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• The unit is fixed on a specific site, generating specificloads.

• The unit is permanently moored (no dry dock isplanned for inspection and maintenance).

• Additional loads generated by the mooring system,risers and topsides.

• Continuously loading and unloading on site (constantvariation of draught), differently of tanker which aregenerally operated in full and ballast conditions.

Therefore, specific assessment of the FPSO hull structureneeds be undertaken taking into consideration suchparticularities. The following limit states are verified for thehull structure components:

Yielding Buckling UltimateStrength Fatigue

Hull Girder

Plating

Ordinarystiffeners

Primarysupporting

Structuraldetails

Hull girder strengthThe first step of the hull structure assessment is in general

the verification of the global hull girder strength. A yieldingcheck is performed, comparing the bending moment applied tothe structure does not exceed the bending moment capacityprovided by the actual hull scantling configuration. Thebending moment applied to the structure is given by themaximum allowable still water bending moment derived fromthe assessment of the loading conditions of the unit (operation,transport, maintenance, etc) and VWBM derived from thewave load parameters analysis.

It should also be checked that the bending moment appliedto the structure is lower than the ultimate bending momentcapacity of the hull girder, taking into account a safety factorincreased compared to ships.

Local strength of plating and ordinary stiffenersThe second step is the verification of the scantlings of the

plating and the ordinary stiffeners. These elements areassessed through a 2D section model loaded with the designload parameters defined by the hydrodynamic analysis. Ayielding check and a buckling check are carried out forstiffeners and plates.

In all cases, local loads are calculated for the expectedmost severe conditions. Each element is analyzed consideringthe compartments as being alternately full or empty, for thepurpose of maximizing the loads induced on that element bythe cargo carried. Similarly, for the elements of the outer shell,

the external sea pressure is calculated at the full load draughtwhen the side tanks are considered empty (cargo loadingconditions), and at the light ballast draught when the sidetanks are considered full (ballast loading conditions).

Strength of the primary structureThe first objective of the finite element analysis is to

determine the stress distribution in the primary supportingmembers. It allows verifying that the scantlings comply withthe yielding and buckling criteria.

Two levels of meshes are generally necessary to assess thestrength of the structure. The first step is the global coarsemesh model (figure 11). The 3D coarse mesh model allowsverify the overall behavior of the primary structure. There aretwo main approaches:• The simpler and faster one is based on a three-cargo

tank model, where the beam theory is used to balancethe model and obtain the desired bending moment andshear force distribution in the mid tank area. VWBM,VWSF, accelerations and relative wave elevationsderived from hydrodunamic analysis are used.

• Complete ship model including the entire hull structureover the unit length, from aft to fore ends can be used.Obviously it is a more time consuming approach. Onthe other end, accelerations and wave pressuredistribution derived from hydrodynamic analysis areapplied along the hull structure model in order toequilibrate it and obtain the correct bending momentand shear force distribution along the hull.

Fine meshes are also performed to get more accurate stresslevels in specific locations (figure 12).

Detailed Stress analyses are in general performed for thefollowing typical primary structures:• horizontal stringers in way of typical oiltight bulkhead• horizontal stringers in way of typical swash bulkhead• typical transverse ring• typical first transverse ring aft and forward oiltight and

swash bulkheads• Longitudinal girders in way of oiltight bulkhead and

swash bulkhead• FPSO specific areas (topside supports, turret structure,

hull connections and other hull attachments).Main scenarios

The tanker and the FPSO phases need be distinguished instructural assessment of the converted FPSO to properconsider the effects of previous corrosion and cumulatedfatigue damage during the trade, but also the floating unitdesign characteristics and operation conditions.

The following scenarios are therefore usually studied:(a) As-built tanker at construction date(b) Actual condition (at conversion date)(c) End of FPSO service life: derived based on the estimated

corrosion wastage cumulated during the FPSO phase.Scenario (a) is mainly used to assess the fatigue damage

cumulated during the tanker phase. It may be also used for afirst screening of the candidate vessel in order to identify the

OTC 17506 9

most loaded areas to be further investigated during inspectionscarried out prior to the hull refurbishment program.

In scenario (b), the existing 2D and 3D finite elementmodels (initially modeled based on the as-built configuration)are updated in order to take into account the FPSO actualconfiguration, new loading conditions, site specificenvironmental loads, structural alterations and topsides addedat conversion. Derived from the latest gauging reports and/orestimated based on typical oil tanker corrosion rates [2], thewastage cumulated during the tanker trade period can becalculated and the hull overall scantlings are updated.Renewals foreseen during the refurbishment may be also takeninto account if already decided. The FPSO hull structurestrength will be verified in accordance with the several limitstates above described.

