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string and the bit. Atop drive eliminates the need for a conventional swivel, a kelly,
a rotating master bushing, and a kelly drive bushing.
Rigs with a top drive, however, still need a rotary table with a master bushing
and bowl to provide a place to suspend the pipe on slips when the bit is not drilling.
Because the rotary table only serves as a place for crew members to place slips on
rigs with top drives, manufacturers make special rotary support tables for top-drive
rigs that are lighter and smaller than regular rotary tables. They are still rugged
enough, however, to support the weight of the drill string. Further, some have built-in
hydraulic motors that can rotate the turntable should the topdrive system
malfunction. Hydraulic motors are considerably lighter in weight than electric motors
and take up less space.
The main advantage of a top drive over a kelly-and- rotary-table system is that
a top drive makes it safer and easier for crew members to handle the pipe. Because of
the way in which a rig with a rotary-table system operates, the crew can add only one
joint of drill pipe at a time as the hole deepens. With a top-drive system, on the other
hand, because it operates differently from the conventional system, the crew can add
pipe three joints at a time, if they choose to do so. Adding three-joint stands of pipe
saves time.

Downhole Motors
In special situations, the rig may use a downhole motor to rotate the bit. Unlike a
rotary-table or a top-drive system, a downhole motor does not rotate the drill pipe.
Instead, it rotates only the bit. Drilling mud powers most downhole motors.
Normally, crew members install the motor in the drill string just above the bit.
To make a mud motor rotate the bit, the driller pumps drilling mud down the
drill string as usual. When the mud enters the motor, however, it strikes a spiral
shaft, which goes inside a tubular housing. The shaft and housing fit in such a way
that mud pressure causes the shaft to turn. Because the bit is attached to the motor
shaft, the shaft turns the bit. The mud exits out of the bit as usual.
Rigs often use downhole motors to drill directional holes. A directional hole is
a hole that is intentionally drilled off- vertical. Sometimes, it is desirable to drill a

hole on a slant because a vertical hole could not reach a desired part of a petroleum
reservoir. Because it is easier to get the bit to drill in the desired direction if the drill
string does not rotate, rigs employ downhole motors. One major instance of
directional drilling is drilling horizontal holes. A horizontal hole drilled through a
formation can, under the right conditions, allow a company to produce the formation
much better than with a vertical hole.

The Drill String
The drill string consists of drill pipe and special, heavy-walled pipe called “drill
collars”. Manufacturers make most drill pipe from steel, but they also use aluminum.

Drill collars, like drill pipe, are metal tubes through which the driller pumps drilling
fluid. They are heavier than drill pipe, however. Drill collars are heavy because they
are used in the bottom part of the string to put weight on the bit. This weight presses

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