Authored by Tadeh Zirakian*
Introduction
Subsea pipelines are externally loaded by a constant source of hydrostatic pressure. Local buckling can occur at pressures much lower than that required for critical buckling (Pcr), and after one region has been compromised, the zone of damage may begin to expand along the length of the pipe leading to eventual collapse of the pipeline [1,2]. During buckling and the ensuing propagation of damage, significant changes are found to occur in the cross section of the pipe geometry. An analysis of post-buckling behavior prior to total collapse should be considered in order to better understand the gradual failure mechanisms of these structures in both the radial as well as longitudinal directions [3-6]. As depicted in Figure 1, the phenomenon of buckling tends to flatten the pipeline in cross section locally, while propagation of this damage longitudinally may be characterized by three zones: the near-end buckled and far-end unbuckled regions, and a transition of pipe length between the two (Figure 1). After buckling initiation, damage propagation reaches a steady state for which the external hydrostatic pressure is constant. Buckling propagation stops when the external hydrostatic pressure is lower than the buckle propagation pressure [7]. According to the elastic stability theory, deformation of pipeline occurs in two stages: pre-buckling and post-buckling [8]. Research has shown that in loads lower than the buckling load, deformation of pipeline is based on a stable pre-buckling path [9,10,12]. As pressure increases, the pipeline section starts to develop plastic hinges and then begins to fail. With the continual increase in pressure, the pipeline undergoes further deformation in the radial direction until the onset of pipeline buckling at a certain external pressure, i.e. the critical buckling load Pcr. Beyond this point, the pipeline behavior is described by the post-buckling path, which may be unstable for pipelines with elasto-plastic materials. The pipeline deformation diagram is portrayed in Figure 2 Illustrated in Figure 3 is also the development of yield lines with buckling propagation along the pipeline section (Figure 2 & 3).Behavior of pipelines in response to external pressure has been the subject of numerous studies, which have shown that length of the pipe and ratio of its diameter to the wall thickness play a decisive role in the pipe’s resistance against buckling. According to Bresse [13], the stability of pipelines under hydrostatic pressure as based on small deflection theory, the critical buckling pressure Pcr can be obtained using the following equation:
Internal and external pressures induce hoop stress, which results in expansion of the pipe’s circular cross-section. On this basis, while studying the fully- or partially-anchored pipelines, one can expect to observe the Poisson effect, i.e. the expansion or compression of material in one direction due to an external force which entails compression or expansion reaction in the other direction [19,20]. Thus, because of the Poisson effect, the hoop stress in an anchored pipeline creates longitudinal stress. Pressure-induced longitudinal stresses in an anchored pipeline can be obtained from the following equation:
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