Jackup

From EnergyWiki

A jackup drilling rig is a bottom-supported MODU consisting of a triangular hull and three legs and a jacking system. Very early versions of jackups were square or rectangular and could have a number legs ranging from four to as many as six, or in one case, eight legs.

Briefly, a jackup rig is towed to the wellsite, its legs are lowered by the jacking system to the seafloor and the hull is elevated (or jacked up) until it is above the water, providing a stable work platform for drilling or other operations. The jackup provides a stable work platform because they are supported by the seafloor and waves pass underneath the hull.

Jackups have been a part of the offshore mobile drilling fleet since the 1950s. In addition to their original purpose of exploration, jackups have been used variously as production units, tender-assisted units, accommodation platforms and work, repair and maintenance platforms.

The first jackups were built for fairly shallow water but have evolved as the offshore industry has moved toward deeper water exploration. Their practical water depth limitation is around 500 ft, although a few jackups are rated to drill in up to 550 ft of water. Most, however, generally are capable of drilling in water depths ranging from 250-350 ft of water. Beyond that water depth, floating rigs become more economical and more efficient.

Jackup rig legs

Jackup rigs come in several varieties and can include cylindrical or lattice type legs, with spud cans or mat-supported footings that support the hull and provide stability when the rig is jacked up. Jackups with cylindrical legs usually are also mat-supported units in which the leg is attached to large mats below the hull that provide stability for the rig. Jackups without mats are typically referred to as independent-leg jackups because each leg can be lowered at different rates of speed and rest at different lengths when on the seafloor. Independent-leg jackups also typically have lattice-type legs. Virtually all modern and newly built jackups presently will have lattice-type legs with spud can footings.

Mat-supported and independent-leg jackups

The main purpose of the spud cans or the mat is to increase the soil bearing area, which enables to jackup to work in relatively weak soil areas. The mats on mat-supported jackups are usually rectangular structures that are flat on top and bottom. The mat contains buoyancy chambers that are flooded when it is submerged to help provide stability. Mat-supported jackups are useful when the seafloor is soft or muddy or where the seafloor can’t support high bearing loads since the large area of the mat reduces the bearing pressure on the seafloor.

Mat-supported jackups are primarily used in certain areas of the Gulf of Mexico. However, because the mat is a large flat structure, it is difficult to use a mat-supported jackup on uneven seafloors or in areas where there are pipelines, boulders or debris on the seafloor.

Lattice-type legs almost always have spud cans at the bottom of each leg to help support the rig. Independent-leg jackups with spud cans are more versatile than mat-supported jackups in that they can operate in more diverse areas, in soft and hard seafloor areas, and in areas with a sloping seafloor. They are also more versatile when it comes to operating in areas with pipelines, boulders and other debris

Slot-type jackups and cantilever jackups

In addition to two types of bottom supported methods (mat-supported and independent-leg), jackup rigs also are divided into slot-type and cantilever units. Slot-type jackups are typically built when also utilizing a mat-support. Slot type jackup rigs are configured for the drilling operations to take place through a slot in the hull below the rig substructure that supports the drilling equipment. The primary reason for using a slot-type rig is that the jackup can be positioned around a small fixed structure to drill additional development wells or to work over existing wells. The rig’s substructure can be moved over the slot to drill different wells without moving the jackup itself.

A cantilever jackup has a feature that permits the drilling platform, or drilling substructure, to be extended out from the hull and over small platforms rather than positioning the jackup’s hull around the platform. This method also allows the jackup to perform drilling or workover operations over pre-existing platforms or structures. However, a cantilever jackup is more versatile than a slot-type unit for several reasons. First, some recent cantilevers can extend up to 75 ft from the rig’s hull, allowing the jackup’s rig to drill over a larger area of the platform. This is useful if a platform’s well bay is near the center of the platform. Additionally, cantilever jackups typically can be positioned closer to a platform, providing a further advantage when drilling or working over wells on a platform.

Cantilever jackups became so popular that few slot-type units were built during the 1990s. In fact, numerous jackups that were built as slot-type units originally were converted to cantilever jackups. Another development was pioneered by Rowan Companies when they developed skid-off technology utilized with their slot-type jackups. In this case, the entire substructure and drilling equipment can be skidded off of the jackup and onto a platform, thus allowing a slot-type jackup to be used on certain platforms that would have required a cantilever unit or a platform rig.