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A tensioner is a device used on a drilling platform or rig, particularly floating rigs in the ocean, to support the risers used for drilling and oil delivery to the rig. Risers connect the drilling platform to the sea floor, and are normally a series of connected pipes. The drilling platform can be held relatively steady during drilling, but storms or high seas can cause the rig to move because of wave action. Damage could result if the risers were rigidly connected to the drilling platform, so they are supported by the tensioner system that can hold the weight of the riser and permit movement separate from the motions of the platform.
Floating oil and gas drilling rigs can be connected to the sea floor by a series of pipes or cables called tension legs. The rig can also float freely and be maintained in a location by global positioning systems, which use satellites and a receiver to determine location, and the rig's engines. Tension legs are pulled upward by the flotation of the rig, which uses ballast tanks filled with air and water to maintain a certain floating height above the water. Ballast systems can be controlled to maintain a steady tension on the drilling rig legs, but there is no way to use ballast to control the riser pipe tension.
Near the top of the riser is an assembly called the slip ring. This system is attached to the riser pipe to provide a gripping point for the tensioner. The upward pull of the tensioner is provided by either cables or rods connected to a group of hydraulic rams or pistons that push or pull upward against the weight of the riser. Electronic controls maintain a constant pull when the drilling rig moves because of waves or weather.
Tensioner systems can be operated with air pressure or hydraulic oil pressure. A series of tanks called accumulators hold air or fluid under pressure, and supply it as needed to the tensioners to maintain the needed riser tension. Emergency disconnect devices are installed that permit the rapid separation of the tensioner pressure system from the risers in case the rig moves violently or must be moved away from the drilling area because of storms. Tensioners are installed with extra, or redundant, units so the riser is protected even if an individual tensioner fails or needs maintenance.
In the late 20th century, designers began experimenting with rigid but flexible elastomers, or rubber-like materials, that could be connected to the risers and provide tension and movement without the need for hydraulic systems. The advantage of elastomeric tensioners is a minimum of moving parts, and the expensive hydraulic systems are not needed. These systems work best where there is less movement of the drill rig, because tension sections have to be added where additional movement is needed, which might be difficult in ocean drilling.