Note: Descriptions are shown in the official language in which they were submitted.
CA 02452708 2005-04-18
HYDRAULIC OIL WELL PUMP DRIVE SYSTEM
Related Application
This application is a division of Canadian Patent Application Serial No.
2,131,192 which was
filed as the Canadian National Phase application of International Application
published
as International Publication No. W093/18306
Technical Field
This invention relates to hydraulic drive systems for oil well pumps.
Background Art
Oil wells vary in depth from a few meters to up to 6100 meters. Oil is lifted
from these
depths by a plunger which reciprocates within a pump barrel at the bottom of
the well. The plunger
is driven by a sucker rod or an interconnected series of sucker rods which
extend down from the
surface of the oil well to the plunger.
Fig. 1 shows a conventional pump jack 10 for driving the sucker rod of an oil
well pump.
Pump jack 10 generally comprises a walking beam 12 which is connected through
a polished rod 14
to an in-hole sucker rod (not shown). Walking beam 12 is pivotally supported
at an intermediate
position along its length by a samson post 16, which is in turn mounted to a
base frame 18. A drive
crank system 20 is also mounted to base frame 18. Base frame 18 is mounted to
a concrete base to
rigidly locate all components relative to the oil well.
Drive crank system 20 has a rotating eccentric crank arm 24. Crank arm 24 is
driven at a
constant speed by an electric or gas motor in combination with a gearbox or
reducers, generally
designated by the reference numeral 26. Eccentric crank arm 24 rotates about a
horizontal axis.
Walking beam 12 has a driven end 30 and a working end 32 on either side of its
pivotal
connection to samson post 16. One or more pitman arms 34 extend from driven
end 30 to a crankpin
35 positioned intermediately along outwardly extending eccentric crank arm 24.
Rotation of crank
arm 24 is translated by pitman arms 34 into vertical oscillation of the
walking beam's driven end 30
and corresponding oscillation of working end 32.
Working end 32 of walking beam 12 has an arcuate cable track or horsehead 36.
A cable 38
is connected to the top of the cable track 36. Cable 38 extends downwardly
along the cable track 36
and is connected at its lower end to polished rod 14. Pivotal oscillation of
walking beam 12 thus
produces corresponding vertical oscillation of polished rod 14 and of the
connected sucker rod.
The arcuate shape of cable track 36 ensures that forces
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between working end 32 and polished rod 14 remain vertically aligned at all
positions of walking beam 12.
The sucker rod of an oil well pump performs its work during an upward
stroke, when oil is liftcd from the well. No pumping is performed during the
downward stroke of the sucker rod. Accordingly, a pump jack such as described
above supplies force to a sucker rod primarily during its upward stroke.
Relatively little force is produced on the downward stroke. To increase
efficiency of a drive system counterbalance weights are utilized to store
energy
during the sucker rod downward stroke and to return that energy to assist in
the sucker rod upward stroke.
In pump jack 10, counterbalance weights 40 are positioned at the
outermost end of crank arm 24. Such weights could also be positioned on the
driven end 30 of walking beam 12. However, a mechanical advantage is
obtained by placing the weights outward along the crank arm from the pitman
arm connection. During the downstroke of the sucker rod the driving motor
must supply energy to raise weights 40 to the top of their stroke. During the
sucker rod's upstroke, however, weights 40 assist the motor and gearbox since
the outward end of crank arm 24 moves downward while the sucker rod moves
upward. The peak energy required by the motor is therefore greatly reduced,
allowing a smaller motor to be used with corresponding increases in
efficiency.
Mechanical pump jacks such as described above have been used for many
years and continue to be used nearly exclusively for driving oil well pumps.
Acceptable substitutes have simply been unavailable. One reason for the
popularity of such mechanical systems is their extreme simpiicity. They do not
involve valves, switches, or electronics, and there are a minimum of moving
parts.
This simplicity results in reliability which is difficult to accomplish with
more
complex systems. Reliability is of utmost importance since oil well pumps are
unattended for long periods, often being located in remote locations.
The very nature of sucker rod displacement created by a reciprocating
pump jack is another apparent reason for its success. An oil well sucker rod
is often over 6100 meters long. While reciprocating, it must not only
accelerate
and decelerate itself, but also a 6100 meter oil column. In addition, it must
accelerate and decelerate oil within an above-surface production line, which
can
be as long as three kilometers. Forces caused by sudden acceleration of the
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sucker rod are thereforc verv, significant. Any such sudden or undue
acceleration
can stretch and snap the sticker rod.
The pump jack described above minimizes acceleration and deceleration
forces on the sucker rod by producing an approximately sinusoidal displacement
at the polished rod. The sinusoidal displacernent results from translation of
rotary crank motion to linear motion at the polished rod. Such sinusoidal
motion significantly reduces strain on the driven, sucker rod.
However, while the pumping action of a mechanical pump jack is
preferable to previously-known alternatives, its physical size creates
significant
jo disadvantages. For instance, the great weight of the walking beam, gearbox,
and
counterbalance weights requires expensive support bases and land site
preparation.
In addition, pump jacks must be attached permanently above a wellhead and are
therefore not easily moved to another site. This results in costly pumping
equipment sitting idle during periods of oil welI inactivity.
While alternative drive systems have been attempted, none have met with
significant commercial success. Fig. 2 illustrates one prior art drive system,
comprising a hydraulic pump drive system which is generally designated by the
reference numeral 50. Drive system 50 includes a hydraulic cylinder 52
containing a piston assembly 54. Piston assembly 54 is designed for reciprocal
vertical motion within cylinder 52. It comprises an elongated center shaft 56
having a pressure piston 58 on its upper end and a working piston 60 at an
intermediate position along its iength. Center shaft 56 has a lower end which
is conr ~;ted through a coupling 62 to a polished rod 64.
Cylinder 52 has a centrally located annular flange 66 which seals against
center shaft 56 between pressure piston 58 and working piston 60 to divide
cylinder 52 into an upper pressure chamber 68 and a lower working chamber 70.
Pressure piston 58 reciprocates within pressure chamber 68 and working piston
60 reciprocates within working chamber 70.
Piston assembly 54 is driven up and down by hydraulic force applied
alternately to the bottom and then the top of working piston 60. A hydraulic
pump 72 supplies hydraulic fluid under pressure from a reservoir 74 to a cross-
over hydraulic valve 76. Valve 76 is in fluid communication with working
chamber 70 through fluid ports both above and below working piston 60. A
lower limit switch 78 and an upper limit switch 80 are actuated by a switch
actuator 82 which travels up and down with center shaft 56. Actuator 82
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actuates lower limit switch 78 at the bottom of desired piston assembly
travel.
causing cross-over valve 76 to supply pressurized hydraulic fluid to working
chamber 70 below working piston 60. This forces piston assembly 54 upward.
Actuator 82 actuates upper limit switch 80 at the top of desired piston
assembly
travel, causing cross-over valve 76 to supply pressurized hydraulic fluid to
working
chamber 70 above working piston 60. This forces piston assembly 54 back down.
Hydraulic fluid displaced by piston 60 from the non-pressurized side of
working
piston 60 is returned through valve 76 into fluid reservoir 74.
Pressure chamber 68 is filled with hydraulic fluid below pressure piston
1o 58 and is connected for fluid communication with an accumulator cylinder
84.
Accumulator cylinder 84 has a free-floating piston 86 which divides
accumulator
cylinder 84 into a hydraulic fluid chamber 88 and a gas chamber 90. Hydraulic
fluid displaced from pressure chamber 68 by the downward movement of pressure
piston 58 is forced into hydraulic fluid chamber 88, forcing free-floating
piston
86 toward gas chamber 90. Gas chamber 90 contains pressurized gas which
opposes such movement.
Hydraulic drive system 50 thus provides a hydraulic mechanism for
alternately moving a sucker rod upward and downward. Furtherrnore, the
opposing pressure of the pressurized gas within gas chamber 90 assists in the
upward stroke of piston assembly 56 and the connected sucker rod. This allows
use of a smaller hydraulic pump than would otherwise be necessary. The drive
system does not, however, address the problems of sudden sucker rod
acceleration
and deceleration. In fact, the significant force applied to the sucker rod is
subject to sudden and complete reversal at both the top and bottom of each
sucker rod stroke. The resulting acceleration and deceleration tends to
greatly
reduce the life of a sucker rod.
Attempts have been made to reduce the sudden acceleration and
deceleration which often occurs at the point of stroke reversal in prior art
hydraulic pump drive systems. For instance, U.S. Patent No. 2,555,426 to W.C.
