Boring or penetrating the earth – Bit or bit element – Rolling cutter bit or rolling cutter bit element
Reexamination Certificate
2000-06-08
2003-08-05
Bagnell, David (Department: 3672)
Boring or penetrating the earth
Bit or bit element
Rolling cutter bit or rolling cutter bit element
C175S420100
Reexamination Certificate
active
06601660
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to roller cone drill bit designs.
2. Background Art
Roller cone rock bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells.
FIG. 1
shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig
10
used to turn a drill string
12
which extends downward into a well bore
14
. Connected to the end of the drill string
12
is a roller cone-type drill bit
20
, shown in further detail in FIG.
2
.
Referring to
FIG. 2
, roller cone drill bits
20
typically comprise a bit body
22
having an externally threaded connection at one end
24
, and a plurality of roller cones
26
(usually three as shown) attached at the other end of the bit body
22
. The cones
26
are able to rotate with respect to the bit body
22
. Disposed on each of the cones
26
of the bit
20
is a plurality of cutting elements
28
typically arranged in rows about the surface of each cone
26
. The cutting elements
28
may be tungsten carbide inserts, superhard inserts such as polycrystalline diamond compacts, or milled steel teeth with or without hardface coating.
The cutting elements
28
on a cone
26
may include primary cutting elements, gage cutting elements, and ridge cutting elements. Primary cutting elements are the cutting elements arranged on the surface of the cone such that they contact the bottomhole surface as the bit is rotated to cut through the formation. Gage cutting elements are the cutting elements arranged on the surface of the cone to scrape the side wall of the hole to maintain a desired diameter of the hole as the formation is drilled. Ridge cutting elements are miniature cutting elements typically located between primary cutting elements to cut formation ridges that may pass between the primary cutting elements to protect the cones and minimize wear on the cones due to contact with the formation.
Significant expense is involved in the design and manufacture of drill bits to produce bits which have increased drilling efficiency and longevity. For more simple bit designs, such as those for fixed cutter bits, models have been developed and used to design and analyze bit configurations which exhibit balanced forces on the individual cutting elements of the bit during drilling. Fixed cutter bits designed using these models have been shown to provide faster penetration and long life.
Roller cone bits are more complex than fixed cutter bits, in that the cutting surfaces of the bit are disposed on roller cones, wherein each roller cone independently rotates relative to the rotation of the bit body about an axis oblique to the axis of the bit body. Because the cones rotate independently of each other, the rotational speed of each cone of the bit is likely different from the rotation speed of the other cones. The rotation speed for each cone of a bit can be determined from the rotational speed of the bit and the effective radius of the “drive row” of the cone. The effective radius of the drive row is generally related to the radial extent of the cutting elements that extend axially the farthest from the axis of rotation of the cone, these cutting elements generally being located on a so-called “drive row”. Adding to the complexity of roller cone bit designs, the cutting elements disposed on the cones of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing. Additionally, most modern roller cone bit designs have cutting elements arranged on each cone so that cutting elements on adjacent cones intermesh between the adjacent cones, as shown for example in FIG.
3
A and further detailed in U.S. Pat. No. 5,372,210 to Harrell. Intermeshing cutting elements on roller cone bits is desired to permit high insert protrusion to achieve competitive rates of penetration while preserving the longevity of the bit. However, intermeshing cutting elements on roller cone bits constrains cutting element layout on the bit, thereby, further complicating the designing of roller cone drill bits.
Because of the complexity of roller cone bit designs, accurate models of roller cone bits have not been widely developed or used to design roller cone bits. Instead, roller cone bits have largely been developed through trial and error. For example, if cutting elements on one cone of a prior art bit are shown to wear down faster that the cutting elements on another cone of the bit, a new bit design might be developed by simply adding more cutting elements to the cone that is known to wear faster in hopes of reducing the wear of each cutting element on that cone. Trial and error methods for designing roller cone bits have led to roller cone bits which have an imbalanced distribution of force on the bit. This is especially true for roller cone bits having cutting elements arranged to intermesh between adjacent cones.
One example of a prior art bit considered effective in the drilling wells is shown in
FIGS. 3A-3D
. This drill bit comprises a bit body
100
and three roller cones
110
attached to the bit body
100
, such that each roller cone
110
is able to rotate with respect to the bit body
100
about an axis oblique to the rotational axis of the bit body
100
. Disposed on each of the cones
110
is a plurality of cutting elements for cutting into an earth formation. As shown in
FIGS. 4
,
5
and
7
, the cutting elements include primary cutting elements
112
and gage cutting elements
156
. The cutting elements
112
,
156
are arranged about the surface of each cone
110
in generally circular, concentric rows substantially perpendicular to the axis of rotation of the respective cone as illustrated for primary cutting elements
112
in FIG.
3
C. In
FIG. 3A
, the profile of each row of cutting elements on each cone are shown in relation to each other to show the intermeshing of the cutting elements between adjacent cones. In this example, the cutting elements comprise milled steel teeth with hardface coating applied thereon. This type of drill bit is commonly referred to as a “milled tooth” bit.
As is typical for modern milled tooth roller cone bits, the teeth of the bit are arranged in three rows
114
a
,
114
b
, and
114
c
on the first cone
114
, two rows
116
a
and
116
b
on the second cone
116
, and two rows
118
a
and
118
b
on the third cone
118
. As shown in
FIG. 3A
, at least one row of teeth on each cone is arranged to intermesh with a row of teeth on an adjacent cone. The first row
114
a
of the first cone
114
is located at the apex of the cone and is typically referred to as the spearpoint of the bit.
The drilling performance of this prior art bit was simulated and analyzed using a method described in a patent application filed in the United States on Mar. 13, 2000, entitled “Method for Simulating the Drilling of Roller Cone Drill Bits and its Application to Roller Cone Drill Bit Design and Performance” and assigned to the assignee of this invention. From this analysis, it was shown that this prior art bit has normalized cone rotation ratios for the first
114
, second
116
and third
118
cones of 1.003:1.077:1, respectively, wherein the second cone
116
was found to rotate approximately 8% faster than the third cone
118
. Other prior art bit designs with cutting elements intermeshing between the cones were found to have rotation ratios with differed by more than 8%. From the analysis of the bit shown in
FIGS. 3A-3D
, it was also observed that the scraping distance of the gage inserts
156
on each cone during drilling significantly differed between the cones. For some prior art bits, the differences between the scraping distance of the gage inserts on each cone is due, in part, to the differences between the rotation speeds of the cones.
BRIEF SUMMARY OF THE INVENTION
The invention is directed to a roller cone drill bit for drillin
Huang Sujian
Singh Amardeep
Xiang Ying
Bagnell David
Dougherty Jennifer R.
Rosenthal & Osha L.L.P.
Smith International Inc.
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