Long-Term Extreme Response Analysis of Marine Structures Using Inverse Reliability Methods

For the assessment of extreme responses needed in design of marine structures, a full long-term analysis is recognized as the most accurate approach. However, due to the very large number of structural response analyses traditionally needed for this approach, the computational effort is usually considered to increase above acceptable levels for complex structures, e.g. floating bridges. In this work, an effort is made towards more efficient, yet accurate, full long-term analyses. This thesis is written as a collection of four papers, where each paper is either published or submitted for journal publication. In the first paper, a new method is presented for efficient calculation of auto- and cross-spectral densities in the stochastic modelling of ocean waves and wave loads. As part of the short-term response analyses, the method contributes to more efficient long-term response prediction. In the second paper, an exact and an approximate formulation for the long-term extreme response of marine structures are discussed and compared. A new method is proposed for the numerical solution of the exact formulation, based on an inverse first order reliability method (IFORM). In the third paper, the IFORM approach is further developed, and the accuracy of the long-term extreme response approximation is improved, using an inverse second order reliability method (ISORM) approach. Finally, in the fourth paper, the developments of the first three papers are demonstrated for a long-span pontoon bridge subjected to wave loads, revealing that characteristic values of the long-term extreme response can indeed be calculated in an efficient manner. Especially the ISORM approach is seen to provide high accuracy, using only a reasonable amount of short-term response calculations. Hopefully, the contributions of this work will make full long-term extreme response analyses more attractive and available for the practical design of marine structures