Trautman ct al. describes use of a gas accumulator connected to a hydraulic
pressure line which feeds a hydraulic drive cylinder. The gas accumulator is
said
to maintain a constant pressure on a polished rod so that the velocity of the
polished rod can vary according to the resistance encountered and produced by
the polished rod and connected sucker rod. However, such an accumulator
produces a great degree of elasticity in the drive system. often resulting in
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uncontrolled and erratic sucker rod displacement. Such uncontrolled
displacement itself is
a cause of unacceptable acceleration and deceleration. The elasticity in the
Trautman
drive system prevents it from producing the constant, sinusoidal motion of a
pump jack,
which experience has proven to be preferable.
The Trautman patent also describes a rather complex valving system intended to
modulate the reversal of hydraulic oil pressure to the drive cylinder.
Recognizing the
desirability of reducing acceleration extremes, Trautman proposes a mechanism
for
decelerating the drive piston rapidly but uniformly at the end of its stroke,
and then
accelerating it as rapidly as possible at the beginning of the next stroke
(column 9, lines
26-34). Using this approach, full hydraulic pressure is applied at the
beginning of each
stroke, causing rapid and uncontrolled acceleration of the polished rod and
connected
sucker rod.
Other patents show similar attempts to provide a workable hydraulic drive
system.
However, no prior attempt has gained any significant acceptance as a
replacement for a
mechanical pump jack. Previous attempts to control acceleration and
deceleration in
hydraulic systems have involved complex valving systems, often requiring
numerous
valves, hydraulic pumps, displacement and velocity sensors, and other
electronic
equipment. Such complexities greatly diminish reliability.
U.S. Patent No. 2,526,388 to W. O. Miller, entitled "Closed Circuit Fluid
Apparatus for Deep Well Pumping with Counter-Balance Cylinder" describes one
attempt
at a simplified hydraulic well pumping system. The Miller device utilizes a
closed fluid
system with two cylinders and a movable piston in each cylinder. One piston is
mounted
atop the end of an oil sucker rod, while the other piston is reciprocated with
a device such
as shown in Fig. 2 of the Miller patent. This produces what Miller refers to
as harmonic
motion. While this system is probably preferable over other prior art systems,
the
mechanical driving mechanism is complex and subject to failure.
SUMMARY OF THE INVENTION
The invention described below eliminates the complexity and unreliability of
previous devices, resulting in a hydraulic drive system which duplicates the
motion of a
mechanical pump jack while requiring no valves or variable restrictions during
its normal
operation. While providing simplicity in both construction and operation, the
preferred
embodiment of the invention includes means for automatically regulating pump
stroke and
for automatically replenishing leaked oil.
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Accordingly, in one aspect the present invention resides in a hydraulic oil
ell pump
drive system for driving an oil well sucker rod, the drive system comprising:
a drive assembly frame (346);
a master cylinder (356);
a master piston (358) which is received within the master cylinder for axial
reciprocation;
a piston drive rod (364) connected to the master piston, the piston drive rod
being
maintained in axial alignment with the master cylinder and master piston, the
piston drive
rod extending axially from the master cylinder;
the master cylinder and the piston drive rod each having a pivotable
connection;
a crank assembly (344) connected to the drive asserribly frame;
a first of the pivotable connections being pivotably connected to the drive
assembly frame;
a second of the pivotable connections being pivotably connected to the crank
assembly to reciprocate the master piston relative to the master cylinder, and
to thereby
produce a bi-directional working fluid flow to and from the master cylinder,
the master
cylinder in fluid communication with a drive assembly for said sucker rod
which is
responsive to said fluid flow, whereby said bi-directional working fluid flow
reciprocally
displaces the sucker rod between upper and lower extremes, the upper and lower
extremes
defining a sucker rod stroke length;
the pivotal connections allowing the master cylinder to pivot angularly in
relation
to the drive assembly frame during reciprocation of the master piston relative
to the master
cylinder.
Further advantages of the invention over both mechanical pumping jacks and
over
prior art hydraulic pump drives will also be apparent from the discussion
below.
Brief Description of the Drawins
Preferred embodiments of the invention are described below with reference to
the
accompanying drawings, in which:
Fig. I is a side view of a prior art oil well pump jack;
Fig. 2 is a schematic view of a prior art hydraulic oil well pump drive;
Fig. 3 is a side view of a hydraulic oil well pump drive system in accordance
with
a first preferred embodiment of the invention:
Fig. 4 is a top view of the drive system shown in Fig,. 3;
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Fig. 5 is a schematic view of a the drive system shown in Figs. 3 and
4;
Fig. 6 is a schematic view of a hydraulic oii well pump drive system in
accordance with a second preferred embodiment of the invention;
Fig. 7 is a schematic view of a hydraulic oil well pump drive system in
accordance with a third preferred embodiment of the invention;
Fig. 8 is a partial cross-sectional view of a master hydraulic cylinder and
piston in accordance with a preferred embodiment of the invention;
Fig. 9 is a cross-sectional view of a single-action fluid injector pump in
accordance with a preferred embodiment of the inivention;
Fig. 10 is a cross-sectional view of a vertically-oriented master hydraulic
cylinder and piston in accordance with a preferred embodiment of the
invention;
Fig. 11 is a side view of a dual-cylinder wellhead hydraulic cylinder
assembly in accordance with a preferred embodiment of the invention, with one
of the cylinders being shown in cross-section;
Fig. 12 is an exploded isometric view of a split bearing assembly for a
wellhead slave cylinder in accordance with a preferred embodiment of the
invention; and
Fig. 13 is a cross-sectional side view of a single-cylinder wellhead hydraulic
cylinder assembly and an wellhead transfer pump ini accordance with a
preferred
embodiment of the invention. '
Disclosare of Invention and Best Modes for CarnJnQ Out the Invention
Figs. 3-5 show a hydraulic oil well pump drive system in accordance with
a first preferred embodiment of the invention, generally designated by the
reference numeral 100. Drive system 100 is located at a conventional oil
wellhead 102. Wellhead 102 has a stuffing box assembly 104 which receives a
polished rod 106 therethrough. Polished rod 106 oscillates or reciprocates in
a
vertical direction, extending downward through a well casing 108 to a sucker
rod
(not shown). The sucker rod extends downward tlhrough well casing 108 to a
plunger (not shown) at the bottom of the oil well. The plunger is oscillated
by the sucker rod to lift oil to the surface and to pump said oil through a
production line 110 to a reservoir or remote location.
A wellhead hydraulic assembly I11 is mounted directly over wellhead 102
to drive the oil well sucker rod. Wellhead hydraulic assembly 111 includes a
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fixed vertical wellhead frame 112 which is mounted or fastened to a concrete
base 114.
Wellhead hydraulic assembly I11 is operably connected to the oil well
sucker rod to alternately displace the sucker rod in opposite vertical
directions.
S It includes a wellhead cylinder assembly 118 having a wellhead slave piston
122
within a welihead slave cylinder 120. An air bleed valve 131 is connected for
fluid communication with the top of slave eylinder 120 to allow escape of
entrapped air within slave cylinder 120. Wellhead cylinder assembly 118
receives
a working fluid flow through a hydraulic supply line 130. The working fluid
flow is bi-directional. alternating in direction between a positive fluid flow
into
slave cylinder 120 and a negative fluid flow out from cylinder 120. The bi-
directional working fluid flow produces relative reciprocal motion between
wellhead piston 122 and wellhead cylinder 120. Positive flow of hydraulic
fluid,
into welihead cylinder 120 through supply line 130, raises wellhead piston 122
at a rate which is directly proportional to the rate of incoming fluid flow.
Negative hydraulic fluid flow, out from wellhead cylinder 120, lowers wellhead
piston 122 at a rate proportional to the rate of outgoing fluid flow.
A piston rod 132 extends downward from wellhead piston 122, through wellhead
cylinder 120, and connects to a connector link 134. Connector link 134 is in
turn connected
to polished rod 106 by a polished rod clamp assembly 136. Wellhead cylinder
120 and
wellhead piston 122 are thus operably connected between wellhead frame 112 and
the oil
well sucker rod to displace the sucker rod alternately up and down at the same
rate as the rate
of hydraulic flow through supply line 130. As shown best in Figure 5, a
cushioning spring
135 surrounds piston rod 132, being positioned beneath slave piston 122 to
tnitibate the
1s
impact of the salve piston which might result from a sudden loss of hydraulic
pressure.
