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Applied Drilling Engineering

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or jourbles, respectively. 
In addition to their height, derricks are rated 
according to their ability to withstand compressive 
loads and wind loads. Allowable wind loads usually 
are specified both with the drillstring in the hole and 
with the drillstring standing in sections in the derrick. 
When the drillstring is standing in the derrick resting 
against the pipe-racking platform, an overturning 
moment is applied to the derrick at that point. Wind 
ratings must be computed assuming wind loading is 
in the same direction as this overturning moment. An-
chored guy wires attached to each leg of the derrick 
are used to increase the wind rating of small portable 
masts. The American Petroleum Institute (API) has 
published standards dealing with derrick specifications 
and ratings. '-3 
To provide working space below the derrick floor 
for pressure control valves called blowout preventers, 
the derrick usually is elevated above the ground level 
by placement on a substructure. The substructure 
must support not only the derrick with its load but 
also the weight of other large pieces of equipment. 
API Bull. D 104 recommends rating substructure 
load-supporting capacity according to (1) the 
maximum pipe weight that can ·be set back in the 
derrick, (2) the maximum pipe weight that can be 
suspended in the rotary table (irrespective of setback 
load), and (3) the corner loading capacity (maximum 
supportable load at each corner). Also, in API 
Standard 4A, 1 three substructure types have been 
adopted. In addition, many non-API designs are 
available. The choice of design usually is governed by 
blowout preventer height and local soil conditions. 
1.4.2 Block and Tackle. The block and tackle is 
comprised of (1) the crown block, (2) the traveling 
block, and (3) the drilling line. The arrangement and 
nomenclature of the block and tackle used on rotary 
rigs are shown in Fig. 1.16a. The principal function 
of the block and tackle is to provide a mechanical 
advantage, which permits easier handling of large 
loads. The mechanical advantage M of a block and 
tackle is simply the load supported by the traveling 
block, W, divided by the load imposed on the 
drawworks, Fr: 
w M=-F/ ........................... (1.4) 
The load imposed on the drawworks is the tension in 
the fast line. 
The ideal mechanical advantage, which assumes no 
friction in the block and tackle, can be determined 
from a force analysis of the traveling block. Consider 
the free body diagram of the traveling block as 
shown in Fig. 1.16b. If there is no friction in the 
pulleys, the tension in the drilling line is constant 
throughout. Thus, a force balance in the vertical 
direction yields 
nF1 =W, 
where n is the number of lines strung through the 
• 
ROTARY DRILLING PROCESS 
TABLE 1.2- AVERAGE EFFICIENCY FACTORS 
FOR BLOCK·AND·TACKLE SYSTEM 
Number of Lines 
(n) 
6 
8 
10 
12 
14 
Efficiency 
(E) 
0.874 
0.841 
0.810 
0.770 
0.740 
traveling block. Solving this relationship for the 
tension in the fast line and substituting the resulting 
expression in Eq. 1.4 yields 
w 
Mi= -- =n, 
Win 
which indicates that the ideal mechanical advantage 
is equal to the number of lines strung between the 
crown block and traveling block. Eight lines are 
shown between the crown block and traveling block 
in Fig. 1.16. The use of 6, 8, 10, or 12 lines is com-
mon, depending on the loading condition. 
The input power Pi of the block and tackle is equal 
to the drawworks load F1 times the velocity of the fast line, v1: 
pi =Ffvf. . ......................... (1.5) 
The output power, or hook power, Ph is equal to the 
traveling block load W times the velocity of the 
traveling block, v b: 
Ph= Wvb . .......................... (1.6) 
For a frictionless block and tackle, W = nFf. Also, 
since the movement of the fast line by a unit distance 
tends to shorten each of the lines strung between the 
crown block and traveling block by only 11 n times 
the unit distance, then v b ~ v11n. Thus, a fricti~nless 
system implies that the ratiO of output power to mput 
power is unity: 
E= ph = (nF1 ) (v11n) = 1. 
Pi Ffvf 
Of course, in an actual system, there is always a 
power loss due to friction. Approximate values of 
block and tackle efficiency for roller-bearing sheaves 
are shown in Table 1.2. 
