Asymmetric Collapse Modes of Pipes under Combined Bending and External Pressure

Abstract

The installation and operation of pipelines in high ambient pressure environments, such as deep water, requires design against the potential of collapse. The subject of pipe buckling and collapse under combined external pressure and bending is revisited in order to investigate the causes of angled buckles observed in recent experiments and the associated scatter in the critical loads. Stainless steel 304 tubes with diameter-to-thickness ratio of 18.3 are bent to collapse under various values of external pressure. Tubes bent at pressures higher than 0.72 exhibited angled buckles which were oriented at 20 deg to 45 deg to the axis of bending. In this pressure regime, the scatter in the results was larger than usual. At lower pressures, the tubes buckled in the expected mode with the flattening being along the axis of bending. A previously developed formulation is extended so that it can handle asymmetric imperfections and buckling modes. The analysis is first used to reproduce the experimental results and subsequently to study the problem parametrically. The orientation of the initial ovality is found to play a role in the final buckling mode and in the value of the critical curvature for bending under high values of pressure. In addition, residual stresses can interact with the initial ovality affecting the critical curvature both positively and negatively. For lower pressures these effects are small.