Note: Descriptions are shown in the official language in which they were submitted.
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HIGH TORQUE ROTATABLE PROGRESSIVE CAVITY
DRIVE RODS AND CONNECTORS
BACKGROUND OF THE INVENTION
1. Field of the Invention.
The present invention relates to rotatable rods and connectors used with
progressive
cavity pumps for pumping oil or other fluids out of wells. More specifically,
the present
invention relates to drive rods having novel pins and connectors specially and
uniquely
designed for high torque rotation and suited for use with progressive cavity
pumps.
2. Prior Art.
For over 100 years, sucker rods have been utilized to pump crude oil and other
liquids
out of wells. Sucker rods typically have threaded pins on each end that are
screwed into
connectors. Connectors attach to a sucker rod on either end. Sucker rods vary
in length but
typically have standard, common gauged threaded pins manufactured to API
Standard II B
specifications.
A string of sucker rods may extend several thou sand feet into a well. They
must,
therefore be very strong. Because of this, they are typically made of metal.
In addition,
lighter sucker rods are typically included at the bottom of the string while
the sucker rods at
the top of the string are often heavier and stronger in oraler to support the
string. The top
of the sucker rod string is attached to a pump jack. The b~ttom of the string
is attached to
a pump. Pump jacks reciprocate the sucker rod string in ;an upward and
downward motion
and subjects the string to compression and tension forces. This reciprocating
motion
operates the pump located at the bottom of the well.
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It is highly desirable that sucker rods do not become unscrewed and detach
from the
connectors between them. If this occurs, the pumping action halts. It is also
difTicult and
labor intensive to stop the pump jack and retrieve the portion of the sucker
rod string in the
bottom of the well. In order to prevent this, sucker rc~d pins and connectors
have been
designed so as not to come unscrewed when a reciprocating mation is applied.
In order to
minimize the risk of detachment, sucker rod pins have evolved into a three
part structure.
First, at the end of the pin is straight threading. This provides for maximum
friction on the
flanks ofthe threading. Just below the threading is an undercut, or stress
relief. Finally, both
the connectors and the sucker rods have shoulders designed to engage one
another. The
presence of the stress relief allows for greater friction between the engaged
shoulders. The
friction between the shoulders prevents the sucker rods and connectors from
disengaging.
The straight threading increases the ability of the sucker rod string to
withstand the stress of
the reciprocating motion.
Recent advances in pump technology have made it desirable to utilize
progressive
cavity (PC) pumps in oil and water wells. PC pumps are well suited for very
viscous liquids,
such as crude oil and for liquids having solids, such as sand, therein They
are also capable
of operating under very high pressure. Examples ofthese pumps may be found in
U. S. Patent
Nos. 1,892,217; 2,085,115 and 2,483,370 issued to Moineau. These PC pumps have
fewer
moving parts than other pumps typically used in wells. They are both sturdier
and less likely
2 o to malfunction. Existing sucker rods are well suiting to withstand the
stress of reciprocating
tension and compression motion. However, standard sucker rods are not ideal
for use with
PC pumps. PC pumps require a rotational motion instead of a reciprocating
motion.
However, they are not ideally suited to withstand high torque rotation.
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It is therefore desirable to provide a drive rod and connector for progressive
cavity
pumps capable of withstanding high torque rotation.
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SLT~~MARY C~F THE INVENTI~I~1
The rods and connectors of the present invention have tapered, rather than
straight,
threading. In the existing art of sucker rods, tapered threading has generally
been
discouraged. When reciprocating motion is applied, it is typically easier to
jar loose sucker
rods having tapered threading. However, because the purpose of these sucker
rods is to
provide a superior rod string for use in high torque, rotational motion,
tapered threading is
actually preferred. The constant, high speed rotation ensures that the sucker
rods and their
connectors remain screwed 3;ogether tightly.
Another reason current sucker rods strings are not suitable for use with 1~C
pumps
that require rotation is that the connectors are not well designed for this
type of stress.
Though well designed to tolerate the stresses of reciprocating motion, the
walls of a
connector are generally too thin to withstand high torque. When exposed to
high rotational
pressure, these connectors will break in half. Because the present invention
provides for
tapered threading on the pins of the sucker rods, the walls of the connectors
are thicker.
Connectors having thicker walls are capable of withstanding greater torsianal
stress.
To further facilitate transfer of the rotational motion between sucker rods
and
connectors, the undercut found on sucker rod pins is eliminated in the present
invention. The
rotational motion of the string constantly tightens the pin within the
connector, thereby
reducing sucker rod's danger of disengagement. The need for the increased
friction on the
shoulders provided by an undercut is therefore eliminated. Furthermore, this
additional
2 0 threading provides for a greater surface area over which the rotational
motion is transferred.
