Abstract

BP Exploration (Alaska)'s Northstar Project is the first offshore arctic field development with a subsea oil pipeline. Twin 10-inch oil and gas pipelines extend 6 miles offshore into the Beaufort Sea to a man-made gravel island in 37 feet water depth. The trenched pipes are designed and constructed to safely withstand potential seabed ice gouge and permafrost thaw settlement loading conditions which may result in displacement-limited non-cyclic plastic bending. This design requirement was addressed by using limit state bending analysis to verify the integrity of the pipelines during their service life. Bending strains up to 2% were found to be acceptable on operational oil and gas pipelines for which the pipe materials, welding, testing, inspection, and operations are controlled to satisfy the limit state design requirements. Winter ice-based construction techniques were used to successfully install the Northstar pipelines. The floating natural sea ice was thickened to safely support conventional onshore pipeline construction equipment. This avoided the need to mobilize offshore pipelay vessels or trenching equipment and allowed work to be performed during a time of year when environmental impacts due to construction are minimal.

Introduction

The Northstar oilfield is located 6 miles offshore the Alaskan Beaufort Sea coast and approximately 11 miles northwest of Prudhoe Bay (Fig. 1). Oil was discovered by Shell in 1983 with exploration wells drilled from Seal Island1. This manmade gravel island in approximately 37-ft water depth was built in 1982 and later abandoned by Amerada Hess in 1994. BP Exploration (Alaska) Inc., (BP) acquired the majority of the Northstar Unit leases in 1995 and is presently developing the field.

Northstar is the first offshore oil field development in the Arctic to use a subsea pipeline. As such, it was necessary to address many technical and permitting challenges in addition to this field's historical economic challenges. The Northstar offshore pipeline design started in 1996. The Northstar Project Final Environmental Impact Statement1 and the State Right-of-Way lease were issued in 1999.

Seal Island was rebuilt and enlarged between January and May 2000 and was connected by pipelines to existing Alaska North Slope facilities during this same winter construction season. The drilling rig, utilities and living quarters modules were moved onto the island during the summer open water season. Production facilities are presently being completed at an Anchorage, Alaska fabrication site and will be sealifted to Seal Island during summer 2001. Developmental drilling started in December 2000 and production start-up is planned for fourth quarter 2001.

Produced crude oil will be processed on Seal Island and shipped via a trenched 10-inch pipeline to the shore crossing at Point Storkersen and then an additional 11 miles overland to the Trans-Alaska Pipeline System (TAPS) Pump Station 1. Natural gas is presently being transported to Seal Island via a second 10-inch pipeline for use as fuel and will be used for future reservoir management purposes.

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