U.S. patent application number 10/488216 was filed with the patent office on 2005-03-03 for drilling apparatus.
Invention is credited to Eddison, Alan Martyn.
Application Number | 20050045380 10/488216 |
Document ID | / |
Family ID | 26246152 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050045380 |
Kind Code |
A1 |
Eddison, Alan Martyn |
March 3, 2005 |
Drilling apparatus
Abstract
Drilling apparatus comprises a cutting structure defined by a
percussive action cutte (42) and at least one other cutter. The
percussive action cutter is adapted to be urged in a drilling
direct ion by hydraulic pressure force, while the other cutter (43)
is urged in a drilling direction by mechanical force.
Inventors: |
Eddison, Alan Martyn;
(Stonehaven, GB) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Family ID: |
26246152 |
Appl. No.: |
10/488216 |
Filed: |
August 31, 2004 |
PCT Filed: |
June 5, 2002 |
PCT NO: |
PCT/GB02/02575 |
Current U.S.
Class: |
175/57 ;
175/267 |
Current CPC
Class: |
E21B 10/40 20130101;
E21B 4/14 20130101; E21B 10/26 20130101; E21B 6/04 20130101 |
Class at
Publication: |
175/057 ;
175/267 |
International
Class: |
E21B 007/28; E21B
010/32 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 5, 2001 |
GB |
0113585.4 |
Jun 19, 2001 |
GB |
0114874.1 |
Claims
1. Drilling apparatus comprising a cutting structure defined by at
least one percussive action cutter and at least one other cutter,
the at least one percussive action cutter being adapted to be urged
in a drilling direction by hydraulic pressure force.
2. The apparatus of claim 1, wherein a plurality of percussive
action cutters are provided.
3. The apparatus of claim 1, wherein the at least one other cutter
comprises a roller cone.
4. The apparatus of claim 1, wherein the at least one other cutter
comprises a plurality of roller cones.
5. The apparatus of claim 1, wherein the at least one other cutter
comprises a fixed cutter.
6. The apparatus of claim 1, wherein the at least one other cutter
comprises a plurality of fixed cutters.
7. The apparatus of claim 5, wherein the fixed cutter is a PDC
cutter.
8. The apparatus of claim 1, wherein, in use, the at least one
other cutter is adapted to support at least the majority of
mechanical weight applied to the apparatus.
9. The apparatus of claim 1, further including means for creating a
percussive force and means for transferring the resulting
percussive force to the at least one percussive action cutter.
10. The apparatus of claim 9, wherein said means for creating a
percussive force comprises a hammer tool.
11. The apparatus of claim 10, wherein the means for transferring
the resulting percussive force to the at least one percussive
action cutter comprises an anvil.
12. The apparatus of claim 10, wherein the hammer tool is
hydraulically actuated.
13. The apparatus of claim 1, wherein the percussive action cutter
defines a flow restriction, such that hydraulic fluid flowing
therethrough experiences a pressure drop, and thus creates a
pressure force on the cutter.
14. The apparatus of claim 13, wherein the restriction comprises at
least one jetting nozzle.
15. The apparatus of claim 14, wherein the flow restriction is
provided in combination with a piston area defined by the
percussive action cutter.
16. The apparatus of claim 15, wherein the piston area serves as an
anvil.
17. The apparatus of claim 1, further comprising a hydraulically
actuated hammer tool defining a fluid pressure responsive piston
area of dimensions comparable to the cutting area of the at least
one percussive action cutter.
18. The apparatus of claim 17, wherein the hammer tool is at least
as large as the cutting area of the at least one percussive action
cutter.
19. The apparatus of claim 1, wherein there is a degree of overlap
in the area swept by the cutting surface of the at least one
percussive action cutter and the at least one other cutter.
20. The apparatus of claim 1, wherein a percussive action cutter is
located to cut a central portion of a bore.
21. The apparatus of claim 1, wherein at least one percussive
action cutter has a cutting face located forwards of the cutting
face of the at least one other cutter.
22. The apparatus of claim 21, wherein said at least one percussive
action cutter includes laterally directed drilling fluid outlets
ahead of the at least one other cutter.
23. The apparatus of claim 1, wherein the at least one percussive
action cutter is retractable or removable.
24. The apparatus of claim 1, wherein the at least one percussive
action cutter is axially moveable relative to the at least one
other cutter.
25. The apparatus of claim 24, wherein movement of the at least one
percussive action cutter beyond a predetermined relative axial
position is operator detectable.
