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Title:
MULTI-CYLINDER HYDRAULICALLY-DRIVEN PUMP SYSTEM
Document Type and Number:
WIPO Patent Application WO/2015/167615
Kind Code:
A1
Abstract:
A hydraulically-driven pumping unit for pumping a working fluid includes a pair of double-acting hydraulic piston-cylinder (HPC) assemblies, each having a central axis, a hydraulic cylinder, a piston, and a rod coupled to the piston for axial movement relative to the first hydraulic cylinder. The pumping unit also includes a hydraulic fluid source and an exhaust path for hydraulic fluid. The two hydraulic cylinders are coupled to the hydraulic fluid source and the exhaust path such that the movements of the two pistons are synchronized. The two HPC assemblies are configured for phase-shifted operation such that when pumping unit is reciprocating the two pistons and rods, the a first pair of the pistons and rods always has a different combination of axial position and direction of travel than does a other pair of the pistons and rods. During phase-shifted operation, at least one of the two rods is moving to extend further beyond the corresponding hydraulic cylinder at all times.

Inventors:
DONNALLY, Robert Benjamin (10000 Richmond Avenue, Suite 100Houston, Texas, 77042-4209, US)
CHENG, Lei (Floor 10-12, Building # 10 Lvzhou Center,Lane 1628, Jin Shan Jiang Road, Shanghai, CN)
Application Number:
US2014/070982
Publication Date:
November 05, 2015
Filing Date:
December 17, 2014
Export Citation:
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Assignee:
NATIONAL OILWELL VARCO, L.P. (7909 Parkwood Circle Drive, Houston, Texas, 77036, US)
International Classes:
F04B5/02; F04B9/105; F04B47/00; F04B47/02; F04B47/06; F04B49/22
Foreign References:
US6454542B12002-09-24
US20110123363A12011-05-26
EP1706643A12006-10-04
DE102008036528A12010-02-11
Attorney, Agent or Firm:
MOSCICKI, Matthew R. (CONLEY ROSE, P.C.1001 McKinney, Suite 180, Houston Texas, 77002, US)
Download PDF:
Claims:
CLAIMS

What is claimed is:

1. A hydraulically-driven pumping unit for pumping a working fluid, the pumping unit comprising:

a first double-acting hydraulic piston-cylinder (HPC) assembly comprising a first central axis, a first hydraulic cylinder, a first piston, and a first rod coupled to the first piston for axial movement relative to the first hydraulic cylinder; a second double-acting hydraulic piston-cylinder (HPC) assembly comprising a second central axis, a second hydraulic cylinder, a second piston, and a second rod coupled to the second piston for axial movement relative to the second hydraulic cylinder;

a hydraulic fluid source; and

an exhaust path for hydraulic fluid;

wherein the first and second hydraulic cylinders are coupled to the hydraulic fluid source and the exhaust path such that movements of the first and second pistons are synchronized;

wherein the first and second HPC assemblies are configured for phase-shifted operation such that when pumping unit is reciprocating the two pistons and rods, the second piston and rod always have a different combination of axial position and direction of travel than do the first piston and rod, and such that at least one of the two rods is moving to extend further beyond the corresponding hydraulic cylinder at all times.

2 The hydraulically-driven pumping unit of claim 1 :

wherein the first piston defines a first extension-chamber and a first retraction- chamber in the first hydraulic cylinder;

wherein the second piston defines a second extension-chamber and a second retraction-chamber in the second hydraulic cylinder;

wherein the first piston is configured to move axially within the first hydraulic cylinder to a first far-extended position, to a first retracted position, and to a first extended position disposed between the first far-extended position and the first retracted position; wherein the second piston is configured to move axially within the second hydraulic cylinder to a second far-extended position, to a second retracted position, and to a second extended position disposed between the first far-extended position and the first retracted position;

wherein the pumping unit is configured with at least these following operational states for the first HPC assembly:

a first state in which the first extension-chamber is in fluid communication with the hydraulic fluid source; and

a second state in which the first extension-chamber is in fluid communication with the exhaust path;

wherein the pumping unit is configured with at least these following operational states for the second HPC assembly:

a third state in which the second extension-chamber is in fluid communication with the hydraulic fluid source;

a fourth state in which hydraulic fluid is exhausted from the second extension- chamber is in fluid communication with the exhaust path; wherein the pumping unit is configured to perform a first transition, transitioning the second HPC assembly from the fourth state to the third state when the first piston reaches the first extended position while the first rod moves to extend further beyond the first hydraulic cylinder, or when the second piston reaches the second retracted position while the second rod retracts;

wherein the pumping unit is configured to perform a second transition, transitioning the first HPC assembly from first state to the second state when the first piston reaches the first far-extended position while the first rod moves to extend further beyond the first hydraulic cylinder;

wherein the pumping unit is configured to perform a third transition, transitioning the first HPC assembly from the second state to first state when the second piston reaches the second extended position while the second rod moves to extend further beyond the second hydraulic cylinder, or when the first piston reaches the first retracted position while the first rod retracts; and

wherein the pumping unit is configured to perform a fourth transition, transitioning the second HPC assembly from the third state to the fourth state when the second piston reaches the second far-extended position while the second rod moves to extend further beyond the second hydraulic cylinder.

3. The hydraulically-driven pumping unit of claim 2 further comprising a rod-end fluid circuit configured to provide fluid communication between the first retraction-chamber in the first hydraulic cylinder and a second retraction-chamber in the second hydraulic cylinder; wherein the rod-end fluid circuit is fluidically-isolated from the hydraulic fluid source in at least one mode of operation.

4. The hydraulically-driven pumping unit of claim 2 :

wherein the first HPC assembly is in the first state during the first transition;

wherein the second HPC assembly is in the third state during the second transition; wherein the second HPC assembly is in the third state during the third transition; and wherein the first HPC assembly is in the first state during the fourth transition.

5. The hydraulically-driven pumping unit of claim 2 :

wherein the first state and the second state are mutually exclusive; and

wherein the third state and the fourth state are mutually exclusive.

6. The hydraulically-driven pumping unit of claim 5 further comprising:

a first control valve fluidically coupled between the first extension-chamber and the hydraulic fluid source and configured to activate and deactivate the first state; and

a second control valve fluidically coupled between the second extension-chamber and the hydraulic fluid source and configured to activate and deactivate the third state.

7. The hydraulically-driven pumping unit of claim 6 further comprising:

a first exhaust valve fluidically coupled between the first extension-chamber and the exhaust path and configured to activate and deactivate the second state; and a second exhaust valve fluidically coupled between the second extension-chamber and the exhaust path and configured to activate and deactivate the fourth state.

8. The hydraulically-driven pumping unit of claim 6

wherein the first control valve is also fluidically coupled between the first extension- chamber and the exhaust path and is further configured to activate and deactivate the second state; and

wherein the second control valve is also fluidically coupled between the second extension-chamber and the exhaust path and is further configured to activate and deactivate the fourth state.

9. The hydraulically-driven pumping unit of claim 1 further comprising:

a first pump driven by the first rod of the first HPC assembly and having a first discharge port;

a second pump driven by the second rod of the second HPC assembly and having a second discharge port; and

a discharge manifold coupled for fluid communication with the first and second discharge ports.

wherein the first and second pump are configured to receive and to pump a working fluid through the discharge manifold.

