Derrick

A derrick is a lifting device composed at minimum of one guyed mast, as in a gin pole, which may be articulated over a load by adjusting its guys. Most derricks have at least two components, either a guyed mast or self-supporting tower, and a boom hinged at its base to provide articulation, as in a stiffleg derrick.

The most basic type of derrick is controlled by three or four lines connected to the top of the mast, which allow it both to move laterally and cant up and down. To lift a load, a separate line runs up and over the mast with a hook on its free end, as with a crane.

Forms of derricks are commonly found aboard ships and at docking facilities. Some large derricks are mounted on dedicated vessels, and known as floating derricks and sheerlegs.

The term derrick is also applied to the framework supporting a drilling apparatus in an oil rig.

The derrick derives its name from a type of gallows named after Thomas Derrick, an Elizabethan era English executioner.

There are various types of derrick based on how the tower or mast is set up and the use of boom:

In an A-frame derrick, the tower is set up from two poles or masts with their bottom ends spread from each other and their top ends joint together. There are crossbars to connect the two poles, forming a letter "A" shape tower. The tower is ground anchored to provide support. A boom is hinged from the lowest cross bar and extends outward and upward. The top end of the boom is connected to a pulley system which is connected to the top of the tower. Manipulating the pulley system will allow the boom to change the angle against the tower. There is another pulley system to connect to the top of the boom for load lifting. When an A-frame derrick is used in a vessel, it is called floating A-frame derrick.

A basket derrick is a derrick without a boom that is supported by a rope-and-pole system that forms a basket. The basket is constructed from a group of poles to form a polygon. There are crossbars between the pole members to strengthen their support. The supporting ropes are tied to the top of the basket poles on one end and joined together on the other end at a lower elevation than the top of the basket poles to form the base for the derrick tower. The derrick tower can be a mast or a post with the bottom hinged at the base where all ropes meet. The top of the tower is secured with multiple reeved guys to position the top of the tower to the desired location by varying the length of the upper guy lines. The load is lifted using a pulley system connected to the top of the tower.

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