Scenario (c) can also be studied, incorporating thecorrosion wastage expected during the FPSO phase, theexpected hull structure condition of the unit at the end of itsservice life is assessed. The reserve of strength of the hullstructure components could be estimated and for thoseelements found critical, the alternatives to mitigate thedeterioration effects along the life of the unit could also beidentified, such as:• Additional renewal and strengthening work at

conversion• Specify corrosion protection system to avoid corrosion

wastage at critical locations.• Improve the IMR plan to ensure the hull structure

integrity

FatigueThe efficiency of the structural connections subjected to

high cyclic stresses needs be checked with respect to possiblefatigue related problems.

Fatigue assessment can be divided into main groups:• connections of the longitudinal ordinary stiffeners with

transverse primary supporting members (transversebulkheads and web frames)

• connections of primary supporting membersThe first group can be assessed by BV program

VeriSTAR/MARS where library of details is used to evaluatethe fatigue strength of end connections of longitudinalstiffeners by mean of fatigue deterministic approach. 2Danalysis is therefore carried out in order to evaluate thestrength of side shell and bottom longitudinal connectionsalong the cargo region due to fatigue flexural mode in way ofeach oil tight bulkheads and in way of ordinary frames.

3D finite element analysis is needed to capture the localstresses in non-standard details for use in the fatiguecalculation. Therefore, very fine mesh models (figure 13) areused, having element size between once and twice thethickness of the structural element.

Assessment of detail connections shall take into accountboth tanker and FPSO phases. Fatigue damage taking intoaccount wave cycles and loading/unloading cycles arecumulated with the damage associated to the tanker phase inorder to estimate the remaining fatigue life at conversion.

Environmental loads for fatigue assessment of the tanker

may be derived based on the analysis of the trading route. Incases where the trade data is not available, standard BureauVeritas Rules wave load parameters for a typical World Widetrade tanker can be used.

In general, tanker analyses are carried out based on thetypical loading conditions: full load and ballast loadingconditions leading to maximum sagging and hogging loadcases. For the FPSO, review of the operational loadingconditions comprising the loading/unloading sequences shouldbe carried out to determine the representative loadingconditions to be considered in the analysis.

Deterministic fatigue strength assessment:The deterministic methodology has been further developed

for the FPSO’s by introducing several draughts and loadingcases over the expected lifetime of the floating unit. Thecalculation method applied today by Bureau Veritas has beencalibrated against spectral fatigue strength assessment and themethodology is applied in-house for screening of the structuraldetails.

The results of the deterministic approach are in generalexpected to be more conservative than those given by thespectral one.

Spectral fatigue strength assessment:In order to assess more precisely the fatigue damage of the

structural details a spectral fatigue strength assessment may becarried out. The spectral analysis is to be carried out in thefollowing three steps:• hydrodynamic analysis• structural analysis• calculation of the fatigue damageThe hydrodynamic analysis determines the loads and the

resulting motions generated by the environmental loads; thesite-specific environmental loads are thus taken into account.

The loads obtained through the hydrodynamic calculationare applied to the structural model. The structural modelprovides the RAO’s of stresses at location of interest, withinthe model. The fatigue damage is then calculated based onstatistics of stress ranges.

At least three draughts (and associated loading conditions)5 headings and 25 frequencies are to be taken into account, butmay be adapted depending on the type of mooring.

The acceptable damage depends on the location, theaccessibility for inspection, maintenance and repair, and on theconsequences of failure.

Loading unloading fatigue assessment:Fatigue damage cumulated during the FPSO phase due to

wave and loading/offloading cycles is also assessed.For the calculation of the stress range due to the loading

unloading, the wave at a return period not less than one day isto be taken into account.

The damage ratio of loading unloading is to be combinedwith the one due to the wave effect.

10 OTC 17506

Considerations about welding of structural membersTo evaluate the standard and quality of welding on

seagoing tankers with respect to possible conversion toFloating Storage Unit, a comparison has been made with anew-built FPSO and a tanker.

In general, welding leg length and scantlings of the FPSO,with the exception of interface structures with Offshoreequipment and topsides, are often made on the basis of Ship'sRule requirements.

Due to welding of interface structures and FPSO-owner'sspecial requirements for particular areas (additional thickness,fatigue life etc.), welding leg length of FPSO seems to bemore significant. Possible difference is coming from the morecomprehensive loading booklet giving higher values of hullgirder still water bending moment and shear forces.

