Descripción de las medidas de mitigación específicas

It is recommended that the structural design be based on the Limit State approach. The limit states to be covered are:

• Ultimate limit state (ULS) using a 10,000 year return period environmental load (see 2.3.2 and 2.3.3 for wind and wave estimates).

• Serviceability limit state (SLS) to assure service performance and habitability. • Fatigue limit state (FLS) using wave data from 2.3.3.

• Accidental limit states (ALS) following credible accident scenarios.

Guidance on application of the limit state approach and partial factors is given in DNV OS-C102 Structural Design of Offshore Ships (ref.37).

A starting-point design may be based on the following classification society rules for trading tankers with the scantlings increased as necessary for FPSO duty e.g. fatigue, sloshing, green water on deck, bow wave impact.

• LR Rules for Ships (ref.38) - part 4, chapters 9 & 10. • ABS Steel Vessel Rules (ref.39) - part 5, chapter 1.

The scantlings should also be verified by strength analysis as follows.

2.4.2 Finite element analysis Modelling

The FEA model should include a representative cargo tank including the boundary transverse bulkheads and one-half of the tanks forward and aft of this tank. A separate model should be made of the turret area. Finer mesh models should be constructed of localised areas of high stress and typical design details e.g. support detail for topsides, longitudinal / transverse web intersection and cutout.

Details of structural analysis and loading cases to consider are typically given in LR (ref.22) Part 4 chapter 4 section 4.2.

Determination of load

From experience, the loading cases that should be examined are:

1. Transit condition with 10year seasonal environmental condition for the tow route. 2. Installation condition

3. Design environmental condition with tank contents and external pressure heads selected to produce maximum differential pressure heads and maximum hog / sag bending effects, coupled with the maximum survival wind / wave / current forces. 4. Design operating condition with tank contents and external pressure heads selected to

produce maximum differential pressure heads and maximum hog / sag bending effects, coupled with the maximum operating (typically 10 year) wind / wave / current forces.

5. Accident conditions, and after remedial action together with 1 year environmental condition.

6. In-situ tank inspection conditions

Permissible stresses and buckling strength

The permissible stresses are given in the above LR (ref.22) Part 4 chapter 5 section 2. Buckling strength of plates and stiffeners is given in Part 4 chapter 5 section 3 and of primary members in section 4.

2.4.3 Fatigue Design

Fatigue design is an important factor because of the high cyclic loads from the harsh environment and the cost / difficulty of detecting and repairing fatigue damage in-situ. This means that a rigorous fatigue assessment, taking account of fabrication quality and details, should be performed for the site-specific environmental parameters with the FPSO weathervaning into the environment.

For converted tankers, allowance should be made for previous usage. The designer should ensure that fabrication details and fatigue analysis are consistent with the required service upgrade.

The areas where fatigue causes problems on an FPSO are:

a) Fatigue design of longitudinals in the side shell in the operating draft range where there is fluctuating water pressure due to wave action. This may require fitting an extra bracket or collar at each longitudinal to transverse web frame connection to improve fatigue lives when modifying an existing tanker design. The problem is worst when the vessel is sitting at a constant draft in the case of oil export by pipeline or via FSU.

b) Hull girder bending. Increased thickness of deck and bottom plate may be necessary when modifying intercept tankers to improve hull girder modulus and reduce cyclic bending stresses.

c) In topsides supporting structure, due to transmission of hull bending stress into topside PAUs.

Guidance on fatigue design and analysis is provided by Classification Societies as follows. The required target fatigue life is 20 years or field life if longer, multiplied by a factor of safety (see below).

LR (ref.22) Part 4 chapter 5 section 5 includes: • Factors which influence fatigue

• Sources of cyclic loading

• Structural areas to be examined for fatigue

• Fatigue damage calculations (using Miner's summation) • Joint classifications and S-N curves (see Appendix A)

• Factors of safety on fatigue life (depending on accessibility for inspection/repair, and consequence of failure)

ABS (ref.23) Chapter 4 section 2.13.5 covers fatigue analysis. ABS (ref.39) Section 5 -1 - 5/7 covers the extent of fatigue analysis.

DNV are managing the FPSO Fatigue Capacity JIP. The first phase examining fatigue performance of butt welds is summarised in HSE report OTN 2001:015 (ref.40). The second phase will examine fillet weld performance and will include a comparison with the earlier MARIN Structural Integrity JIP.

2.4.4 Local strength

Formulae for local strength are given by LR (ref.22) part 4, chapter 6: • Design heads

• Watertight shell boundaries • Deck structure

• Helicopter landing areas • Wheeled vehicle loading • Bulkheads

• Double bottom structure

• Superstructures and deckhouses

• Bulwarks and other means for protection of crew

2.4.5 Reliability based design

Recent work by HSE in OTO 98:164 has examined Reliability Based Design and Assessment of FPSO Structures (ref.41). This has adopted the approach to reliability design of fixed jacket structures where statistical data on loading, materials, fabrication and rigorous analysis methods are applied. The approach has been used on a recent UKCS FPSO to assess the extreme loading states the vessel will encounter and a non- linear finite element analysis, incorporating post-yield and post-buckling behaviour of

elements, has been performed to assess the ultimate capability of the structure under this loading.

2.4.6 Material Selection

Steel grade is of course predetermined for conversion of existing or intercept tankers. For new-build custom-designed FPSOs, it is possible to optimise steel grade for the particular requirements of FPSO duty. The choice lies between a completely mild steel hull (yield stress = 235 N/mm2) and a hull where the deck and bottom is high tensile steel (yield stress normally 355 N/mm2) and sides and longitudinal bulkhead in mild steel. The issues affecting material selection are as follows:

• High tensile steel produces a lighter hull (subject to the other constraints below) and allows greater crude oil cargo deadweight to be carried. This is normally of benefit for dense crude oil where the crude oil capacity is limited by deadweight. For lighter crude oils, especially where a double bottom is fitted, the crude oil capacity is limited by volume and therefore a saving in hull weight may not be directly beneficial.

• Fatigue is a major design issue for harsh environment FPSOs and the use of high tensile steel may produce greater cyclic stress ranges than the structural details permit for fatigue. This constraint may not permit the full advantage of reduced scantlings and structural weight from HTS to be realised.

• HTS hull is more flexible (reduced modulus) so that more care needs to be taken in topside/piping design for hull flexibility.

• Weldability requirements for HTS are more onerous than mild steel.

2.4.7 Welding and structural detailing

Welding requirements and structural detailing are covered in detail in classification society rules e.g. LR (ref.22) Part 4 Chapter 8.

It is important to note that due to the difficulty of in-situ structural repair, high standards of welding and structural detailing are required to prevent initiation of fatigue cracks. Particular attention should also be paid to quality of field welds between block sub- assemblies where problems have occurred in the past.

2.5 GREEN WATER