This paper presents a combined-table approach for calculating longterm extreme values of wave-induced hull girder loads on an FPSO in complicated wave conditions. Usually, the long-term extreme responses are determined using a wave scatter diagram associated with a family of wave spectra of a simple and uniform model. However, in areas like West of Africa and Offshore Brazil, both swells and seas (local windor storm- generated waves) exist and propagate in separate directions. The traditionally used wave scatter diagram approach is not applicable anymore. The proposed approach is essentially suitable for any conditions using any wave spectrum, including measured numerical spectrum. Besides, two additional parameters are introduced into a combined spectrum with Ochi-Hubble's six-parameters for directionality of swells and seas, separately. Comparisons of extreme values of the hull-girder vertical bending moment confirm that artificial uncertainties are introduced if the simplified spectral model is used.
In the design of a ship-shaped Floating Production, Storage and Offloading system (FPSO), some chosen dominant loading parameters determine the hull-girder scantlings (ABS, 2000, 2002). The dominant design load parameters include, for example, the extreme wave-induced vertical and horizontal bending moments. Unlike the design of a tanker that applies the unrestricted wave conditions regardless of the service routes of the tanker, the design of an FPSO requires the application of site-specific wave conditions. In determining extreme values, designers prefer different approaches as indicated in API Recommended Practice 2T, Chapter 7 (API, 1997), depending on their understanding and available sources. For instance, the extreme significant wave height with a loo-year return period and the associated peak period, (Hs, Tp)100, can be obtained based on longterm wave statistics; then a two-parameter wave spectrum defined by the (Hs, Tp)100 is utilized to calculate the extreme responses based on short-term statistics (referred to as short-term approach).