Rig Glossary: A–C



Two large support legs that when standing they form a steel A-frame structure that supports the derrick when it is standing. They are also the support frames for the drilling line when raising or lowering the derrick.


A storage device that delivers hydraulic fluid under pressure from nitrogen to operate the blowout preventers.


Used in surface mud systems to suspend solids and maintain homogeneous mixture throughout the system. A mechanical agitator is driven by an explosion-proof motor, coupled to a gear box that drives the impeller shaft. The impellers (turbines) transform mechanical power into fluid circulation or agitation. The objective is to obtain a uniform suspension of all solids.

Air Compressor

A machine that takes air, compresses it, and stores it in a tank so it can be distributed around the rig to various air driven equipment such as clutches and tuggers.

Air Hoist

A compressed air powered winch-like device, usually mounted on the rig floor, used to lift heavy objects or tubulars to the rig floor. It is also called a “tugger” or a “winch”.

Annular Blowout Preventer

A well control device, usually installed above the ram preventers, that is used to seal the annulus space between any shape of tubular and the well bore or, if no tubular is present, it will seal on an open wellbore.


The open downhole space between a tubular and the well bore wall. Also called the “annular space”.




Unexpected reverse motion of the tongs, left on a pipe or collar, during the pipe spinning operation.

Bale Eyes

The end of the bale bars that form an enclosed eye.


Long, heavy steel bars with an eye on each end. One end attaches to the ears of the travelling blocks and the other end attaches to the ears of the elevators.


Weighting material used to increase the density of a liquid drilling fluid system.


A steel structural member; usually an I-beam, angle iron, square, or round tube. Also called a “Girt”.


The storage area for the bottom end of a stand of tubulars when they are stored vertically in the derrick. The birdbath usually has rows of mounting pegs for each stand referred to as alligator teeth. A birdbath also captures the drilling fluid that drains out of the stand of tubulars.


A drill element used in drilling oil and gas wells that cuts or bores through the fromations. Can be either a tri-cone bit or a PDC bit.

Bit Breaker

A plate-like device the drill bit sits in; that fits in the rotary table bushings and holds the drill bit in place while the torque is broken from the bit connection and it is being unscrewed from the bit sub or drill collar.

Bit Sub

A crossover sub inserted between the drill collar and the bit. A bit sub is usually identified by the double box ends.

Blowout Preventer Stack (BOP Stack)

The stacked arrangement of well control equipment including annular preventers, ram preventers, diverter spools, values, and nipples connected to the top of the wellhead.


A closed vessel in which water or other fluid is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.

Bolt Pins

Bolts placed through a pin that prevents the pin from sliding out of the hole it is in.

Bottom Hole Assembly (BHA)

The portion of the drilling assembly below the drill pipe. It can be very simple, composed of only the bit and drill collars, or it can be very complex and made up of several specialty components like: Jars, Shock Subs, Reamers, Stabilizers, Cross-over subs.

Box End

The female end of a tool joint with internal threads inside.


The braking device on the drawworks or air hoist to suspend or stop a load being lifted or lowered. It is a device for arresting the rotational motion of a mechanism, usually by means of friction, as in the drawworks and air hoist brakes. Usually there are four types of braking systems on a rig: electric brakes, auxiliary brakes, hydromatic brakes and disc brakes.

Brake Band

The part of the brake mechanism consisting of a flexible steel band lined with a material that grips a drum when the bands are tightened. On drawworks, the brake band acts on the drum to control or stop the lowering of the traveling block and the drill string.


To unscrew a connection of two tubular components from each other, especially the drill string while it is being removed from the wellbore. During this operation, the tongs are used to break the torque from the connection and start the unscrewing motion.

Breakout Device System

A hydraulic or mechanical device that stands alone or is connected to the drawworks. It is used as a pulling power source for the tongs when making or breaking tubular connections.

Breakout Line

A chain or cable with one end attached to the end of the breakout tong arm and the other end attached to the breakout device or cylinder; used to apply a pulling force to the tongs when breaking out a connection.

Breakout Tong

Large wrench like tool suspended from the derrick on the mud tank side of the rotary table that is used to break the torque from the drill string connection; and acts as a backup gripping device as the connection is rotated out.

Bridle Line

A cable used to pull the derrick into the upright position. It is strung through the derrick and attached to the blocks.

Bumper Block

Timbers wrapped with wire mesh or other retaining medium located below the crown to act as a cushion and protect the crown sheaves in the event the travelling block is raised too far. Can also mean a bound set of timbers or a solid rubber block attached to the end of the catwalk to stop the travel of a tubular that is sliding down the
V-Door onto the catwalk.

