Contribution of supposed wave condition on the long-term distribution of a wave-induced load

This report is concerned with the statistical analysis of the long-term distribution of a wave-induced load, and examines which factors influence the long-term distribution of the load level, e.g., the significant wave height, the mean wave period of the supposed wave condition, and the relative angle between the ship's course and the wave direction. The long-term distribution is broken down into these factors, and a contribution rate analysis method for each factor in each load level in the long-term distribution is introduced. Based on the method used, the contribution rate of a specific mean wave period and a wave angle encountered is clarified, when the long-term distribution is larger than other wave periods and wave angles. The specific mean wave period and wave angle encountered are defined as the wave condition which governs the long-term distribution. The maximum wave-induced load in the vicinity of a probability of exceedance of around 10⁻⁸ in the long-term distribution is decided by the most severe short-term wave condition which has the largest significant wave height with a specific mean wave period. Based on S–N curves and Miner's rule, the relation between the fatigue damage and the supposed wave condition is examined. The contribution rate analysis method for fatigue damage is introduced. The governing wave condition and the most severe short-term wave condition also have an important effect on the fatigue damage. A simple estimation method for the long-term distribution, described by the Weibull distribution from the statistical properties of the most severe short-term wave condition, is introduced. Several examples show the applicability of the estimation method. Received: November 22, 2001 / Accepted: January 9, 2002