In the search for oil and natural gas under the ocean, three general types of drilling rigs are used. A “jackup” drilling rig is a floating barge with drilling equipment on its deck and long support legs, and is used in shallow waters up to 300 feet (90 meters). A semi-submersible is the most common type of offshore drilling rig, used for drilling in waters more than 300 feet (90 meters) deep. Semi-submersibles are floating vessels supported on large pontoon-like structures submerged below the sea surface. Semisubmersibles are attached to the ocean floor using strong chains or wire cables. Farther offshore, specially designed rigs mounted on ships can drill a well in waters over 10,000 feet (3050 meters) deep. These rigs float and can be attached to the ocean bottom using traditional mooring and anchoring systems or they maintain their position by using thrusters to counteract winds, waves and currents.
Each drilling systems is designed to withstand the wide range of wind and wave forces, including severe winter storms and hurricanes. Following is a description of 7 most common types of platforms.
A Fixed Platform (FP) consists of a jacket (a tall vertical section made of tubular steel members supported by piles driven into the seabed) with a deck placed on top, providing space for crew quarters, a drilling rig, and production facilities. The fixed platform is economically feasible for installation in water depths up to 1,500 feet (455 meters).
A Compliant Tower (CT) consists of a narrow, flexible tower and a piled foundation that can support a conventional deck for drilling and production operations. Unlike the fixed platform, the compliant tower withstands large lateral forces by sustaining significant lateral deflections, and is usually used in water depths between 1,000 and 2,000 feet (305 and 610 meters).
A Mini-Tension Leg Platform (Mini-TLP) or Sea Star (SStar) is a floating mini-tension leg platform of relatively low cost developed for production of smaller deep water reserves which would be uneconomic to produce using more conventional deep water production systems. It can also be used as a utility, satellite, or early production platform for larger deep water discoveries. The world’s first mini- TLP was installed in the Gulf of Mexico in 1998.
A Floating Production Systems (FPS) is a semisubmersible drilling rig containing 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 drill ships. With a floating production system, the wellhead is actually attached to the seafloor once the drilling is completed, rather than being attached up to the platform. The extracted petroleum is transported via risers from this wellhead to the production facilities on the semisubmersible platform. These production systems can operate in water depths of up to 6,000 feet (1830 meters).
A Tension Leg Platform (TLP) consists of a floating structure held in place by vertical, tensioned tendons connected to the sea floor by pile-secured templates. Tensioned tendons provide for the use of a TLP in a broad water depth range with limited vertical motion. The larger TLPs have been successfully deployed in water depths approaching 4,000 feet (1220 meters).
A Subsea System (SS) is located on the sea floor, as opposed to at the surface. Just as in a floating production system, the petroleum is extracted at the seafloor, and then ‘tied-back’ to an already existing production platform. The well is drilled by a moveable rig, and instead of building a production platform for that well, the extracted natural gas and oil are transported by 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 (2135 meters) or more, and do not have the ability to drill, only to extract and transport.
A Spar Platform (SP) is the largest offshore platforms in use. These huge platforms consist of a large cylinder supporting a typical fixed rig platform. The cylinder 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. Its cylinder measured 770 feet (235 meters) long and was 70 feet (20 meters) in diameter.