Page 85 - Air and Gas Drilling Manual

Chapter 3: Downhole Equipment 3-13
dynamic and is more effective than the quasi-static crushing action of tri-cone and
single cone drill bits. Therefore, air hammer drilling operations require far less
WOB as comparable drilling operations using tri-cone or single cone drill bits.
The air hammer is made up to the bottom of the drill string and at the bottom
of the air hammer is the air hammer bit. The air hammer drill string must be rotated
just like a drill string that utilizes tri-cone or single cone drill bits. The rotation of
the drill string allows the inserts (i.e., tungsten carbide studs) on the bit face to
move to a different location on the rock face surface. This rotation allows a different
position on the rock face to receive the impact load as the upper end of the hammer
bit is struck by the hammer. In direct circulation operations, air flow passes through
the hammer section, through the drill bit channel and orifices to the annulus. As the
air passes into the annulus, the flow entrains the rock cuttings and carries the
cuttings to the surface in the annulus. Direct circulation air hammers are available in
a wide variety of outside housing diameters (3 inches to 16 inches). These air
hammers drill boreholes with diameters from 3 5/8 inches to 17 1/2 inches.
There are also reverse circulation air hammers. These unique air hammers allow
air pressure in the annulus to actuate the hammer via ports in the outside housing of
the hammer. The reverse circulation air hammer bits are designed with two large
orifices in the bit face that allow the return air flow with entrained rock cuttings to
flow to the inside of the drill string and then to the surface. Reverse circulation air
hammers are available in larger outside housing diameters (6 inches to 24 inches).
These air hammers drill boreholes with diameters from 7 7/8 inches to 33 inches.
Figure 3-11 shows two typical air hammer bits that would be used with direct
circulation air hammers. The larger bit (standing on its shank end) is an 8 5/8 inch

Figure 3-11: Two typical air hammer bits with concave face (8 5/8 inch diameter bit on
end, 6 inch diameter on side) (courtesy of Rock Bit International Incorporated).