Since the first jackup was built in 1954, jackups have become the most popular type of mobile offshore drilling unit (MODU) for offshore exploration and development purposes. In fact, there are hundreds of jackups stationed around the world performing drilling and workover operations in just about every environment.
One of two types of bottom-supported MODUs (the other being the much less popular submersible rig), jackup rigs rest on the sea floor rather than float. The premise of a jackup rig is that it is self-elevating; here, the legs are stationed on ocean floor and the drilling equipment is jacked up above the water's surface.
Providing a very stable drilling environment, in comparison to other types of offshore drilling rigs, jackups can drill in waters up to 350 feet deep. Once drilling is required in waters that are deeper than the capabilities of a jackup, semisubmersibles and drillships become a more logical choice for exploration and development operations.
When their legs are not deployed, jackups float, which makes these types of MODUs quite easily transported from one drilling location to another. While some are capable of self-propulsion and do not need an outside source for movement, most jackups are transported via tug boats or submersible barges. While towing is easily performed, barges are the transportation of choice when the jackup needs to be moved quickly or over a longer distance.
Whether a jackup design involves three or four legs, there are two main types of legs that are used on jackups. The first incorporates open-truss legs into the design that resemble electrical towers. Open-truss legs are made of tubular steel sections that are crisscrossed, making them strong, but lightweight.
Jackups can also have columnar legs made of huge steel tubes. While columnar legs are less expensive than open-truss legs to fabricate, they are less stable and cannot adapt to stresses in the water as well as open-truss legs. For this reason, columnar-legged jackups are not used in waters that measure more than 250 feet deep.
In addition to their legs, jackups are supported by two different systems of stabilization. Jackup legs are supported on the sea floor via either mats or spud cans.
A more logical choice for drilling environments that have soft floors, mat-supported jackups distribute the weight of the rig across the ocean's bottom, like a snow shoe. Usually shaped like an "A," mat supports are connected to the bottom of each leg of the jackup, ensuring that the rig does not punch-through the ocean's bottom.
Usually used on independent-legged jackups, spud cans are cylindrically shaped steel shoes with pointed ends, similar to a cleat. Spud cans are attached to the bottom of each leg, and the spike in the can is driven into the ocean floor, adding stability to the rig during operations.
Additionally, there are two types of elevating devices. Once the jackup is on location, the legs are lowered to the ocean's floor and the rig hull and drilling equipment is elevated well above the water's surface and away from any potential waves.
The first type of elevating device uses hydraulic cylinders equipped with moving and stationary pins. These cylinders extend and retract to climb up and down the legs of the jackup. The other type of elevating device employs a rack and two pinion gears that are turned to move the legs up and down.
Whichever type of leg that is used in the design, the legs of the jackup rise up through holes in the hull of the drilling rig. Here, a deck is used to support the drilling derrick and other equipment.
There are two ways to mount the drilling equipment on the hull. The most popular design for drilling equipment is a cantilevered jackup. Here, the drilling derrick is mounted on an arm that extends outward from the drilling deck. With a cantilevered jackup, drilling can be performed through existing platforms, as well as without them. Because of the range of motion that the cantilever provides, most jackups built in the last 10 years have been cantilevered jackups.
The other type of jackup is the slot-type jackup, also known as a keyway jackup. Drilling slot jackups are built with an opening in the drilling deck, and the derrick is positioned over it. While exploration wells can be drilled with drilling slot jackups, the rig can also be jacked up over another smaller facility, drilling through its hull.