非线性波浪作用下深水浮式结构物大幅运动的时域分析

Deep-water floating structures often experience large-amplitude motions due to large waves. In order to solve this complicated engineering problem, the current numerical methods based on the small wave steepness and small body motion assumptions are not applicable. Therefore, a hydrodynamic analysis theory which includes both the incident wave nonlinearity and the body nonlinearity is badly needed. In the present project, a higher-order boundary element method (HOBEM) based on time-domain Green function will be adopted. The instantaneous free-surface boundary condition is satisfied on the mean free-surface by using Taylor series expansion and the incident wave nonlinear terms are considered by introducing “weak-scatter” hypothesis. The exact body boundary condition is satisfied on the instantaneous wetted surface of the moving body. A “Time-frequency transform method” is proposed to calculate the convolution including the time-domain Green function, and a stretching/compressing dynamic grid technique is introduced to re-mesh the instantaneous wetted body surface. A series of model tests and numerical simulations are performed to validate the new numerical model, and determine the conditions and limits to apply different analysis theories. The large-amplitude drifting motions of a deep-water mooring FPSO exerted by irregular waves will be simulated to analyze the effects of wave parameters on the motion responses of the floating structures, wave forces and mooring forces, and further to reveal the mechanism and influencing factors of large-amplitude drifting motions.

深水浮式结构物在作业时经常会遭遇波高很大的非线性波浪，并产生大幅度的运动响应。研究这一复杂的工程问题，现有的基于小波陡和小位移假定的数值方法已不再适用，必须建立一个计及入射波浪和运动响应非线性影响的水动力分析理论。本项目拟采用基于时域格林函数的高阶边界元方法，利用泰勒级数展开，将非线性的瞬时自由水面边界条件变换到平均自由水面上满足，引入“弱散射假定”，保留其中与入射波有关的非线性项；精确的物面边界条件在物体运动的瞬时湿表面上满足。提出一个计算时域格林函数卷积的“时频转换算法”，引入弹性伸缩动网格技术对瞬时物面进行重新剖分。分别开展物理模型试验和数值模拟研究，验证本项目所建模型的正确性，明确现有分析理论的适用范围和应用条件。开展不规则波作用下深水系泊FPSO大幅慢漂运动问题的研究，分析波浪要素对浮式结构物的运动响应、波浪力及系泊力的影响和规律，揭示大幅慢漂现象的形成机理和影响因素。

深水浮式结构物在作业时经常会遭遇波高很大的非线性波浪，并产生大幅度的运动响应。研究这一复杂的工程问题，现有的基于小波陡和小位移假定的数值方法已不再适用。本项目以数值模拟为主要手段，建立了一个计及入射波浪和物面条件非线性的时域数值模型，提出了计算时域格林函数的“人工小参数”方法和计算时域格林函数卷积的“时频转换”方法，推导了弹性动网格的计算公式，解决了大型结构物计算、长时间数值模拟和网格重构等关键问题。建立了研究三维系泊浮体大幅运动问题的物面非线性时域模型，开展了风和不规则波联合作用下系泊FPSO水动力问题的研究，揭示大幅慢漂现象的形成机理和影响因素。通过物理模型实验测定了单层及多层垂荡板的阻尼系数及附加质量系数，研究了升沉板对Spar平台水动力性能的影响。