Long term structural assessment of an FSU ship based on the spectral fatigue method

Abstract

For the safety operation of the offshore FSU floating storage units, the structural design must be assessed on the short-term yielding stress criterion and on the long-term fatigue strength criterion. This paper presents the fatigue strength assessment for a large FSU structure with length of 292 m, based on the wave loads from Western North Atlantic at zero speed in the operation area. Besides the waves loading cycles, the oil tanks loading and unloading low cycles are taken into account. The short-term stress analysis is based on a coupled hydrodynamic and structural 3D model. The spectral fatigue analysis is based on the long-term D damage cumulative factor method, considering 100 years as the design exploitation time reference. The numerical results from this study are point out the structural details of the FSU hull which do not comply with the fatigue strength criteria.