This paper presents results from an ongoing development project which aims at increasing the knowledge for the estimation of extremes conditions and fatigue assessment of structural element (i.e. topside) of Ship Shaped Floating Units (SSFU) in a complex environment-load conditions. The method can be extended for the calculation of total bending moment to connections and hull-girder system.

In some world geographical context, such as those of West Africa and Offshore Brazil, the environmental conditions are characterized by co-existence of waves and swells propagating in different directions. In these conditions load history for SSFU are strictly linked to comprehension knowledge of site specific environment. Loads pertain to various frequency ranges, for instance the long-period response of a morning, quasi-static response to swell, dynamic response to shore-crested sea and structural resonance of flexible components as flares etc.

This paper aims at developing a new method for establishing load history at site in case of metocean climate including combination of several (up to 3 or 4) sea state components, such as those of main swell, secondary swell and wind sea. The method is applicable to both extreme conditions and fatigue assessment as results of a “combined-event approach”.

The results are compared and discussed. They show that the use of different approaches, long and a short term analysis with deterministic and probabilistic computation of vessel heading, provide a reasonably conservative estimate of the vessel responses. Differences from the conventional method, i.e. unimodal spectrum and equivalent sea state, are also presented.

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