Note: Descriptions are shown in the official language in which they were submitted.
CA 02550066 2006-06-09
IMPROVED WELLHEAD DRIVE BRAKING MECHANISM
Field of the Invention
This invention relates to oil production equipment. In particular, the
invention relates to an improved braking mechanism for a wellhead drive system
and
a wellhead drive system including said improved braking mechanism.
Background of the Invention
In the past, many conventional oil wells were serviced by a downhole
pump at or close to the bottom of the well, the pump being of a conventional
reciprocating kind actuated by a rod string, in turn reciprocated vertically
by a pump
jack.
Many of these older reciprocating pumps have been replaced by rotary-
drive progressive cavity pumps. Such rotary pumps are particularly suited for
the
production of crude oil laden with sand and water.
However, because of the typical depth of an oil well, the torque applied at
the top of the rod string, and the resistance of the pump at the bottom, can
cause the
rod string to wind up like a spring, thus storing the torque energy. Whenever
there is a
power failure or the system is shut down, this stored torque energy, along
with the
energy created by the fluid head on the pump, must release itself. Without any
control
on the rate of backspin of the rod string, serious problems have occurred, for
example:
- the motor, connected to the rod string through a reducer and a sheave and
pulley
arrangement, may reach reverse speeds exceeding safe limits. These speeds tend
to
damage the motor, and may even cause it to explode;
- one or both of the sheaves can reach speeds exceeding their limits;
- on drive configurations in which the polish rod extrudes out the top of the
drive, the
projecting portion can bend and break, and the broken-off portion will then be
flung
away from the installation, due to centrifugal force; and/or
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without some form of braking, the rod string could uncouple, with the result
that the rod
string and the pump would be lost down the hole.
U.S. patent no. 6,152,231 issued November 28, 2000 to Grenke, addresses the
above problem and describes a braking mechanism for controlling the release of
energy in
a rod string for a down-well rotary pump. The braking mechanism incorporates a
rotary
member positioned in the energy loop from a motor to the top end of the rod
string,
requiring that the rotary member rotate at a consistent speed ratio and
direction with
respect to the top end of the rod string. The rotary member drives a fluid
pump through a
slip clutch so that when the top end of the string rotates in the normal
direction, the clutch
slips and does not run the fluid pump. However, when the top end of the rod
string seeks
to rotate in the opposite direction, for example on shut down or power
failure, the fluid
pump is operated to pump fluid from a reservoir and back to the reservoir in a
closed loop
which includes a mechanism for restricting fluid. The fluid pump is separate
from the
wellhead drive system and connected via the clutch and couplings.
Although an effective safety feature, the slip clutch requires additional
moving
parts, increasing the cost and maintenance requirements for the drive device.
Brief Description of the Drawings
In drawings which illustrate by way of example only a preferred embodiment
of the invention,
Figure 1 is a sectional view of the wellhead drive system according to the
preferred embodiment; and
Figure 2 is a sectional view of the braking mechanism according to the
preferred embodiment, taken at right angles to the section of Figure 1.
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Detailed Description of the Invention
In one aspect, the present invention provides a braking mechanism which,
without using a slip clutch, slows the backspin of a rod string by using the
rotational
energy of the rod string to turn a fluid pump to pump fluid through a
constriction
orifice in a closed fluid circuit. As compared to the prior art described in
the
background, the present invention reduces the space required for the braking
mechanism and moreover reduces the number of moving parts within the braking
mechanism which in turn may reduce the likelihood that the braking mechanism
will
require maintenance or fail, as well as reduce the manufacturing cost.
In another aspect, the present invention provides a wellhead drive system
which employs the inventive braking mechanism in a wellhead drive system for
driving a pump, for example a progressive cavity pump.
A preferred embodiment of the braking mechanism of the present
invention is accomplished by providing for use with a pumping system in which
a
downhole pump has a rotor which is rotated by a rod string which is in turn
rotated by
a motor, a braking mechanism for slowing the release of torsional potential
energy in
the rod string when the motor stops, the mechanism comprising: a rotary member
rotationally coupled to the top end of the rod string; a reservoir containing
a
dampening fluid; a fluid pump driven by the rotary member, interposed into a
closed
fluid circuit with the reservoir, the fluid circuit comprising: a one-way
valve allowing
dampening fluid to flow between the reservoir and the fluid pump in a first
direction
corresponding to normal operation of the downhole pump, the one-way valve
including a valve orifice having a valve orifice area that does not
substantially restrict
the free flow of the dampening fluid during normal operation of the downhole
pump;
and a constriction orifice allowing the dampening fluid to flow between the
reservoir
and the fluid pump in a direction opposite to the first direction, the
constriction orifice
having an orifice area which substantially restricts the flow of the dampening
fluid
through the fluid circuit as potential torsional energy is released from the
rod string;
whereby if the motor stops the potential torsional energy in the rod string is
dissipated
by controlled rotation of the rod string in a direction opposite to the first
direction.