10. The hydraulically-driven pumping unit of claim 2 :

wherein the pumping unit is configured with the following operational states:

a fifth state in which the first extension-chamber chamber of the first HPC assembly is not in fluid communication with the hydraulic fluid source and is not in fluid communication with the exhaust path; and a sixth state in which the second extension-chamber of the second HPC assembly is not in fluid communication with the hydraulic fluid source and is not in fluid communication with the exhaust path;

wherein the first HPC assembly is in the fifth state between the first state and the second state during the second transition; and

wherein the second HPC assembly is in the sixth state between the third state and the fourth state during the fourth transition. A method for operating a fluid-driven pumping unit, the method comprising:

supplying a flow rate of a driving fluid during an operation period;

dividing the flow rate of the driving fluid between members of a plurality of double- acting hydraulic piston-cylinder (HPC) assemblies, each assembly having a movable rod;

operating at least two of the HPC assemblies using synchronized, phase-shifted cycles, wherein the corresponding two movable rods extend and retract periodically, such that the two rods always have a different combination of axial position and direction of travel and such that at least one of the two rods is extending at all times;

providing a plurality of pumps configured to deliver a working fluid, each pump driven by a rod of the plurality of rods; and

pumping a working fluid using the plurality of pumps.

The method of claim 1 1 further including:

interconnecting the plurality of HPC assemblies so the extension of a first actuator causes the retraction of a second actuator and the extension of the second actuator causes retraction of the first actuator.

The method of claim 12 further including:

operating the HPC assemblies such that the pistons perform smooth reversals, not reaching the maximum extent of their strokes.

Description:
MULTI-CYLINDER HYDRAULICALLY-DRIVEN PUMP SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. provisional patent application Serial No. 61/984829 filed April 27, 2014, and entitled "Multi-Cylinder Hydraulically-Driven Pump System," which is hereby incorporated herein by reference in its entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND

[0003] This disclosure relates generally to reciprocating pumps. More particularly, it relates to reciprocating drilling fluid pumps used for drilling wells in the oilfield industry.

[0004] When forming an oil or gas well, a bottom hole assembly (BHA), including a drill bit, is coupled to a length of drill pipe to form a drill string. The drill string is then positioned adjacent the earth or inserted downhole, where drilling commences. During drilling, a drilling fluid, or "mud," is circulated down through the drill string to lubricate and cool the drill bit as well as to provide a vehicle for removal of drill cuttings from the borehole. After exiting the bit, the mud returns to the surface through the annulus formed between the drill string and the surrounding borehole wall.

[0005] Instrumentation for taking various downhole measurements and communication devices are commonly mounted within the drill string. Many such instrumentation and communication devices operate by sending and receiving pressure pulses through the annular column of drilling mud maintained in the borehole.

[0006] Reciprocating mud pumps are commonly used to deliver the mud through the drill string during drilling operations. Reciprocating pumps include one or more piston-cylinder assemblies having a piston slidably enclosed in a cylinder. The cylinder is hydraulically coupled to a suction valve and a discharge valve to control the flow of a working fluid (e.g., the mud) into and out from the cylinder. Many conventional mud pumps are single-acting triplex pumps having three piston-cylinder assemblies driven by a "power end" having a large rotating crankshaft and a mechanical gear drive. [0007] While pressurizing and delivering the drilling mud, conventional reciprocating pumps impart unwanted pressure pulsations into the fluid. These pulsations may disturb the downhole instrumentation and communication devices by degrading the accuracy of measurements taken by the instrumentation and hampering communications between downhole devices and control systems at the surface. Over time, the pulsations may also cause fatigue damage to the drill string pipe and other downhole components.

[0008] The conventional triplex mud pumps are heavy and require a change in the cylinder diameter to make a significant change to the discharge pressure. The change may be accomplished by adding or removing an annular liner during maintenance. This type of modification is required to make the best use of the pump power end capacity while obtaining as close as practical to the optimum flow and pressure required for maximum down-hole drilling performance. For example, to achieve a higher pressure, a smaller diameter liner is used; however, this alteration reduces the flow capacity of the pump. When, instead, the situation requires a higher flow rate and a higher pressure capacity, the installation of a larger pump may be necessitated, and the size and weight of the power section increases substantially.

[0009] Accordingly, there is a need for improved designs and improved methods for reciprocating pumps to address the pulsation and size challenges of current designs.

BRIEF SUMMARY OF THE DISCLOSURE

[0010] These and other needs in the art are addressed in one embodiment by a hydraulically- driven pumping unit. In an embodiment, the hydraulically-driven pumping unit for pumping a working fluid includes a first double-acting hydraulic piston-cylinder (HPC) assembly having a first central axis, a first hydraulic cylinder, and a first piston, and a first rod coupled to the piston for axial movement relative to the first hydraulic cylinder. In addition, the pumping unit includes a second double-acting hydraulic piston-cylinder (HPC) assembly having a second central axis, a second hydraulic cylinder, and a second piston, and a second rod coupled to the piston for axial movement relative to the second hydraulic cylinder. Further, the pumping unit includes a hydraulic fluid source. Still further, the pumping unit includes an exhaust path for hydraulic fluid. In this embodiment, the first and second hydraulic cylinders are coupled to the hydraulic fluid source and the exhaust path such that the first and second HPC assemblies are synchronized. Also in this embodiment, the first and second HPC assemblies are configured for phase-shifted operation such that when pumping unit is reciprocating the two pistons and rods, the second first piston and rod always have a different combination of axial position and direction of travel than do the first piston and rod, and such that at least one of the two rods is moving to extend further beyond the corresponding hydraulic cylinder at all times.

[0011] Also disclosed is method for operating a fluid-driven pumping unit. In an embodiment, the method includes supplying a flow rate of a driving fluid during an operation period; dividing the flow rate of the driving fluid between members of a plurality of double- acting hydraulic piston-cylinder (HPC) assemblies, each assembly having a movable rod; and operating at least two of the HPC assemblies using synchronized, phase-shifted cycles, wherein the corresponding two movable rods extend and retract periodically, such that the two rods always have a different combination of axial position and direction of travel and such that at least one of the two rods is extending at all times. In this embodiment, the method also includes providing a plurality of pumps configured to deliver a working fluid, each pump driven by a rod of the plurality of rods; and pumping a working fluid using the plurality of pumps.

[0012] Thus, embodiments described herein include a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The various features and characteristics described above, as well as others, will be readily apparent to those of ordinary skill in the art upon reading the following detailed description, and by referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] For a detailed description of the disclosed embodiments of the disclosure, reference will now be made to the accompanying drawings in which:

[0014] Figure 1 is side view of a hydraulically-driven pumping unit having multiple pumping piston-cylinder assemblies coupled to multiple hydraulic piston-cylinder assemblies in accordance with the principles disclosed herein;

[0015] Figure 2 is an end view of the hydraulically-driven pumping unit of Figure 1 in accordance with the principles disclosed herein; [0016] Figure 3 is a top view in partial cross-section showing the hydraulically-driven pumping unit of Figure 1 along section A-A of Figure 1 in accordance with the principles disclosed herein;

[0017] Figure 4 is a schematic diagram of the hydraulically-driven pumping unit of Figure 1 in accordance with the principles disclosed herein;

[0018] Figure 5 is a tabulated diagram disclosing various transitional events related to the operation of the hydraulic piston-cylinder assemblies of the pumping unit of Figure 1 in accordance with the principles disclosed herein; and

[0019] Figure 6 is a diagram illustrating a method to operate a fluid-driven pumping unit having multiple pumping piston-cylinder assemblies in accordance with the principles disclosed herein.

NOTATION AND NOMENCLATURE

[0020] The following description is exemplary of embodiments of the disclosure. These embodiments are not to be interpreted or otherwise used as limiting the scope of the disclosure, including the claims. One skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and is not intended to suggest in any way that the scope of the disclosure, including the claims, is limited to that embodiment.

[0021] The drawing figures are not necessarily to scale. Certain features and components disclosed herein may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. In some of the figures, in order to improve clarity and conciseness of the figure, one or more components or aspects of a component may be omitted or may not have reference numerals identifying the features or components that are identified elsewhere. In addition, like or identical reference numerals may be used to identify common or similar elements.

