Compound electro-hydraulic frac pumping system

An electrically driven oilfield pumping system may include a compound electro-hydraulic fracturing (frac) pump system that has a primary electric motor that selectively delivers power to one or more of at least two fracturing (frac) pumps. The primary electric motor may be a constant speed alternative current (AC) motor with a fixed rated speed.

BACKGROUND OF THE INVENTION

Field of the Invention

The preferred embodiments relate generally to the field of hydrocarbon recovery from the earth and, more specifically, to oilfield pressure pumping systems for fracturing underground formations to enhance recovery of hydrocarbons.

Discussion of the Related Art

Hydraulically fracturing subterranean formations with oilfield pressure pumping systems to enhance flow in oil and gas wells is known. Hydraulic fracturing increases well productivity by increasing the porosity of, and thus flow rate through, production zones that feed boreholes of the wells that remove underground resources like oil and gas.

Oilfield pressure pumping systems include heavy-duty industrial-type components to create the extreme hydraulic pressures, for example, 10,000 psi or more, which are needed to fracture (frac) the subterranean geological formations. Positive displacement, high pressure, and/or plunger pumps are used as fracturing (fracking or frac) pumps to generate the extreme hydraulic pressures that are capable of fracturing subterranean geological formations.

Flow and pressure of frac fluids from frac pumps must be closely regulated at the various fracturing stages in order to adequately control the fracturing process. Accordingly, prime movers that deliver power to the frac pumps are variable speed devices, since driving the frac pumps at variable speeds at least partially provides the flow and pressure control.

Typically, the prime movers are high horsepower stationary diesel engines that deliver power to the frac pumps through multi-speed gearboxes or transmissions. High horsepower stationary diesel engines are expensive and require maintenance and operational attention, such as refueling.

Other attempts have been made to use variable speed electric motors to power frac pumps. Variable speed electric motors can vary flow and pressure of the frac pumps through speed-varying motor controls, which facilitates control of the fracturing operation. Variable speed electric motors either directly drive the frac pumps at the motors' variable speeds or with an intervening single-speed gearbox or transmission. Such variable speed electric motors include shunt wound, variable speed, DC (direct current) traction motors and variable speed, for example, variable frequency, AC (alternating current) electric motors. Although variable speed electric motors can require less operational attention than high horsepower stationary diesel engines, they are expensive and require sophisticated motor controls.

Constant speed AC motors are more straightforward than variable speed electric motors but have not been used to deliver power to frac pumps. That is because the fixed speed(s) of constant speed AC motors do not provide the desired amount of flow and pressure control of the frac pumps to allow operators to suitably control the fracturing operation. Typical multi-speed gearboxes are unable to resolve this problem with constant speed AC motors because they are unable to shift under full load and have range ratios that are ill-suited to provide a sufficient variety of output shaft speeds or corresponding frac pump flow and pressure control.

Furthermore, constant speed AC motors of high-enough horsepower ratings to power frac pumps are difficult to start because they require extremely high starting currents as in-rush (locked rotor) currents to begin their rotations.

Additionally, like pressure pumping systems that use internal combustion engines, pressure pumping systems that are electrically driven require substantial amounts of jobsite space, typically implemented as multiple trailer-mounted frac pump systems that collectively provide the pressurized frac fluid for delivery into a well. Each electric motor that drives a frac pump requires large conductors or electrical cables to transmit electrical power from an electrical power system to the motor. These large electrical cables are heavy and expensive. Furthermore, cable management or routing the large electrical cables in a tidy manner at an oilfield site can be challenging.

Therefore, what is needed is a prime mover for high pressure pumping applications, like powering frac pumps, employing a constant speed AC motor, but without the above-noted drawbacks primarily directed to flow and pressure control.

SUMMARY AND OBJECTS OF THE INVENTION

The preferred embodiments overcome the above-noted drawbacks by providing an electrically driven oilfield pumping system with a compound electro-hydraulic fracturing (frac) pump system that has a primary electric motor that selectively delivers power to one or more of at least two fracturing (frac) pumps. The compound electro-hydraulic frac pump system may incorporate a constant speed AC motor and a pair of frac pumps that can individually or together selectively receive power from the AC motor.