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A preferred embodiment of the wellhead driving system of the present
invention is accomplished by providing a wellhead drive system for driving a
downhole pump including a rotor comprising: a rod string having a top end and
a
bottom end, the bottom end being connected to, supporting and rotating the
rotor; a
motor providing torque energy for rotating the top end, whereby torsional
potential
energy is stored in the rod string during operation; and a braking mechanism
for
slowing the release of torsional potential energy in the rod string when the
motor
stops, the mechanism comprising: a rotary member rotationally coupled to the
top end
of the rod string; a reservoir containing a dampening fluid; a fluid pump
driven by the
rotary member, interposed into a closed fluid circuit with the reservoir, the
fluid
circuit comprising: a one-way valve allowing dampening fluid to flow between
the
reservoir and the fluid pump in a first direction corresponding to normal
operation of
the downhole pump, the one-way valve including a valve orifice having a valve
orifice
area that does not substantially restrict the free flow of the dampening fluid
during
normal operation of the downhole pump; and a constriction orifice allowing the
dampening fluid to flow between the reservoir and the fluid pump in a
direction
opposite to the first direction, the constriction orifice having an orifice
area which
substantially restricts the flow of the dampening fluid through the fluid
circuit as
potential torsional energy is released from the rod string; whereby if the
motor stops
the potential torsional energy in the rod string is dissipated by controlled
rotation of
the rod string in a direction opposite to the first direction.
Figure 1 illustrates the wellhead drive system 5 according to the preferred
embodiment. The wellhead drive system 5 operates with a pump, for example a
progressive cavity pump (not shown), to pump well fluids to the surface. Rod
string
34 comprises a long rod that transfers the torque derived from the motor 10 to
the
progressive cavity pump down the well. In the preferred embodiment, rod string
34
passes through a main hollow shaft 35 in gear box 36 and is connected to the
top of
the hollow shaft 35 by a drive cap 37 or clamp.
In the preferred embodiment shown in Figure 1, the motor 10 has a
substantially vertical drive shaft 12 carrying a first sheave 14. The main
hollow shaft
comprises an elongate and substantially vertical body carrying a second sheave
18.
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A belt (not shown) is entrained over the first sheave 14 and the second sheave
18 so
that the motor 10 may drive the rotation of the rod string 34. The motor 10
may
alternatively drive the main hollow shaft 35 via any suitable means, including
a chain
drive or a gear drive.
During operation of the wellhead drive system 5, when the motor 10 is
initially started the rod string 34 makes a number of turns before the
progressive
cavity pump starts to turn, building up torsional potential energy along the
length of
the rod string 34. When the system is shutdown for planned maintenance or due
to a
power outage, this torsional potential energy is released via the backspin of
rod string
34.
In addition, the fluid head above the progressive cavity pump creates a
back-pressure on the pump which adds to the backspin of the rod string 34.
Depending
on the viscosity of the fluid being pumped, this fluid head can push down on
the
progressive cavity pump with enough force to cause the downhole pump to act as
a
motor, rotating the rod string 34 in a direction opposite the direction of
rotation during
normal operation. The braking mechanism of the present invention allows for
the
torsional potential energy to be released in a controlled manner. As long as
there is
enough fluid above the downhole pump to cause the pump to turn in reverse, the
braking mechanism will continue to operate, slowing the rate of backspin.
A braking mechanism 7 according to the preferred embodiment is
illustrated in Figure 2. The braking mechanism 7 includes a rotary member such
as a
pinion shaft 16. A pinion gear 15 is mounted on the pinion shaft 16. The
pinion gear
15 engages a gear wheel 19 mounted to the main hollow shaft 35. The pinion
shaft 16
is an elongate shaft mounted parallel to the rod string 34, and extends
through the
interior of a reservoir such as reservoir 20. The reservoir 20 includes a
bottom wall 28.
The pinion shaft 16 passes through the bottom wall 28, but is sealed
thereagainst to
prevent leakage.
Shafts 16 and 35 maybe operatively interconnected in various ways. The
preferred embodiment, as shown in Figure 1, uses a variant where each of the
shafts
16 and 35 carries a gear, the two gears meshing in such a way that the ratio
of rotation
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between the shafts 16 and 35 remains constant (with the shafts rotating in
opposite
directions). Another variant involves the provision of a sprocket on each of
the shafts
16 and 35, along with a chain engaging both sprockets. In this second variant,
the
shafts 16 and 35 would rotate in the same direction. Other transmission means
may
also be suitable to rotate the pinion shaft 16 as the main hollow shaft 35
rotates.
