You can't work on an oil rig unless you've completed your emergency response training. That includes practical exercises in escaping from an upturned helicopter ditched in the sea, and fire-fighting training.
Very few firms provide free training to potential roustabouts. (They might train graduate engineers, who're in short supply, but there's no shortage of people wanting to start at the bottom as roustabouts, before moving on to become derrickmen). Most people pay for their own training (which costs thousands of pounds) before seeking employment. Even after completing the training, it can be hard to find work initially, as most employers only take on experienced staff.
See here for further information:
http://www.learndirect-advice.co.uk/helpwithyo urcareer/jobprofiles/profiles/profile1099/
Chris