Second-order frequency-domain coupled analysis for moored offshore structure dynamics

摘要

This article aims at proposing an efficient frequency-domain approach for fully coupled analysis of offshore structures. In this method, the nonlinear dynamic items in governing equations including velocity-squared drag force and geometric deformation of mooring lines are extracted and linearized up to second-order perturbation transformations in frequency domain. Moreover, the line-seabed interaction term is considered effectively and the bottom-lying portion is modeled with a reduced spring coefficient related to the floater motion results. By matching the continuity requirement at boundary points, the first- and second-order differential governing equations are solved numerically by Goodman-Lance approach to avoid prohibitive calculations, so that estimations of frequency response functions can be more computationally intensive. Using time-domain (TD) simulation as a benchmark, one case of a moored semi-submersible is employed to investigate the performance of this frequency -domain (FD) approach. The numerical results show that (i) the predicted vessel motion and mooring tension responses match well with that of the fully coupled time-domain analysis software SIMO and RIFLEX, and (ii) the bottom line-seabed interaction can be modeled accurately, which provide almost identical response estimates as those of the time-domain method, especially for the mooring-line segment near touch-down point (TDP).