U.S. patent application number 11/083540 was filed with the patent office on 2005-11-17 for drill bit having increased resistance to fatigue cracking and method of producing same.
This patent application is currently assigned to Smith International, Inc.. Invention is credited to Bramlett, Kenneth, Griffo, Anthony, Liang, Dah-Ben, Potticary, Adam.
Application Number | 20050252691 11/083540 |
Document ID | / |
Family ID | 35308332 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050252691 |
Kind Code |
A1 |
Bramlett, Kenneth ; et
al. |
November 17, 2005 |
Drill bit having increased resistance to fatigue cracking and
method of producing same
Abstract
The present invention relates to a roller cone drill bit that
includes a bit body adapted to be rotated about a longitudinal
axis, where the bit body has at least one leg depending therefrom,
wherein the leg comprises a treated portion that provides a
residual compressive stress, and a roller cone rotatably mounted on
a journal. The treated portion treatment may comprise one selected
from shot peening, laser-shock peening, and hammer peening.
Further, the present invention relates to a method of manufacturing
a roller cone drill bit that includes inducing a compressive
stress, through plastic deformation, in at least a portion of at
least one leg depending from a bit body. The inducing a compressive
stress may comprise one selected from shot peening, laser-shock
peening, and hammer peening.
Inventors: |
Bramlett, Kenneth; (Conroe,
TX) ; Griffo, Anthony; (The Woodlands, TX) ;
Liang, Dah-Ben; (The Woodlands, TX) ; Potticary,
Adam; (Calgary, CA) |
Correspondence
Address: |
OSHA LIANG L.L.P.
1221 MCKINNEY STREET
SUITE 2800
HOUSTON
TX
77010
US
|
Assignee: |
Smith International, Inc.
Houston
TX
|
Family ID: |
35308332 |
Appl. No.: |
11/083540 |
Filed: |
March 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60554633 |
Mar 19, 2004 |
|
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|
Current U.S.
Class: |
175/374 |
Current CPC
Class: |
E21B 10/08 20130101;
B23K 26/356 20151001 |
Class at
Publication: |
175/374 |
International
Class: |
E21B 010/00 |
Claims
What is claimed is:
1. A roller cone drill bit comprising: a bit body adapted to be
rotated about a longitudinal axis, the bit body having at least one
leg depending therefrom, wherein the at least one leg comprises a
treated portion that provides a residual compressive stress; and a
roller cone rotatably mounted on a journal.
2. The roller cone drill bit of claim 1, wherein a treated portion
of the at least one leg comprises a leg backtum face.
3. The roller cone drill bit of claim 1, wherein the treated
portion is a plastically deformed area.
4. The roller cone drill bit of claim 3, wherein the plastically
deformed area extends inward about 0.035 inch.
5. The roller cone drill bit of claim 1, wherein the treated
portion treatment comprises one selected from shot peening,
laser-shock peening, and hammer peening.
6. The roller cone drill bit of claim 5, wherein the peening
provides 100 percent coverage.
7. The roller cone drill bit of claim 1, further comprising a
hardface coating disposed on a region adjacent the treated
portion.
8. The roller cone drill bit of claim 2, wherein the treated
portion of the leg backturn face of the at least one leg comprises
p-features.
9. The roller cone drill bit of claim 8, wherein the p-features
comprise inserts.
10. A method of manufacturing a roller cone drill bit comprising:
inducing a compressive stress, through plastic deformation, in at
least a portion of at least one leg depending from a bit body.
11. The method of claim 10, wherein the portion comprises a leg
backturn surface.
12. The method of claim 10, wherein inducing a compressive stress
comprises one selected from shot peening, laser-shock peening, and
hammer peening.
13. The method of claim 10, further comprising applying a hardface
coating to a region adjacent the treated portion.
14. The method of claim 12, wherein the shot peening comprises shot
having a hardness between 45 and 52 Rc.
15. The method of claim 12, wherein the shot peening comprises shot
having a size between 10 and 18 mesh.
