Note: Descriptions are shown in the official language in which they were submitted.
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1 "SHEAR COUPLING ASSEMBLY WITH BACKOFF PREVENTION"
2
3 FIELD OF THE INVENTION
4 Embodiments of the invention relate to a shear coupling assembly
used for connecting a downhole pump to a terminal end of an actuating rod
6 string in reciprocating pumped wells, and more particularly to a shear
coupling
7 having means for substantially preventing back-off of a threaded connection
8 therebetween.
9
BACKGROUND OF THE INVENTION
11 Downhole reciprocating pumps are positioned and actuated in a
12 wellbore by a rod string extending from surface. The rod string is
typically either
13 one continuous member or a plurality of sucker rods, connected end-to-end
14 through standard threaded couplings.
It is known that downhole pumps may become lodged or stuck in a
16 wellbore, often by sand deposited and packed around the pump, either at the
17 downhole pumping location or as the pump is being tripped out of the
welibore.
18 Conventionally, the rod string is removed from the pump by applying a
pulling
19 force on the rod string to sever the rod string from the pump.
A shear coupling assembly is typically used to connect between
21 the pump and a downhole end of the rod string. The shear coupling primarily
22 functions to provide a means for separating the rod string from the pump so
as to
23 release and remove the rod string from the wellbore and permit specialized
24 equipment to be inserted into the well annulus to free the pump. Use of the
shear
coupling at the interface between the rod string and the pump provides a
CA 02630086 2008-04-29
1 specified location at which the pump and rod string are separated and the
shear
2 coupling can be constructed to actuate under a desired design load which is
3 highly predictable. Without the shear coupling, the rod string would sever
at a
4 location along the rod string that is unknown and largely unpredictable and
which
can be problematic for retrieval of the pump.
6 It is known to use a shear coupling comprising transversely
7 extending shear pins for joining male and female coupling members between
the
8 pump and the rod string. The shear pins are known to be prone to premature
9 fatigue which arises from cyclic compressive stress induced in the shear
pins if
the rod string "taps down" at the base of each reciprocating stroke. Further,
as
11 the shear pins break, fragments fall downhole into the pump, resulting in
further
12 problems in freeing the pump.
13 In an effort to solve the problems associated with previous shear
14 coupling designs, shear couplings, such. as taught in Canadian Patent
1298715
to Mann et al, are known to utilize a threaded connection between a pin
coupling
16 member, having an externally threaded head, and an internally threaded box
17 coupling member. Either of the pin coupling member or the internally
threaded
18 box coupling member is connected to the pump and the other is connected to
19 the downhole end of the rod string. The threaded head of the pin coupling
member threadedly engages the internal bore of the internally threaded
coupling
21 member for operatively connecting therebetween. The pin coupling member
22 further comprised a shear neck of reduced diameter between the head and a
23 body of the pin coupling member which is designed to shear under design
load
24 to free the pump from the rod string.
2
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1 In operation, a pretension is applied to the neck of the pin coupling
2 member during threaded connection to the box coupling. The box coupling
seats
3 on a shoulder of the pin coupling so as to maintain the neck in tension
during
4 normal operation of the pump for preventing premature fatigue of the shear
neck.
One major problem associated with such threaded connections is
6 "backing off" of the threads due to operation and vibration of the well
equipment
7 resulting in subsequent loosening of the threaded connection and a loss of
the
8 pretension in the shear neck. Once the pretension is lost, premature failure
of
9 the shear coupling is highly imminent as the reduced diameter neck becomes
exposed to cyclic compressive stresses associated with the rod string "tapping
11 down" at the bottom of each downstroke, impact loading associated with the
12 "fluid pound" phenomenon and bending stresses associated with flexure of
the
13 rod string. Due to the coarse nature of the threads used in the connection,
the
14 pretension can be lost with a turn in the connection of less than 180
degrees.
