Pipeline Built to Survive Extremes Can’t Bear Slow Oil Flow

[IsailedawayfromFR]

Here at the top of the world, January brought a glimpse of the anxious future facing Alaska’s once-mighty oil pipeline.

The 800-mile Trans Alaska Pipeline System was built for extreme conditions. But as the state’s oil production declines, the pipeline faces a new challenge: flows so sluggish operators worry the line may become unusable, cutting off access for hundreds of North Slope oil wells.

With the mercury dipping as low as -60 Fahrenheit, workers in January fired up heating units across the system. It worked, but if the brutal cold had lasted or the oil flow had slowed further, the pipeline would have been in uncharted territory. Four decades after it opened, Alaska’s pipeline -- once a symbol of independence for an oil-strapped nation -- is facing a midlife crisis. The line now moves a quarter of the volume it carried at its peak. And as the flows slow, the risks are rising.

“We’re already at the stress point," said Tom Barrett, president of the Alyeska Pipeline Service Co., which operates the system. “We don’t have the kind of cushion you’d like to have."
https://www.bloomberg.com/news/articles/2017-04-10/pipeline-built-to-survive-extremes-can-t-bear-slow-flow-of-oil

[thackney]

http://alyeska-pipeline.com/TAPS/PipelineOperations/LowFlowOperations

Wax

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.

[IsailedawayfromFR]

This sounds daunting, but what do you think about a smaller line, maybe plastic, run inside that can be the main conduit of fluid movement to increase velocity? Maybe another insulating fluid in annulus?

Just thinking out loud.

[EC]

This sounds daunting, but what do you think about a smaller line, maybe plastic, run inside that can be the main conduit of fluid movement to increase velocity? Maybe another insulating fluid in annulus?

Just thinking out loud.

Wouldn't a smaller line inside block the pigs? Far as I know they're self propelled (or pushed along by the oil) and almost fill the pipe.

[thackney]

This sounds daunting, but what do you think about a smaller line, maybe plastic, run inside that can be the main conduit of fluid movement to increase velocity? Maybe another insulating fluid in annulus?

Just thinking out loud.

It has to contain the pressure of the pumping stations.  I don't think you would get good results of a plastic pipe without any outside sidewall pressure in a much bigger opening.

The 71 gate valve stations would be a significant issue.  Plus the tie-in at every pump station.  Not to mention all the instruments installed on the existing pipewall.  I don't know how you could push it through the 800 miles in any reasonable cost and shutdown time.

[thackney]

Wouldn't a smaller line inside block the pigs? Far as I know they're self propelled (or pushed along by the oil) and almost fill the pipe.

The pigs are pushed by the oil.  The exterior of the pig matches the interior of the pipe.  If a smaller pipe was used a smaller pig would have to be used.


http://youtu.be/dufOVyzj-3Q

[IsailedawayfromFR]

It has to contain the pressure of the pumping stations.  I don't think you would get good results of a plastic pipe without any outside sidewall pressure in a much bigger opening.

The 71 gate valve stations would be a significant issue.  Plus the tie-in at every pump station.  Not to mention all the instruments installed on the existing pipewall.  I don't know how you could push it through the 800 miles in any reasonable cost and shutdown time.

I know it is tough to do,  but I have seen it done in a lot less complicated situations

[thackney]

I know it is tough to do,  but I have seen it done in a lot less complicated situations

The majority of the newer fields from Western Alpine into NPRA have been very light, 35~45° gravity.  More of these would be the best solution.

I suspect they would also add heat with the North Slope Nat Gas before re-pipe.  There would be a lot of politics against greatly reducing future flow rates.  It may never be reality, but many, many believe expansion of flow will eventually climb again.