Sketch of a jack up rig with its leg lowered
Jack-up rig is a type of a drilling rig used in offshore locations which is basically made up of a barge (hull) and legs that can be raised or lowered onto the seafloor to provide support for the rig. Jackup rigs belong to a class of drilling rigs called mobile offshore drilling units (MODU).
MODUs are majorly deployed in offshore locations for the exploration of hydrocarbons. Of all types of mobile offshore drilling units available, Jackup rig is the most popular. The very first Jackup rig was deployed in 1954 and ever since then, they have become the drilling rig of choice in shallow water (less than 300 ft or 91 m) to relatively deep water environments (up to 550ft or 167m).
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Once the offshore location to position the rig is known, the Jackup is mobilized and towed to location. To achieve this, the rig is floated on its barge section while the legs are raised up above the water so the barge can float freely. Upon arrival at the drilling location, the legs are then lowered onto the seafloor.
When the rig’s legs are raised up during rig mobilization, the rig is said to be in a “jack up” position; when the legs are lowered, the rig is in a “jack down”position. Hence, the name “Jackup” rig.
Understanding how the Jacking Action Works
Jack up rig with leg raised
Most Jackup rigs have 3 legs; however, it is not uncommon to find some with 4, 6 or 8 legs. The barge of the jackup rig has “holes” through which these legs can be lowered onto the seafloor or raised to the air as required.
The jack-up-jack-down action is made possible through elevating devices. There are two types of elevating devices used with jackup rigs; the hydraulic cylinders type and the rack type.
The hydraulic cylinder type has stationary and moving pins which help extend and retract the cylinder. The extension and retraction of the cylinder is behind the jacking up and jacking down of the legs respectively.
The rack type has two gears which rotate to jack the legs up or down. The legs of a jack-up rig can be of the open-truss or the column type. If the legs of a jack up rig looks like those of electrical towers (with tubular steel sections crisscrossed around one another) then it is the open-truss type.
On the other hand, if the legs are simply made up of huge long steel pipes, more like a pole, then it is the column type. The open-truss type is more common than the column type since they offer better stability and flexibility than the column type.
Types of Jackup Rig
There are two types of jackup rigs based on how the rig rests on the seafloor; the independent-legged Jackup and the mat-supported jackup.
Independent-legged jackups have cylindrically shaped steel shoes with pointed ends called spud cans. These spud cans are attached to the bottom of each leg and when the legs are jacked-down, the pointed ends of the spud cans are driven into the ocean floor to provide bottom support and rig stability.
Independent legged Jack up rig type with spud cans attached to the legs
Driving the legs down involves “preloading” the legs by adding weight onto the legs in stages and driving the leg deeper each stage until it cannot go any further. This ensures that the rig remains stable and the legs will not penetrate the seafloor any further during operation. The jacking down mechanism also raises the upper portions (barge and drilling structure) above the sea level.
The other type of jack-up rig, the mat-supported jackup, attaches a steel shoe shaped in the form of an “A” to the bottom of the legs. Thus, the legs firmly rest on the seafloor instead of penetrating into the seafloor like the independent-legged type.
A type of mat supported jack up rig where all the legs are connected to one common footing
Since the legs of the mat-supported jackup are merely resting on the seafloor, the support they provide is more susceptible to failure. The mat-supported type is relatively cheap compared to the independent-legged type.