Conventionally, thread-locking adhesive or epoxy such as
16 LOCTITE (available from Henkel Technologies, USA) has been used to
17 strengthen the threaded connection between the coupling members. However,
18 degradation of the thread-locking epoxy often results from prolonged
exposure to
19 elevated temperatures and chemical compounds, such as pentanes and
hexanes, found in the well environment. Further, machining oil residues on the
21 connecting threads may prevent proper adhesion of the epoxy to the metal
22 threads. Additionally, if the epoxy does not evenly fill the crevices in
the thread
23 pattern, an inconsistent bonding may develop between the threaded surfaces
24 resulting in a weaker bond.
3
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1 Therefore, there is interest in the industry for a shear coupling
2 assembly and a method of connection therein which provides a reliable
locking
3 mechanism for permanently locking the connecting threads in place to prevent
4 backing-off and ensuring maintenance of the pretension which is applied to
the
neck of the shear coupling during assembly.
6
7 SUMMARY OF THE INVENTION
8 An improved pin coupling member for a shear coupling assembly
9 comprises discontinuities formed in a top portion of an insert member of the
pin
coupling member which permit the top portion of the insert member to be
11 expanded radially outward after operative engagement with a box coupling
12 member so as to lockingly engage the threaded connection therebetween to
13 prevent backing-off of the connection. Thus, pretension applied to a neck
of the
14 pin coupling member during operative engagement with the box coupling
member is maintained during use.
16 In one broad aspect of the invention, a pin coupling member for a
17 shear coupling assembly adapted for connecting a downhole pump to a
18 downhole end of a rod string, the pin coupling member comprises: a
cylindrical
19 body; a cylindrical insert member extending axially outwardly from the
cylindrical
body and having an extemally threaded surface adapted to engage an internally
21 threaded axial bore of a box coupling member; a shear neck connecting
between
22 the cylindrical body and the insert member; a counterbore formed in the
insert
23 member; and two or more radial discontinuities formed in a top portion of
the
24 insert member being of sufficient axial depth for permitting the top of the
insert
member to flex radially outwardly in response to a force applied to the
4
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1 counterbore for lockingly engaging the threaded connection between the pin
2 coupling member and the box coupling member.
3 In another broad aspect of the invention, a shear coupling
4 assembly adapted for connecting between a downhole pump and a downhole
end of a rod string, comprises: a box coupling member having a tubular body;
6 and an internally threaded axial bore formed therethrough; and a pin
coupling
7 member having a cylindrical body; a cylindrical insert member extending
axially
8 outwardly from the cylindrical body and having an externally threaded
surface for
9 engaging the intemally threaded axial bore of the box coupling member; a
shear
neck connecting between the cylindrical body and the insert member; a
11 counterbore formed in the insert member; and two or more radial
discontinuities
12 formed in a top portion of the insert member and being of sufficient depth
for
13 permitting radially outward flexing of the top portion in response to a
force
14 applied to the counterbore for lockingly engaging the threaded connection
between the pin coupling member and the box coupling member.
16 In another broad aspect of the invention, a method of assembling a
17 shear coupling assembly comprises: threading an insert member of a pin
18 coupling member into a threaded axial bore of a box coupling member, the
insert
19 member having a counterbore and two or more radial discontinuities formed
therein at a top portion for permitting outward radial expansion of the top
portion;
21 applying pretension to a neck of the pin coupling member during the
threading of
22 the box coupling member to seat on the pin coupling member and extending
23 axially between a cylindrical body of the pin coupling assembly and the
insert
24 member; and thereafter inserting an expansion member into the counterbore
for
expanding the top portion of the insert member radially outward within the
axial
5
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1 bore of the box coupling member for lockingly engaging the threaded
connection
2 therebetween.