26. The apparatus of claim 25, wherein movement of the at least one
percussive action cutter beyond a predetermined relative axial
position at least restricts fluid flow through a fluid port.
27. The apparatus of claim 25, wherein said movement corresponds to
said at least one other cutter cutting ahead of said at least one
percussive action cutter.
28. The apparatus of claim 25, wherein forward movement of the at
least one percussive action cutter beyond a predetermined axial
position relative to the at least one other cutter corresponds to
said at least one percussive action cutter having cut ahead of said
at least one other cutter.
29. The apparatus of claim 28, further comprising means responsive
to said forward movement of the at least one percussive action
cutter, said means being effective to at least reduce the
percussive action of said at least one cutters in response to said
forward movement.
30. The apparatus of claim 24, wherein the at least one percussive
action cutter is normally retracted from a fully extended
configuration.
31. The apparatus of claim 30, wherein the at least one percussive
action cutter is spring biassed towards the normally retracted
position.
32. The apparatus of claim 1, wherein the at least one percussive
action cutter comprises a mounting member in sliding sealing
contact with a bore defined by a supporting body, whereby fluid
pressure in the bore creates a pressure force acting on the
cutter.
33. The apparatus of claim 1, wherein the apparatus is in the form
of a shoe.
34. A drilling method comprising the steps: providing drilling
apparatus comprising a cutting structure defined by at least one
percussive action cutter and at least one other cutter; and urging
the least one percussive action cutter in a drilling direction by
hydraulic pressure force while urging the at least one other cutter
in a drilling direction by mechanical force.
Description
FIELD OF THE INVENTION
[0001] This invention relates to drilling apparatus, and in
particular to drilling apparatus for use in drilling bores in earth
formations.
BACKGROUND OF THE INVENTION
[0002] Conventional drill bits, as used for example in the drilling
of bores in the oil and gas exploration and production industry,
feature a number of toothed roller cones. In use, weight is applied
to the rotating bit, and the cones roll over the circular face
forming the end of the bore. The cutting of the rock is achieved by
a number of mechanisms, including a crushing action, the crushed
rock then being removed from the cutting face by the action of
drilling fluid exiting the drill bit via appropriately directed
jetting nozzles. In addition, by varying the orientation of the
rotational axes of the cones, it is possible to provide a scraping
or scouring action as the cones rotate.
[0003] A more recent development, allied to advancements in
drilling and materials technology, has lead to the development of
drill bits featuring fixed cutting faces provided with relatively
hard materials, typically polycrystalline diamond compact
(PDC).
[0004] Traditionally, drilling of the deep bores required in the
oil and gas industry relied solely upon applied weight, which
weight may be applied from surface, or achieved simply by the mass
of the drill string, drill collars and other tools and devices
above the bit. However, in recent years drilling has commenced in
areas featuring particularly hard rock, in which drilling using
conventional methods is still possible, but relatively slow. In
attempts to overcome this problem, there have been various
proposals for percussive or hammer drill bits; while air-powered
hammer drills are in common use in, for example, the construction
trades and in mining, drilling of deep bores with non-compressible
drilling fluid poses many different and varied problems. One
difficulty has been in reliably achieving the necessary impact
forces at the bit. The limited diameter available downhole limits
the size of the hammer tool, resulting in excessive loading and
wear on the tool in order to achieve the required impact force.
Another difficulty that has been experienced in percussive bits is
the tendency for the bits to "lose gauge", that is the outer edges
of the bits wear prematurely, with the result that the bit drills a
smaller diameter bore than intended, or must be replaced at
frequent intervals.
[0005] SU 1730420 A1 (Leningrad Scientific-Research & Design
Institute "Gipronikel") discloses a combined percussive rotary
drilling tool. In addition to a conventional roller-cutter, the
tool features a central percussive bit which cooperates with a
piston actuated by a supply of compressed air. Clearly, such an
arrangement would not be suitable in downhole operations where
there is no supply of compressed air available; conventionally,
drilling "mud" is supplied to the drill bit, which liquid could not
be utilised to operate a piston as disclosed in this document.
[0006] U.S. Pat. No. 3,807,512 discloses a percussion-rotary
drilling mechanism with a rotary drill bit and a percussion drill
bit. A mud drive turbine is utilised to generate reciprocal motion
of the percussion drill bit via an arcuate cam and cam follower
drive mechanism or a rotatable eccentrically weighted wheel drive
mechanism.
