Dynamic response analysis of offshore cable structures under wave forces

本文提出了海洋结构缆索在潮流和/或波浪力作用下的动力响应分析方法，该缆索用于系泊、支撑或系泊近海结构物。利用作者过去建立的三结点曲线有限元，推导了拉索杆件在流体力作用下的运动方程。采用有限元分析方法，对海洋缆索连接的桁架杆件、刚架杆件和浮体进行了分析，并编制了实用计算程序。该程序具有三个主要功能：(1)大变形的三维静力学分析；(2)特征值分析；(3)动力响应时程分析。此外，时程分析的数值结果可以在自动过程中以图形方式显示，这使得计算结果易于阅读。通过大量的算例分析，充分检验了数值解的精度和非线性迭代的收敛速度，证实了程序的实用性。为了更好地使用该程序，对程序进行了进一步的改进。通过建立缆索模型，给出了数值算例，这些缆索模型应该用于实际的海洋结构物中。研究中得到的静力潮流作用下拉索的大变形和时变波浪力作用下拉索的动力响应的数值结果揭示了海洋结构物系缆或系缆的基本特性。

A method of dynamic response analysis of marine structural cables, which are used for the purpose of mooring, staying or guying the offshore structures under the action of tidal current and/or wave forces, is presented. Equations of motion of cable members under the action of fluid forces are derived by employing three node curvilinear finite elements which were formulated by the authors in the past. Truss members, rigid frame members and floating bodies, which are to be connected with marine cables, are also analyzed in the form of finite element formulation.Computer program based on the above theory is developed for practical use. The completed program provides three main functions, i.e., (i) three dimensional statics with large deformation, (ii) Eigenvalue analysis and (iii) time history analysis of dynamic response. Further, the numerical results of time history analysis can be displayed graphically in an automatic process, which makes it easy to read the results of computation. The accuracy of numerical solutions and convergence of nonlinear iteration were fully checked and the practicability of the program was cofirmed after many case studies. Further improvement is intended for the better usage of the program.Numerical examples are shown by setting up cable models which are supposed to be ones utilized in practical offshore structures. Numerical results obtained in this research as to large deformation of cables under the action of statical tidal current and dynamic response of cables subject to time-varying wave forces show fundamental behavior of staying or mooring cables of offshore structures.