BACKGROUND

[0001] The subject matter of this disclosure relates generally to motor-driven pumps, and more particularly, to a system and method for converterless operation of motor- driven pumps.

[0002] The conventional approach to controlling the speed of motor-driven pumps is through the use of a variable speed drive (VSD) that is fed by a fixed frequency AC supply. The VSD synthesizes voltages and currents of such frequency as is necessary to operate the pump in the desired manner. In the oil and gas industry, the voltage output by the VSD is usually stepped up to a medium voltage using a transformer because high voltage motors are deployed in wells to reduce the size of the power cable needed to supply the motor.

[0003] Figure 1 illustrates a conventional system 10 that is known in the oil and gas industry for operating electric submersible pumps (ESPs) 12 in an off-grid application. One or more prime movers that are directly coupled to generators 14 produce an AC voltage having a fixed frequency and amplitude to supply electrical loads 15. The prime movers may comprise, for example, a reciprocating engine that is fueled by either natural gas or diesel fuel, or a turbine. The generated AC power is fed to a VSD 16 that is responsible for regulating the operation of the ESPs 12 subsequent to stepping up the AC voltage to a medium voltage level that is supplied to ESP motor(s) 18 via a suitable transformer 19.

[0004] There is a need in the oil and gas industry to provide a system for operating ESPs that is less complex, less costly, and that has a smaller footprint. A system that reduces the capital expense, weight and footprint size will advantageously reduce the time it takes to put a well into production using power generated on-site when compared with the time it takes to put a well into production using utility power because of the delays associated with getting the utility to install necessary power lines. [0005] It is possible to use the natural gas produced by the well to support operation of the generator, thereby reducing the operating expense of the system. Depending on the selection of the generator and the prime mover, it may be necessary to couple the generator and the prime mover through a gearbox. It is generally possible to select a gearbox with a fixed ratio, thereby avoiding the need for changing gear ratios during system operation.

BRIEF DESCRIPTION

[0006] According to one embodiment, a converterless motor-driven pump system comprises: at least one off-grid prime mover comprising a rotational driveshaft and operating in response to a throttle control command to control a rotation speed of the rotational driveshaft; at least one electric power generator driven by the at least one off-grid prime mover to generate AC power; at least one variable speed motor directly powered by the at least one electric power generator; at least one electric submersible pump driven by the at least one variable speed motor, wherein one or more operating characteristics associated with the at least one electric submersible pump are monitored by one or more corresponding sensors; a system controller programmed to generate the throttle control command in response to the one or more pump operating characteristics such that the at least one off- grid prime mover, the at least one electric power generator, and the at least one variable speed motor together operate to regulate a pressure at the inlet of the at least one electric submersible pump; and monitoring and protection equipment comprising circuit breakers to ensure safety to personnel around the system, and to provide protection to the prime mover, generator, and variable speed motor during system starting, or in response to equipment failure or in response to occurrence of one or more unforeseen events.

[0007] According to another embodiment, a method of operating an electric submersible pump comprises: controlling a driveshaft speed of an off-grid prime mover in response to a throttle control command; controlling an AC power output of an electric power generator in response to the driveshaft speed of the off-grid prime mover; controlling a speed of a variable speed motor directly in response to the AC power output of the electric power generator; and monitoring operating characteristics of the electric submersible pump and generating the throttle control command in response thereto such that together the off- grid prime mover, the electric power generator, and the variable speed motor operate to regulate a pressure at an inlet to the electric submersible pump.

DRAWINGS

[0008] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein:

[0009] Figure 1 illustrates a conventional electrical submersible pump (ESP) system that is known in the art;

[0010] Figure 2 illustrates a converterless ESP system according to one embodiment;

[0011] Figure 3 is a block diagram illustrating a system controller interfacing with and controlling a converterless ESP system according to one embodiment; and

[0012] Figure 4 is a block diagram illustrating a method of providing off-grid power to a motor-driven submersible well pump according to one embodiment.

[0013] While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.

DETAILED DESCRIPTION

[0014] The embodiments described herein are directed to control of motor- driven pumps in applications that are operating independently of a utility power grid, and combine the control of a prime mover and an AC generator to provide substantially similar functionality as a variable speed drive (VSD) to reduce system complexity, cost and footprint size. Such embodiments are particularly useful in the oil and gas industry where the usual control objective is to regulate the pressure at the inlet of the motor driven submersible pump, although other control objectives, including without limitation, temperature, speed, or vibration can be applied in like fashion.

[0015] Figure 2 illustrates a converterless ESP system 20 according to one embodiment. In this embodiment, the prime mover(s) 21 is/are directly controlled to regulate the pump inlet pressure. More specifically, the ESP system 20 comprises one or more prime movers 21 that are coupled to one or more generators 22, means 24 for electrically connecting the output of the generator(s) 22, and a motor-driven pump 26. The prime mover 21 is typically a reciprocating engine that is fueled by either natural gas or diesel fuel, but is not so limited, as other types of prime movers such as, without limitation, turbines may also be employed as the prime mover 21. Depending on the selection of the prime mover 21 and the generator 22, it may be desirable to use a gearbox to match the shaft speeds of the prime mover 21 and generator 22. It is preferable to use a fixed ratio gearbox to keep the system 20 as simple as possible. The motor-driven pump 26 is typically located within a well for purposes of artificially lifting a fluid from the well. The fluid could be, without limitation, water, gas or oil in a well, or a combination thereof. It is likely some amount of solids, such as sand or proppant, will be entrained with the fluid.

