Strength optimization design of spherical hulls for deep-sea submersibles: A hydraulic autofrettage approach in external pressure vessels

The spherical pressure hull is the most important structure to ensure the safety of people in a deep-sea submersible. Its current main design trend is to increase the shell thickness to support greater depths and reduce overall stress. However, increased thickness greatly raises the submersible weight and the stress difference between the inner and outer surfaces. To achieve a balance between structural strength and weight, an autofrettage approach of strengthening the external pressure vessels has been studied. Firstly, an analytical solution of the autofrettage pressure of the spherical external pressure hull is derived based on elastic-plastic mechanics. Secondly, a finite element model is established to validate the analytical solutions and investigate the autofrettage process. Both theoretical and numerical results reveal that the overall stress in the spherical hull has been decreased significantly, and the stress distribution between the inner and outer surfaces becomes more even through the autofrettage approach. Additionally, a method has been developed to determine the optimum autofrettage pressure of the spherical hull for deep-sea manned submersibles which is found to be dependent on working pressure and shell size. This study could contribute to the strength optimization design of spherical hulls for deep-sea manned submersibles without increasing weight.