Drive system 100 also includes a master hydraulic source or supply
assembly 140 for driving wellhead cylinder assembly 118. Supply assembiy 140
forms means for displacing a working fluid such as hydraulic oil or fluid to
produce a bi-directional working t7uid flow, wherein the direction of the
working
fluid flow alternates between an positive, outward displacement of hydraulic
fluid
from supply assembly 140 and an negativc, inward displacement into supply
assembly 140. The rate of the hi-directional working fluid flow is sinusoidal
as
a r-_sult of the unique driving mechanism described below. Suppiy assembly 140
is in fluid communication with wellhead cylinder assembly 118, supplying the
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working tluid llow to slave cvlinder 120 through supply line 110. Wcllhead
cylinder assembly 118 is directly responsivc to the working Eluid flow to
reciprocate the sucker rod at the same sinusoidal rate as the working fluid
flow.
Supply assembly 140 has a master drive assembly frame 146. Master drive
assembly frame 146 is shown in Figs. 3 and 4 as a mobile or portable trailer
assembly. Other types of frames are of course pc>ssible, including stationary
frames. Supply assembly 140 also includes one or more master cylinder
assemblies 142. Each of the master cylinder assemblies 142 is mounted at one
of its ends to frame 146 and is driven at its other end by an eccentric crank
or crank arm 144. While two master cylinder assemblies 142 are shown, only
one is required. Each master cylinder assembly 142 is pivotally mounted at its
first end to an anchor bearing assembly 148 on frame 146. The second end of
each master cylinder assembly 142 is connected to a crank pin 147 on the
outward end of its associated crank arm 144.
1s Each crank arm 144 is rotatably connected to frame 146 by a crank drive
which rotates crank arm 144 at a constant rotational speed. More specifically,
each crank arm 144 is driven by a crankshaft 149 of a gearbox or reducer 150.
A motor 152 is connected to drive gearbox 150 by a belt 154 or by other
suitable means. Crank arm 144, gearbox 150, and motor 152 are conventional
2c devices such as available for use in existing mechanical pump jack drives.
Motor
152 can be an electric motor, a gasoline or diesel engine, or a hydraulically-
powered motor. A hydraulically-driven motor might be desirable, for example,
to provide variable speed capability to the drive systetn.
As shown best in Figure 5, each master cylinder assembly 142 includes a master
hydraulic cylinder 156 and a master piston 158. Master cylinder 156 has first
and second
axial end sections 160 and 162, corresponding to a first, pressure end of
cylinder 156 and
a second, working end of cylinder 156. Each of sections 160 and 162 comprises
a tubuiar sleeve which is closed on one end and open on the other. The two
sections are connected together by flanges at their open ends, to form a
30 cylindrical compartment within which master piston 158 reciprocztes. A
center
seal or center seal assembly 168 is positioncd at the abutment of the two end
sections near the axial midpoint of master cylinder 156. Master piston 158 is
siidabiy received through center seal assembly 168 for axial a:splacement or
reciprocation between the closed ends of first and second end sections 160 and
35 162. Center seal 168 seals against master piston 158, defining with master
piston
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158 a working chamber 170 in the working end of master cylindcr 156 and a
pressure chamber 172 in the pressure end of master cylinder 156. Hydraulic oil
or fluid is contained within working chamber 170 and pressure chamber 172.
Master piston 158 has a piston drivc rod 164 which extends through a
S sealing aperture in the closed end of second end section 162. Crankpin 147
of
crank arm 144 is pivotally connected to drive rod 164 to reciprocate master
piston 158 within master cylinder 156. Each of the working and pressure
chambers 170 and 172 defines a fluid volume which varies with the axial
displacement of master piston 158 within master cylinder 156. Working chamber
1o 170 is in fluid communication with wellhead hydraulic cylindcr assembly 111
through supply line 130. A hydraulic cooling chamber 174 in supply line 130
cools hydraulic =oil passing therethrough. Cooling chamber 174 is optional,
and
will not be used in many cases. The components described above form a closed
working fluid communication system which preferably does not contain a
pressure
15 accumulator or any accumulator-like element. The presence of an accumulator
would add undesired elasticity to the drive system. Because of the closed
communication between working chamber 170 and slave cylinder 120, the vertical
position of slave piston 122 relates directly to the axial position of master
piston
158 within master cylinder 156.
20 Crankshaft 149 and crank arm 144 are driven by motor 152 at a constant
rotational speed. The rotational motion of crankshaft 149 is transiated into
axial
and reciprocal motion of master piston 158 by the pivotal connection of
crankpin
147 to piston drive rod 164. This method of driving master piston 158 results
in a sinusoidal rate of master piston displacement. Displacement of master
25 piston 158 causes- a corresponding displacement of hydraulic fluid into or
out
from working chamber 170, which results in a working flow of hydraulic fluid
through supply line 130. The rate and direction of the working fluid flow is
related directly to the rate and direction of master piston displacement.
Accordingly, the working fluid flow is bi-directional. alternating between a
positive
30 fluid flow from working chamber 170 and a negative fluid flow back into
working
chamber 170. The rate of working fluid displacement is sinusoidal, in response
to the sinusoidal rate of master piston displacement. Welihead cylinder
assembly
118 is directly responsive to the master piston reciprocation. by virtue of
the
working fluid flow caused by such reciprocation, to alternately displace the
sucker
33 rod in opposite directions at a sinusoidal rate.
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In addition to the components described above, supply assembly 140
includes pressure accumulator means for applying upward biasing force to the
sucker rod to assist in producing upward displacement of the sucker rod. The
accumulator means preferably comprises a gas accumulator 176 which is in fluid
5 communication with pressure chamber 172 of master cylinder 156 through a
pressure fluid line 178. Pressure chamber 172 and gas accumulator 176 contain
a volume of hydraulic oil 180. The volume of hydraulic oil within pressure
chamber 172 varies with the axial position of master piston 158. As master
piston 158 moves toward the pressure end of master cylinder 156, it displaces
lo oil from pressure chamber 172, out through pressure fluid line 178, and
into
accumulator 176. Oil is drawn back into pressure chamber 172 as master piston
158 moves toward the working end of master cylinder 156. Accumulator 176
contains an excess of hydraulic oil over that required by pressure chamber
172,
so that a minimum level of oil is always present in accumulator 176. A volume
1s of pressurized gas 182 such as nitrogen is also coritained within gas
accumulator
176 over hydraulic oil 180. The pressure of gas 182 is adjusted through an air
valve 183 on top of accumulator 176. Hydraulic oil displacement from pressure
chamber 172 and into accumulator 176 is opposed by the gas within accumulator
176. The pressurized gas subsequently assists in displacement of the master
piston toward the master cylinder working end.
The portion of the master piston stroke corresponding to the downward
stroke of wellhead piston 122, during which little f'orce is required to move
the
sucker rod, is opposed by the pressurized gas within accumulator 176. During
the subsequent upward stroke of wellhead piston 122, during which maximum
force must be produced. the compressed gas acts through hydraulic oil 180 to
assist in moving master piston 158 toward the working end of master
cylinder 156, effectively biasing the sucker rod upward and assisting in
producing
its upward displacement.
The gas volume within gas accumulator 176 is ideally large enough to
avoid any significant pressure variance resulting from gas compression and
expansion as hydraulic oil enters and exits the accumulator. In practice,
however, a certain amount of compression and expansion will occur. As a
result, accumulator pressure increases as the master piston moves toward the
mastcr cylinder pressure end, corresponding to downward movement of the sucker
rod. Accumulator pressure decreases as the master piston moves toward the
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master cylinder working end, corresponding to upward movement of the sucker
rod. The effect is greatest at the extremes of master piston displacement.
However, the crank drive has a mechanical advantage at displacement extremes,
essentially producing greater driving force near the ends of the sucker rod
s strokes. The greater driving force at displacement extremes overcomes and
largely negates the variable pressure supplied by the pressure accumulator.
The unique combination of hydraulic and mechanical elements described
above drives an oil well sucker rod at a sinusoidal rate, duplicating the
motion
of a conventional mechanical pump jack. In addition, a hydraulic equivalent to
a conventional counterweight system is provided by the gas accumulator working
against the master piston. In contrast to prior art hydraulic drive systems,
however, the wellhead hydraulic assembly and the master hydraulic cylinder
working chambcr form a closed system which requires no valving and which
allows no pressure elasticity other than that produced by the sucker rod
itself.
Modulating the rate of the working fluid flow to the wellhead hydraulic
cylinder
is accomplished entirely by the natural sinusoidal reciprocation of the master
piston, resulting from its connection to an eccentric crank drive. The system
is
dramatically simpler than prior art hydraulic drive systems. While some of the
additional mechanisms to be described below include valves and valve control
mechanisms, such valves do not cycle with each sucker rod reciprocation and
are
not required to produce such 'sucker rod reciprocation. Rather, such valves
and
valve controls are necessary only for repienishing oil supplies or for
correcting
overstrokc conditions. Even with the additional mechanisms to be described,
the
drive system is much simpler than previous hydraulic drive systems.
