OFFSHORE OIL RIG AND PLATFORM TYPES
Different types of offshore oil rigs and platforms are used depending on the offshore oil/gas field water-depth and situation. Rigs are used for the drilling of the wells and platforms are installed in the field for extracting oil/gas operation. Main types of rigs and platforms are briefly explained as follows: Drilling for natural oil/gas offshore, in some instances hundreds of miles away from the nearest landmass, poses a number of different challenges from drilling onshore. With drilling at sea, the sea floor can sometimes be thousands of feet below sea level. Therefore, while with onshore drilling the ground provides a platform from which to drill, at sea an artificial drilling platform must be constructed.
Moveable offshore drilling platforms/rigs
There are two types of offshore drilling rings/platforms. The first type is moveable offshore drilling rigs that can be moved from one place to another and the second type is the fixed rigs/platforms.
Drilling barges are used mostly for inland, shallow water drilling. This typically takes place in lakes, swamps, rivers, and canals. Drilling barges are large, floating platforms, which must be towed by tugboat from location to location. Suitable for still, shallow waters, drilling barges are not able to withstand the water movement experienced in large open water situations.
Jackup rigs are similar to drilling barges, with one difference. Once a jackup rig is towed to the drilling site, three or four ‘legs’ are lowered until they rest on the sea bottom. This allows the working platform to rest above the surface of the water, as opposed to a floating barge. However, jackup rigs are suitable only for shallower waters, as extending these legs down too deeply would be impractical. This rig type can only operate to 500 feet in the depth of water. These rigs are typically safer to operate than drilling barges, as their working platform is elevated above the water level.
Submersible rigs, also suitable for shallow water, are like jackup 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 submerges 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.
This is an offshore oil rig that has a floating drill unit that includes columns and pontoons that, if flooded with water, will cause the pontoons to submerge to a depth that is predetermined. Semi-submersible rigs are the most common type of offshore drilling rigs, combining the advantages of submersible rigs with the ability to drill in deep water. Semi-submersible rigs work on the same principle as submersible rigs; through the ‘inflating’ and ‘deflating’ of its lower hull. 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. Semi-submersible rigs are generally held in place by huge anchors, each weighing upwards of ten 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.
Semi-submersible rigs can also be kept in place by the use of dynamic positioning.
Semis-submersible rigs can be used to drill in much deeper water than the rigs mentioned above. Now with a leap in technology, depths of up to 6,000 feet (1,800 m) can be achieved safely and easily. This type of rig platform will drill a hole in the seabed and can be quickly moved to new locations.
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 called a “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. This offshore oil rig can drill in very deep waters.
Drillships use ‘dynamic positioning’ systems. Drillships are equipped with electric motors on the underside of the ships hull, capable of propelling the ship in any direction. These motors are integrated into the ships 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.
In certain instances, in shallow water, it is possible to physically attach a platform to the sea floor. This is what is shown above as a fixed platform rig. The ‘legs’ are constructed of concrete or steel, extending down from the platform, and fixed to the seafloor with piles. With some concrete structures, the weight of the legs and seafloor platform is so great, that they do not have to be physically attached to the seafloor, but instead simply rest on their own mass. There are many possible designs for these fixed, permanent platforms. The main advantages of these types of platforms are their stability; as they are attached to the sea floor, there is limited exposure to movement due to wind and water forces. However, these platforms cannot be used in extremely deep water; it simply is not economical to build legs that long.
Template (jacket) platforms
This type of fixed platform is the one usually installed in the Persian Gulf, the Gulf of Mexico, Nigeria, and California shorelines and is made of steel (Sadeghi 1989, 2001). Template platforms mainly consist of jacket, decks and piles.
All of the petroleum platforms installed in the Persian Gulf are of the Template (Jacket) type. At the present time about 145 template platforms belonging to Iran and about 130 template platforms belonging to Arabian countries are installed in the Persian Gulf. Figure 2 shows one template platform.
