To identify the mechanism of ringing of offshore structures, non-linear loads on vertical small diameter cylinder in freak waves, which are generated in a two-dimensional BEM-based numerical wave tank, were calculated and analyzed in this paper. Numerical study demonstrated that the loads on vertical small diameter cylinder under the action of freak waves were highly nonlinear and impulsive, and they contained high frequency components. The analysis of numerical calculation results indicated that the non-linear loads on vertical cylinders under the action of freak waves may be one of the mechanisms of ringing of offshore structures.
The study of Natvig (1994) indicated that ringing response of vertical cylinder and tether of Tension Leg Platforms, TLPs, were induced by highly nonlinear and asymmetric freak waves. So it is a key problem to identify the wave force that may induce the ringing of structures and the structures responses to wave force in the study of ringing of offshore structures. In the study of wave forces on structures, the structures are classified into two types, large scale structures and small scale structures based on whether the scale of structures affect the wave motion evidently or not. Many offshore platforms are supported by vertical cylinders. In routine design, the diagnostic scale, i. e. diameter of vertical cylinder and tether of TLPs, D, is about 20m, while in the severe sea state the wave height and wave length are about 10m and 200~400m respectively. Then the ratio of structures characteristic scale and the wave scale is D L < 0.2, so the structures can be regard as small diameter structures. To study the mechanism of ringing of offshore structure theoretically, only the response of the vertical small diameter cylinder due to the freak wave in medium water depth is carried out in this paper.