Gas Leak Detection

Alyeska releases TAPS low-flow impact study

HOUSTON, July 1 -- Alyeska Pipeline Service Co.’s ‘Low Flow Impact Study’ of the Trans-Alaska Pipeline System warns of potential operations and safety issues if flow rate on the line drops below 350,000 b/d. TAPS, completed in 1977, transported 2 million b/d at its peak. It currently carries 650,000 b/d of oil from Alaska’s North Slope to Valdez, from where it is shipped to US West coast refineries.

The study identified water dropout and corrosion, ice formation, wax deposition, geotechnical, and other concerns posing operational risks to TAPS at throughput between 600,000 b/d and 300,000 b/d, but concluded the line could be operated safely down to 350,000 b/d if a number of issues are addressed.

These included:

• Water dropout and corrosion. Unless the crude is heated, as flow rates drop below 550,000 b/d freezing of the water in the oil is very likely during winter months, potentially icing and disabling check valves and other pipe features and instruments and creating ice slugs during pig passage. Icing could manifest itself at rates of 780,000 b/d if the hot residuum currently returned to the pipeline by refineries at North Pole, Alas., were to become unavailable, according to the study.

• Wax deposition. Assuming no heating of the oil, the study expects ice lens formation in certain soil conditions at a throughput of 350,000 b/d, causing differential movement of the pipe via frost heave, and warns that unacceptable pipe displacement limits and possible overstress conditions would be reached at 300,000 b/d.

Other operational issues include increased difficulty of pigs to remove wax, reduction in pipeline leak detection efficiency, and shutdown and restart issues related to potential ice blockages.

The study places TAPS’s reliable operating throughput at 550,000 b/d absent mitigation of these issues and 350,000 b/d with mitigations in place. The study also listed a number of issues likely to be experienced at rates below 350,000 b/d and said additional work would be required to determine potential mitigation of these issues.

Safest operations

For safe operations at 350,000 b/d, however, the study made a number of recommendations, including:

• Minimize the risk of ice formation by adding heat sources if necessary, enhancing insulation, and establishing a minimum temperature of 105° F. for crude entering TAPS from North Slope fields.