190 Sample issue The Journal of Pipeline Engineering
3rd Quarter, 2009 191 Advanced numerical modelling tools aid Arctic pipeline design by Kenton Pike JP Kenny, Houston, TX, USA A THREE-dimensional (3D) finite-element (FE) simulator tools has been developed to deal with common challenges in Arctic regions: ice gouging and permafrost. The Arctic oil and gas market has garnered much renewed attention as of late, and the team at J P Kenny is developing tools to help ensure pipeline designs for future Arctic developments are as safe and economical as possible. REMOTE LOCATIONS, extreme cold and harsh weather conditions, lack of infrastructure, difficult transportation of materials, goods and services, sensitive environments, and limited construction windows are merely some of the challenges faced when designing, constructing, and operating in Arctic regions. As a result of the high costs and long cycle times associated with developing oil and gas in the Arctic, break-even oil prices can be as high as $61/ barrel. Safety of course takes precedence; however, unnecessary conservatism should be avoided to reduce expenses when technically and practically feasible. Ice gouging Ice gouging (or ice scouring) occurs when environmental forces drive ice features (icebergs or ice-ridges) that extend deeper than the water depth through the seabed soil. Ice gouging occurs offshore in Arctic and sub-Arctic regions, such as in the shallow Beaufort Sea and offshore Newfoundland. With the majority of estimated Arctic oil and gas reserves being held offshore, ice gouging could potentially govern the design of pipelines and subsea architecture for many future field developments. Current practice in pipeline design to mitigate the ice gouging hazard is to bury the pipeline deep enough to reach Author’s contact details: tel: +1 281 675 1045 e-mail: kenton.pike@jpkenny.com a safety target against pipeline system failure. Contact with the keel of the ice feature is avoided; however, pipelines are installed in a region where some soil displacement can be transferred to the pipeline. It is therefore critical to accurately predict sub-gouge soil displacement. J P Kenny utilizes the Coupled Eulerian Lagrangian (CEL) FE method, available in ABAQUS/Explicit, to model the ice gouge process and has carried out extensive validation work to ensure its models behave accurately. The major advantage realized by this modelling technique is that it overcomes mesh distortion and convergence issues experienced by other methods. In the CEL FE formulation, the seabed soil is modelled using an Eulerian material that is allowed to freely flow throughout a fixed mesh. Because the mesh does not distort, very large deformations experienced during the ice gouge process can be realistically simulated. Sample issue The model (Fig.1), consisting of a rigid ice keel, Eulerian seabed, and Lagrangian pipeline, provides a fully-coupled numerical solution for ice-soil-pipeline interaction events. In running the model, the first step of the analysis allows the soil to reach an in-situ initial stress state; during the second step, the ice keel is translated through the seabed causing soil failure and displacement. The soil forms a frontal mound, displaces to the side, creating berms, and also displaces below the gouge, imposing strains on the buried pipeline. As pipeline strain demand and response are determined explicitly, the results of the model can be used to optimize pipeline burial depth requirements based on limit state-based design criteria. J P Kenny has performed
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