16. The method of claim 10, wherein the shot peening provides 100
percent coverage.
17. The method of claim 10, wherein the shot peening has a peening
intensity between 7 and 9 C.
18. The method of claim 10, wherein the inducing a compressive
stress is performed on at least one portion of at least one leg
with p-features.
Description
BACKGROUND OF INVENTION
[0001] 1. Field of the Invention
[0002] The invention is related generally to the field of roller
cone drill bits. More particularly, the invention is related to
methods for increasing the durability of a roller cone bit.
[0003] 2. Background Art
[0004] Drill bits used to drill wellbores through earth formations
generally fall within one of two broad categories of bit
structures. Shear cutter bits are configured with a multitude of
cutting elements directly fixed to the bottom, also called the
face, of the drill bit. The shear bit has no moving parts, and its
cutters scrape or shear rock formation through the rotation of the
drill bit by an attached drill string. Shear cutter bits have the
advantage that the cutter is continuously in contact with the
formation and see a relatively uniform loading when cutting the
gage formation.
[0005] Furthermore, the shear cutter is generally loaded in only
one direction. This significantly simplifies the design of the
shear cutter and improves its robustness. However, although shear
bits have been found to drill effectively in softer formations, as
the hardness of the formation increases, it has been found that the
cutting elements on the shear cutter bits tend to wear and fail,
affecting the rate of penetration (ROP) for the shear cutter
bit.
[0006] In contrast, roller cone rock bits are better suited to
drill through harder formations. Roller cone rock bits are
typically configured with three rotatable cones that are
individually mounted to separate legs. The three legs are welded
together to form the rock bit body. Each rotatable cone has
multiple cutting elements such as hardened inserts or milled
inserts (also called "teeth") on its periphery that penetrate and
crush the formation from the hole bottom and side walls as the
entire drill bit is rotated by an attached drill string, and as
each rotatable cone rotates around an attached journal. Thus,
because a roller cone rock bit combines rotational forces from the
cones rotating on their journals, in addition to the drill bit
rotating from an attached drill string, the drilling action
downhole is from a crushing force, rather than a shearing force. As
a result, the roller cone rock bit generally has a longer life and
a higher rate of penetration through hard formations.
[0007] FIG. 1 depicts a roller cone drill bit 30 which comprises a
bit body 32 that is adapted to rotate about a longitudinal axis L.
Three legs 36 extend downwardly from the bit body 32. The legs 36
are spaced 120 degrees apart along the circumference of the bit
body 32. The upper end of the bit body 32 includes a threaded pin
38 which can be coupled to another tool, usually a drill string
(not shown). A roller cone 40 is rotatably coupled to each leg 36.
The roller cones 40 have cutting elements 42 which deform earth
formation as the drill bit 30 is rotated about the longitudinal
axis L. Although the drill bit 30 is shown as having three legs 36,
those of ordinary skill will appreciate that other numbers of legs
may also be used.
[0008] FIG. 2 shows a partial cross section of one of the legs 36
shown in FIG. 1. The leg 36 terminates in a shirttail portion 44. A
bearing pin 46 extends from the shirttail portion 44. The bearing
pin 46 includes a journal 50, an axial thrust face 52, and a nose
pin 54. The journal 50 forms a main bearing surface 56 for the
roller cone 40. The roller cone 40 has a bearing surface 58 which
provides a bearing for the main bearing surface 56. The nose 54
forms a bearing surface 60 which is retained within a complementary
surface 62 within the roller cone 40.
[0009] Lubricant is fed between the bearing surfaces 56 and 58
through one or more lubrication ports (not shown) in the journal 50
to minimize friction between the bearing surfaces. Friction between
the bearing surfaces 56 and 58 may also be minimized by placing a
low-friction bearing material, such as a low-friction pad 64, a
roller bearing (not shown), a ball bearing (not shown), or other
type of anti-friction bearing between the bearing surfaces. The
lubrication ports (not shown) in the journal 50 communicate with a
lubrication passage 66, which is connected to receive lubricant
from a grease reservoir 67 (shown in FIG. 1) in the upper part of
the leg 36. A seal 68 is provided to retain the lubricant between
the bearing surfaces 56 and 58.
