Draw-works, also called drawworks, are hoisting equipment used specifically in the earth drilling industry. The main function of a draw-works is to raise and lower drilling equipment up and down the bore hole. Drilling gear includes a drill bit, drill pipe or stem, drilling lines and other specialty components. Gravity powers the reeling out of the drilling line, and an electric motor or engine powers the reeling in. Some draw-works systems access the power from the motors (referred to as prime movers) through the transmission to drive the rotary table on the rig.
Numerous configurations of power sources, transmission, brakes, hoisting drum and block assembly allow the rig designer to outfit a draw-works to closely match the projected drilling conditions.
On modern drilling rigs, the power source or prime mover consists of one to four or more diesel engines producing up to several thousand horsepower. Usually, the diesel engines are connected to electric generators. The generated electrical power is then sent to AC or DC motors that operate the draw-works. Strictly mechanical systems transmit power directly from the engines through a system of mechanical belts, chains and clutches. Variable Frequency Drive (VFD) AC motors can operate up to 3000 rpm and eliminate the need for a multi-speed transmission. An advantage of an AC driven draw-works is the internal braking capability of the AC motor. DC motors are limited to roughly 1200 rpm and require shifting transmissions to achieve the correct speed for different conditions.
A gearbox transfers energy from the motor(s) to the winch or spool. Larger rigs may have more gears in the draw-works transmissions. Torque converters used in draw-works are intended to absorb shocks in the system 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 draw-works.
Brakes control the speed of the drill line while it is being reeled out or lowered. They also stop and hold the line. The braking system is in constant use during a round trip up and down the bore. The main brake of a draw-works is a mechanical friction brake. Since using friction brakes for extended periods will create excessive heat, an auxiliary brake is often used to slow and control movement allowing the mechanical brake to stop and hold. Auxiliary bakes can be hydrodynamic or electromagnetic. Regenerative electromagnetic braking is sometimes present to harness and store energy during the descent of the drilling equipment.
Also known as a hoisting drum, a spool is often a grooved cylinder with wire rope drilling line carefully wound with a spooling or reeling mechanism. As a key component of the draw-works, drum design is an important consideration; from the standpoint of power requirements for hoisting, the ideal drum would have a small a diameter and as great a width as great as possible. Consequently, hoisting drum designs are optimized around these measurements.
Drilling line is the wire rope that reels in and out while the drilling assembly is lowered and raised. It withstands the tension created by the weight of the drill assembly. Drilling line is made by braiding together several strands of high-strength steel. From the drum, the line is run through the pulleys in the crown block and traveler block. This threading process is known as “reeving”.
Crown Block and Traveling Block
This configuration is known more generally as a block and tackle. The crown block sits on the top of the mast or derrick. The traveling block has a drilling hook attached to the bottom and moves up and down with the drilling assembly. This multi-looped block and tackle layout gives mechanical advantage to the system and substantial savings in size and energy usage of the motor(s) powering the draw-works.
Catheads are small rotating spools located on the sides of the draw-works. They are used as a power source to perform regular operations on 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, and operating a sand reel. A sand reel is a small hoisting drum that carries a light wire rope line (sand line) through the crown to carry out pulling work in the vicinity of the rig.