3
4 BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a sectional side view of a prior art shear coupling
6 assembly;
7 Figure 2 is a partial sectional side view of an alternate embodiment
8 of the prior art shear coupling assembly of Fig. 1;
9 Figure 3A is an exploded perspective view of a shear coupling
assembly according to an embodiment of the invention;
11 Figure 3B is a side view of an assembled shear coupling assembly
12 according to Fig. 3A;
13 Figure 4 is a plan view of the pin coupling member according to
14 Fig.3A;
Figure 5 is a side view of the pin coupling member according to
16 Fig.3A;
17 Figure 6 is a 90 degree rotated side view of the pin coupling
18 member according to Fig. 5;
19 Figure 7 is a longitudinal sectional view of the assembled shear
coupling according to Fig. 3A, along section lines A-A of Fig. 4, after
insertion of
21 an expansion member;
22 Figure 8 is a longitudinal sectional view of the assembled shear
23 coupling according to Fig. 3A, along section lines B-B of Fig. 4, after
insertion of
24 an expansion member;
6
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1 Figures 9A - 9E are plan views of the pin coupling member
2 illustrating alternate slot patterns in a top of an insert portion; and
3 Figures 10A-10C are side views of expansion members according
4 to embodiments of the invention.
6 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
7 Embodiments of the invention relate to improvements to prior art
8 shear coupling assemblies to substantially prevent backing-off of a threaded
9 connection within the shear coupling assembly when in use. The improvements
are described in the context of a conventional shear coupling assembly having
a
11 male and a female member threadedly connected therebetween. A description
of
12 the conventional shear coupling assembly and method of assembly is provided
13 to assist in understanding the embodiments of the invention.
14
Prior Art Shear Couplin4 Assembly
16 Having reference to Figs. 1 and 2, a conventional prior art shear
17 coupling assembly 1 comprises two members, a pin coupling member 10 and a
18 box coupling member 12. Either of the pin or box coupling member 10, 12 can
19 be connected to either of a pump or a rod string (not shown) for permitting
connection therebetween.
21 The pin coupling member 10 comprises a cylindrical body 14
22 having an externally threaded insert member 16 extending axially outwardly
23 therefrom and connected to the body 14 by a shear neck 18 typically having
a
24 relatively reduced section. The insert member 16 is cyiindrical and smaller
in
diameter than the cylindrical body 14. The reduced section of the shear neck
18
7
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1 has a known cross-sectional area and acts as a preferential point of parting
in
2 the connection between the rod string and the pump under design loading. A
top
3 20 of the insert member 16 is bored with an intemally threaded axial
counterbore
4 22 adapted for use for pretensioning the neck 18 during assembly, as
described
below. The pretension in the neck 18 is maintained through a seating interface
6 between the pin and box coupling members 10, 12 along a radial contact
7 shoulder 24 formed at a top of the cylindrical body 14. The body 14 further
8 comprises a threaded connection 26 at an end opposite the insert member 16
for
9 threaded connection to either the rod string or the pump and can be either a
female connection (Fig. 1) or a male connection (Fig. 2) as shown.
11 The box coupling member 12 comprises a tubular body 28 having
12 an internal threaded axial bore 30 that co-operates with a thread profile
of the
13 externally threaded insert member 16 and which extends substantially a full
14 length of the bore 30. A first end 32 of the tubular body 28 is connected
to the
pin coupling member 10 at the externally threaded insert member 16. A second
16 end 34 of the body tubular body 28 is threadediy connected to either the
rod
17 string or the pump.
18
19 Prior Art Method of Assembly
Prior to assembling the pin and box coupling members 10,12, a
21 thread-locking epoxy or adhesive is typically applied to the extemally
threaded
22 insert 16 and the radial contact shoulder 24 of the pin coupling member 10.
The
23 externally threaded insert 16 is inserted into either the first or second
end 32, 34
24 of the box coupling member 12 and is advanced along the intemally threaded
axial bore 30 until the radial contact shoulder 24 of the pin coupling member
10
8
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1 approaches, but does not yet fully contact, the first or second end 32,34 of
the
2 box coupling's tubular body 28.
3 A tensioning rod commonly called a ready rod or bolt (not shown),
4 having an external thread at one end matching the profile of the internally
threaded counterbore 22, is inserted through the axial bore 30 of the box
6 coupling member 12 and is threaded into the internally threaded counterbore
22.
