Collapse response analysis of a ship's hull girder in cyclic focused waves using a hydro-elasto-plastic beam model

Large waves and whipping effects cause cyclic bending moments on the hull girder that may cause some regions of the structure to exceed the linear limit. In this event nonlinear structural damage/defect (e.g., plasticity and buckling deformation) is accumulated in the structure, which subsequently degrades the ultimate/post-ultimate capacity of the girder. A cyclic load-shortening curve model has been proposed to track the response of stiffened panels under cyclic loading considering the cumulative degradation due to elastoplastic buckling. Separately, a coupled hydrodynamic FE/Smith method (HFS method) has been developed to predict post-collapse response of the hull girder in waves. In this paper the cyclic load-shortening curve is incorporated into HFS method and verified through a case study example. A container ship is excited in a series of focused waves that causes a cyclic response beyond the hull-girder ultimate strength. The effect of different load-shortening curve models on the post-ultimate response of the girder are studied. These show the sensitivity of the reduced hull girder capacity. The results imply that even if the hull-girder collapse stops with partial damage, the hull-girder capacity could be reduced by the cyclic plasticity and the collapse could rapidly progress in subsequent waves.