[0010] A number of different forces can result in early failure of
roller cone bits. In particular, much attention has been paid to
the stresses that are applied to the inserts and the cones during
typical drilling operations. These stresses tend to be very high
and are cyclical. The cyclical nature of the stress results from
the fact that each cutting element is only in contact with rock
being drilled for a portion of the time when the drill bit is being
rotated. As a result, it is common for the cone to crack within
some of the sockets. Cracking in one or more of the sockets can
result in loss of the inserts pressed into the sockets that undergo
cracking, or can result in total cone failure.
[0011] Various methods have been developed to adjust the
distribution of stresses in roller cones with the objective of
reducing cracking and insert loss. For example, U.S. Pat. No.
4,181,187 issued to Lumen describes cutting stress relief grooves
in the bottom of the sockets. U.S. Pat. No. 3,970,158 issued to
Black describes placing a compressible (malleable) material at the
bottom of the sockets to absorb some of the cyclic stress applied
to the inserts. The malleable material extrudes against the sides
of the socket to reduce the incidence of cracking.
[0012] In addition, U.S. Pat. No. 6,598,689 discloses treating the
interior surface of a socket to provide a residual compressive
stress near that portion of the socket. In one embodiment, that
patent discloses providing residual compressive stress inside an
interior surface of the socket by a shot peening process.
[0013] However, while various improvements have been made in
reducing the failure rate on the cones, as described in the above
patents, less attention has been paid to the other surfaces of the
bit. In particular, the legs are subject to tensile stresses which
lead to cracks in the surface, potentially resulting in breakage
and failure of the bit. In particular, a portion of backturn
surface of the leg has a low section modulus, which leads to a high
failure rate.
[0014] What is still needed, therefore, are techniques and bits
that have improved durability and fatigue resistance.
SUMMARY OF INVENTION
[0015] In one aspect, the present invention relates to a roller
cone drill bit that includes a bit body adapted to be rotated about
a longitudinal axis, where the bit body has at least one leg
depending therefrom, wherein the leg comprises a treated portion
that provides a residual compressive stress, and a roller cone
rotatably mounted on a journal. The treated portion treatment may
comprise one selected from shot peening, laser-shock peening, and
hammer peening
[0016] In one aspect, the present invention relates to a method of
manufacturing a roller cone drill bit that includes inducing a
compressive stress, through plastic deformation, in at least a
portion of at least one leg depending from a bit body. The inducing
a compressive stress may comprise one selected from shot peening,
laser-shock peening, and hammer peening.
[0017] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 shows a prior art roller cone drill bit;
[0019] FIG. 2 shows a cross-section of one leg of the prior art
drill bit of FIG. 1;
[0020] FIG. 3a shows an outside view of a leg illustrating a back
turn area treated in accordance with one embodiment of the present
invention; and
[0021] FIG. 3b shows an inside view of a leg illustrating a back
turn area treated in accordance with one embodiment of the present
invention.
DETAILED DESCRIPTION
[0022] In one aspect, the present invention relates to a method for
improving the durability of a roller cone bit. In another aspect,
the present invention relates to a drill bit having improved
durability when compared to prior art bit. In particular, the
methods and bits of the present invention comprise treating at
least a portion of a roller cone bit leg surface.
[0023] In one embodiment, the treating comprises inducing a
residual compressive stress in a backturned surface of at least one
roller cone drill bit leg. In one specific embodiment, inducing the
residual stress comprises applying a shot peening process to the
selected area (i.e., at least a portion of the leg backturn area).
As those having ordinary skill in the art are aware, shot peening
is a cold working process in which the surface of a part is
bombarded with small spherical media called shot. Shot-peening is a
controlled cold work process in which the surface of a part is
bombarded with controlled impingement of a stream of high velocity
shot, causing plastic deformation on part surface and resulting in
the surface being compressively stressed.