Drive system 100 preferably includes overstroke correction means, preferably
comprising a hydraulic fluid injector for preventing excessive downward
displacement of the sucker rod. Such excessive downward displacement would
typically occur because of insufficient oil volume forming the working fluid
flow,
caused by leakage of hydraulic oil from master cylinder 156 or wellhead
cylinder
120. The overstroke correction means functions by sensing excessive downward
sucker rod displacement and by injecting additional oil into the working fluid
flow in response.
The overstroke correction means or fluid injector is formed by an injector
subsystem 186 and a mechanically-actuated and normally closed two-way fluid
line
valvc 188. Oil injcction subsystem 186 supplies pressuriZed hydraulic oil
through
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an injection supply line 190 to oil line valve 188. Oil linc valve 188 is
connected, in turn, to selectively supply pressurized hydraulic oil to
welihead
cylinder 120.
A valve actuating finger 192 is attached to wellhead piston rod 132 for
reciprocal motion corresponding to the reciprocal motion of the oil well
sucker
rod. Finger 192 and two-way valve 188 are adjustably positioned relative to
each
other so that finger 192 actuates or enables two-way valve 188 upon downward
overstroke of piston rod 132 and the oil well sucker rod. Upon being enabled,
valve 188 injects pressurized hydraulic fluid into the wellhead cylinder 120.
The
io additional oil injected into the working fluid tlow, raises the operating
level of
wellhead piston 122, thereby preventing further overstroking in the downward
direction.
A guide finger 194 extends laterally behind actuating finger 192. Guide
finger 194 is received along a vertically-extending guide bar 196. Guide bar
196
prevents rotation of actuating finger 192 around wellhead piston rod 132 and
ensures continued alignrnent of actuating finger 192 with two-way valve 188.
Oil injection subsystem 186 comprises a hydraulic fluid reservoir 200, a
fixed displacement hydraulic pump 202, a nitrogen-charged hydraulic
accumulator
204, a hydraulic pressure unloading valve 206, and a closed-center, three-way
manual directional control valve 208. Hydraulic pump 202 is connected through
a one-way check valve 210 to supply a low volume of high-pressure hydraulic
fluid from reservoir 200 to injection supply line 190. Accumulator 204 is
connected to injection supply line 190 to level pressure fluctuations.
Unloading
valve 206 is also connected to supply line 190 to regulate the pressure in
supply
line 190.
Three-way valve 208 is connected to manually increase or decrease the
volume of hydraulic oil in the working fluid flow. Valve 208 is used primarily
during initial set-up of the drive system to set the desired range of travel
of
wellhead piston 122. Initial set-up begins by opening air bleed valve 131 and
opening three-way valve 208 to inject oil into the working fluid flow. Air
bleed
valve 131 is closed when it begins to pass hydraulic oil rather than air.
Three-
way valve 208 remains open to inject the estimated appropriate volume of
hydraulic oil into the working fluid flow. Motor 152 is then energized to
begin
reciprocation of the mastcr piston. Three-way valve 208 is subsequently used
to
add or subtract oil from the working fluid flow as required to obtain the
desired
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travel of wellhead piston 122. During normal operation. leakage from the
working fluid flow is restored by operation of valve 188. In addition, the
nitrogen pressure within accumulator 176 is adjusted through air valve 183 to
obtain the desired counterbalancing force as required to adequately oppose the
downward stroke of the oil well sucker rod and to assist in its subsequent
upward stroke. The accumulator pressure is calculated and adjusted to subject
motor 152 to an approximately equal load ciuring both the upstroke and
downstroke of wellhead piston 122.
The overstroke correction means could alternately comprise a selectively
activated and electrically powered hydraulic pump connected through a one-way
check valve to the working fluid flow. The pump could be switched on by an
electrical limit switch activated by actuating finger 192. Flow of pressurized
hydraulic fluid into the working fluid flow could likewise be initiated by an
electrical limit swicch connected to open an electrically activated solenoid
valve.
Fig. 6 illustrates a second preferred embodiment of a pump drive system
in accordance with the invention, generally indicated by the reference numeral
220. The components shown are mounted in a manner similar to that already
described above with reference to Figs. 3 and 4. Drive system 220 includes a
wellhead or slave hydraulic assembly 222 driven by a master hydraulic source
assembly 224. It also includes a gas accumulator 226 which supplies an upward
bias to the oil well sucker rod to assist in upward strokes of the sucker rod.
However, gas accumulator 226 operates directly on wellhead hydraulic
assembly 222 rather than on master hydraulic source 224.
Wellhead hydraulic assembly 222 includes a fixed vertical welihead
frame 228 which is mounted to a concrete base over a wellhead to drive an oil
well sucker rod. Wellhead hydraulic assembly 222 comprises a welihead cylinder
assembly 230 having a two-stage wellhead slave piston 234 positioned within an
upper primary wellhead slave cylinder 232 and a lower, secondary welihead
slave
cylinder 246 for vertical displacement therein. Slave piston 234 includes an
upper. primary section 236 and a lower, secondary section 238. Upper
section 236 and lower section 238 are aligned concentrically about a vertical
axis.
Lower section 238 has a smaller diameter than upper section 236, and extends
downwardly from upper section 236.
Upper section 236 of slave piston 234 is driven by a working hydraulic
fluid flow to reciprocate verticallv within upper slave cylinder 232. A seal
240
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extends about upper slave cylinder 232 at an approximate midpoint of upper
cylinder 232. Upper section 236 of slave piston 234 is slidably received
within
seal 240. dividing slave cylinder 232 into an upper, working chamber 242 and a
lower, pressure chamber 244 at the upper and lower ends of slave cylindcr 232,
respectively.
Lower section 238 of slave piston 234 extends downward from primary
cylinder 232 into secondary slave cylinder 246. Secondary slave cylinder 246
defines a lower working chamber 248. A piston rod 250 is connected to a
polished rod (not shown) by a connector link 252. Wellhead piston 234 is thus
operably connected between wellhead frame 228 and the oil well sucker rod to
alternately reciprocate the sucker rod.
Master hydraulic supply assembly 224 comprises a master cylinder
assembly 258 having first and second working chambers 260 and 262. A master
piston 264 is positioned within master cylinder assembly 258 for sinusoidal
reciprocation. Such reciprocation produces two separate flows of working fluid
which are communicated to the uppcr and lower working chambers of wellhead
cylinder assembly 230, respectively. Each of the working fluid flows is
isolated
from the other. Each working fluid flow has a bi-directional and sinusoidal
flow
rate resulting from the reciprocal and sinusoidal displacement of master
piston 264 within master cylinder assembly 258. However, the working fluid
flow
rates are generally opposite to each other at any moment.
Master cylinder assembly 258 includes a master hydraulic cylinder 266.
Cylinder 266 is pivotally mounted at one of its ends by an anchor bearing
assembly 268. A center seal or center seal assembly 270 is positioned at an
2s approximate axial midpoint of master cylinder 266. Master piston 264 is
positioned within master cylinder 266, being slidably received through center
seal
assembly 270 for axial displacement or reciprocation between the two axial
ends
of master hydraulic cylinder 266. Center seal 270 seals against master
piston 264. defining with the master piston the first and second working
chambers 260 and 262 in the two ends of master cylinder 266. Hydraulic oil
or fluid is contained within the two working chambers.
Master piston 264 has a piston drive rod 272 which extends through a
sealed aperture and bearing surface in the end of imaster cylinder 266
opposite
its pivotal mounting connection. An eccentric crank arm 274 is pivotally
3-5 connected to piston drive rod 272 at its crankpin 275. Crank arm 274 is
driven
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at a constant speed to reciprocate master piston 264 within master cylinder
266.
This mechanism for driving master piston 264 results in a sinusoidal rate of
master piston axial displacement. Such displacement causes a corresponding
displacement of hydraulic fluid alternately into and out from working
5 chambers 260 and 262, resulting in bi-directional and sinusoidal working
fluid
flows from master cylinder assembly 258.
First master working chamber 260 communicates with upper slave working
chamber 242 through a fluid supply line 276. Second master working
chamber 262 communicates with lower slave working charnber 248 through a
10 similar fluid supply line 278. A cross-over relief valve 279 is connected
between
supply lines 276 and 278 to relieve excessive levels of hydraulic pressure.
Cooling chambers 282 and 284 are also connected in series with supply lines
276
and 278 to cool hydraulic oil passing therethrough.
The two working chambers of master cylinder assembly 258 are thus
15 coupled directly to the two working chambers of welihead hydraulic assembly
222.
