Boring or penetrating the earth – Bit or bit element – Rolling cutter bit or rolling cutter bit element
Reexamination Certificate
2000-06-08
2003-10-28
Tsay, Frank (Department: 3672)
Boring or penetrating the earth
Bit or bit element
Rolling cutter bit or rolling cutter bit element
C175S431000
Reexamination Certificate
active
06637527
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 borehole
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
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. 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.
Typically, roller cone bits, especially bits with milled steel teeth, include one or more cutting elements arranged about the apex of at least one cone to cut through formation near the center of the bit. The cone apex having cutting elements arranged thereon is commonly referred to as a “spearpoint” of the bit. One example of a spearpoint on one cone of a roller cone drill bit is shown at
114
a
in FIG.
3
A.
Some bits exist which do not include a spearpoint to cut formation near the center of the bit. These bits are commonly referred to as “coring bits” and are used for drilling a borehole with an uncut center (or core) within the hole. Coring bits differ from conventional roller cone bits in that coring bits are purposefully designed to form a core within in the borehole as the borehole is drilled. On the other hand, conventional roller cone bits are designed to drill the entire formation in the borehole, wherein formation near the center of the bit is drilled by the spearpoint of the bit, typically located at the apex of one cone.
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 substantially 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 faster worn cone 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 which have cutting elements arranged to intermesh between adjacent cones and a spearpoint on one of the cones.
One example of a prior art bit considered effective in the drilling wells is shown in
FIGS. 3A-3D
. This drill bit comprises abit body
100
and three roller cones
110
attached thereto, such that each roller cone
110
is able to rotate with respect to the bit body
100
about an axis oblique to the bit body
100
. Disposed on each of the cones
110
is a plurality of cutting elements
112
for cutting into an earth formation. The cutting elements are arranged about the surface of each cone in generally circular, concentric rows substantially perpendicular to the axis of rotation of the respective cone as illustrated in FIG.
3
C. In
FIG. 3A
, the profiles 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 modem 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
, the teeth of the bit are arranged on the cones such that at least one row of teeth on each cone intermeshes with a row of teeth on an adjacent cone.
As is typically for milled tooth roller cone bits, the first row of teeth
114
a
on the first cone
114
is located at the apex of the cone to cut formation at the center of the bit, proximal to the bit axis of rotation, as shown in FIG.
3
B. This row of teeth located at the apex of the first cone is referred to as the spearpoint of the bit, as described above. To avoid contact with the spearpoint on the first cone, the apexes of the other two cones
116
,
118
are truncate
Huang Sujian
Singh Amardeep
Rosenthal & Osha L.L.P.
Smith International Inc.
Tsay Frank
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