SUMMARY OF THE INVENTION
[0007] It is among the objectives of embodiments of the present
invention to provide an improved drilling apparatus which provides
or at least facilitates a greater degree of control in a drilling
apparatus having a percussive action cutter provided in combination
with another form of cutter.
[0008] According to a first aspect of the present invention there
is provided drilling apparatus comprising a cutting structure
defined by at least one percussive action cutter and at least one
other cutter, the at least one percussive action cutter being
adapted to be urged in a drilling direction by hydraulic pressure
force.
[0009] The combination of a percussive action cutter with a cutter
of another form, such as a roller cone or a fixed cutter, such as a
PDC cutter, offers many advantages over conventional drilling
apparatus and drill bits. For example, the hammer action of
percussive bits is most effective working with relatively light
weight applied to the bit, however this may limit the cutting
effectiveness of the bit, such that in conventional hammer bits a
balance must be struck between these two requirements. However, in
the present invention the other cutter may bear a significant
proportion of the weight applied to the bit, allowing the
percussive action cutter to operate more effectively. Indeed, in
preferred embodiments of the present invention, substantially all
of the mechanical force normally applied to the apparatus, whether
by force applied to a drill string from surface or due to the mass
of drill collars, the drill string and the like, is applied to or
borne by the other cutter. The "weight" applied to the percussive
action cutter is a function of applied hydraulic pressure,
equivalent to the "pump-open force", and thus may be controlled
independently of the weight applied to the other cutter, and solely
with a view to maximising the effectiveness of the percussive
action cutter. This division of force between the cutters also
serves to utilise the different forces available, that is
mechanical force and hydraulic force, in an efficient and effective
manner.
[0010] Furthermore, as the percussive action cutter only provides a
proportion of the area of the cutting face, for a given available
hammer or percussive force the impact pressure force applied by the
relatively small area percussive cutter to the rock may be
relatively high. This effect may be further accentuated by the
ability to create a hammer or percussive action within the body of
the apparatus over an area which is relatively large when compared
to the cutting area of the percussive action cutter. In
conventional percussive drill bits, the hammer tool diameter is
always substantially smaller than the cutting area of the bit, as
the bit has to cut a bore of a gauge large enough to accommodate a
drilling fluid return annulus and the body of the tool. In
embodiments of the present invention the hammer tool may be at
least as large as the cutting area. Of course other embodiments of
the invention may achieve a hammer or percussive effect in other
ways, as will be apparent to those of skill in the art.
[0011] Preferably, the apparatus includes means for creating a
percussive force and means for transferring the resulting
percussive force to the at least one percussive action cutter.
Typically, the means for creating a percussive force will comprise
a hammer tool, and the percussive action cutter will define an
anvil. Conveniently, the hammer tool will be hydraulically
actuated, although other forms of actuation may be employed if
desired.
[0012] Preferably, the percussive action cutter defines a flow
restriction, such that hydraulic fluid flowing therethrough
experiences a pressure drop, and thus creates a pressure force on
the cutter. The restriction may take any appropriate form, and will
typically be provided by one or more jetting nozzles. The flow
restriction may be provided in combination with a piston area,
which piston area may also serve as an anvil.
[0013] Preferably, there is a degree of overlap in the area swept
by the cutting surface of the percussive action cutter and the
other cutter. Thus, if there is a loss of gauge through wear to the
percussive action cutter, this will be accommodated by the other
cutter.
[0014] The percussive action cutter may be located to cut a central
portion of the bore. Due to the relatively low speed of a rotating
drill bit at the bit centre, there are often difficulties
experienced in cutting the centre of the bore. Thus, by locating
the percussive action cutter centrally, the enhanced cutting action
provided by the hammer drill effect will avoid this difficulty.
Furthermore, it is generally desirable to rotate percussive action
cutters at relatively low speed (10 to 40 rpm), the primary reason
for rotation being to expose fresh formation to the individual
cutter elements, with higher speeds leading to excessive or
accelerated wear of the cutting elements. Merely by locating the
percussive action centrally, the linear speed experienced by the
cutter elements is of course relatively low in comparison to the
other cutters located radially outwardly of the percussive action
cutters, which other cutters typically benefit from higher cutting
speeds (150 to 200 rpm). Alternatively, or in addition, the
percussive action cutter may be biassed rearwardly, conveniently by
means of a spring, or otherwise configured, such that the
percussive action cutter is normally held slightly off bottom and
thus remains in contact with the formation only for the duration of
the hammer impact or impulse. The percussive action cutter will
thus only make contact with the formation periodically, and for
only a fraction of the time the other cutter remains in contact
with the formation. This will reduce the rubbing action and wear
experienced by the percussive action cutter, even at higher rotary
speeds, allowing the apparatus to be rotated at speeds suited to
the other cutter without damaging the percussive action cutter.