[0024] Each piece of shot striking the surface acts as a tiny
peening hammer, which imparts a small indentation or dimple on the
surface. Below the surface, the compressed grains try to restore to
its original shape, which produces a hemisphere of cold-worked
material, that is stressed in compression. By providing compressive
stress, cracks are less likely to be initiated or to propagate. As
a result, by inducing a compressive stress on at least a portion of
at least one leg, the present invention reduces the likelihood that
cracks will occur on the surface of a rock bit leg.
[0025] Turning to specific embodiments, the present inventors have
discovered that the leg backtum area, as shown in the Figures
below, is an area that is subject to high tensile stress during
drilling operations and, therefore, is more prone to leg breakage,
especially when stress risers in the area such as P-features,
ballhole weld and hardfacing interfaces, etc. are present. In
addition, the corrosive environment in which the drill bits
operate, due to the presence of O.sub.2, CO.sub.2, and H.sub.2S
gases, for example, can cause corrosion of the legs, which also may
lead to early failure. In addition, the drilling fluids (especially
alkaline fluids) used in drilling operations can cause corrosion of
the bit legs.
[0026] The present inventors have further discovered that inducing
compressive stress is effective in retarding, and in many cases,
preventing corrosion, e.g. pitting corrosion, stress corrosion
cracking and corrosion fatigue. Moreover, the present inventors
have discovered that inducing a compressive stress may improve
other aspects of performance, for example improving resistance to
thermal fatigue, axial fatigue, bending fatigue, and torsional
fatigue. Also, an induced compressive stress may provide improved
performance in the regions affected by welding, carburization, or
other physical changes on the leg.
[0027] FIG. 3a illustrates a first embodiment of the present
invention. In FIG. 3a, an outside view of a single leg 100 of a
roller cone bit (not separately numbered) is shown. A roller cone
106 is rotatably coupled to the leg 100. A backturn area 130 is
disposed between the roller cone 106 and a threaded end 110 of the
roller cone bit. In addition, FIG. 3a shows that a portion of the
leg 116 has been covered by hardfacing. The application of
hardfacing 116 causes additional stress on the leg 100. In
addition, FIG. 3a shows p features 122 disposed on the leg 100. The
"p features" comprise inserts that are pressed in to give
additional wear resistance to the leg backface. Also shown in FIG.
3a is ballhole weld 132 which is the point at which the cone is
locked to the leg 100. In this embodiment, at least a portion of
the backtum area 130 has an induced compressive stress ("treated
portion"). As the term is used herein, "treated portion" refers to
an area that has been plastically deformed to create a surface
compressive state.
[0028] In one embodiment, the compressive stress is induced by shot
peening a select portion of the backtum area 130. The select
portion of the backturn area 130 may be the entire region, or may
be significantly less than all of the backtum area 130. However, in
certain embodiments the region adjacent to the hardfacing 116 is
treated in order to increase the resistance of the leg 100 to
fatigue cracking.
[0029] Further, in certain embodiments, reservoir 120 and seal area
112 are protected, by means known in the art, such as by masking
off with a selected area with industrial (heavy-duty) masking tape,
from the shot peening process (or other treatment methods, as noted
below). Another suitable means for protecting an area may be a
plastic covering, or plug. Those having ordinary skill will
appreciated that other types of protection may be used. The
reservoir 120 and the seal area 112 can be damaged by the treatment
process, if care is not taken to protect those regions. Similarly,
as shown in FIG. 3b, dome vent hole 134 should be protected from
the treatment process, in order to avoid damaging the dome vent
hole 134.
