Prediction of extreme responses of floating structures using a hybrid time/frequency domain coupled analysis approach

The dynamic analysis of a deepwater floating system has many complexities, which include the dynamic coupling between the platform and the moorings/risers, coupling between the first-order wave frequency (WF) and second-order low-frequency (LF) motions, hydrodynamic/geometric nonlinearities and non-Gaussian response statistics. These complexities can be automatically captured in a fully coupled time domain analysis, but at the expense of an enormous computational cost. In a recent work, an efficient hybrid approach is developed, in which the LF and WF responses are solved separately in the time domain and frequency domain, respectively. The analyses are coupled through special feedback mechanisms. It is found that the hybrid method is nearly as accurate as coupled time domain analysis in terms of predicting the response standard deviations, but requires only one-tenth of the computational effort. The aim of this paper is to extend the hybrid method for predicting the extreme vessel motions and line tensions. To this end, a procedure is outlined for calculating the mean upcrossing rates based on the time history of the LF response and spectral moments of the WF response. The crossing rates thus obtained are found to be in good agreement with those extracted from coupled time domain simulation.