Stress triaxiality as a postponer of the collapse pressure of thick-walled submarine pipes

Abstract

One of the main concerns of pipeline design engineers for ultra-deepwater applications is to predict the pipes’ collapse capacity due to buckling caused by the hydrostatic pressure. DNV-ST-F101 is one of the most used guides for such systems, and presents the equations for the computation of the collapse pressure of submarine pipelines considering a limited diameter-to-thickness (D/t) range, not taking into account thick-walled structures. This work aims to investigate the accuracy of DNV-ST-F101 equations for submarine pipes with low D/t ratios. We show that, for these specific configurations, the analytical equations provided tend to underestimate the collapse capacity, which might result in the design of unnecessarily thick structures. Using finite element analyses we conclude that thick-walled pipes show a high level of stress triaxiality inside its wall at the moment of collapse, making it necessary a different analytical approach than that of DNV-ST-F101. Finite element simulations of four different metallic materials and five pipe configurations (D/t ratios) are performed to develop an alternative approach for the computation of the plastic collapse of pipes under hidrostatic pressure that takes into account the stress triaxiality at the moment of the pipes’ collapse. Our results improve the accuracy of the original DNV design equation for pipe diameter-to-thickness ratio within the range commonly found in the pre-salt offshore exploration.