The jackup-type mobile offshore drilling unit (MODU) has become the premier bottom-founded drilling unit, displacing submersibles and most platform units.
The primary advantage of the jackup design is that it offers a steady and relatively motion-free platform in the drilling position and mobilizes relatively quickly and easily. Although they originally were designed to operate in very shallow water, some newer units, such as the “ultra-harsh environment” Maersk MSC C170-150 MC, are huge (Fig. 1) and can be operated in 550 ft in the GOM. With 673.4-ft. leg length, a hull dimension of 291×336×39 ft, and a variable deck load (VDL) of 10,000 long tons, it is mammoth and rivals some of the larger semis. This type of unit can be commercially competitive only in the North Sea and in very special situations.
Types of jackup unit
There are two basic types of jackups:
|Types of Jackup Units||Description|
|The Independent-Leg Type Jackup||Usually three legs with lattice construction|
|The Mat Type Jackup||The legs are attached to a very large mat that rests on the ocean bottom|
Both types of jackups have a hull, float onto location, jack the legs to the ocean bottom, and then jack the hull out of the water.
Independent-leg type jackup unit
For the independent-leg units, “preloading” is required to drive the legs into the ocean bottom before the hull is completely jacked out of the water. During this procedure, the jackup MODU is at risk from weather and leg “punch through”; i.e., one leg breaks through a hard crust, putting the other legs in a large bending movement. Generally, 5-ft swells and/or a combined sea of 8 ft are the maximum seas in which these units can jack out of the water. If the hull should roll, pitch, and heave to an extent that the legs come into contact with the ocean bottom, particularly if it is hard, the legs can be severely damaged.
The preload sequence is usually done in stages, with the hull never rising more than 5 ft out of the water to safeguard against having a leg punch through. If the ocean bottom is soft and consists of clay, it is not uncommon to take 7 or more sequences, with each sequence taking 7 to 12 hours. The unit’s pumps seawater into its preload tanks, adding weight to the hull and driving the legs. After the legs are driven and the hull goes into the water, the seawater is dumped overboard and the sequence is begun again. This process occurs until the legs no longer penetrate the ocean bottom. The concept is to load the legs to a level above that which the unit will encounter in the harshest predicted environment.
The newer, enhanced premium units do a single preload in which the jacking system is strong enough to jack the unit with all the preload water onboard, the basic weight of the hull, and the full transit VDL. This is a significant advantage in that a much smaller “weather window” can be acceptable to move the unit. Jackups are most susceptible to major damage or loss when they are floating.
Mat-type jackup unit
The mat-type jackup also usually consists of three legs that are cylindrical and are from 8 to 12 ft in diameter (Fig. 2). The mat is carried just under the hull during mobilization, usually with ≈ 5-ft gap. When the unit comes onto location, it jacks the mat down to the ocean bottom, and because of its low bearing pressure, usually under 500 to 600 psf, the unit jacks the hull out of the water without going through the preload sequence required for independent-leg units. Bethlehem Steel Corp. built most of these units from the 1950s through the 1980s. Their key advantages are that they were relatively inexpensive to build and leave no footprint at the drilling location.
Unfortunately, the Mat-Type Jackup unit also has several disadvantages:
- They are very susceptible to damage from any object on the ocean bottom.
- They tow very slowly because the mat and hull are large and create a lot of drag. Their mats are susceptible to being gouged by workboat propellers.
- Their upper hull has limited open deck storage space.
- Their legs sometimes form a wind-induced leg vibration known as vortex shedding at high winds, which can cause them to fail.
- Vortex shedding is a form of severe vibration seen with smoke stacks without spoilers.
- Most mat rigs have cylinders for legs and are structurally limited to shallower water depths, usually less than 250 to 275 ft.
- Only a few units have reached 300 ft, and these units have lattice-type legs.
For these reasons, mat jackups have fallen into disfavor, although they are relatively inexpensive and for some well types are more than adequate.
