Note: Descriptions are shown in the official language in which they were submitted.
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SWIVELLING DEVICE FOR A DOWNHOLE ROD PUMP
This invention is in the field of devices for releasing stuck drive strings in
a downhole
and more particularly to a device that absorbs longitudinal motion and
rotational motion
of a drive string while the drive string is becoming unstuck.
BACKGROUND
lo Downhole rod pumps, such as rotary pumps and stroke pumps, for extracting
oil from an
oil well are typically driven by a drive string extending down the downhole of
the oil
well. The drive string is a number of sections of tubes or rods connected end
to end and
passing down the downhole. Because the downhole can be quite deep, the drive
string
can be made up of numerous sections of these tubes or rods.
Progressive cavity pumps, one type of rotary pump, has a stator attached to
the end of the
production tubing lining the downhole and a corkscrew-shaped rotor attached to
a bottom
end of the drive string at the bottom of the downhole. The top end of the
drive string is
connected to a polish rod. The polish rod has a smooth outer surface to
correspond with
an inside surface of the production tubing lining the downhole and the top end
of the
polish rod connects to a drive unit. The drive unit rotates the polish rod and
the drive
string. Progressive cavity pumps are operative to pump fluid up the down hole
by using
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the drive unit to rotate the polish rod which in tum rotates the drive string
and the
corkscrew-shaped rotor attached at the bottom end of the drive string. This
rotation of
the corkscrew-shaped rotor forces fluid up the annulus formed between the
drive string
and the inner surface of the production tubing, lining the downhole, and up to
the ground
surface.
During pumping, the downhole end of the drive string can become jammed by
accumulation of debris, high viscosity of the medium being pumped, or
obstiuctions in
the downhole. When the drive string becomes jammed, the drive string at the
end of the
downhole stops rotating. The drive string continues to rotate a bit, twisting
the drive
string all the way up the downhole until the polish rod connected to the end
of the drive
string applies enough torsional force to the drive unit to trigger overload
sensors in the
drive unit. When the overload sensors trigger the drive unit to shut down, the
drive unit
typically includes a braking system that controllably releases torsion built
up in the
twisted drive string by "back spinning" the polish rod and the drive string in
a controlled
fashion using the braking system.
It is this twisting of the top end of the drive string while the bottom end of
the drive string
is stuck that causes torsional forces to build up in the drive string. Due to
the fact that the
drive string can be quite long, substantial torsional forces can be built up
in the drive
string when the bottom end of the drive string becomes jammed.
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The braking system in theory should remove all this built up torsional force
however it
does not always release all of the torsional force stored up in the drive
string. Often
additional torsional force must be released by "picking up" the drive string.
To "pick up"
the drive string, the drive unit is disconnected from the polish rod, and the
polish rod and
attached drive string is pulled up the downhole a few centimeters. This
releases the stuck
drive string and can cause stored up torsional forced to suddenly release in
an
"uncontrolled" manner by the polish rod and attached drive string back
spinning until the
drive string is once again untwisted.
io This "picking up" is typically done by attaching a device to the polish rod
that can be
attached to a support system, capable of lifting the drive string, and still
allow the polish
rod and the attached drive string to rotate. A conventional method uses a pony
rod, a rod
elevator and a rod hook for a device. The pony rod, rod elevator and rod hook
are
attached to the polish rod and the rod hook has a swivel which allows it to
rotate with the
polish rod and drive string, allowing the string to back spin and release the
built up
torsional forces. The polish rod and drive string are pulled a short distance
up the
downhole, attached to the swivelling hook, and any built up torsional force is
released
causing the drive string and polish rod to swivel on the hook.
Previously these hooks were often not symmetrical around their axis which
caused
"wobbling" as the drive string spun. Additionally, the way the hook attaches
to the
polish rod often results in a loose connection which allows the polish rod to
move
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radially while it is attached to the hook. This "wobbling" is hard on the
devices and can
result in failure of the device which can cause damage to surrounding
equipment and
even physically injure workers nearby.
Stroke pumps are also commonly used to pump oil from an oil well. Stroke pumps
use a
pump attached at the end of the drive string to pump fluid up the downhole.
Rather than
rotating the drive string, the drive string is driven up and down inside the
down hole to
force fluid up the downhole to the ground surface. A drive unit and lever
system is
attached to the polish rod, which in turn is attached to the drive string. The
lever system
pushes and pulls the polish rod up and the attached drive string up and down
the
downhole to cause the pumping action.
However, stroke pumps can also become stuck in the downhole. If a stroke pump
becomes stuck, the drive string must be stripped out of the downhole in order
to remove
the drive string from the downhole. However, as the drive string becomes freed
by
backing it off, the drive string can suddenly twist or jerk longitudinally in
the downhole.
This jerking or jumping can be quite violent and may damage the equipment
being used
or even injure people who are nearby.
.20 US Pat. No. 6,253,844 to Walker discloses a swivelling device for a
downhole rod pump
for releasing torsional forces on a drive string in an "uncontrolled" manner.
