Contribution of Supposed Wave Condition on Long-term Distribution of Wave Induced Load

Abstract

This study is the second report concerned with statistical analysis on long-term distribution of wave induced load. In the first report, authors examined which factor gives influence by load revel in the long -term distribution, such as significant wave height, average wave period of supposed wave condition and relative angle between ship course and wave direction. The long-term distribution was decomposed into every these factors and a contribution rate analysis method for these factors in each load revel was introduced. Based on the method, it was clarified that the maximum wave induced stress value in the vicinity of the probability of exceedance around 10-8 can be governed by the most sever short-term wave condition which has the largest value of short-term wave induced Raleigh distribution parameter.In this report, the shape of the long-term distribution of the wave induced load for evaluation of fatigue strength is studied. Based on S-N curves and Miner's rule, the relation between the fatigue damage and supposed wave condition to calculate the long-term distribution is examined. The contribution rate analysis method for fatigue damage from supposed wave condition, such as significant wave height, average wave period of supposed wave condition and relative angle between ship course and wave direction, is introduced. It is shown that the most sever short-term wave condition, defined by the former report, also plays the most important role in the fatigue damage. The simple estimation method for long-term distribution described by Weibull distribution from the statistical propaty of the most sever short-term wave condition is introduced. Several examples show the applicability of the estimation method and estimation errors of the maximum value with probability exceedance 10-8 are less than 5% and of the fatigue damage rates are within 1030% large value.