Axial Rock Berm-Pipeline-Seabed Interactions

Abstract

Abstract Rock berms installed over discrete sections of high temperature, high pressure (HTHP) subsea pipelines may be used to provide axial restraint with a view to minimising the end expansions or to improve the later-life buckle performance. For such applications, the paramount consideration for an efficient rock berm design is the interaction of the rock berms with the pipeline and simultaneously, of the pipeline with the seabed during operation. This paper re-introduces the mechanism of stress re-distribution in the berm or ‘arching’ occurring during pipeline operation-driven cyclic embedment. The development of cyclic pipeline embedment due to shearing and re-consolidation of the soils around the pipeline is quantified at different times during the pipeline operational life. This is assessed analytically by consideration of the generated pore pressures during episodic shearing and intervening consolidation in a framework described herein. Further, the results from Finite Element (FE) analyses that correlate the pipeline embedment and the effective rock berm-pipeline contact stress, are combined with predictions of the cyclic pipeline embedment from the analytical framework presented here. This allows to form-find the rock berm cross-profile and screen the rock volume requirements in a reliable manner.