Coupled dynamic analysis of a moored Spar in random waves andcurrents (time-domain vs

Nonlinear coupled responses of a moored spar in random waves with and without colinear currents are investigated in both time and frequency domains. The first and second-order wave forces, added mass and radiation damping, and wave drift damp-ing are calculated from a hydrodynamics software package called WINTCOL. The total wave force time series (or spectra) are then generated in the time (or frequency) domain based on a two-term Volterra series model. The mooring dynamics are solved using the software package WINPOST, which is based on a generalized-coordinate-based finite element method. The mooring lines are attached to the platform through linear and rotational springs and dampers o that various boundary conditions can be modeled using proper spring and damping values. In the time-domain analysis, the nonlinear drag forces on the hull and mooring lines are applied at the instanta-neous position. In the frequency-domain nalysis, nonlinear drag forces are stochas-tically linearized, and solutions are obtained by an iterative procedure. The time-domain results are systematically compared with the frequency-domain results.