[0016] A sensor package 28 is attached to the motor-driven pump 26 that may comprise, for example, one or more temperature sensors and one or more pressure sensors to provide an indication of various pump operating temperatures and pressures. An important pressure is the inlet pressure to the pump 26, since this pressure provides a direct indication of whether the well is being operated at the proper loading for maximizing well production. The sensor package 28 may further comprise one or more vibration sensors configured to monitor various pump vibration characteristics and to provide an indication whether a predetermined vibration level is exceeded. At least one speed sensor may be included in the sensor package 28 in order to accurately monitor the rotational speed of the pump. Other types of sensors may be included in the sensor package 28 depending on the particular application requirements.

[0017] The converterless ESP system 20 advantageously i) eliminates the need for a variable speed drive and transformer, simplifying the system, resulting in improved system reliability, ii) can use pumped gas via the pump 26 itself as the fuel to run the prime mover 22, resulting in very low fuel costs, and iii) operates independently of a utility power grid.

[0018] It can be appreciated that there may be reasons to retain a transformer between the generator 22 and the motor driven pump 26. Such reasons may include, without limitation, minimizing system cost and/or maximizing operational flexibility. According to one aspect, the decision to retain or remove the transformer from the system 20 may be made on the basis of system optimization rather than conceptual operation of the system 20.

[0019] Figure 3 is a block diagram illustrating the flow of power and information for a converterless ESP system 30 according to one embodiment. The power flows from the prime mover 21 through the generator 22 and cable 32 to the motor 34 and subsequently the pump 26. The power between the prime mover 21 and the generator 22 is mechanical driveshaft power, as is the power between the induction motor 34 and the pump 26. A gearbox between the prime mover 21 and the generator 22 may advantageously be employed for purposes of system optimization, as stated herein.

[0020] The programmable system controller 36 is responsible for monitoring the pump operating conditions, including without limitation input and output pressures, pump temperature(s), pump vibration levels, and pump rotational speed, and commanding the throttle position control 38 of the prime mover 21 that will drive the pump 26 output to the desired pump operating point in response to one or more of the monitored operating conditions. According to one aspect, the system controller 36 also monitors the shaft speed of the prime mover 21 and commands the generator exciter 39 of the synchronous generator 22 accordingly.

[0021] The programmable system controller 36 may comprise, without limitation, one or more computers and/or data processors/devices and associated display devices. The data processors/devices may comprise one or more CPUs, DSPs and associated data storage devices, data acquisition devices and corresponding handshaking devices that may be integrated with the system controller 36 and/or distributed throughout the converterless ESP system 30. The system controller 36 may communicate with a remote operations center 37 that is able to monitor system operation and modify system operating objectives without requiring action of a local operator.

[0022] According to another aspect, the system controller 36 monitors the voltage, frequency and current being supplied to the motor 34, and generates the prime mover throttle control command in response to the monitored information to modify control of the prime mover 21. The rate of change in prime mover driveshaft speed, for example, might be controlled to maintain the generator current below a specified value by limiting the current being supplied by the generator 22. Such operation can help reduce stress on the system, thereby making the converterless ESP system 30 more reliable.

[0023] According to another aspect, the generator 22 may be a permanent magnet generator that does not require excitation. It can be appreciated that use of a permanent magnet generator would further simplify the converterless ESP system 30 without sacrificing performance.

[0024] It can be appreciated that the pump motor 34 may be any electric motor that can be line started, including not only induction motors, but also a special class of permanent magnet motors known as line-start permanent magnet motors. [0025] In summary explanation, a converterless ESP system eliminates the variable speed drive and, potentially, its associated transformer from a motor driven submersible pump system, resulting in a simpler system that reduces capital expense, weight and system footprint. The use of power generated on-site advantageously reduces the time it takes to put a well into production resulting from delays in getting the utility to install requisite power lines. Further, the use of natural gas produced by the well itself advantageously reduces the operating expense.

[0026] Since the output of the generator 22 is substantially sinusoidal when compared with the output of a variable speed drive, a filter is not required between the generator 22 and the motor 34. The output of a variable VSD, for example, contains significant high frequency content, the result of chopping up DC voltage/current to produce AC voltage/current. This chopping action disadvantageous ly creates high frequency components called harmonics that are detrimental to the motor driving the pump. A filter is usually installed between the VSD and the motor; however, anecdotal data suggest that even such a filter may not always adequately filter out the harmonics, leading to accelerated aging of the insulation systems in the transformer 19, cable 32, and motor 34. This disadvantageously reduces the life of the ESP system.

