Environmental Loadings and Geotechnical Considerations for the Northstar Offshore Pipelines

BP Exploration (Alaska) Inc. completed the installation of the first subsea Arctic oil production pipeline in April 2000 for the Northstar Development Project. The drilling and production facilities are located at Seal Island, approximately 10 km offshore of the Alaskan Beaufort Sea coast. Twin 273.1 mm (10-inch) oil and gas pipeline systems run approximately 10 km from Seal Island, through a lagoon area, to a shore crossing, and then overland for approximately 18 km. The unique aspects of this design included the pipeline environmental loadings, geotechnical considerations, and the use of limit state design procedures for extreme loading conditions. Environmental loadings and geotechnical conditions (in-situ and backfill) along the pipeline route were a major factor in the design of the offshore portion of the pipelines. Data collection of environmental conditions (e.g. ice gouging and strudel scour) and proper evaluation of the same were required to provide appropriate design data. Comprehensive field and laboratory programs were undertaken to generate the necessary geotechnical data for design. The evaluation of and design for unique Arctic environmental loading conditions including ice gouging, offshore permafrost, upheaval buckling, and strudel scour are described. Trenching and backfilling aspects of the pipeline design are also discussed. The paper closes with a general overview of the pipeline operations since the start of oil production in November 2001.