The compound electro-hydraulic frac pump system may further include a transmission system with a pair of transmissions that selectively deliver power from the AC motor to a pair of frac pumps. The AC motor may have a pair of outputs that deliver power to the pair of transmissions.

The system may include multiple APUs (auxiliary power units) that can, for example, provide power into or through various system components while the AC motor is de-energized. The multiple APUs may include a pair of start-APUs mounted to the pair of transmissions. Each of the start-APUs may be individually used to provide a torque that pre-rotates a main shaft of the de-energized AC motor to rotate it to its rated speed before the AC motor is energized during a single-APU starting mode. Both start-APUs may be simultaneously used to provide a torque that pre-rotates the AC motor's main shaft to rotate it to its rated speed before the AC motor is energized during a compound-APU starting mode.

The multiple APUs may include a pair of slow-frac-APUs mounted to the pair of transmissions. During a single-APU slow frac mode, one of the slow-frac-APUs may be individually used to provide a torque that drives a corresponding frac pump at a de-rated speed or slower than the frac pump can be driven by the AC motor during a normal speed frac mode. Both start-APUs may be simultaneously used to provide a torque that simultaneously drives both frac pumps during a compound-APU slow frac mode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring toFIG.1, one embodiment of the invention is an electro-hydraulic high-pressure pumping system, shown as an electrically driven oilfield pumping system or pumping system10. The pumping system10is shown here implemented as a compound electro-hydraulic frac pumping system12, which includes an electro-hydraulic drive system13that delivers power to a fracturing (frac) pump system14that includes at least a pair of frac pumps15,16. Each frac pump15,16can be a positive displacement, high-pressure, plunger pump or other suitable pump that can deliver high flow rates and produce high pressures, for example, 10,000 psi or more. This oilfield site is shown with multiple compound electro-hydraulic frac pumping systems12, each of which has two frac pumps15,16in their frac pump systems14, that operate together for a subterranean geological formation fracturing or fracking operation to stimulate well production.

Within each compound electro-hydraulic frac pumping system12, the respective frac pumps15,16can be activated or brought online and implemented separately or together, depending on the particular pumping needs for a given fracking operation or operational stage, as intra-system activation states. Each of the compound electro-hydraulic frac pumping systems12is typically implemented as a singularly-packaged unit, for example, mounted on a trailer that can be towed by a semi-tractor or other tow vehicle. Each frac pump15,16receives fracturing fluid or frac fluid18that is stored in a frac fluid storage system20and delivers the frac fluid18to the frac pumps15,16through frac fluid delivery lines22. Pressurized frac fluid18is delivered from the frac pumps15,16, through manifold delivery lines24, to manifold26that delivers the pressurized frac fluid18through a manifold outlet line28to a wellhead30. At the wellhead30, the frac fluid18is directed to flow through a borehole that extends through a well casing32for fracturing the subterranean formation.

Still referring toFIG.1, compound electro-hydraulic frac pumping system12selectively receives electrical power through conductors34from electrical power system36. Electrical power system36includes a generator and prime mover such as a combustion engine which may be a gas turbine engine. A frac site control system represented as control system40includes a computer that executes various stored programs while receiving inputs from and sending commands to the compound electro-hydraulic frac pumping system12for controlling, for example, energizing and de-energizing various system components as well as bringing the compound electro-hydraulic frac pumping system12online for fracking the subterranean formations by controlling the various electronic, electromechanical, and hydraulic systems and/or other components of each compound electro-hydraulic frac pumping system12. Control system40may include the TDEC-501 electronic control system available from Twin Disc®, Inc. for controlling the compound electro-hydraulic frac pumping system(s)12. By controlling the compound electro-hydraulic frac pumping system12, control system40can change and/or control operating modes by varying operational states of various corresponding components and/or subsystems within system10.

Referring now toFIG.2, compound electro-hydraulic frac pumping system12includes a primary electric motor42, and a transmission system44. Primary electric motor42is typically implemented as a high-powered constant speed AC motor, for example, about 6,000 HP (horsepower) or having an equivalent torque rating of about a 6,000 HP diesel engine. Primary electric motor42may operate at a relatively slow fixed rotational speed, such as a fixed rated speed of about 1,800 RPM (rotations per minute). A pair of outputs or first output48and second output50are defined at opposite ends of primary electric motor42. Outputs48,50are typically implemented as a pair of output shafts or first output shaft52and second output shaft54that are coaxially aligned with each other and extend from opposite ends of the primary electric motor42.