Deck Transverse Capacity and Topsides ModulesIntegration

Hull structure elements of oil tankers built after 1985 are ingeneral more optimized than in previous designs, particularlythe 70’s tankers often used in previous conversion projects. Inaddition, double hull tankers have in general higher stresslevels than single hull tankers. Therefore top tank structurestrength and their integration with the topsides modules needto be carefully investigated in order to identify needs foradditional strengthening.

Topside modules should be supported in way of transverseand longitudinal bulkheads and deck transverses. Finiteelement models, as described above, are used to verifystrength of such structures and specify eventual additionalstrengthening of the under deck structure. Top down analysisusing VeriSTAR Hull is an efficient way to perform suchverifications taking into consideration local loads induced bytopsides, but also the hull behavior due to sea loads andinternal liquid loads. Global loads induced by the hull girderbending are also taken into account.

Refurbishment ProgramRefurbishment work of the oil tanker hull may be

undertaken before or at conversion, depending on the FPSOproject strategy. The scope of work should be defined basedon the hull structure condition assessment and on detailedinspection of internal and external part of the hull, supportedby finite element analysis.

The refurbishment specification is expected to contain atleast the following main contents:

Outcomes from the hull structural assessment shouldprovide• Plates and stiffeners to be renewed and/or strengthened

based on the FPSO hull structure verification• Plates and stiffeners to be renewed and/or strengthened

based on the future corrosion study• Detail connections to be renewed, modified and/or

strengthened based on fatigue analysis, taking intoconsideration the cumulated damage during the tankertrade and FPSO phases

Despite information incorporated in the previous surveyand inspection reports may be taken into consideration in the

structural analysis, there are a number of parameters that canbe proper assessed only by means of inspections.• Pitting on tank bottom plating, stiffeners and other

horizontal members.• Brackets and web stiffeners may present heavily

corroded.

Plate renewalRenewal thickness of plates and stiffeners needs to be

defined in order to determine the total steel renewal required atconversion. The expected corrosion during the FPSO servicelife is to be taken into account. Therefore the renewalthickness at conversion is defined as follows:

tLFTT FPSOquirednewal ×+= ReRe

Where:TRenewal = renewal thickness at conversionTRequired = required thickness derived from rules or by mean of

hull structure calculations as described above.LFFPSO = required FPSO service lifet = yearly corrosion rate for the studied element

According to class rules, substantial corrosion margin isdefined as 75% of the allowable corrosion margin. Therefore,those elements found during the FPSO service life havingsubstantial corrosion will be subject to further inspections,more extensively and more often. Therefore, in order to avoidthe increase the scope of inspections during the unit servicelife, the renewal thickness at conversion should also includesufficient margin to avoid substantial corrosion of suchelements found critical in the hull condition assessment.

FatigueReturn of experience shows that the most common actions

in order to address fatigue cracks are the following:• fitting of backing brackets on side shell and bottom

longitudinal stiffeners in way of transverse bulkheadsand ordinary web frames

• changes of toes of horizontal girders, longitudinalgirder ends and web frames joints

Inspections should confirm whether the critical fatiguedetails are the same as indicated in the as-built drawings.Previous repairs as indicated in the tanker repair specificationsand survey records should be verified as well.

Fatigue analysis is in general undertaken by means of finiteelement calculations, in general to determine the cause of thedamage and test the alternatives of repair configurationobtained by improving of the detail design and reducing ofstress concentrations.

OTC 17506 11

ConclusionsThe selection of vessels for conversion into FPSO will

probably face two main types of design: single hull tankersbuilt between 1985-95 and double hulls built after 1996.

The return of experience from previous conversion projectsshows that both designs are prone to defects due to fatigue andcorrosion wastage. Despite the particularities inherent to eachhull design, corrosion and fatigue deterioration cumulatedalong the trade period will affect the FPSO hull structureintegrity and performance along its service life.

Thought that FPSO conversion projects requires fast trackschedules, a detailed hull structure assessment by means offinite element analysis may not be available at the selection ofthe vessel to be converted. Therefore, the various aspectsconcerning the hull design characteristics, ship’s historicalinformation, including operation conditions, maintenanceprocedures and records of survey and inspections should bereviewed at least in qualitative way during the hull selectionprocess.

In the same way that the hull structure may vary infunction of the shipyard design and fabrication standards, thehull condition may change depending on the trade, operationconditions and maintenance procedures. Therefore a completehull structure assessment by means of finite element analysisand consideration of the tanker trade phase degradation modesseems the most efficient way to proper assess the FPSO hullstructure design and screening of the critical areas andcomponents of the hull. Therefore, as far as possible, firstresults should be available early to assist the selection of thehull and the specification of the refurbishment program.

