A jack-up rig is a type of mobile platform that is able to stand still on the sea floor, resting on a number of supporting legs. The most popular designs use 3 independent legs, although some jackups have 4 legs or more. On "mat-type" jackups the legs are connected to a submerged hull.
A jackup is a floating barge fitted with long support legs that can be raised or lowered. The jackup is maneuvered (self-propelled or by towing) into location with its legs up and the hull floating on the water. Upon arrival at the work location, the legs are jacked down onto the seafloor. Then "preloading" takes place, where the weight of the barge and additional ballast water are used to drive the legs securely into the seabottom so they will not penetrate further while operations are carried out. After preloading, the jacking system is used to raise the entire barge above the water to a predetermined height or "air gap", so that wave, tidal and current loading acts only on the relatively slender legs and not on the barge hull.
Jackup rigs can only be placed in relatively shallow waters, generally less than 400 feet (120 m) of water. However, a specialized class of jackup rigs known as premium or ultra-premium jackups are known to have operational capability in water depths ranging from 500 to 625 feet.
This type of rig is commonly used in connection with oil and/or natural gas drilling. There are more jackup rigs in the worldwide offshore rig fleet than any other type of mobile offshore drilling rig. Other types of offshore rigs include semi-submersibles (which float on pontoon-like structures) and drillships, which are ship-shape vessels with rigs mounted in the centers. These rigs drill through holes in the drillship hulls, known as moon pools.
Jackup rigs also refer to specialized barges that are similar to an oil and gas platform but are used as a base for servicing other structures such as offshore wind turbines, long bridges, and drilling platforms.