The effect of initial imperfection on the hull girder ultimate strength of intact and damaged ships

It is important to clarify the residual hull girder strength of damaged ships by collision or grounding in order to ensure their safety. There are several methods to calculate the hull girder ultimate strength, such as the Smith's method and the finite element method (FEM). Up to now, many studies have been performed for the hull girder ultimate strength of intact ships. However, only a few studies have been performed for damaged ships. The main objective of this study is to develop the simplified calculation method of hull girder ultimate strength for not only intact but also damaged ships. In this study, the simplified calculation program of hull girder ultimate strength based on the procedure of Common Structural Rules (CSR) for bulk carriers, which is a kind of the Smith's method, was developed. In this program, the incremental-iterative method and the nonlinear stressstrain relation (σ-ε curve) of structural members, such as the longitudinal stiffened plates, are utilized in order to calculate the hull girder ultimate strength. To calculate the hull girder ultimate strength of damaged ships after collision or grounding by the Smith's method, the stress-strain relation (σ-ε curve) of damaged members is necessary to be obtained beforehand. Therefore, the behavior of damaged members under the tensile and compressive condition had been calculated by the finite element analysis (FEA), and the idealized stress-strain relation (σ-ε curve) of damaged members was proposed. Moreover, the inclined effect of neutral axis in the mid-ship section due to the asymmetrical damage by collision or grounding is also taken into account in this developed simplified calculation program. In this paper, utilizing this developed simplified calculation program, the hull girder ultimate strength is calculated for both intact and damaged ships, and the reduction rates of hull girder ultimate strength by damage are investigated. Also, to examine the accuracy of this developed simplified calculation method, the calculation results are compared with the FEA results for both intact and damaged ships. Furthermore, the effect of initial imperfection on the hull girder ultimate strength of intact and damaged ships is also investigated.