Page 600 - Air and Gas Drilling Manual

11-38 Air and Gas Drilling Manual
The drill bit speed of the motor, N , when operating on air is approximately
.
N m 231 016 q mav m
s
(231 .016 ) (414 . )
3
N m
462 .032
N m 207 2 rpm
.
The above drill bit speed is quite close to the 200 rpm required. If 9/32 inch nozzles
are placed in the drill bit, the bit speed would be lower. And if 11/32 inch nozzles
are placed in the drill bit, the bit speed would be higher.
The use of the 10/32 inch nozzles in the drill bit controls the drill bit speed and
the expansion of the air inside the motor when the motor is under load (with WOB).
When the load is removed from the motor (no WOB), the motor will go to run away
speeds and damage of the elastomer flexible sheath will result. Thus, when the
motor load is removed, either the air flow to the drill string must be shut down, or a
weight actuated by-pass must be used to divert the air to the annulus when the
weight on bit is removed.
Geometry Inside the Drill String
Starting at the bottom of the drill string, the total length of the drill collars
length increment, H 5, in the openhole section is
H 500 ft
5
The inside diameter of the drill collars length in this openhole section is
.
d 5 2 8125 inches
d 5
D
5
12
.
D 5 0 234 ft
The total length of the drill pipe tool joints lumped geometry increment, H 4, in
the openhole section is
H 4 125 ft

The inside diameter of the drill pipe tool joints lumped geometry in this openhole
section is
d 6 350 inches
.