Estimation of ultimate hull girder strength with initial imperfections

摘要

The estimation of the ultimate strength of the ship hull is very important for its structural safety against applied loads. Various methodologies have been developed to evaluate the ultimate hull girder capacity, for example Caldwell (1965), Paik and Mansour (1995), International Association of Classification Societies Common structural rules (IACS CSR, 2006), etc. These methods do not usually include initial imperfections like initial deflection and initial residual welding stresses in the plating between stiffeners. The IACS CSR, introduced in April 2006, suggest the analytical incremental-iterative method for determining ultimate strength estimation of ship hull, which ignores welding residual stresses. In the present study, the stress-strain relationship for stiffened plate given in IACS CSR method is extended to account for the initial imperfections. Our previous work (Vhanmane and Bhattacharya 2007) on the stress-strain relationship of plate between stiffeners under axial loads, including imperfections like initial deflection and initial residual welding stresses, is used to determine the effective width of attached plating. The proposed methodology is applied to six benchmark cases: a double hull very large crude carrier and a capesize bulk carrier under three different levels of imperfection (slight, average, and severe) for both initial deflection and residual stresses. The ultimate strengths thus obtained are compared with published results that use two different methods (idealised structural unit method and finite element analysis) and it appears that the proposed methodology is simple yet robust in estimating hull girder ultimate strength under initial imperfections.