that seems to have been forgotten in this <terrorists are everywhere> piece.
"The purpose of terrorism, is to terrorise," Attributed to Lenin.
And you 'terrorise' people, not fish. Blowing up a farmhouse is much safer than a tube station or train. But that won't 'terrorise' people, so you would have failed.
9/11 worked because the American people, and their government, panicked. The whole country changed because of a single terrorist attack. The terrorists won and the world lost.
7/7 didn't work so well, people were getting on buses and tubes the next day. But the UK had already been bloodied by the IRA attacks, and knew what was most likely to happen next, nothing.
Blowing up a gas tanker in a harbour will only work if it is somewhere like the London docks. It is the population density that terrorists have to look for.
Blowing up an oil rig, won't affect most people and so won't 'terrorise', but it will seriously piss off some seriously rich people. People who have the means, and, possibly, the will to make things tough for the sponsors of terrorism.
So there is a real reason why even such a soft target as an oil rig hasn't been targeted by terrorists, it isn't worth it.