Even if the hull is not the expensive part of the productionproject, as a large investment is made on the topsides andsubsea equipment, special attention needs to be paid to the hulleffective design life. Safety margins should be estimated andthe necessary procedures to ensure the hull structure integrityshould be incorporated in the unit’s IMR plan.

AcknowledgementThe authors wish to thank Bureau Veritas for permission to

publish this paper. The views expressed are those of theauthors and do not necessarily reflect those of Bureau Veritas.

References[1] Guidance Manual for the Inspection and ConditionAssessment of Tanker Structures – Tanker Structure Co-operative Forum, Witherby & Co. Ltd., 1986.

[2] Condition Evaluation and Maintenance of TankerStructures - Tanker Structure Co-operative Forum, Witherby& Co. Ltd., 1992.

[3] Guidance Manual for Tanker Structures – Tanker StructureCo-operative Forum, Witherby & Co. Ltd.

[4] Guidelines for the Inspection and Maintenance of DoubleHull Tankers - Tanker Structure Co-operative Forum,Witherby & Co. Ltd., 1995.

[5] Floating Production Systems, assessment of the outlook forFPSO vessels, production semis, TLPs and spars –International Maritime Associates, Inc.. New Orleans, USAMarch 2004.

[6] Otegui J., Orsini M.: Converted FPSO’s. Making aWorthwhile Conversion – DOT 2004.

[7] Arselin E., Cambos P., Frorup U.: New FPSO Rules Basedon Return of Experience. – OMAE – FPSO’04 – 0091.Houston, USA 2004.

[8] Rule Note on Hull Structure of Production, Storage andOffloading Surface Units – NR 497, October 2004 – BureauVeritas.

[9] Rules for the Classification of Steel Ships – November2004. Bureau Veritas.

12 OTC 17506

Figure 1: hull type of VLCC’s built between 1973-2004.

Figure 2: oil tankers operating per year of build and average deadweight.

Hull Type 1973-2004

6.7%

36.0%

56.4%

0.9%

SH 73-84SH 85-95DH 96-04Others

Ships in Service - build 1973-2004

0

5

10

15

20

25

30

35

40

45

1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003

Year

Qty

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

500000

Ave

rage

Dw

t

OTC 17506 13

Ship Sales 2000-04 (USD)> 250,000 dwt

0.0

20.0

40.0

60.0

80.0

100.0

120.0

2000 2001 2002 2003 2004year

aver

age

(m$)

1973-85

1986-95

1996-00

2001-04

source: CW Kellock & Co Ltd

Figure 3: average sale price of VLCC’s between 2000-04.

Figure 4: double hulls typical structural arrangement arrangement

Figure 5: detail of longitudinal stiffener connections.

X

Y

ZX

Y

Z

0 . 00E+00

1 . 41E+01

2 . 82E+01

4 . 23E+01

5 . 64E+01

7 . 05E+01

8 . 46E+01

9 . 87E+01

1 . 13E+02

D I SPLACEMENTS; LOAD CASE 9 :PART I AL CARGO LC8 , HEAD, S, D, TROUGH, VBM

STRESSCOMPONENTS

LOAD CASE9PANEL SIGVM

A

X

Y

ZX

Y

Z

- 1 . 52E+01

0 . 00E+00

1 . 52E+01

3 . 04E+01

4 . 56E+01

6 . 08E+01

7 . 60E+01

9 . 12E+01

1 . 06E+02

D I SPLACEMENTS ; LOAD CASE 9 :PART I AL CARGO LC8 , HEAD , S , D , TROUGH , VBM

STRESS COMPONENTS

LOAD CASE 9PANEL SIGVM

B

Transverseelement

Stiffenerflange

Stiffenerweb

Longitudinalplating

Flat bar: Detail whenlow stress area

One backing bracket:improved design

Two backing brackets:reinforced design

14 OTC 17506

Figure 6: scallops of longitudinal stiffener connections.

Figure 7: angle connection Figure 8: cruciform joint

Figure 9: shear mode induced crack

no collar plate collar plate full slot

Shear introduced in the web of primarymembers by longitudinal stiffeners under localpressure: ‘local shear’

Shear in the web of primary member itself due toglobal displacements: ‘global shear’Shear mode:

Typical crack attransverse web frame

OTC 17506 15

Figure 12: fine mesh model – detailed stress analysis

Figure 11: Coarse mesh model

Figure 13: very fine mesh model – fatigue analysis

Flexural mode:Typical crack at transverse webframe

Figure 10: flexural modeinduced crack