Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02763162 2013-04-26
HYDRAULIC OILFIELD LIFT PUMP
BACKGROUND OF THE INVENTION
1. Field of the Invention.
This invention relates to lift pumps suitable for producing hydrocarbons and
other liquids
from the earth. In particular, this invention is drawn to a very long stroke
hydraulically
operated lift pump using a combination of single and double acting hydraulic
cylinders that
are actuated with an energy efficient hydraulic fluid accumulator system.
2. Description of the Related Art.
Hydraulically operated Oilfield lift pumping units are well known and have
been used in
the industry for many years. Typically, these units mimic the non-hydraulic
mechanical
'walking beam' design similar to the pumping unit 10 illustrated in Figure 1.
These
conventional units are more commonly driven directly by electric motors
through gear
boxes, but many are also hydraulically driven as shown for example in U.S.
Patents
4,201,115; 4,198,820; 3,405,605. These hydraulic units are typically
configured similarly to
the motor driven units, but have hydraulic cylinders fitted in place of the
linkages and
gearing of the mechanical system.
Limitations of the 'walking beam' design include limited stroke variability,
the need for a
rotating counter weight of roughly the same weight as the sucker rods and
other devices
suspended in the borehole, and a stroke limited by the length of the 'walking
beam' and/or
the height above the ground of the mast upon which the beam is mounted.
Nonetheless, these 'walking beam' pumps are ubiquitous in the industry and are
readily
visible from many major thoroughfares, particularly in Texas and western
Canada.
- 1 -
CA 02763162 2011-11-22
WO 2010/141405
PCT/US2010/036807
Vertical pumping units, as shown for example in U.S. Patents 4,761,120;
4,512,149;
4,698,968; 4,762,473, typically use hydraulic cylinders aligned vertically
which typically
connect directly to the sucker rod string and are controlled by complex
hydraulic systems.
These vertical systems have attempted, and often times succeeded in overcoming
many of
the above mentioned limitations. However, these units tend to have very
complex
hydraulic systems because the mechanical design requires the hydraulic system
to perform
many different functions. Furthermore, these units also tend to have very
uneven power
cycles, causing a very cyclic loading on the prime mover, and requiring it to
have a much
higher horsepower rating to power through the peak loads, than the overall
average
horsepower consumed.
BRIEF SUMMARY OF THE INVENTION
Described herein is a new type of hydraulically driven oilfield lift pump unit
which features
a simplified hydraulic system. This system comprises a hydraulic power section
and a
wellhead mounted lifting section. The hydraulic power section is typically,
but not limited
to, a flow controlled, pressure compensated hydraulic pump system that can
readily be
optimally sized to meet the desired lifting speed and weight parameters of the
lift section
and an electrically controlled 4-way type variable displacement control valve,
or other
types of suitable control valves. These types of 'power units' are well known
and are in
common use in the industry.
The lifting section comprises a plurality (typically a pair) of single acting
hydraulic
cylinders, and a plurality (typically a pair) of double acting hydraulic
cylinders. The
cylinders are mechanically mounted on common end plates such that they all
must extend
and retract simultaneously.
The cap ends of the double acting cylinders are 'closed-coupled' to one or
more hydraulic
accumulators so that in normal operation all the flow into and out of the cap
ends of the
cylinders is into or out of the hydraulic accumulators. The accumulators are
pre-charged
with gas at a pressure such that when the cylinders are fully extended and the
accumulators
nearly depleted of hydraulic fluid, the double acting cylinders are
nonetheless able to
statically hold a high percentage, of the lifting load.
- 2 -
CA 02763162 2011-11-22
WO 2010/141405
PCT/US2010/036807
This may also effectively balance the pump motor's load between the upstroke
and the
downstroke cycles, and may permit use of a prime mover motor or engine to run
under a
relatively constant load (compared to conventional pumps) regardless of
whether in both
the upstroke mode and the downstroke mode. Typical load variations may be in
the range
of 70% to 130% of the average load, but as will be described, these variations
may be
reduced even more than this, if desired. When compared to other pump units of
this type,
the load balancing effect of the accumulators allows the use of a motor with a
significantly
less horsepower rating (or capacity) than the prior art units. Furthermore,
the lower peak
power consumption may allow for a lower effective electric rate, as in many
locations the
electric rate is based on, or at least affected, by the peak load instead of
the average load.
During the "up" stroke, pressurized fluid from the hydraulic power unit is
applied only to
the 'cap' ends of the single acting cylinders, causing all of the cylinders to
extend together.
The pair of single acting hydraulic cylinders can only be powered in the
extended direction.
