ABSTRACT:

A reliable and economical hybrid integral-equation method is proposed for predicting wave-induced dynamic responses of ocean platforms of complex geometry This is based on combining a direct boundary element solution of the fluid region close to the body with an eigenfunction representation of the far field behaviour To achieve accuracy and economy, the boundary surfaces are discretised into quadratic isoparametric elements The validity and accuracy of the proposed method are demonstrated through several numerical examples, including the results for the ITTC semi-submersible. It is shown that the use of quadratic isoparametric elements leads to significant improvement of accuracy and efficiency of the hybrid integral-equation method, compared with classical boundary integral approaches based on constant element idealizations

INTRODUCTION

The design of huge ocean platforms depends critically upon the analysis of wave induced loads and motions. Thus, over the last fifteen years, substantial efforts have been devoted to the development of reliable and economical methods for predicting these effects, Computer programs based on three-dimensional diffraction theory are now available for evaluating the hydrodynamic coefficients, loads and motions for structures of practical form For large bodies with relatively simple geometry such as a vertical cylinder or sphere, these programs generally provide accurate predictions which compare well with analytical solutions and model test results. Recent surveys for the ITTC (Takagi et al., 1985) and the ISSC (Eatock Taylor and Jefferys, 1986), however, suggested that this was not the case for more complicated structures such as a semi-submersible and tension-leg platform (TLP) The ITTC Ocean Engineering Committee has performed a comparative study of motions of a semi-submersible, which has been published by Takagi et al (1985). The three-dimensional diffraction programs seemed to overestimate the heave added mass, leading to longer natural periods than those predicted by Monson" s formula or experiment.

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