The device
uses a rod with a threaded end to connect to a mating thread on the poiishing
rod. This
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connecting of a threaded end of a rotating shaft to a corresponding threaded
end of a
polish rod causes the polish rod to rotate around the center of axis of the
device, greatly
reducing the chance that wobblc will occur in the drive string as the drive
string back
spins to release built up torsional forces.
The device taught in 6,253,844 also uses a shock mechanism to allow the device
to also
handle longitudinal force placed on the swivelling device as it back spins or,
if the device
is being used with a stroke pump to dislodge a stuck drive string, absorb
sudden jerks the
drive string caused by the drive string coming unstuck. The shock mechanism
uses a
spring placed inside the housing and surrounding the rotating shaft. The shaft
can
therefore move longitudinally relative to the housing using the spring to
absorb any
longitudinal motion. However, in order for the device to function, either the
shaft or
bearing supporting the shaft must slide relative to the housing which can
prematurely
wear the bearing and limit the size and type of bearings that can be used.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a device for releasing a
stuck drive
string that overcomes problems in the prior art.
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In a first aspect, a swivelling device for coruiection to a drive string is
provided. The
device comprises: a housing having an opening in a bottom end; a shaft mounted
within
the housing and defining an axis, the shaft rotatably mounted within the
housing and
fixed longitudinally relative to the housing, a bottom end of the shaft
passing through the
opening in the bottom end of the housing and connectable to a drive string
with no point
of articulation between the shaft and the drive string; a support frame
connectable to an
external support, the support frame moveably connected to the housing such
that the
housing can be longitudinally displaced, along the axis, away from the support
frame;
and at least one biasing mechanism biasing the housing towards the support
frame.
In a second aspect, a swivelling device for connection to a drive string is
provided. The
device comprises: a housing having an opening in a bottom end; a shaft mounted
within
the housing using a first bearing and a second bearing and defining an axis,
the fust
bearing positioned towards the bottom end of the housing and suitable to
support a radial
load imposed on the first bearing by the shaft, the second bearing being a
thrust bearing
and positioned at a top end of the shaft preventing downward motion of the
shaft relative
to the second bearing such that downward motion of the shaft causes the
housing to move
downwards in conjtutction with the shaft, a bottom end of the shaft passing
through the
opening in the bottom end of the housing and connectable to a drive string
with no point
of articulation between the shaft and the drive string; a support frame
connectable to an
external support, the st-pport frame moveably connected to the housing such
that the
housing can be longitudinally displaced, along the axis, away from the support
frame;
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and at least two biasing mechanisms biasing the housing towards the support
frame, the
two biasing mechanisms positioned on substantially opposing sides of the
housing
In a third aspect, a swivelling device for cormection to a drive string is
provided. The
device comprises: a housing having an opening in a bottom end; a shaft mounted
within
the housing and defining an axis, the shaft rotatably mounted within the
housing and
fixed longitudinally relative to the housing, a bottom end of the shaft
passing through the
opening in the bottom end of the housing and connectable to a drive string
with no point
of articuiation between the shaft and the drive string; a support frame
connectable to an
extemal support, the support frame moveably connected to the housing such that
the
housing can be longitudinally displaced, along the axis, away from the support
frame;
and at least two coil springs, each coil spring positioned within a sleeve
connected an
outside surface of the housing and biasing the housing towards the support
frame.
In an aspect, a device is provided for allowing a drive string to move
longitudinally and
rotate as it is being picked up or backed off. The device has a shaft
rotatably mounted in
a housing so that the shaft can freely rotate but can not move radially or
axially relative to
the housing. A pair of biasing mechanisms comprising springs are located on
each side
of the housing and are attached at a top end to a frame support. The frame
support has an
aperture to be attachable to a support system capable of lifting up the drive
string.
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The biasing mechanisms allow the housing containing the rotating shaft to move
relative
to the frame support. In this way longitudinal motion such as sudden upward or
downward jerks of the drive string that places force on the shaft is
transferred to the
hotising, without requiring the shaft to slide in one or more of the bearing
and subject the
bearing surfaces to frictional forces between the shaft and the bearings or
the bearings
and the housing. The housing can move relative to the frame support using the
biasing
mechanism and the biasing mechanisms therefore absorb longitudinal forces
placed on
the shaft by the drive string.
DESCRIPTION OF THE DRAWINGS
While the invention is claimed in the concluding portions hereof, preferred
embodiments
are provided in the accompanying detailed description which may be best
understood in
conjunction with the accompanying diagrams where like parts in each of the
several
diagrams are labeled with like tiumbers, and where:
Fig. 1 is a bottom perspective view of the device, in accordance with the
present
invention;
Fig. 2 is a top perspective view of the device show in Fig. 1;
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Fig. 3 is a top view of the device shown in Fig. 1; and
Fig. 4 is a side sectional view of the device shown in Fig. 1.
