Page 77 - Air and gas Drilling Field Guide 3rd Edition

68 CHAPTER 4 Downhole Equipment

rock formation). Figure 4-7b shows the three roller cones at the bottom of a bore-
hole. This latter schematic shows the offset of the cones. The offset is the degree
the cones of the bit are designed to depart from a true rolling action on the rock
face. Offset indicates that two or more cones of the bits do not have their center-
lines of rotation passing through the center of bit rotation. Figure 4-7b shows a
bit with no unique center point through which all three cone centerlines pass [5].
Tricone drill bits can be used to drill a wide variety of rock formations.
Figure 4-8 shows a typical insert tricone drill bit. The “insert” term for this bit
refers to the fact that the “teeth” or “buttons” are tungsten-carbide studs that
are inserted (shrink-fit) into holes drilled in the cone material. The insert tricone
drill bits are used to drill medium-to-hard rock formations.
Most tricone drill bits are fabricated for use with drilling muds. However,
most manufacturers produce a few of their drill bit styles for air drilling opera-
tions. These tricone drill bits are designed with special internal air passages to
provide the bit bearings with the appropriate cooling from the less dense com-
pressed air or natural gas. Figure 4-9 shows a cutaway of a tricone drill bit used
for air operations.
Tricone drill bits used for air and gas drilling are usually designed with
increased gauge protection (relative to their mud drilling counterparts). This

FIGURE 4-8. Insert tooth 7 7/8-in (200 mm)-diameter tricone roller cutter bit IADC Code 517
(courtesy of Reed Rock Bit Company).