As the demand for energy resources grew, the oil and gas industry looked north for exploration and development of offshore hydrocarbon resources. Significant discoveries were made in Arctic and sub-Arctic regions; however, these environments can be particularly unforgiving on pipelines and offshore structures with a host of geohazards unique to the area. Burial under the seabed became the common practice of protecting offshore Arctic pipelines. Innovative design considerations including installation techniques, leak detection monitoring of the pipelines during operation, and bundling pipelines together aided in the successful completion of these projects.
As design engineers on the three subsea pipelines completed in the US Arctic - Northstar (BP), Oooguruk (Pioneer), and Nikaitchuq (ENI) - all in the Beaufort Sea on the North Slope of Alaska, INTECSEA have been able to progressively improve upon design evaluations, building upon learnings from each of the previous projects.
Traditional subsea pipeline design is a stress-based approach with individual pipelines either laying on the seabed or buried to a shallow depth to protect against hazards including fishing gear or dragged anchors or for protection against upheaval buckling. In Arctic pipeline design, the extreme environmental loadings tend to require deeper burials, and due to the short installation windows and high cost of installation on-ice, may lead to the use of pipeline bundles to facilitate installation. Strain-based design is generally used for Arctic subsea pipelines due to the extreme displacement-controlled loading conditions.
This paper will discuss the evolution of the design considerations and methodologies for Arctic subsea pipelines subjected to the unique Arctic environmental loadings such as strudel scour, permafrost thaw settlement, and ice gouging. These evolving design methodologies are comprised in INTECSEA's in-house suite of Arctic Tools and pipeline design standards. Focus is also placed on the importance of appropriate environmental and geotechnical data collection, the use of limit states design, and the various trenching and backfilling aspects of pipeline design.