Jackup rig consists of a buoyant hull with retractable legs. The number of legs varies depending on design, three or four being the norm. Jackups are mobile, and vessels can tow them between locations with the legs fully extended above the hull. Upon arrival, it lowers the legs down to the sea bottom. Then, it elevates its hull out of the water till reaching the desired clearance above the water line. The unit then provides a fixed platform for drilling and carries the total rig and supply loads.
Afloat, the barge-like hull has to have enough buoyancy to support the additional weight of the legs. These may weigh as much as 350 tons each and are tubular (round or square) or open-fabricated (truss-type design). Tubular legs are cheaper but become too heavy for water depths exceeding 60 meters. They also present too much-projected area against waves. Therefore many prefer the truss type for greater water depths. These have either a triangular or square cross-section.
The water depth in which a specific offshore drilling rig type can operate depends on several factors:
- The length of the legs of the unit.
- The anticipated penetration of the legs into the sea bed.
- Environmental conditions for the area.
- The maximum wave height for the area. This value determines the required air gap between the bottom of the hull and the mean sea level.
Jackup Rig Types
Many offshore drilling companies use Jackup rig type, representing the largest group of offshore rigs. Using a jackup in preference to a floating rig is usually advantageous whenever possible. The jackup provides a stable drilling platform and will be less susceptible to shutdown during severe weather. Also, the operation itself is greatly simplified as the B.O.P. stack is above water. Operating water depth ranges from 3 meters for minimum draft shallow water units to 125 meters for the new generation ultra harsh environment units offering a drilling capability of 10,000 meters, maximum hook load of 2×106 lbs, and 15,000 psi B.O.P. stack equipment. Jackups typically operate in the Gulf of Mexico, South East Asia and India, the North Sea, the Arabian Gulf, and West Africa. World Oil’s Marine Drilling Rig ’95 Summary listed 387 rigs, which has remained relatively steady. You might be interested in World’s Biggest Oil Rigs.
Jackups fall into two main generic types, mat-supported and independent leg; the latter is more common. Within these types are two main designs: slot type and Cantilever.
Mat supported Jackup
Many oil and gas companies use mat-supported jackup for exceptionally soft unconsolidated sea bed conditions as in tidal deltas. With this type of rig, they replaced the spud cans with a large mat, often covering a sea bed area more significant than the vessel’s hull. This supporting mat distributes the unit’s weight over a large area, thus eliminating point loading on the sea bed. The sea bed must be relatively level and debris-free to limit damage to the mat. Also, we must consider tidal solid currents in shallow waters, which may cause sliding of the mat along the sea bed. Because of the mat jackups, a shallow draft can be used in water as shallow as 3 meters.
World Oil’s Marine Drilling Rig ’95 Summary list approximately seven of these units, most of which operate in the Gulf of Mexico. Figure 2 gives an example of a mat-supported, slot-type jack-up rig.
Independent leg jackup
Independent leg jackup rigs are the most common type of unit. Each leg’s base is equipped with a “spud can” intended to distribute the load evenly over a portion of the seafloor. The spud cans are generally circular or polygonal, designed with a heavy point to penetrate the most challenging sea floor and ease retrieval from deep penetration of a soft sea bed. Independent leg-type rigs are designed for various bottom conditions, including uneven or slanting bottoms. A major limiting factor in using an independent leg jackup is excessive spud can penetration, which may vary from seven to ten feet in some places in the North Sea to fifty to seventy feet in soft bottom areas. Figure 3 gives an example of an independent leg, cantilever-type jackup rig.
Slot type jackup
Slot-type jackups were the industry norm until the mid-seventies when designs moved more towards cantilever-type jackups, offering increased flexibility. A slot-type jackup fully supports the derrick on the rig hull’s deck. The main limitations of a slot-type jackup are:
- A limited skid plan for development drilling.
- Limited deck space.
- Limited access to existing platforms.
Cantilever Jackup
The Cantilever is the system by which the derrick and substructure can be skidded over the barge’s side (usually the stern). Doing so allows the rig to work over platforms with the jackup positioned alongside. With lateral and longitudinal traversing ability, the jackup can drill and work over multi-well platforms with great flexibility (however, precise rig placement is vital).
Two main skid beams are mounted parallel to the rig’s axis on the main deck. The cantilever deck with the derrick and substructure unit is skidded along these beams either to the inboard position during tows or the outboard position during drilling operations. The further the cantilever deck is skidded outboard over the skid beams, the greater the bending moment exerted on them and the greater the overturning moment on the jackup itself.
If the center line of the rotary table is close to the transom bulkhead of the jackup, then the rig can work to its designed maximum working load. However, as the derrick substructure skids away from the transom bulkhead, the allowable maximum cantilever load decreases. The center line of the cantilever structure provides the most even load distribution point. However, the allowable cantilever load decreases markedly if we skid the derrick laterally such that the rotary table is significantly from the center line. We must consider this factor when handling a large casing string type or stuck pipe. Each unit’s operations manual will display the allowable loads for each position of the derrick set on the skid beams. The variation in the capacity as the derrick set moves around its envelope can be seen in Figure 3
Jackup Rig Components
Jackups are basically made up of four main components (Figure 4). Starting from the bottom, those are:
- Footing
- Leg
- Jacking system
- Hull
Legs
The legs of a jackup are made of steel and serve to support the hull when the unit is elevated. They also provide stability to withstand lateral loads. Some independent leg units can tilt the legs in deep water to increase resistance against overturning. The legs can be either:
- Trussed legs or
- Cylindrical legs/columnar legs.
