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6 Part I Liquid Drilling Systems

1.3 DRILL STRINGS
Figure 1.5 shows a drill string used in the oil and gas industry. The
components of a drill string are described in this section according to the
sequence in which they are run into the hole. A drill bit is installed at
the bottom of the drill string. Three types of drill bits are used in the oil and
gas industry: drag bits, cone bits (roller cutter bits), and PDC bits. Figure 1.6
shows two tricone bits, one with milled teeth and one with inserted teeth.
Figure 1.7 shows a cutoff view of a tricone bit. Most bits are designed to
install nozzles of different sizes. Drilling fluid passes through the bit nozzles at
high velocity to clean the bit teeth and remove cuttings from the bottom of
the hole. Figure 1.8 shows several types of bit nozzles, which are made of
hard metal to resist erosion. Bit nozzle diameters often are expressed in 32nds
of an inch. For example, if the bit nozzles are described as “12-13-13,” this
12
denotes that the bit contains one nozzle with a diameter of / 32 inch and two
13
nozzles with diameters of / 32 inch. Odd-numbered nozzles are not available
for nozzle sizes above 20.

Swivel
Drill Pipe
Swivel Sub

Kelly Cock

Crossover Sub

Drill Collar
(BHA)
Kelly

Bit Sub

Drill Bit

Figure 1.5 A drill string used in the petroleum industry.