In this paper, design formulae of local and global mode ultimate strength currently available are reviewed and evaluated based on the results of finite element analysis, considering their capabilities of accounting for effects of initial hull deflection and variation of hull thickness. As initial hull imperfection data, actual imperfections measured from two small-sized and a mid-sized test models are used. Strain data measured from the test model are discussed and compared with the results of the finite element analysis. Collapse pressures calculated from FEA are compared with those estimated from the ultimate strength design formulae for cylindrical shells. Effects of the initial imperfection on the collapse pressure of stiffened cylindrical shells are also investigated.
Submersible pressure hulls usually consist of stiffened cylindrical shells with conical or hemispherical ends. They are mainly designed based on the concept of ultimate limit state. In order to evaluate ultimate strength of submersible pressure hull structures, several approaches are generally conducted which are model test under high pressure, finite element analysis, and estimation of ultimate strength using design formulae available, etc. Especially, reliable prediction using the formulae is needed in the initial design stage. The concept of using ring stiffeners to increase the strength of cylindrical shells for external pressure was confirmed when von Mises (1933) discovered that the buckling strength of cylinders varied with unsupported length. If stiffened cylinders are sufficiently long, all the rings buckle simultaneously in a general instability mode: the strength is determined by isolating one typical stiffener and applying the wellknown ring-buckling analysis of Levy (1884), using radial loads given by von Sanden and Günther (1952) and Salerno and Pulos (1951). This method to predict the general instability pressure for an infinite stiffened cylinder has been used to approximate ring strength, although experimental verification is lacking.