STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Hoisting System 529
Transmission and Clutch. The transmission in the drawworks generally has
six to eight speeds. Large rigs can have more gears in the drawworks transmission.
More gearing capacity is available when the compound is used. This transmission
uses a combination of sprockets and roller chain drives and gears to accomplish
the change of speeds and torque from the prime movers (via the compound).
The clutches used in the transmitting of prime mover power to the drawworks
are jaw-type positive clutches and friction-type clutches. In modern drawworks,
nearly all clutches are pneumatically operated from the driller’s console. The
driller’s console also controls the shifting of gears within the drawworks.
Torque converters used in most drawworks are designed to absorb shocks from
the prime movers or the driven equipment and to multiply the input torque.
Torque converters are used in conjunction with internal combustion prime
movers when these engines are used directly to drive the drawworks. More
modern drawworks are driven by electric drives since such prime movers usually
simplify the drawworks.
Brakes. The brake systems of the drawworks are used to slow and stop the
movement of the large weights that are being lowered into the borehole. The
brake system will be in continuous use when a round trip is made. The principal
brake of the drawworks is the friction-type mechanical brake system. But when
this brake system is in continuous use, it would generate a great deal of heat.
Therefore, an auxiliary brake system is used to slow the lowering speeds before
the friction-type mechanical brake system is employed to stop the lowering
motion. Hydraulic brake system and electromagnetic brake system are the basic
types of auxiliary brake systems in use. The hydraulic brake system uses fluid
friction (much like a torque converter) to absorb power as equipment is lowered.
The electromagnetic brake system uses two opposed magnetic fields supplied
by external electrical current to control the speed of the hoisting drum. The auxiliary
brake system can only control the speed of lowering and cannot be used to
stop the lowering as does the mechanical friction-type brake system.
Catheads. The catheads are small rotating spools located on the sides of the
drawworks. The cathead is used as a power source to carry out routine operations
on the rig floor and in the vicinity of the rig. These operations include
making up and breaking out drill pipe and casing, pulling single joints of pipe
and casing from the pipe rack to the rig floor. The sand reel is part of this
mechanism. This small hoisting drum carries a light wire rope line (sand line)
through the crown to carry out pulling operations on the rig floor or in the
vicinity of the rig.
Power Rating
In general, the drawworks is rated by its input horsepower. But it used to be
rated by depth capability along with a specific size of drill pipe to which the
depth rating pertains. The drawworks horsepower input required HPi, for
hoisting operations is
wv h
HP, = 33,000ehe,
(4-18)
where W is the hook load in lb, vh is the hoisting velocity of the traveling block
in ft/min, e,, is the hook-to-drawworks efficiency, and e,,, is the mechanical
efficiency within the drawworks and coupling between the prime movers and
the drawworks (usually taken as about 0.85).