I can't imagine any small nation having the ability to target pipelines in the middle of an ocean. That's not to say that it's impossible of course, but it's probably unlikely.
With that said, there are 3 obvious candidates.
The Russians themselves, in order to justify a pre-planned nuke attack on Ukraine.
The Chinese, purely because they'd love to cause shit between Russia and the west, get involved, and win a war.
Or Biden, who clearly isn't a man for turning, and would love to destroy the Kremlin.

Whodunnit?