半潜水式海洋構造物の不規則波中での長周期運動の数値シミュレーション

Abstract

The paper reports the results of time-domain simulation of the slow-drift motions of a semi-submersible in irregular seas. The simulation is based on exact evaluation of the second-order wave exciting forces including the contribution from the second-order velocity potential. The validity of the simulation method is confirmed by comparing the simulated time histories of motions with the experimental data measured in model tests. The conclusions arising from limited simulation results reported herein are summarised as follows: (i) The sway and roll responses of the semi-submersible in irregular beam waves are dominated by the slow-drift resonance motions excited by the low-frequency component of the second-order wave forces. This is due to the very low natural frequencies and low damping possessed by the system. (ii) The slow-drift sway motion can be predicted fairly well with the knowledge of mean drift forces in regular waves, as suggested by Newman (Ref. 1)). (iii) The contribution of the second-order velocity potential to the slow-drift roll moment is significant. Exact evaluation of the second-order wave forces are therefore essential for correct prediction of the slow-drift roll motion of the semi-submersible. (iv) The present simulation method based on exact evaluation of the second-order wave forces can reproduce with reasonable accuracy the motions of the experimental model measured in a wave flume.