Fatigue crack is one of the most important failure modes for hull structures and so it is imperative to verify that the calculated fatigue life is beyond the vessel service life for fatigue prone structures in design stage. However, the amount of uncertainties lying in the analysis is quite large for such kind of analysis that the analysis often fails to explain the real phenomena correctly. To manage such a risk, design fatigue factor is usually introduced and for the determination of proper level of design fatigue factor, a quantitative assessment of the fatigue reliability is very useful. In this paper, a fatigue reliability analysis for typical upper deck penetration structures of a compact drillship is presented. Possible uncertainties in inputs and analysis models have been investigated and the variation of important factors has been quantitatively estimated. The estimated failure probability of the current practice design has shown that about 3.4% cases may have fatigue crack within 20-year service life. Proper design fatigue factors for current and higher level of load uncertainties have been investigated and suggested as 2.0 to 6.0 according to the desired safety level.

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