Knowledge of the block and tackle efficiency 
permits calculation of the actual tension i~ !he fa~t 
line for a given load. Since the power efficiency 1s 
given by 
~ Derrick ----l;l Leg L_j 
'----oead Line 
/Lines to 
/ Block 
•••• 
•••• 
Line 
Fig. 1.17-Projection of drilling lines on rig floor. 
then the tension in the fast line is 
w 
9 
F1=-. . ......................... (1.7) En 
Eq. l. 7 can be used to select drilling line size. 
However, a safety factor should be used to allow for 
line wear and shock loading conditions. 
The line arrangement used on the block and tackle 
causes the load imposed on the derrick to be greater 
than the hook load. As shown in Fig. 1.16c, the load 
Fd applied to the derrick is.the sum of the hoo~ lo~d 
W, the tension in the dead hne, Fs, and the tensiOn m 
the fast line, Ff: 
Fd= W+FJ+Fs. . .................. (1.8a) 
If the load, W, is being hoisted by pulling on the fast 
line, the friction in the sheaves is resisting the motion 
of the fast line and the tension in the drilling line 
increases from Win at the first sheave (deadline) to 
WI En at the last sheave (fast line). Substituting these 
values for F1 and Fs in Eq. 1.8a gives 
W W (1+E+En) Fd= W+- +- = W . .. (1.8b) 
En n En 
The total derrick load is not distributed equally over 
all four derrick legs. Since the drawworks is located 
on one side of the derrick floor, the tension in the 
fast line is distributed over only two of the four 
derrick legs. Also, the dead line affects only the leg to 
which it is attached. The drilling lines usually are 
arranged as in the plan view of the rig floor shown in 
Fig. 1.17. For this arrangement, derrick Legs C and 
D would share the load imposed by the tension in the 
fast line and Leg A would assume the full load im-
posed by the tension in the dead line. The load 
10 APPLIED DRILLING ENGINEERING 
TABLE 1.3- EXAMPLE CALCULATION OF DERRICK LEG LOAD 
Load Source Total Load Leg A 
hook load w W/4 
fast line W!En 
dead line W;n W!n 
W(n +4)/(4n) 
TABLE 1.4- NOMINAL BREAKING STRENGTH OF 6 x 19* 
CLASSIFICATION WIRE ROPE, BRIGHT(UNCOATED) 
OR DRAWN-GALVANIZED WIRE, INDEPENDENT 
WIRE· ROPE CORE (IWRCf 
Nominal 
Diameter 
(in.) 
1/2 
9/16 
5/8 
3/4 
7/8 
1 
1 1/8 
1 1/4 
1 3/8 
1 1/2 
1 5/8 
1 3/4 
1 7/8 
2 
Approximate 
Mass 
(lbm/ft) 
0.46 
0.59 
0.72 
1.04 
1.42 
1.85 
2.34 
2.89 
3.50 
4.16 
4.88 
5.67 
6.50 
7.39 
Nominal Strength 
Improved 
Plow Steel 
(lbf) 
23,000 
29,000 
35,800 
51,200 
69,200 
89,800 
113,000 
138,800 
167,000 
197,800 
230,000 
266,000 
304,000 
344,000 
Extra Improved 
Plow Steel 
(lbf) 
26,600 
33,600 
41,200 
58,800 
79,600 
103,400 
130,000 
159,800 
192,000 
228,000 
264,000 
306,000 
348,000 
396,000 
"Six strands having 19 wires per strand. 
Correct way to measure the 
diameter of wire rope. 
Incorrect way to measure the 
diameter of wire rope. 
Fig. 1.18- Measurement of wire rope diameter.7 
Load on Each Derrick Leg 
Leg B Leg C Leg D 
W/4 W/4 W/4 
W!2En W!2En 
W/4 W (En+ 2)14En W(En + 2)14En 
distribution for each leg has been calculated in Table 
1.3. 
Note that for £"?::.0.5, the load on Leg A is greater 
than the load on the other three legs. Since if any leg 
fails, the entire derrick also fails, it is convenient to 
define a maximum equivalent derrick load, F de, 
which is equal to four times the maximum