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In the last 10 years, the importance of whipping on the extreme hull girder loads has received much attention from designers and classification societies. The most common practice to evaluate the ultimate strength of a relatively soft floating structure is to compare the maximum dynamic vertical bending moment after a slamming event with the quasi-static hull girder capacity. Some aspects regarding the current procedure remain unclear, like the capability of the current hydro-elastic methods to accurately predict the extreme dynamic response on the basis of a linear elastic structural model. Moreover, the whipping-induced stresses have a higher frequency than the ordinary wave-induced stresses; hence, the dynamic effects may provide additional strength reserves for the structure and should be investigated. Therefore, the aim of this paper is to investigate the dynamic ultimate strength of stiffened panels considering real loading scenarios, associated with wave loads and whipping response.

Nowadays direct Finite Element Method (FEM) calculation using partial or full length model is necessary for checking the structural integrity of ship and offshore structures under given environmental conditions. The main advantage of using hydro-structure analysis on partial model is to obtain better accuracy than usual computation based on rule loads and also a consistent decrease of the time necessary to build a complete ship model. The comparison of different three cargo hold models with the complete ship model and the improvement of our partial FEM models are the main objectives of the work. Unlike the classical partial FEM models approach, our hydro-structure analysis is based on creating an equivalent full FEM model from the partial model. The equivalent full FEM model is built by adding to the partial model two concentrated masses in the center of gravity of missing aft and fore parts. The mass and inertia properties of the equivalent full FEM model are the same as full ship FEM model. By using an equivalent full FEM model the problem of balancing the partial model transforms into the same problem for the corresponding full model. Instead of using the traditional method for interpolating the pressure from hydrodynamic mesh to structural mesh, the pressure components are recalculated over structural mesh. The inertial loads are then determined by motion equations integrating all pressure loads. In this way, the structural model is fully balanced. The balancing of the 3D FEM structural models represents one important issue to avoid unphysical structural response induced by an unbalanced structural model. This paper is focused on the validation of hydro-structure analysis methodology by comparing the results on a FSO unit using an equivalent full FEM model and a complete ship model.

The main topic of this paper is the 3D-FEM global strength analysis of a offshore supply vessel (OSV) under the following loads: still water and equivalent quasi-static head waves pressure, eigen ship and cargo weight. Two loading cases are selected for this analysis: full loading condition and ballast condition. The 3D-FEM (Finite Element Method) model extends over the whole ship length, the floating and trim equilibrium condition, in vertical plane, are obtained using eigen iterative numerical procedures. The buckling and yielding criteria are used to compare the numerical results with the allowable values according to classification societies.