This paper presents results on the torsional behaviour of stiffener outstands in ring stiffened cylindrical shells subjected to external pressure and longitudinally stiffened panels under axial loading, with geometries typical of those found in current offshore design. The study has been carried out using a full non-linear finite element (FE) package and plastic mechanism techniques. The FE package has been used to obtain load deflection curves, including peak loads, unloading curves and critical buckling loads for a wide range of stiffened panel geometries and stiffener shapes. The effects of stiffener and panel slenderness have been investigated. Comparison with experimental results has been carried out in order to establish the validity of the mesh and the boundary conditions adapted in the FE study. A plastic mechanism approach has also been developed to predict the unloading response of a tripping failure in a flat-bar ring stiffener in a cylindrical shell subject to external pressure. The results have been compared with existing design guidance, which mostly takes the form of simple slenderness limits and a minimum inertia requirement, and comments on these limits are made.