STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

872 Drilling and Well Completions
The major reason most turbine motors are designed with various add-on motor
sections is to allow flexibility when applying turbine motors to operational situations.
For straight hole drilling the turbine motor with the highest possible torque
and the lowest possible speed is of most use. Thus the turbine motor is selected
such that the motor produces the maximum amount of power for the lowest
possible circulation flowrate (i.e., lowest speed). The high power increases rate
of penetration and the lower speed increases bit life particularly if roller rock
bits are used.
For deviation control drilling the turbine motor with a lower torque and the
shortest overall length is needed.
Example 1
Using the basic performance data given in Table 4-1 10 for the 6 +-in. outside
diameter turbine motor with 2 12 stages, determine the stall torque, maximum
horsepower and pressure drop for this motor if only one motor section with
106 stages were to be used for a deviation control operation. Assume the same
circulation flow rate of 400 gal/min, but a mud weight of 14 lb/gal is to be used.
Stall Torque. From Table 4-110 the stall torque for the turbine motor with 212
stages will be twice the torque value at optimum speed. Thus the stall torque
for 10 lb/gal mud weight flow is
Ms = 2( 1421)
= 2,842 ft-lb
From Equation 4-138 it is seen that stall torque is proportional to the number
of stages used. Thus the stall torque for a turbine motor with 106 stages will
be (for the circulation flowrate of 400 gal/min and mud weight of 10 lb/gal)
M, = P,842(g)
= 1,421 ft-lb
and from Equation 4-146 for the 14-lb/gal mud weight
M, = 1,4,1( g)
= 1,989 ft-lb
Maximum Horsepower. From Table 4-1 10 the maximum horsepower for the
turbine motor with 212 stages is 217. From Equation 4-143 it can be seen that
the maximum power is proportional to the stall torque and the runaway speed.
Since the circulation flowrate is the same, the runaway speed is the same for
this case. Thus, the maximum horsepower will be proportional to the stall
torque. The maximum power will be (for the circulation flowrate of 400 gal/
min and mud weight of 10 lb/gal)