Still referring toFIG.2, primary electric motor42is typically implemented in a sandwiched configuration with respect to transmission system44. Transmission system44typically includes a pair of transmissions, shown as first transmission62and second transmission64, that have input shafts (not labeled) which respectively receive power from the primary electric motor's output shafts52,54. Output shafts52,54may be defined by opposite ends of a single or common axially extending main shaft of primary electric motor42, which rotates in unison with its rotor or armature. Transmissions62,64are shown here in a close couple mounting configuration with respect to primary electric motor42, which may be implemented as flexible couplings66,68at the transmission inputs without intervening torque converters or clutches. Regardless of the particular connecting components between primary electric motor42and transmissions62,64, each transmission62,64has a heavy-duty industrial gearbox or transmission. Typically, each transmission62,64is a multi-speed transmission having multiple ranges that provide multiple substantially evenly spaced drive ratios to facilitate close regulation of rotational speed of the transmission output shaft and, correspondingly, operational speed of components within frac pump system14and output flow and pressure from frac pump system14. A suitable transmission includes, for example, a model TA90-7600, available from Twin Disc®, Inc., which is capable of changing ranges while the frac pump15,16is fully loaded. Driveshafts70,72transmit torque from transmissions62,64to frac pumps15,16of the frac pump system14. Clutches71,73are shown between the transmission62,64outputs and the driveshafts70,72and are configured to selectively disconnect power transfer between the respective transmission62,64and frac pump15,16, shown upstream of the driveshafts70,72. It is understood that clutches71,73may be arranged between other adjacent power-transmitting components in order to, for example, prevent or allow power transfer from primary electric motor42to either or both of frac pumps15,16. In one example, clutches71,73provided between the primary motor output shafts52,54and transmission62,64inputs, which may be done instead of, or in addition to, the flexible couplings66,68.

Still referring toFIG.2, transmissions62,64are shown with PTO towers or sections with respective pairs of pump pads84,86and88,90for mounting and mechanically delivering power to or receiving power from various components, for example, hydraulic components. Each of the lower illustrated pump pads84,88is shown supporting a respective pair of transmission pumps or charging and lube pumps92,94and96,98which may be configured to, for example, supply pressurized oil for transmission lubrication and controlling hydraulically actuated components within the transmission. Although transmissions62,64and their various components and/or accessories may be identical, typically, one of the pair of charging and lube pumps92,94and96,98is internally modified. The internal modification of the charging and/or lube pump(s)92,94or96,98may be a mirror image reconfiguration compared to the non-modified configuration, to allow its/their rotation in an opposite direction of the non-modified configuration. This allows for driving the charging and/or lube pump(s)92,94or96,98in a common direction even though transmissions62,64receive power through rotational inputs in opposite directions because of the opposed relationship of transmissions62,64with respect to the (single) primary electric motor42.

Still referring toFIG.2, an auxiliary power unit (APU) system100selectively delivers power through one or both transmission(s)62,64. APU system100is shown with electro-hydraulic-start auxiliary power units (APUs) or start-APUs102,104at transmissions62,64and electro-hydraulic-slow-frac auxiliary power units (APUs) or slow-frac-APUs103,105at transmissions62,64. Start-APUs102,104include hydraulic starting motors106,108, each of which may be a high speed, low torque, hydraulic motor. Hydraulic starting motors106,108are shown respectively mounted to the charging and/or lube pump(s)92,94or96,98and therefore transmissions62,64by way of pump pads84,88. Within the start-APUs102,104, electric motors are provided as APU electric starting motors110,112that selectively deliver torque to hydraulic starting motors106,108. Each of the APU electric starting motors110,112may be a variable speed AC motor that is substantially smaller than primary electric motor42, with APU electric starting motors110,112rated at, for example, about 50 HP. Energizing APU electric starting motor110activates hydraulic starting motor106and energizing APU electric starting motor112activates hydraulic starting motor108, which rotates various gear train or other components of transmissions62,64and correspondingly rotates the shaft(s)52,54of primary electric motor42when the primary electric motor42is de-energized.

