Collapse Design Optimisation of Ring-Stiffened Cylindrical Pressure Hulls

Abstract

The pressure hull is a vital structural component of the underwater vehicle. It enables the underwater vehicle to withstand environmental loadings such as hydrostatic pressure. Pressure hull design seeks maximum strength capacity with minimum structural weight and is usually constructed as a ring-stiffened cylindrical shell. The shell has the potential to buckle and reduce its loading capacity. Hence, the critical collapse analysis of the pressure hull represents one of the most appealing areas to optimise in structural design.

This thesis aims to optimise the collapse performance of a ring-stiffened cylindrical pressure hull under uniform external hydrostatic pressure. This will be done by studying the influence of the design parameters on the two constraints, which are the critical buckling pressure and the corresponding weight of the steel construction. The optimisation process contains four design variables which include shell cylinder thickness, unsupported spacing, ring-stiffener height and ring-stiffener thickness. A pressure hull model was developed with ANSYS software, and the collapse design analysis is performed using the finite element method. The design influence of the performance output is investigated using eigenvalue analysis, and a parameter correlation study was performed to analyse the coefficient of correlation between the design variables and the performance output. Additionally, a probabilistic analysis was conducted for the pressure hull model in order to investigate the reliability of the critical buckling pressure due to uncertainties in the design parameters.

The findings from this study revealed that the design factor was reduced with 16.4% and the critical buckling pressure was increased by 7.09% when considering parameter uncertainties of cylinder thickness, diameter, unsupported spacing, ring-stiffener height and ring-stiffener thickness. The input design parameters with a significant influence on the performance of the ring-stiffened cylindrical pressure hull are the unsupported spacing between the ring-stiffeners and the cylinder thickness. The cylinder thickness also showed a strong coefficient of correlation with the critical buckling pressure and the corresponding weight of the pressure hull.

This thesis presents an in-depth insight into the design of an important component in the underwater vehicle. The method and analysis framework presented in this thesis can also be utilised in other structural applications.