.5
DETAILED DESGRIPTION OF THE ILLUSTRATED EMBODIMENTS
Figs. 1 and 2 are perspective views of a device 10 and Fig. 3 is a top view of
the device
10, in accordance with the present invention. The device 10 comprises: a
housing 12; a
support frame 14; a shaft 16; a cover plate 22; and a pair of biasing
mechanisms 24.
A drill string to be picked up or backed off is attached to device by
attaching a polish rod
to the threads 42 at a bottom end 19 of the shaft 16. A support system (not
system) is
connectable to an aperture 40 in the support frame 14.
Fig. 4 is a sectional front view of the device 10 along line A-A of Fig. 3.
The shaft 16 is
supported in the hoiLsing 12 by a primary bearing 1.8 and a secondary bearing
20. Each
biasing mechanism 24 comprises: a spring 30 and a structural bolt 32.
The shaft 16 is rotatably mourtted inside the housing 12 using the primary
bearing 18 and
the secondary bearing 20. The primary bearing 18 and the secondary bearing 20
maintain
the shaft 16 at an axis C of the device 10 and prevent radial motion of the
shaft 16.
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Typically, the primary bearing 18 is sized larger than the secondary bearing
20 in order to
support the majority of the radial force placed on the primary bearing 18 and
secondary
bearing 20 by the shaft 16. The secondary bearing 20 predominantly supports a
top end
17 of the shaft 16 and is typically also a thrust bearing to prevent axial
movement of the
shaft 16. In this manner, the primary bearing 18 and secondary bearing 20
allow the shaft
16 to rotate relative to the axis C but prevent radial and axial movement of
the shaft 16
relative to the housing 12.
The biasing mechanisms 24 are positioned on either side of the housing 12 and
allow the
support frame 14 to move relative to the housing 12 along axis C. Each
structural bolt 32
is provided with threads 33 that screw into mating threads 41 on the support
frame 14
(either by having the mating threads 41 cut right into the support frame 14 or
by using
nuts). The other end 35 of the structural bolt 32 abuts the end of the spring
30. The other
end 35 of the structural bolt 32 and spring 30 are housed within sleeves 31
connected to
the housing 12. When a longitudinal downward force is applied along axis C, to
the end
19 of the shaft 16, the shaft 16 places force on the secondary bearing 20,
which is a thni.st
bearing, causing the housing 12 to displace downward with the shaft 16. Stops
37 push
downwards on the tops of the springs 30, compressing the springs 30 positioned
on the
structural bolts 32 and allowing the housing 12 to move relative to the
support frame 14.
The biasing mechanisms 24 allows the housing 12 and the shaft 16 to be
displaced along
axis C, while the suppoit frame 14 remains in substantially the same position
along axis
C.
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When the housing 12 is no longer being forced downwards away from the support
frame
12 by a downward force on the shaft 16, the tops of the springs 20 place an
upwards force
on the stops 37 moving the housing 12 back towards the support frame 12.
In operation with a rotary pump, when a drive string gets stuck and a drive
unit rotating
the drive string has stopped and released any built up torsional forces that
it can by back
spinning the drive string using a breaking system, the drive unit is
disconnected from the
polish rod connected to the drive string and the device 10 is attached to the
end of the
polish rod. The device 10 is connected to the polish rod by attaching the
threaded end 42
of the shaft 16 to con=esponding threads on the polish rod. The polish rod and
attached
drive string is then "picked up" by using some type of support structure to
hoist up on the
structural frame 14 of the device 14, which will in turn hoist up on the shaft
16, the polish
rod and the drive string. The rotating shaft 16 allows the polish rod and
attached drill
string to release any built up torsional force by back spinning as the polish
rod and drive
string is picked up. The rotation of the shaft 16 along the centerline of the
device 10
prevents the inducing of wobble by the rotating shaft 16. The biasing
mechanisms 24
absorb vibrations and abrupt longitudinal motion of the drive string.
In operation with a rod pump, when a drive string gets stuck the threads 42 on
the end 19
of the rotating shaft 16 are used to connect the shaft 16 of the device 10 to
the polish rod.
The threads 42 on the end 19 of the rotating shaft 16 can either be connected
directly to
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the polish rod by corresponding threads or through the use of an adapter
suitable to
connect shaft 16 to the polish rod. The drive string and polish rod are then
backed off
using the device 10. Any rotation of the drive string that occurs during the
backing off
will be handled by the rotatable shaft 16 and the shock absorbing mechanism 34
will
allow the drive string to be loaded progressively during the backing up and
absorb any
sudden longitudinal motion of the drive string.
The foregoing is considered as illustrative oinly of the principles of the
invention.
Further, since numerous changes and modifications will readily occur to those
skilled in
to the art, it is not desired to liniit the invention to the exact
construction and operation
showa and described, and accordingly, all such suitable changes or
modifications in
structure or operation which may be resorted to are intended to fall within
the scope of
the claimed invention.
