Mobile Offshore Drilling Units, (MODU) are vessels that are deployed in drilling activities for the exploration of natural resources deep beneath the sea-bed. These natural resources include gaseous or liquid hydrocarbons, salt or sulphur. Various types of MODU have been made over time and they differ in size, type and efficiency of the offshore units.
They include jack-ups, submersibles, semi-submersibles, drillships and barges, etc. Most of the offshore exploration wells, deep-water wells and development wells are drilled using these units. These types of Mobile Offshore Drilling Units are manufactured and installed based on the economical, technical, safety and governmental requirements of the drilling operations.
Jack-Up Rigs
A jack-up rig or a self-elevating unit is a type of mobile platform that consists of a buoyant hull fitted with a number of movable legs, capable of raising its hull over the surface of the sea. The buoyant hull enables transportation of the unit and all attached machinery to a desired location. Once on location the hull is raised to the required elevation above the sea surface supported by the sea bed. The legs of such units may be designed to penetrate the sea bed, may be fitted with enlarged sections or footings, or may be attached to a bottom mat. Generally jack-up rigs are not self-propelled and rely on tugs or heavy lift ships for transportation. They are considered safer than some other types of moveable rigs, such as drilling barges, since their surface facilities are elevated from the water and less susceptible to waves and weather.
The primary advantage of the jack-up 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 generally less than 120 metres (390 ft) of water, however, a specialized class of jack-up rigs known as premium or ultra-premium jack-ups are known to have operational capability in water depths ranging from 150 to 190 meters (500 to 625 feet).
Submersible Rigs
Submersible rigs, also suitable for shallow water, are like jack-up rigs in that they come in contact with the ocean or lake floor. These rigs consist of platforms with two hulls positioned on top of one another. The upper hull contains the living quarters for the crew, as well as the actual drilling platform. The lower hull works much like the outer hull in a submarine - when the platform is being moved from one place to another, the lower hull is filled with air - making the entire rig buoyant. When the rig is positioned over the drill site, the air is let out of the lower hull, and the rig submerses to the sea or lake floor. This type of rig has the advantage of mobility in the water, however once again its use is limited to shallow water areas.
Submersibles are attractive in shallow water of less than 14 to 20 ft and/or where the ocean bottom is very soft. Submersibles also have other advantages in that their variable deck load (VDL) or well-consumable load-carrying ability is usually much higher than for comparable shallow-water jack-ups. They also do not leave a “footprint” like an independent-leg jack-up does with its spud can holes. These footprints can cause significant structural leg problems when another jack-up with different leg spacing is jacked up in the same area. Even if the second rig jacks up, it may slide into the previous spud can holes and lose its position over the platform, possibly causing significant leg damage.
Semisubmersible
Rigs
Semisubmersible rigs are the most common type of offshore drilling rigs, combining the advantages of submersible rigs with the ability to drill in deep water. A semisubmersible rig works on the same principle as a submersible rig: through the 'inflating' and 'deflating' of its lower hull. The main difference with a semisubmersible rig, however, is that when the air is let out of the lower hull, the rig does not submerge to the sea floor. Instead, the rig is partially submerged, but still floats above the drill site. When drilling, the lower hull, filled with water, provides stability to the rig. Semisubmersible rigs are held in place by huge anchors, each weighing upwards of 10 tons. These anchors, combined with the submerged portion of the rig, ensure that the platform is stable and safe enough to be used in turbulent offshore waters. Semisubmersible rigs can be used to drill in much deeper water than the rigs mentioned above.
They are the most dependable, motion-free, and capable of all the MODUs. Their cost is generally higher than that of a jack-up, but in water depths exceeding that for which jack-ups are rated, they are the unit of choice. Most Semisubmersibles are not self-propelled and hence have to be towed. Anchored or moored Semisubmersibles can drill in water up to 1500 meters while dynamic positioning Semisubmersibles can drill beyond 6000 meters.
Drillships
Drillships are exactly as they sound: ships designed to carry out drilling operations. These boats are specially designed to carry drilling platforms out to deep-sea locations. A typical drillship will have, in addition to all of the equipment normally found on a large ocean ship, a drilling platform and derrick located on the middle of its deck. In addition, drillships contain a hole (or 'moonpool'), extending right through the ship down through the hull, which allows for the drill string to extend through the boat down into the water. Drillships are often used to drill in very deep water, which can often be turbulent. Drillships use what is known as 'dynamic positioning' systems. Drillships are equipped with electric motors on the underside of the ship's hull, capable of propelling the ship in any direction. These motors are integrated into the ship's computer system, which uses satellite positioning technology, in conjunction with sensors located on the drilling template, to ensure that the ship is directly above the drill site at all times.
They are most commonly used in remote areas where long-distance mobilizations between drilling locations are the rule. This allowed drillers to take advantage of the mobility and transit speed of the self-propelled drillships, as well as their greater deck load capacities. They were also concentrated in areas with mild to moderate weather conditions because their greater water-plane exposure made them inherently less stable than the semisubmersibles, with more pronounced response motions, especially in roll and heave. This was particularly true of drillships with conventional multipoint spread moorings, which locked the vessels into a specific heading and meant that they would occasionally take the weather abeam on the side of the hull.