[0022] The terms "including" and "comprising" are used herein, including in the claims, in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to... ." Also, the term "couple" or "couples" is intended to mean either an indirect or direct connection. Thus, if a first component couples or is coupled to a second component, the connection between the components may be through a direct engagement of the two components, or through an indirect connection that is accomplished via other intermediate components, devices and/or connections. In addition, if the connection transfers electrical power or signals, whether analog or digital, the coupling may comprise wires or a mode of wireless electromagnetic transmission, for example, radio frequency, microwave, optical, or another mode. So too, the coupling may comprise a magnetic coupling or any other mode of transfer known in the art, or the coupling may comprise a combination of any of these modes. The recitation "based on" means "based at least in part on." Therefore, if X is based on Y, X may be based on Y and any number of other factors. The terms "fluidically coupled" and "coupled for fluid communication" are used interchangeably.

[0023] In addition, as used herein, including in the claims, the terms "axial" and "axially" generally mean along or parallel to a given axis (e.g., central axis of a body or a port), while the terms "radial" and "radially" generally mean perpendicular to the axis. For instance, an axial distance refers to a distance measured along or parallel to the axis, and a radial distance means a distance measured perpendicular to the axis.

[0024] Furthermore, as used herein, including in the claims, the following terms are defined:

[0025] The term "open-state-exclusivity" means that the operations of a supply valve and the operation of an exhaust valve, each fluidically coupled to a same fluid transfer device (e.g., a chamber of a hydraulic piston-cylinder assembly), are coordinated so that the two valves may be simultaneously closed to restrict fluid communication with the transfer device but will not be simultaneously open . That is to say, at most, only one of the supply valve and the exhaust valve will be open at any given time

[0026] The term "open-state-assurance" means that the operations of a plurality of supply valves, each coupled to a same fluid source, are coordinated so that at least one of the supply valves will be open at given time while the system containing the valves is operating. Not all the supply valves will be simultaneously closed, and during operation at least a portion of the flow rate from the fluid source is always supplied to at least one of the fluid recipients.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

[0027] This specification discloses a reciprocating, hydraulically-driven fluid pumping unit having two or more hydraulic piston-cylinder assemblies as the "power end" for two or more pumping piston-cylinder assemblies. The power to weight ratio of a hydraulic piston- cylinder assembly is beneficially higher than the power to weight ratio of a power end for various pumps having a rotating crankshaft driven by a mechanical gear drive. Consequently, the flow and pressure characteristics of the disclosed pump can be altered with engineering or economic advantages. As an example, the stroke length of the disclosed pump can easily be built longer for reduced suction and discharge valve cycling. Building a traditional mud pump with increased stroke length reaches a practical limit much more quickly because the weight and size of the crankshaft drive system grows exponentially with stroke length increase. As another example, larger diameter pistons and liners may be used for the working fluid than used with conventional pumps. This can allow slower piston travel speeds for reduced liner and seal wear.

[0028] In at least one embodiment disclosed herein, two hydraulic piston-cylinder assemblies are configured to drive two pumping piston-cylinder assemblies. In at least one mode of operation, the two hydraulic piston-cylinder assemblies are coordinated, and the total flow of hydraulic fluid driving the two assemblies is constant. Therefore, in such embodiments, the speeds and accelerations of the two pistons within the two assemblies "mirror" each other. When one piston is accelerating the other piston is decelerating. When one piston is traveling forward full speed, the other piston is full speed in the reverse direction. This mode of operation is accomplished by having the total flow of hydraulic fluid directed through one valve, or multiple valves coordinated as one, to feed both hydraulic piston-cylinder assemblies. The valve(s) directs the full flow to either the first assembly or the second assembly or to a combination of the two assemblies. During operation, the flow of hydraulic fluid is always directed onto the at least one of the hydraulic piston-cylinder assemblies.

[0029] Also disclosed herein is a hydraulic fluid control system and a technique for sequencing the operation of the two or more hydraulic piston-cylinder assemblies coupled to the two or more pumping piston-cylinder assemblies. In various embodiments, the result is a smoother discharge flow for the working fluid, e.g., drilling mud as compared to the discharge flow from a conventional pump. A smoother flow of a fluid is generally characterized by a steadier flow rate, by fewer or reduced pressure pulsations, or a combination of these results.

[0030] Figure 1 and Figure 2 show a side view and an end view, respectively, of a hydraulically-driven pumping unit 100 having multiple hydraulic piston-cylinder assemblies configured to drive multiple pumping piston-cylinder assemblies. In particular, for the embodiment shown, pumping unit 100 includes two double-acting hydraulic piston-cylinder assemblies 120, which will be distinguished as assembly 120A and assembly 120B in the figures and in the text when advantageous, and two reciprocating pumping piston-cylinder assemblies 140, which are each driven by an assembly 120.

[0031] Each assembly 120 is aligned with and coupled to an assembly 140 by means of a cylinder union 138. The assemblies 120, 140 are further coupled to a mounting plate 105. Hydraulically-driven pumping unit 100 also includes a fluid accumulator 260, specifically a hydraulic accumulator 260, in fluid communication with the two assemblies 120. Pumping unit 100 further includes a working fluid reservoir 160 to contain water, drilling mud, or another suitable fluid to be pressurized and discharged by pumping piston-cylinder assemblies 140. Pumping unit 100 further includes a variable displacement hydraulic pump 170 mechanically coupled to an electric motor 172 and coupled for fluid communication between a hydraulic reservoir 250 and the two piston-cylinder assemblies 120. Driven by electric motor 172, pump 170 supplies hydraulic fluid from reservoir 250 to piston-cylinder assemblies 120. In this embodiment, pump 170 is an axial piston swash plate-type pump, with variable volume control. In this embodiment, electric motor 172 is a single speed A/C motor. Other embodiments may use other types of hydraulic pumps and other types of motors. For example, other embodiments of pump 170 may be coupled to a variable speed A/C motor coupled to a variable frequency drive (VFD) or may be coupled to a variable speed D/C motor having an appropriate electrical drive to control rotational speed and maintain a selected pumping flow rate or delivery pressure with the help of a flow meter or pressure transducer.

[0032] Pump 170 is an example of a source for pressurized hydraulic fluid for pumping unit 100 configured to supply a flow rate at an operating pressure. In other embodiments, another suitable source for pressurized hydraulic fluid may be used. In general, the flow rate from the source for pressurized hydraulic fluid may be set or established by making adjustments to the source for pressurized hydraulic fluid or by selecting another source for pressurized hydraulic fluid, one configured to provide a different discharge rate for the anticipated operating conditions. A hydraulic fluid control system 200 is coupled between hydraulic pump 170 and piston-cylinder assemblies 120 to govern the flow of hydraulic fluid therebetween, synchronizing the reciprocating motions of the two assemblies 120A, 120B in phase-shifted cycles, which will be explained below.

[0033] Figure 3 presents a sectional view of hydraulically-driven pumping unit 100 along section A-A of Figure 1 to illustrate various features of the two double-acting hydraulic piston-cylinder assemblies 120, the two pumping piston-cylinder assemblies 140, and the cylinder union 138. The various components or features of the two hydraulic piston-cylinder assemblies 120A, 120B will likewise be distinguished by the suffix letters "A" and "B" in the figures and the text when advantageous. So too, various references to these same components or features will exclude any suffix letter "A," "B" when potentially associated with either assembly 120A, 120B. Thus, for example, the reference numeral 125 may refer to either or to both reference numerals 125 A, 125B, as used elsewhere in the text and in the figures.