This further enhances the sucker rod string's ability to withstand high
torsional stress. The
synergistic effect of tapered threading, thicker connector rods
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and elimination ofthe undercut provides for rods and connectors having
superior tolerance
to rotational stress.
In the present invention, the pins of sucker rods are modified so that they
may better
withstand the stress of high torque in excess of 1.,200 foot pounds.
5 It is therefore an object of the present invention to provide sucker rod
strings capable
of withstanding the stress of high torque rotation.
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BRIEF DESCRIPTION OF THE IJRAWINGS
Figure 1 shows a diagrammatic illustration of a drive rod string and connector
being
utilized to drive a progressive cavity pump at the bottom o.f a well.
Figure 1 A shows an enlarged portion of the drive rod string of Figure 1.
Figure 2 shows a side plan view of one end of a typical sucker rod currently
in use.
Figure 3 is a side plan view of the drive rod of the present invention.
Figure 4 is a cross sectional view of a drive rod connectar of the present
invention.
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DETAILED DESCRIPTION OF THE DRAWINC~rS
The present invention is an improved drive rod and connector capable of high
torque
rotation. Three of the key features of the present invention work together
synergistically to
improve the amount of rotational stress that a drive rod can withstand. These
improvements
consist of drive rod pins having tapered threading and little or no undercut.
This also results
in strengthened connectors having thicker sidewalk. The lack of undercut
increases the
surface area by which rotational motion may be transferred from a drive rod to
a connector
to a subsequent drive rod. The present invention is especially useful for
driving a progressive
cavity pump.
Drive rods are comprised of approximately three components: The first, the
body, is
a long, usually metal, shaft typically about 40 feet in length. The other two
components are
the pins located at each end of the shaft. The pins are threaded so that they
may be screwed
into connectors. One rod is screwed into the bottom of a connector while
another is screwed
into the top. This is repeated until the rod string is of the dlesired length.
This can sometimes
be several thousand feet and require numerous rods.
Recently, pumps known as progressive cavity pumps have been developed for use
in
pumping oil and other fluids out of wells. These pumps are especially suited
for pumping
thick viscous liquids such as crude oil. A relatively simple design makes them
both sturdy
and reliable. This has resulted in a need for a sucker rods better suited for
rotational rather
than reciprocating tension and compression motion.
2 0 Figure 1 illustrates a typical well having a progressive cavity pump
wherein the
present invention is employed. Oil well 10 has a PC pump located at the bottom
of it. At the
top of the well is a powered drive head 12. Drive head 12 is rotated by a
motor. This results
in rotation of the polished rod 14 which projects downward through stuffing
box 16.
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Stuffing box 16 creates a water proof seal such that pumped fluids will not
exit through the
top of stuffing box 16. Directly below the stuffing box is a Ilowline 18.
Pumped oil or other
fluid exits the well through this flowline. Below this is a tubing head 19
located directly
above casing head 20. Tho;>e skilled in the art of well drilling will be
familiar with tubing
head 19 and casing head 20 as well as the casing itself 26: The polished rod
14 is connected
to the top end of drive rod string 22. Drive rods in the string are held
together by connectors
24 as will be described.
Figure l A shows an enlarged view of sucker rod string 22 and a connector 2.4
holding
2 rods 30 and 32 together. For simplicity, this diagram shows use of only 2
sucker rods.
1 o Those skilled in the art will appreciate that in practically a:ll wells,
the rod string is comprised
of several rods and connectors. The pins of drive rods 30 and 32 are not seen
in Figure 1 or
Figure 1A. This is because the pins themselves have been screwed into the
connector and
are not visible.
Figure 2 shows a pin of a typical rod common in the art prior to introduction
of the
present invention. Sucker rod pin 36 is comprised of a cylindrical threaded
portion 38 and
an undercut portion 40. Cylindrical threaded portion 38 has a constant
diameter and is
known in the art as straight threading. Undercut 40 has a diameter slightly
less than that of
threaded portion 3 8 and extends to shoulder 42. Shoulder 42 has a greater
diameter than the
other portions of the shaft. It has an engaging portion 44~ that comes in firm
contact with a
2 o complimentary shoulder on the connector. The friction created by the
contact between
engaging portion 44 or shoulder 42 and the shoulder of a connector discourages
the
unscrewing of a sucker rod from a connector. The presence of an undercut 40
allows
threaded portion 3 8 to be screwed into a connector more tightly, thus
increasing the static
friction created by the shoulders. Without undercut 40; the connector and
sucker rod are
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connected more loosely and are more likely to unscrew. Therefore, the present
art teaches
away from eliminating undercut 40. Underneath shoulder 42 is a square bolt
portion of 46.