The columnar legs, also known as cylindrical legs, are constructed from hollow steel tubes made of structural steel that are between 1½ to 2¾ inches thick. Each leg has a diameter of 10-12 feet and ranges from 225-312 feet in length. At every 6-foot interval, there are rings with six pin holes spaced at 60° intervals along the length of the leg. These holes are intended to be used to engage the pins in the jacking assembly. Additionally, these legs require less deck area.
The legs of a truss can be either triangular or square in shape and are made up of chords and braces. The braces allow the leg to resist shear forces, while the chords provide strength against axial and flexural loads. At every corner of the trussed legs (as shown in Figure 5), racks are present for engaging with the gears or pinions in the jacking assembly. These racks, which look like threads cut on a flat bar, are made from special high-yield strength steel with good impact resistance. Trussed legs are bigger than columnar legs and therefore require a larger deck area.
Footings
At the bottom of a jackup’s legs are footings that aim to increase the area that the legs can bear. This reduces the weight the soil needs to support, providing a stable foundation for the jackup. There are two types of footings: spud cans/tanks and mats. Depending on the type of footing used, jackups can be classified into two categories.
- Independent leg type
- Mat-supported type
The Hull
The hull of a jackup unit is a watertight structure that supports or houses the equipment, systems, and personnel, thus enabling the unit to perform its tasks. When the jackup unit is afloat, the hull provides buoyancy and supports the legs’ weight, footings equipment, and variable load.
The hull is generally built of stiffened plate. The structure is configured to efficiently transfer loads acting on the various hull locations into the legs. Axial and horizontal loads are transferred into the legs through the hull leg interface connections and chords. Hence, a bulkhead terminates at each leg chord location.
Jacking System
Two types of jacking or elevating systems are used for lifting and lowering the hull.
- Rack-and-pinion type
- Electro-hydraulic type
We will talk about it in more detail in the Jackup operations article.
Jackup Rig Advantages
Some of the main advantages of jackup type rigs are:
- They provide a fixed platform and thus experience minor ‘shutdown for the weather’ during drilling operations.
- They can provide a work deck for simultaneous operations while development drilling.
- Once set, the units resist lateral shifting on the bottom.
- They are relatively cheap in comparison with floating drilling units.
- Drilling operations are relatively similar to operations on land and require standard surface-installed wellhead and well control equipment.
Jackup Rig Disadvantages
There are some disadvantages as well:
- Towing speed is low, about three knots or less. Therefore long moves are costly.
- Since jacking speed is slow, there is an appreciable time interval when the leg bottoms (spud cans) are touching or nearly touching the sea floor during positioning and removal operations. The spud cans are then subject to pounding on the bottom, even in very moderate sea conditions. A failure of the jacking system at this time would be severe. We cannot perform jacking operations safely with a wave height of more than 5 feet.
- With the platform floating, the legs will extend several hundred feet into the air. This makes jackups prone to overturning in lousy weather. We can remove leg sections for long moves to lower the unit’s center of gravity and increase stability.
- The uneven sinking of the legs into the ocean floor during the time on location can cause severe stress concentrations, which could lead to failures. Inclination alarms and indicators will enable this detection and correction.
- It is a must to have a reasonable weight distribution to prevent the overloading of any one leg. We should know, record and limit the offshore rig variable loads compared to floating rigs.
- There are deck loading and P.O.B. restrictions during rig moves that are more acute than for other types of mobile units.
- Handling shallow gas is necessary at the surface utilizing a surface diverter system, and the rig cannot move off location like floating rigs in case of a shallow gas blow-out.
- Cratering of the sea bed can cause catastrophic problems to the rig’s stability.
Jackup Rig Operational Aspects
Jackup drilling rigs will not usually compete with floating rigs since their operating parameters are complementary rather than conflicting. Having said this, there are areas where a large jackup or small semi may be considered for a particular job. Also, using a bottom-supported submersible or drilling barge may provide an alternative to a jackup in shallow water. The choice of the type of drilling unit to be used will largely depend on the operating characteristics, limitations, availability of rigs, environmental conditions, and, ultimately, economics.
The specific drilling location is one of the most important factors to consider when deciding on a jackup rig. In shallow waters, jackup rigs are ideal for their ability to be positioned in areas where depths reach as low as 20 feet. However, other rig types may be more suitable when working in deeper waters. The design of the jackup rig must also take into account any specific current loading conditions that exist at the drilling location. This is because offshore drilling units are subject to significant forces generated by wind and sea currents, which can create additional strain on the entire barge and its components. To meet these demands, jack-up rigs utilize pinion gears that drive spud can legs into the seabed.
This section will describe some jackup operations systems, procedures, and design requirements. The following subjects will be addressed:
- Jacking systems
- Pre-drilling operations
- Operational design requirements
- Other operational matters
External Links:
- Semi-Submersible Rigs
- “World Jackup Drilling Rig Fleet.” Archived from the original on 2009-07-27. Retrieved 2010-02-07.
- “Rowan Companies marks 50th anniversary of landmark LeTourneau jackup“(PDF). Drilling Contractor.61(5): 34–36. 2005.ISSN0046-0702. Retrieved 22 September 2022.
- Offshore Petroleum Drilling and Production – Sukumar Laik – CRC Press – Taylor & Francis Group
