Experimental study of rock-breaking with an offset single cone bit
Experimental study of rock-breaking with an offset single cone bit
0 School of Mechanical and Electronic Engineering, China University of Petroleum , Beijing 102249 , China
An experimental study of rock-breaking with an offset single cone bit was completed on the bit bench test equipment. Data such as transmission ratio, weight on bit (WOB), rate of penetration (ROP) and torque on bit were acquired in the experiments. Based on analyzing the experimental results, several conclusions were drawn as follows. The transmission ratio of the offset single-cone bit changed slightly with rotary speed of bit, weight on bit and offset distance. The rate of penetration of the offset singlecone bit increased with increase of WOB and offset distance. The torque on bit increased with increase of offset distance under the same WOB and bit rotary speed, decreased with increase of bit rotary speed under the same WOB. The rock-breaking mechanism of the offset single-cone bit was a scraping action. This indicates that the offset single-cone bit is a chipping type bit.
Offset single-cone bit; transmission ratio; rate of penetration; torque on bit
In order to accelerate the exploration and development of
China’s oil and gas resources, especially the deep offshore
reservoirs and complex reservoirs, it is necessary to develop
new drilling technology and equipment. Since the 1990s,
the slim hole drilling technology is the focus of research for
new drilling technology, the development of a new type of
bit is the key to slim hole drilling technology
Langford Jr, 1999; Li and Tan, 2000; Qiu, 2000; Wu and Liu,
2000; Yu et al, 1994)
Compared with conventional single-cone bit (with
no offset), the offset single-cone bit has new structural
characteristics, providing more selections of bit types for
modern slim hole drilling
(Deng et al, 2001; Ma et al,
2004; Moran and Witman, 2004; Witman et al, 2006; Xia
et al, 2002; Yu and Yu, 2007)
. However, the rock-breaking
mechanism and drilling characteristics of the offset
singlecone bit are still issues to be studied.
The transmission ratio (i.e. the ratio of cone rotary speed
to bit rotary speed) is a key parameter for bit design, which
is essential to analyzing the trajectory of teeth on the cone.
Therefore, it is a vital parameter for the design of cutting
(Yu et al, 1995)
. In general, transmission ratio
for offset single-cone bit under practical conditions cannot
be obtained from theoretical study, so the effect of offset
distance on transmission ratio can be obtained only through
Compared with conventional single-cone bit, the offset
single-cone bit is characterized by an asymmetrical structure,
so the rate of penetration (ROP), torque and rock-breaking
characteristics must be investigated experimentally through
bench tests. At the same time, the bottom hole tracks crushed
or cut by offset single-cone bits can also be analyzed.
2 Experimental apparatus and procedures
The experiment was performed on bit bench test
equipment, developed by Southwest Petroleum University.
Fig. 1 shows its schematic diagram. The offset single-cone bit
used is shown in Fig. 2.
The bit bench test equipment is composed of a
threephase silicon-controlled recti er control box 1, direct current
electromotor 2, gear box 3, lower rotary table 4, sensors 7
and 8, joint 9, measuring joint 10, hydraulic cylinder 11,
centralizer 12, upper rotary table 13, kelly bar 14, balance
beam 15, and a swivel 16.
During testing, the upper rotary table 13 was locked by
pin and was stationary. The lower rotary table 4, on which the
rock 5 was held by a clamping device, was driven by a direct
current electromotor 2 through gearing. The rotational speed
of the electromotor was adjusted by a three-phase
siliconcontrolled rectifier. Then the bit could rotate relative to the
rotary table. The hydraulic cylinder 11 could provide the
given weight on bit (WOB) and control the vertical movement
of bit. Sensors 7 and 8 were installed to monitor rotary speed
of the bit and cone.
WOB and torque were measured by resistance strain
gauges on the measuring joint, and the linear strain was then
converted into voltage signals with dynamic strain gauges.
3 Analysis of the experimental results
Four single-cone bits with offset distances, d, of 0, 3.2, 5.8,
1-Silicon-controlled rectifier control box; 2-Direct current electromotor;
3-Gear box; 4-Lower rotary table; 5-Rock; 6-Single-cone bit; 7, 8-Sensor;
9-Joint; 10-Measuring joint; 11-Hydraulic cylinder; 12-Centralizer; 13-Upper
rotary table; 14-Kelly bar; 15-Balance beam; 16-Swivel
9.0 mm, were tested respectively. The journal angles of these
bits used were 40o, and their diameters were 152.4 mm. The
rock sample was limestone.
