STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

864 Drilling and Well Completions
Figure 4-190. Basic turbine motor design principle. (Courtesy Smith
International, Inc.)
Design
The rotational energy provided by the flowing fluid is used to rotate and
provide torque to the drill bit. Figure 4191 shows the typical complete downhole
turbine motor actuated with an incompressible drilling fluid.
In general, the downhole turbine motor is composed of two sections: (1) the
turbine motor section and; (2) the thrust-bearing and radial support bearing.
These sections are shown in Figure 4-191. Sometimes a special section is used
at the top of the motor to provide a filter to clean up the drilling mud flow
before it enters the motor, or to provide a by-pass valve.
The turbine motor section has multistages of rotors and stators, from as few
as 25 to as many as 300. For a basic motor geometry with a given flowrate, an
increase in the number of stages in the motor will result in an increase in torque
capability and an increase in the peak horsepower. This performance improvement,
however, is accompanied by an increase in the differential pressure
through the motor section (see Table 4-109). The turbine motor section usually
has bearing groups at the upper and lower ends of the rotating shaft (on which
are attached the rotors). The bearing groups only radial load capabilities.
The lower end of the rotating shaft of the turbine motor section is attached
to the upper end of the main shaft. The drilling fluid after passing through
the turbine motor section is channeled into the center of the shaft through large
openings in the main shaft. The drill bit is attached to the lower end of the
main shaft. The weight on the bit is transferred to the downhole turbine motor
housing via the thrust-bearing section. This bearing section provides for rotation
while transferring the weight on the bit to the downhole turbine motor housing.
In the thrust-bearing section is a radial support bearing section that provides
a radial load-carrying group of bearings that ensures that the main shaft rotates