畸形波作用下二阶波浪载荷对张力腿平台动力响应的影响

Effects of second order wave load on dynamic response of a TLP under freak waves

畸形波易对海上的建筑物造成极大的危害。为研究畸形波作用下张力腿平台的动力响应特性,考虑张力腿平台的六自由度运动与张力腿非线性恢复刚度,建立非线性耦合运动方程。结合随机频率相位角调制法生成的畸形波波面时历,计算在畸形波条件下平台所受的一阶及二阶和、差频波浪载荷,并采用数值方法求解平台六自由度的运动。结果表明,在畸形波作用下,一阶、二阶波浪载荷均受到畸形波的影响,但各波浪载荷成分的增幅在不同自由度上并不相同。以纵荡为代表的低频运动主要受二阶差频波浪载荷影响,以垂荡、纵摇为代表的高频运动受二阶和频波浪载荷的影响。而由平台水平面内运动引发的下沉运动在二阶差频波浪载荷的作用下显著增大,从而诱发了垂荡运动产生了显著增加,因此在畸形波作用下垂荡运动同时受到二阶和频及差频波浪载荷的影响。此外,由于畸形波具有冲击载荷的特性,不同自由度运动幅值出现时刻并不相同。

Freak waves can cause destructive effects on offshore structures. In order to study the dynamic response of a tension leg platform( TLP) under freak waves,nonlinear equations for its coupled motions were established based on the platform 's 6-DOF motions and the leg 's nonlinear restoring stiffness. Time histories of freak wave surface were generated using the random frequency phase angle modulation method. The 1 st order,2 nd order difference frequency and sum frequency wave loads exerted on the platform were calculated under freak waves. The platform's 6-DOF motions were solved with numerical method. The results showed that freak waves affect both the 1 st order and the 2 nd order wave loads,increases in amplitude of different wave loads are different in different DOFs; the lower frequency motion,such as,surge of TLP,is mainly affected by the 2 nd order difference frequency wave load; the higher frequency motions,such as,heave and pitch of TLP,are mainly affected by the 2 nd order sum frequency wave load; under the action of the 2 nd order difference frequency wave load,the negative motion caused by motions in horizontal plane of TLP increases significantly to cause an obvious increase in heave motion; therefore,heave motion under freak waves is affected by both the 2 nd order difference frequency wave load and the 2 nd order sum frequency wave load; besides,due to the impact characteristic of freak waves,the amplitude of each DOF motion appears at different time instants.