Hydroelastic Response of a Ship Structural Detail to Seakeeping Loads Using a Top-Down Scheme

In order to investigate the local response of a ship structure, it is necessary to transfer the seakeeping loading to a 3DFEM model of the structure. A common approach is to transfer the seakeeping loads calculated by a BEM method to the FEM model. Following the need to take into account the dynamic response of the ship to the wave excitation, some methods based on a modal approach have been recently developed that include the dry structural modes in the hydro-structure coupling procedure and allow to compute the springing and whipping response of the ship structure to the seakeeping loads. In the context of the fatigue life assessment of a structural detail, a very fine FE model is required. A very large number of seakeeping loading cases also need to be considered to account for all the conditions encountered by the ship through its life. It becomes then clear that because of the CPU time issue, the whole FE model can not be very fine. This is why a hierarchical top-down analysis procedure is commonly used, in which the global ship structure is modelled in a coarse manner using one finite element between web frames. The structural details are modelled separately using a fine meshing. Such top-down methods are commonly used for the estimation of the quasi-static response of structural details to the seakeeping loads. This paper presents a methodology in which a top-down method is used to estimate the springing response of a ship structural detail loaded with wave pressure, and its fatigue life. The global dry structural modes are transferred to the detail fine model using the shape functions of the finite elements of the global model. The hydrodynamic pressures are computed directly on the fine mesh model, avoiding any interpolation error. The imposed displacements at the fine mesh boundary are computed using the same method that is used to transfer the structural mode shapes, and the local pressure induced loads and inertia loads are applied on the fine mesh nodes. This method is applied for the calculation of the elongation of a strain gauge which is installed in the passage way of an ultra large container ship.