Common factors that impact both types of jackups
Air gap, or the distance from mean water level to the bottom of the hull while the unit is jacked up in the operating condition, is a critical issue. The bottom of the hull must have a large enough air gap that the largest wave crest will not hit the hull and turn over the rig. Air gaps usually are 35 to 50 ft, with the larger air gaps in shallower water, because wave heights build as water depth decreases. If a unit should work over a platform with a very high deck, air gaps of up to 100 ft are not uncommon; however, this obviously reduces the water depth rating. Jackup water depth ratings generally use a minimal leg penetration of 15 to 25 ft, which may not be the case in actual operation.
Independent- and mat-leg jackups also come in two types of drill floors, slot and cantilevered. As previously discussed, slot units were initially built in the 1950s through the late 1970s; however, with bigger platforms, the ability to cantilever the drill floor over the platform had an advantage over the slot units, which could only “swallow” minimal-size platforms. As the cantilever moves out to position itself over a well, it generally loses combined drillfloor load rating. The combined loading consists of the hook, setback, rotary, and drive-pipe tension if that tension is hung off the drill floor substructure. Generally, a minimum cantilever length (≈ 14 to 20 ft) is required for moving blowout preventers (BOPs) and other items next to the hull. Full rating is usually accomplished at center positions, but decreases as the cantilever moves further out and the drillfloor moves either side of center (usually ±15 ft). The rating on the extreme cantilever and extreme off-center can decrease by as much as 80%, leaving the unit capable of only light workovers.
Technological advances of jackup unit
Unlike typical earlier 1-million-lbm cantilever load units, the new premium jackups have ratings of at least 2 million lbm. With the advent of extended reach wells (ERWs), deeper gas wells, and high-pressure/high-temperature requirements, the higher load ratings are required, so many older jackups have been upgraded and enhanced, although not to the extent of some of the newer premium units built in the late 1990s and early 2000s. The Atwood Beacon (Fig. 3) is shown in the process of setting a small platform. This unit has features:
- Accommodations for a crew of 120
- 2-million-lbm cantilever load rating
- 7,500-psi-working-pressure mud system
- 70-ft cantilever
- 400-ft water depth rating
- 7,500-kip VDL (which is typical of the dozen or so units like the Atwood Beacon)
Fig. 3—Atwood Beacon enhanced premium jackup typical of the large and very capable vintage units of the 2000s. Note that the rig is setting a small fixed platform by jacking up to a 161-ft air gap, at which time the platform will be righted to vertical, and then the rig will jack down and set the platform on the sea bottom. Courtesy Atwood Oceanics.
There are more jackup-type MODUs than any other type of MODU. Table 1 shows general information about the various types of major units. Marathon Le Tourneau (now Le Tourneau) has designed and built more of these units than any other designer and builder. As shown, the size and capabilities of these units vary widely, with the general trend being for them to get bigger and more expensive with higher drilling and marine capability.
Advantages of jackup units
Unlike platform rigs, submersibles, and ships, jackups and semis are upgradeable from a technical and commercial standpoint. Rowan Co. and Noble Drilling, both large offshore drilling contractors with large jackup fleets, have done extensive upgrades and enhancements to units built in the 1970s and 1980s. Upgrading usually consists of:
- Converting slot to cantilever units
- Leg strengthening and lengthening with more preload tanks
- Increasing environmental capability
- Updating the drilling package with higher hook loads and installation of top drives
Originally, MODUs were considered to have a life of 12 to 15 years, but, through rigorous hull and equipment maintenance and technological updating, some 30-year-old units are considered “modern” and well fit for select purposes.
Why use a jackup? For water depths of 25 to 300 ft, there are many units to choose from. Some can be used in greater than 400-ft water depth. The jackup is the rig type of choice in certain water depths because of its:
- Stable work platform,
- Relatively inexpensive mobilization costs
Noteworthy papers in OnePetro
B.L. Miller, P.A. Frieze 1993, Motion And Impact Responses Of Jackups Moving Onto Location, Offshore Technology Conference, 3 May-6 May. 7301-MS. http://dx.doi.org/10.4043/7301-MS
2. R. Brennan, H. Diana 2006. Installing Jackups in Punch-Through-Sensitive Clays, Offshore Technology Conference, 1 May-4 May. 18268-MS. http://dx.doi.org/10.4043/18268-MS