7 The shear neck 18 is placed into tension by pulling upwardly on the
tensioning
8 rod. With the tensioning rod and shear neck 18 under tension, the box
coupling
9 member body 12 is further advanced along the externally threaded insert 16
until
the tubular body 28 firmly contacts the radial contact shoulder 24 of the pin
11 coupling member 10. Contact between the tubular body 28 of the box coupling
12 member 12 and the radial contact shoulder 24 of the pin coupling member 10
13 acts to maintain the pretension in the neck 18. The tensile load on the
tensioning
14 rod is then released and the tensioning rod is unthreaded and removed from
the
assembly 1. The assembled shear coupling I is unused for sufficient time to
16 permit the thread-locking epoxy to dry and harden.
17
18 Embodiments of the invention
19 Having reference to Figs. 3A - 6 and in an embodiment of the
invention, a shear coupling assembly 100 utilizes an improved pin coupling
21 member 102.
22 The pin coupling member 102, as in the prior art, comprises a
23 cylindrical body 114 having an externally threaded insert member 116
extending
24 axially outwardly therefrom and connected to the body 114 by a shear neck
118
designed to part under design loads. In the embodiment shown herein, the shear
9
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1 neck 118 has a reduced section. While referred to in the industry as a shear
2 neck, it is believed the parting is a tensile failure. The insert member 116
is
3 bored with an internally threaded axial counterbore 122 used to pretension
the
4 neck 118 during assembly, as described for the prior art shear coupling
assembly 1.
6 In embodiments of the invention, a top portion 120 of the insert
7 member 116 comprises two or more radial discontinuities 124, such as slots
or
8 grooves, formed radially across a radius of the top portion 120 of the
insert
9 member 116, from an outer threaded surface 126 of the insert member 116 to
join the internally threaded axial counterbore 122. The radial discontinuities
124
11 extend axially into the insert member 116 sufficiently to permit radially
outward
12 flexing of at least the top portion 120 thereof.
13 As shown in Figs. 7 and 8, the improved pin coupling member 100
14 is threadedly connected to the inner threaded surface 129 of the box
coupling
member 12 as described for the prior art shear coupling assembly 1.
16 After release and removal of the tensioning rod from the insert after
17 pretensioning of the neck 118, the insert member 116 is locked to the box
18 coupling member 12. An expansion member 130, such as a ball or a wedge or
19 the like, is positioned at an opening 132 of the internally threaded axial
counterbore 122 of the insert member 116. Pressure is applied downwardly to
21 the expansion member 130 to move the expansion member 130 into the
22 counterbore 122 sufficient to flex the top portion 120 of the insert 116
radially
23 outwardly. The outer threaded surface 126 of the insert member 116 is
caused
24 to more strongly engage or lock with the inner threaded surface 129 of the
box
coupling member 12, substantially preventing backing-off of the threaded
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1 connection during use. Applicant believes that local deformation of the co-
2 operating threads of the insert member 116 and threaded inner bore 129 of
the
3 box coupling member 12 may assist to ensure that the threaded connection
4 cannot loosen during use and further acts to maintain the pretension in the
shear
neck 118 substantially preventing premature failure of the shear neck 118.
6 In an embodiment of the invention, best seen in Figs. 3A, 7 and 8,
7 the expansion member 130 is a sphere, such as a ball bearing, which has a
8 diameter slightly larger than a major diameter of the internal thread of the
insert's
9 counterbore 122.
Alternately, as shown in Figs. 10A-C, the expansion member 130 is
11 shaped so as to have a greatest extent 131 being slightly larger than the
major
12 diameter of the internal thread of the insert's counterbore 122 formed
along a
13 length of the expansion member 130.
14 A chamfer 134 is cut around the opening 132 or upper periphery of
the counterbore 122 so as to provide a seat to aid in concentrically
positioning
16 the ball bearing 130 prior to the application of downward force thereto and
to
17 ease the entry of the ball bearing 130 into the counterbore 122. In an
18 embodiment, force is applied to the ball bearing 130 until a major diameter
of the
19 ball bearing 130 is below a top face 121 of the insert 116. In an
embodiment, the
batl bearing 130 is inserted until a top of the ball bearing 130 is flush with
the top
21 face 121 of the insert 116.
