Page 87 - Air and Gas Drilling Manual

Chapter 3: Downhole Equipment 3-15
bits. Although the air hammer bit faces are somewhat uniform in design, the shafts
are different for each air hammer manufacturer. The air hammer face and shafts are
integral to the bit, thus, manufacturing air hammer bits is complicated. Fortunately,
the air hammer has proven in the past decade to be very effective in drilling deep
boreholes. This has given rise to competition among traditional drill bit
manufacturers to provide improved air hammer bits for deep drilling operations.
This competition has in turn resulted in an increase in the quality and durability of
air hammer bits (over the traditional air hammer manufacturer-supplied air hammer
bits) in the more hostile environments of the deep boreholes. Operational use of the
air hammer will be discussed in detail in Chapter 11.

Figure 3-15: Air hammer bit face profile designs and application to rock formation
abrasiveness and hardness (courtesy AB Sandvik Rock Tools).
3.2.4 Classification of Drill Bits
The International Association of Drilling Contractors (IADC) has approved a
standard classification system for identifying similar bit types available from various
manufacturers [5]. Table 3-1 gives an example IADC classification chart for insert
drill bits (first column). In general, the classification system adopted is a three-digit
code. The first digit in the bit classification code is the rock formation series
number. The letter “D” precedes the first digit if the bit is diamond or PDC drag
type bit. The first digit 1 to 3 are reserved for milled tooth bits in soft, medium,
and hard formation categories. The first digit 5 to 8 are for insert bits in soft,
medium, hard and extremely hard formation categories.
The second digit is called the type number. Type 0 is reserved for PDC drag
bits. Types 1 to 4 designate a formation hardness subclassification from the softest
to the hardest formations with each series category.
The third digit is the feature number. These numbers are interpreted differently
for drag bits and roller cutter bits. For diamond and PDC drag bits the features
numbers are 1 to 8 and refer respectively to: step type, long taper, short taper, non-
taper, downhole motor, side-track, oil base, and core ejector.
The third digit feature numbers for roller cutter bits are 1 to 7 and refer
respectively to: standard roller bearings, standard roller bearing for air applications,
standard roller bearing with gauge protection, sealed roller bearings, sealed roller