Computer aided reliability based design of ring-stiffened cylindrical shells under external pressure

A Level I code format is proposed for the buckling design of ring-stiffened cylindrical shells under external pressure. Depth independent partial safety factors to be applied to the resistance (collapse pressures), are proposed for the four relevant collapse modes Unterframe Shell Collapse, Frame Yield, Plate Yield and Frame Tripping), covering design and fabrication factors. A partial safety factor to be applied to the load (external pressure), and varying with the design pressure and the maximum expected overdiving, is proposed to cover operational factors. For deep diving vessels or in cases in which the risk of overdiving is not relevant, it is proposed that the overall safety factors used in design may be smaller than those presently recommended. In order to obtain such partial safety factors, different aspects of strength modelling and Structural Reliability had to be addressed. On the strength modelling side, the work was focused on the frame collapse modes. Seventy two experimental results were compiled, corresponding to machined models failing by elastic General Instability. Finite Element (FE) meshes were validated in view of mesh studies and experimental results and further used in parametric studies. The effect of boundary conditions on the elastic General Instability pressure pn was investigated in view of both experiments and results of the FE models. Statistical properties were obtained for the model uncertainty associated with pn. Thirty five experimental results were compiled corresponding to welded models failing by General Instability. FE models were validated in view of the most relevant of these experiments as well as in view of other numerical results found in the literature.