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This is of course inspired by the Nordstream gas pipelines in the Baltic Sea. When it was decided that Nordstream 2 would not start operating after construction finished they still filled it with natural gas at full pressure (a little above 100 bars iirc). When Nordstream 1 was shut down it was also kept full of natural gas at full pressure.

To a lay person this sounds like a reasonable thing to do, after all these pipes are meant to hold and transport natural gas. Then I read in a blog that this apparently is a terrible idea because it will lead to hydrate plugs. Cleaning these out is very complicated and if you do it the wrong way or sloppy there is a high risk of an explosion. The sensible way to shut down a pipeline for a few months is to fully empty it of natural gas and then instead fill it with some inert gas like nitrogen. However I've never seen any talk about this in general news media (so far) and all the news about the Nordstream explosions went from unknown cause to clearly external sabotage within the last two days.

So when the Nordstream gas pipes were shut down, was filling them with natural gas a sensible thing to do or is this something where experts on operating gas pipes should have screamed warnings?

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The blog is wrong . It requires higher pressures and/or lower temperatures to make hydrates. Hydrates can be a problem at high pressure gas production wells; such as above 5000 psi. That high pressure must be reduced in one or more chokes to deliver less than 2000 psi gas for pipeline transport. There is substantial cooling produced by the expanding gas ; This can cause hydrates at the chokes. So the chokes are heated ( I don't know how ,never worked with it). The gas hydrate can remain solid up to about 70F with high pressure.( I am sure Wikipedia has more info for anyone interested). Nat gas would be the practical material to fill the pipe with because it is available. That quantity of nitrogen would be a logistics problem. Footnote ; there are unimaginable amounts of hydrate in the deeper areas ( high pressure ) of the oceans.

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