[0034] Each double-acting hydraulic piston-cylinder assembly 120 includes a hydraulic cylinder 125 having a central axis 128, a first or cap-end 126, and a second or rod-end 127 positioned opposite cap-end 126 and coupled to cylinder union 138. Each assembly 120 also includes a piston 130 slidably disposed within cylinder 125 and a rod 134 coupled to piston 130. The rod 134 extends beyond rod-end 127, through cylinder union 138, and into a coupled pumping piston-cylinder assembly 140. Piston 130 will also be called power piston 130.

[0035] Referring to Figure 3 and Figure 4, within the first hydraulic piston-cylinder assembly 120A, the movable power piston 130A separates hydraulic cylinder 125A, or simply "cylinder A," into two chambers having variable volumes. More specifically, an extension- chamber 135A extends between cap-end 126A and piston 130A, and a retraction-chamber 136A extends between piston 13 OA and rod-end 127 A. A cap-end port PI is located adjacent cap-end 126A for fluid communication with chamber 135A. A rod-end port P2 is located adjacent rod-end 127A for fluid communication with chamber 136A. In the example of Figure 3, two cap-end ports PI and two rod-end ports P2 are shown. Likewise, in the second hydraulic piston-cylinder assembly 120B, a power piston 130B separates a hydraulic cylinder 125B, or simply "cylinder B," into two chambers having variable volumes, namely extension-chamber 135B and retraction-chamber 136B, with fluid communication provided by a cap-end port Ql and a rod-end port Q2, respectively.

[0036] Each pumping piston-cylinder assembly 140 includes cylinder 145 having a central axis 148, a cap-end 146, a rod-end 147 positioned opposite cap-end 146 and coupled to cylinder union 138. Each assembly 140 also includes a piston 150 slidably disposed within cylinder 145 and coupled to the rod 134 that extends from the aligned and corresponding hydraulic piston-cylinder assembly 120. Piston 150 will also be called a pumping piston 150. The movable pumping piston 150 defines a variable-volume pumping chamber 155 proximal the cap-end 146 of cylinder 145. Proximal the cap-end 146 of cylinder 145, an inlet port 156 is coupled to a one-way check valve 157 to perform as a suction valve, and a discharge port 158 is coupled to a one-way check valve 159 to perform as a discharge valve. Ports 156, 158 provide fluid communication for pumping chamber 155 in cylinder 145. The inlet port 156 of each pumping piston-cylinder assembly 140 is coupled to the working fluid reservoir 160. The discharge port 158 of each pumping piston-cylinder assembly 140 is coupled to a discharge manifold 165 having an outlet portl66 to supply working fluid to a process or a destination external to pumping unit 100. In the example embodiment, pumping system 100 is a component of a drilling mud recirculation system (not shown), and the manifold 165 is capable of supplying drilling mud to a well drilling operation. As configured, each pumping piston-cylinder assembly 140 may also be called single-acting pumping unit. Acting together, the two pumping piston-cylinder assemblies 140 may be called a single-acting duplex pump.

[0037] As best shown in Figure 3, cylinder union 138 couples to the first hydraulic piston- cylinder assembly 120A at its rod-end 127A and couples to the first of the pumping piston- cylinder assemblies 140 at rod-end 147 with central axes 128, 148 co-linearly aligned. Cylinder union 138 also couples to the second hydraulic piston-cylinder assembly 120B at its rod-end 127B and couples to the second pumping piston-cylinder assembly 140 at its rod-end 147 with the corresponding central axes 128, 148 co-linearly aligned. Thus, cylinder union 138 also laterally couples the first pair of aligned assemblies 120A, 140 with the second pair of aligned assemblies 120B, 140 for improved integrity and rigidity of pumping unit 100.

[0038] Fluid or hydraulic accumulator 260 includes a pressurized, contained gas held in an expandable and contractible chamber and a transfer fluid in an adjacent, second chamber, which is also expandable and contractible. At least in this embodiment, the total volume of the accumulator 260 is fixed, and the transfer fluid is hydraulic fluid. The second chamber of accumulator 260 couples to ports P2, Q2 for simultaneous fluid communication with chambers 136A, 136B. A rod-end fluid circuit 265 is defined by at least accumulator 260, chambers 136A, 136B, ports P2, Q2, and tubing coupling these components. At least during some portions of routine operation, the rod-end fluid circuit 265 is separate and does not communicate with the fluid circuit that includes pump 170, and the rod-end fluid circuit 265 is separate and does not communicate with the fluid circuit that includes gear pump 220. Thus, rod-end fluid circuit 265 is fluidically-isolated from a hydraulic fluid source in at least one mode of operation. However, in some embodiments, make-up fluid is introduced to the rod-end fluid circuit 265. In some embodiments, the rod-end fluid circuit 265 is fluidically- isolated from pump 170.

[0039] At least in this embodiment, the contained gas within hydraulic accumulator 260 has a pre-charged pressure that is less than the operating pressure of pump 170, greater than the pressure required to start and to continue moving the first power piston 13 OA when the port PI is in fluid communication with a hydraulic reservoir 250, and greater than the pressure required to start and to continue moving the second power piston 130B when the port Ql is in fluid communication with the hydraulic reservoir 250. In various instances, the pre-charged pressure of the contained gas within hydraulic accumulator 260 is set to a value that allows power pistons 130A, 130B to travel at speeds within a selected range of speeds and greater than a minimum speed associated with the pistons 130A, 130B.

[0040] In the schematic of Figure 4, the hydraulic fluid control system 200 is designated by a polygon drawn with dash-dot lines. When advantageous in the figures and in the text, components of control system 200 will be distinguished by a suffix letter "A" or "B," depending on the piston-cylinder assembly 120A, 120B served by the component. So too, various references to these same features may exclude any suffix letters "A" or "B" when potentially associated with either assembly 120A, 120B.

[0041] Fluid control system 200 includes a supply valve 205 and an exhaust valve 210 for each hydraulic cylinder 125. In the example embodiment, valves 205, 210 are "normally closed," spring-return poppet valves having a hydraulically-driven control mechanism. The selection of the open position or the closed position of each valve 205, 210 is governed by a solenoid-operated two-position three-way valve 215, which are supplied with pressurized hydraulic fluid from reservoir 250 by a gear pump 220. Pilot lines for hydraulic fluid, indicated by dashed lines in Figure 4, extend from gear pump 220 to the valves 215 and from each valve 215 to the corresponding valve 205, 210. The embodiment of Figure 4 has two supply valves 205, two exhaust valves 210, and four of the solenoid-operated valves 215. Any of the valves 205, 210, 215 may be called a "control valve." Each valve 215 has an energizing position 215p configured to transfer pressure from gear pump 220 to the control mechanism of the coupled valve 205, 210 in order to open the respective valve 205, 210. Each valve 215 also has an exhaust position 215x configured to relieve pressure from the control mechanism of the coupled valve 205, 210, allowing the respective internal spring to close the valve 205, 210. When a supply valve 205 is open, it is configured to pass pressurized hydraulic fluid from pump 170 to a coupled chamber 135A, 135B in a corresponding cylinder 125A, 125B. When an exhaust valve 210 is open, it is configured to pass pressurized hydraulic fluid from a coupled chamber 135A, 135B to an exhaust path 218 that includes reservoir 250; passing the fluid to path 218 reduces the pressure inside the corresponding extension-chamber 135A, 135B.

