STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

776 Drilling and Well Completions
is perpendicular to the pilot pen and the thrust button is designed to take
outward thrust. The ball bearings allow the cutter to take inward thrust. When
other bearing parts are worn out, the balls will also take some radial and
outward loading.
Air Circulating Ball and Roller Bearings. When air, gas or mist are used as a
drilling fluid, nonsealed ball land roller bearing bits are used. The design allows
a portion of the drilling fluid to be diverted through the bearing for cooling,
cleaning and lubrication. Since free water in contact with loaded bearing surfaces
will reduce their life, bits are equipped with a water separator to prevent this
action in cases where water is injected into the air or gas.
Also available for the prevention of bit plugging are backflow valves that
prevent cuttings suspended in water from backing up through the bit into the
drill pipe when the flow of air or gas is interrupted.
The “ring lock” bearing is a newer friction bearing design which is also
classified under Columns 6 or 7 on the IADC chart. Instead of ball bearings, a
snapring retainer holds the cone shell in place. This provides greater load-bearing
area and cone shell thickness in the region where the ball bearing race has been
eliminated. A compressed O-ring seal prevents drilling mud from contaminating
the bearing grease.
Steel Tooth Cutting Structure Design
The designs of steel tooth bits cutting structure are shown in Figure 4-141 [44].
Steel tooth bits are employed in soft formations where high rotary speeds can be
used. All steel tooth cones have tungsten carbide hardfacing material applied to
the gage surface of the bit body and to the teeth as dictated by the intended use
of a specific roller cone design. Tooth hardfacing improves wear resistance but
reduces resistance to chipping and breaking, For this reason, hard formation steel
tooth cones usually have gage hardfacing only, while soft formation steel tooth cones
usually have hardfacing on tooth surfaces as well as the gauge surface.
Soft Formation Bits. Bits for drilling soft formations are designed with long,
widely spaced teeth to permit maximum penetration into the formation and
removal of large chips.
Medium Formation Bits. Medium and medium-hard formation bits are designed
with more closely spaced teeth, since the bit cannot remove large pieces of the
harder rock from the bottom of the borehole. The teeth also have slightly larger
angles to withstand loads needed to exceed formation strength and produce chips.
Hard Formation Blts. The heel or outermost row on each cone is the driving
row, that is, this row generates a rock gear pattern on the bottom of the borehole
that, in the case of these strong rocks, is not easily broken away from the wall
of the borehole. The numbers of heel row teeth used on each of the three cones
are selected to prevent the heel teeth from “tracking,” or exactly following in
the path of the preceding cone, which would cause abnormally deep rock tooth
holes on the borehole bottom.
Insert Bit Tooth Design
The companion of insert bits cutting structure is shown in Figure 4142 [44].
Initially, the tungsten carbide tooth bit was developed to drill extremely hard,
abrasive cherts and quartzites that had been very costly to drill because of the