Post-buckling Analysis Of Subsea Pipelines And The Buckle Propagation

Under the action of hydrostatic external pressure subsea pipelines may generate local buckling. Once initiated the local buckle will propagate along the pipelines ff the external pressure is higher than a critical value defined as the propagation pressure. Buckle propagation flattens the pipeline, creating collapse of pipeline along its passage. It eventually leads to the structural failure of the entire pipeline, causing great economic loss and environmental pollution. The phenomenon of buckle propagation is very much complex, so the causes to induce it. In this paper we deeply analyze the essence and the mechanism of the buckling and buckle propagation of pipelines using the theory of non-linear stability. The steady-state buckle propagation is regarded as the translation of the buckling mode along the pipeline, thus related to the problem of post-buckling of pipeline subjected to external pressure. The length of transition zone in buckle propagating pipeline is determined from post-buckling analysis.In this paper the undersea pipeline is modeled as a cylindrical shell subjected to external pressure and analyzed using shell theory instead of ring models which were often used to analyze the buckle propagation previously. According to laboratory results, we consider that a local buckling occurs in the pipeline, causing deformation in part of the pipeline region that is described by assuming a deflection with dynamic boundary conditions at the two ends. Using the non-linear large deflection theory, the analytical expression to calculate the critical load is derived from variational principle. Numerical calculations are conducted to obtain the post-buckling paths for different ratios of diameter-to-thickness. The results show that when buckling occurs the load-carrying capability falls precipitously with a very slight increment of the deflection. The decreasing is slow down and become quilt smooth until the two opposite points of the pipeline contact with each other. The profile of the cross section at the contact place changes into dog-bone shape. Such observations coincide with those from experiments. Further analyses indicate that the length of transition zone identified as the span of post-buckling mode due to membrane stretching in axial direction depends not only on the material properties but also on the ratio of diameter to thickness. This paper presents a methodology to analysis the mechanism of post-buckling failure of the pipeline subjected to external pressure, to investigate the intrinsic relation between the problem of post-buckling and the phenomena of buckle propagation, and to determine the length of transition zone in buckle propagating pipeline. It can be a good reference for updating the designing codes of pipelines.