tions to transverse primary structures are a typical defect as the fatigue strength and cor- rosion margins are affected by the less stiff structural panels. Notably, hulls built with ST355 steel type are especially prone to defects and probably have had to be rein- forced with additional brackets after con- struction. Angle stiffeners used in the side shell and bottom panel, in a so-called asym- metric profile, also have a high probability of failure and deserve special attention during the assessment of candidate conversion hulls. Double hulls The standardised double-hull designs, which started emerging from Japanese and Korean shipyards in the late 1990s, on the other hand, benefit from advances in fatigue assessment methodologies and improvements in connection details and fabrication standards. In vessels built after 1997 different positioning for the cross-ties used in the centre or side cargo tanks mean that atten- tion must also be given to side and inner shell longitudinal stiffener connections, horizontal girders, hopper connections and inner hull structures. For designs where cross-ties are fitted in side tanks, a decrease in side shell structural deflections and stress levels can be expected. In contrast, the deflections in the longitudinal bulkheads will be higher, especially at alternate load- ing conditions. When an FPSO is converted on the basis of classification society requirements, it is often the case that the durability of the T here are approximately 97 FPSOs (Floating, Production, Storage and Offloading systems) in operation or avail- able worldwide of which over 60% are con- version projects. The two main options open for new conversion projects comprise single hull tankers built between 1985-95 and dou- ble hulls built after 1996. Vessels built after 1985 tend to have highly optimised designs and use high tensile steel (HTS) more exten- sively than the 1970s tankers typically selected for earlier conversion projects. Single hulls There are two types of structural arrangement traditionally used in single hull tankers: a longitudinal ring stiffener system, which comprises deep girders with- in centre and side tanks that support the horizontally stiffened transverse bulkheads, and does not require horizontal stringers, and a horizontal system, which typically consists of four stringers within centre and side tanks that support the vertically stiff- ened transverse bulkheads and a longitudi- nal ring for the centre line girder. In these older generation vessels, fatigue was not explicitly addressed during the design of connections for primary or sec- ondary elements, however this was effec- tively compensated by a stiffer structural arrangement, larger corrosion margins and restricted use of HTS. Since 1985 the structural design of tankers changed substantially with the intro- duction of finite element calculations to opti- mise hull form and HTS construction to reduce the weight of steel. However, the extensive use of HTS, particularly in the side shell area around the neutral axis, has been identified as a weakness for FPSO conver- sion. Fatigue cracks of longitudinal connec- In selecting a tanker for FPSO conversion, hull girder strength as well as yielding, buckling and fatigue of primary and secondary structure must be verified* Candidates * This article is based on a presentation pre- pared by P. Biasotto, V. Bonniol and P. Cambos of Bureau Veritas Royal Belgian Institute of Marine Engineers
Transcript
tions to transverse primary structures are atypical defect as the fatigue strength and cor-rosion margins are affected by the less stiffstructural panels. Notably, hulls built withST355 steel type are especially prone todefects and probably have had to be rein-forced with additional brackets after con-struction. Angle stiffeners used in the sideshell and bottom panel, in a so-called asym-metric profile, also have a high probability offailure and deserve special attention duringthe assessment of candidate conversion hulls.
Double hullsThe standardised double-hull designs,
which started emerging from Japanese andKorean shipyards in the late 1990s, on theother hand, benefit from advances infatigue assessment methodologies andimprovements in connection details andfabrication standards.
In vessels built after 1997 differentpositioning for the cross-ties used in thecentre or side cargo tanks mean that atten-tion must also be given to side and innershell longitudinal stiffener connections,horizontal girders, hopper connections andinner hull structures. For designs wherecross-ties are fitted in side tanks, a decreasein side shell structural deflections and stresslevels can be expected. In contrast, thedeflections in the longitudinal bulkheadswill be higher, especially at alternate load-ing conditions.
When an FPSO is converted on thebasis of classification society requirements,it is often the case that the durability of the
There are approximately 97 FPSOs(Floating, Production, Storage andOffloading systems) in operation or avail-able worldwide of which over 60% are con-version projects. The two main options openfor new conversion projects comprise singlehull tankers built between 1985-95 and dou-ble hulls built after 1996. Vessels built after1985 tend to have highly optimised designsand use high tensile steel (HTS) more exten-sively than the 1970s tankers typicallyselected for earlier conversion projects.
