Note: Descriptions are shown in the official language in which they were submitted.
CA 02282822 1999-09-20
Swivelling Device for a Dowahole Rod Pump
Background of the Invention
Field of the Invention
The present invention is directed to a swivelling device for pulling pump
drive
strings which may be under torque.
Downhole rod pumps, particularly rotary pumps such as progressive cavity pumps
(PCP), and stroke pumps, are generally driven by a drive string which extends
through a
concentrically arranged production tubing string. The drive string is made up
of a
plurality of rods or tubes which are connected together end-to-end by rod
boxes.
Alternatively the drive string may consist of one continuous rod cut and
pinned to the
desired length.
Progressive cavity pumps generally include a stator affixed to the production
tubing
and a corkscrew-shaped rotor connected to and supported at the working end of
the drive
string. The drive string is connected at its top end to a smooth rod, called a
polish rod,
which allows an effective seal to be created between the outer production
tubing and the
inner drive string. The polish rod is attached to a drive unit which, in
operation, rotates
the drive string. Upon actuation of the pump by rotation of the drive string,
fluids are
forced to the ground surface through an annular space provided between the
drive string
and the production tubing.
In operation, often the working end of a drive string will become jammed by
such
things as accumulation of debris, too high viscosity of the surrounding
medium, or
obstructions in the pump hole. When this happens with a rotary pump the end of
the drive
string may stop rotating, but the polish rod and remaining rods of the drive
string continue
to rotate until the drive unit is stopped by overload sensors; then a braking
system that is
built into the drive unit controllably releases back spin. The rotation of the
drive string
without rotation of its end causes the accumulation of torque, through
twisting of the drive
string. The accumulated torque, which may be substantial, due to the length of
the drive
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CA 02282822 1999-09-20
string, may be released by "back spin", spinning the drive unit backwards.
Under normal
circumstances, the braking system releases in a controlled manner, thereby
allowing the
release of the torque accumulated in the drive string by "back spin". However,
if the
braking system locks on solid, the accumulated torque may be released
uncontrolled once a
drive clamp is disconnected from the drive unit by picking the polish rod up a
few
centimetres. The back spin is then released in an uncontrolled manner. In
conventional
systems, to pull the drive string and release torque, the drive unit is
disconnected and the
polish rod attached to a pony rod, a rod elevator and a rod hook. The rod hook
contains a
swivel which allows the drive string to "back spin", for the release of
accumulated torque.
Such conventional rod pulling devices, using a rod hook connected to a polish
rod by a
rod elevator, are not symmetrical along the axis of rotation. This means that
the known
devices are prone to 'wobble", which can lead to stress under the high angular
velocities
attained when back spin is released uncontrolled. The "wobble" is exacerbated
by the fact
that the rod elevator and the rod hook form a point of articulation which can
bend under
the centrifugal force of rotation, allowing the bottom of the hook and the top
of the
elevator to "swing out" from the axis of rotation. The point of articulation
between the
rod elevator and the rod hook is a common cause of failure. Conventional
devices also
have moving parts for attaching the elevator to the rod hook, in the form of a
locking
finger on a hook portion of the rod hook, which is usually spring biassed in a
closed
position. The moving parts complicate the manufacture and assembly of the
device, and
provide weak points for failure. These factors contribute to make conventional
rod pulling
devices extremely unstable under conditions of high angular velocities, such
as are
encountered in the release of back spin. The conventional devices may fail
during use,
leading to equipment damage and human injury when the components fly apart.
Pumps other than rotary pumps, such as a stroke pump, may become sanded in or
stuck. If this situation occurs the only way to pull the pump rod string out
of the hole is
to strip it out. This is achieved conventionally by installing tubing tongs
over the rod
string. A device called a "back off tool" is then clamped to the rod string.
The tubing
tongs are then used to rotate the back off tool and rod string. The rod string
then backs
off at the weakest connection. The swivel is then removed and a rod hook and
elevator
are installed for pulling off the rods. If there is no tubing drain, the fluid
inside the tubing
(surrounding the rod string) must then be swabbed out down to the top of the
remaining
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rods (i.e. the rods below the weakest connection). Once this is achieved the
tubing is
pulled out of the hole to the remaining rods. The back off procedure is
repeated until all
rods and tubing are removed from the well.
Because the rods are backed off at the weakest connection the sudden rotation
of
the rods and/or torque created to back the rods off often causes the rod to
jump violently.
On some occasions the elevator can jump out past a safety latch of the rod
hook, dropping
the elevator onto the tubing tongs, causing equipment damage or possible
injury to the
working crew. The torque generated to back the rods off varies from type of
pump, for
example PCP (which rotates to the right under normal conditions) to a rod pump
on a
to pump jack which strokes up and down. The pump jacks are not usually as
tight requiring
less torque to back the rods off, but still require substantial torque. The
sudden back off
causes sudden and very quick back spin that usually lasts a few seconds.
