深水浮式平台垂荡运动与水下柔性立管涡激振动的动力耦合

Impact of top-end heave on vortex-induced vibration of submarine riser for deep-water platform

基于有限元数值模拟,进行了"平台垂荡-顶张力立管涡激振动"整体系统的动响应数值模拟。动响应模型考虑了立管尾迹流场的水动力与结构动力的耦合和垂荡引起的立管结构刚度的时变特性;分析了平台垂荡运动的频率、模态阶数等因素对水下顶张力立管涡激振动的影响。数值结果表明:与不考虑平台运动相比,立管的动响应位移会增大;立管响应幅值随着模态阶数的降低而增大;在响应过程中,尤其对于低阶模态,会出现响应的模态转换现象。鉴于在平台垂荡和涡激振动的共同作用下,立管的动响应会大于涡激振动、参数激励分别单独作用的响应,建议在立管实际工程设计中应该考虑平台运动和涡激振动耦合激励作用下的结构动响应。

The dynamic coupling between top-end motion（ heave） and top-tensioned riser ＇s vortex-induced vibration（ VIV） is explored by our finite element simulations. First,a coupled hydrodynamic force approach,concerning vortex-induced lift force along with fluid drag force,is developed,which regards the interaction between instantaneous riser motion and fluid dynamics. The structural stiffness is changing during dynamic response. Then the dynamic responses of the integrated system involving both floating top-end and a top-tensioned riser undergoing VIV are presented to examine impacts of platform heave,in terms of heave frequencies and mode order number,on riser＇s VIV. Our numerical results show that the dynamic response displacement of riser becomes several times larger than the displacement for the case without top-end motion. The impact of top heave on riser＇s VIV gets larger as modal order number drops.Moreover,an interesting phenomenon,called the mode transition,is observed particularly at lower vibration frequencies due to the natural dynamic characteristics of the slender riser. Based on our results,it is suggested that,in practices of riser design,a combined excitation needs to be considered for accurate dynamic analysis of slender marine structures subjected to both top-end motion and VIV.