Slave piston 234 is directly responsive, through communication of the working
fluids through supply lines 276 and 278, to the reciprocal and sinusoidal
motion
of master piston 264 within master cylinder asse:mbly 258. Drive system 220
therefore produces sinusoidal reciprocation of the oil well sucker rod in
emulation of a mechanical pump drive system. The wellhead hydraulic assembly
and master hydraulic cylinder working chambers form closed hydraulic system~
which preferably do not include accumulators, in order to avoid adding
elasticiR
to the drive system. Because of the direct and closed communication betweer,
working chambers 260 and 262 and slave cylinder working chambers 242 and 248,
the vertical displacement or position of slave piston 234 relates directly to
the
axial displacement or position of master piston 264 within master cylinder
266.
Gas accumulator 226 is connected for fluid communication with slave
pressure chamber 244, forming an accumulator meaiis for applying upward
biasing
force to the sucker rod. Downward displacement of slave piston 236 displaces
hydraulic oil from pressure chamber 244 and into gas accumulator 226. A
volume of compressed gas such as nitrogen is contained within gas
accumulator 226 to supply a biasing pressurc on hydraulic oil in pressure
chamber 244 and a corresponding upward biasing force on slave piston 234. The
pressure of the compressed gas within accumulator 226 is adjusted through a
gas
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16
valve 245 to provide appropriate or dcsircd counterbalancins of the oil well
sucker rod.
The gas volume within gas accumulator 226 is ideally large enough
accommodate the changing volume of contained hydraulic fluid without
significant
gas compression or expansion. Any such compression and expansion which does
occur tends to be negated by the mechanical advantage obtained by the crank
drive at the extremes of sucker rod displacement.
Drive system 220 also includes fluid injection means, comprising a hydraulic
fluid reservoir 286, a fixed displacement hydraulic pump 288. and a relief
lo valve 290 for regulating the minimum pressure of hydraulic fluid supplied
by
hydraulic pump 288. Hydrautic pump 288 supplies pressurized hydraulic fluid to
supply lines 276 and 278 through one-way check valves 292 and 294,
respectively.
Hydraulic pump 288 and relief valve 290 define and maintain a minimum
pressure in each of working chambers 260 and 262. An effect of this pressure
maintenance is to replenish oil which leaks from the various working chambers
and fluid conduits.
Drive system 220 provides a simple hydraulic oil well drive which emulates
the sinusoidal motion of a conventional mechanical pump jack. It also provides
a counterpressure system which is the functional equivalent of conventional
pump
jack counterweights. Because of the closed working fluid communication system,
there are no valves or variable restrictions required to modulate the
hydraulic
fluid flow. Accordingly, the system is much simpler and reliable than prior
art
hydraulic drives.
Fig. 7 illustrates a third embodiment of an oil well pump drive system in
accordance with the invention, generally designated by the reference numeral
300.
Drive system 300 is located over a conventional oil welihead 302. Wellhead 302
has a stuffing box 304 which slidably receives a polished rod 306. Polished
rod 306 oseillates or reciprocates in a vertical direction, extending downward
through a well casing and production tubing to a sucker rod. The sucker rod
extends through the well casing and production tubing to a plunger at the
bottom of the oil well. The plunger is driven by the sucker rod to lift oil to
the surface and to pump said oil through a production line.
A wellhead hydraulic assembly 311 is mounted directly over wellhead 302
to drive the oil well sucker rod. A fixed vertical wellhead frame 312 connects
wellhead hydraulic assembly 311 to wellhead 302. Wellhead hydraulic
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assembly 311 includes a wellhead cylinder assembly 318 having a wellhcad slave
cylinder 320 and a reciprocating wellhcad slave piston 322. It receives a
working
fluid flow through a hydraulic supply line 330. The working fluid flow is
bi-directional, alternating in direction between positive, inward flow to
cylinder
s assembly 320 and negative, outward flow from cylinder 320. The bi-
directional
working fluid flow produces relative reciprocal motion between the wellhead
piston and cylinder. Positive flow of hydraulic fluid to wellhead cylinder
assembly 318 through supply line 330 raises polished rod 306 at a rate which
is directly proportional to the rate of positive fluid flow. Negative flow of
io hydraulic fluid from wellhead cylinder assembly 318 through supply line 330
lowers polished rod 306 at a rate proportional to the rate of negative fluid
flow.
Further details regarding preferred designs of wellhead cylinder assemblies
will be
described in more detail below.
Drive system 300 includes a master hydraulic source or supply
1s assembly 340 for driving wellhead hydraulic assembly 311. Supply assembly
340
displaces a working fluid such as hydraulic oil or fluid to produce a working
fluid flow at a bi-directional and sinusoidal flow rate. Supply assembly 340
is
in fluid communication with wellhead hydraulic assembly 311, supplying a
working
fluid flow through supply line 330. Wellhead hydraulic assembly 311 is
directly
20 responsive to the working fluid flow to reciprocate the sucker rod at the
same
sinusoidal rate as the working fluid flow.
Supply assembly 340 has a master drive assembly frame 346. A master
cylinder assembly 342 has one end which is pivotally mounted to frame 346.
It has another end which is driven by an eccentric crank or crank arm 344.
2s Crank arm 344 is rotatably connected to frame 346 by a crank drive
mechanism
which rotates crank arm 344 at a constant rotational speed. More specifically,
crank arm 344 is driven by a crank shaft 349 of a gear box or reducer 350.
A motor 352 is connected to drive gear box 350 by a belt 354 or other suitable
means. Crank arm 344, gear box 350 and motor 352 are conventional devices
30 such as are available for use in existing mechanical pump jack drives.
Motor 352 can be an electrical motor, a gasoline or diesel internal combustion
engine, or a hydraulically-powered motor.
Mastcr cylinder assembly 342 includes a master hydraulic cylinder 356 and
a mastcr piston 358. Mastcr cylinder 356 has first and second axial end
35 sections 360 and 362, corresponding to a first, pressure end of cylinder
356 and
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a second, working end of cviinder 356. respectiveh=. Each of sccond scctions
360
and 362 comprises a tubular sleeve which is closed on onc end and open on
the other. The two sections are connected together with their open ends
towards each other to form a cylindrical compartment within which master
piston 358 reciprocates.
Fig. 8 shows a center seal or a center seal assembly 368 which is
positioned at the abutment of the two end sections at an approximatc axial
midpoint of master cylinder 356. Center seal assembly 368. divides master
cylinder 356 into a working chamber 370 and a pressure chamber 372. Center
seal assembly 368 comprises a pressure end hydraulic seal 402 and a working
end
hydraulic seal 404. Pressure end and working end hydraulic seals 402 and 404
are conventional "U-Cup" or "poly-pack" seals which surround and seal against
master piston 358. Parker Seal Group, of Salt Lake City, Utah, manufactures
a'U-Cup' hydrauiic seal which is particularly adapted for long life in a
is reciprocating environment such as encountered within cylinder assembiv 342.
The
seal is sold under the part designation "SLC 4300 BS U-CL'P."
Hydraulic seals 402 and 404 are axially spaced from each other. with
working end hydraulic seal 404 being spaced toward the master cylinder working
end from pressure end hydraulic seal 402. Pressure end hydraulic seai 402
restricts hydraulic tluid passage from pressure chamber 372 of the master
cviinder.
Working end hydraulic seal 404 restricts hydraulic nuid passage from the
working
chamber 370 of master cviinder 356.
A dividing seal surrounds master piston 358 between pressure end
hydraulic seal 402 and working end hydraulic seal 404. The dividing
seal comprises an inner ring 406 of Teflorf surrounded by a Nroprene loader
407, The inner Teflon ring surrounds and receives inaster piston 35$, being
urged into sliding engagement with master piston 358 by !oader 407= The
dividing seal defines a pressurc end seal gap between the dividing seal and
pressure end hydraulic seal 402. It also defines a working end seal r,ap
between
the dividing seal and working end hydraulic seal 404.
More specifically, center seal assembly 368 includcs a steel seal retaining
ring 408 with inner periphery approximately complemcntary in diameter to the
ostcr periphery of master piston 358. Center seal asscmbly 368 slidably
receives
the mastcr piston while providing a hydraulic seal separating wcrkins chamber
170 anci pressure chamber 372. Seal retaining rinc has an annuiar groovc 410
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which extends completely about its inner pcriphery. The dividing seal is
received
within annular groove 410 to surround master piston 358. Pressure end
hydraulic
seal 402 and working end hydraulic seal 404 are spaced axially from opposite
sides of the dividing seal adjacent opposite sides of seal retaining ring 408.
s Pressure end section 360 of cylinder 356 includes a radially-extending
pressure end flange 412 about its open end. Working end section 362 includes
a radially-extending working end flange 414 about its open end. Pressure end
flange 412 has an inner surface with an annular groove extending thereabout
for
receiving pressure end hydraulic seal 402. Seal retaining ring 408 abuts
flange 412, retaining pressure end hydraulic seal 402 within the annular
groove.
