A direct method for calculating hydro elastic responses of a very large floating structure (VLFS) is investigated with two different numerical schemes. The direct method means solving the integral equation for the pressure distribution beneath a structure simultaneously with the vibration equation of a freely-floating plate. In the first numerical scheme, the pressure is represented using bi-cubic B-spline functions and the structural deflection is represented by an expansion in the dry eigenmodes for bending of a uniform beam with free ends. This scheme can be shown by an appropriate transformation to be exactly the same as the mode-expansion method. The second numerical scheme adopts bi-cubic B-spline functions for representing both the pressure and the structural deflection, which is simpler and easier in coding the method. A numerical convergence test for local deflections shows that the results from the two different schemes are practically the same, and those results are in good agreement with experimental measurements. Not only the structural deflection but also the wave profile around a VLFS is computed, and the effects of hydroelastic motions on reflected and transmitted waves are discussed.

INTRODUCTION

Very large floating structures (VLFS) are recently considered for various purposes, such as airports, storage or manufacturing facilities, habitation, and so on. The configuration considered as an airport in Japan is of barge type, and its size is of order of 5 km long, 1 km wide, and a few meters in depth. Therefore the flexural rigidity of this type of structure is relatively small and thus hydroelastic responses are more important than the rigid-body motions. Several methods for calculating hydro elastic responses have been proposed; those are categorized roughly into the mode-expansion method (e.g. Maeda et al. (1995), Takaki et al. (1996), Kashiwagi et al. (1997), Nagata et al. (1997), and Ohmatsu (1997)) and the direct (FEM-BEM combined) solution method (e.g. Yago et al. (1996) and Yasuzawa et al. (1996)).

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