At the early design stage of a large floating structure, it is firstly important to know the hydroelastic response characteristics in waves. For this purpose, the structure is modeled by three-dimensional grillage, and hydroelastic analysis is performed in order to estimate the overall behaviour. At this stage, main design parameters are: floater shapes, their arrangement and rigidity distributions. They are optimized by referencing to the hydroelastic responses estimated by the analysis. As the design work develops, more detailed modelling is possible. At the final design stage, the design must be confirmed by checking the response against criteria. The structure is re-modeled by shell FE elements for skin structures and beam elements for stiffeners. It is considered that the more correct estimations are performed by employing the refined model. However, there might be significant differences in the modelling and also in the resultant estimations between the first and final stages even when the subject structure is identical. Then, it is necessary to evaluate the differences between the results estimated by using these two models in order to assure the actual response level estimated by using the beam model at the early design stage. In this paper, three-dimensional grillage and shell FE structural models of a large floating structure are prepared. Hydroelastic analyses are performed on the two models. The results are compared in terms of motion, member force and stress responses.

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