Apertures are positioncd to allow fluid communication between pressure chamber
372 and the cup of pressure end hydraulic seal 402. Working end hydraulic
seal 404 is received within an annular slot 416 formed about seal retaining
ring 408. Working end flange 414 abuts seal retaining ring 408 to retain seal
retaining ring 408 between flanges 412 and 414. Bolts 418 extend through
flanges 412 and 414 about the periphery of master cylinder assembly 342 to
secure the two end sections 360 and 362 to each other. An 0-ring 419 is
received between flanges 412 and 414. An 0-ring 421 is received between
flange 414 and retaining ring 408.
An annular bronze bearing 423 surrounds master piston 358, providing a
bearing surface against master piston 358. Bronze bearing 423 is received
within
a relief in the inner wall of cylinder end section 362 at an axial position
against
seal retaining ring 408. The bearing also abuts working end hydraulic seal 404
to retain it within its annular slot. Apertures are provided in the bronze
bearing to communicate pressurized hydraulic oil from working chamber 370 to
the cup of working end hydraulic seal 404.
Seal retaining ring 408 has a pair of fluid passages extending outward
from its inner periphery to communicate with corresponding passages in
cylinder
flanges 412 and 414. More specifically, a pressure end fluid passage 420
extends
from the pressure end seai gap between the dividing seal and the pressure end
hydraulic seal 402. A working end fluid passage 422 extends from the working
end seal gap betwecn the dividing seal and working end hydraulic seal 404.
Corresponding pressure cnd and working end flange passages 424 and 426 are
formed in flanges 412 and 414 between fluid passages 420 and 422 and the
3S outer periphery of flanges 412 and 414. The fluid passages describcd above
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allow hydraulic fluid which escapes or leaks past hydraulic seals 402 and 404
to
be collected in respective reservoirs through fluid passages 420 and 422, and
through flange passages 424 and 426. In addition, a fluid injection port 403
allows hydraulic fluid to be injected into pressu:re chamber 372 during device
5 operation.
Referring again to Fig. 7, master piston 358 is positioned within master
cylinder 356, and is slidably received through center seal assembly 368 for
axial
displacement or reciprocation between the closed ends of first and second end
sections 360 and 362. Seal 368 and master piston 358 define working
lo chamber 370 in the working end of master cylinder 356 and pressure
chamber 372 in the pressure end of master cylinder 356. Hydraulic oil or fluid
is contained within working chamber 370 and pressure chamber 372.
Master piston 358 has a piston drive rod 364 which extends through the
closed end of working end section 362. A crankpin 347 at the outer end of
1s crank arm 344 is pivotally connected to piston drive rod 364 to reciprocate
master piston 358 within master cylinder 356. Each of the working and pressure
chambers 370 and 372 defines a fluid volume whicti varies with the
reciprocation
of master piston 358 within master cylinder 356. Working chamber 370 is in
fluid communication with wellhead hydraulic assembly 311 through supply line
330.
20 An air bleed valve 451 is optionally positioned at an intermediate position
along
supply line 330.
Crank shaft 349 and crank arm 344 are driven by motor 352 and
gearbox 350 at a constant rotational speed which is translated into axial and
reciprocal motion of master piston 358. This results in a sinusoidal rate of
master piston displacement, causing a corresponding displacement of hydraulic
fluid
into or out from working chamber 370, which in turn results in a working flow
of hydraulic fluid through supply line 330. The rate of the working fluid flow
is directly related to the rate of master piston displacement. Accordingly,
the
working fluid flow is bi-directional, alternating between positive and
negative fluid
flow in relation to working chamber 370. The ratc of fluid flow is sinusoidal,
in direct response to the sinusoidal displacement of the master piston.
Wellhead
hydraulic assembly 311 is directly responsive to the working fluid flow caused
by
the master piston reciprocation to alternately displacc the sucker rod in
opposite
directions at a sinusoidal rate. Because of the closed communication between
working chambcr 370 and slave cylindcr 320. the vcrtical position of slavc
piston
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322 rclates directly to the axial position of master piston 358 within master
cylinder 356.
A gas accumulator 376 is connected directly to and above the working
end of hydraulic cylinder 356. Accumulator 376 is in fluid communication with
s pressure chamber 372 of master cylinder 356 through a connecting passage
378.
Pressure chamber 372 contains a volume of hydraulic oil which varies with the
axial position of mastcr piston 358. As master piston 358 moves toward the
pressure end of master cylinder 356, it displaces oil from pressure chamber
372
and into gas accumulator 376. Hydraulic oil is drawn back into pressure
chamber 372 as master piston 358 moves toward the working end of master
cylinder 356. Accumulator 376 contains an excess of hydraulic oil so that a
minimum level of oil is always present in accumulator 376. A volume of
compressed gas such as nitrogen is also contained within gas accumulator 376,
over the hydrauiic oil. The pressure of the gas is adjusted through an air
valve 383 on top of accumulator 376. The compressed gas maintains an
equivalent pressure in the hydraulic oil within pressure chamber 372, and a
corresponding biasing force on master piston 358 toward the working end of
hydraulic cylinder 356. The biasing force assists in displacement of the
master
piston toward the master cylinder working end, effectively biasing the sucker
rod
upward.
The gas volume within gas accumulator 376 is ideally large enough to
accommodate the changing volume of contained hydraulic fluid without resulting
in sic -ificant gas compression or expansion. Any such compression and
expansion
which does occur tends to be negated by the mechanical advantage obtained by
the crank drive at the extremes of sucker rod displacement.
To monitor and maintain proper fluid levels within pressure chamber 372,
gas accumulator 376, and working chamber 370, fluid recovery means are
provided for receiving hydraulic fluid which leaks past pressure end hydraulic
seal 402 and working end hydraulic seal 404. Specifically, a working end fluid
reservoir 440 is in fluid communication with the working end seal gap through
flange passage 426 and fluid passage 422 to receive fluid which leaks past
working end hydraulic seal 404 from working chamber 370. A pressure end fluid
reservoir 442 is likewise in fluid communication with the pressure end seal
gap
through flange passage 424 and fluid passage 420 to rcceive hydraulic fluid
which
leaks past pressure end hydraulic seal 402 from pressure chamber 372. Working
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end fluid reservoir 440 and pressure end fluid reservoir 442 each have a fluid
level indicator, such as a sight window 444, to indicate the leaked fluid
volume
received from working chamber 370 and pressure chamber 372, respectively. In
addition. manually operated working and pressure end fluid injectors 446 and
448
are connected to receive oil from fluid reservoirs 440 and 442, respectively,
and
to inject hydraulic oil back into the working fluid flow and into pressure
chamber 372. Working end fluid injector 446 is used primarily upon initiating
drive system operations, to fill the various working chambers. It is connected
through an injection line 450 to inject oil into supply line 330. Pressure end
1o fluid injector 448 is used during operation of the system to restore leaked
hydraulic fluid to pressure chamber 372. It is connected through an injection
line 452 to inject oil into pressure chamber 372. The sight window in pressure
end fluid reservoir 442 allows fluid injection into the appropriate fluid
chambers
when the leaked fluid volume exceeds a predetermined limit. Alternatively, a
float actuator (not shown) could be located within pressure end fluid
reservoir
442 to automatically actuate a fluid injector such as an electrically powered
pump
or a solenoid valve connected to a pressurized source of hydraulic fluid.
Manual shut-off valves 454 and 456 are positioned downline of each of
fluid injectors 446 and 448 to isolate them from the pressurized hydraulic
fluid
2o as desired. In addition, manually operated bypass valves 458 and 460,
connected
between injection lines 450 and 452 and the hydraulic reservoirs, allow the
levcl
of oil in the working fluid flow and in the pressure chamber to be decreased
as required. Electrical pressure switches 462 and 464 are located in injection
lines 450 and 452 to shut down the system in the case of a drop in hydraulic
pressure below a'predetermined limit.
