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naraburns  nihil supernum  1yr ago (text post)   29600 thread views

Culture War Roundup for the week of February 6, 2023

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Well from that link earlier the pipeline holds ~300 million c.m. when in storage mode -- that's indeed a couple days pumping, but:

  1. You don't need to pump it enough to equalize the pressure completely, just enough so the plug doesn't move quite so violently -- I think this getting the plug to move is actually the goal in this kind of operation?

  2. Related, you probably would not allow the pressure to go anywhere as low as 10 bar when your instruments are telling you it's 100 at the other end; I guess this depends on how stupid you think the Russians are, but they did build two of the things and operate the other one for quite a while without blowing it up

  3. In 17 hours the pressure on the low side would be quite a lot higher than previously assuming somebody said "oh shit" and started the pumps again after incident 1 -- so you'd expect incident 2 to be quite a bit less destructive even if the plug did move some in the end, which does not seem to be the case.

To put some numbers around it:

The internal cross-section of the pipe is approximately 1m2, so each bar of pressure differential will push a plug with 100kN of force. That's enough to shoot 10 tons of hydrate at g-like acceleration. Sounds difficult at the best of times.

Also the plug is stuck until it isn't. When you depressurise you move back across that phase diagram until the solid sublimates, which happens radially from the outside, in. The plug is stuck until it shrinks from the walls enough to move (upon which you don't want it to move) and can be melted and cleaned up with pigging and glycol.

Just running the numbers based on the source you linked to above its pretty clear that you can rule out methane hydrate.

I) First determine how much water is in the pipeline to begin with. Per your source the gas is injected at 220 bara and 6 C, which we can use to determine the partial pressure of water in the inlet gas. Per the steam table linked below the partial pressure of water vapor at 6 C is 0.00935 bara.

Since the partial pressure of a gas component in a given mixture is its mol fraction * the total pressure we can determine that the mol fraction of water in our gas stream is 0.00935 bar / 220 bar = 4.25E-05.

Conveniently that same Petro Skills blog linked also had another page that calculates the total kmol inside the pipe during storage equilibrium as 12017147 kmol. With the mol fraction calculated above we can determine the total kmol in the pipe at storage equilibrium to be 12017147 kmol * 4.25E-05 = 510.95 kmol = 9197.05 kg of water present in the pipeline at storage equilibrium of 165 bara.

II) Next determine the how much water is actually in any given mass of methane hydrate. Based on the chemical structure given in wikipedia we can determine the mass fraction of water in a given mass of methane hydrate:

methane hydrate -> 4CH4 *23H2O

mass fraction of water:

= [mass methane hydrate] / [(mass methane) + (mass water)]

= [kmol methane * MW methane] / [(kmol methane * MW Methane) + (kmol methane * MW water)]

= [23 mol H2O * (18 kg / kmol)] / [(23 mol H2O) * (18 kg / kmol) + (4 mol CH4) * (16 kg / kmol) ]

= 414 / 478 = 0.866

III) We can use the answers from 1 & 2 to check the feasibility of an accident caused by a methane hydrate plug based on water availability. Just taking your estimate of a 10 ton methane hydrate plug:

10 ton = 20,000 lbs = 9197.05 total kg methane hydrate.

Which contains 9197.05 kg * 0.866 = 7856 kg of water, or 85% of all water in the entire pipeline. For reference the pipeline is 1224 km long, and at a cross section of 1 m2 & density of 900 kg/m3 the total length of that plug would be about 10 meters if a perfect cylinder. Since I think that there is literally no way that 85% of the water in a 1,224,000 meter pipeline would somehow freeze up in a single 10 meter (or 100 m or 500 m) section I think its safe to rule out a 10 ton hydrate plug. Even if it was one ton that's still 8.5% of the water stuck in a 1 meter section.

A pertinent question would be: why does this 10 m section have such wildly different conditions that the other 1224 km of pipe? Even such that its this 10 m section across 2 different 1224 km pipelines? It might be a valve but I'm figuring that they would have mentioned something about a valve. And valves are expensive, especially when they are 1m diameter and cause a large pressure drop and you just spent all of this effort to get 1224 km of pipe down to 6 micron smoothness to reduce pressure drop because you have 490,000 HP of compression. Also I think relatively quickly someone would have pointed out that the explosion happened at a valve location so maybe that had something to do with it. Given that we have not heard about a valve and the huge cost of having one in the first place I think we can rule out a valve. If it was a hole or something in the insulation I think you have to ask why did the hole happen in relatively the same place to two pipelines a mile apart that are ~1200 km long?

Even if it was a more gradual build up on a larger section you still have to answer how all the water ended up in a pretty small section. And the longer the section is the less its going to plug the pipe. And there really isn't enough water to go around, and how did it end all end up freezing in such a relatively small section? If it happened during pressurization they would have caught it then by noticing the massive pressure drop from the plug forming.

But overall there just isn't enough water in the gas to begin with to support the methane hydrate plug hypothesis. Especially since the Germans are on the other end of the pipeline and would have noticed as soon as any significant amount of water started showing up in the gas. Or if there was water being added to the gas. Or if the Russians shut swapped out dry gas with wet when it was shut off. But maybe my math is off, feel free to correct.

I would like to read the original source with the idea though if you have it?

https://www.thermopedia.com/content/1150/

https://petroskills.com/en/blog/entry/april2022-totm-Transportation-of-Natural-Gas-in-Dense-Phase%E2%80%93Nord-Stream-1

http://www.jmcampbell.com/tip-of-the-month/2022/06/nord-stream-long-distance-gas-pipeline-part-3-application-of-basic-and-aga-equations-for-estimating-maximum-gas-flow-in-a-long%E2%80%90distance-pipeline/

https://en.wikipedia.org/wiki/Methane_clathrate

If the plug comes unstuck, the pressure differential will equalize pretty rapidly -- 1g is not that much if it doesn't go on for too long.

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