Page 375 - Air and Gas Drilling Manual

8-58 Air and Gas Drilling Manual
surface location elevation, the derated input power available must be greater than the
total actual shaft power needed. Figure 4-15 shows that for 4,000 ft elevation the
input power of a turbocharged prime mover must be derated by approximately 10
˙
percent. The derated input horsepower, W , available from the prime mover is
i
.
W = 740 1( − 0 10)
i
˙
W = 666 .0
i
For this illustrative example, the prime mover of each of the two compressor
units derated input power is greater than the total actual shaft horsepower needed,
thus, these compressor units can be operated at this 4,000 ft surface location
elevation (while drilling at a depth of 10,000 ft).
Fuel Consumption
The anticipated drilling rate of penetration is estimated to be 60 ft/hr. The
vertical interval section to be drilled is from a depth of 7,000 ft to depth of 10,000
ft, or a 3,000 ft length of borehole. Therefore, the estimated actual drilling time to
drill this interval is approximately 50 hours.
The injection pressure into the top the drill string while drilling at 10,000 ft of
depth was found to be approximately 293.9 psia. Using similar calculations, the
injection pressures for depths from 7,000 ft to 10,000 ft can be obtained. Figure 8-
11 shows these injection pressures as a function of drilling time (or drilling depth).
These compressor system units are a unique combination of a rotary screw
primary compressor (with a fixed pressure ratio) and a reciprocating piston booster
compressor driven by a single Caterpillar Model 3412 prime mover. Since the
injection pressure required (293.9 psia) is greater than the fixed ratio derated output
pressure of the rotary screw primary compressor (176.6 psia), then the reciprocating
piston booster will match its output with the required injection pressure. Thus, for
this operational situation Figure 8-11 shows only one curve indicating the injection
pressure is the same as the pressure output of the compressor units. (Note that if the
injection pressure is less than the derated output pressure of the rotary screw primary
compressor, there would be two curves on Figure 8-11. One flat horizontal line for
at 176.6 psia for the primary compressor pressure output and another curve
representing the actual back pressure which is the injection pressure. The
compressor system fuel consumption is determined from the primary compressor
output pressure.)
Above it was found that the total actual shaft horsepower required by the
combined primary and booster compressor system in each of the two units to
compress air to 293.9 psia was approximately 467.9 (at 10,000 ft of depth). Also in
the above the derated horsepower available from the Caterpillar Model 3412 prime
mover at the surface elevation of 4,000 ft was found to be approximately 666.
Drilling at a depth of 10,000 ft, the prime mover power ratio is

.
PR = 467 9 ( 100) = 70 3
.
666 0
.