Northstar Sub Sea Pipeline Design of Metallurgy, Weldability, and Supporting Full Scale Bending Tests

BP Exploration (Alaska) Inc. is developing the Northstar oil field 9.7 km offshore the Alaskan Beaufort Sea coast (Owen et al. 2001). The development has become economically feasible largely due to the successful design and construction of the first Arctic offshore sub sea pipelines. The pipelines are two: an oil export and a gas field reinjection, both 27.31-cm diameter connecting Seal Island production facilities, in 11 m water depth, to the coastline at Point Storkersen. These pipelines are the first to be designed for operational bending strains up to 1.8% using limit state design. This paper presents the main aspects of the pipeline strain based design, the environmental conditions, the metallurgy selection of the offshore pipe metallurgy to achieve the fracture toughness requirements, and the fit- for-purpose welding criteria required. Also included is the full scale bending and weld testing required to demonstrate the integrity of the design, to the State of Alaska and Federal Government permitting requirements.