How survivable would the average offshore platform be against missiles and gunfire?
Depends: are they currently full of oil or not?
Aside from that, those rigs are designed to withstand conditions that most ships run from. As long as you offload the flammables, the rig should be able to take one or two hits- which is about what any warship would be able to take. Remember, the rig is generally 'hollow', and a lot of the blast force that would be contained in a ship will be lost in atmosphere on the rig!