22 In one embodiment, a hydraulic press (not shown) is used to apply
23 force to the ball bearing 130 to force the ball bearing 130 into the
counterbore
24 122.
11
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1 As shown in Figs. 4-6 and 9A and in one embodiment, four radial,
2 orthogonal discontinuities or slots 124 are formed about a circumference of
the
3 top portion 120 of the insert 116. In this embodiment, the radial slots 124
extend
4 axially to a depth sufficient to permit radial expansion of at least the top
portion
120 of the insert 116 upon insertion of the ball bearing 130. In one
embodiment
6 the radial slots 124 extend about one half the depth of the insert member
116.
7 Altemately, the radial slots 124 may extend further into the top portion 120
to
8 ensure flexure.
9 In an embodiment as shown in Fig. 9B, two radial slots 124 are
formed at 180 degrees from each other and effectively extend across the
11 diameter of the insert's top portion 120.
12 In an embodiment as shown in Fig. 9C, three radial slots 124 are
13 spaced evenly about the circumference of the top portion 120 of the insert
116.
14 In an embodiment as shown in Fig. 9D, six radial slots 124 are
spaced evenly about the circumference of the top portion 120 of the insert
116.
16 In an embodiment as shown in Fig. 9E, eight radial slots 124 are
17 spaced evenly about the circumference of the top portion 120 of the insert
116.
18 Applicant believes that additional slots 124 may be added as required to
radially
19 deflect the top portion 120 of the insert member 116.
Diametrically opposed slots 124, formed across the top portion 120
21 of the insert 116 can aid in the formation of the slots.
22
12
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1 EXAMPLE
2 In an embodiment of the invention as shown in Figs. 7 and 8, a
3 shear coupling assembly 100 sized for 3/<" sucker rod, includes both a box
4 coupling member 12 and a pin coupling member 102. The shear coupling
assembly 100 is typically manufactured using 4140 AISI, HTSR alloy steel or
6 other suitable structural material. The threaded surfaces for connection to
the
7 pump and the sucker rod are standard API threaded connections.
8 In this embodiment, the total length of the pin coupling member
9 102 is 4.687" and the insert member 116 is 0.843" in length. Four radial,
orthogonal slots 124 are cut in the top portion 120 of the insert member 116
of
11 the pin coupling member 102, forming a cross-shaped discontinuity in the
top
12 portion 120 of the insert member 116. The slots 124 extend axially about
0.375"
13 into the insert member 116 and are about 1/16" to 1/32" in width. The
insert
14 member's counterbore 122 has a diameter of 7/16" and is threaded using a
7/16"-14 UNC Class 2B thread at an effective depth of 5/8". The chamfer 134
16 formed at the opening 132 of the counterbore 122 is at 45 X 1/16".
17 The expansion member 130, used to radially expand the
18 discontinuous top portion 120 of the insert member 116, is a'/~" ball
bearing 130
19 made of a high compressive strength material, having a suitable hardness
and
surface finish.
21 In operation, the ball bearing 130 is inserted through the axial bore
22 30 in the box coupling member 12 after the removal of the tensioning rod.
The
23 box coupling member 12 is 4.05 inches in length. At a starting depth, a top
of the
24 ball bearing 130 resting on the opening 132 of the counterbore 122 of the
insert
member 116 engaged therein is at about 2.25 inches from the top of box
13
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1 coupling 12. Force is applied to the ball bearing 130, such as using a
hydraulic
2 press, to force the ball bearing 130 into the counterbore 122. The travel of
the
3 press is about 0.37 + 0.03 inches to position the major dimension of the
ball
4 bearing 130 flush with the top face 121 of the insert member 116. The top of
the
ball bearing 130 after insertion is at about 2.46 inches from the top of box
6 coupling 12 and about 0.16 inches above the top of the insert member 116.
14
