- published: 26 Jan 2016
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A derrick is a lifting device composed of one tower, or guyed mast such as a pole which is hinged freely at the bottom. It is controlled by lines (usually four of them) powered by some means such as man-hauling or motors, so that the pole can move in all four directions. A line runs up it and over its top with a hook on the end, like with a crane. It is commonly used in docks and onboard ships. Some large derricks are mounted on dedicated vessels, and are often known as "floating derricks".
The device was named for its resemblance to a type of gallows from which a hangman's noose hangs. The derrick type of gallows in turn got its name from Thomas Derrick, an English executioner from the Elizabethan era.
Another kind of derrick is used over oil wells and other drilled holes. This is generally called an oil derrick and is a complex set of machines specifically designed for optimum efficiency, safety and low cost. This is used on some offshore oil and gas rigs.
The centerpiece is the archetypical derrick tower, used for lifting and positioning the drilling string and piping above the well bore, and containing the machinery for turning the drilling bit around in the hole. As the drill string goes deeper into the underlying soil or rock, new piping has to be added to the top of the drill to keep the connection between drill bit and turning machinery intact, to create a filler to keep the hole from caving in, and to create a conduit for the drilling mud. The drilling mud is used to cool the drilling bit and to blow rock debris clear from the drill bit and the bottom of the well. The piping joints sections—usually each about 10 meters (30 ft) long—have threaded ends, so they can be screwed together. The piping is hollow to allow for the mud to be pumped down into the drilling hole, where it flushes out at the drilling bit. The mud then proceeds upwards towards the surface on the outside of the piping, carrying the debris with it.