The pressure required to operate the single acting cylinders will steadily
increase as the
cylinders extend because the lifting capability of the double acting cylinders
decreases as
the accumulators gas volume expands. In the preferred embodiment, the load on
the down
stroke may typically be about two-thirds of the load on the up stroke. This
difference is
caused by a transfer of fluid column weight onto the down hole lifting valves.
During the 'down' stroke, the pressurized fluid from the hydraulic power unit
is applied
only to the 'rod' ends of the double acting hydraulic cylinders. The
combination of the
weight of the load and this hydraulic pressure on the top end of the cylinder
causes all the
cylinders to retract. This in turn forces the hydraulic fluid on the cap ends
of the double
acting cylinders back into the accumulators ¨ and therefore filling them. The
relative
horsepower required for each of the 'extend' and 'retract' cycles may be
changed and/or
more nearly equalized by adjusting the pre-charge pressure of the accumulators
to account
for variations in the average lifting load, permitting usage of the smallest
possible engine or
motor for the hydraulics. Furthermore, this may also help reduce large load
fluctuations,
reducing stress on the system. However, because in some instances the actual
loading may
vary over time, an intermediate pre-charge pressure may be chosen.
- 3 -
CA 02763162 2011-11-22
WO 2010/141405
PCT/US2010/036807
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is side view of a typical 'walking beam' oilfield lift pump of the
prior art.
Figure 2 is an overall perspective view of the oilfield lift pump of the
present invention.
Figure 3 is a perspective view of just the mast section of lifting section of
the oilfield lift
pump of the present invention.
Figure 4 is a hydraulic schematic diagram of the hydraulic power unit of the
present
invention.
Figure 5 is a portion of the hydraulic diagram of the lifting portion of the
lifting portion of
the oilfield pump of the present invention showing the hydraulic flow
arrangement for the
"up stroke" portion of the pumping cycle.
Figure 6 is a portion of the hydraulic diagram of the lifting portion of the
oilfield pump of
the present invention showing the hydraulic flow arrangement for the "down
stroke"
portion of the pumping cycle.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to figures 2 and 3; the hydraulically driven surface oil well
pumping unit 12
of the present invention comprises a base portion 14, including a hydraulic
power unit (not
shown) and a mast portion 16 comprising a pair of single acting hydraulic
cylinders 18 and
a pair of double acting hydraulic cylinders 20. Figure 2 shows the hydraulic
cylinders 18,
20 of the hydraulically driven oil field pumping unit 12 fully extended, and
figure 3 shows
the mast portion 16 separated from the rest of the oilfield pumping unit 12 of
figure 2 with
the hydraulic cylinders 18, 20 fully retracted. The new oil well pumping unit
12 of the
present invention is useful in reciprocating a "load" in the wellbore;
particularly a load
which is substantially higher as the load is being raised, than when the load
is being
lowered ¨ as is typically the case when pumping oil in an oil well.
- 4 -
CA 02763162 2013-04-26
The cap ends 22a and 22b of the hydraulic cylinders 18, 20 are shown mounted
to a common
rigid base plate 24. The rods 26a 26b extending from the hydraulic cylinders
18, 20 are
shown mounted to a common rigid end plate 28. It is anticipated that in
operation the
hydraulic cylinders 18, 20 will typically oriented with their cap ends 22a and
22b secured
to the base plate 24, but it is also possible to mount them such that the cap
ends 22a and 22b are
mounted to the end plate 28, and use tension members to 'suspend' the load.
The lifting
mechanism is then suspended in a framework (not shown) using an intermediate
support
structure. It is preferred, however, to mount the cylinders 18, 20 with the
cap ends 22a and
22b secured to the base plate 24 as shown, as this allows the forces from the
'load'
to pass in compression directly from the cap ends 22a and 22b to the base
plate 24. A frame
with sufficient strength - or a separate base - may be needed to support the
mast.
Referring now to figure 4, the hydraulic schematic 40 of the hydraulic power
unit (shown
generally as 30) of the hydraulically driven oil field pumping unit 12 is
shown in figure 4
and is typical for pressure compensated power units. The hydraulic power unit
30 typically
comprises but is not limited to one or more pumps 42, 44 powered by one or
more engine 46
(or alternatively, motors). Hydraulic oil contained in a reservoir 48 is
pumped into a control
valve 50 which may be a proportional-volume controlled pressure balanced valve
as
illustrated, or may be a solenoid valve that is essentially open or closed.
The valve 50 is configured to reversibly provide pressurized hydraulic fluid
to either of line
52a or 52b with the other line being the return. The operation of this valve
50 controls the
extension and retraction of the hydraulic cylinders 18, 20. This hydraulic
configuration may
be comprised of, but not limited to a closed loop, solenoid operated hydraulic
pumping unit.
