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I wonder with what these pipelines are filled during construction?

An inert gas like Nitrogen or Argon is probably too expensive, natural gas might be too dangerous when welding is still necessary. Seawater is probably too dirty and aggressive.

And as a related question: Should the pipeline fill up with sea water completely (for example because of large scale damage and loss of pressure) is it possible to make the pipeline usable again?

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That's actually why I looked this up in the first place. I think it's just air from what I could gather. The three sections of the pipe are laid on the seafloor like this (with the open end staying out of the water): https://www.wermac.org/nordstream/html_img66.html.

And then those three sections are welded together using an "air-tight habitat" surrounding them.

From what I've heard salt water getting into the pipe could make it really difficult to repair, and it might have to be scrapped altogether.

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