In addition to the mechanisms described above, wellhead hydraulic
assembly 311 includes a mechanically driven injector pump 472 forming
overstroke
correction means for preventing excessive downward displacement of the sucker
rod. Injector pump 472 is preferably a piston pump which is actuated by
depressing a vertically-extending plunger. Welihead hydraulic assembly 311
includes a push rod 474 which extends upwardly above slave cylinder 320, being
slidably received at its upper end by a guide arm 475. The lower end of push
rod 474 is aligned with the plunger of injector pump 472. The top of slave
piston 322 includes a laterally extending member 477 which reciprocates with
polished rod 306. The length of the rod is chosen so that extending member
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477 strokes or depresses push rod 474 and injector pump plunger 482 whenever
downward polished rod displacement exceeds a predetcrmined limit. Injector
pump 472 is connected to receive hydraulic oil from working fluid reservoir
440
and to supply or inject hydraulic oil. when driven by push rod 474, into the
s working fluid flow. Excessive downward displacement of polished rod 306 is
therefore corrected by injection of additional hydraulic oil into the working
fluid
flow whenever excessive downward movement of polished rod 306 is encountered.
The mechanism automatically corrects for leakage from the working fluid flow.
Fig. 9 shows an example of a single-action piston injection pump 472.
1o Injection pump 472 includes a base housing 474 with a cylindrical inner
chamber 476 containing hydraulic oil. A sleeve bearing 478 fits within inner
chamber 476 at its upper end. Sleeve bcaring 478 has a central cylindrical
inner
bore 480 which is concentric with inner chamber 476. A piston 482 extends
from inner chamber 476, through sleeve bearing 478, and upward to form a
15 pump plunger.
Inner bore 480 has an inner diameter which is approximately
complementary to the outer diametcr of piston 482. A hydraulic seal 486 is
received about sleeve bearing 478 to surround and seal against piston 482 as
it
exits base housing 474. A cap 488 retains sleeve bearing 478 within base
20 housing 474. A spring 490 extends from the bottom of inner chamber 476 to
urge piston 482 upwardly. Piston 482 is retained within base housing 474 by
a washer assembly 492 at the lower end of piston 482.
Inner chamber 476 communicates with working end fluid reservoir 440
through an intake line 494. Pressurized hydraulic fluid is supplied from inner
25 chamber 476 to slave cylinder 320 through a pressure outlet line 495. Check
valves 496 and 497 are positioned in series with intake line 494 and outlet
line 495. respectively, to ensure that working fluid flow occurs only in the
direction from intake line 494 to outlet line 495. Stroking piston 482
downward
forces oil out through outlet line 495. Check valve 496 prevents hydraulic
fluid
30 from escaping through intake line 494. During the subsequent upstroke of
piston 482, outlet check valve 497 closes while intake check valve 496 opens
to
ailow hydraulic fluid to enter inner chamber 476 from fluid reservoir 440.
The injection pump described above is merely an example of a
mechanically-actuated pump which could be used in combination with a welihead
35 hydraulic cylinder. Othcr tvpcs of pumps are also possiblc and may be
desirable.
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A mechanically-actuated injection pump is in many situations superior to
valve-actuated or electrically-actuated systems described because of its
simplicity.
Fig. 10 shows an alternative embodiment of a master cylinder assembly,
generally designated by the reference numeral 500. Master cylinder assembly
500
is generally similar to master cylinder assembly 342 described above with
reference to Figs. 7 and 8. However, master cylinder assembly 500 includes a
master hydraulic cylinder 502 which is oriented generally vertically, with its
pressure end positioned generally above its working end. Rather than
communicating with a separate gas chamber, a gas chamber or pressure
lo accumulator is formed within the pressure chamber of master hydraulic
cylinder 502. The pressure chamber contains a volume of hydraulic oil, and
also
a volume of gas above the hydraulic oil. The gas is precharged to an
appropriate pressure to bias the master pistoni toward the working end of
cylinder 502.
More specifically, master cylinder assembly 500 is pivotally mounted at its
upper end to a frame member 503. A master piston 506 is positioned within
master cylinder 502 for axial displacement therein. Master piston 506 is
surrounded at a midpoint of master cylinder 502 by a center seal assembly 508
such as already described with reference to Fig. 8. Master piston 506 has a
piston drive rod 512 which extends downward from master piston 506 and
through the lower end of master hydraulic cylinder 502. A sleeve bearing 514
and a hydraulic seal 516 surround piston drive rod 512 at the iower end of
cylinder 502. Piston drive rod 512 is connected to an eccentric crank arm 518
which rotates at a constant speed to reciprocate master piston 506 within
master
hydraulic cylinder 502. The connection of piston rod 512 to eccentric crank
arm 518 results in reciprocation of master piston 506 within master cylinder
502
at a sinusoidal rate.
Master piston 506 is positioned within master cylinder 502, being slidably
received through center seat assembly 508 for axial displacement or
reciprocation
within master cylinder 502. Center seal assembly 508 seals against master
piston 506, defining with master piston 506 a working chamber 522 in the lower
end of master cylinder 502 and a pressure chamber 524 in the upper end of
master cylinder 502. Working chamber 522 is Cilled with hydraulic fluid which
is communicated to and from a welihead cylinder assembly through a hydraulic
fluid supply line 520. Reciprocal displacement of master piston 506 causes a
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corresponding displacement of hydraulic fluid through fluid supply line 520.
The
wellhead cylinder assembly responds as already described to reciprocate an oil
well sucker rod at a sinusoidal rate.
Pressure. chamber 524 contains a small volume of hydraulic oil. The
5 purpose of such hydraulic oil within pressure chamber 524 is to lubricate
and
insure proper sealing between master piston 506 and the hydraulic seals in the
center seal assembly 508. Pressure chamber 524 also contains a pressure-
charged
gas such as nitrogen. Such gas maintains a downward biasing force against
master piston 506, acting as a counterbalance similar to the counterbalance
1o weight of a mechanical pump jack. Pressure chamber 524 is preferably
charged
through a gas charge valve 526 atop master cylinder 502. The gas pressure
within pressure chamber 524 is adjusted irnpose an approximately equal load on
a driving power source during both upstroke and clownstroke of a driven oil
well
sucker rod. Cylinder 502 also has an oil level check plug 527 for checking and
15 adding to the level of hydraulic oil within pressure chamber 524.
The gas volume within pressure chamber 524 is ideally large enough
accommodate the changing volume within pressure chamber 524 without resulting
in significant gas compression or expansion. However, any such compression and
expansion which does occur tends to be negated by the mechanical advantage
20 obtained by the crank drive at the extremes of stieker rod displacement.
Master cylinder assembly 500 includes fluid communication ports for
cooperation with a leaked fluid recovery system such as dcscribed above. For
instance, fluid recover lines 509 and 510 communicate from the center seal
assembly seal gaps to appropriate hydraulic fluid reservoirs to recovery any
25 hydraulic fluid which leaks past center seal assembly 508. Fluid injection
port
528 communicates with pressure chamber 524 to allow the fluid level within
pressure chamber 524 to be adjusted during operation.
Master cylinder assembly 500 has the advantage of being simpler than
other embodiments described herein. having an integral compression chamber
which does not required an external housing. Moreover, the vertical profile of
the resulting hydraulic source may be desirable in some situations. lt is also
possible to incline cylinder assembly 500 to some degree, as long as
sufficient
hydraulic oil is prescnt within pressure chamber 524 to surround centcr seal
assembly 508.
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Fig. l1 shows a preferred embodiment wellhead slave cylinder assembly,
generally designated by the reference numeral 600. Wellhead hydraulic
assembly 600 is located at conventional oil wellhead 602. Wellhead 602 has a
stuffing box assembly 604 which receives a polished rod 606 therethrough.
S Polished rod 606 oscillates or reciprocates in a vertical direction,
extending
downward through a well casing 608 to a sucker rod. The sucker rod extends
downward through well casing 608 to a plunger at the bottom of the oil well.
The plunger is driven by the sucker rod to lift oil to the surface and to pump
said oil through a production line 610.
Wellhead hydraulic assembly 600 is mounted to a wellhead flange around
the top of well casing 608 to alternately displace the sucker rod in opposite
vertical directions. It includes a pair of identical wellhead cylinder
assemblies 618 which are laterally spaced from each other about polished rod
606. Each wellhead cylinder assembly 618 has a reciprocating outer cylinder
620
and a stationary slave piston or inner rod 622. In contrast to conventional
wellhead hydraulic cylinders, however, cylinder assemblies 618 are inverted.
More
specifically, wellhead slave rods or pistons 622 are mounted by a base plate
624
directly to wellhead 602. Outer cylinder 620 has an inner diameter which is
slightly larger than the outer diameter of stationary inner rod 622, and is
slidably
received over stationary inner rod 622 to reciprocate vertically in response
to a
working fluid flow.
A lower sleeve bearing 623 is affixed to the lower end of outer cylinder
620 to provide a sliding inner bearing surface against stationary inner rod
622.
An upper split sleeve 626 bearing is also retained by inner rod 622 between
its
outer surface and the inner surface of outer cylinder 620, as shown in Fig.
12.
