ONSTREAM JIP FPSO Design Optimisation through Structural Reliability

Analysis Methods

Valerie Quiniou-Ramus and Brian Campbell

Noble Denton Europe Ltd., Noble House, 131 Aldersgate Street, London EC1A 4EB, UK

Tel: +44 (0)20 7606 4961 – Fax: +44 (0)20 7606 5035 onstream@nodent.co.uk, http://www.nobledenton.com

Abstract. Walk-on discussion for the Rogue Waves 2000 Workshop – Brest, 23-30/11/2000. This paper presents the OnStReAM Joint Industry Project and how it relates to Rogue Waves and to the need for a better knowledge and modelling of the environment.

1 Presentation of the JIP

Noble Denton Europe Ltd is about to launch the ONSTREAM JIP with the support of the Health and Safety Executive (HSE) in the UK. The aim of the JIP is to provide guidance on FPSO design Optimisation through Structural Reliability Analysis Methods.

The project team will benefit from Noble Denton’s unique blend of actual floating production project experience (e.g. Roncador, Agbani, Banff, Foinhaven and Schiehallion) together with cutting edge risk, safety and reliability based research. The project team will also include external consultants in order to provide input on issues regarding overall FPSO design and operation, naval architecture and structural reliability analysis.

At present the “fast track” nature of the FPSO developments have meant that “good practice” from a number of specialisms has been brought to bear on the complete system design without having the opportunity to integrate or balance the reliability levels of different sub-systems which combine to produce the FPSO. The ramifications of this lack of integration are amplified by the fact that there is a relatively large number of structural limit states present in FPSO design, when compared to fixed structures, leading to greater difficulty in understanding true safety levels. To name but a few, Hull Midship Section design, Station Keeping design, Bow Structure design against Slamming, Deck and Topsides design against Greenwater, etc.

This lack of integration, at the design stage, and resulting problems are typified by the fact that a number of FPSOs currently operating in the UKCS were not completed either within budget or schedule. If this is not significant enough, even after coming

on stream a number of FPSOs have suffered from either falling short of expectations or experiencing structural damage during operation, leading to production downtime.

As a result, the need has been identified to ascertain the reliability of each critical structural limit state relating to FPSO design and to evaluate the consequences of failure on personnel, environment, production and repair / replacement. This will make it possible to integrate the risks associated with each limit state, and therefore to calculate the overall system reliability of an FPSO and to pin-point the most critical limit states in order to optimise the whole system.

Obvious benefits of this study include an optimisation of the system safety and reliability, together with the enhancement of the system performance and the achievement of a balanced risk throughout the field life of the development. This will induce capital and/or operating cost benefits on FPSO components leading to more efficient designs and a reduction in repair costs and their associated loss of production periods.

2 Assessment of the Probability Distribution Function Associated to the Wave

The reliability assessment of each limit state starts with the identification of all parameters involved in their design, in order to evaluate all possible sources of uncertainty. Major parameters are typically those which are environment related and in particular the wave. When conducting reliability analysis, probability distribution functions are associated to the different variables. Environment distributions are usually derived from metocean data deemed relevant to the locations under consideration. Hindcast environmental data may be supplied for this task. Where appropriate, joint probability models, e.g. linking wind, wave and current events, are developed and applied to the analysis in order to avoid conservatisms. In most cases, an FPSO is designed to withstand a 100-year return environment, and the use of typical wave spectrum (JONSWAP, Pierson-Moskovitch) is a common practice. Nevertheless, exceptional storms and expected wave events are possible: how do they compare with the statistical 100-year return wave event? Are they still predictable using second order wave modelling?

For illustration, let us focus first on hull midship section design. The evaluation of this limit state is a balance between the resistance – the hull midship section’s strength – and the load – still water and wave-induced bending moments and shear forces.

For this limit state, the main relevant input parameters are: FPSO main characteristics (L, B, lightship displacement and Centre of Gravity), structural data (steel yield, scantling and stiffener distribution, welds, corrosion), wave data (Tp and Hs, spectrum, direction) and operational data (storage amount and distribution).

Few of the International Rules are tailor-made for the FPSO. It therefore, often becomes necessary to refer to the Rules for Ships (sea-going vessels) or Mobile Offshore Units, which have a larger empirical / historical background and are broadly approved “rules of thumb” but are not always relevant for the issues encountered by FPSOs. They usually provide the engineers with several formulae for the evaluation

of design Wave Bending Moments (hog and sag) and Shear Forces and minimum required section modulus or strength, as a function of an effective wave height which is dependent on the vessel’s length only. The Rules also influence the choice of the hull scantling. They give some nominal indications on the plate thickness decreasing with time to make allowances for corrosion. Finally, permissible stresses have to satisfy the Rule requirements (safety factors).

In the document HSE OTO 98164 (Faulkner, [1]), it is claimed that though IACS (International Association of Classification Society) Unified Standard aims to unify the Rules’ requirements, there is still a large diversity in the design wave-induced bending loads and in their interpreted reliability. In that document, 8 rules have been compared (see Fig.). For a given probability of exceedance Pe, say 10-8/wave, the ratio of the highest calculated design wave-induced moment Mw to the lowest is 1.8. In addition, for a given Mw, Pe varies by 4 orders of magnitude in sag and 3 orders in hog.

It is worth noticing however that the Rules now offer computer-based design (3D Finite Element Analysis) as an alternative. Use of this direct design in place of the usual rule of thumb, and provided the computer programs are accurately checked, is likely to improve the consistency in the reliability levels achieved through design.

Nevertheless, the reliability level itself could be improved only if the environmental loading is better assessed. This is even more necessary for Bow Structure design against Slamming loads or Deck and Topside design against Greenwater, where the wave height and profile are critical parameters.

Fig. 1. Eight classification society plots of different wave bending probabilities for the same container ship and route (copied from report HSE OTO 98164)

To cope with the lack of understanding on probability of exceedance of a given sea-state and a given vessel response, Research Institutes and companies in the Oil Industry ought to work together. There is a need to improve our knowledge on these unexpected or “Rogue” waves, to develop adapted joint distribution functions for these events, and to develop design tools to be used when designing an FPSO. This would considerably help in optimising FPSO designs.

References

1. Faulkner, D.: Reliability Based Design and Assessment of FPSO structures. HSE OTO 98164 (November 1998)