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

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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.