But the bubbles aren´t big enough to be a threat. How big can they be if just one pipeline is their origin, even if the gas expands?

The methane bubble area in the image has a diameter of 200 meters. That's enough for an entire aircraft carrier. Expansion is not necessary, the turbine(s) just have to be strong enough to keep pushing air into the pipeline.

Will you rip open the pipeline over several dozen meters?

You do that with a valve every 100 meters or so. I should have mentioned that in the text. :-)

They also get diluted once they rise up.

That's ok. It just has to be enough to replace ~20% of the water below the ship. Then it sinks.