Drill string on a drilling rig is the assembly of the drill bit, drill collars, and other drill stem subs which make up the "bottom hole assembly" (BHA), columns of drill pipe, and various drilling tools. The drill string is used to carry drilling fluid and apply torque to the drill bit. Drilling fluid is pumped down the drill string via mud pumps. Torque is either applied to the drill string by the kelly drive or top drive.
Drilling fluid is pumped down the drill string to help cool the drill bit and soften the ground that is being bored through. This increases the overall lifespan of the bit. The drilling fluid and drill cuttings are circulated back up the annulus where the drilling fluid is recovered and drill cuttings are removed.
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