They would need access to powerful explosives, a submersible, and detailed knowledge of the location of the pipeline. Hard to imagine any non-government actor being behind this.
A dumb way of finding pipeline location is by looking at history AIS data. The pipe laying vessel must broadcasting its position via AIS while in operation. Since the water is only 70 meters deep, it is reasonable to assume the underwater pipeline is directly below pipe laying vessel.
And 9/11 required a person to fly a commercial airliner. Crazier things have happened.