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 $US61 per bbl. Safety of course takes precedence; however, unnecessary conservatism should be avoided to reduce expenses when technically and practically feasible.

J P Kenny, part of international energy services company John Wood Group, has developed three-dimensional (3D) finite element (FE) simulator tools to deal with common challenges in Arctic regions: ice gouging and permafrost.

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, Canada. 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 oil and gas field developments.

Current practice in pipeline design to mitigate the ice gouging hazard is to bury the pipeline deep enough to reach 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 utilises the Coupled Eulerian Lagrangian (CEL) FE method, available in ABAQUS/Explicit – FE analysis software that simulates events involving impact – to model the ice gouge process and has carried out extensive validation work to ensure its models behave accurately. The major advantage realised 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.

The developed model 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 optimise pipeline burial depth requirements based on limit-state-based design criteria. J P Kenny has performed validation, sensitivity and hypothetical design studies that have demonstrated the reliability and applicability of the tool.

Determining pipeline burial depth to protect against encroaching ice keels has traditionally been a conservative aspect of Arctic pipeline design due to analysis procedures and a lack of explicit criteria. Realistic 3D simulation can help reduce the unknowns associated with ice-soil-pipeline interaction, safely reduce unnecessary conservatism, and ultimately save on trenching costs.

Permafrost

Permafrost is permanently frozen soil, covering about half of Canada and Russia, and 85 per cent of Alaska. The existence of permafrost presents a significant challenge to the design, construction and operation of pipelines on Arctic terrain. Pipelines transporting warm hydrocarbons can transfer heat to the surrounding soil, causing the ground to thaw over years of operation and lose load-carrying capacity. Differential ground settlement is likely to occur, over stressing pipelines and inducing bending strains.

J P Kenny has developed a 3D FE model for investigating the effects of permafrost on Arctic pipelines. The model predicts unsteady-state heat transfer, thaw settlement, and global deformation processes of a pipeline buried in permafrost soil.

The model is designed to give accurate predictions of the amount of thawed ground around the pipe and corresponding strain on the pipeline.

The thaw settlement of the pipeline is assessed based on the actual growing size of the thaw bulb instead of the commonly-used total thickness of the thaw-unstable permafrost layers. The predictions of the current model provide clear scenarios of pipeline thaw-settlement evolution and corresponding bending strains over its lifetime, and helps avoid over-conservative designs.

The developed model can be used during the initial pipeline design phase to improve safety and cost-effectiveness; the evaluation can aid the selection of proper construction and implementation methods used to minimise the impact to the permafrost and the surrounding environment. In addition, the model can be used to assess existing pipelines embedded in permafrost soils from both thermal and mechanical perspectives.

J P Kenny in the Arctic engineering industry

Currently, J P Kenny is involved in the DNV ICE Pipe joint industry project, which is aimed at evaluating and presenting design methods and recommendations specifically related to the installation, operation and maintenance of offshore pipelines in areas of extreme cold and ice. As part of its involvement, the company is using its ice gouge simulator tool to contribute to a major study on numerical ice-soil-pipe interaction.

In addition to developing advanced simulation tools, J P Kenny has provided its Arctic expertise to a number of projects, including a consortium comprising Gazprom, Total and StatoilHydro for a trunkline project that will bring gas from the Shtokman field in the Barents Sea to Northern Russia. J P Kenny collaborates with Wood Group sister company IMV Projects Atlantic and with the Memorial University of Newfoundland (MUN) in St John’s, Canada, to develop leading-edge solutions for engineering in Arctic regions. Wood Group formed a partnership with MUN in January 2009 and is donating $US461,883 for the sponsorship of the Wood Group Chair in Offshore Engineering for Arctic and Harsh Environments, focused on oil and gas engineering in cold regions.