LAUSR

Abstract 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 α 1 has significant effect on the motion responses of the cylinder, especially for surge and heave. However, α 2 , α 3 , α 4 have not such significant effect on the motion components. With α 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 α 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 0heave  ˜ 1.0, 1.5, 2.0 and T p /T 0pitch  ˜ 1, respectively. With the natural periods and α 1  = 2–2.2, surge, heave and pitch under freak waves were approximately 44%, 40% and 30%, respectively, larger than those under irregular waves.