Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CASING EXIT MILL ASSEMBLIES WITH REPLACEABLE BLADE SLEEVE
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Application No. 14/698320, filed
on April
28, 2015, which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The invention relates generally to the design and construction of
downhole
milling tools used to perform casing exits and other metal cutting operations.
2. Description of the Related Art
[0002] Conventional large size casing exit mills have a cylindrical body with
a larger
diameter section that transitions to a smaller diameter pipe on both sides at
a taper angle.
Blades are welded upon the taper and an enlarged section. The blades are
typically brazed
with crushed tungsten carbide particles or a tungsten carbide insert of a
particular shape
suitable for cutting metal. Blades are welded onto the body, typically with a
3/8" fillet weld.
The brazing of carbide onto the blades is carried out usually at temperatures
that are from
about 1650 F to 1700 F. If not performed in a uniform manner, that high
temperature heating
process results in bowing of the body and softening of the steel adjacent to
the blades.
Though mills are often heat treated after welding and brazing, consistency in
the mechanical
properties on the surface of the mill body is doubtful. Every heat cycle on
the mill body and
associated components changes the mechanical strength of the surface fibers.
During a casing
exit operation, the mill is subjected to cyclic bending loads and intermittent
torsional loads.
These loads induce cyclic bending stress and torsional stress in the surface
fibers. The
superimposition of the axial component of torsional stress and bending stress
causes the
surface fibers to fail resulting in cracks on the body. Hence, it is necessary
for the surface
fibers to maintain their mechanical strength during each casing exit
operation. The higher the
number of heat cycles, the higher the tendency for surface fibers to become
softer, thus
making the body susceptible to cracking under low bending stress. This
reduction in the
strength of surface fibers reduces the fatigue life of the body.
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SUMMARY OF THE INVENTION
[0003] The invention provides improved designs for the construction of
downhole
mills. In other aspects, the invention provides methods of assembling a mill.
An exemplary
casing exit mill assembly is described in which a central shaft body is
provided with a
mounting portion that is shaped to prevent rotation of a blade sleeve mounted
thereupon. In
described embodiments, the mounting portion presents a plurality of arcuately
curved contact
surfaces. Adjacent contact surfaces on the mounting portion adjoin each other
at angled
corners. There are at least three curved contact surfaces. According to
preferred
embodiments, there are between three and twelve contact surfaces. The cross-
sectional shape
of each of the contact surfaces is defined as an arcuate segment from a circle
having a radius
which is greater than the radius of the contact surface upon the shaft body
mounting portion.
[0004] A bearing and a blade sleeve surround the mounting portion. The bearing
could be a separate component from the shaft and blade sleeve. In other
embodiments, the
bearing is a coating formed upon either the mounting portion of the shaft body
or upon the
inner surface of the central opening of the blade sleeve. The blade sleeve is
provided with a
central opening that is shaped and sized to be complementary to the mounting
portion of the
shaft body. Preferably, the components are secured together using a press fit
or interference
fit. The modular construction of the mill assembly permits the blade sleeve to
be easily
replaced when worn or damaged. Alternatively, the blade sleeve could be
replaced by a blade
sleeve having a different diameter or design of cutting structures.
[0005] The inventor has found that the use of a mounting portion having
arcuately
curved contact surfaces and a blade sleeve having a complementarily shaped
engagement
surface is advantageous. Torque forces can be effectively transmitted between
the
components while minimizing the stress that might result from other
interfaces.
[0006] In certain embodiments, the shaft body of the mill assembly includes a
radially
enlarged portion which is located proximate the blade sleeve. A removable
protection blade
sleeve radially surrounds the radially enlarged portion. Like the blade
sleeve, the protection
blade sleeve is preferably press fit and features a tapered interface with the
radially enlarged
portion.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0007] For a thorough understanding of the present invention, reference is
made to
the following detailed description of the preferred embodiments, taken in
conjunction with the
accompanying drawings, wherein like reference numerals designate like or
similar elements
throughout the several figures of the drawings and wherein:
[0008] Figure 1 is a side, cross-sectional view of an exemplary casing exit
mill
assembly constructed in accordance with the present invention.
[0009] Figure 2 is an enlarged side, cross-sectional view of portions of the
mill of
Figure 1.
[0010] Figure 3 is an axial cross-section of the mill assembly taken along
lines 3-3 in
Figure 2.
[0011] Figure 4 is an axial cross-section of the mill assembly of Figures 1-3,
now
being subjected to torsional force.
[0012] Figure 5 is a schematic view of the mounting portion of the mill
assembly
shown in Figs. 1-4 in comparison to a larger circle.
[0013] Figure 6 is a side view of the shaft body of an alternative mill
assembly
wherein the mounting portion of the shaft body has six contact faces.
[0014] Figure 7 is a side view of an alternative mill assembly which
incorporates the
shaft body shown in Figure 6.
[0015] Figure 8 is an axial cross-section taken along lines 8-8 in Figure 6.
[0016] Figure 9 is an axial cross-section taken along lines 9-9 in Figure 7.
