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up) the drill string, the kelly drive bushing slides along the kelly as the hoisting
system raises the kelly. When the drive bushing reaches the bottom of the kelly, the
kelly tool joint, being bigger than the bushing's opening, keeps the bushing from
sliding off the kelly.
In general, a hexagonal kelly is stronger than a square kelly. Consequently,
contractors tend to use hexagonal kellys on large rigs to drill deep wells because of
their extra strength. Small rigs often use square kellys because they areless expensive.
Manufacturers make most square and hexagonal kellys to American Petroleum
Institute (API) specifications. The API is a trade association that sets oilfield
standards and specifications. A standard API kelly, either square or hexagonal, is 40
feet (12.2 metres) long, although an optional length of 54 feet
(16.5 metres) is also available. Most rigs use 40-foot kellys.

Swivel

The fifth principal part of a rotary-table system is the swivel. The swivel
interfaces the rotary system with the hoisting system. A heavy-duty bail, similar to
the bail, or handle, on a water bucket but much larger, fits into a big hook on the
bottom of the traveling block. The hook suspends the swivel and attached drill string.
Crew members make up the top of the kelly to the swivel. The kelly screws onto a
threaded fitting – the stem, – that comes out of the swivel. This stem rotates with the
kelly, the drill string, and the bit. At the same time, drilling mud flows through the
stem and into the kelly and drill string.
Near the top and on one side of the swivel is a gooseneck. The gooseneck is a
curved, erosion-resistant piece of pipe. It conducts drilling mud under high pressure
into the swivel stem. A special hose – the rotary, or kelly, hose – attaches to the
gooseneck. The rotary hose conducts drilling mud from the pump to the swivel.
To summarize the kelly-and-rotary-table system: (a) the turntable in the rotary
table rotates the master bushing; (b) the master bushing rotates the kelly drive
bushing; (c) the kelly drive bushing rotates the kelly; (d) the kelly rotates the attached
pipe and bit; and (e) the swivel suspends the pipe, allows it to rotate, and has a

passage for drilling mud to enter the kelly and pipe.

Top Drives
While many rigs use the kelly-and-rotary-table system to rotate the drill
string and bit, a large number use a different system. This system does away with
the kelly and thus the kelly drive bushing and a rotating master bushing. Instead, a
top drive, also called a “power swivel,” rotates the drill string and bit.
Like a regular swivel, a top drive hangs from the rig’s large hook and it has a
passageway for drilling mud to get into the drill pipe. However, a top drive comes
equipped with a heavy-duty electric motor (some large top drives have two motors).
Drillers operate the top drive from their control console on the rig floor.The
motor turns a threaded drive shaft. The crew stabs (inserts) the drive shaft into the
top of the drill string. When the driller starts the top drive’s motor, it rotates the drill

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