Bushing Puller

A tool used to hook into each table bushings insert to remove or set them in the rotary table. Also called a “Hairpin”.


A set of split wedged inserts that fit into the rotary table that help force the slips to grip the tubular when the drill string is suspended in the rotary table.




A wire rope consisting of multiple strands of wire.

Cable Clamp

A fastening device to secure or hold together two ropes.

Cantilever Mast

A mast is assembled horizontally at ground level and pinned onto the substructure. Hydraulic cylinders or the draw works is used along with an A-frame support to raise the mast to working position.


Steel pipe placed in an oil or gas well to prevent the wall of the hole from caving in; to prevent movement of fluids from one formation to another; to prevent drilling fluid leaking into the formation and to aid in well control.

Casing Shoe

A short assembly, which is screwed to the bottom of a casing string. The rounded profile helps guide the casing string past any ledges or obstructions that would prevent the string from being correctly located in the wellbore.

Catch Samples

The process of gathering drilled formation cuttings coming over the shale shakers from the wellbore. These formation samples are taken to the geologist for analysis.

Cats Asses

A term to describe the look of a steel cable that becomes twisted and tangled.


The elevated work area at the bottom of the V-door ramp on a drilling rig where pipe is stored to be lifted to the rig floor by the tugger or by an air winch.


The dug out area in the ground under the substructure that provides additional height between the rig floor and the wellhead to accommodate the installation of stacked blowout preventer components. It also may collect drainage water and other fluids for subsequent disposal.

Centrifugal Pump

A pump with an impeller or rotor, an impeller shaft, and a casing, which discharges fluid by centrifugal force. An electric submersible pump used to pump out the cellar, pits or ditch is a centrifugal pump.


A component of solids removal equipment that removes fine and ultra-fine solids.

Chain Tong

A special hand tool that consists of a handle and chain that resembles the chain on a bicycle. The chain component wraps around and grips the tubular item and when force is applied to the handle, it delivers a rotating force to the tubular. This tool can be used to tighten or loosen connections.

Check Valve

A mechanical device that permits fluid to flow or pressure to act in one direction only.

Chemical Barrel

A barrel used mix chemicals in. It is located on the pits and usually caustic or lime is mixed in it.


A block or wedge placed under the edge of pipe to keep it from moving.


A flow restricting valve-like component that the opening adjusts to create a desired back pressure that transmits back down the wellbore to maintain an overbalance pressure against the formations.


Choke Line

A line, or pipe, through which fluid from the wellbore is flowed from the blowout preventer stack to the choke manifold.

Choke Manifold

The arrangement of piping and special valves, called chokes, through which drilling mud and the influx is circulated through when the blowout preventers are closed to control the pressures encountered during a kick.


A coupling used to connect and disconnect a driving and a driven part of a mechanism, especially a coupling that permits the former part to engage the latter gradually and without shock. In the oil field, a clutch permits gradual engaging and disengaging of the equipment driven by a prime mover.


1. A coupling device used to join two lengths of pipe, such as casing or tubing. A combination collar has left-hand threads in one end and right-hand threads in the other.
2. A drill collar.

Conductor Casing

Generally, the first string of casing in a well. A short, large bore piece of pipe lowered into a drilled wellbore near the surface and cemented in place; it may be driven into the ground by a special pile driver. Its purpose is to prevent the loose formations near the surface from caving in and to provide a flow path for drilling mud from the bottom of the well to the surface when drilling starts.


The action of adding a joint of pipe to the drill string as drilling progresses.

Connection Break

The area where the threaded ends of two drill string components are connected.


In piping, a metal collar with internal threads used to join two sections of external threaded pipe.

Crossover Sub

A sub that allows different sizes and types of drill string components to be joined. For example: drill pipe to drill collars.

Crown Block

An assembly of sheaves or pulleys mounted on beams at the top of the derrick. The drilling line is run over the sheaves down to the hoisting drum.

Crown Saver

A device mounted near the drawworks drum to keep the driller from inadvertently raising the traveling block into the crown block. A probe senses when too much line has been pulled onto the drum, indicating that the traveling block may strike the crown. The probe activates a switch that simultaneously disconnects the drawworks from its power source and engages the drawworks brake.


The fragments of rock dislodged by the bit and transported to the surface in the drilling mud. Washed and dried cuttings samples are analyzed by geologists to obtain information about the formations drilled.

Courtesy of Ensign Drilling Inc.