Experimental study on motion responses of a moored rectangular cylinder under freak waves (I: Time-domain study)

Extensive experiments on the motion responses of a rectangular cylinder under random and freak waves have been conducted in the present study. The effects of the relative wave height, relative period and freak wave parameters on the motion responses were investigated. Analysis in time-domain revealed that the freak wave parameter alpha(1) has significant effect on the motion responses of the cylinder, especially for surge and heave. However, alpha(2), alpha(3), alpha(4) have not such significant effect on the motion components. With alpha(1)= 2-2.8, the maximum surge and heave under freak waves were 2.5 and 1.5 times, respectively, larger than those under irregular waves. In addition, the maximum pitch under freak waves was approximately 1.3 times of that under irregular waves. The difference in motion response under freak waves and irregular waves decreased with the increasing relative wave heights. With Hs/d = 0.03-0.1 and alpha(1)= 2-2.2, the maximum surge and heave under freak waves were approximately 30-60% and 20-40%, respectively, larger than those under irregular waves. For the effect of the relative period, the critical surge, heave and pitch occur at period around T-p/T-0neave similar to 1.0, 1.5, 2.0 and T-p/T-0pitch similar to 1, respectively. With the natural periods and alpha(1)= 2-2.2, surge, heave and pitch under freak waves were approximately 44%, 40% and 30%, respectively, larger than those under irregular waves.