[0042] Referring to Figure 3 and Figure 4, at least one position sensor 230 is coupled to each hydraulic piston-cylinder assembly 120. Therefore, this exemplary embodiment includes two sensors 230, one for piston 130A and one for piston 130B. In this embodiment, each position sensor 230 is a linear displacement transducer (LDT) configured to give position data or signals describing each power piston 130 as it extends or retracts axially within cylinder 125. The velocity and acceleration of the respective piston 130 can be determined based on the response of the LDT position sensor 230. For example, in some embodiments, the LDT's (i.e., the position sensors 230) integrate position data to provide the velocity or acceleration of the piston 130 as well as position. In various other embodiments, a computational processor external to the LDT's integrates position data to provide the velocity or acceleration of the piston 130.

[0043] Referring now to cylinder 125A in Figure 4 as an example, at least three specific axial locations or positions 231A, 232A, 233A along cylinder 125A are defined in relation to the travel of piston 130A between cylinder ends 126A, 127A and in relation to the amount of extension of rod 134A beyond the cylinder's rod-end 127A. The positions 231A, 232A, 233A are associated or indicated by the position sensor 230 of cylinder 125A. For convenience, the positions 231 A, 232A, 233A will be discussed in terms of the piston 130A, but these positions could equally be described in terms of an exterior portion of the rod 134A. An extended position 231A is generally proximal the rod-end 127A, and a far-extended position 232A is closer to rod-end 127A, lying between extended position 231A and rod-end 127A. When piston 130A is at far-extended position 232A, chamber 136A still has more than its minimum volume, meaning that piston 130A could still travel closer to rod-end 127A after reaching far-extended position 232A. Thus, when piston 130A reaches position 232A in cylinder 125A, piston 130A and the coupled rod 134A are near but not at full extension. The location of position 232A is selected to allow time for supply valve 205A to close and exhaust valve 21 OA to open before piston 130 reaches its full extension, i.e., its maximum stroke length. This arrangement causes piston 130 to change direction without stopping abruptly adjacent rod-end 127A. In at least some embodiments, pumping system 100 is configured so that piston 130A and the coupled rod 134A change direction without reaching their full extension during an operational cycle to cause the change of direction to proceed more smoothly. This event may be called a "smooth reversal." In a smooth reversal the piston does not reaching the maximum extent of its stroke, but rather changes direction before reaching the maximum extent of its stroke.

[0044] The other position of piston 130A to be associated or indicated by position sensor 230 is proximal the first or cap-end 126A of cylinder 125A and will be called the retracted position 233A. When piston 130A reaches position 233A in cylinder 125A, piston 130A and the coupled rod 134A are near but not at full retraction," meaning sufficient room still exists in the shrinking chamber 135A to allow time for exhaust valve 210A to close and supply valve 205A to open before piston 130A must change directions. In at least some embodiments, pumping system 100 is configured so that piston 13 OA and the coupled rod 134A change direction without reaching their full retraction during an operational cycle to cause the change of direction to proceed more smoothly. This event is another example of a smooth reversal. Thus, a smooth reversal for piston 130A may occur near either end 126A, 127 A of cylinder 125 A. The ability of piston 13 OA to perform a smooth reversal when changing direction is based on the selected locations established for one or more of the positions 231A, 232A, 233A, 23 IB, 232B, 233B, the response time of one or more of the several valves that supply hydraulic fluid, and the momentum of the piston. Piston 130B in hydraulic piston-cylinder assembly 120B may also perform smooth reversals for some embodiments. Positions 231 A, 232A, 233A correspond to the variable size of chambers 135A, 136A during various stages of operation of piston-cylinder assembly 120A.

[0045] Continuing to reference Figure 4, the locations designated as positions 231 A, 232A, 233 A are axially adjustable along a hydraulic cylinder 125 A to test or tune the performance of hydraulically-driven pumping unit 100. For example, in some embodiments that implement position sensor 230 as an LDT, the positions 231 A, 232A, 233A are adjustable within the circuitry or logic of control circuit 225. In some instances, these adjustments are made in response to input from a human user. As a further example, control circuit 225 may dynamically adjust the location of any of the positions 231A, 232A, 233 A during operation of pumping unit 100 in response to system performance based on status data relating to any of the position sensors 230, the solenoid operated valves 215, the supply valves 205, the exhaust valves 210, or another component of pumping unit 100, for example. In some embodiments, additional instrumentation may be added to allow control circuit 225 to monitor the status data of the particular component or components of pumping unit 100. Adjusting the location of any of the positions 231A, 232A, 233A influences the timing of when valves 205A, 210A open and close and may influence other events related to the components of pumping unit 100. The same method is applicable to positions 23 IB, 232B, 233B along hydraulic cylinder 125B.

[0046] It is contemplated that the distance between the extended position 231A and the rod- end 127A (i.e., the position of full extension at rod-end 127) will be approximately 20% of the stroke length of the hydraulic piston-cylinder assembly 120A. In other embodiments, the distance between the extended position 231 A and the rod-end 127A may range between 15% and 25% of the stroke length, while a different value or values may be used in still other embodiments.

[0047] It is contemplated that the distance between the retracted position 233A and the cap- end 126A (i.e., the position of full retraction at cap-end 126A) will be approximately 10% of the stroke length of the hydraulic piston-cylinder assembly 120A. In other embodiments, the distance between the retracted position 233 A and the cap-end 126A may range between 5% and 15% of the stroke length, while a different value or values may be used in still other embodiments. In at least some embodiments, the distance between the far-extended position 232A and rod-end 127A is set to a distance corresponding in the same range as described for the distance between retracted position 233A and cap-end 126A.

[0048] Still referring to Figure 4, the other position sensor 230 coupled to piston-cylinder assembly 120B also has at least three specific locations or positions along the respective cylinder 125B associated with the travel of piston 130B between cylinder ends 126B, 127B. An extended position 23 IB, a far-extended position 232B, and a retracted position 233B for position sensor 230 are defined and established similar to the positions 231A, 232A, 233A along the corresponding cylinder 125A. Positions 23 IB, 232B, 233B correspond to the variable size of chambers 135B, 136B during various stages of operation of piston-cylinder assembly 120B.

[0049] Fluid control system 200 also includes a control circuit 225 electrically coupled to the four solenoid-operated valves 215 and to the two position sensors 230 on the two hydraulic piston-cylinder assemblies 120. Control circuit 225 electrically drives the positions of the various solenoid-operated valves 215 between the energizing position 215p and the exhaust position 215x based on signals from position sensors 230, as will be explained. Thereby, control circuit 225 governs the open or closed state of each supply valve 205 and each exhaust valve 210.

[0050] Control circuit 225 is configured so that during normal operation, the supply valve 205 and the exhaust valve 210 coupled to any one hydraulic cylinder 125 are not both open at the same time. For example, supply valve 205 A and exhaust valve 21 OA, which are both coupled to chamber 135A on hydraulic cylinder 125A, are controlled by control circuit 225 so that valves 205A, 210A are not both open at the same time. For instance, during operation, if supply valve 205A is open at a point in time when the corresponding position sensor 230 indicates that exhaust valve 21 OA should be open, control circuit 225 first closes valve 205 A before opening valve 21 OA. Similarly, in other instances, control circuit 225 first closes valve 210A before opening valve 205A. Therefore, control circuit 225 implements open-state-exclusivity to coordinate the operation of supply valve 205A and exhaust valve 210A for extension-chamber 135A to allow valves 205 A, 210A to communicate alternately but not simultaneously with port PI and chamber 135A. The open-state-exclusivity of the valve pair 205A, 21 OA prevents hydraulic fluid supplied by pump 170 from by -passing cylinder 125A and going directly to reservoir 250. Control circuit 225 governs the valve pair 205B, 210B for extension-chamber 135B in a similar manner to operate hydraulic cylinder 125B, applying open-state-exclusivity to the valve pair 205B, 210B. In this manner, open- state-exclusivity is applied to each hydraulic piston-cylinder assembly 120 to control the flow of hydraulic fluid entering and exiting the corresponding cylinder 125.

