What’s a tensioner?

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A turnbuckle is a device used on a drilling platform or drilling rig, especially rigs floating in the ocean, to support the risers used for drilling and to deliver oil to the rig. The risers connect the drilling rig to the sea bed and are normally a series of connected pipes. The drilling rig can be kept relatively stable while drilling, but storms or high seas can cause the rig to move due to wave action. Damage could occur if the risers were rigidly connected to the drill rig, so they are supported by the tensioning system which can support the weight of the riser and allow for movement separate from the rig movements.

Floating oil and gas drilling rigs can be connected to the seabed by a series of pipes or cables called tension legs. The rig can also be free-floating and be held in a position by global positioning systems, which use satellites and a receiver to determine position, and by the rig’s motors. The tension legs are pulled up by the buoyancy of the rig, which uses ballast tanks filled with air and water to maintain a certain buoyancy height above the water. Ballast systems can be controlled to maintain a constant tension on the rig legs, but ballast cannot be used to control riser pipe tension.

Near the top of the riser is a group called a slip ring. This system is attached to the riser tube to provide a gripping point for the tensioner. The tensioner’s upward pull is provided by cables or rods connected to a group of rams or hydraulic rams which push or pull up against the weight of the riser. Electronic controls maintain constant traction when the drill moves due to waves or weather.

Tensioning systems can be operated with air pressure or hydraulic oil pressure. A series of reservoirs called accumulators hold air or fluid under pressure and supply it as needed to the tensioners to maintain the necessary riser tension. Emergency disconnect devices are installed which allow for quick separation of the turnbuckle pressure system from the risers in the event the rig moves violently or needs to be removed from the drilling area in a storm. Turnbuckles are installed with add-on or redundant drives so the riser is protected even if a single turnbuckle fails or needs maintenance.

In the late 20th century, designers began experimenting with stiff but flexible elastomers, or rubber-like materials, that could be attached to braces and provide tension and movement without the need for hydraulic systems. The advantage of elastomeric tensioners is a minimum of moving parts and expensive hydraulic systems are not required. These systems work best where there is less rig movement, because sections of tension must be added where additional movement is needed, which could be difficult in ocean drilling.

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