http://alyeska-pipeline.com/TAPS/PipelineOperations/LowFlowOperationsWax
ANS Crude oil naturally contains up to 2 percent wax by volume. There are two issues with wax: First, when the pipe walls are colder than 70 degrees and colder than the oil, wax crystals gravitate to the pipe wall and stick to it. Second, wax precipitates out of the crude oil. Less turbulence, cooler crude temperature and slower flow all may result in more wax sticking to pipe walls and more wax dropping out of the oil and settling in the pipeline. Wax deposits must be removed by running cleaning pigs.
Less throughput = more challenges
Less oil → slower flow → crude spends more time in pipe, and less turbulence
Slower flow/less turbulence → more wax may accumulate in the pipe, requiring more frequent ‘pig’ cleaning
More time in pipe → Crude loses heat → higher risk of ice problems, more wax forms
TAPS is currently moving an average of 513,000 BPD (2014 daily average)
Challenges are immediate
No hard and fast thresholds; a continuum of challenges requires corresponding actions to address them
Ultimately may need shift to intermittent flow
The pipeline today
Alyeska and its owner companies have analyzed the risks, options and challenges of declining throughput. Some mitigations are already in place, while engineers are validating other potential steps through laboratory and field tests.
Transitional fix: more heat, more pigs
For the immediate future, Alyeska is adding heat to keep the crude warm and to prevent small amounts of water from freezing in the line. The cleaning pig program has been modified – with frequent pigging and redesigned pigs as needed – to keep the pipe clean of wax.
Heat is added through recirculation at Pump Stations 3, 4 and 9. Pump Station 7, which was previously decommissioned, is now back online to recirculate oil. Infrastructure for recirculating oil at that station is also enhanced. A schedule is in place for adding more heat as the crude continues to cool due to declining throughput.