This is the portion of the shaft that is engaged by tools that tighten the
sucker rods'
connections to the connectors. Main body 48 of the sucker rod is substantially
cylindrical
and extends the length of the sucker rod. An identical pin, not shown, is
located on the other
end of the sucker rod main shaft 48.
connectors 24 are essentially elongated bolts having a bore and an interior
threading
complimentary to threaded portions 3 8. The bore has a length that is at least
slightly greater
than the length of two pins 36. This prevents pins 36 from contacting each
other and allows
the rods to be screwed into the connectors more tightly.
Figure 3 shows the drive rod of the present invention from a similar angle as
the prior
art in Figure 2. It shows modified pin 60 having a threaded portion 62. As
shown in Figure
3, threaded portion 62 is slightly tapered and frustoconical. In a preferred
embodiment, the
threaded portion tapers slightly inward at about 3.5 °. In this
particular embodiment, there
is a very narrow undercut 64. This particular embodiment: shows slight
undercut 64 in order
to illustrate that it is possible in the present invention to utilize an
undercut. It is desirable
to have undercut 64 as small as possible. It is however substantially easier
to machine a pin
having a slight undercut than machining a pin having no undercut at all.
Tapered threading
offers a variety of advantages. Threaded tapering offers better balance of
strength from the
2 0 connection, easier stabbing during make-up, quicker release on break-out
and greater ease
of re-working.
In the past, the art has taught away from such tapering because they are
easier to pull
apart and become disconnected. However, when the sucker rods are rotated, the
tapered
threading and the pin tightens its engagement to the connector resulting in a
very strong
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connection. Furthermore, tapering causes the pin to be placed in radial and
circumferential
compression and the socket to be placed in radial compression and
circumferential tension.
This radial tension allows rotational movement to be transferred from a rod to
a connector
to a subsequent rod in a smoother fashion. This increases the amount of torque
and
rotational stress that the sucker rod string may be subjected to without
failure.
In the present invention, rotational motion may be transferred not only by the
shoulder, as in traditional sucker rods, but also by the entire pin itself.
This increase in
surface area over which torque is transferred reduces overall stress on the
pin and drive rod
as a whole. Those skilled in the art will appreciate that b~y utilizing the
entire pin as well as
1 o the shoulder to transfer rotational movement, overall strain on the drive
rod is reduced.
Typical one inch rods cannot withstand more than 1,200 foot pounds of torque.
One inch
drive rods according to the present invention, however, may withstand up to
1,500 foot
pounds of torque. This provides sturdier, more reliable and faster withdrawal
of oil or other
fluids from a well.
Figure 4 shows a connector of the present invention suitable for use with a
drive rod
disclosed in Figure 3. Connector 70 is essentially an elongated bolt having a
cylindrical
outside wall 72 penetrated by bore 80. Bore 80 has tapered threading
complimentary to pin
60 in Figure 3. In this particular embodiment, a slight undercut 76 is
included in bore 80 in
order to accommodate the slight undercut 64 on pin 60 shown in :Figure 3 , As
stated above,
2 o this undercut is unnecessary but the invention may be easier to machine in
this fashion. In
this particular embodiment, connector 70 is approximately 4 inches long. This
is more than
long enough to adequately accommodate two pins 60 which are each approximately
1.75
inches long. This allows the threading inside the connector 70 to tighten upon
rotation of the
sucker rods string. One of the advantages of the design of the present
invention is that
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connector wall 72 is thicker than those of more traditional connectors without
increase of the
outside diameter. This allows connector 70 to withstand more torsional stress
than a
standard connector. The added thickness of wall 72 adds strength to the
connector. In
addition, the tapered threading allows the connector to transfer a rotational
motion through
both shoulder 82 and bore 80. As explained above, spreading the transfer of
rotational force
over a larger surface area allows the connector, like the pin, to withstand
more force.
Although these modifications may not appear significant, those skilled in the
art will
appreciate that the present invention has unique mechanical properties not
found in existing
sucker rods. Modifications made to the pins and connectors allow at least 25%
more torque
to be safely supplied thraugll a rod string. Those skilled in the art will
appreciate that this
significantly improves the use of progressive cavity pumps.
The present progressive cavity rod will provide greater torque values than
existing
sucker rod strings as may be observed from the following table:
TYPICAL RECOMMENDED TORQUE VALUES
Atl torque values are in foot pounds
Itod Size Grade D (78) Iiigt~ Strength (97) PC Rod
1" 1100 1200 1500'
1 " w 7/8 pin thread 1100 NIA N/A
1-1/8" 1570 1700 2200*
2 0 I- 1/4" 2000 2500 3000*
'Actual torque vatues from test data
**Projected torque values - actual tests
in process
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Whereas, the present invention has been described in relation to the drawings
attached
hereto, it should be understood that other and further modifications, apart
from those shown
or suggested herein, may be made within the spirit and scope of this
invention.
