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There are basically four types of mobile drilling rigs used in offshore oil and natural gas activity. These include submersibles, jackups, semisubmersibles and drillships. 

Submersibles are fitted with ballast tanks, so that they can be floated to shallow-water locations. Once on location, the vessel is ballasted to sit on the seabed and provide a stable drilling base. After the well is completed, the water is pumped out of the ballast tanks, the vessel is refloated and then towed to the next drilling site. 

Jackups (see top photo to left) are usually towed by ocean-going tugs to their drilling locations, with their legs up and barge-like sections floating on the water. Once on-site, the jackup's three legs are mechanically jacked down to the seabed. This raises the hull and its main floor clear of, and above, the water for drilling purposes. Most of the drilling equipment and related supplies are contained on the main floor. Jackups are used in various water depths, to a maximum of 400 feet.

Semisubmersibles (middle photo to left) are floating vessels with superstructures supported by columns on large pontoon-like structures submerged below the sea surface. The operating decks are usually elevated 100 feet or more above the pontoons. This has the advantage of minimizing loading from waves and wind. Semisubmersibles are anchored with six to 12 anchors tethered by strong chains and wire cables. These items are computer-controlled to maintain the vessel's position above the wellsite. Semisubmersibles can operate in a wide range of water depths, including deepwater tracts. Some of these vessels have their own propulsion, while others are towed by ocean-going tugs. For transport mode, the hulls are deballasted to the surface. Semisubmersibles are remarkably stable in rough weather, and they can be used in medium-to-deep waters.

Drillships (bottom photo to left) are ship-shaped vessels, on which the drilling derricks are usually placed in the middle. Drilling is conducted through a hole in the hull below the derrick, known as a moonpool. A few drillships are equipped to drill over the side. Drillships usually have their own propulsion. They are either anchored on-site or kept in position by what is called dynamic positioning. This process employs computer-controlled propellers or "thrusters" along the hull to continually correct drift in any direction. Drillships typically carry larger payloads than semisubmersibles, but the trade-off is that they have inferior motion characteristics. Drillships are typically used in medium-to-deep waters, although they can suffer from some instability in rough weather.