[0051] Control circuit 225 is further configured so that during normal operation the supply valve 205 for at least one cylinder 125 is open at all times. Thus, in the example embodiment, valves 205A, 205B are not both closed at the same time. As implemented, in various instances of the operation cycle, valve 205B opens before valve 205A closes, and in various other instances of the operation cycle, valve 205A opens before valve 205B closes. Therefore, control circuit 225 implements open-state-assurance to cause at least a portion of the flow rate of hydraulic fluid from pump 170 to be always supplied to one or both of the cylinders 125A, 125B of the hydraulic piston-cylinder assemblies 120A, 120B, respectively.

[0052] In an exemplary embodiment, during operation of the hydraulic piston-cylinder assemblies 120A, 120B, the application of open-state-exclusivity and open-state-assurance together causes all or substantially all the flow rate of hydraulic fluid from pump 170 to be always supplied to one or both cylinders 125A, 125B without a portion by-passing a cylinder 125A, 125B and moving directly to reservoir 250. Thus, the flow rate established for pump 170 is fully utilized and none is wasted. During non-routine situations that result in an unacceptably high pressure in the supply line extending from pump 170, a pressure relief safety valve (PRV) 222 releases some hydraulic fluid.

[0053] In some embodiments, control circuit 225 comprises and utilizes electrical relays, wiring arrangements, or pilot line arrangements for valves 215 to govern the operation of pumping unit 100 in accordance with the principles disclosed herein. In various other embodiments, control circuit 225 includes an electronic control unit such as a programmable logic controller (PLC), a computer, or another suitable computational processor to govern the operation of pumping unit 100. In various embodiments, control circuit 225 includes or is coupled to a non-transitory computer-readable storage device or a graphical user interface for input and output of data and commands. In various embodiments, control circuit 225 and the non-transitory computer-readable storage device transfer machine readable code or instructions to establish or to adjust the operation of pumping unit 100. Various components of control circuit 225 may be remotely located away from the other components of pumping system 100.

[0054] Operation of an Embodiment of a Multi-Cylinder Hydraulically-Driven Pumping Unit

[0055] Various aspects of the operation of the hydraulically-driven pumping unit 100 will be explained with reference to Figure 4. This discussion gives an example of the operation that may occur in various circumstances. During operation, the reciprocal motion of power piston 130A in hydraulic piston-cylinder assembly 120A drives a reciprocal motion of pumping piston 150 in the coupled pumping piston-cylinder assembly 140 shown in the top right corner of Figure 4.

[0056] Similarly, the reciprocal motion within hydraulic piston-cylinder assembly 120B drives a reciprocal motion within the coupled pumping piston-cylinder assembly 140 shown in the lower right corner of Figure 4. The operation of assemblies 120A, 120B are coordinated to provide smoother flow of working fluid, e.g. drilling mud, from the pumping piston-cylinder assemblies 140 and manifold 165. Pump 170 is configured to supply hydraulic fluid at a selected or established flow rate and an operating pressure to the group of piston-cylinder assemblies 120, directly powering or driving the pistons 130 and rods 134 within the various assemblies 120 as they periodically extend during the synchronized, phase- shifted cycles governed by fluid control system 200, which will be described below. In some operational conditions, the flow rate from pump 170 is substantially constant. The rod-end fluid circuit 265, including accumulator 260, powers or drives the return stroke, i.e., the retraction, of the pistons 130 and rods 134.

[0057] The hydraulic piston-cylinder assembly 120A and the hydraulic piston-cylinder assembly 120B shown in the embodiment of Figure 4 are synchronized and configured for phase-shifted operation such that when pumping unit 100 is reciprocating the two pistons 130A, 130B and rods 134A, 134A B, the second first piston 130B and rod 134B always have a different combination of axial position and direction of travel than do the first piston 130A and rod 134A, and such that at least one of the two rods 134A, 134B is moving to extend further beyond the corresponding hydraulic cylinder 125A, 125B at all times.

[0058] In order to cause the pistons and rods of the hydraulic piston-cylinder assemblies 120A, 120B to reciprocate in synchronized, phase-shifted cycles, hydraulic fluid control system 200 applies a plurality of operational states related to assembly 120A and assembly 120B. Some of the operational states overlap with other operational states during the operation of pumping unit 100.

[0059] In an operational State 1A (one-A), control system 200 places the extension-chamber 135A of assembly 120A in fluid communication with pump 170, allowing rod 130A to extend. The State 1A is achieved by placing supply valve 205 A in its open position while exhaust valve 210A is in its closed position, insuring open-state-exclusivity for supply valve 205 A in relationship to exhaust valve 21 OA. Thus, the supply valve 205 A is a control valve that is fluidically coupled between the extension-chamber 135A and the hydraulic fluid source, i.e. pump 170, and is configured to activate and deactivate the State 1A.

[0060] In an operational State 2A, control system 200 places the extension-chamber 135A in fluid communication with exhaust path 218, including hydraulic reservoir 250, allowing rod 130A to retract. The State 2A is achieved by placing supply valve 205A in its closed position and then placing exhaust valve 21 OA in its open position. Thus, exhaust valve 21 OA, being fluidically coupled between the extension-chamber 136A and the exhaust path 218, is configured to activate and deactivate State 2A.

[0061] In an operational State IB, control system 200 places the extension-chamber 135B of assembly 120B in fluid communication with pump 170, allowing rod 130B to extend. The State IB is achieved by placing supply valve 205B in its open position while exhaust valve 21 OB is in its closed position, insuring open-state-exclusivity for supply valve 205B in relationship to exhaust valve 210B. Thus, supply valve 205B is a control valve that is fluidically coupled between the extension-chamber 135B and the hydraulic fluid source and is configured to activate and deactivate the State IB. The State 1A and State IB may be individually activated or deactivated. In some instances, the State 1A and State IB may overlap, meaning the State IB may be active during at least a portion of the time that the State 1 A is active, in keeping with the concept of open-state-assurance. Thus for a period of time, hydraulic fluid may be supplied simultaneously to both supply valves 205A, 205B to exert an extension force on both rods 130A, 130B simultaneously.

[0062] In an operational State 2B, control system 200 places the extension-chamber 135B in fluid communication with exhaust path 218, including hydraulic reservoir 250, allowing rod 130B to retract. The State 2B is achieved by placing supply valve 205B in its closed position and then placing exhaust valve 210B in its open position. Thus, exhaust valve 210B, being fluidically coupled between the extension-chamber 136B and the exhaust path 218, is configured to activate and deactivate State 2B.

[0063] Additional operational states can be defined to help describe the operation of the pumping unit 100. For example, in a State 3 A the first extension-chamber chamber of the first HPC assembly is not in fluid communication with the hydraulic fluid source and is not in fluid communication with the exhaust path. In a State 3B the second extension-chamber of the second HPC assembly is not in fluid communication with the hydraulic fluid source and is not in fluid communication with the exhaust path.

[0064] Referring to Figure 5, the coordinated operation of hydraulic piston-cylinder assemblies 120A, 120B may also be described in terms of transitional events governed by fluid control system 200. Figure 5 tabulates the locations of power pistons 13 OA, 130B during four transitional events (Transitions 1, 2, 3, 4) in which the open and closed positions of the fours valves 205A, 205B, 210A, 210B are periodically selected in accordance with open-state-exclusivity and open-state-assurance as previously defined. The four states of operation previously described will be evidenced by the positions of the four valves 205 A, 205B, 21 OA, 210B. Prior to a first transitional event, or Transition 1 (one), supply valve 205A is open, exhaust valve 210A is closed, supply valve 205B is closed, and exhaust valve 210B is open. Therefore, prior to Transition 1, piston 130A and rod 134A in cylinder 125A extend while piston 130B and rod 134B in cylinder 125B retract. The extension of piston 130A and rod 134A is driven by hydraulic fluid from pump 170. The retraction of piston 130B and rod 134B is driven by piston 130A pushing fluid from chamber 136A toward accumulator 260 and chamber 136B.

