Advanced Modelling of Clamped Mattress for Pipeline Walking Prevention and Mitigation

This paper is based on experience obtained during design of Payara Project, offshore Guyana, where 10" production and 16" water injection pipelines were found to be potentially susceptible to walking. Offshore pipelines that are subject to HT/HP conditions, riser tensions and seabed slopes can be susceptible to walking, which may jeopardize the integrity of connected subsea structures. Should the cumulated walking during the operating life of the pipeline exceed the maximum allowed displacement, an anchoring system could be required to mitigate the movement. However, predicting pipeline walking is a complex matter, depending on parameters often affected by significant uncertainty, such as pressure and temperature conditions, heating and cooldown cycles, soil pipe interaction, planned buckle evolution and route modifications. For this reason, a walking assessment performed during detail design can lead to uncertain results, and question arises as to the best timing to install walking mitigation structures (WMS). A Wait-and-See approach represents a good compromise, mediating between CAPEX and OPEX needs, and allowing a risk-based decision process based on pipeline behavior as monitored during planned surveys. However, the installation of WMS during operation may require ancillary structures to be installed on ‘day one’ that could nullify the strategic advantage given by the Wait-and-See, not to mention the mob/demob cost of the vessel used for installation. The Pipe Clamping Mattress (PCM), patented by Shell and commercialized by MMA Offshore, has been proposed as a convenient and effective way to anchor the pipeline, either at day one or during operation. The PCM comprises a two-winged mattress fabricated by pivoting two concrete blocks around a central hinge, and which is shaped to engage and clamp the pipe. A log mattress is then positioned to provide additional clamping force. The overall weight of PCM provides the clamping grip required to prevent pipeline slippage as well as the additional axial resistance needed to mitigate or arrest walking. An advanced FE Model for PCM performance study has been developed in ABAQUS based on the CEL methodology (Coupled Eulerian-Lagrangian). In comparison to more conventional models, this technique models the seabed as a deformable Eulerian domain, in which PCM and pipeline can penetrate. Seabed settlement and soil displacement due to system motion can be fully calculated, estimating the evolution of axial resistance and the growth of berms at the PCM sides. The objective of this paper is to improve the understanding of pipeline/PCM interaction, in particular with respect to: Verifying the overall embedment of the pipeline/PCM system.Verifying and assessing the PCM restraining capacity against axial displacement.Assessing the minimum number of PCMs required to ensure walking prevention/arrest.