The kelly is a long, four- or six-sided steel bar with a fluid path drilled through its middle for the passage of drilling fluid. The kelly transfers rotating motion to the drill string from the rotary table or kelly bushing and allows the drill string to be raised or lowered as it rotates,.
Trenchlesspedia Explains Kelly
The kelly provides a connection between the rotary table, also called the kelly bushing, to the drill string that allows the drill string to turn around its longitudinal axis and move up and down along its length. The hole in the kelly is matched by a hole in the kelly bushing that allows for the passage of drilling fluid into the bore. A kelly is used in fluid or oil assisted bore projects.