Pages 133-140
It is very important to estimate exactly low frequency motions in directional waves in order to design rationally mooring systems of floating offshore structures, as the real sea consists of directional waves. We have developed the numerical simulation method of the moored body motions in multi-directional waves with low frequency motions. However, we have not yet clarified the sufficient number of directions of incident multi-directional waves for estimating the low frequency motions in directional waves. In this paper, we carry out the numerical simulation of motions of a moored semi-submersible structure in 1,3,5 and 7 directional waves and discuss the mean and significant amplitudes of the low frequency surge and sway motions. From the results, it became clear that the low frequency motions in directional waves can estimate using the motion simulation in 7 directional waves. Furthermore, very important characteristics of low frequency motions in directional waves were cleared that the low frequency sway motions in directional waves the principal direction of which is that of head seas are larger than the low frequency surge motions in the same waves.