Single hullsThere are two types of structural
arrangement traditionally used in singlehull tankers: a longitudinal ring stiffenersystem, which comprises deep girders with-in centre and side tanks that support thehorizontally stiffened transverse bulkheads,and does not require horizontal stringers,and a horizontal system, which typicallyconsists of four stringers within centre andside tanks that support the vertically stiff-ened transverse bulkheads and a longitudi-nal ring for the centre line girder.
In these older generation vessels, fatiguewas not explicitly addressed during thedesign of connections for primary or sec-ondary elements, however this was effec-tively compensated by a stiffer structuralarrangement, larger corrosion margins andrestricted use of HTS.
Since 1985 the structural design oftankers changed substantially with the intro-duction of finite element calculations to opti-mise hull form and HTS construction toreduce the weight of steel. However, theextensive use of HTS, particularly in the sideshell area around the neutral axis, has beenidentified as a weakness for FPSO conver-sion. Fatigue cracks of longitudinal connec-
In selecting a tanker forFPSO conversion, hullgirder strength as wellas yielding, bucklingand fatigue of primaryand secondary structuremust be verified*
hull structure is not properly assessedagainst the required service life. As a conse-quence, these units might arrive at the firstor second five yearly class renewal surveyswith substantial structural corrosion andincreased risk of buckling and fatigue.Unless remedial action is taken, the conse-quences could include reduction of storagecapacity, non-planned shutdown forrepairs, or structural failure.
Anomalies frequently found in convert-ed FPSOs include excessive pitting of hori-zontal structures, knife edging as well asfatigue induced cracks. While cracks can bedue to the high stress experienced duringthe loading and offloading cycle, non-oper-ational factors including imprecise design,below standard workmanship and corro-sion of welded joints will contribute toaccelerated cracking.
Design lifeThe evaluation of VLCC candidates for
FPSO conversions will, of course, includeappraising its adequacy for storage capaci-ty and cargo tank arrangement. However,consideration of effective service life is also
a crucial element of the assessment process.While methods for testing structuralstrength against yield and buckling areproven, accurate measurement of deteriora-tion modes such as corrosion and fatigue isless straightforward.
Fatigue is related to cyclical loads dueto wave and cargo loading and offloadingas well as structural stiffness, fabricationstandards and workmanship. Furthermore,the design life will be directly affected byoperation and maintenance regimes and thefatigue damage and corrosion wastage dur-ing the candidate hull's previous incarna-tion as a tanker.
A hot spot map derived from a finiteelement analysis of the hull structure is use-ful for evaluating hull condition but is notalways available. In such cases a more qual-itative review is required. For long termprojects this should encompass as-builtdrawings, as well as survey and inspectionrecords in order to identify problem areas.Moreover, repair specifications should beevaluated for scantling renewal extensions(signifying corrosion incurred duringtanker service) as well as other steps taken
In double hull
tankers, fitting
cross-ties can
reduce side-shell
structure
deflections and
stress levels
conversionconversionfor
The Sanha, built by
IHI Marine United,
represents a foray
into the FPSO
newbuild market by
Japan, a country well
known for its
standardised double
hull tankers used in
conversion projects
to mitigate defects and/or strengthen thehull. The tanker selection process shouldalso take into account operation and main-tenance practices, including type of oil,temperature, washing and corrosion pro-tection.
Support connectionsOwing to their stress concentration,
alignment and discontinuity, connections ofthe longitudinal ordinary stiffeners withtransverse primary supporting members,and connections of primary supportingmembers, are critical areas during the eval-uation process.
Ordinary stiffeners are subjected tohigh cyclic loading and constitute a majorfatigue problem area — repair work torenew or strengthen these details can signif-icantly affect the tanker refurbishmentschedule. The fatigue strength of connec-tions is influenced by numerous parametersincluding location, number, shape and sizeof brackets as well as the longitudinal stiff-ener profile, the existence of misalignment,the use of HTS in side plates and the typeof scallop.