The conventional method for backing off of a rod pump on a pump jack is
therefore dangerous, and suffers the same drawbacks as mentioned above for
backing off
15 of a rotary pump.
The "flush by" is a frequent procedure with PCP pumps. PCP pumps may need to
be flushed because of debris causing the rotor to become sticky or the fluid
may become
too thick or heavy for the pump to force it to the ground's surface. A flush
by is achieved
by picking the polish rod up a few centimetres to unlock the polish rod clamp
from the
2o drive unit. Fluid is then pumped down the annular space between the
production tubing the
drive string; this causes the drive string to rotate backward, i.e. "back
spin". The back spin
may be very fast, for long periods of time causing equipment to fly apart.
Braef Summary of the Invention
The present invention provides a safe and easy to use swivelling device for
use
25 with a rod bottom hole pump.
In accordance with one aspect of the present invention there is provided a
swivelling device for a rod bottom hole pump for connection to a drive string,
comprising:
a housing having an opening at a bottom end, and having means for attachment
to an
external support at a top end, said means for attachment being rigidly fixed
to the housing
3o to prevent relative movement therebetween; a rotatable shaft being
partially housed within
the housing, extending through the opening, and projecting from the bottom end
of the
housing, the shaft having at its bottom end means for attachment to the drive
string; and
CA 02282822 2004-07-16
means for mounting the shaft rotatably within the housing, the means for
mounting being
enclosed in the housing; wherein the shaft and means for attachment to the
drive string are
substantially symmetrical about their common axis of rotation, and provide no
point of
articulation between the shaft and the drive string.
In accordance with another aspect of the present invention there is provided a
method for backing off a drive string in a rod bottom hole pump, comprising
attaching a
rotatable shaft to the drive string via a polish rod connected to a back end
of the drive
string; wherein the shaft is attached to the drive string symmetrically about
the axis of
rotation of the shaft, and there is no point of articulation between the shaft
and the drive
to string, and wherein a top end of the shaft is enclosed in a housing; and
pulling on the drive
string while allowing the shaft to rotate in response to torque in the drive
string.
In accordance with a further aspect of the present invention, there is
provided a
swivelling device adapted for connection to a drive string of a downhole rod
pump, the
device comprising: a housing having an opening at a bottom end, and having
means for
1 s attachment to an external support at a top end, said means for attachment
to said external
support being rigidly fixed to said housing without possibility of relative
movement there
between; a rotatable shaft having most of its length housed within the
housing, and
extending through the opening and projecting only from the bottom end of the
housing, the
shaft including an integral means for attachment to the drive string at a
bottom end of the
2o shaft, said shaft and said means for attachment to the drive string being
substantially
symmetrical about a common axis of rotation and being such that said means for
attachment to the drive string connects to said drive string with no point of
articulation
between the shaft and the drive string; and means for mounting the shaft
rotatably within
the housing, the means for mounting being enclosed within the housing.
25 Detailed Description of Preferred Embodiments
The invention will be described in terms of a device for backing off a rod
bottom
hole pump (a rod bottom hole pump being any pump which operates by a drive
string),
particularly for releasing back spin in a progressing cavity pump, or for
backing off a
stroke pump, however it is to be understood that the device of the invention
extends to use
3o in any application in which it is desired to lift a load which is under
torque, which rotates,
or which it is desired to rotate.
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CA 02282822 2004-07-16
The device of the present invention allows the pulling of a drive string and
if
necessary the release of back spin, with considerably enhanced safety when
compared to
known devices. The use of a housing for the shaft and bearings confers
enhanced stability,
even under conditions of high angular velocity. A number of connections are
eliminated,
thus decreasing possible failure points. The symmetry of the rotating elements
about the
axis of rotation, and the lack of any point of articulation between the shaft
and the drive
string mean that the device provides a straight direct pull, when pulling a
drive string,
decreasing "wobble" during rotation, which in turn decreases stress.
to In use with a progressive cavity pump, the shaft of the device is attached
to the
drive string by a polish rod. The device reduces polish rod burn scratches and
scoring
damage to the drive unit. The enclosure of all the rotating parts of the
device, with the
exception of the bottom end of the shaft, means that in the event of failure
the components
are unlikely to fly out and cause injury and equipment damage. The device may
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CA 02282822 1999-09-20
optionally include shock absorbing means for damping longitudinal
displacements of the
shaft caused by vibrations and longitudinal displacements of the shaft caused
by
movements of the drive string, which can be abrupt and are often the cause of
"wobble" in
conventional rod hooks, leading to failure. The device of the invention
eliminates the
need for a pony rod, rod elevator and rod hook, all of which are potential
points of failure
of conventional devices.