Still referring toFIG.2, the start-APUs102,104can be operated separately or together to rotate the primary motor shaft(s)52,54during a primary electric motor starting mode. When operated separately to individually rotate the primary motor shaft(s)52,54, the start-APUs102,104may be used alternatively during subsequent primary motor shaft(s) rotations in order to reduce instances of usage of each. Otherwise, start-APUs102,104may be used simultaneously to reduce the load burden on each while rotating the primary motor shaft(s)52,54. Regardless of the particular control strategy for implementing one or both of the start-APUs102,104, primary electric motor's shaft(s)52,54is rotated by at least one of the start-APUs102,104to bring it sufficiently close to its rated fixed speed or synchronous speed before the primary electric motor42is energized during a starting mode of system10. Hydraulic starting motor(s)106,108can correspondingly rotate at about 1,800 RPM or at an appropriate speed that can rotate the primary electric motor's shaft(s)52,54at 1,800 RPM or other speed, depending on the particular rated or synchronous speed of primary electric motor42. Rotating the primary electric motor42with hydraulic starting motor(s)106,108to achieve the synchronous speed of primary electric motor42allows connection to the electrical power source DoL (Direct on Line) while avoiding the motor's high in-rush (locked rotor) current that would otherwise be required to start the primary electric motor42. The primary electric motor42is therefore able to be started at essentially its normal running current, when pre-driven to its synchronous speed by hydraulic starting motor(s)106,108.

Still referring toFIG.2, slow-frac-APUs103,105include slow frac hydraulic motors114,116, each of which may be configured to, for example, supply slow speed or low flow operation of the respective frac pump15,16. Slow frac hydraulic motors114,116may be low speed, high torque, hydraulic motors that are mounted to the pump pads86,90. The rotational speed(s) of slow frac hydraulic motors114,116may be a fraction of the rotational speed of hydraulic starting motors106,108. Clutches118,120are shown arranged between the slow frac hydraulic motors114,116and pump pads86,90and are configured to disconnect power transfer between the slow frac hydraulic motors114,116and their respective transmissions62,64. Each clutch118,120may be an overrunning clutch or an actuatable or other clutch to passively or actively connect or disconnect power flow between the slow frac hydraulic motor114,116and its respective transmission62,64to correspond to different operational states of the fracking system. It is understood that instead of or in addition to implementing clutches118,120, when the slow frac hydraulic motor(s)114,116is not being implemented, it can be locked against activation, which may include binding or holding the pistons in the motor(s) fixed, depending on its configuration.

Electric motors, implemented as APU slow frac electric motors122,124of slow-frac-APUs103,105, selectively deliver torque to slow frac hydraulic motors114,116. Like electric motors110,112, each APU slow frac electric motor122,124may be a variable speed AC motor that is substantially smaller than primary electric motor42, with APU slow frac electric motors122,124rated at, for example, about 50 HP. Energizing APU slow frac electric motor122activates slow frac hydraulic motor114and energizing APU slow frac electric motor124activates slow frac hydraulic motor116. When activated, slow frac hydraulic motors114,116rotate various gear train or other components of their respective transmissions62,64. The activated slow frac hydraulic motor114may correspondingly rotate driveshaft70through transmission62and the activated slow frac hydraulic motor116may correspondingly rotate driveshaft72through transmission64when the primary electric motor42is de-energized. In either situation, the activation of slow frac hydraulic motor114or the activation of slow frac hydraulic motor116, the hydraulic motor may rotate the output shaft(s)52,54through the mechanical coupling of the output shaft(s)52,54with the transmissions62,64. In this way, either one (or both, simultaneously) of the slow frac hydraulic motors114,116can be activated to rotate primary electric motor's shaft(s) at slow and precisely controlled speeds to deliver torque through the transmission(s)62,64and correspondingly precisely control the frac pump(s)16to provide high-pressure low speed fracking. The rotational speed of slow frac hydraulic motor(s)114,116is typically between about 800 RPM to 1,100 RPM or at an appropriate speed that can rotate the primary electric motor42shaft at between about 800 RPM to 1,000 RPM or other speed, depending on the particular speed required to produce the desired flow rate of frac pump15,16for high pressure low speed fracking. Regardless, the precise slow speed control of slow frac hydraulic motor60may be achieved using a closed-loop controller (e.g., proportional integral derivative (PID) controller) within the control system40(FIG.1) that controls rotational speed of the APU slow frac electric motors122,124that power the slow frac hydraulic motors114,116.