[0030] In an embodiment of the invention using shot peening, a rain
of metallic shot impinges at high speed, on the backturned surface
of a rock bit leg. Those having ordinary skill will recognize that
ceramic, glass, or other suitable types of shot may be used. The
plastically deformed portion extends inwards from several
thousandths of an inch to a few hundredths. The specific amount of
cold working (i.e., the imparted compressive strength) depends
principally on the plastic work done by the pellets, which in turn
is dependent on the size and speed of the pellets and the total
number of impacts. For different materials and locations, there are
different combinations of shot size, speed, duration, which may be
varied depending on the particular application.
[0031] Coverage is defined as the extent (in percept) of uniform
and complete dimpling or obliteration of the original surface of
the part or work piece. Inspection of percent coverage can be
accomplished using a ten power (10.times.) magnifying glass. In one
embodiment, the treated portion has 100 percent coverage. 100
percent coverage is reached when the original surface of the
material is obliterated entirely by overlapping peening
dimples.
[0032] Calibration of the impact energy or peening intensity of the
shot stream is essential to controlled shot peening. The energy of
shot stream is a function of the media size, material, hardness,
velocity and impingement angle. In order to specify, measure and
calibrate peening impact energy, J. O. Almen of General Motors
Research Lab developed a method utilizing SAE 1070 spring steel
specimens which he called Almen strips. In his method, an unpeened
Almen strip is fastened to a steel block and exposed to a stream of
peening shot for a given period of time. Upon removal from the
block, the residual compressive stress and surface plastic
deformation produced by the peening impacts will have caused the
Almen strip to curve convexly on the peened surface. The height of
this curvature when measured in a standard Almen gauge is called
arc height. There are 3 standard Almen strips currently in use: "A"
strip 0.051" thick, "C" strip 0.094" thick and "N" strip 0.031"
thick. Intensity designations should include both arc height and
the type of Almen strip used: e.g. 9C intensity=0.009" arc height
on the "C" strip.
[0033] In a preferred embodiment, the shot peening operation is
performed in accordance with AMS-S-13165 using a shot peening
machine, such as specified by AMS S-13165. Further, in certain
embodiments, a shot size that is used is specified as "cast steel
shot 460" which has hardness at 45-52 Rc and shot size between
10-18 (0.0787" and 0.0394") mesh per AMS S-13165. Additionally, in
one embodiment, the treated region has 100 percent coverage. In one
embodiment, the shot-peening intensity is between 0.007" and
0.009", or between 7 and 9 C. Those having ordinary skill will
recognize that these standards for shot peening are publicly
available, and that other methods of shot peening may be used.
Moreover, those having ordinary skill in the art will appreciate
that shot peening is only one method to induce a residual
compressive strength and that other methods may be used that do not
depart from the scope of the present invention.
[0034] Those having ordinary skill in the art will realize that the
above considerations are merely examples, and that no limitation on
the scope of the invention is intended thereby. Depending on the
material to be treated, the thickness of the material, the coverage
required, and other known considerations, a wide range of methods,
shot sizes, and shot material may be used in order to induce the
compressive stress.
[0035] Although various embodiments of this aspect of the invention
are described in terms of shot peening the backturned surface of at
least one roller cone leg, it should be understood that any
treatment which provides a residual compressive stress on the
backturned surface of at least one roller cone leg can also be used
in accordance with this aspect of the invention. Two such treatment
methods are hammer peening and laser peening. Laser peening and
hammer peening are known in the art for providing residual
compressive stress in materials. In some embodiments, the hammer
peening and laser peening may be performed over the same portions
of the backturned surface of at least one roller cone leg as
described for shot peening.
[0036] Two bits that have been treated according to embodiments of
the invention on two of the three legs were field tested. Both bits
had severe cracks on the untreated legs, while the treated legs did
not develop any significant cracks.
[0037] Advantageously, embodiments of the present invention induce
compressive residual stresses in roller cone legs, which offset
tensile stresses caused by drilling operations and, therefore,
provide increases in bit life. In addition, embodiments of the
present invention may be effective in retarding, and in many cases,
preventing corrosion, e.g. pitting corrosion, stress corrosion
cracking and corrosion fatigue.
[0038] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
* * * * *