[0015] A centrally located percussive action cutter may also have a
cutting face located forwards of the cutting face of the other
cutter, such that the percussive action cutter effectively cuts a
pilot hole in the centre of the bore. Of course this facilitates
the dislodgement of rock by the following cutter. Furthermore, by
providing lateral fluid outlets on the percussive action pilot
cutter, ahead of the other cutter, drilling fluid may be injected
into the rock formation ahead of the other cutter, facilitating the
release of cuttings by the following cutter.
[0016] In light of the possibility of the different forms of cutter
achieving different cutting speeds, the apparatus may include one
or more of the following features.
[0017] Means may be provided for indicating that the other cutter
is cutting at a faster rate than the percussive action cutter,
allowing, for example, the weight applied to the other cutter to be
reduced, thus reducing the cutting speed of the other cutter,
improving the cutting efficiency of the percussive action cutter,
and preventing premature damage and wear to the percussive action
cutter, which may have been experiencing excessive applied weight.
Such means may take the form of fluid outlets which are closed if
the percussive action cutter experiences elevated weight and is
forced rearwardly into the body of the apparatus. The resulting
change in back pressure will be detectable at surface, allowing
remedial action to be taken.
[0018] In circumstances where the percussive action cutter is
cutting faster that the other cutter, extension of the cutter
beyond a predetermined relative position may result in the
associated percussive tool ceasing hammering, allowing the other
cutter to catch up; it is a standard feature in many percussion
tools that if the tool is picked up off bottom the tool ceases
hammering. Alternatively, in a hammer and anvil arrangement the
anvil on the cutter may simply move beyond the end of the hammer
stroke.
[0019] A centrally located percussive action cutter may be
retractable or removable, to allow the cutting of cores by the
remaining cutter, to allow passage of other tools or devices
through the apparatus, or to facilitate flow of, for example,
cement slurry, through the apparatus. Thus, the drilling apparatus
may be utilised as a casing shoe. In such applications it is likely
that the shoe will be provided with fixed reaming cutters,
typically PDC cutters, which tend to require a high applied torque
to rotate the cutters to ream out obstructions to the passage of
the shoe and following casing; however, casing, and casing threads,
tend not to be capable of accommodating elevated torques. By
providing a percussive action cutter in the shoe, centrally or
otherwise located, the torque required to rotate the shoe may be
reduced.
[0020] According to another aspect of the present invention there
is provided a drilling method comprising the steps:
[0021] providing drilling apparatus comprising a cutting structure
defined by at least one percussive action cutter and at least one
other cutter; and
[0022] urging the least one percussive action cutter in a drilling
direction by hydraulic pressure force.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] These and other aspects of the present invention will now be
described, by way of example, with reference to the accompanying
drawings, in which:
[0024] FIG. 1 is a sectional view of a part of a drilling apparatus
in accordance with a first embodiment of the present invention;
[0025] FIG. 2 is a view from below of the apparatus of FIG. 1;
[0026] FIG. 3 is an alternative sectional view of the drilling
apparatus of FIG. 1;
[0027] FIGS. 4 and 5 are sectional views of a part of a drilling
apparatus in accordance with a second embodiment of the present
invention;
[0028] FIG. 6 is a sectional view of a part of a drilling apparatus
in accordance with a third embodiment of the present invention;
and
[0029] FIG. 7 is a sectional view of a part of a drilling apparatus
in accordance with a fourth embodiment of the present
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0030] Reference is first made to FIGS. 1, 2 and 3 of the drawings,
which show a drilling apparatus 10 in accordance with a first
embodiment of the present invention. The apparatus 10 comprises a
bit 12 mounted via a pin and box connection 14 to the lower end of
a hammer tool 16.
[0031] Mounted on the lower end of the drill bit are two roller
cones 18, 19; when the bit 12 is rotated, the cones 18, 19 will
roll over the rock formation below the bit 12, crushing or
otherwise dislodging cuttings from the rock. The cutting structure
of the bit 12 is further defined by two circular cutting faces
defined by the ends of a pair of percussive action cutters 20, 21.
As may be seen from FIG. 2, the roller cones 18, 19 are positioned
on opposite sides of the bit 12, and the percussive action cutters
20, 21 are located in the quadrants between the roller cones 18,
19.
