Elastic Response Analysis of Very Large Floating Structures in Waves Using Spectrum Finite Element Method

本项目的目的是发展一种超大型浮式结构物在波浪中弹性响应分析的简化方法，以评估初始设计阶段弹性响应与结构参数(如质量和刚度分布)之间的关系。为此，采用弹性地基梁弯曲和弯扭变形的动力形函数，建立了一维谱有限元。该方法可以用较少的单元数求出梁在给定外力作用下的动力平衡方程的精确解。利用标准有限元程序可以方便地考虑结构质量和刚度的变化。对浮箱式和半潜式超大型浮筒结构进行了一系列结构响应分析，研究了质量和刚度分布对结构响应的影响。结果表明：(1)减小结构端部质量是避免浮箱式超大型浮体结构在波浪中发生共振的有效途径。(2)增大弯曲刚度可以有效地降低超大型浮体结构的弯曲响应，包括挠度和弯曲应力。(3)竖向剪切刚度对VLFS弯曲响应的影响可以忽略不计。(4)对于宽长比较大的VLFS，实际中板的变形是沿宽度方向发生的。相应地，梁模型往往会高估结构的刚度。因此，将梁模型应用于VLFS的响应分析必须受到一定的限制。

The objective of the present project is to develop a simplified method of elastic response analysis of a Very Large Floating Structure (VLFS) in waves to evaluate the relationship between elastic response and structural parameters, such as mass and stiffness distributions, in the initial design stage. For this purpose, one-dimensional spectrum finite element using the dynamic shape function of bending and flexural-torsional deformation of a beam placed on an elastic foundation was derived. In the proposed approach, the exact solution of dynamic equilibrium equation of a beam under given extrernal loads can be obtained by small number of elements. The change in mass and stiffness can be easily considered by using standard finite element procedures.A series of structural response analysis of pontoon-type and semi-submersible type VLFSs was performed, and the influences of mass and stiffness distributions on structural responses were investigated. It has been found that :(1) The reduction of mass of the end part of structures is an effective way to avoid resonance in waves for pontoon-type very large floating structures.(2) The increase of bending stiffness is effective to reduce the bending response of a VLFS,including deflection and bending stress. The same holds for flexural-torsional responses.(3) The influence of vertical shear stiffness on the bending response of VLFS is only negligible.(4) For the VLFS with a large width to length ratio, the deformation as a plate takes place in width direction in reality. Accordingly, a beam model tends to overestimate the stiffness of structures.Some limitation must be therefore imposed on the application of a beam model to the response analyzes of a VLFS.