Trans-Alaska Pipeline System

From Academic Kids

Map of the pipeline
Map of the pipeline

The Trans-Alaska Pipeline System (TAPS), usually called the Alyeska Pipeline in Alaska or the Alaska Pipeline elsewhere, is a major U.S. oil pipeline connecting oil fields in northern Alaska to a sea port where the oil can be shipped to the Lower 48 states for refining.

Oil was discovered at Prudhoe Bay in 1968. A pipeline was considered the only viable system for transporting the oil to the nearest ice-free port, almost 800 miles (1,287 km) away at Valdez. The oil companies with exploitation rights grouped together as the Alyeska consortium to create a company to design, build, and then operate the pipeline. US President Richard Nixon signed the Trans-Alaska Pipeline Authorization Act into law on November 16, 1973, which authorized the construction of the pipeline. Although the pipeline is actually about 799 miles long, it is usually referred to as 800 miles long.

The single 48 inch (1.22 m) diameter pipeline was built between March 27, 1975 and May 31, 1977 at a cost of around US$8 billion. The pipe was constructed in six sections by five different contractors employing 21,000 people at the peak of work; 31 were killed in accidents during construction.

Missing image
An elevated section with typical zig-zag configuration

The 800 mile (1,287 km) route presented special challenges. As well as the harsh environment, the need to cross three mountain ranges and many rivers and streams, the permafrost of Alaska meant that almost half of the pipeline's length had to be elevated rather than buried as normal to prevent the ground melting and shifting. There were five years of surveying and geological sampling before construction began. During construction archeological teams were repeatedly called in to investigate previously unknown sites which were disturbed by excavation.

Along the pipeline there are eleven pump stations, each with four pumps. Each electric pump is powered by diesel or natural gas generators. Twelve pump stations were planned but Pump Station 11 was never built, the southward numbering system for the pump stations retains a place for this nonexistent station. Usually only around seven stations are active at one time, and future plans to replace the existing pumps with newer high-efficiency pumps may reduce the number of active stations even further.

In areas where thaw-sensitive permafrost exists but the line must be buried, such as highway crossings or avalanche-prone areas, the pipe is encased in an insulated, refrigerated ditch. Nearby refrigeration plants pump cold brine through 6 inch (15 cm) pipes which absorb heat and keep the soil cooled. Other areas of burial are either conventional covered ditches or unrefrigerated but insulated ditches, depending on the sensitivity of the surrounding soil.

Some elevated parts of the pipeline have radiators called "heat pipes" within the Vertical Support Members (VSMs) which use passive convection of anhydrous ammonia to disperse heat from the oil traveling through the pipeline. The oil emerges from the ground at up to 180 °F (80 °C), and travels through the pipeline at temperatures above 120 °F (49 °C). Without the radiators, the heat from the oil would conduct from the pipe through the VSMs and would melt the permafrost in which the VSMs are embedded. This would cause the pipeline to sink and possibly sustain damage. Since the radiators are passively cooled by convection, the ammonia within absorbs heat and vaporizes at the base of the supports, then rises to the radiator fins where the cooler air causes the vapor to condense. Once condensed the ammonia falls back down to the base of the supports where the cycle continues. Since ammonia has a very low boiling point in comparison to the permafrost, the passive convection works throughout the year even during the coldest winter nights. This relatively simple convection cooling system is felt by TAPS engineers and maintainers to be the greatest technological innovation associated with the pipeline.

Another innovation associated with the pipline is the zig-zag configuration aboveground. Since pipe shifts around far more easily aboveground than it does when buried, the zig-zag path of the pipeline allows the pipe to move somewhat from side to side and lengthwise. This movement may be caused by earthquakes or by temperature-related expansion and contraction. The VSMs also include special "shoes" to allow for this horizontal or lateral movement, and crushable blocks to absorb sudden shocks from earthquakes, avalanches, or vehicles.

Oil began flowing on June 20, 1977. Since then over 13 billion barrels (2.1 billion m³) have been pumped, peaking at 2.1 million barrels (330,000 m³) per day in 1988. Around 16,000 tankers have been loaded at the Marine Terminal at Valdez. The terminal has berths for four tankers and cost almost US$1.4 billion to build. The first tanker to leave the terminal was the ARCO Juneau on August 1, 1977.

Missing image
A caribou walks next to a section of the pipeline north of the Brooks Range.

The pipeline is surveyed several times a day, mostly by air. Due to the placement of the surveillance bases, the pipeline can be surveyed in just two hours, but most surveys take longer to ensure thoroughness. Other methods of surveying include regular Pipeline Inspection Gauges, or pigs, sent through the line. Some pigs are used to remove buildup of paraffin on the insides of the pipe, others have complex electronics which relay radar scans fluid measurements as they travel down the line.

The pipeline has been damaged on a number of occasions. Although it was built with earthquakes in mind, it is vulnerable to intentional attack and potentially to forest fires. The highest losses from the pipeline was in February 1979, when a deliberate explosion led to more than 16,000 barrels (2,500 m³) leaking out at Steele Creek, near Fairbanks. However, no one was charged. From 1977 to 1994 there were 30 to 40 spills a year on average, the worst years in terms of number of incidents were 1991 to 1994 when there were 164 spills, although none were major. Since 1995 the number of spills has been sharply reduced with total losses from 1997 to 2000 totalling only 6.89 barrels (1 m³).

The steel pipe is resistant to gunshots, but on October 4, 2001, a drunken hunter shot a hole into a weld, causing a spill of about 6,000 barrels (950 m³). The hunter was later arrested.

See also

External links

hu:Transzalaszkai Csővezeték


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