Compliant Towers (Tower platforms)
Compliant towers are much like fixed platforms. They consist of a narrow tower, attached to a foundation on the seafloor and extending up to the platform. This tower is flexible, as opposed to the relatively rigid legs of a fixed platform. This flexibility allows it to operate in much deeper water, as it can ‘absorb’ much of the pressure exerted on it by the wind and sea. Despite its flexibility, the compliant tower system is strong enough to withstand hurricane conditions.
Seastar platforms are like miniature tension leg platforms. The platform consists of a floating rig, much like the semi-submersible type discussed above. A lower hull is filled with water when drilling, which increases the stability of the platform against wind and water movement. In addition to this semi-submersible rig, however, Seastar platforms also incorporate the tension leg system employed in larger platforms. Tension legs are long, hollow tendons that extend from the seafloor to the floating platform. These legs are kept under constant tension, and do not allow for any up or down movement of the platform. However, their flexibility does allow for side-to-side motion, which allows the platform to withstand the force of the ocean and wind, without breaking the legs off. Seastar platforms are typically used for smaller deep-water reservoirs, when it is not economical to build a larger platform. They can operate in water depths of up to 3,500 feet.
Floating production systems
Floating production systems are essentially semi-submersible drilling rigs, as discussed above, except that they contain petroleum production equipment, as well as drilling equipment. Ships can also be used as floating production systems. The platforms can be kept in place through large, heavy anchors, or through the dynamic positioning system used by drillships. With a floating production system, once the drilling has been completed, the wellhead is actually attached to the seafloor, instead of up on the platform.
The extracted petroleum is transported via risers from this wellhead to the production facilities on the semi-submersible platform. These production systems can operate in water depths of up to 6,000 feet.
Tension leg platforms
Tension leg platforms are larger versions of the Seastar platform. The long, flexible legs are attached to the seafloor, and run up to the platform itself. As with the Seastar platform, these legs allow for significant side to side movement (up to 20 feet), with little vertical movement. Tension leg platforms can operate as deep as 7,000 feet.
Subsea production systems are wells located on the sea floor, as opposed to at the surface. As in a floating production system, the petroleum is extracted at the seafloor, and then can be ‘tied-back’ to an already existing production platform. The well can be drilled by a moveable rig, and instead of building a production platform for that well, the extracted oil and natural gas can be transported by a riser or even undersea pipeline to a nearby production platform. This allows one strategically placed production platform to service many wells over a reasonably large area. Subsea systems are typically in use at depths of 7,000 feet or more, and do not have the ability to drill, only to extract and transport.
Spar platforms are among the largest offshore platforms in use. These huge platforms consist of a large cylinder supporting a typical fixed rig platform. The cylinder however does not extend all the way to the seafloor, but instead is tethered to the bottom by a series of cables and lines. The large cylinder serves to stabilize the platform in the water, and allows for movement to absorb the force of potential hurricanes. The first Spar platform in the Gulf of Mexico was installed in September of 1996. It’s cylinder measured 770 feet long, and was 70 feet in diameter, and the platform operated in 1,930 feet of water depth.
Design of offshore fixed platforms The most commonly used offshore platforms in the Gulf of Mexico, Nigeria, California shorelines and the Persian Gulf are template type platforms made of steel, and used for oil/gas exploration and production (Sadeghi 1989, 2001).
The design and analyses of these offshore structures must be made in accordance with recommendations published by the American Petroleum Institute (API).
The design and analysis of offshore platforms must be done taking into consideration many factors, including the following important parameters:
- Environmental (initial transportation, and in-place 100-year storm conditions)
- Soil characteristics
- Code requirements (e.g. American Institute of Steel Construction “AISC” codes)
- Intensity level of consequences of failure The entire design, installation, and operation must be approved by the client.
Different analyses needed for template platforms
Different main analyses required for design of a template (jacket) type platform are as follows (Sadeghi 2001):
- In-place analysis
- Earthquake analysis
- Fatigue analysis
- Impact analysis
- Temporary analysis
- Loadout analysis
- Transportation analysis
- Appurtenances analysis
- Lift/Launch analysis
- Upending analysis
- Uprighting analysis
- Unpiled stability analysis
- Pile and conductor pipe drivability analysis
- Cathodic protection analysis
- Transportation analysis
- Installation analysis.