Drag forces acting on mooring lines and viscous damping forces of a floating structure are essential to estimate a magnitude of a slow drift oscillation of semi-submersibles. In this paper a time domain calculation method of dynamic tensions of mooring chains and experimental results on viscous damping forces of semi-submersibles model are presented with the view of investigating the damping force for the slow drift oscillation. The calculated results of the slow drift oscillation of floating structure and dynamic tensions of mooring lines obtained by using this calculation method are compared with the experimental ones. The calculated results are in good agreements with experimental ones.
A lot of research on responses of semi-submersibles in regular waves have been done, and it is shown that the responses can be precisely estimated by a linear theory (Tasai et al, 1970 and Takezawa et al, 1984). However, we think that a method for quantitatively correct estimation of slow drift oscillation is not established though research efforts have been made (Koterayama et al, 1987 and Takezawa et al, 1988) since Pinkster (1974) pointed out the existence of a slowly drifting force. The reason is that understanding of damping forces are not clear. At first we studied damping forces acting on a floating structure. It was known that the viscous drag coefficient of semi-submersibles in the simple harmonic oscillation is different from the viscous drag … coefficient in the slow drift oscillation which is multiharmonic oscillation. However, such a numerical method is not practical in a preliminary design of an ocean structure because of very long computation time. One of the authors proposed an Approximate Method (Koterayama, 1978) for calculating the dynamic tension of mooring lines taking added mass forces and drag forces into consideration.