[0032] Both percussive action cutters 20, 21 are mounted on the end
of a mandrel 22 which extends through the bit body and into the
hammer tool 16. The mandrel 22 is splined, at 24, and co-operates
with the bit body 13 such that the mandrel 22 may move axially
relative to the body 13, but cannot rotate relative to the bit 12.
An upper portion of the mandrel 22 defines an annular recess 26
which co-operates with a ring 28 trapped between the hammer tool
body 29 and the bit body 13, to limit the axial movement of the
mandrel 22.
[0033] The upper end face of the mandrel 22 defines an anvil 30
against which impacts a hammer 32. Reciprocal movement of the
hammer 32 may be achieved by any appropriate means, and may utilise
pressure forces created by the pressure of drilling fluid within
the drill string. The drilling fluid passes through the apparatus
10 and exits the apparatus through various appropriate jetting
nozzles adjacent the cutters 21,22 and the roller cones 18,19. The
drilling fluid passes through the mandrel 22 and, with the pressure
drop resulting from the fluid passing through the nozzles, creates
a pressure force which acts on the upper area of the anvil 30 and
thus urges the cutters 21,22 in a drilling direction with a force
proportional to the drilling fluid pressure.
[0034] In use, the apparatus 10 will be run downhole on the end of
an otherwise conventional drillstring. Drilling fluid will be
pumped from surface and will travel through the string, the hammer
tool 16 and the bit 12, to exit the bit through appropriate jetting
nozzles (not shown in FIG. 1 to 3), as noted above. The bit 12 is
rotated such that the roller cones 18, 19 are rolled over the end
face of the bore, cutting the rock formation therebelow in a
conventional manner. However, the impact of the hammer 32 on the
anvil 30 will produce a percussive or hammer action which is
transferred via the mandrel 22 to the percussive action cutters 20,
21. Thus, as the drill bit 12 rotates, the cutters 20, 21 are urged
into the rock formation with an impact force.
[0035] It is believed that the combination of the cutting action
provided by the roller cones 18, 19 and the percussive hammer
action of the cutters 20, 21 will be more effective than the action
of, for example, a drill bit featuring only roller cones,
particularly when drilling through relatively hard formations.
Furthermore, the mechanical weight applied to the bit 12 will be
supported by the roller cones 18, 19, such that the applied weight
may be relatively high without detracting from the hammer action of
the cutters 20, 21: as noted above, the "weight" applied to the
cutters 20, 21, and on top of which the impact or impulse is
applied, is a function of the hydraulic pressure of the drilling
fluid, and may be controlled independently of the applied
mechanical weight to provide efficient percussive drilling.
Furthermore, the cutting area of the percussive action cutters 20,
21 is relatively small when compared to the size of the hammer tool
16, such that the percussive or impact pressure force applied to
the rock formation will be relatively high.
[0036] Reference is now made to FIGS. 4 and 5 of the drawings,
which illustrates drilling apparatus 40 in accordance with a second
embodiment of the invention. The apparatus 40 shares a number of
features with the apparatus 10 described above, however in this
apparatus 40 only a single percussive action cutter 42 is provided,
and the cutter 42 is located centrally of the drill bit 43. Thus,
in the course of a drilling operation, the percussive action cutter
42 will cut a pilot bore, and the following roller cone cutters 44,
45 will effectively provide a reaming operation to bring the bore
out to gauge.
[0037] The cutter 42 is mounted on the lower end of a mandrel 46,
the upper end of which defines an anvil 48 which is struck by the
hammer 50 of the percussive tool 52 provided above the apparatus
40. The tool body 54 defines a bore 56 and the anvil 48 features a
seal 58 which provides a sliding seal between the bore 56 and the
anvil 48; as with the previous embodiment, the "weight" or force
normally applied to the cutter 42 is therefor dependant on the
internal fluid pressure acting on the seal area of the anvil
48.
[0038] The figures also illustrate a central bore 60 passing
through the mandrel 46, the bore 60 leading to appropriately
located jetting nozzles 62, 64. In use, the jetting nozzles 62
located in the face of the percussive action cutter 42 will inject
pressurised drilling fluid into the walls of the pilot bore created
by the cutter 42, thus facilitating cuttings removal by the cones
44, 45. If desired, jetting nozzles may be provided on the sides of
the cutter 42, such that drilling fluid is directed laterally of
the cutter 42, directly into the surrounding formation.
[0039] The other jetting nozzles 64 are directed towards the cones
44, 45, and are normally located below the end of the bit body bore
66, which forms a continuation of the hammer tool body bore 56.
