Study On The Bending Failure Characteristics Of Submarine Pipeline Under Combined Loadings

Submarine pipelines, which are the lifeline of offshore oil and gas fields, play an important role in the development of oil and gas resources, serving as the most economical and reliable transporting way. Submarine pipelines are typically subjected to complex loadings in operation, including internal pressure, axial force and bending moment mainly. At present, part of the operating submarine pipelines might be experiencing problems more or less or be in the middle-to-late services stage already, indicating that the failure rates and leakage accidents are increasing. To avoid such accidents and make the correct decision whether the pipeline need to be repaired or replaced, it is necessary to predict the capacities accurately. One of major failure modes is bending rupture due to bending moment. When pipelines are subjected to the seabed movements, seismic activities, frost heaves and thaw settlements, bending moment would apply to the pipelines. Consequently, study of the bending failure characteristics of the submarine pipelines under combined loadings is of great significance. The main contents and results of the study are drawn as follows:The failure tests for full-scale pipe with small ratio of radius to thickness under the combined loads including internal pressure, axial force and bending moment are carried out in Dalian University of Technology. The ultimate bending capacities, the moment-curvature curve, the pipeline failure modes, and the relationship between strain and bending moment at the different positions are obtained. The experimental facility, the loading ways acting on the operating pipelines, the test procedures and the processing of experimental data are discussed in details. Meanwhile, the distributions of the wall thicknesses and diameters of the specimens along the axial and the hoop directions are measured by a pipeline measuring apparatus prior to the full-scale failure tests, and the effect of the manufacturing deviation on the ultimate bending capacities is studied.Based on the Mohareb's analytical solution, analytical solutions for steel submarine pipelies subjected to combined bending moments, axial forces and internal pressure is derived, considering the effect of ovalization on the plastic region, and including both the neutral axis is in and out of the cross-section. Using C++ language, a program is developed to determine the ultimate bending capacities according to the analytical solutions. Both lower and upper bound solutions are proposed by selecting a general modulus of pipe material, namely strain-hardening modulus and Young's modulus, respectively. The ultimate bending capacities, as well as the moment-curvature relationship and the stress distribution along the pipe section, can be obtained from the analytical solutions.The comparisons of the analytical solutions with the experimental results of this paper, the experimental results by the other researchers and the numerical results are studied comprehensively. From the analysis, the analytical results such as the ultimate bending capacities, as well as the moment-curvature relationship and the stress distribution along the pipe section are typically in the range of the lower bound solutions and the upper bound solutions. Therefore, the proposed analytical solutions can be used to predict the response of steel pipelines under combined loadings.Based on the analytical solutions, and considering the factors such as initial internal pressure, the initial axial force, the radio of diameter to thickness, the yield strength of the pipe material, the generalized modulus after yielding, the anisotropy coefficient and the ovalization of the pipe section, sensitivity analysis of the upper and lower ultimate bending capacities are performed, respectively. Some useful conclusions are drawn. Considering the complexity of the analytical solutions, the simple expressions of the lower bound and the upper bound bending capacity are regressed based on the the Mohareb solution. Then, compared with experimental results, the approximate expressions are validated.