FPSOs are being increasingly used for deeper water and for a variety of functions related to offshore drilling, production and storage. Some FPSOs are being converted from tankers which have been designed using a traditional Class Rule type approach, while other FPSOs are being designed according to a more advanced hydrodynamic/Finite Element (FE) type approach. The traditional design procedures of FPSOs are based on simplifying component-based approaches that may underestimate the loading (in comparison to advanced hydrodynamic models) but provide a large factor of safety on the capacity. Results from studies for certain UK Central North Sea FPSOs seem to indicate that the safety factor for the two approaches is similar, since the advanced hydrodynamic/FE approach tends to result in higher loadings but also higher capacities. However it is not clear to what extent these results can be applied to other FPSOs or to other areas. Another issue that is addressed is the apparent variation in reserve strength against hogging and sagging bending moments. This is particularly true for double hull FPSOs which tend to have a higher capacity at the bottom of the hull in comparison to the deck. The purpose of this paper is to collate available data on the range of variation of safety factors of currently used FPSOs, and to consider how more consistent margins of reserve strength for various FPSO configurations under various loading conditions might be achieved.

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