Influence of geometric parameters on the structural strength of reinforced cylindrical shells under external hydrostatic pressure

Different combinations of structural geometrical parameters (stiffener spacing and thickness of shell plating) are associated with different pressures and failure modes of the hull of a submarine. Considering construction limitations (maximum and minimum thicknesses of the plating and dimensions of the stiffeners), the optimal combination will be that which results in a lighter structure, better operating performance of the submarine and lower material costs. Furthermore, geometric imperfections (local and global) and residual stresses affect the structural strength of the hull and interact differently with their modes of collapse. In order to study the influence of the geometric parameters and imperfections on the collapse of reinforced cylindrical shells under external pressure, parametric numerical models were developed in the ANSYS finite element program. The models considered nonlinear kinematics and material behaviour. The results were compared with the experimental results available in the literature. In addition, these results were compared to results obtained from semi-empirical formulas of pressure hull design codes. The collapse pressures obtained by varying the spacing between stiffeners and the thickness of the plate, with the most detrimental geometric imperfections, were adjusted to a failure surface using the least method squares. Finally, a case study was developed simulating the structural design of a pressure hull for a given maximum operating depth, in order to optimize the weight of the structure.