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Applied Drilling Engineering

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leg load. 
For the usual drilling line arrangement shown in Fig. 
( n+4) Fde= -n- W. .................... (1.9) 
A parameter sometimes used to evaluate various 
drilling line arrangements is the derrick efficiency 
factor, defined as the ratio of the actual derrick load 
to the maximum equivalent load. For a maximum 
equivalent load given by Eq. 1.9, the derrick ef-
ficiency factor is 
For the block and tackle efficiency values given in 
Table 1.2, the derrick efficiency increases with the 
number of lines strung between the crown block and 
traveling block. 
The drilling line is subject to rather severe service 
during normal tripping operations. Failure of the 
drilling line can result in (1) injury to the drilling 
personnel, (2) damage to the rig, and (3) loss of the 
drillstring in the hole. Thus, it is important to keep 
drilling line tension well below the nominal breaking 
strength and to keep the drilling line in good con-
dition. The nominal breaking strength (new) for one 
type of wire rope commonly used for drilling line is 
shown in Table 1.4 for various rope diameters. The 
correct method for measuring wire rope diameter is 
illustrated in Fig. 1.18. 
Drilling line does not tend to wear uniformly over 
its length. The most severe wear occurs at the pickup 
points in the sheaves and at the lap points on the drum 
of the drawworks. The pickup points are the points in 
the drilling line that are on the top of the crown block 
sheaves or the bottom of the traveling block sheaves 
when the weight of the drillstring is lifted from its sup-
ports in the rotary table during tripping operations. The 
rapid acceleration of the heavy drillstring causes the 
most severe stress at these points. The lap points are 
the points in the drilling line where a new layer or lap 
of wire begins on the drum of the drawworks. 
Drilling line is maintained in good condition by 
following a scheduled slip-and-cut program. Slipping 
the drilling line involves loosening the dead line 
anchor and placing a few feet of new line in service 
from the storage reel. Cutting the drilling line in-
volves removing the line from the drum of the 
drawworks and cutting off a section of line from the 
end. Slipping the line changes the pickup points, and 
cutting the line changes the lap points. The line is 
sometimes slipped several times before it is cut. Care 
must be taken not to slip the line a multiple of the 
distance between pickup points. Otherwise, points of 
maximum wear are just shifted from one sheave to 
the next. Likewise, care must be taken when cutting 
the line not to cut a section equal in length to a 
multiple of the distance between lap points. 
API 18 has adopted a slip-and-cut program for 
drilling lines. The parameter adopted to evaluate the 
amount of line service is the ton-mile. A drilling 
line is said to have rendered one ton-mile of service 
when the traveling block has moved 1 U.S. toll. a 
distance of 1 mile. Note that for simplicity this 
parameter is independent of the number of lines 
strung. Ton-mile records must be maintained in 
order to employ a satisfactory slip-and-cut program. 
Devices that automatically accumulate the ton-miles 
of service are available. The number of ton-miles 
between cutoffs will vary with drilling conditions and 
drilling line diameter and must be determined 
through field experience. In hard rock drilling, 
vibrational problems may cause more rapid line wear 
than when the rock types are relatively soft. Typical 
ton-miles between cutoff usually range from about 
500 for 1-in.-diameter drilling line to about 2,000 for 
1.375-in.-diameter drilling line. 
Example 1 .2. A rig must hoist a load of 300,000 
lbf. The drawworks can provide an input power to 
the block and tackle system as high as 500 hp. Eight 
lines are strung between the crown block and 
traveling block. Calculate (1) the static tension in the 
fast line when upward motion is impending, (2) the 
maximum hook horsepower available, (3) the 
maximum hoisting speed, (4) the actual derrick load, 
(5) the maximum equivalent derrick load, and (6) the 
derrick efficiency factor. Assume that the rig floor is 
arranged as shown in Fig. 1.17. 
1. The power efficiency for n = 8 is given as 0.841 
in Table 1.2. The tension in the fast line is given by 
Eq. 1.7. 
w 300,000 
Ff= En = 0.841 (8) =44,590 lbf. 
2. The maximum hook horsepower available is 
Ph =E·pi =0.841 (500) =420.5 hp. 
3. The maximum hoisting speed is given by 
33,000 ft-lbf/min) 420.5 hp 
ph hp 
vb = W = ------------
300,000 lbf 
= 46.3 ft/min. 
To pull a 90-ft stand would require 
t= =1.9min. 
46.3 ft/min 
4. The actual derrick load is given by Eq. 1.8b: 
( 1 +E+En) Fd= W En 
- ( 1 +0.841 +0.841(8)) 
- 0.841(8) (300,000) 
= 382,090 lbf. 
5. The maximum equivalent load is given by Eq. 
n+4) 8+4 F de = -n- W = -
- (300,000) = 450,000 lbf. 
6. The derrick efficiency factor is 
Fd 382,090 
Ed=- = =0.849 or 84.907o. 
Fde 450,000 
1.4.3 Drawworks. The drawworks (Fig. 1.19) pro-
vide the hoisting and braking power required to raise 
or lower the heavy strings of pipe. The principal parts 
of the drawworks are (1) the drum, (2) the brakes, (3) 
the transmission, and (4) the catheads. The drum 
transmits the torque required for hoisting or braking. 
It also stores the drilling line required to move the 
traveling block the length ofthe derrick. 
The brakes must have the capacity to stop and 
sustain the great weights imposed when lowering a 
string of pipe into the hole. Auxiliary brakes are used 
to help dissipate the large amount of heat generated 
during braking. Two types of auxiliary brakes 
commonly used are (1) the hydrodynamic type and 
(2) the electromagnetic type. For the hydrodynamic 
type, braking is provided by water being impelled in a 
direction opposite to the rotation of the drum. In the 
electromagnetic type, electrical braking is provided 
by two opposing magnetic fields. The magnitude of 
the magnetic fields is dependent on the speed of 
rotation and the amount of external excitation 
current supplied. In both types, the heat developed 
must be dissipated by a liquid cooling system. 
The drawworks transmission provides a means for 
easily changing the direction and speed of the 
traveling block. Power also must be transmitted to 
catheads attached to both ends of the drawworks. 
~ ;:; 
Fig. 1.19- Example drawworks used in rotary drilling. 
Fig. 1.20- Friction-type cathead.12 
Fig. 1.21- Tongs powered by chain to cathead. 
Friction catheads shown in Fig. 1.20 turn con-
tinuously 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 un-
screw 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. 
1.5 Circulating System 
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