Active Pressure of Rock Berms on Pipelines

Rock berms are frequently used to provide additional resistance against pipeline lateral and vertical displacements. As a pipeline buried under a berm attempts to move, it will push against the rock, thus mobilising the berm’s passive resistance. If it moves axially, however, passive resistance is not mobilised – the resistance is frictional only. The additional axial resistance provided by the rock is only due to the increased (active) contact pressure on the rock-pipe and soil-pipe interfaces. If the pipe still moves, and in particular if it moves cyclically backwards and forwards over the seabed, it may cause the stresses in the rock volume to redistribute reducing this contact pressure, and consequently the additional axial resistance provided by the berm. This fact is commonly ignored in pipeline design that banks on the rock berm restraint. The paper presents results of numerical analyses demonstrating this loss of ‘grip’ mechanism. For the assessed cases, the observed loss in additional axial resistance varied between 20% and 70%. The stiffness contrast between the rock berm and seabed materials was found to dictate the degree to which the rock berm restraint is reduced.