[0017] Figure 10 is a further axial cross-section of a mounting portion with
six
contact faces illustrating exemplary geometric features of the mounting
portion.
[0018] Figure 11 is a side, cross-sectional view of another enlarged portion
of the
shaft body.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] Figures 1-2 depicts an exemplary casing exit mill assembly 10
constructed in
accordance with the present invention. The mill assembly 10 includes a central
shaft body 12
which defines an axial fluid passage 14 along its length. Threaded connections
16 are
provided at each axial end of the shaft body 12 to permit the mill assembly to
be incorporated
into a downhole work string. The shaft body 12 is preferably cylindrical in
shape along its
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length except where otherwise indicated. A mounting portion 18 of the shaft
body 12 has a
shaped outer radial surface upon which a bearing 20 and a blade sleeve 22 are
mounted. The
outer radial surface of the mounting portion 18 is shaped to preclude rotation
of the bearing
20 and blade sleeve 22 with respect to the shaft body 12. With further
reference to Figures 3-
4, it can be seen that the mounting portion 18 presents three arcuately curved
convex contact
faces 24 which adjoin one another at corners 26. The mounting portion 18 is
designed to
ensure that the blade sleeve 22 and bearing 20 do not rotate upon the shaft
body 12. It should
be appreciated with reference to Figures 3 and 4 that the mounting portion 18
is generally
nearly round but still provides corners 26 that will prevent rotation of the
surrounding bearing
20 and blade sleeve 22 upon the shaft body 12. It is noted that, in axial
cross-section (i.e.,
Figures 3-4) the contact faces 24 of the mounting portion 18 preferably each
form an arcuate
segment of a circle that has a radius greater than the radius 25 of the
mounting portion 18 at
its widest point. Figure 5 illustrates a larger circle 27 of which a contact
face 24 forms an
arcuate segment. The larger circle 27 has a radius 29 that exceeds the radius
25 of the
mounting portion 18 at its widest point. In certain preferred embodiment, the
radius 29 is at
least twice as large as the radius 25.
[0020] Referring once again to Figure 1, it is noted that the shaft body 12
also
includes a radially enlarged portion 31 which is located proximate, but a
spaced distance from,
the mounting portion 18. A protection blade sleeve 33 radially surrounds the
radially enlarged
portion 31 and presents a hardened protruding portion for engaging a
surrounding tubular
during casing window cutting. The radially enlarged portion 31 and the
protection blade
sleeve 33 are preferably press fit.
[0021] Preferably, the mounting portion 18 is axially tapered to allow for
ease of
assembly. As illustrated in Figure 2, the mounting portion 18 is tapered at an
angle "r". In
currently preferred embodiments, the angle of taper "r" is about 2 degrees, as
measured from
the central axis of the shaft body 12. This taper facilitates a press fit
securing of the bearing
20 onto the mounting portion 18. As best seen in Figures 3-4, the blade sleeve
22 presents a
plurality of cutting blades 28 which project radially outwardly from the
sleeve 22. The cutting
blades 28 are formed of or brazed with carbide cutters or other hardened
cutting structures.
[0022] The bearing 20 is preferably formed of a material that is softer than
the
material making up the shaft body 12 and the blade sleeve 22. In preferred
embodiments, the
bearing 20 is formed of copper, manganese or bronze, or alloys which include
these materials.
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In alternative embodiments, the bearing 20 is formed of a viscoelastic
material which provides
an effective damper for torsional vibrations and shocks. The bearing 20 may be
in the form of
a separate component that is disposed between the shaft body 12 and the blade
sleeve 22.
Alternatively, the bearing 20 may be in the form of a coating that is applied
to either or both of
the shaft body 12 and/or the blade sleeve 22.
[0023] Figures 3 and 4 help illustrate the resistance of the blade sleeve 22
to rotation
in response to torsional loading. Figure 3 illustrates the mill assembly 10
having no torsional
loading applied. In Figure 4, a torsional force is being applied to the blade
sleeve 22, as
indicated by arrow 30. Bearing 20, if made of a viscoelastic material, deforms
slightly to
accommodate the loading. Absorption of torque spikes by the bearing 20 will
help damp
torsional impact loads on the blades 28. The absorption of torque spikes will
essentially result
in less wear upon the blades 28 and increase the life of the mill assembly 10.
[0024] The mill assembly 10 can be constructed by first sliding the bearing 20
onto
the shaft body 12 and over the mounting portion 18. Thereafter, the blade
sleeve 22 is slid
onto the shaft body 12 to overlie the bearing 20 so that the bearing 20 is
located radially
between the shaft body 12 and the blade sleeve 22. A press fit, or
interference fit, affixes the
three components together.
[0025] It is noted that the modular construction of the mill assembly 10
permits users
to easily replace a worn blade sleeve 22 or to replace the blade sleeve 22
with a blade sleeve
having a larger or smaller outer diameter or having a different type or design
of cutting blades
or structures.
[0026] In operation, the mill assembly 10 is constructed as described above
and is
then incorporated into a work string. Thereafter, the work string is disposed
into a wellbore.
