Page 575 - Air and Gas Drilling Manual

144
P
p
a5
a5
2
,
P
a5 29 448 lb/ft abs Chapter 11: Specialized Drilling Equipment 11-13
Using the air hammer to drill the borehole does not alter the annulus solution
obtained in Illustrative Example 8.3b. However, it will alter the pressure
calculations from the bottom of the annulus to the top of the inside of the drill
string (the injection pressure).
Air Hammer Bit Orifice Passages
As discussed above, the air flow through an air hammer is curtailed over
approximately one half of the cycle time period of the piston motion. Therefore,
since the weight rate of flow of compressed air being injected into the inside of the
top of the drill string is constant and must be the same as the weight rate of flow
exiting the blooey line, then the flow through the air hammer over half the cycle
period must be twice the flow rate through the overall circulation system. Thus, the
pressure should be determined in the various air hammer constrictions with twice the
flow rate.
The air hammer bit used in this example operation has three open orifice
passages, each with an inside diameter of 1.125 inches and length of 3.0 inches.
Thus, the inside diameter of the open orifice, D op, is
.
d op 1 125 inches
d op
D op
12
.
D op 0 0938 ft
The length of the orifice passages, L , is
op
.
l op 30 inches
l op
L op
12
.
L 0 250 ft
op
Equations 6-67, 6-68, 6-72, and 6-73 can be used altered and used to evaluate
the flow through the three orifice passages in the air hammer bit. The inside surface
of these orifice passages have the surface roughness of commercial steel. Therefore,
over the length of the passages the surface roughness is
e 0 00015 ft
.
p