Pipe Soil Interaction During Cyclic Buckling: The Importance of Site-Specific Seabed Properties

Abstract

Knowledge of how an unburied pipeline interacts with the seabed during operational buckling is critical both to check the fatigue life of a pipeline in the buckling zone and to make accurate predictions of the pipeline end-expansions and walking. This paper explains how site-specific seabed conditions affect pipe-soil interaction (PSI) for buckling pipelines during their operational life. The paper first demonstrates that for many soil conditions, cyclic PSI models that only consider the ‘berm push’ resistance fail to capture all the key elements of the pipeline-seabed interaction during cyclic buckling. An additional mechanism associated with the changing pipeline trajectory which develops after many pipeline lateral movements (‘sweeps’) is identified. For some soil and pipeline conditions this second mechanism provides the main component of the seabed resistance after many operational cycles. Appropriate analysis methodologies are discussed which allow the ‘berm-push’ and trajectory mechanisms to be considered in light of soil conditions and pipeline input conditions. Secondly, published design approaches to assess cyclic PSI during buckling have typically assumed that the seabed around pipelines is not mobile (i.e. unscourable). However, in some situations, the combination of pipeline geometry, seabed properties and metocean conditions may lead to seabed scour around the pipeline and surrounding soil berms. This may therefore affect how the lateral soil resistance develops during the operational lifetime of a pipeline and should be considered in design. The paper discusses this issue, presenting results from CFD scour analyses of a particular berm geometry for different metocean conditions. The implications of berm scour on pipe-soil interaction for this geometry is explained.