On Irregular Motions and Structure Response of a Moored Semi-Submersible Rig Including Slowly Varying Phenomena

It was well known that slowly varying phenomena in irregular motions of a moored semi-submersible rig could not be neglected in irregular waves or in actual ocean waves. On the other hand, the structural responses of members of offshore structures were usually treated only by the linear response functions owing to the wave frequency.This paper shows at first the clear experimental results including the slowly varying structural response which are obtained by the model experiments of a standard semi-submersible rig with chain mooring in irregular waves.And the theoretical linear transfer functions of the motions and structural responses are compared with the measured values, then usefulness of these practical calculation methods is confirmed.It is investigated that the structural responses with very low frequency are caused by the slowly varying motions and can be approximately estimated by the application of the free oscillation tests results in still water.Furthermore, the statistical properties of the irregular time histories with the slowly varying phenomena are clarified, and the statistical predictions of the motions and structural responses with the slowly varying phenomena become practical.