Referring now toFIG.3, an exemplary simplified hydraulic schematic layout is shown. At each transmission62,64, hydraulic components of the system10share a common tank or sump, shown here as reservoir130within transmission62and reservoir132within transmission64. Hydraulic power packs134,136control flow of hydraulic fluid through various components within transmissions62,64. Each of the mode selector valves138,140of hydraulic power packs134,136provides three discrete flow paths of hydraulic fluid out of the respective hydraulic power pack134,136. The mode selector valves138,140may be, for example, solenoid actuated spool valves that provide the three discrete positions, represented as positions142,144,146and148,150,152, to selectively allow flow out of three corresponding outlets and provide three corresponding flow paths out of the hydraulic power packs134,136. Actuating the mode selector valves138,140allows for selectively activating and permitting hydraulic fluid flow through the hydraulic starting motor(s)106,108, slow frac hydraulic motor(s)114,116, or none of them.

Still referring toFIG.3, a starting mode of system10can be achieved in different ways by controlling the start-APUs102,104to force a pre-rotation of the de-energized primary electric motor42, bring it to approximately its rated speed before its energization. The different ways include a single-APU starting mode in which only one of the start-APUs102,104pre-rotates the de-energized primary electric motor42during the starting mode and a compound-APU starting mode in which both of the start-APUs102,104pre-rotate the de-energized primary electric motor42during the starting mode. As a first example of a single-APU starting mode, when start-APU102is utilized as the pre-rotation APU, mode selector valve138is at a first position shown as position142, which directs hydraulic fluid to hydraulic starting motor106. This rotates hydraulic starting motor106, which delivers torque to rotate the output shaft52of the de-energized primary electric motor42to achieve the motor's synchronous speed in preparation for its energization by connecting to the electrical power source DoL. In another example of a single-APU starting mode, when start-APU104is utilized as the pre-rotation APU, mode selector valve140, is in its first position148to rotate hydraulic starting motor108. Hydraulic starting motor108delivers torque to rotate the output shaft54of the de-energized primary electric motor43to achieve the motor's synchronous speed in preparation of its energization. When in the compound-APU starting mode, both start-APUs102,104share the starting load of pre-rotating the de-energized primary electric motor42, with both mode selector valves138,140in their first positions142,148. This directs hydraulic fluid to and activates both hydraulic staring motors106,108to simultaneously rotate the output shafts52,54of the de-energized primary electric motor42.

Still referring toFIG.3, a slow speed frac mode of system10can be achieved in different ways by controlling the slow-frac-APUs103,105to serve as a prime mover(s) instead of primary electric motor42to deliver frac fluid at a high-pressure but slow speed into the subterranean formation. The different ways include a single-APU slow speed frac mode in which only one of the slow-frac-APUs103,105drives a frac pump15,16and a compound-APU slow speed frac mode in which both of the slow-frac-APUs103,105drive frac pumps15,16. Typically, during any of the slow speed frac modes, the slow-frac APU(s)103,105drives its respective frac pump15,16at an underdrive speed, which is slower that the frac pump15,16can be driven by the primary electric motor42. Also in any of the slow speed frac modes, the slow-frac-APU(s)103,105used as the frac pump-powering prime mover(s) typically transmits power through the de-energized primary electric motor's shaft(s) as a passively driven torque-transmitting component(s) that transmits torque back into the respective transmission(s)62,64to drive the corresponding frac pump(s)15,16.

