Derrick (lifting device)

A derrick is a lifting device composed of one mast or pole which is hinged freely at the bottom. It is controlled by (usually 4) lines powered by some such means as man-hauling or motors, so that the pole can move in all 4 directions. A line runs up it and over its top with a hook on the end, like with a crane (machine). It was commonly used in docks.

The device was named after Thomas Derrick, an England execution (legal) from the Elizabethan era because of its resemblance of the frame from which a hangman s noose hangs.

= Derricks over drilled holes =

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.

The centerpiece is the archetypical derrick tower, used for lifting and positioning the drilling bit and piping above the well head, and containing the machinery for turning the drilling bit around in hole. As the drill goes deeper into the underlying soil or rock, new piping has to be added to the top of drill to keep both the connection between drill bit and turning machinery intact, to create a filler to keep the hole from caving in somewhere halfway between the drill bit and the surface, and to create a conduit for the drilling mud, which is needed to cool down the drill and used for pumping out rock debris created by the drilling. For this purpose the piping sections, usually each about 10 meters (30 feet) long, have threaded ends, so they can be screwed or bolted together.The piping is hollow to allow for the mud to be pumped down into the drilling hole, where it flushes out near the drilling bit. This cools the drilling bit down, and blows rock debris clear from the drill bit and the bottom of the well. The mud then proceeds upwards towards the surface on the outside of the piping, carrying the debris with it.

At the same time the derrick is used to control the pressure on the drilling bit, because the drill bit works at an optimum rate only when it is pushed at an exact amount of pressure on the rock beneath it. Too high a pressure can break the drilling bit, and too low a pressure will at least increase costs by prolonging drilling time. Because the weight of the pipes above the drill bit will increase the pressure on it the deeper it goes, and because when starting drilling at the surface there will be hardly any weight on the drill at all, the derrick has to push the drill at the start of drilling, then ease up pressure when piping sections are added, and eventually lift the entire drill-and-piping complex to prevent too high a pressure as the well goes deeper.

Because of all these different requirements, done under high cost and time pressures, in almost any climate and virtually any place in the world, combined with the fact that the purpose is to find flammable or explosive fossil fuels, oil drilling is a complex and dangerous business.

The people who do this hard and dirty work do so under dangerous circumstances and often in considerable isolation, especially when on an offshore oilrig.