基于势流理论的内孤立波与立管作用数值研究

内孤立波是一种发生在水面以下的在世界各个海域广泛存在的大幅波浪, 其剧烈的波面起伏所携带的巨大能量对以海洋立管为代表的海洋结构物产生严重威胁, 分析其传播演化过程的流场特征及立管在内孤立波作用下的动力响应规律对于海洋立管的设计具有重要意义. 本文基于分层流体的非线性势流理论, 采用高效率的多域边界单元法, 建立了内孤立波流场分析计算的数值模型, 可以实时获得内孤立波的流场特征. 根据获得的流场信息, 采用莫里森方程计算内孤立波对海洋立管作用的载荷分布. 将内孤立波流场非线性势流计算模型与动力学有限元模型结合来求解内孤立波作用下海洋立管的动力响应特征, 讨论了内孤立波参数、顶张力大小以及内部流体密度对立管动力响应的影响. 发现随着内孤立波波幅的增大, 海洋立管的流向位移和应力明显增大. 由于上层流体速度明显大于下层, 且在所研究问题中拖曳力远大于惯性力, 因此管道顺流向的最大位移发生在上层区域. 顶张力通过改变几何刚度阵的值进而对立管的响应产生明显影响. 对于弱约束立管, 内部流体的密度对管道的流向位移影响较小. 

THE NUMERICAL INVESTIGATION OF THE RESPONSE OF RISERS INDUCED BY INTERNAL SOLITARY WAVES BASED ON POTENTIAL FLOW THEORY1)

Internal solitary waves are large waves that occur below the ocean surface and are widely present in all sea areas of the world. The huge wave profile undulations and energy pose a serious threat to marine structures like marine risers. Analysis of the flow field characteristics in the propagation and evolution process of internal solitary waves and the dynamic response law of the risers under the action of internal solitary waves are of great significance to the design of the marine risers. Multi-domain boundary element method is adopted to establish a numerical model to analyse and calculate the flow field of internal solitary wave based on the nonlinear potential flow theory in stratified fluids in this paper, and the flow field characteristics of internal solitary wave in real time can be obtained. The Morison equation is used to calculate the load distribution including inertia force and drag force induced by the internal solitary wave on the marine risers according to the flow field information calculated using the numerical simulations. The nonlinear potential flow calculation model of the internal solitary wave flow field is coupled with the dynamic finite element model to solve the dynamic response characteristics of the marine riser under the action of the nonlinear internal solitary wave. The influences of the internal solitary wave parameters, the top tension and the internal fluid densities on the dynamics of the riser are calculated and discussed. It is found that the displacement in flow direction of the ocean riser increases significantly as the amplitude of the internal solitary wave increases. The top tension has a significant impact on the response of the marine risers by changing the value of the geometric stiffness matrix. However, the density of the internal fluid has little effect on the displacement of the pipeline at flow direction for the weakly restrained risers.