The jack-up rig is a movable offshore platform, which is basically being designed to work at any location, worldwide for given maximum water depths and sea states, combined with different sea bed conditions. Moreover the platform is often transported with the legs in fully raised position, from one offshore location to another, resulting in completely different loads on the structure. The combination of extreme environmental loads with a small crack in structural joints due to fabrication process or fatigue degradation may cause unstable crack extension and fracture failure. To calculate the failure probability through the fracture failure mode, two methods based on the failure assessment diagram of level 2 recommended by the British Standard Institute (BS7910[1]) and formulation recommended by Dijkstra et al. [2] are presented and investigated. The results of BS7910 gives the lower reliability index than the Dijkstra formulation for several secondary to yield stress ratios and service times but the maximum difference between them is restricted to 17% in the case of secondary stress to yield stress ratio equal one.

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