ABSTRACT

Influences of sea-bottom topography on the hydroelastic behavior of a Very Large Floating Structures(VLFS) are investigated experimentally and numerically. To calculate the sea-bottom effects more rigorously, finite-element method based on the variational method is used in fluid domain. Floating structure is modeled as a thin plate with uniform stiffness. The hydroelastic responses of pontoon-type VLFS are analyzed by the mode superposition method. To enhance the satisfaction of shear and moment free condition a set of orthogonal function is adopted as the mode function instead of free-free beam mode function.

INTRODUCTION

At the design stage of VLFS, there have been many researches(Watanabe et al.,2004, Kashiwagi,1998,1999) on developing the analysis of hydroelastic responses of the VLFS. For the most parts of the previous studies however, the effects of sea-bottom topography on the hydroelastic response of VLFS have been ignored for a computational simplicity. The assumption of flat sea-bottom has a good reason when the mean water depth is very deep to the incident wave length or the VLFS is very small compared to the sea-bottom variation. When the VLFS is designed to construct near the shore the sea-bottom effects on the hydroelastic response of VLFS should be carefully considered because it is very difficult to find a site of vast flat seabottom near the shore.

One of the conventional numerical methods for VLFS has been the boundary element method(BEM) based on the free-surface Green function. For example, Hong et al.(2002) have introduced a Higher-Order Boundary Element Method(HOBEM) for investigating the effects of floating breakwater on the hydroelastic responses of mat-type VLFS with non-zero draft. But in case of considering the sea-bottom variation, the BEM requires the discretization of whole closed boundaries, which increase the storage requirement for computation and calculation time is proportional to the squared number of unknowns.

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