Still referring toFIG.3, as a first example of a single-APU slow speed frac mode, when slow-frac APU103is utilized as the slow frac APU, mode selector valve138is at a second position shown as position144and hydraulic fluid directed to slow frac hydraulic motor114. This rotates an output shaft of slow frac hydraulic motor114that delivers torque through the primary motor's output shaft52(FIG.2) to rotate the primary motor's main shaft and deliver torque through transmission62and to the frac pump15to achieve high-pressure, slow speed, fracking in the slow frac mode of system10. In another example of a single-APU slow speed frac mode, when slow-frac APU105is utilized as the slow frac APU, mode selector valve140is in its second position150to direct hydraulic fluid to slow frac hydraulic motor116. This rotates an output shaft of slow frac hydraulic motor116that delivers torque through the primary motor's output shaft54(FIG.2) to rotate the primary motor's main shaft and deliver torque through transmission64and to the frac pump16to achieve high-pressure, slow speed, fracking in the slow frac mode of system10. When in the compound-APU slow speed frac mode, both slow-frac-APUs103,105share the frac pump driving task(s), with both mode selector valves138,140in their second positions144,150. This directs hydraulic fluid to and activates both of the slow frac hydraulic motors114,116, to deliver torque through the output shafts52,54of the de-energized primary electric motor42and various transmission components to drive the frac pumps15,16to achieve high-pressure, slow speed, fracking in the slow frac mode of system10.

Still referring toFIG.3, at each of the mode selector valves138,140, it is at a third position shown as neutral position146,152, hydraulic fluid that would otherwise be directed to hydraulic starting motor106,108or slow frac hydraulic motor114,116is instead directed to tank or reservoir130,132of the respective transmission62,64. Each selector valve138,140is actuated to or held in this neutral or third position146,152when, for example, for the particular associated start-APU102,104or slow-frac APU103,105is the inactive APU during a single-APU mode in which only one is being used. Both selector valves138,140are actuated or held in the neutral or third position146,152when, for example, primary electric motor42is energized and driving frac pump(s)15,16through transmission(s)62,64and shaft(s)70,72, which provides default or normal fracking operation as a default frac mode or normal speed frac mode of system10. During normal speed frac mode, positioning selector valves138,140to their neutral or third positions146,152avoids any non-desired pumping through hydraulic starting motor(s)92,108or slow frac hydraulic motor(s)114,116by preventing flow to or through them. Such inadvertent passive pumping can be yet further prevented with respect to slow frac hydraulic motor(s)114,116by, for example, clutch(es)118,120(FIG.2) that either allows the rotating mechanism(s) of pump pad(s)86,90to overrun the slow frac hydraulic motor(s)118,120or disengage a selective driving engagement between the pump pad(s)86,90and the slow frac hydraulic motor(s)114,116.

Still referring toFIG.3, normal speed frac mode of system10can be achieved by controlling, for example, the clutches71,73(FIG.2) and/or transmissions62,64, to selectively deliver power to and deliver frac fluid18(FIG.1) from the frac pumps15,16. The different ways include a normal-speed single-pump frac mode in which only one of the frac pumps15,16is activated and a normal-speed compound pump frac mode in which both of the frac pumps15,16are activated. Referring again toFIG.2, as a first example of a normal speed single pump frac mode, in which frac pump15is the active frac pump, clutch71is engaged to establish a power flow from primary electric motor output shaft52and transmission62through shaft70to drive frac pump15and clutch73is disengaged to disconnect power transmission from primary electric motor42to frac pump16. As a second example of a normal speed single pump frac mode, in which frac pump16is the active frac pump, clutch73is engaged to establish a power flow from primary electric motor output shaft54and transmission64through shaft72to drive frac pump16and clutch71is disengaged to disconnect power transmission from primary electric motor42to frac pump15. When in the normal-speed compound pump frac mode, both of the clutches71,73are engaged so that power is transmitted from each of the primary electric motor's output shafts52,54, through transmissions62,64, and to both active frac pumps15,16.

