FPSOs are generally designed to remain at their offshore location for 15-20 years, without the need for dry-docking. Over the past few years, orders for newbuild FPSOs have risen significantly, in particular for use in harsh environment zones or for deepwater developments.
However, with most of the newbuilding shipyards lacking spare slots, tanker conversions are becoming increasingly attractive for fast-track projects. Conversions remain a popular solution for milder environments such as West Africa, Southeast Asia, Australia, and Brazil.
Structural issues reported on both purpose-built FPSOs and conversions after entering service reveal the importance of hull strength and fatigue verification during the design/conversion stage. In particular, more feedback is becoming available concerning the corrosion pattern of single and double-hull tankers, and the typical failure of structural components.
Below we report some lessons learned regarding newbuilds and conversions, the methodology Bureau Veritas is employing to assess structural strength. Bureau Veritas also has examples of situations where we have acted as a consultant to assist our client with solving problems on units in service.
Corrosion build-up
Fatigue and coating are two interrelated phenomena. The higher stresses concentrated in certain parts of the hull can lead to coating break-up and the early onset of corrosion. It is essential to keep the coating efficient as long as possible because the fatigue life of uncoated areas is normally around half that of coated areas.
In double hulls, the cargo temperature can be up to 20° C higher than in single hulls. Higher temperatures, and temperature fluctuations both influence the corrosion process. Humidity - water vapor in the air space above the ballast or cargo - also creates a need for careful protection against creeping corrosion.
However, there are often conflicting interests between the owner’s priorities of a well-built, long-lasting vessel with minimal operating costs, and those of the shipyard. The standard guarantee for a ship only applies within one year of delivery, so the yard will rarely face a claim for coating failure if the coating is reasonably well applied. For this reason, IACS has devised harmonized Common Ship Rules for double-hulled tankers and bulk-carriers where the IMO performance standard for protective coating applies as a condition of class. The offshore industry should benefit from these developments.
Construction trends
Thirty years ago, designs for conversions were still based mainly on the use of mild steel. The traditionally applied classification rules were based on empirical experience without the imposition of explicit and consistent safety goals. A fatigue check was not compulsory from the outset.
Later designs were based on the use of high tensile steels with optimization techniques that involved checking mainly yielding and buckling criteria, leading to elastic structures with increased stresses. Converted vessels of these designs have to be checked carefully for fatigue performance, since part of their life has already been consumed during service as a tanker. Plate renewals and fatigue enhancements may be requested following structural analysis.
As for newbuild FPSOs, the major oil company operators also have vast experience as large tanker owners. Based on their service experience with both types of vessels, they are imposing new design specifications:
- For hull construction, high tensile steel is normally requested for the deck and bottom, with mild steel for the remainder of the hull
- A preference for certain structural details validated through previous experience
- Provision of access openings and means of access for inspection and rescue purposes
- Fatigue verification to provide results that can be used to plan quality assurance and inspection priorities during construction and operation
- Scantling requirements specified according to the owner’s experience and strategy (i.e. striving for higher corrosion margins to minimize maintenance costs).
Methodology of analysis
Bureau Veritas’ hull analysis starts with a direct hydrodynamic calculation, taking into account metocean data and mooring conditions at the offshore location, the purpose being to define sea loads on the structure. The study is also aimed at determining the values of wave-induced loads and motions.
Sea pressures on complete model of FPSO.
A unit operating in the North Sea needs to be reviewed with increased wave loads compared with standard ship rules for “unrestricted service” notation. The environmental conditions off, say, West Africa, are clearly less severe than the rule values corresponding to conditions in the North Atlantic.
The analysis inputs environmental site parameters such as wave direction, the wave spectrum with all its various parameters, the relative headings between all components, and the water depth. Wind force and direction and current data are also factored in.
A calculation is conducted for a minimum of three draughts: full load, ballast, and intermediate. We also analyse the loading condition giving rise to maximum shear force in still water, as experience shows there is a correlation between still water shear force and wave-induced shear force.