[0065] At Transition 1, rod 134A has reached the extended position 231 A, which is indicated by a signal from the corresponding position sensor 230... Exhaust valve 210B is initially open (State 2B is active), and rod 134B has reached the retracted position 233B, which is indicated by a signal from the corresponding position sensor 230, and rod 134B is near but not at full retraction, being proximal cap-end 126B of cylinder 125B. The signal corresponding to position 231 A generated by position sensor 230 of cylinder 125A directs control system 200 to close exhaust valve 210B (deactivate State 2B) and then to open supply valve 205B (activate State IB) for cylinder 125B, in accordance with open-state-exclusivity for these two valves. The signal corresponding to position 233B confirms that rod 134B has retracted sufficiently. Supply valve 205 A remains open (State 1A is active), and exhaust valve 21 OA remains closed (State 2A is inactive).

[0066] As a result of Transition 1, rod 134A continues to extend, and rod 134B reverses direction and begins to extend. Because both supply valves 205A, 205B are open (State 1A and State IB are simultaneously active), and share the constant flow from fluid from pump 170, in some embodiments, the average extension speed of the two rods 134A, 134B after Transition 1 may be approximately one half the previous extension speed of rod 134A while rod 134B was retracting. In other embodiments, depending on the combined characteristics and the loading of the hydraulic piston-cylinder assemblies 120A, 120B, 140, the speed of the two rods 134A, 134B after Transition 1 may vary in a different manner. Because both pairs of pistons 130 and rods 134 are extending, the volumes of chambers 136A, 136B are simultaneously reducing in volume. Therefore, accumulator 260 receives and accumulates fluid from both chambers 136A, 136B.

[0067] Assuming, for convenience of discussion, an embodiment in which the pressure drop between pump 170 and chamber 135A equals the pressure drop between pump 170 and chamber 135B, both chambers 135A, 135B and both pistons 130A, 130B receive an equal portion of the hydraulic fluid flow rate and experience the same pressure coming from pump 170 following Transition 1. In this assumed situation, both pistons 130A, 130B exert generally the same force on the respectively coupled pumping piston-cylinder assembly 140, and the flow rates of working fluid leaving the two discharge ports 158 are therefore generally equal, following Transition 1 and prior to the second transitional event.

[0068] At a second transitional event, or Transition 2, rod 134A has reached the far-extended position 232A, which is indicated by a signal from the corresponding position sensor 230, and rod 134A is near but not at full extension, being proximal rod-end 127A of cylinder 125A. Rod 134B is between the retracted position 233B and extended position 23 IB, moving toward rod-end 127B of cylinder 125B. The signal corresponding to position 232A of cylinder 125A directs control system 200 to close supply valve 205A (deactivate State 1A) and then to open exhaust valve 21 OA (activate State 2A) for cylinder 125 A, in accordance with open-state-exclusivity for these two valves. Supply valve 205B remains open (State IB is active) in accordance with open-state-assurance for supply valves 205A, 205B, and exhaust valve 210B remains closed (State 2B is inactive).

[0069] As a result of Transition 2, rod 134A reverses direction and begins to retract, receiving fluid from accumulator 260 and from chamber 135B as rod 134B continues to extend. The extension speed of rod 134B is approximately twice as fast as it was following Transition 1 because supply valve 205B feeds all hydraulic fluid from pump 170 to chamber 135B.

[0070] A third transitional event, or Transition 3, is the inverse or the "mirror" of Transition 1 with the events for assemblies 120A, 120B swapped. At Transition 3, rod 134B has reached its extended position 23 IB. Rod 134A has reached the retracted position 233A and is near but not at full retraction. The signal corresponding to position 23 IB of cylinder 125B directs control system 200 to close the exhaust valve 21 OA (deactivate State 2A) and then to open the supply valve 205A (activate State 1A) for cylinder 125A, in accordance with open- state-exclusivity for these two valves. The signal corresponding to position 233 A confirms that rod 134A has retracted sufficiently. Supply valve 205B remains open (State IB is active), and exhaust valve 210B remains closed (State 2B is inactive).

[0071] At Transition 3, rod 134B continues to extend, and rod 134A reverses direction and begins to extend. Because both supply valves 205A, 205B are open (State 1A and State IB are simultaneously active), in some embodiments, the extension speed of each rod 134A, 134B after Transition 3 may be approximately one half the previous extension speed of rod 134B while rod 134A was retracting. In other embodiments, depending on the combined characteristics and the loading of the hydraulic piston-cylinder assemblies 120A, 120B, 140, the speed of the two rods 134A, 134B after Transition 3 may vary in a different manner. Accumulator 260 receives and accumulates fluid from both chambers 136A, 136B, which are shrinking in volume at this stage of the operation.

[0072] A fourth transitional event, or Transition 4, is the inverse or the "mirror" of Transition 2. At Transition 4, rod 134B has reached its far-extended position 232B and is near but not at full extension. Rod 134A is between the retracted position 233A and extended position 231A, moving toward rod-end 127A of cylinder 125A. The signal corresponding to position 232B of cylinder 125B directs control system 200 to close supply valve 205B (deactivate State IB) and then to open exhaust valve 210B (activate State 2B) for cylinder 125B, in accordance with open-state-exclusivity for these two valves. Supply valve 205A remains open (State 1A is active) in accordance with open-state-assurance for supply valves 205A, 205B, and exhaust valve 21 OA remains closed (State 2A is inactive).

[0073] As a result of Transition 4, rod 134B reverses direction and begins to retract, receiving fluid from accumulator 260 and from chamber 136A as rod 134A continues to extend. The extension speed of rod 134A is approximately twice as fast as it was following Transition 3 because supply valve 205 A feeds all hydraulic fluid from pump 170 to chamber 135A.

[0074] In at least one embodiment, Transition 1, Transition 2, Transition 3, and Transition 4 occur in sequence, as numbered. The inverse or mirrored relationship between Transition 1 and Transition 3 as well as the inverse or mirrored relationship between Transition 2 and Transition 4 cause the two pairs of pistons 130 and rods 134 within the assemblies 120A, 120B to reciprocate in synchronized, phase-shifted cycles governed by fluid control system 200. Thus, as previously stated, the hydraulic piston-cylinder assemblies 120A, 120B are directly powered by pump 170 during their extension strokes. Assemblies 120A, 120B are powered during their return strokes by rod-end fluid circuit 265 (Figure 4), i.e., by fluid coming from the opposite assembly 120 or coming from accumulator 260, exchanged through ports P2, Q2.

[0075] As stated, the signals of sensors 230 corresponding to positions 233A, 233B confirm that rods 134A, 134B retract sufficiently. The information these signals provide is used to control the amount of fluid in the rod-end fluid circuit 265. Occasionally, make-up fluid is added to the rod-end circuit 265 to compensate for leakage. In some embodiments, the monitoring and adjusting of the amount of fluid in circuit 265 is accomplished automatically by hydraulic fluid control system 200 with the aid of one or more additional valves (not shown) and a pump, for example pump 170, gear pump 220, or an additional hydraulic pump. In some other embodiments, a change to the amount of fluid in circuit 265 is accomplished manually during maintenance. The signal data from sensors 230 that is monitored by control system 200 is available to guide the maintenance operation, the data being presented in any suitable manner, for example using a graphical user interface coupled to control system 200.

