Fatigue life of bulk carrier side shell frame lower toes as a function of ship length and loading condition

New methods and tools have led to optimized ship structure, but poorly designed details persist, leading to fatigue failure. The cost of optimizing all details is prohibitive. A way is needed to parametrically assess fatigue life. Fatigue is a function of many variables ranging from design and fabrication to operational issues and little work has been done to quantify the relative effects. This work focuses on a narrow problem: fatigue life of bulk carrier side shell frame lower bracket toes. The frame was reduced to a simple two dimensional beam, subject to heave induced inertial and buoyant loads. The ship was assumed to be operating in an ITTC wave spectrum. It was assumed that the speed of advance is zero; the ship was flexible and the seas were from ahead. Boundary conditions were modeled using spring supports and fixity parameters. Fatigue analysis was performed using a hotspot SN approach and Miner's Rule. Results show a rapid decrease in fatigue life for ships greater than 100m.; A loading parameter was defined to model the difference between homogeneous and alternate hold loading. Results indicate homogeneous hold loading is preferable for ships less than approximately 290m in length and alternate hold loading is preferable for ships greater than 290m in length. This is thought to be because the heave natural frequency tends to be higher than the wave spectrum peak for small ships. Increasing length and unit loading causes a lowering of the heave natural frequency to a value closer to the forcing frequency of the waves for larger ships.