In drilling, the standard derrick has 4 legs standing at the corners of the substructure and reaching to the crown block. The substructure is an assembly of heavy beams used to elevate the derrick and provide space to install blowout preventers, casingheads, and so forth.
Derricks are usually pyramidal in shape, and offer a good strength-to-weight ratio. If the derrick design does not allow it to be moved easily in one piece, special ironworkers must assemble them piece by piece, and in some cases disassemble them if they are to be moved.
The classic oil derrick pump is known colloquially as a sucker rod pump, named for the plunger-like mechanics it uses to pump oil from underground wells up to the surface. It uses a series of gears and cranks to pump a polished rod up and down an oil well in a piston-like motion, albeit much slower.
The Derrickman or Derrickhand is the person who sits atop the Derrick on a drilling rig. The primary responsibilities include assisting with operations to lift and sink casing and drill pipe, as well as drill collars. Their job is to guide the stands of the drill pipe into the fingers at the top of the Derrick.
Other duties might include monitoring pH and calcium levels, viscosity and the mud weight (density), adding chemicals and oil based fluids, and being responsible for the shale shakers and mud pump.
Some modern drilling rigs have automated pipe-handling equipment such that the Derrickman controls the machinery rather than physically handling the pipe. In an emergency, the derrickman can quickly reach the ground by an escape line often called the Geronimo line.