ABSTRACT

The thin-walled beam theory and the three dimensional potential theory are applied to the hydroelastic analysis of ship hulls with large deck openings on the basis of dry mode method. A thin-walled beam FEM model has been developed to obtain dry natural frequencies and modes of ship hulls. The hydrodynamic problem is formulated according to the potential theory, and the boundary integral equation is solved through the higher-order boundary element method (HOBEM). Vibration experiments of an aluminum model ship are conducted in both air and water. Comparisons between experimental results and computational results show the effectiveness and efficiency of this method.

INTRODUCTION

In the last decade, ship hulls with large deck openings, such as container ships, are becoming more and more popular due to the convenience in loading and unloading operations. Consequently, their torsional stiffness decreases so dramatically that their natural frequencies drop significantly and the coupled bending and torsional vibrations of ship hulls become the dominant vibration mode. When the wave frequency synchronizes with the first natural frequency, serious elastic vibrations may happen which are known as springing. The elastic vibrations of large floating structures in water were solved using the hydroelastic theory (Bishop & Price, 1979). The hydroelastic analysis on the basis of dry mode method involves two steps (Wu, 1984; Lu, 1992): the first is the computation of natural frequencies and modes of structures in air which are known as dry modes; the other is the solution of the boundary integral equation to obtain hydrodynamic coefficients and loads. Dry modes of ship hulls can be calculated by the three dimensional finite element method, but it is extremely time-consuming to establish the FEM model, perform computations and post-process.

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