Effect of Stress Triaxiality On the Computation of the Collapse Pressure of Submarine Pipes

Abstract

The linepipe steel engineered for ultra-deepwater fluid transportation undergo extreme conditions due the high pressure. One major concern of pipeline engineers when designing pipelines for such environment is to predict the onset of buckling caused by the static external hydrostatic pressure. DNV-ST-F101 standard features as one of the most popular guides for the pipelines project and presents the design equation for the prediction of the collapse capacity of submarine pipelines covering a limited diameter-to-thickness (D/t) range leaving heavy-walled pipes, such as those demanded for the presalt exploration, out of the scope. The accuracy of the DNV-ST-F101 design equation is investigated in the scope of this research paper for pipes featuring D/t lower than 15, defined as the lower bound covered by the standard, and it is found that it underestimates the maximum collapse capacity in those cases possibly driving the pipe operators to define pipe walls excessively thick causing the cost increasing of the operation. We sustain that the major cause for such inadequacy falls into the standard’s equation for the pipe’s plastic collapse, which uses the material’s yield strength to set the beginning of the plastic collapse. Using the finite element method we conclude that the high level of stress triaxiality inside the pipe’s wall at the moment of collapse calls for the necessity of a different parameter to measure the onset of plastic failure. Numerical data of five different metallic materials and five different pipe configurations are used to propose an alternative equation to compute the plastic collapse of pipes subjected to external pressure, and our results consistently improve the accuracy of DNV’s approach for moderate to thick pipes without compromising the results of thinner structures.