On the predictions of hydroelastic behaviours of a huge floating structure in waves -3rd report-

Abstract

A prediction method of hydroelastic behaviours of a very large floating structure (VLFS) that may be used for such purposes as an international airport or an offshore city is presented. In the present analysis, the structure is divided into small substructures and the continuous deformation of the structure is represented by the succession of the discrete displacement of each substructure.The present one is our 3rd report under the same title and shows two major new developments in the hydroelastic analysis of a VLFS. The first one is the evaluation of the structural constraints on the motions of substructures. They are evaluated by a finite difference scheme instead of the differentiation of mode functions, which had been adapted in the previous two reports. In this way the force-free as well as the moment-free conditions at the rims of a VLFS can be rationally satisfied in the analysis. Besides, as will be shown in the paper, an approximation that exploits the fact that the structure is very large can be incorporated in the analysis fairly easily. The other development is a new mathematical technique for the evaluation of hydrodynamic interactions among substructures. The diffraction/ radiation characteristics of a certain number of substructures, which hereafter is called a sub-group, are first determined by the hydrodynamic interaction theory of Kagemoto and Yue (1986). Then the diffraction/radiation characteristics of a group of these sub-groups are determined by the same interaction theory while treating each of the sub-groups as a single floating structure. After repeating this procedure several times, interaction characteristics of a quite large number of substructures can be evaluated with reasonable computational effort. Moreover, exploiting the periodicity of the substructures, this procedure is further simplified because the same diffraction/radiation characteristics can be used for most of the sub-groups.The efficaciousness of the presented new techniques are discussed through comparisons with experimental results. Several example calculations are also carried out for the hydrodynamic analyses of a VLFS supported on more than 10, 000 legs, for which, to the knowledge of the authors, no numerical results have been successfully obtained because the application of conventional numerical techniques is practically prohibitive due to the required huge computational burden.