Page 611 - Air and Gas Drilling Manual

The diesel fuel consumption rate (in United States gallons) for the drilling depth of
10,000 ft is
222 8 . Chapter 11: Specialized Drilling Equipment 11-49
q 32 8 . gal/hr
f
(. )( .
0 8156 8 33)
Since there are two compressor units providing compressed air to this example
air hammer drilling operation, the total diesel fuel consumption rate while drilling at
the 10,000 ft of depth is approximately 66 gal/hr.
11.2.3 Downhole Pneumatic Turbine Motors
The downhole pneumatic turbine motor is a rather new innovation in air drilling
downhole equipment. This motor was developed in the early 1980’s and was first
used in the Geysers geothermal fields in Northern California [4, 5]. Figure 11-13
shows a schematic of the typical downhole pneumatic turbine motor. These
downhole motors are actuated by a single stage aerodynamic turbine device. The
turbine speed is controlled by a governor which is attached to the top of the turbine
shaft. The aerodynamic turbine runs at high speed when exposed to a high weight
rate of flow of compressed air or other gases. The high weight rate of flow of
compressed air (or gas) is needed to carry the drill bit generated cuttings to the
surface in the annulus.
The downhole turbine motor develops power mainly from the kinetic energy of
the weight rate of flow through the turbine stator and rotor. There is a small
pressure drop through the turbine, but with the high weight rate of flow through the
turbine the pressure drop does not contribute greatly to the power. The single stage
turbine converts the air flow energy to high speed mechanical shaft energy. This
high mechanical shaft speed must be reduced to operate the drill bit. The downhole
turbine motor has a sealed small compact gear reduction system that is directly
coupled to the bottom of the turbine output shaft (i.e., the high speed coupling).
The downhole motor can be fitted with several gear box configurations which can
give reductions from 40 to 1 to as high as 120 to 1. The gears are grease lubricated
and, therefore, their speed is limited. The governor on the turbine provides
protection for the gears by controlling the input to the gear box from the turbine.
The output shaft of the gear box is directly coupled to the main drive shaft (with
thrust and radial bearing assembly) with a high torque capability coupling. The high
torque coupling allows high non-transient torque to be transferred from the output
shaft of the gear box to the main drive shaft and, thus, to the tri-cone drill bit. Any
transient impact torques are absorbed by the flexible spring system in the coupling,
thus, protecting the gear teeth in the gear box from damaging impact forces.
In general, the downhole pneumatic turbine motor is used with roller cone drill
bits. The motors can be actuated on compressed air and other compressed gases and
the motors do not require any lubricant to be injected into the air or gas flowing to
the motor. In essence, the motor can be run with dry air or gas. On the other hand,
the motor can also be operated with unstable foam (misting).