[0076] In the operation scenario explained, the average retraction speeds of the pistons 130 are faster than the average extension speeds of the pistons 130. For example, the retraction of piston 130A is completed during and between Transition 2 and Transition 3, but the extension of piston 13 OA starts during Transition 3 and continues until the subsequent Transition 2. As similar analysis is true of piston 130A for this embodiment. For some embodiments, the higher retraction speed may be due, at least in part, to the over-pressurizing of the accumulator 260 of circuit 265 (Figure 4) that occurs during the brief time when both pistons extend at the same time, e.g. following Transition 1 and following Transition 3. A short time later, when one of the pistons begins to retract during Transition 2 or Transition 4, the now-retracting piston 130 receives an initial burst of fluid from the accumulator 265 as well as the steady flow of hydraulic fluid that continues to come from the other piston 130 that is still extending. As retraction continues, the retraction speed may reduce below its initial magnitude.

[0077] In at least some embodiments, the first HPC assembly is in the State 3A between the State 1A and the State 2A during the Transition 2. In at least some embodiments, the second HPC assembly is in the State 3B between the State IB and the State 2B during the Transition 4.

[0078] Figure 6 presents a method 400 for operating a fluid-driven pumping unit in accordance with the principles described herein. At block 402, method 400 includes supplying a flow rate of a driving fluid during an operation period. Hydraulic fluid is an example of a driving fluid. Supplying a flow rate of the driving fluid is accomplished in the example previously described by a source for pressurized hydraulic fluid such as, for example, pump 170. At block 404, method 400 includes dividing the flow rate of the driving fluid between members of a plurality of double-acting hydraulic piston-cylinder (HPC) assemblies, each assembly having a movable rod. The synchronized, phase-shifted cycles are implemented by a fluid control system. At block 406, method 400 includes operating at least two of the HPC assemblies using synchronized, phase-shifted cycles, wherein the corresponding two movable rods extend and retract periodically, such that the two rods always have a different combination of axial position and direction of travel and such that at least one of the two rods is extending at all times. In the example of pumping system 100, two double-acting hydraulic piston-cylinder assemblies 120 drive the two pumping piston- cylinder assemblies 140 to discharge a working fluid, such as drilling mud, through the outlet port 166 of manifold 165. At block 408, method 400 includes providing a plurality of pumps configured to deliver a working fluid, each pump driven by a rod of the plurality of rods. At block 410, method 400 includes pumping a working fluid using the plurality of pumps. At block 412, method 400 includes interconnecting the plurality of HPC assemblies so the extension of a first actuator causes the retraction of a second actuator and the extension of the second actuator causes retraction of the first actuator. In pumping system 100, for example, rod-end fluid circuit 265 serves to accomplish the step described in block 408.

[0079] The steps or actions depicted in Figure 6 or otherwise described for method 400 may be performed in the order shown or in a different order, and two or more of the actions may be performed in parallel, rather than serially. The actions of method 400 may be performed by control circuit 225, which in some embodiments includes a computational processor. Various embodiments of method 400 may include additional steps based on any of the concepts presented in this written description, including the figures. For example, open-state- exclusivity or open-state-assurance, as previously defined, may be applied to the operation of the fluid-driven pumping unit.

[0080] Embodiments consistent with the present disclosure have been presented. In addition, numerous modifications and variations are possible, such as those stated here.

[0081] In addition to being useful for supplying a working fluid to a process or a destination, in various embodiments, the disclosed hydraulically-driven pumping unit is suited for extracting or recovering a fluid from an above ground or underground reservoir, such as extracting hydrocarbons from an operating oil well.

[0082] Although in the earlier discussion the sensing of various locations of pistons 130 within hydraulic cylinders 125 is accomplished using an LDT position sensor 230 coupled to each cylinder 125 or, similarly, coupled to each piston 130, in some other embodiments the position sensor 230 is implemented with another suitable technology. For example, three proximity sensors may instead be used for each cylinder 125. One proximity sensor is placed at each position 231 A, 232A, 233A for cylinder 125A, and one proximity sensor is placed at each position 23 IB, 232B, 233B for cylinder 125B. The proximity sensors detect the presence or movement of piston 130A, 130B or another recognized, movable feature. In some embodiments, the proximity sensors are mounted in a configuration that allows them to be selectively moved in an axial direction along a hydraulic cylinder 125A, 125B to simplify testing or tuning of the performance of hydraulically-driven pumping unit 100. Depending on space constraints or other considerations, in other embodiments, proximity sensors acting as position sensors 230 are mounted on each pumping piston-cylinder assembly 140 and detect the presence or movement of pumping pistons 150 or another recognized, movable feature, such as a selected locations adjacent the paths of travel of the rods 134A, 134B outside cylinder 125A, 125B. The presence or movement of pumping pistons 150 are then correlated to the movement of the coupled piston 130A, 130B. Any suitable type of proximity sensor may be used. For example, proximity sensors that operate based on sensing changes in capacitance or proximity sensors that transmit and receive acoustic signals, or sensors that rely on electrical inductance or physical contact with the moving piston may be used in various embodiments. Thus various position sensor(s) 230 that utilize a mechanical phenomenon, an electrical phenomenon, or another suitable phenomenon may be used to detect the localized presence, i.e., the position, of piston 130 along cylinder 125.

[0083] In some embodiments, fewer than three positions 231 A, 232A, 233A along hydraulic cylinder 125A are instrumented by position sensor 230, and the presence or movement of piston 130A at some of the positions 231A, 232A, 233A is determined by timing circuitry or logic in control circuit 225, possibly in conjunction with status data from a valve or another component of pumping unit 100. For example, the determination of the presence of piston 130A at position 232A may be implemented by a timing delay triggered by the opening of second supply valve 205B and/or by the status of the solenoid operated valve 215 that supplies pressurized hydraulic fluid to valve 205B. A similar technique is applicable to position 232B along hydraulic cylinder 125B, and a similar technique may be applicable to another position 231 A, 233A, 23 IB, 233B.

[0084] Although the described embodiment includes two of the hydraulic piston-cylinder assemblies 120, some other embodiments include three or more double-acting hydraulic piston-cylinder assemblies 120 coupled to three or more pumping piston-cylinder assemblies 140, and include one or more rod-end fluid circuits 265. The operation of each hydraulic piston-cylinder assembly 120 is controlled by open-state-exclusivity so that the assembly's supply valve and exhaust valve are not simultaneously in an open position or open state. The operation of the three or more assemblies 120 is coordinated according to open-state- assurance so that the supply valve for at least one of the assemblies is open at any given time.

[0085] In various embodiments, the separate supply valve 205 and exhaust valve 210 for each cylinder 125 is replaced by a two-position three-way control valve having "pressurizing position" to place the corresponding port PI, Q 1 in fluid communication with the source for pressurized hydraulic fluid, e.g., pump 170, and a "discharge position" to place the same port PI, Ql in fluid communication with the hydraulic reservoir 250. For example, an alternative embodiment of pumping unit 100 of Figure 1 would have two such control valves. Each of the two-position three-way control valves may be coupled to a solenoid operated two-position three-way valve 215 or may have an integrated control mechanism that may be fluid-operated or electrically-operated.

[0086] In various embodiments, a pumping piston-cylinder assembly 140 may be a double- acting piston-cylinder assembly having two pumping chambers 155, one pumping chamber on either side of the enclosed piston 150, and having an inlet port 156 and a discharge port 158 for each of the two pumping chambers 155. The two pumping chambers 155 in a single assembly 140 configures the assembly 140 to pump fluid when piston 150 moves in either direction, providing higher capacity of fluid delivery or potentially providing a smother flow of fluid to manifold 165.

[0087] While exemplary embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the disclosure. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. The inclusion of any particular method step or action within the written description or a figure does not necessarily indicate that the particular step or action is necessary to the method. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.