Referring again to the preferred embodiment of the braking mechanism 7
shown in Figure 2, a fluid pump such as a gear pump 40 is interposed into a
closed
fluid circuit with the reservoir 20. Gear pump 40 is disposed in a gear pump
chamber
72 in fluid communication with a first conduit 42 and a second conduit 44. The
gear
pump 40 is connected directly to the pinion shaft 16, and rotates as the
pinion shaft 16
is rotated by the main hollow shaft 35. It will be appreciated that the fluid
pump does
not need to be a gear pump, and a different type of pump may be used. A
different
transmission mechanism may also be used.
The second conduit 44 includes two orifices: a constriction orifice 75 and a
free-flow orifice formed by a one-way valve such as a check valve 70. The
cross-
sectional area of the check valve 70 is preferably at least as large as the
cross-sectional
area of the conduits 42, 44, while the cross-sectional area of the
constriction orifice 75
is considerably smaller, to restrict the flow of dampening fluid in the manner
described below. The check valve 70 and the constriction orifice 75 are in
communication with a conduit 49 (shown in Figure 1) in fluid communication
with
the reservoir 20.
The reservoir 20 is filled with a preferably viscous dampening fluid, such
as oil, to a level that immerses the pinion gear 15 in the fluid. The fluid is
used to
dampen or retard the backspin of the rod string 34 in the manner described
below.
Gear pump 40 is able to turn in both directions. During operation of the
wellhead drive system 5, when the gear pump 40 is turning during normal
operation
the dampening fluid is suctioned from the reservoir 20 through the conduit 49
and
check valve 70, through the second conduit 44 and into the gear pump chamber
72.
Fluid will also be suctioned through the orifice 75, which also flows through
the
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second conduit 44 and into the gear pump 40 chamber 72. The gear pump 40 then
pumps the dampening fluid back to the reservoir 20 through the first conduit
42.
When the wellhead drive system 5 shuts down for any reason, the rod
string 34 will attempt to spin backwards as the torsional potential energy is
released.
This will cause rotation of the hollow shaft 35, which in turn will rotate the
pinion
shaft 16 through the meshing gears. When the rod string 34 goes into backspin,
the
gear pump 40 suctions dampening fluid from the reservoir 20 through first
conduit 42
and into the gear pump chamber 72. The pinion shaft 16 rotates in reverse
under the
force of the back-spinning rod string 34, which drives the gear pump 40 in
reverse,
causing the dampening fluid to be suctioned from the reservoir 20 through the
first
conduit 42 and into the gear pump chamber 72. However, the check valve 70 does
not
permit the dampening fluid being discharged into conduit 44 to flow through
the large
orifice, so the gear pump 40 is constrained to pumping the fluid through the
constriction orifice 75 in the second conduit 44 and back to the reservoir 20.
Since the
constriction orifice 75 has an orifice area that is significantly less than
the valve
orifice area of the orifice of the check valve 70, fluid flow rate is
restricted
commensurately and the resistance to the flow of the dampening fluid opposes
the
backspinning force of the rod string 34, causing the rod string 34 to unwind
slowly,
releasing the torsional potential energy in a controlled fashion.
Since both the first conduit 42 and the second conduit 44 communicate
with the interior of the reservoir 20, through sealed openings (not shown),
which in
the embodiment shown contains the pinion gear 16, the lubricating oil for the
wellhead drive system 5 (which is used to lubricate the pinion gear 15 anyway)
can be
advantageously used as the dampening fluid. Thus, in the preferred embodiment
a
lubrication conduit 80 located downstream of the gear pump discharge during
normal
operation of the wellhead drive system 5 communicates a small portion of the
oil to
lubricate the top bearings 88 of the gearbox 36. Lubrication conduit 80 also
includes a
small one-way valve such as a check valve 90 that keeps air from entering the
integral
gear pump 40 during backspin of the rod string 34. However, in alternate
embodiments a separate reservoir can be employed to contain a dampening fluid
that
is separate from the lubricating fluid.
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According to the preferred embodiment of the braking mechanism 7, an
adjustable flow control such as flow control valve 46 may be interposed
upstream of
the orifice 75 in the second conduit 44. This provides a manually adjustable
control to
the braking mechanism 7 in order to controllably restrict the flow of
dampening fluid
through orifice 75. Actuation of the adjustable flow control 46 allows the
backspin of
the rod string 34 to be varied from a controlled backspin to a virtual
standstill. When
the flow control valve 46 is substantially fully opened, the rod string 34
will be
allowed to backspin at a relatively slow rate of rotation, as fluid from the
reservoir 20
is continuously pumped in a closed loop by the gear pump 40. When the flow
control
valve 46 is substantially closed, the rod string 34 will be brought to a
virtual standstill
because the gear pump is stalled by the inability to pump fluid through the
second
conduit 44. The addition of the adjustable flow control 46 is optional and
allows for
visual inspection of the drive system 5 following a fault condition.
Various embodiments of the present invention having been thus described
in detail by way of example, it will be apparent to those skilled in the art
that
variations and modifications may be made without departing from the invention.
The
invention includes all such variations and modifications as fall within the
scope of the
appended claims.
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