The structural response of buried subsea pipelines to ice gouging

Ghorbanzadeh, Alireza (2024) The structural response of buried subsea pipelines to ice gouging. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (11MB)

Abstract

Ice gouging significantly threatens the integrity of subsea pipelines in Arctic regions. While burial offers protection from direct ice contact, the shear resistance of seabed soil and resulting subgouge deformations can still threaten the stability of buried pipelines. Determining an optimal burial depth that balances safety and economic considerations presents a complex engineering challenge and necessitates a comprehensive understanding of the factors involved in the ice gouging process. Conventional design approaches often simplify the seabed as a uniform material domain, neglecting the potential complexities introduced by natural and human-made non-uniformities. Trenching and backfilling the pipeline creates such non-uniformities, as the backfill material typically possesses stiffness characteristics different from those of the native soil. These pipeline-backfill-trench wall interaction can significantly affect failure mechanisms around the pipe and, consequently, pipeline response. Additionally, layered seabeds, common in many Arctic regions, are often simplified to uniform medium, potentially leading to inaccurate predictions. This study simulated ice gouge events in cohesive soils under various non-uniform conditions using the Coupled Eulerian-Lagrangian (CEL) framework within Abaqus/Explicit. Unlike conventional models that idealize seabed soil as elastic, perfectly plastic, this work incorporated the strain-rate dependency and strain-softening effects of cohesive soils through a VUSDFLD user subroutine to improve prediction accuracy. Following mesh sensitivity analysis and model validation against published data, a comprehensive set of scenarios explored the impact of trenching/backfilling, backfill stiffness and type, trenching methods and trench geometry, and layered soil configurations on keel reaction forces, soil failure mechanisms, and pipeline response (deformations, stresses, axial strains, and ovalization). The results demonstrated that trenching and backfilling significantly influence seabed failure mechanisms around the pipe. Through careful backfill selection and trench design, pipeline safety can be enhanced. Furthermore, the study revealed that simplifying a layered seabed can lead to misleading predictions. Interactive mechanisms between soil layers can substantially alter soil failure patterns near the ice keel and along the soil layers interface, and it could cause unexpectedly large pipeline deformations at deeper burial depths. The findings of this research offer valuable practical insights for pipeline design in the Arctic, leading to several recommendations for improving the safety and integrity of subsea pipelines in these challenging environments.

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