SS: Atlantic Canada
Authors: Phillips Ryan Douglas , Barrett John , Al-Showaiter Aiman 
Published: July 2010

Moving ice features impose significant challenges on engineering aspects of offshore oil and gas field development in the Arctic and sub-Arctic regions, particularly related to their interaction through the seabed with submarine installations such as pipelines. This paper compares continuum finite element modelling of ice keel-seabed interaction against physical model data and evaluates the current state-of-the-art. It identifies areas requiring further research and development, and evaluate potential limits on ice gouging in soft to stiff clays.

Introduction

Environmental conditions impose significant challenges on engineering aspects of oil and gas field development in the offshore Arctic and sub-Arctic regions. One major challenge is the burial depth requirements for marine pipelines against a gouging ice keel. King et al (2009) present a probabilistic burial analysis for pipeline protection against ice gouging of the seabed. The analysis combines a probabilistic characterization of the ice gouge regime with a deterministic analysis of pipeline strain response for a range of ice gouge widths, depths and ice keel/pipeline crown clearances, allowing a pipeline burial depth to be defined that satisfies specified stress/strain limits and target reliability levels. Such analyses require a relatively simple model of the ice - soil - pipe interaction to consider the multitude of different load cases. This simple model must be validated against available physical model test data and complex numerical analyses of a much smaller number of load cases, as recently recognized by DnV (2008).

Continuum explicit finite element eulerian-based analyses provide an adequate framework for consideration of the ice - seabed - pipe interaction as demonstrated by Nobahar et al (2007), Liferov et al (2007), Konuk & Yu (2007) and Abdalla et al. (2009). These analyses focused on clay seabeds and can be calibrated using appropriate material parameters, but need to be validated against physical data. Limited validations to date in these papers have included comparisons to lateral subgouge deformation (SGD) profiles on the gouge centreline measured from PRISE centrifuge model tests described by Phillips et al (2005).

This paper extends this validation to consider steady state conditions, gouge forces, frontal berm formation, seabed failure mechanisms and both vertical SGD and transverse distributions of lateral SGD below the gouge depth. The pipeline is not considered in this present validation. A validated numerical analysis can be used to extend the understanding of the seabed response to ice gouging and associated parametric influences such as keel geometry and seabed soil properties.

Physical Model Test Data

Konuk & Yu (2007), Liferov et al (2007) and Abdalla et al. (2009) compared their numerical modelling predictions to the physical model test data produced from PRISE. The Pressure Ridge Ice Scour Experiment (PRISE) joint industry research program was led by C-CORE, Phillips et al (2005). It investigated the stresses and soil deformations during ice gouging events and was a proprietary program designed to develop the engineering framework to allow for pipeline installation in arctic regions with the understanding of soil deformations and ice loads seen during ice gouging events. The program included a series of small scale physical model tests of ice gouging conducted in a geotechnical centrifuge. Centrifuge modeling provides an effective alternative to large scale physical models by stress scaling to study the mechanics of the ice gouging process.