Alaskan Beaufort Sea Pipeline Design, Installation and Operation

Abstract

Abstract The BP Northstar, Pioneer Oooguruk and Eni Nikaitchuq pipeline projects provide a significant experience base for designing, installing and operating future offshore arctic pipelines. Each pipeline is located in the nearshore zone of the Beaufort Sea, offshore the North Slope of Alaska. Maximum pipe diameters range up to 18 inches, water depths to 37 ft and lengths to 6 miles. Unique offshore arctic environmental loading conditions influenced each pipeline design differently for defining the required pipe bundle configuration, thermal insulation and trenching requirements. Similar differences affected the winter construction procedures used for each project and would be expected to impact future pipeline projects in deeper waters. A total of 13 years of pipeline operational experience has been gained and key features such as routine seabed surveys, in-line-inspection pigging and effective leak detection systems are important to the continued safe operations of these pipeline systems. Introduction Development of offshore oil and gas reserves in Arctic and sub-Arctic regions of the world have progressed relatively slowly (INTECSEA, 2011). This slow pace results largely from the remote field locations, unique arctic environmental load conditions and the lack of existing hydrocarbon export facilities. Subsea pipeline systems are often needed to connect offshore fields with onshore pipeline networks, for intrafield flowlines or for export tanker loading lines. Safe and economical design, construction and operational procedures are needed to address the unique conditions encountered for offshore arctic pipelines. Three pipeline systems have been installed in the Beaufort Sea, offshore the North Slope of Alaska for BP's Northstar, Pioneer's Oooguruk and Eni's Nikaitchuq field development projects (Figure 1). The Northstar and Oooguruk projects have been operating for a combined total of over 13 years and the Nikaitchuq project is scheduled to start operations during 2011. Each of these offshore arctic pipeline systems transports oil and gas from gravel island structures to shore. This Beaufort Sea pipeline experience can be applied on future Arctic and sub-Arctic projects having first-year sea ice, multi-year ice or iceberg load conditions. Seasonal sea ice is found at varying latitudes in both marine and inland lake locations, not just north of the Arctic Circle (or surrounding Antarctica). Other potential applications of Beaufort Sea pipeline experience include projects having complex thermal interactions with the local environment and offshore arctic projects requiring conventional marine construction procedures during the short Arctic summer season. Additionally, pipeline operations, maintenance and potential repair procedures form an integral part of an economical, safe and reliable offshore arctic pipeline system. Beaufort Sea Environmental Conditions The nearshore Alaskan Beaufort Sea ice typically breaks up or melts in place starting in June. Summer open water conditions then prevail from late July through mid-October. The summer polar pack ice edge recedes varying distances offshore based on the site location, water depth and year to year variations or long-term climate trends. Even during the open water season, however, ice floes are often present and their movement is influenced by winds and currents. Along much of the Alaskan North Slope coast, low barrier islands prevent multi-year polar pack ice from entering the nearshore lagoon areas. Ice can affect offshore marine operations outside the barrier islands at any time during the summer.