A RATIONALISATION OF FAILURE SURFACE EQUATION FOR THE RELIABILITY ANALYSIS OF FPSO STRUCTURES

Article type: Research Article

Authors: Maerli, A. | Das, P.K. | Smith, S.N.

Affiliations: University of Glasgow, Scotland | Amerada Hess Limited, Scotland

Abstract: Floating Production Storage and Off-loading (FPSO) Vessels have been chosen for an increasing number of field developments in recent years. High payload capacity, short development schedule and in-built cargo storage capacity are some important advantages that make 'Ship Shaped' FPSOs very attractive for field developers. The operation and maintenance profiles of a FPSO differ from those of a traditional merchant ship, and these differences will significantly influence the structural reliability of the vessel. This paper presents the structural reliability analysis of the hull girder ultimate strength for a ship shaped FPSO. The ultimate strength of the hull girder was calculated using a component approach, where the behaviour of the hull is evaluated based on the behaviour of the single structural components. Three collapse conditions were investigated; failure initiated by plate compression, failure initiated by stiffener tension and failure initiated by stiffener compression. Only vertical bending moment has been considered and, the hull girder loads are divided into still water induced and wave induced components. The two loading components have been considered independent and Ferry Borges-Castenheta load combination method has been applied to obtain load combination factors for the Full Load, Partial Load (50% loaded) and Ballast condition. The distributions of the extreme values of the vertical wave bending moments (VWBM) were calculated, based on linear strip theory and a long-term formulation. The vertical mooring forces are small and they were considered to have an insignificant influence on the bending moment response. The reliability analysis was carried out using a SORM analysis. Annual reliability indices (β) and probabilities of failures were calculated for hogging and sagging conditions. The calculated β values were higher than the annual reliability indices proposed in DNV Classification Notes 30.6 for serious failures in redundant structures. This indicates that the design is safe and reliable for operation in this particular location. The sensitivity of β with respect to the different variables in the limit state function was investigated and further reliability analyses were performed based on other locations in the North Sea and various stochastic models. Some of these results are included in the paper to highlight the importance of formulating a rational failure surface equation.

Journal: International Shipbuilding Progress, vol. 47, no. 450, pp. 215-225, 2000