Analysis of influence of deck longitudinal fatigue strength on hull girder design section modulus based on CSR rules

Abstract：Objectives This study investigates the impact of fatigue strength requirements on the design section modulus of the hull girder, focusing on the deck longitudinals of CSR tankers and bulk carriers, to address the design section modulus requirements in the actual ship design process under Common Structural Rules (CSR). Methods Based on the fatigue assessment methodology specified in the CSR, the relationship between the fatigue strength of deck longitudinals and the hull girder section modulus was calculated and analyzed for actual tankers of various tonnages. An empirical regression formula was proposed considering practical scenarios. A comparative analysis of the key influencing parameters in the fatigue assessment for bulk carriers between the CSR-BC rules and the current CSR rules was conducted, presenting the fatigue assessment results for the deck longitudinals and an analysis of the differences. Results The research indicates that with the increasing application ratio of high-strength steel in current CSR tankers, the hull girder section modulus requirement governed by the fatigue strength of deck longitudinals may potentially exceed the requirement determined by the hull girder's design bending capacity itself. Furthermore, the fatigue requirements for CSR bulk carriers are significantly enhanced compared to those in the CSR-BC era.Conclusions In the design of CSR tankers utilizing higher grades of high-strength steel, situations may arise where the section modulus required for fatigue strength is higher than that required for the hull girder's ultimate longitudinal strength. The derived simplified formula can be referenced for trial calculations and preliminary predictions. Due to adjustments in the CSR fatigue assessment framework, the fatigue strength requirements for CSR bulk carriers have been substantially increased compared to the CSR-BC period. Subsequent follow-up studies incorporating specific ship operational conditions are necessary for further tracking and research aligned with rule developments.