FPSOs, moored permanently at deep seas, are more vulnerable to fatigue failures due to cyclic loads from environmental conditions and operating cycles. In welded structures, the fatigue crack initiation strongly depends on the structural geometry and the stress concentration. In this work, a parent conventional hull is taken and an equivalent hemispherical hull, which could be a refined shape hull structure with smooth transition joints, is created with equal volumes. In both the hulls, the common critical location selected for comparison is a stiffener on central girder of the hull structures and the fatigue damage at full load and ballast load conditions are calculated, for design life of 25 years, by using simplified fatigue assessment procedure and S-N curve. It is observed that the hemispherical hull structure components have higher fatigue capacity than the conventional hull structure.

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