There are no set requirements, but experience in construction or engineering could help you get a job. You must be over 18 years. You'll need an Offshore Medical Certificate and emergency safety training. You could get into this career through an apprenticeship. My Oil and Gas Career has further advice on careers in this area.
You'll live and work on a rig for 2 or 3 weeks, then have 2 or 3 weeks at home (56 days on and 28 days off or 28 days on and 28 days off rotation). You'll work up to 12 hours a day in shifts, covering days and nights. Companies are giving on/off both rotation salaries.