Petroleum production - Deepwater, Ultradeepwater, Drilling: In deeper, more open waters up to 5,000 feet (1,524 metres) deep over continental shelves, drilling is done from free-floating platforms or from platforms made to rest on the bottom. Floating rigs are most often used for exploratory drilling and drilling in waters deeper than 3,000 feet (914 metres), while bottom-resting platforms are usually associated with the drilling of wells in an established field or in waters shallower than 3,000 feet. One type of floating rig is the drill ship, which is used almost exclusively for exploration drilling before commitments to offshore drilling and production are made. This is an