Split sleeve bearing 626 comprises two semicircular halves 628 which are
received
about a corresponding relief 630 formed near the upper end of stationary inner
rod 622. This construction allows sleeve bearing 626 to be assembled around
relief 630 before outer cylinder 620 is slid over stationary inner rod 622.
Once
assembled, split sleeve bearing 626 is vertically retained by rclicf 630 and
held
around inner rod 622 by the inner walls of outer cylinder 620.
Stationary inner rod 622 has a hollow interior which is connected at its
lower end to fluid supply line 624. The uppcr end of inner rod 622 is open
for fluid communication with the interior of outer cylinder 620. The combined
interiors of inner rod 622 and outer cylinder 620 form a slave cylinder
working
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27
chamber having a volume which varies with the vertical displacement of outer
cylinder 620 in relation to stationary inner rod 622. A hydraulic seal 632 at
the
lower end of outer cylinder 620 surrounds and scals against stationary inner
rod
622 to prevent escape of hydraulic oil from the slave cylinder working
chamber.
Because of the inverted construction of the cylinder assembly, only one seal
is
required for each cylinder. Air bleed valves 631 are connected for fluid
communication with the slave cylinder working chamber to allow accumulated gas
to be discharged from the working chamber.
Outer cylinders 620 are connected together to reciprocate in unison. A
yoke plate 634 extends laterally between cylinders 620 to connect the
cylinders
together at their lower ends. A tie plate 636 extends similarly between the
top
ends of cylinder's 620. Polished rod 606 is connected to yoke ptate 634 midway
between the two wellhead cylinder assemblies by a rod clamp 630. The
connection of polishcd rod 606 is at an elevation at or near the lower end of
outer cylinders 620. This prevents torsion which might otherwise bind the
cylinder assemblies.
The specific construction of the wellhead hydraulic assembly described
above provides at least two significant advantages. First, the oil well
polished
rod is connected between individual hydraulic cylinder assemblies rather than
directly in line with a reciprocating member. This reduces or -eliminates the
need for precise lateral alignment of the hydraulic assembly with the
wellhead.
Second, the polished rod is connected at or near the lower end of the cylinder
assembly reciprocating member, rather than at its upper end as has been the
case with prior art devices. This prevents binding of the side-by-side
hydraulic
cylinders.
Fig. 13 shows another preferred embodiment wellhead hydraulic assembly,
generally designated by the reference numeral 700. Wellhead hydraulic assembly
700 includes both a wellhead slave cylindcr assembly 702 and a wellhead
transfer
pump 704 which operates synchronously with cylinder assembly 702.
~o Wellhead slave cylinder assembly 702 includes a stationarily-mounted slave
cylinder 706. A slave piston 708 is positioned there:in for vertical
reciprocation
in response to a bi-directional fluid flow supplied through a fluid supply
line
710. Cylinder 706 has a cylindrical interior which forms a slave cylinder
working
chamber 712. Slave piston 708 extends upward Grom working chamber 712,
through a sleeve bearing 714, a hydraulic seal 716. and a wiper seal 718. A
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split sleeve bearing 720, such as that described above with reference to Fig.
12,
surrounds slave piston 708 within the interior of working chamber 712. Slave
piston 708 has a reduced diameter lower portiori 721 which extends downward,
through a sleeve bearing 730, a hydraulic seal 732, and a wiper seal 734 in
the
lower end of slave evlinder 706.
A rod arm 738 extends laterally from slave piston 708 above slave cylinder
706. A guide rod or pump actuator rod 740 is adjustably mounted by arm 738
to extend downward alongside the exterior of cylinder 706. A guide arm 742
extends laterally from the upper end of slave cylinder 706, having a guide
aperture 744 through which the actuator rod is rer,eived. Pump actuator rod
740
is adjusted vertically to depress or otherwise drive an injector pump operator
or
plunger (not shown) to inject additional hydraulic oil into working chamber
712
upon excessive downward displacement of slave piston 708. Guide arm 742
maintains the desired rotational alignment of slave piston 708, cnsuring that
actuator rod 740 is aligned over the injector pump plunger.
A polished rod 736 is received through an axial aperture formed in the
center of slave piston 708, being connccted at the top of slave piston 708 by
a polished rod clamp 737. A hydraulic seal 722 seals between the axial
aperture
in slavc piston 708 and the received polished rod 736. Polished rod 736
connects at its lower end to an oil well sucker rod to drive an oil well pump
plunger.
Wellhead transfer pump 704 is aligned below wellhead slavc cylinder
assembly 702, concentric with slave piston 708 and polished rod 736. It has a
cylindrical pump chamber 750 which communicates through an inlet check valve
751 and a transfer line 752 with an oil well casing and production tube 754.
Pump chamber 750 also communicates with an oil production output line 756
through an outfet check valve 757. Inlet check valve 751 allows production oil
into pump chamber 750 from the oil well while preventing passage of production
oil from pump chamber 750 back into the oil well. Outiet check valve 757
allows production oil to be pumped out of pump chamber 750 and into
production line 756. while preventing flow of production oil in the reverse
direction, or back into pump chamber 750 from production line 756. The
reduced diametcr lower section of slave piston 708 forms a pump piston within
pump chamber 750 which varics the internal lluid volume of pump chamber 750.
Diameter of the slave cylinder reduced section is of necessity slightly larger
than
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the diameter of the oil well plunger so that a slight vacuum is created during
the upward stroke of the oil wcll plunger. Polished rod 736 passes through a
sleeve bearing 760. a hydraulic seal 762. and a wuper seal 764 at the bottom
of
pump chamber 750. Hydraulic seal 732 surrounds slave piston 708 to seal
working chamber 712 from pump chamber 750.
During the upstroke of slave piston 708 and the connected polished and
sucker rods, the fluid volume within pump chamber 750 is increased, drawing
oil
from casing or tubing 754, through transfer line 752 and inlet check valve
751,
and into pump chamber 750. During the downstroke of slave piston 708, the
1o fluid volume within pump chamber 750 is decreased, forcing oil out through
outlet check valve 757 and production line 756. The pumping motion of
wellhead transfer pump 704 is synchronized with the reciprocal motion of the
oil
well pump so that oil is drawn into pump chamber 750 during the upward,
pumping stroke of the oil well plunger, and is pumped out of pump chamber
750, against inlet check valve 751, during the non-pumping downward stroke of
the oil well plunger.
The wellhead slave cylinder assembly 702 can be uscd over a welihead
with or without wellhead transfer pump 704. In either case, it can replace the
traditional stuffing box usually required at the top of a well casing.
2o Furthermore, wellhead transfer pump 704 can be used independently of slave
cylinder assembly 702. Specifically, transfcr pump 704 can be used in
conjunction with any mechanism which reciprocally drives an oil well polished
rod. The transfer pump need only bc located as shown over a welihead, or
with its internal piston operably connected for synchronization with a
reciprocating polished rod.
The embodiments described above provide a number of readily apparent
advantages over prior art attempts to hydraulically drive an oil well sucker
rod.
One important characteristic of the drive system is that it produces a
sinusoidal
sucker rod motion. This type of motion has proven to be much gentler on
sucke- rods, prolonging their life greatly over drive systems which rapidly
reverse
a driving force. Another important characteristic of the drive systems
described
abovc is their extreme simplicity. They can be implemented without any
electronic control and without complex valving mechanisms. The direct coupling
between a master cylindcr working chamber and a slave cylindcr results in a
simplicity which has not previously been suggested. In contrast, prior art
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attempts havc concentrated on more and more complicated hydraulic control
schemes which increase cost while decreasing rcliability.
Furthermore, the preferred embodiments described above effectively recover
leaked hydraulic fluid and function to maintain proper fluid levels within the
5 working and pressure systems, largely without operator monitoring or
intervention.
The mechanisms described for maintaining the oil levels are simple and
reliable.
The devices described above are uniquely suited for long periods of unattended
operation, such as often encountered in oil well pumping applications.
Many variations of the above devices are of course possible, and are
1o intended to fall within the scope of this disr,losure. For instance, it is
contemplated that a two master hydraulic assemblies might connected to drive a
single wellhead hydraulic assembly. Furthermore, more than one master cylinder
assembly might be driven by a single gearbox. with each master cylinder
assembly
being coupled to a different wellhead hydraulic assembly. Coupling between the
15 master assembly and the wellhead assembly can bc provided by pipes, tubing,
or
flexible hose, allowing remote location of the master assembly relative to the
wellhead assembly. Such remote coupling is particularly attractive in the
context
of off-shore oil pumping, in which the master hydraulic assembly can be placed
on-shore or on a drilling platform, to communicate through flexible hosing
with
20 an underwater, slave cylinder.
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