Two types of results are transferred to the structural models: fatigue loads, and extreme/operational loads for strength verification. For the strength verification, the hydrodynamic analysis provides the following parameters which are valid both for the vessel at the intended offshore location, and over the hull’s full length:
- Wave-induced bending moment
- Wave-induced shear force
- Total accelerations in all directions, at the center of gravity of each compartment, and at relevant positions in topsides areas
- Relative wave elevation.
For the fatigue verification, the hydrodynamic calculation provides loads to be applied on the structural model in order to determine the RAOs of stress ranges.
Strength calculation
Structural assessment takes into account the results of the hydrodynamic analysis and includes:
- Global strength assessment (yielding and ultimate)
- Structural assessment of primary and secondary members
- Impacts (bow impact, flat bottom reinforcements, sloshing)
- Spectral and deterministic fatigue assessment
- Collisions, explosions, dropped objects.
Extreme wave loads corresponding to 100-year return period are included in the calculation.
Midship - ultimate bending moment plot.
Structural analysis starts with a 2D analysis and scantling verification. The first step is to verify the global hull girder strength. A yielding check is then performed. Rules require also a check that the bending moment applied to the structure is lower than the ultimate bending moment capacity of the hull girder, bearing in mind suitable safety factors.
The second step is to verify the scantlings of the plating and the ordinary stiffeners, with yielding and buckling checks for the stiffeners and plates. Even at this stage, a fatigue check can be conducted for the end connections of longitudinal stiffeners.
For the primary supporting members, structural analysis normally involves use of the 3D finite element method that is mandatory according to BV rules. In general, the entire ship is modeled. Each hold is analyzed via a three-hold length finite element model. Calculations are also performed on the integrated model (completed hull and topsides). This allows evaluation of interaction between the hull and topsides when both are subject to deforming. Various loading patterns are also considered, representing the different loading conditions the vessel will be subjected to, namely normal operation, accidental, repair, inspection, and towing. Each of these conditions is prepared with appropriate sea loads determined via the hydrodynamic analysis.
Normal operating conditions are evaluated with loads corresponding to a return period of 100 years. Some load reductions may also be allowed for inspection or repair conditions. The finite element model serves also to assess both the hull and other areas of the FPSO such as topside supports, turret structure, hull connection, all represented in fine mesh models. Calculations are performed in “net scantlings”, i.e. the scantlings that according to the rules criteria are needed to sustain the loads acting upon them, without any implicit margin for corrosion.
For conversions, a recent thickness measurement report is mandatory. Present thickness is the base for future corrosion, and may also be used for corrosion pattern trends.
Stresses are evaluated via the top-down technique on coarse models, where the mesh size is typically equal to the spacing of the stiffeners. Buckling is verified taking into account the panel dimensions. In stress concentration areas, the mesh size will be adapted to the stress gradient down to a mesh size of 50 x 50 mm. Allowable stresses on such fine mesh models are higher and may be taken according to the new IACS Common ship rules for double hull oil tankers.
For newbuild FPSOs, there are two distinctive periods - transit and service on site. With conversions, the tanker phase must be analyzed also for damage build-up and to determine whether detail renewals or modifications are needed.
Two main causes of fatigue must be considered when analyzing fatigue performance on site: wave-induced loads and loading/unloading sequences. With the spectral approach, the wave-induced loads calculated in the hydrodynamic analysis are applied to the structure’s finite element model.
The structural model provides the RAOs of the stresses. Fatigue damage is then calculated, based on statistics of stress ranges. At least three draughts and associated loading conditions, five headings, and 25 frequencies, must all be taken into account. However, these can be adapted, depending on the type of mooring.
Stress ranges on structural details are assessed through the 3D finite element models with very fine meshes, the mesh size typically being equal to the plate thickness. The “acceptable” level of damage depends on the location, the accessibility for inspection, maintenance and repair, and on the consequences of failure.
This is an edited version of a paper prepared for the Offshore Meditteranean Conference in Ravenna, Italy, March 2007.
