In this paper, a simulation method is proposed to analyze the slow drift motion as well as the fast motion of a floating structure including coupled effect of mooring dynamics. Hydrodynamic forces are evaluated by 3- dimensional panel method. Morison type approach is adopted for viscous drag forces. Lumped mass model is used for mooring lines. The fourth order Runge-Kutta method is employed for the time integration of the equation of motion while Newmark method is used for solving the mooring line equations. A DDF(Deep Draft Floater) is chosen for numerical application, which had been used as one of standard models for comparative study in FPS 2000 workshop(1989, Norway). The results show that there is a big difference in slow motion amplitude depending on the presence of the wave frequency motion in the calculation of viscous drag. This fact implies that the fast motions also contribute to viscous damping in slow motion. Using FFT analysis of drag forces in bichromatic waves, it is shown that there exist additional low and high frequency components. The main part of the low frequency component is found to be 2ωi - ωj. Coupled dynamic effect of mooring line seems to be more important in tension variation rather than in body motion. In spite of increase in dynamic tension, the resulting motions do not show noticeable changes.
Recently, deep sea exploration has increased rapidly, and various types of floating structures have been developed to meet the demand. Prediction of slow drift motion behavior of such floating structures is of great importance to insure their safety and operational requirements as well as down time analysis in offshore operation. Therefore damping mechanism of slow motion has been one of hot issues in offshore hydrodynamics for the accurate estimation of slow motion amplitude. Wave drift damping and mooring line damping are one of the typical damping mechanisms of the slow motion(Wichers, 1988).
