Several things wrong with that plan that I see.
A) Constructing transport helicopters. Now, that in itself wouldn't be much of an issue (in fact, it probably isn't), what with us having magitek motors, but. We likely need some levels in Air Superiority or somesuch in order for whatever we create to have any sort of chance of surviving a combat encounter. We'll need to train pilots who are able to fly those things, too, because unlike driving, if you screw up flying you're dead. And we'll need to develop parachutes or other methods of rapid vertical delivery of troops, because if those rigs are turned into fortresses, then even if they still have their helipads, anything we try to land there is swiss cheese by the time it touches down. Developing an actual combat dropship that can withstand several minutes of sustained AP fire (a flying T-Tank, if you will) would take us inordinate amounts of time. (though to be fair, it's not like we're in any kind of hurry at this point)
B) We'll need humongous numbers of troops to pull off a coordinated simultaneous attack. I'm not sure how many raiders there are, but considering the description, I'm presuming somewhere in the low hundreds - that's what it'd take to occupy and defend several drilling platforms as well as run the not-insignificant number of ships these raiders must use to get around and stage their attacks. We'll need help from quite a few cities if we're to build a force equivalent to that. Our 15 Elites won't cut it, and the T-Tank is still landlubber-y. That's not to mention this plan still hinges on pipes being intact and empty enough to let our strike teams through. And sort of hiccup, anywhere, will lead to disaster.
Bottom line, I still think we should go the "gunboat diplomacy" route. If we get good rolls on research, maybe with an actual gunboat this time, but if not, announcing that we have all their rigs, erm.. rigged to blow, displaying the truthfulness of that statement to any dissenters, and letting them choose to either surrender or face fiery death should work well enough.