The mill assembly 10 is run in to a desired location within the wellbore and
rotated, in a
manner known in the art, so that the blade sleeve 22 of the mill assembly 10
mills or cuts away
desired material.
[0027] In order to remove the blade sleeve 22 from the mill shaft body 12, as
well as
the bearing 20, axial force is applied to the blade sleeve 22 proximate the
larger end of the
taper of the mounting portion 18. Arrow 32 in Figure 2 illustrates the
application of such
force. The force applied must be sufficient to overcome the frictional force
of the press fit,
thereby causing the blade sleeve 22 and perhaps the bearing 20 to be unseated
from the mill
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shaft body 12. Thereafter, the blade sleeve 22 and/or the bearing 20 could be
replaced as
necessary.
[0028] Figures 6-10 illustrate features of an alternative embodiment for a
mill
assembly 50 constructed in accordance with the present invention. Except where
otherwise
described, the mill assembly 50 is constructed and operates in the same manner
as the mill
assembly 10 described previously. The shaft body 12' of the mill assembly 50
includes a
tapered mounting portion 18'. The mounting portion 18' has six contact faces
24' rather than
three. The contact faces 24' adjoin each other at corners 26'. As Figure 9
depicts, a bearing
20' may be disposed between the mounting portion 18' and the blade sleeve 22'.
[0029] Figures 8 and 10 help to illustrate geometric features associated with
the
contact faces 24' and the mounting portion 18' of the mill assembly 50. A true
circle 52 is
depicted in Figures 8 and 10 superimposed around the outer perimeter of the
mounting
portion 18'. Figure 10 shows that a gap 54 is defined between the contact
faces 24' and the
true circle 52. The gap 54 results from the fact that the central portions 55
of the contact
faces 24' are defined as arcuate segments of a circle having a radius 56 which
is larger than the
radius 58 of the mounting portion 18' at its widest point. In other words, the
contact faces
24' have a larger radius of curvature than circle 52. Figure 10 also
illustrates an exemplary
hexagonal outer surface 60 superimposed on the mounting portion 18' and drawn
with its
vertices at the corners 26'. It can be seen that the contact faces 24' extend
radially outwardly
beyond the sides 62 of the hexagonal outer surface 60.
[0030] In preferred embodiments, the contact faces 24' are further shaped to
present
contoured transition surface portions 64 that are located proximate the
corners 26' of the
mounting portion 18'. The transition surface portions 64 have a radius of
curvature 66 smaller
than either the true circle 52 or the central portions 55 of the contact faces
24'.
[0031] It is further noted that the mounting portion 18' is preferably axially
tapered,
as illustrated by Figure 6, in the same manner as described for mounting
portion 18. The
tapering will aid in assembly and repair of the mill assembly 50.
[0032] The inventor has determined that the use of arcuately curved contact
faces in
a mounting portion along with angled corners which adjoin the contact faces is
an
advantageous design for transmission of torque forces between the shaft body
and the
surrounding blade sleeve. Stress concentrations which are associated with
other designs are
avoided. In particular, shaft body/blade sleeve interfaces constructed in
accordance with the
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present invention provide increased contact area between the contact faces 24,
24' and the
surrounding blade sleeve 22, 22', thereby increasing the ability of torque
forces to be
transmitted between the components and supplementing force transmission
between the
corners 26, 26' and the surrounding blade sleeve 22, 22'. According to certain
preferred
embodiments, contact faces, such as contact faces 24' have outer radial
portions with different
radii of curvature. For example, the central portion 55 of each contact face
24' has a radius of
curvature 56 that is greater than the radius of curvature 58 of the mounting
portion 18', as
measured from its widest point. Lateral portions 64 of the contact face 24',
however, have a
radius of curvature 66 that is smaller than the radius of curvature 58 for the
mounting portion
18' and the radius of curvature 56 of the central portion 55 of the contact
face 24'.
[0033] In particular embodiments, the mounting portions 18, 18' have at least
three
contact faces or the type described previously. In preferred embodiments,
there are from three
to twelve contact faces. In particularly preferred embodiments, there are from
three to six
contact faces.
[0034] Figure 11 illustrates the radially enlarged portion 31 and the
protection blade
sleeve 33 in greater detail. The radially enlarged portion 31 preferably
presents an outer radial
surface that is shaped with contact faces and corners similar to those
described previously with
respect to the mounting portions 18, 18'. The largest diameter on this axially
tapered enlarged
portion 31 is preferred to be equal to or smaller than the smallest diameter
of the mounting
portion 18, to aid the axial passage of the smallest inner diameter of the
blade sleeve 22.
Preferably also, the radially enlarged portion 31 is axially tapered in the
same manner as the
mounting portions 18, 18'. In certain embodiments, a bearing 68 is disposed
between the
radially enlarged portion 31 and the protection blade sleeve 33. During
operation, the
protection blade sleeve 33 will provide a hardened contact point for the mill
assembly 10 or 50
to assist in casing window exit operations.
[0035] Those of skill in the art will recognize that numerous modifications
and
changes may be made to the exemplary designs and embodiments described herein
and that the
invention is limited only by the claims that follow and any equivalents
thereof
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