Friction catheads shown in Fig. 1.20 turn continuously and can be used to assist in lifting or moving equipment on the rig floor. The number of turns of rope on the drum and the tension provided by the operator controls the force of the pull. A second type of cathead generally located between the drawworks housing and the friction cathead can be used to provide the torque needed to screw or unscrew sections of pipe. Fig. 1.21 shows a joint of drillpipe being tightened with tongs powered by a chain from the cathead. Hydraulically or air-powered spinning and torquing devices also are available as alternatives to the conventional tongs. One type of power tong is shown in Fig. 1.22.
A major function of the fluid-circulating system is to remove the rock cuttings from the hole as drilling progresses. A schematic diagram illustrating a typical rig circulating system is shown in Fig. 1.23. The drilling fluid is most commonly a suspension of clay and other materials in water and is called drilling mud. The drilling mud travels (1) from the steel tanks to the mud pump, (2) from the pump through the high-pressure surface connections to the drillstring, (3) through the drillstring to the bit, (4) through the nozzles of the bit and up the annular space between the drillstring and hole to the surface, and (5) through the contaminant-removal equipment back to the suction tank.
The principal components of the rig circulating system include (1) mud pumps, (2) mud pits, (3) mud-mixing equipment, and (4) contaminant-removal equipment. With the exception of several experimental types, mud pumps always have used reciprocating positive-displacement pistons. Both two-cylinder (duplex) and three-cylinder (triplex) pumps are common. The duplex pumps generally are double-acting pumps that pump on both forward and backward piston strokes. The triplex pumps generally are single-acting pumps that pump only on forward piston strokes. Triplex pumps are lighter and more compact than duplex pumps, their output pressure pulsations are not as great, and they are cheaper to operate. For these reasons, the majority of new pumps being placed into operation are of the triplex design.
The advantages of the reciprocating positive-displacement pump are the (1) ability to move high-solids-content fluids laden with abrasives, (2) ability to pump large particles, (3) ease of operation and maintenance, (4) reliability, and (5) ability to operate over a wide range of pressures and flow rates by changing the diameters of the pump liners (compression cylinders) and pistons. Example duplex and triplex mud pumps are shown in Fig. 1.24.
The overall efficiency of a mud-circulating pump is the product of the mechanical efficiency and the volumetric efficiency. Mechanical efficiency usually is assumed to be 90% and is related to the efficiency of the .prime mover itself and the linkage to the pump drive shaft. Volumetric efficiency of a pump whose suction is adequately charged can be as high as 100%. Most manufacturers' tables rate pumps using a mechanical efficiency, Em, of 90% and a volumetric efficiency, Ev, of 100%.
Generally, two circulating pumps are installed on the rig. For the large hole sizes used on the shallow portion of most wells, both pumps can be operated in parallel to deliver the large flow rates required. On the deeper portions of the well, only one pump is needed, and the second pump serves as a standby for use when pump maintenance is required.
A schematic diagram showing the valve arrangement and operation of a double-acting pump is shown in Fig. 1.25. The theoretical displacement from a double-acting pump is a function of the piston rod diameter dn the liner diameter dh and the stroke length Ls. On the forward stroke of each piston, the volume displaced is given by di2 Ls
Similarly, on the backward stroke of each piston, the volume displaced is given by
Thus, the total volume displaced per complete pump cycle by a pump having two cylinders is given by
where Ev is the volumetric efficiency of the pump. The pump displacement per cycle, Fp, is commonly called the pump factor.
For the single-acting (triplex) pump, the volume displaced by each piston during one complete pump cycle is given by ir ,
Thus, the pump factor for a single-acting pump having three cylinders becomes
The flow rate q of the pump is obtained by
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