The housing is adapted at its top end for attachment to a support. For this
purpose,
the top end of the housing may, for example, be in the form of a ring or loop,
through
which a hook or other support means may be passed. If desired, the top end of
the
housing can be closed, for example, by a dust cap.
The shaft is rotatably mounted in the housing. In a preferred embodiment the
shaft
can move longitudinally in response to vibrations and abrupt movements of the
drive
string. A preferred means for mounting the shaft is by using a floating
bearing. It is
particularly preferred to use a floating flat roller bearing. If desired, a
stopper ring can be
placed about the bottom portion of the shaft, below the housing, to prevent
the shaft from
rising too high in the housing. In a preferred embodiment, the stopper ring is
machined
into, and forms part of the shaft. For safety reasons, it is preferred to have
a stopper ring.
The svvivelling device is often required to pull heavy weights, and if the
polish rod should
become suddenly inadvertently detached and there is no stopper ring, the shaft
can shoot
out through the top of the device.
For the purpose of stability, it is preferred that the greater part of the
shaft be
within the housing.
For use with a rotary pump such as a progressive cavity pump the shaft is
provided, at its bottom end, with means for attaching to a polish rod. The
attachment
means can be any means which does not deviate substantially from symmetry
about the
common axis of rotation of the shaft and the attachment means, and which does
not
provide a point of articulation between the shaft and the polish rod (i.e. the
shaft and the
attachment means, when attached to the polish rod, cannot bend significantly
from the axis
of rotation under the influence of centrifugal or other forces inclined to the
axis of
rotation). This avoids the disadvantageous arrangement found in known devices
wherein
the attachment means is non-symmetrical about the axis of rotation, and/or
wherein the rod
pulling device must be attached to the polish rod via an elevator and a pony
rod. The
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CA 02282822 1999-09-20
arrangement of the known devices can lead to "wobble" during rotation. In a
preferred
embodiment, the shaft and the means of attachment to the polish rod are
constructed as a
single piece.
The attachment means can be, for example, in the form of a female socket at
the
end of the shaft to receive a complementary male part on the polish rod, a
male part on
the end of the shaft to be received by a complementary female socket on the
polish rod, or
a screw thread on the end of the shaft to be received by a complementary screw
thread on
the polish rod. In the case of a PCP, it is preferred that the attachment
means be a screw
thread fitting provided at the bottom end of the shaft, which can be fitted
into a
complementary screw thread fitting on a polish rod. The screw thread on the
shaft can be
female or male.
Preferably the attachment means attaches directly to the polish rod, although
connection through adaptors is also possible, provided the adaptors are also
symmetrical
about the axis of rotation, and provide no point of articulation between the
shaft and the
polish rod.
The attachment of the device of the invention directly or through adaptors to
a
polish rod means that there is no need for the use of a rod hook, rod elevator
and pony
rod. This eliminates a point of articulation between the rod hook and the rod
elevator.
This point of articulation is a source of "wobble" causing polish rod burn and
wear, and
increasing the change of equipment failure.
In the case of other rod pumps, such as a stroke pump, the device can be
attached
to the drive string by screwing onto a thread on the polish rod of the drive
string. If the
rod string pin thread is different from that of the device, an adapter can be
used.
Another situation in which the device of the invention is useful is in
"flushing by"
or "pumping by". Flushing by is an additional procedure for freeing a rod
string that has
become jammed. The rod string is backed off slightly, as described previously,
and fluid
is pumped under pressure into the space between the rod string and the outer
tubing
surrounding the rod string, in order to dislodge debris which may have caused
the rod
string to jam. Usually, when the debris dislodges, the rod string will
abruptly move, under
either applied torque, in the case of a stroke pump, or accumulated torque, in
the case of a
rotary pump. Again, the device of the invention allows the torque to be
released without
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CA 02282822 1999-09-20
the hazards associated with traditional tools used for backing off, such as
rod tongs and a
rod elevator.
In a preferred embodiment, the device is provided with shock absorbing means
for
damping longitudinal displacements of the shaft. The shock absorbing means
can, for
example, be provided by a hydraulic shock absorber or a spring or a plurality
of springs.
It is preferred to use a spring, particularly a coiled spring. It is
particularly preferred to
use a coiled spring, with the shaft passing through the longitudinal axis of
the helix of the
spnng.
The materials which may be used to construct the device of the invention are
limited only in that they must be of sufficient strength to lift the required
load, and to
withstand the angular momentum generated by rotation. For a PCP, it is
preferred to use
steel for the housing and the shaft.
A particularly preferred embodiment will be described with reference to the
figures,
which depict:
Brief Description of the Drawings
Figure 1 shows a partially cut-away view of a device of the invention.
Figure 2 shows a device of the invention on the left, and a conventional rod
hook,
elevator and pony rod on the right.
