Trans-Alaska Pipeline System - Technical Details

Technical Details

Oil going into the Trans-Alaska Pipeline comes from one of several oil fields on Alaska's North Slope. The Prudhoe Bay Oil Field, the one most commonly associated with the pipeline, contributes oil, as do the Kuparuk, Alpine, Endicott, and Liberty oil fields, among others. Oil emerges from the ground at approximately 120 °F (49 °C) and cools to 111 °F (44 °C) by the time it reaches Pump Station 1 through feeder pipelines that stretch across the North Slope. North Slope crude oil has a specific gravity of 29.9 API at 60 °F (16 °C). In 2008, the pipeline carried approximately 700 thousand barrels per day (110,000 m3/d), less than its theoretical maximum capacity of 2.14 million barrels per day (340,000 m3/d) or its actual maximum of 2.03 million barrels per day (323,000 m3/d) in 1988. From Pump Station 1 it takes an average of 11.9 days for oil to travel the entire length of the pipeline to Valdez, a speed of 3.7 miles per hour (6.0 km/h).

The minimum flow through the pipeline is not as clearly defined as its maximum. Operating at lower flows will extend the life of the pipeline as well as increasing profit for its owners. The 2012 flow of 600,000 bbd is significantly less than what the pipeline was designed for. Low flowrates require that the oil move slower through the line, meaning that its temperature drops more than in high-flow situations. A freeze in the line would block a pig in the line, which would force a shutdown and repairs.. A 2011 engineering report by Alyeska stated that, to avoid freezing, heaters would need to be installed at several pump stations. This report noted that these improvements could bring flow as low as 350,000 bbd, but it did not attempt to determine the absolute minimum. Other studies have suggested that the minimum is 70,000 100,000 bbd with the current pipeline. Alyeska could also replace the 48" pipeline from Prudhoe Bay to Fairbanks with a 20" pipeline and use rail the rest of the way, which would allow as little as 45,000 bbd.

Pumping stations maintain the momentum of the oil as it goes through the pipeline. Pump Station 1 is the northernmost of 11 pump stations spread across the length of the pipeline. The original design called for 12 pump stations with 4 pumps each, but Pump Station 11 was never built. Nevertheless, the pump stations retained their intended naming system. Eight stations were operating at startup, and this number increased to 11 by 1980 as throughput rose. As of December 2006, only five stations were operating, with Pump Station 5 held in reserve. Pump Stations 2 and 7 have a capacity of moving 60,000 gallons/minute (227,125 l/min), while all other stations have a capacity of 20,000 gal/min (75,708 l/min). The pumps are natural-gas or liquid-fueled turbines.

Between the pipe stations and the end of the line is 800.3 miles (1,288.0 km) of 48-inch (1,200 mm) welded steel pipeline. The pipeline crosses 34 major streams or rivers and nearly 500 minor ones. Its highest point is at Atigun Pass, where the pipeline is 4,739 feet (1,444 m) above sea level. The maximum grade of the pipeline is 145%, at Thompson Pass in the Chugach Mountains. The pipeline was created in 40 and 60-foot (12.2 and 18.3-meter) sections. Forty-two thousand of these sections were welded together to make a double joint, which was laid in place on the line. Sixty-six thousand "field girth welds" were needed to join the double joints into a continuous pipeline. The pipe is of two different thicknesses: 466 miles (750 km) of it is 0.462 inches (1.17 cm) thick, while the remaining 334 miles (538 km) is 0.562 inches (1.43 cm) thick. More than 78,000 vertical support members hold up the aboveground sections of pipeline, and the pipeline contains 178 valves.

At the end of the pipeline is the Valdez Marine Terminal, which can store 9.18 million barrels (1,460,000 m3) of oil. Eighteen storage tanks provide this capacity. They are 63.3 feet (19.3 m) tall and 250 feet (76 m) in diameter. They average 85% full at any given time—7.8 million barrels (1,240,000 m3). Three power plants at the terminal generate 12.5 megawatts each. Four tanker berths are available for mooring ships in addition to two loading berths, where oil pumping takes place. More than 19,000 tankers have been filled by the marine terminal since 1977.