Analysis on Low Frequency Heave, Roll and Pitch Motions of a Deepwater Semisubmersible

Low frequency heave, roll and pitch motions of a semisubmersible should be evaluated carefully, since the air gap and the drilling operation are affected directly. A drilling semisubmersible in operating and survival conditions is investigated numerically and experimentally. Results of RAOs, power spectrums and statistic values are presented and analyzed. Viscous forces are significant for roll and pitch but negligible for heave. The natural periods of roll and pitch are relatively large and low frequency response exists due to excitations by the second order wave drift force. By comparison with the wave frequency response, the low frequency roll and pitch are dominative in operating condition and the same magnitude in survival condition. For heave motion, the low frequency response doesn’t exist in survival storm with long wave periods and exists obviously in operational storm with short wave periods. The effect of wind spectrum is small for heave but very significant for roll and pitch. The low frequency roll and pitch in wind spectrum are much larger than ones in steady wind. It indicated that the resonant roll and pitch induced by the slowly varying wind drift force are much larger than ones induced by the wave drift force.