The drawworks is one of the largest and heaviest piece of equipment on a rig. It has a spool-shaped revolving drum around which crew members wrap the wire rope they call drilling line. It also has several shafts, clutches, brakes, and chain-and-gear drives. The shafts, clutches, and drives allow the driller to engage and disengage equipment on the drawworks. The driller can also change the speed with which the drum revolves, thereby varying the speed with which the drawworks raises the traveling block and hook. The driller controls the drawworks from a panel, or console, near the drawworks.
The drawworks also has a heavy-duty main brake. Large bands on both rims of the drum stop the drum from turning when the driller engages the brake. When the brake is disengaged, the drawworks drum lets out drilling line to lower the traveling block. An auxiliaryhydraulic or electric brake assists the main brake when the draw-works is raising or lowering heavy loads. The auxiliary brake absorbs some of the momentumcreated by a heavy load. The main brake
thereby works more efficiently.
Fig. 4. Hoisting system
A catheadis a winch, or windlass, on which a line, such as rope, cable, or chain, is coiled. When activated, the cathead reels in the line with great force. Pulling on a rope, cable, or chain is vital to screwing and unscrewing (making up and breaking out) drill pipe.
A friction cathead is a steel spool a foot (30 centimetres) or so in diameter. It revolves as the catshaft revolves. Crew members used to employ friction catheads and a length ofcatline—a large-diameter rope made out of plant fiber, or hemp—to move heavy equipment around the rig floor. One floorhand rigged up one end of the catline to the object they wished to move. Another wrapped the other end of the catline a couple of times around the cathead. (Rig hands called this operation "taking wraps around the cathead.") This second crew member gripped the line near the turning cathead and, by pulling hard or not so hard on the line, adjusted the amount of friction the cathead applied to the line. When the crew member tightened the line—applied more friction—on the cathead, the cathead pulled on the catline and lifted the object. When the crew member loosened the catline wraps on the cathead— released the friction—the cathead stopped pulling on the catline.
Anautomatic cathead also pulls on a wire rope or, in some cases, on a chain, but in a way very different from a friction cathead. Instead of having to manually adjust tension on a line, the driller simply moves a control lever on the console to engage or disengage an automatic cathead. The automatic cathead on the driller's side of the drawworks is the makeup cathead. The automatic cathead on the opposite side of the drawworks is the breakout cathead, which looks exactly like the makeup cathead.
Drilling line is wireline, or wire rope. Drilling line is, however, considerably larger than the wire rope on the tongs. Wire rope is what most of the world calls cable. Wire rope manufacturers make drilling line by braiding several steel wires together. It looks like cloth, or fiber, rope except that it is made from steel wire rather than plant or plastic fibers. Drilling line ranges in diameter from ¾ to 2 inches, or about 22 to 51 millimetres.
First, assume that the derrick is lying horizontally in its cradle in the substructure. The drawworks is on the rig floor and the rig's engines are running. To begin, workers take one end of the rope off the supply reel, which is resting on the ground near the substructure. They pull the line from the reel to the top of the derrick. There the rig builders installed a large set of pulleys, or sheaves, termed the crown block. Crown blocks have several sheaves over which workers string the drilling line. They thread, or reeve, the drilling line over a groove in a crown block sheave. Then they pull the end of the line to another set of sheaves placed near the middle of the derrick. This sheave set is the traveling block.
The driller raises and lowers the traveling block in the derrick as drilling progresses. During string-up, however, the traveling block is also stationary until workers complete the string-up job. To continue stringing up, they reeve the end of the drilling line through one of the traveling block sheaves. Then they pull the line back to the crown block. They reeve the line over another sheave in the crown block and pull it back to the traveling block. There, they reeve the line through another traveling block sheave and pull it back to the crown.
The number of times the workers reeve the line through the blocks depends on the weight the line has to support and on the size of the crown and traveling blocks. Crown blocks and traveling blocks vary in the size and the number of sheaves they contain.
After the workers reeve the line for the last time over the crown block sheaves, they pull the end of the line to the drawworks. They secure the end of the drilling line to the drum in the drawworks. A worker then powers up the drawworks and takes several wraps of line around the drum, much as an angler reels in fishing line after a cast. Since the traveling block is resting on a support that holds it stationary, the line runs through it without moving it. The part of the drilling line running from the drawworks to the crown block is the fast line. It moves when the rig is operating. That is, after the drilling crew readies the rig, the fast line moves on or off the drawworks drum when the driller raises or lowers the traveling block.
Fig. 5. Several sheaves make up the crown block.
Fig. 6.Travelling block
During the drilling of a well, the drilling line carries many tons of drill pipe and other tools in and out of the hole over a distance of several miles; or it uses many newtons of force moving drill pipe over a distance of thousands of metres. The crew therefore rates drilling line use in ton-miles or megajoules. (When a line has moved 1,000 newtons of load over a distance of 1,000 metres, the line has given i megajoule of service.) The driller keeps careful track of how many ton-miles (megajoules) of wear occur over time. By consulting specially prepared tables, the driller knows when it is time to slipthe line. Slipping the line places unworn line on the wear points where the line goes through and over sheaves in the traveling and crown blocks. The drilling line also wears where it spools off the drawworks drum.
(Baker R. “A Primer of Oil Well Drilling”, 2001, Austin, Texas)