Yeah, let's spark up a welder next to a gas pipeline. Maybe we'll go smoke at a gas pump or oil the brakes on my car too.

Now we've gone from seconds to fix to hours to fix. If you wanted better I'd say to target pumping stations. If you could cause some serious damage to the pumps that push the oil, you could take the entire line out of commission for a good long while.

Of course, the pumping stations are guarded, and it isn't a good look for ecoterrorists to kill people. But maybe you could pull a Stuxnet.