[dark_lord]

Yep. Sounds easy. But. I say again. The corporations that run the tankers pay big bucks to security consultants, who employ and are owned by ex-CIA, ex-DIA, ex-FBI, ex-spec ops, ex-cops, etc. Those consultants come up with all the ideas - just like the way you proposed but in more detail. Then they come up with counters. Counter-counters. Etc.
So - it is not so easy as you think.

[DB]

After one spends billions for more than a decade on making the "bomb" the relative difficulties of delivering it are small.