Having a prior experience in O&G is a definite prerequisite. If you notice, there are not many such ads in the newspaper for positions on the rig. Normally by words or mouth by colleagues, poaching, or if you are a equipment vendor / contractor working on the rig and know someone high up, this helps in getting in. The thing is, rigs are expensive to build, maintain, operate and to hire. They need to ensure having a set of experienced crew for safety is paramount to personnel and rig. A day's downtime could cost the owner US$500k in lost charter.

To be fair, I need to sound a word of caution. This is not an industry without incidents. You probably have heard of Deepwater Horizon, West Atlas, Piper A. Work safety has bucked up a lot in recent years though due to these incidents.

Good luck in your job search.