Type of offshore oil and gas Jackup Rigs
The basis of a jackup rig is that it is self-elevating where the legs are placed on sea bed and the drilling equipment is jacked up above the sea surface. There are two types of Jackup rigs which are bottom-supported Mobile Offshore Drill Units (MODUs), jackup rigs rest on the sea floor rather than float like semisubmersible rigs.
Offshore jackup rig gives a very stable drilling setup, in comparison to other types of offshore drilling rigs. The jackups can drill in waters up to 350 feet deep. The Ultra Harsh Environment jack-up rig can even go deeper upto 450ft to 499ft. When drilling is required in seas that are deeper than the capabilities of a jackup rig, semisubmersibles rigs and drillships become a more suitable technique for offshore exploration and development operations.
From the time when the first jackup rig was built in 1954, jackup rigs have become the most popular type of mobile offshore drilling unit (MODU) for offshore exploration and development purposes. There are hundreds of jack-up rigs deployed around the world performing drilling and work over operations in all types of sea conditions and regions.
When jackup rig is not standing on seafloor, then its legs are taken up and jackup floats, which makes these, types of MODUs very easily to be transported from one drilling location to another. Jackups rigs are transported on huge submersible barges or by gigantic float-on/float-off ships (rig mover ships) or towed by tug boats. While towing is easily performed, submersible barges or rig mover ships are the also deployed for transportation when the jackup rig needs to be moved speedily or over a longer distance. While some jackup rigs have its own propulsion and do not need an external source for shipping,
Whether a jackup rigs design involves three or four legs or some time even more legs, there are two main types of legs that are used upon jackups. The first type integrates open-truss legs into the design that is similar to electrical power towers. Open-truss legs are made of tubular steel segments that are fabricated crisscrossed, making them stronger, but light weighted.
Furthermore, in addition to their legs, jackup rigs are supported by two different types of stabilisation systems. Jackup rigs legs are supported on the sea bed by means of either mats or spud cans.
Mat footing usually are rectangular structures like letter A, flat on the upper and lower sides, and contain buoyancy chambers which are flooded when the mat is submerged. For drilling operation in region of soft sea bed, mat supported jackup rigs are more suitable which distribute the weight of the rig across the ocean's bed, like a snow shoe. Typically 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 floor. Mat footings connect all the Jackup unit's legs to one common footing.
Spud cans are normally designed to be free flooding when submerged, though they can be pumped dry for internal inspection. Spud cans are like conical structures, with sloping tops and bottoms. Its sloping top helps in sloughing off mud that collect on top of the spud-can in the event of deep penetration. The sloping bottom helps ensure that there will be some penetration even in very hard soils.
Spud cans are attached to the bottom of each leg, and the spike in the can is plunged into the ocean floor, adding stability to the rig during drilling operations. Generally used on independent-legged jackup rigs, spud cans are cylindrically shaped steel shoes with pointed ends, similar to a cleat.
The jackup rigs have two types of uplifting machines. Once the jackup is on location, the legs are lowered to the sea's bed and the rig hull and drilling equipment is elevated well above the water's surface and away from any possible 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 rig.
The second type of uplifting equipment uses a rack and two pinion gears that are turned to move the legs up and down. Whatever type of leg that is used in the jackup design, the legs of the jackup rise up through holes in the hull of the drilling rig. A deck is used to support the drilling derrick and other equipment.
There are two techniques to mount the drilling equipment on the hull of a rig. Primary 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. These types of jackup rigs are used for exploration wells; can be drilled with drilling slot jackups, the rig can also be jacked up over another smaller facility while drilling through its hull.
But secondary type of jackup is the most standard design for drilling equipment is a cantilevered jackup. Because of the range of movement that the cantilever facilitates, most jackups built in the last decade has been cantilevered jackup rigs. 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.