[0027] A VSD also draws nonsinusoidal currents from its supply, unless an active front end is applied to the VSD. These resulting harmonics are detrimental to the generator supplying the VSD. Many system designs oversize the generator so that it can better tolerate the harmonic currents drawn by the VSD. Other system designs will use an active power filter to source the harmonic currents drawn by the VSD, thereby alleviating the generator from having to supply them. Either of such approaches adds to the cost and complexity of the system.

[0028] The principles described herein with reference to the various embodiments include reduced capital expense and more timely well production. The off-grid converterless system embodiments advantageously allow putting a well into production sooner since there is frequently a substantial waiting period for the utility to install supply lines to the well site, as stated herein. At such time as utility power is available, the well operator can remove the prime mover and generator, replacing them with a variable speed drive and transformers if desired. [0029] Figure 4 is a block diagram illustrating a method 40 of providing off-grid power to a motor-driven submersible well pump 26 according to one embodiment. A prime mover 21 driveshaft is coupled directly or indirectly to a generator 22; while the generator 22 is electrically coupled to a motor that may be a line start motor such as an induction motor or permanent magnet motor 34 via a power cable 32 that may be, for example, without limitation, an electrical submersible pump cable; and the motor driveshaft is directly coupled to the submersible well pump 26, as represented in block 42. The prime mover 21 is turned-on to rotate its driveshaft, causing the generator 22 to produce AC power sufficient to power the motor 34, that subsequently drives the submersible well pump 26, as represented in block 44. A sensor package 28 that may comprise, without limitation, various pressure sensors, temperature sensors, vibration sensors, and speed sensors associated with the submersible well pump 26 function to monitor operating conditions including without limitation, pump inlet pressure, pump vibration levels, pump rotational speed, and temperatures at desired points associated with the submersible well pump 26, as represented in block 46. The monitored operating data is acquired by a system controller 36 that determines whether the prime mover driveshaft should be rotating at a different speed. The system controller 36 then transmits an appropriate throttle control command 38 to the prime mover 21, causing the prime mover driveshaft to rotate faster or slower as necessary to ensure the submersible well pump 26 is operating at the desired operating point, as represented in block 48. According to one embodiment, the system controller 36 also monitors the rotational speed of the prime mover driveshaft via one or more speed sensors 25 associated with the driveshaft of the prime mover 21, and commands the exciter 39 of a generator 22 to supply an appropriate level of excitation to the generator 22 when the generator 22 is a synchronous generator, as represented in block 50.

[0030] For reasons of safety and system protection, system elements may be included that are responsible for monitoring the operation of the system equipment, with means to instruct the controller 36 to shut down the system 30 if a failure or external event causes an exception to intended operation. Exemplary system elements may include, without limitation, one or more pump pressure sensors, pump speed sensors, pump temperature sensors, pump vibration sensors, pump viscosity sensors, pump gas volume fraction sensors, specific gravity sensors, motor current sensors, motor temperature sensors, motor voltage sensors, and motor frequency sensors. A pump gas volume fraction sensor, for example, may be employed to determine a volumetric ratio of liquid versus gas that is flowing through the pump(s). External events causing a system shutdown may include, for example, i) a volume fraction that gets too large, i.e., too much gas for pump to handle, ii) a motor temperature that gets too high, or iii) a clogged pump, causing pump pressure to get too high. Monitored sensor signals are transmitted to the system controller 36 that ensures that the motor 34 and pump 26 are operating within prescribed design, safety, specification and/or threshold limits.

[0031] Another embodiment includes monitoring the voltage, frequency, temperature and current being supplied to the motor 34 via the generator 22, and acquiring the monitored information, as represented in block 52. The acquired motor supply voltage, frequency, temperature and current information is used by the system controller 36 to determine whether the prime mover driveshaft should rotate at a different speed. If a different prime mover driveshaft speed is required, the system controller 36 transmits an appropriate throttle command 38 to the prime mover 21, causing a change in the running speed of the prime mover 21, as represented in block 54. This embodiment can be employed in applications where it might be of interest to, for example, limit the current being supplied by the generator 22; so the rate of change in prime mover speed could be controlled to keep the generator current less than a specified value.

[0032] Since some applications may employ a permanent magnet generator that does not require excitation, it can be appreciated that a generator exciter will not be required in such applications. The use of a permanent magnet generator further simplifies the converterless ESP system 30 without sacrificing performance, as stated herein.

[0033] Although particular embodiments have been described herein with application to electric submersible pumps, the principles described herein can just as easily be applied to other applications including without limitation, geothermal applications. In such applications, gas turbines or reciprocating engines can be employed to rotate the generator. [0034] The principles described herein can be applied to a motor generator set feeding a plurality of ESPs (i.e., when an existing oil field is to be expanded to include, for example, 50% more wells, wherein the wells are in close proximity to each other). The controller 36 in this application is further programmed according to one embodiment to provide for load balancing among the ESP motors, thereby reducing unwanted losses.

[0035] The controller 36 may further be configured with synchronization logic and programmed according to yet another embodiment to generate a control signal that activates an auxiliary/spare generator to provide a parallel operation capability.

[0036] While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.