Alyeska releases TAPS low-flow impact study

Christopher E. Smith
OGJ Pipeline Editor

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. Water is typically entrained at current throughputs, but will start separating out at flows below roughly 500,000 b/d, according to the study, increasing the potential for internal corrosion at the bottom of the pipe.

Ice formation. 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. Wax deposition increased in the mid-1990s as the crude shipped dropped below its wax-appearance temperature of roughly 75° F. The study says deposition will continue at current levels as throughput declines, even if the oil is heated in the future. Wax will then be collected and hardened by scraper pigs.

Geotechnical concerns. 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.

• Mitigate freezing of water in the pipeline during extended winter shutdowns, including bypass of back pressure control at the Valdez Marine Terminal.

• Develop procedures to reduce risk of throughput interruption leading to crude temperature falling below the freezing point.

• Modify current water specification to prohibit slugs above 0.35%. At throughput below 400,000 b/d reduce the water specification to 0.2% to reduce accumulation of water even in flowing conditions.

• Implement contingency procedures, practices, and facilities to reduce potential formation of ice stemming from extended pipeline shutdowns and reduced throughputs in winter.

• Reduce the risk of internal corrosion by regularly injecting corrosion inhibitor and biocides.

• Implement practices and facilities to manage continued or increased wax deposition via pigging.

• Revisit the pipeline’s cold restart analysis and implement a continuing cold-restart evaluation program, with the analysis revisited every 5 years.

• Use current curvature pig-monitoring program to monitor pipeline frost heave.

• Perform a detailed analysis of field instrument and leak detection capabilities at low flows.

• Conduct a probability analysis to determine winter design shutdown duration and associated credible minimum ambient temperatures.

• Supplement the Department of Revenue forecast for timing of low-flow related mitigation projects with the forecasting algorithm developed by the study team based on past throughput decline rates. Update the algorithm yearly.

Contact Christopher E. Smith at chriss@ogjonline.com.

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