Page 111 - Air and Gas Drilling Manual

determine the maximum allowable axial tension force for the drill pipe. Thus,
Equation 3-3 is
F = 0 9. (595 004, ) Chapter 3: Downhole Equipment 3-39
a
F = 535 504, lb
a
The factor of safety is determined from Equation 3-4. Equation 3-4 gives
,
FS = 535 504
226 890
,
.
FS = 236
Most land double and triple rotary drilling rigs do not have lifting capacities as
high as 535,504 lb. Assuming a rotary drilling rig is selected with a maximum
hoisting capacity of 300,000 lb, then Equation 3-5 can be used to determine the
MOP. Equation 3-5 gives

MOP = 300 000 − 226 890
,
,
MOP = 73 110 lb
,
From the calculations above, the additional 73,110 lb (above the weight of the drill
string) or, a total axial tension force of 300,000 lb can be safely pulled by the rig
hoisting system with the assurance that the drill string will not structurally fail.

Illustrative Example 3.2 The George E. Failing Company Star 30K self-
propelled drilling rig (shown in Figure 1-3) is to be used to air drill a shallow well
with dual wall pipe (reverse circulation). The well is to be vertical with a depth of
820 ft and will use 5.50 inch concealed inner-tube pipe (see Table 3.6). The drilling
operation will drill a borehole having a 6 1/4 inch diameter. The hoisting capacity
of this drill rig is 30,000 lb. Determine the factor of safety for this drill string and
the MOP for this drilling operation.
The 5.50 inch dual wall pipe has a weight per unit length of 27.4 lb/ft (see
Table 3-6). Since the drilling fluid is air, then γ m = 0, and K b = 1.0. The maximum
axial tension force in the top dual wall pipe element of the drill string is when the
depth is 820 ft and when the drill bit is lifted off the bottom of the well (after the
target depth has been reached). The maximum axial tension force is determined from
Equation 3-1. This is

F = (820 ) (27 4. ) (1 0. )

F = 22 468 lb
,