Structural Failure Of Pipelines Subjected To The Combined Loads Of Axial Tension And External Pressure

Subsea pipelines subjected to hydrostatic pressure may experience local collapse，till thetwo opposite points on the inner surface of the shell contact each other. The collapse of thepipeline will initiate a post buckling mode and such mode maypropagate along the pipelinewhen the external pressure is greater thana critical hydrostatic pressure called bucklepropagation pressure,causing the overall structural failure of the pipeline.Buckle propagationwill cause great economic loss and environmental pollution. Upon propagation, the pipeline isdivided into three segments: unbuckled region, transition zone and buckled region.Subsea pipelines in complex environment is not only subjected to hydrostatic pressure,but also bear other effects, such as axial tension, axial pressure, bending and so on. This paperconsiders the effect of the axial tension in addition to hydrostatic pressure on the structuralfailure of the pipelines. Nonlinear stable theory of shells with large deflection is used to studyand analysis the mechanism of local buckling and buckling propagation of submarine pipelinein-deep sea. In this paper we consider that a local buckling mode is first initiated then itspreads along the pipeline in a constant speed, i.e. it is a steady state propagation. The papermainly contains:First, the pipelines is simplifiedas a cylindrical shell. Basic assumptions are proposedaccording to Karman`s nonlinear theory and the fundamental geometric andphysical equationsof the cylindrical shell are established by analyzing the deformation and static equilibrium of adifferential element.Then buckling of the pipeline subjected to both axial tension and external pressure is analyzed.Governing equations are derived and are solved by choosing an appropriate flexuraldisplacement. From the principle of minimum potential energy, the post-buckling equilibriumpath is obtained and the calculation formula forthe length of transition region is derived.Numerical studies are performed to obtain theVariation curves between the axial tensioncoefficient and the axial tension and the Variation curves between pipeline buckling load andaxial tension. It is found that length of the transition zone in the pipeline increases with theincreasing of the axial tension.Finally based on the solutions of the transition zone, the propagation of the buckle in thepipeline is investigated. Considering linear elastic linear hardening material properties, we getstrain components in the transition. According to the plastic stability theory, the strain energydue to bending, due to membrane stretching and virtual work done by the external forces intransmission zone is derived. The buckle propagation pressure is conducted from principle ofvirtual work. The effect of R/h on the strain-hardening coefficient mhand the yieldcoefficientmy, the effect of R/h and the axial tension on the propagation pressure areinvestigated by performing the numerical studies.