Through-Life Modelling of Rock Berm-Pipe-Seabed Interactions

Abstract

Abstract This paper portrays salient features of the rock-berm pipe seabed interaction affecting the through-life axial restraint of rock berms installed intermittently over High Temperature High Pressure (HTHP) pipelines including: (i) axial displacement cycles of a pipeline leading to cyclic compaction of the soil and associated to this, downwards movement of the pipe and unloading of the rock berm-pipe contact. This is followed by a gradual re-gain in the system axial restraint by the process of (ii) cyclic soil hardening, and (iii) the process of ‘arch’ collapse promoted by the axially moving pipeline. Earlier numerical studies (in Carneiro et al., 2017, Borges-Rodriguez et al., 2018) demonstrated unloading of the rock berm-pipe contact by ‘arching’ of the stresses in the rock berm as the pipe penetrates the seafloor. Observations of unloading of the rock berm-pipe contact and subsequently, of the axial load re-gain due to pipe movements, have been made in a suite of centrifuge physical model tests (in O'Beirne et al., 2022a, b). An analytical tool has been developed to capture these effects and is presented here. More specifically, the model describes the evolution, in a cycle-by-cycle manner, of the axial restraint by the rock berm-pipe-seabed system, allowing the tracking of the axial restraint of the combined system across the operational life. Features of the analytical tool include the prediction of soil volume compaction and downwards displacement of the pipe with cycles, and of an initial and significant ‘drop-off’ in the axial resistance associated with the pipe penetration. The analytical model also reproduces the re-gain in system axial resistance that develops through the combined effects of soil hardening through the process of episodic shearing and re-consolidation, and by the pipe axial movements disrupting the force ‘arches’ set-up along the berm. The analytical tool has been calibrated through dedicated numerical analysis and the results of model tests in the public domain. The tool permits analysis of a range of pipe and soil properties and rock berm geometries and seabed materials. The tool's modelling of the arching phenomenon and post-arching recovery in axial resistance is through a sound theoretical framework. Hence, the analytical tool provides a robust basis for screening the concept of rock berm installation to manage pipeline axial end-expansion, buckle-to-buckle feed-in and arresting the walking propensity. Finally, and based on the demand for axial restraint, the tool can be used for form-finding of the berm for the accurate evaluation of rock volume requirements.