Page 153 - Introduction to Petroleum Engineering

140 DRILLING

systems, which replaced most mechanical systems, use internal combustion engines
to generate electricity that is converted to DC with SCR. The DC electricity powers
motors to run the draw works, rotary table or top drive, pumps, and so on. Diesel‐
electric AC systems with variable frequency drives (VFD) are preplacing the SCR
systems. With VFD, motor control is superior to that with DC power. Instead of
diesel for fuel, some rigs may use natural gas, and some rigs in populated areas have
connected directly to the electric power grid.
Overall power efficiency of an engine can be calculated from the input power and
output shaft power. The shaft power P is
sp

P = ω T (8.1)
sp
where ω is the angular velocity of the shaft and T is the output torque. The overall
power efficiency η is
sp
power output
η = (8.2)
sp
power input
Power output is shaft power P . Power input P for internal combustion engines
in
sp
depends on the rate of fuel consumption  m and the heating value of fuel H so that
f
m H
P =  f (8.3)
in
Therefore, the overall power efficiency is

P ωT
η = sp = (8.4)
sp

P in mH
f
example 8.1 Shaft Power
A diesel engine rotates a shaft at 1000 rev/min to provide an output torque of
1500 ft⋅lbf. What is the shaft power in hp?

Answer
rev rad
Angular velocity ω = 2 π ×RPM = 2 π ×1000 = 6283
min min
Power output
⋅

⋅
P = ω T = 6283 rad  ×1500 ft lbf = 94210 6 ft lbf
×
.
sp  
 min  min
⋅
ft lbf 1 hp
×
.
P = 94210 6 × = 286 hp
⋅
sp
min 33000 ft lbf