Figure 3 shows a partially cut-away view of a rod hook of the prior art.
In the preferred embodiment of a device of the invention depicted in figure 1,
the
housing (1) is an approximate cylinder, closed at the top end by means of a
dust cap (2),
with a grease nipple (3) disposed for easy lubrication of the device. The
housing ( 1 ) has a
loop shaped handle (4), which, in use, is attached to support means. The shaft
(5) is
rotatably mounted in the housing (1) by means of a floating flat roller
bearing (6). A
stopper ring (7) about the bottom portion of the shaft (5), of greater
diameter than the
opening at the bottom of the housing, prevents the shaft from rising above a
height in the
housing (1) at which the stopper ring (7) abuts the bottom end of the housing
(1). The
floating flat roller bearing (6) is sandwiched between two washers (8, 9).
About the shaft,
above the washer (8) is placed a lock nut (10). The top portion of the shaft
(5) is
provided with a safety pin (11) for the lock nut (10) that sits above the lock
nut (10). A
coiled spring ( 12) surrounds the shaft (5) and abuts at its top end the
bottom washer (9).
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CA 02282822 1999-09-20
The bottom end of the spring (12) abuts a ridge (13) that is provided on the
inside of the
bottom end of the housing. Although in the embodiment depicted the spring sits
below
the floating bearing, it can equally well sit above the floating bearing, in
which case means
must be provided at the top of the housing to abut the top end of the spring.
In other
embodiments the shock-absorbing means is provided by a plurality of springs,
which abut
the floating bearing without the shaft passing through their coils. The spring
provides
shock absorbing means for damping longitudinal displacements of the shaft
caused by
vibrations and abrupt movements of a load attached to the shaft (5). The shaft
is provided
at its bottom end with a male screw thread fitting (14), which, in operation,
is attached to
a corresponding female screw thread fitting on a polish rod of a drive string.
In operation, in use with a rotary pump, such as a progressive cavity pump the
screw thread on the shaft is fitted to a polish rod attached to the back end
of a drive
string. The shaft rotates when the drive string rotates. The rotation of the
shaft permits
the release of torque in the drive string through back spin. Abrupt
longitudinal movements
of the drive string and vibrations are absorbed by the spring. The spring
allows the gentle
picking up of a load, such as a polish rod attached to a drive string. The
device minimizes
"wobble". The minimization of wobble greatly decreases the chances of failure
due to
centrifugal force. The housing ensures that all rotating parts are enclosed,
so that in the
event of failure the parts are unlikely to fly out causing injury and
equipment damage.
The use of means for attachment directly to the polish rod eliminates the need
for a pony
rod, and a rod elevator which contribute to "wobble" during rotation, and
which provide
weak points for failure during high speed rotation.
In operation in use with a rod pump other than a progressive cavity pump, the
screw thread on the shaft is fitted to a complementary screw thread, either on
the drive
string itself, or on an adapter suitable to be connected to the drive string.
The rods of the
drive string are then backed off, and any abrupt motions of the drive string
are absorbed
either by the rotation of the shaft, or the damped longitudinal movement of
the shaft.
In figure 3, a rod hook of the prior art is shown. It shares some features
with the
device of the invention, however in the prior art device, the rotatable shaft
(5) terminates
at its bottom end in a hook ( 15). In use, the hook ( 15) is attached to a rod
elevator (not
shown), which is attached to a polish rod via a pony rod (both not shown). As
can be
seen from figure 3, the hook (15) is unsymmetrical in shape and mass about the
axis of
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CA 02282822 1999-09-20
rotation. It furthermore comprises two moving parts: the locking finger (16),
and the lock
releasing latch (17), which are possible points of failure, and provide small
parts which
may fly out under the influence of centrifugal force, or abrupt longitudinal
movements.
When attached to the rod elevator a point of articulation is formed between
the rod hook
and the rod elevator (i.e. bending out of the axis of rotation is possible
between the rod
hook and the elevator). This point of articulation permits the bottom of the
rod hook and
the top of the elevator to "swing out" under the influence of centrifugal
force during
rotation. This leads to stress on the device, with possible failure. The use
of a rod
elevator further increases the likelihood of failure. Most rod elevators
comprise several
moving parts with inherent weaknesses at their points of connection.
Figure 2 shows the device of the invention on the left, and a conventional
arrangement on the right. The top half of the conventional arrangement
consists of a rod
hook, as shown in Figure 3. The rod hook shown in figure 2 has roll pins (18).
The top
roll pin allows the pivoting of the locking finger (16), and the bottom roll
pin allows the
I S pivoting of the releasing latch (17). The roll pins can fly out or release
during back spin,
with consequent equipment damage or user injury.
The lower half of the conventional arrangement in figure 2 consists of a rod
elevator.
9