The mast 16 lifting section comprises a pair of single acting hydraulic
cylinders 18, and a
pair of double acting hydraulic cylinders 20. As shown in Figs. 5 and 6, the
cylinders 18, 20
are mechanically mounted on common end plates 24, 28 such that they all must
extend
and retract simultaneously.
The cap ends 54, 56 of the double acting cylinders 20 are 'closed-coupled' to
one or more
hydraulic accumulators 58, 60 so that in normal operation all the flow into
and out of the cap
ends 54, 56 of the cylinders flows into or out of the hydraulic accumulators
58, 60.
- 5 -
CA 02763162 2011-11-22
WO 2010/141405
PCT/US2010/036807
The accumulators 58, 60 are pre-charged with an inert gas to a pressure
depending upon the
weight of the load such that when the double acting cylinders 20 are fully
extended they are
able to hold a high percentage of the lifting load.
As illustrated in Figure 5, during the "up" stroke, pressurized fluid from the
hydraulic
power unit 30 is applied only to the cap ends 66, 68 of the single acting
cylinders 18
through line 52a, causing all four of the cylinders 18, 20 to extend. The pair
of single
acting hydraulic cylinders 18 can only be powered in the extended direction.
The pressure required to operate the single acting cylinders 18 will steadily
increase as the
cylinders continue to extend because the load capability of the double acting
cylinders 20
decreases as pressure drops in the accumulators 58, 60 as they empty their
hydraulic fluid.
The hydraulic pressure required to move the load is typically higher than the
pressure
required to 'hold' the load. So, even at the end of their strokes, the reduced
pressure
available from the accumulators 58, 60 maybe adequate to keep the cylinders
firmly and
fully extended as the cycle reverses and the cylinders 18, 20 begin to
retract. This
'overcharging' of the accumulators solves a known problem in the prior art
hydraulic lift
pumps, which have been known to 'drop' occasionally as the cylinders reverse
and begin to
retract. 'Drops' like this, if repeated frequently, may cause fatigue, and
perhaps even
failure within the members comprising the "load".
As illustrated in Figure 6, during the 'down' stroke, the pressurized fluid
from the
hydraulic power unit 30 is applied only to the 'rod' ends 62, 64 of the double
acting
hydraulic cylinders 20 through line 52b. The combination of the weight of the
load and
this hydraulic pressure on the top end of the cylinder causes the cylinders to
retract. This in
turn forces the hydraulic fluid on the cap ends 54, 56 of the double acting
cylinders 20 back
into the accumulators ¨ and therefore recharging them, and completing the
cycle.
In operation, the pre-charge pressures of the accumulators 58, 60 are 'tuned'
so that in
conjunction with the weight of the load, the horsepower to raise and lower the
load is
substantially the same throughout the entire extend/retract cycle. This allows
optimal
sizing of the engine(s) 46 (or motors) driving the hydraulic pumps 42, 44; and
may
improve the overall useful lifetime of the oil well pumping unit 12, by
reducing the
accompanying cyclic fatigue. In some selected embodiments the lowering load
may be set
- 6 -
CA 02763162 2011-11-22
WO 2010/141405
PCT/US2010/036807
within a range of about 60% to 95% of the raising load. In other selected
embodiments it
may be preferable to limit the lowering load to no lower than about 70% of the
raising load.
In still other selected embodiments, it may be desirable to maintain the
lowering load in a
range of 75% to 85% of the raising load.
In some selected embodiments, this ratio of lowering load to raising load may
be further
adjusted by varying the pre-charge pressures among the accumulators 58, 60. In
this
embodiment three or four or more than four accumulations with varying pre-
charge
pressures may be useful.
In other selected embodiments the ratio of lowering load to raising load may
be adjusted
by varying the distribution of swept volumes among the accumulators. In still
other
selected embodiments the ratio of lowering load to raising load may be
adjusted by using
different volume capacities among the accumulators.
For those instances where precise tuning and 'leveling' of the horsepower is
important,
accumulators 58, 60 may be arranged with both different pre-charge pressures,
and with
different volume capacities, which may 'tuning' even more precise.
Although the system described herein is disclosed as having two single acting
hydraulic
cylinders 18 and two double acting hydraulic cylinders 20, it would be
apparent to those
skilled in the art that the system only requires one or more of each of the
single 18 and/or
double 20 acting cylinders to operate in the manner described. The second
cylinders 18, 20
allow for a more compact 'footprint' and provides for greater mechanical
stability to the
structure.
Whereas the present invention has been described in particular relation to the
drawings
attached hereto, it should be understood that other and further modifications
apart from
those shown or suggested herein, may be made within the scope and spirit of
the present
invention.
- 7 -
