They can’t do any damage that would seriously disrupt the EU without making it extremely likely that Article 5 would be invoked

Article 5, as defined in scope by Article 6, doesn’t cover infrastructure in international waters. It does cover vessels flying the flag of member countries in the North Atlantic area, which I assume includes the Baltic Sea, but pipelines aren’t vessels.

https://www.nato.int/cps/en/natolive/official_texts_17120.htm

Article 6

For the purpose of Article 5, an armed attack on one or more of the Parties is deemed to include an armed attack:

• on the territory of any of the Parties in Europe or North America, on the Algerian Departments of France 2, on the territory of Turkey or on the Islands under the jurisdiction of any of the Parties in the North Atlantic area north of the Tropic of Cancer;

• on the forces, vessels, or aircraft of any of the Parties, when in or over these territories or any other area in Europe in which occupation forces of any of the Parties were stationed on the date when the Treaty entered into force or the Mediterranean Sea or the North Atlantic area north of the Tropic of Cancer.

I actually went looking a while back and there isn’t really anything in customary international law or treaties protecting pipelines in international water. I also found some articles stating that there really isn’t law on the matter. Undersea cables have a late-1800s treaty signed in IIRC Paris that was intended to cover telecommunications cables, but I expect probably also extends to power cables; undersea power transmission cables weren’t a thing back then, but the actual wording of the treaty doesn’t limit the treaty to telecom cables, just uses the term “cable”. But there was never a corresponding treaty signed for pipelines. Companies just started building them without first building up legal support for them.

googles

Yeah, here’s the cable treaty. From 1884:

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