ESTIMATION OF NONLINEAR LONG-TERM EXTREMES OF THE VERTICAL BENDING MOMENTS IN SHIPS

This paper deals with estimation of the long-term extreme of hull girder loads in ships for a given return period. In principle, the response is obtained by combining the response in all the sea states. The long-term response for a linear system can be effectively obtained by determining the response for each sea state, specified by wave height and peak period in the frequency domain. However, if the response is nonlinear, time domain simulation would be required and long time series is required to limit the statistical uncertainty. Long-term analysis has become complicated and time consuming, and it is crucial to improve the efficiency in calculation. In this work it is shown that the long-term extremes can be estimated by considering only a few short-term sea states. A long-term analysis, based on identifying the most important sea state, defined by the coefficient of contribution, using linear analysis is applied. An interaction procedure is used to find the nonlinear long-term extreme values. It is concluded that only a limited number of sea states are necessary to get an acceptable estimate of the nonlinear D-year response as long as the most important sea states are included.