On the behaviour of rock berms to arrest axial walking of offshore pipelines

Subsea pipelines are used to transport gas (and fluids) from subsea wells to processing facilities. Over the life of a project, cycles of operation and shutdown can lead to axial ‘walking’ of pipelines and potential damage to facilities. Rock berms may be used to mitigate walking, by adding weight to increase the frictional restraint of the pipeline against the seabed. Recent studies (by others) have identified the potential for ‘arching’ to occur within the berm, linked to pipeline embedment, which reduces the stress applied to the pipeline and may thereby reduce the magnitude of axial restraint for a fixed volume of rock. To overcome reductions in axial restraint, longer rock berms may be required thereby increasing project costs.The objective of the current study is to investigate the extent to which arching may impact the effectiveness of rock berms to mitigate walking. The method used was physical modelling in a geotechnical centrifuge. The results presented in this paper specifically investigate whether pipeline embedment can lead to arching in ‘real’ rock berms, and whether associated axial movement may lead to ‘collapse’ of the arch and reinstatement of the restraint. Results were obtained using a specially designed trapdoor model and are presented from testing of different berm geometries at centrifuge accelerations of 20g and 10g. The study concludes that arching can occur in response to pipeline embedment, but that this can ‘recover’ in response to axial movement of the pipeline. The degree of recovery depends on the amount of embedment adopted.