3.1 Measured signals of WOB, torque on bit and bit rotary speed
60 rpm and 2 tonnes respectively. As shown in Fig. 3, the
actual acquired signal of WOB had similar tendency with
that of torque, while the signal of bit rotary speed had reverse
tendency with that of torque. This is consistent with the actual
situation, and helps indicate that the data acquired in the
laboratory were reliable.
3.2 Transmission ratio
Data on WOB, torque, rotary speed of bit and rotary speed
of cone for the single-cone bits with different offset distances
were measured under different experimental conditions. The
experimental data and calculated results are shown in Table 1
As shown in Tables 1 and 2, the transmission ratio for
single-cone bits with different offset distances changed
slightly with the bit rotary speed or WOB. Moreover, for the
single-one cone bit, when bit rotary speed and WOB were
kept unchanged, the offset distance had a slight in uence on
3.3 Rate of penetration of the offset single-cone bit
The rate of penetration
( R O P ) o f s i n g l e - c o n e
bits with different offset
d i s t a n c e s w e r e o b t a i n e d
under different WOB values,
as shown in Fig. 4, where
journal angle was 40° and
bit rotary speed was 60 rpm.
Fig. 4 shows that for the
offset single-cone bit, ROP
increased with increase of
WOB. The depth of teeth
of penetration into rock
increased with WOB, as a
result the rock crushed or
cut by cone teeth would be
For the offset single-cone bit, ROP increased with
increase of offset distance when the bit rotated at 60 rpm and
the applied WOB was the same. Under the same experimental
conditions, rock crushed or cut by teeth decreased with
increase of offset distance, therefore the rate of penetration
3.4 Torque on the single-cone bit
Fig. 5 shows the torque on single-cone bit under different
bit rotary speeds when the WOB was kept at 3 tonnes.
Fig. 6 shows the torque on offset single-cone bit under
different WOB when the bit rotary speed was kept at 60 rpm.
The torque on the single-cone bit increased with increase
of offset distance under the same experimental conditions.
The distance between cone center and bit axis increased with
increase of offset distance, as a result the force applied at
borehole wall by the cone became larger, and therefore the
For the offset single-cone bit, the torque on bit decreased
with increasing rotary speed when the applied WOB was
the same, this is consistent with the conventional
singlecone bit. The torque on bit increased with increasing WOB
when the rotary speed of bit was kept unchanged. A possible
explanation is that the cutting depth increased with increasing
WOB, therefore the torque increased.
4 Bottom hole track pattern of the offset single-cone bit
Four bottom holes were obtained by using single-cone
bits with different offset distances of 0, 3.2, 5.8 and 9.0mm,
respectively, in bench tests (as shown in Fig. 7). After crushed
or cut by single-cone bits with different offset distances,
there were different track patternson on the rock surface. The
probability that broken rock was crushed again by the teeth
decreased with increase of offset distance, thus improving
the rock-breaking efficiency. The teeth track shows that the
rock-breaking mechanism of the offset single-cone bit was a
scraping action. This indicates that the offset single-cone bit
was a chipping type bit.
Fig. 7 Pictures of bottom hole crushed by single-cone bits with different
1) For single-cone bits with different offset distances,
the transmission ratio changed slightly with rotary speed of
bit and weight on bit. The designers need not to consider
the influence of offset distance when designing the cutting
structure of single-cone bit.
2) The rate of penetration of the offset single-cone
bit increased with increase of WOB and offset distance.
Designers could increase offset distance properly in order to
increase ROP when designing the bit.
3) The torque on bit increased with increase of offset
distance under the same experimental conditions, so the
influence of torque on bit on the strength of drilling pipe
should be considered.
4) The bottom holes drilled with single-cone bits with
different offset distances had different track patterns. The
probability that broken rock was crushed again by teeth
decreased with increase of offset distance, thus increasing
rate of penetration and drilling ef ciency.
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