Structural Revision of an Independent Cargo Tank: Investigation of Ultimate Limit State and Fatigue Limit State

Abstract

This thesis is dealing with Ultimate Limit State and Fatigue Limit State of an independent cargo tank for bitumen carriers. The cargo tank has been investigated with main focus on the upper and lower edge of the longitudinal watertight tank bulkhead. The main question is how can the stress concentrations in those specific connections be lowered. Additionally the fatigue life has to be above 25 years. This can in theory archieved by decreasing the stiffness in the connection allowing more deformation as the bulkhead hinges instead of transferring moments in the stiff connections. Thus mainly going from a fixed connection to a simply supported. To assess and improve the performance of the design, FE-analysis in MSC APEX is used. For FLS the FE-analysis was used as input in the hotspot method to be able to calculate fatigue life. All methods used are in accordance to the classification rules of Bureau Veritas. The results show that the stress concentrations can be lowered through a change of the position of the top tank transverse stiffener and the bottom tank transverse stiffener as well as through the removal of a bracket and the relocation of the anti flotation device. Further changes had to be made to meet the requirement for fatigue life of 25 years. This was done by increasing the thickness of the bottom plate.