In a scallop connection, a relativelylarge cut-out of the primary member meanswelding for the secondary stiffener is onlypossible on one side. While fitting a collarplate enables welding on the other side, thiscould lead to possible problems withcracks. The profiled slot method, mean-while, involves removing a section from thetransverse member minutely larger than theweb plate of the secondary stiffener. Thispermits welding from both sides and resultsin better stress transmission.
Microbe attackIn some cases excessive pitting has been
reported in cargo tank plating in single hulltankers due to microbial attack in areaswhere coating or anode protection is notprovided. Also, residual water from oilcargo can cause grooving and pitting corro-sion in horizontal structures like stringersand bottom plating.
Accelerated corrosion has also been iden-tified in the cargo tanks of double hull tankers,again due to microbial attack from bacteria inthe cargo oil. Temperatures in double hulltankers can be up to 20°C higher than singlehulls owing to the insulation provided by the
inner hull. This brings about the necessaryconditions for microbes to remain activelonger and thereby produce more corrosiveacidic compounds. Higher temperatures alsoresult in greater humidity, increasing theamount of water vapour in the air space abovethe ballast and cargo tanks. This means thetank coating remains continuously wet, fur-ther increasing the risk of microbial attack.
The total surface area to be coated indouble hulls can be up to three times largerthan a single hull. Consequently, the main-tenance of coating systems is one of themost important aspects regarding the hullstructure condition.
Operating environmentIn assessing candidate hull structures for
conversion, Bureau Veritas recommends thatconsideration should also be given to theoperational environment of the floatingunits, as these factors will interact with ves-sel dimensions, shape and load distribution.
Hydrodynamic models enable globalhull girder loads (wave bending momentsand shear forces), relative wave elevationand vessel acceleration to be determined,which can then be utilised to assess scant-ling structure. The analysis typically teststhree loading conditions comprising mini-mum, intermediate and maximum draughtas well as the influence of the selectedmooring system. In a spread mooreddesign, the unit is maintained in a constantposition independent of the sea and currentheading, which is in contrast to a turretmoored design where the unit is free toweathervane and has a natural tendency toorientate in the direction of the most severeenvironmental component.
As explained above, in assessing thesuitability of a FPSO hull structure, it isnecessary to verify that the tanker hullstructural strength meets the project speci-fication and that due consideration is givento storage capacity, additional topsideweight, environmental loads as well asintended service life. In practice, the analy-
sis is based on design load parametersestablished from hydrodynamic investiga-tion and comprises a multi-step procedure,where global coarse mesh analysis are fol-lowed by local fine mesh analyses at criticallocations identified from the coarse meshresults. In conversion projects, this must becarried out twice, once for tanker configu-ration and once for FPSO configurationdue to their different operating profiles.
The first step is the verification of glob-al hull girder strength, which involves ayielding check to confirm that the bendingmoment applied to the structure does notexceed the bending moment capacity pro-vided by the actual hull scantling configu-ration. The second step entails yielding andbuckling tests on scantling elements includ-ing plates and stiffeners. Local loads arecalculated for the most severe conditionsenvisaged and for full and empty cargoconditions. External sea pressure is alsodetermined for elements of the outer shell.
The primary structure is subject to afinite element analysis, which allows thestress distribution in the primary support-ing members to be established as well asprovide verification that the scantlingscomply with the yielding and buckling cri-teria. The coarse mesh model can be basedon a three-cargo tank model, where beamtheory is used to balance the model andobtain the desired bending moment andshear force distribution in the mid-tankarea, or based on a complete ship model.While the latter is more time consuming, itprovides greater accuracy in bendingmoment and shear force distribution alongthe hull. Fine meshes are typically per-formed for:� Horizontal stringers for typical oil-tight
bulkhead� Horizontal stringers for typical swash
bulkhead� Typical transverse ring� Typical first transverse ring aft and for-
ward of oil-tight and swash bulkheads� Longitudinal girders for oil-tight and
swash bulkheads� FPSO specific areas.
Future developmentsThe new rules for the Hull Structure of
Production, Storage and OffloadingSurface Units launched by Bureau Veritasindicate a key focus going forward will beplaced on reinforcing crack focal pointswhile taking into account that FPSOs haveto remain onsite without dry-docking for25 years.
