The design of pipelines using controlled lateral buckling is sensitive to the resistance caused by soil berms. To investigate the behaviour of these berms, an experiment was carried out using a model pipe photographed through a window in a centrifuge strongbox. The processed images, using particle image velocimetry (PIV), indicated a failure mechanism consisting of a bearing-sliding zone in the intact seabed, plus a sliding failure at the berm base. During cyclic movements, distinct failure planes exist between the ‘slices’ of berm added in each cycle, and are reactivated in subsequent cycles. It is also shown that the equivalent lateral friction factor (referenced to the horizontal and vertical directions) is influenced by the local slope of the seabed. However, the intrinsic equivalent friction between the pipe and the seabed, based on forces referenced to local seabed slope, remains constant (away from large soil berms). This observation highlights an important effect on the pipe-soil resistance.

1. Introduction

Pipelines in deep waters are normally laid on the seabed and often transport hot products at high pressure, tending to cause elongation. However, this is restrained by axial friction between the pipe and the seabed, causing high axial compressive forces to be generated within pipelines. These can be relieved by planned buckling. The displacement of the crown of a buckle can be up to 10 or 20 pipe diameters. Pipelines scrape some soil in front as they displace laterally, forming an active berm. On changing the displacement direction, the pipe leaves the active berm behind to become a dormant berm and creates another active berm against the opposite face. During each operation and shutdown cycle, pipelines buckle and straighten, scraping more soil on either sides of the pipe and creating a trench with two dormant berms along the edges.

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