STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 769
pipe unit weights and steel grades and, finally, the design that produces the
lowest cost should be selected.
The maximum load that can be suspended in the slips, from Equation 4-83
(assume K = 2.36, Ls = 12 in.) is
wmm =
501090
v2
= 346,056 lb
Total weight of string = 238,727 Ib.
The drill pipe will not be crushed in the slips. The drill string design satisfies
the specified criteria.
DRILLING BITS AND DOWNHOLE TOOLS
Classification of Drilling Bits
Numerous individual rotary bit designs are available from a number of
manufacturers. All of them are designed to give optimum performance in various
formation types. There is no universal agreement on this subject; variations in
operating practices, type of equipment used or hole conditions require an
experimental approach. It has been noted in development drilling that those
operators who consistently drill the “fastest” wells usually employ several types
of bits.
All manufacturers use their own classification numbers for their bits. This
results in mass confusion about which bit to use in what formation and whose
bit is better. The International Association of Drilling Contractors (IADC) has
addressed this classification problem through the development of a unified
system. But whose bit is better is left to trial-and-error experimentation by the
individual operator.
Rotary drilling bits are classified into the following types:
1. Roller rock bits (milled tooth bits)
2. Tungsten carbide insert roller bits
3. Diamond bits and core bits
4. Polycrystalline diamond compacts (PCD) bits
The cutting mechanics of different types of bits are shown in Figure 4-135 [43].
IADC Classification Chart and Bit Codes
In 1987, IADC developed a revised standard nomenclature for roller bits
which includes a classification chart and a four-character bit code. All manufacturers
must classify their bits in a prescribed manner on the IADC classification
chart. The classification includes four categories: series, types, feature,
and additional features. Figure 4-136 shows an IADC classification chart. A letter
used in the fourth position of the four-character IADC code indicates additional
design features specified in Table 4-91.
Series. Numbers 1, 2 and 3 are for milled tooth bits and designate soft, medium
and hard formations, respectively. Numbers 4, 5, 6, 7 and 8 are for insert bits