Referring now toFIG.4and with background reference toFIG.2to show various components, different component states are shown for various modes and sub-modes of system10. Within the chart, cells with a letter designation “A” represent active components and cells with a letter designation “I” represent inactive components. In the group of the top three rows representing the starting mode, it is shown that during all versions of starting modes, both slow-frac-APUs103,105are inactive, primary electric motor42is inactive, and both frac pumps15,16are inactive. During each of the single-APU starting modes, only one of the start-APUs102,104is active and during the compound-APU staring mode, both of the start-APUs102,104are active. In the group of the middle three rows representing the slow speed frac mode, it is shown that during all versions of the slow speed frac modes, both start-APUs102,104are inactive and primary electric motor42is inactive. During each of the single-APU slow speed frac modes, a paired set of a slow-frac-APU103,105and a cooperating frac pump15,16are active. During the compound-APU slow speed frac mode, both slow-frac-APU103,105and both frac pumps15,16are active. In the group of the bottom three rows representing the normal speed frac mode, it is shown that during all versions of the normal speed frac modes, the primary electric motor42is active, both start-APUs102,104are inactive, and both slow-frac-APU103,105are inactive. During each of the single-pump normal speed frac modes, only one of the frac pumps15,16is active and during the compound pump normal speed frac mode, both frac pumps15,16are active.

A method200of fracking using the above-described systems of the preferred embodiments is set forth inFIG.5, with background reference toFIGS.1and2for various components. Method200includes determining if a primary electric motor42, such as that described previously, is energized at Block202. If energized, at evaluation Block204, the system evaluates whether the current normal speed frac mode is appropriate based on an underlying control methodology or predetermined fracking session parameters. This may include, for example, evaluating a timer that corresponds to different normal speed mode phases, and transitions between, according to the fracking procedure's control strategy or based on evaluated performance characteristics of the system, the well, or other aspects of the oilfield. At Block206, if the system determines that a different normal speed frac mode should be implemented at that particular time, then the system commands a mode change which may include controlling, for example, one or both of the clutches71,73and/or transmissions62,64. Normal speed frac mode changes include transitioning from one of the single-pump normal speed frac modes to a compound-pump normal speed frac mode, from a compound-pump normal speed frac mode to one of the single-pump normal speed frac modes, or from one of the single-pump normal speed frac modes to the other to switch between which of the frac pumps15,16is active. At Block208, one or both of the frac pumps15,16is driven, based on the particular normal speed frac mode being implemented at that time.

Still referring toFIG.5, If, on the other hand, the primary electric motor42is not energized, during method200, the system determines whether it is already in or should engage a slow frac mode, in Block210. If not, the system in initiates a starting mode in Block212. In evaluation Block214, the system evaluates whether the current starting mode is appropriate for the starting task. This may include comparing a rotational speed of the primary electric motor's main shaft to determine if it is being appropriately accelerated toward its fixed rated speed. If not, then at Block216, the system commands a starting mode change. This may include changing from the activation of one of the start-APUs102,104to the other one as a switch from one single-APU starting mode to the other or change to a compound-APU starting mode by activating both start-APUs102,104. Once the start-APU(s)102,104sufficiently pre-rotates the primary electric motor's main shaft to bring it to its rated fixed or synchronous speed, a connection is made to the electrical power source DoL (Direct on Line) in Block218. Once connected to the DoL, primary electric motor can drive the frac pump(s) of the system in Block208.

Still referring toFIG.5, when in slow frac mode, at Block220, the system evaluates whether the current slow frac mode is appropriate based on an underlying control methodology or predetermined fracking session parameters. Like with the normal speed frac mode evaluation(s), this may include, for example, evaluating a timer that corresponds to different slow speed mode phases, and transitions between, according to the fracking procedure's control strategy or based on evaluated performance characteristics of the system, the well, or other aspects of the oilfield. At Block222if the system determines that a different slow speed frac mode should be implemented at that particular time, then the system commands a mode change which may include controlling, for example, one or both of the slow-frac-APUs103,105and/or transmissions62,64. Slow speed frac mode changes include transitioning from one of the single-APU slow speed frac modes to a compound-APU slow speed frac mode, from a compound-APU slow speed frac mode to one of the single-APU slow speed frac modes, or from one of the single-APU slow speed frac modes to the other to switch between which of the slow-frac-APUs103,105is active. Returning to Block208, one or both frac pumps15,16are driven, based on the particular slow speed frac mode being implemented at that time. The system determines if the fracking session is or should still be performed at Block224and, if so, continues as described above and, if not, the fracking session ends.