Definition
Kelly — A kelly is a square or hexagonal length of pipe at the top of the drill string on a rotary-table rig that transmits rotation from the rotary table to the string while allowing it to slide down as the hole deepens.
Also known as: kelly, kelly joint, square kelly, hexagonal kelly
What a kelly is
A kelly is the drive member at the top of the drill string on a rotary-table rig. It has a square or hexagonal outer cross-section that fits through matching bushings in the rotary table; as the table turns, it turns the kelly and therefore the whole string, yet the flat sides let the kelly slide down through the bushings as drilling deepens the hole.
The kelly connects at the top to the swivel (which supplies drilling fluid and hangs from the hook) and at the bottom to the drill string. On modern rigs the top drive has largely replaced the kelly, but kelly systems remain in wide use.
Associated equipment
- Kelly cock — a valve above and/or below the kelly to shut in drill-string pressure.
- Kelly saver sub — a sacrificial sub below the kelly that takes make-up wear instead of the kelly threads.
- Kelly bushing and drive bushing — transmit torque from the rotary table to the kelly.
Renting kelly equipment
Kelly systems and their subs, cocks and bushings are readily rented for rotary-table operations. Explore kelly equipment or contact us to outfit your rotary drive.