However, if the cutter mandrel 46 moves rearwardly into the bit
body 68, the nozzles 64 are closed, and which may be identified at
surface by an increase in the back pressure of the drilling
fluid.
[0040] The presence of the percussive action cutters 42 in the
centre of the bit 44 avoids the difficulties normally associated
with drilling the centre of the bore using a conventional bit.
Furthermore, in the event of loss of gauge of the cutter 42, it is
apparent from FIGS. 4 and 5 that such a reduction in diameter of
the cutter 42 would have no significant effect, as the swept area
of the roller cones 44, 45 overlaps the outer edge of the cutter
42.
[0041] Furthermore, if the cutting rate of the cones 44, 4S should
exceed the cutting rate of the cutter 42, and the cones 44, 45
"catch up" with the cutter 42, as illustrated in FIG. 5, the action
of the hammer tool may be affected, and the cutter 42 may
experience a potentially damaging increase in applied weight. In
this event, as noted above, the nozzles 64 are closed and an
increase in back pressure will be noted at surface. The operator is
therefore alerted to reduce the applied weight, slowing the cutting
speed of the cones 44, 45, for example by reducing the mechanical
weight applied to the cones 44, 45, and allowing the cutter 42 to
move ahead of the cones 44, 45, to the normal, optimum drilling
position.
[0042] On the other hand, if the cutting rate of the cutter 42
should exceed the cutting rate of the cones 44, 45, the anvil 48
will move towards or even beyond the end of the stroke of the
hammer 50, such that the energy transferred to the cutter 42 will
be reduced, and the cutting rate of the cutter 42 will decrease. In
other embodiments, the percussion tool may include a control which
stops the hammering action when the reaction weight or force
experienced by the tool or tool bit, in the form of cutter 42,
falls below a predetermined level.
[0043] Reference is now made to FIG. 6 of the drawings, which
illustrates, in section, a view of a part of a drilling apparatus
70 in accordance with a third embodiment of the present invention.
The apparatus 70 shares many features with the apparatus 40
described above, and additionally includes a spring 72 provided
between a shoulder on the anvil 74 and the ring 76 trapped between
the lower end of the hammer tool body 78 and the upper end of the
bit body 80. The spring 72 is selected such that the percussive
action cutter 82 is normally held slightly off bottom 84, as
illustrated. The cutter 82 thus only contacts the bore bottom 84
when the hammer 86 strikes the anvil 74 and drives the bit ahead
such that the cutter 82 impacts the formation. The cutter 82 is
thus touching the bottom of the hole only for the duration of the
hammer blow, and when the hammer 86 moves away from the anvil 74
the cutter 82 springs back off the bottom of the hole.
[0044] This feature of the apparatus 70 reduces the rubbing action
and wear experienced by the cutter 82, such that the apparatus 70
may safely be rotated at a relatively high speed better suited to
the cutting action of the cone cutters 90, 92; typically 150 to 200
rpm, rather than the relatively slow speeds (10 to 40 rpm) normally
utilised in percussive drilling.
[0045] Reference is now made to FIG. 7 of the drawings, which
illustrates drilling apparatus 100 in accordance with a third
embodiment of the present invention. The apparatus 100 shares a
number of similarities with the embodiments described above,
however rather than featuring roller cones, the apparatus 100 is
provided with a fixed cutting structure provided with
polycrystalline diamond compacts (PDCS) 102.
[0046] As with the apparatus 40 described above, the apparatus 100
features a centrally located percussive action cutter 104 which
will, in use, cut a pilot bore ahead of the PDC cutters 102.
[0047] Conventional drill bits provided with aggressive PDC cutters
normally require application of relatively high torques, which may
create difficulties in certain drilling situations. However, in
this embodiment of the present invention, the provision of the
percussive action cutter 104 will tend to reduce the torque
necessary to rotate the apparatus 100, as a proportion of the
cutting is being performed by the percussive action cutter 104,
which requires a relatively low input torque.
[0048] It will be clear to those of skill in the art that the above
embodiments of the present invention, in which one or more
percussive action cutters are combined with other cutters in a
single drilling apparatus, offers numerous advantages over the
prior art.
[0049] It will also be apparent to those of skill in the art that
the above embodiments are merely exemplary of the present
invention, and that various modification and improvements may be
made thereto without departing from the scope of the invention. For
example, the particular configuration of the apparatus may vary
from the configurations described above, and embodiments of the
invention may be provided in forms other than drill bits, for
example as reamers or shoes.
* * * * *