Application of average sea-state method for fast estimation of fatigue damage of offshore structure in waves with various distribution types of occurrence probability

The fatigue strength of offshore structures operating in ocean must be determined to ensure their structural integrity. All the possible environmental conditions must be considered to accurately estimate the fatigue damage. Thus, analyses for estimation of fatigue damage can consume a significant amount of time as considering all the environmental conditions. Average sea-state method for fast estimation of fatigue damage was proposed in a previous study, and this method considers only one analysis for the representative values of significant wave height and modal period, considering wave occurrence probabilities. The proposed method can significantly reduce computation time required for fatigue analyses. In this study, the proposed method is applied to various wave occurrence probability distributions to determine the applicability to distribution forms. The seafastening structure which secures the topside on transportation vessel is used as the example model. The structural responses of seafastening structures are assessed by conducting coupled analyses considering the coupled behaviors of topside and vessel. The fatigue damage and life for each environmental probability distribution are estimated and compared with common solution. From the error comparisons, it is confirmed that the proposed method is suitable to be applied to a probability distribution form with a relatively small variance. •The applicability of average sea-state method on wave occurrence probability distribution types is studied.•The coupled analysis method is applied to assess structural responses of structures, considering coupled behaviors in waves.•Four of wave occurrence probability distribution types are made and analyzed for estimating fatigue damage of the structure.•The proposed method could be usefully applied for occurrence probability conditions of small variances.