Full long-term extreme response analysis of marine structures using inverse FORM

An exact and an approximate formulation for the long-term extreme response of marine structures are discussed and compared. It is well known that the approximate formulation can be evaluated in a simplified way by using the first order reliability method (FORM), known for its computational efficiency. In this paper it is shown how this can be done for the exact formulation as well. Characteristic values of the long-term extreme response are calculated using inverse FORM (IFORM) for both formulations. A new method is proposed for the numerical solution of the IFORM problem, resolving some convergence issues of a well-established iteration algorithm. The proposed method is demonstrated for a single-degree-of-freedom (SDOF) example and the accuracy of the long-term extreme response approximations is investigated, revealing that the IFORM methods provide good estimates in a very efficient manner. The reduced number of required short-term response calculations provided by the IFORM methods is expected to make full long-term extreme response analysis feasible also for more complex systems.