STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Downhole Motors 867
Circulation Rate 400 gpm
-
Mud Weioht 10 ppg
-
\
-c Preaaure
300
1500
- Horsepower
-
-
*O0.
;
a
E
r
100
a
500 lo00 1500 2000
Bit Speed (rpm)
Figure 4-192. Turbine motor, 6Yi-h. outside diameter, two motor sections,
21 2 stages, 400 gal/min, 1 0-lb/gal mud weight. (Courtesy Eastman-Christensen.)
For this example, the stall torque of the motor is 2,824 ft-lb. The runaway
speed is 1,614 rpm and coincides with zero torque. The motor produces its
maximum horsepower of 217 at a speed of 807 rpm. The torque at the peak
horsepower is 1,412 ft-lb, or one-half of the stall torque.
A turbine device has the unique characteristic that it will allow circulation
independent of what torque or horsepower the motor is producing. In the
example where the turbine motor has a lO-lb/gal mud circulating at 400 gal/
min, the pressure drop through the motor is about 1,324 psi. This pressure drop
is approximately constant through the entire speed range of the motor.
If the turbine motor is lifted off the bottom of the borehole and circulation
continues, the motor will speed up to the runaway speed of 1,614 rpm. In this
situation the motor produces no drilling torque or horsepower.
As the turbine motor is lowered and weight is placed on the motor and thus
the bit, the motor begins to slow its speed and produce torque and horsepower.
When sufficient weight has been placed on the turbine motor, the example
motor will produce its maximum possible horsepower of 217. This will be at
a speed of 807 rpm. The torque produced by the motor at this speed will be
1,412 ft-lb.
If more weight is added to the turbine motor and the bit, the motor speed
and horsepower output will continue to decrease. The torque, however, will
continue to increase.
When sufficient weight has been placed on the turbine motor and bit, the
motor will cease to rotate and the motor is described as being stalled. At this
condition, the turbine motor produces its maximum possible torque. Even when
the motor is stalled, the drilling mud is still circulating and the pressure drop
is approximately 1,324 psi.