Thanks.
I can only relate what I have knowledge of.
We charged the pipe with water for weld integrity, then Nitrogen for gas-leaks.
Water has a high compressibility so if any part of the pipe is gonna burst, it will with water.
Finally, ultra dry Nitrogen to sweep any moisture and finally brought it to line pressure with product for FERC certification.
Note FERC doesn’t oversee EU pipelines.
We use a very low pressure nitrogen “blanket” atop the oil in large substation transformers. Like 1-3 psi, just enough to keep a positive pressure and prevent ingress of moisture into the transformer. Usually provided by a nitrogen cylinder. Typically the cylinder pressure is pretty high, regulated down, and will last anywhere from a week to 2 or 3 months.
It’s been a long time since I swapped out a nitrogen cylinder.
I bet FERC wishes it had oversight on EU pipelines. Such is the case with regulators - they want more and more control over that which they regulate.