Patent Publication Number: US-2016248364-A1

Title: Variable speed drive with topside control and subsea switching

Description:
TECHNICAL FIELD 
     The present disclosure relates to control of subsea electric motors. More particularly, the present disclosure relates to control of subsea electric motors driven by a subsea variable speed drive. 
     BACKGROUND 
     In subsea applications, for example subsea fluid processing in the oil and gas industry, subsea motors are used to carry out certain fluid processing tasks such as to power fluid pumps and compressors. Conventionally, the subsea motor is controlled by a topside variable speed drive (VSD) that generates varying frequency three-phase power transmitted through an umbilical cable system to the subsea location where the subsea motor is located. Additionally, especially in cases of long umbilical cable distances, a topside step-up transformer and subsea step-down transformer may be used to reduce energy losses through the umbilical. 
     Entire VSD systems can be deployed subsea, which can be beneficial, for example, in cases of very long tiebacks and/or multiple consumers. With a subsea VSD, power in form of A/C or D/C electricity is transmitted through the umbilical, and a subsea VSD converts the power to variable A/C for driving the subsea motor. With subsea VSDs, however, the control circuitry needs to be qualified for long-term subsea installation, since any failures may result in very costly intervention procedures. Proposed designs include control electronics which generate the pulse width modulated signals, etc. and are located in an air-filled atmospheric enclosure. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     According to some embodiments, a system is described for operating a subsea electric motor. The system includes: a subsea electric induction motor installed in a subsea location; a surface control system configured to adjust operating speed of the subsea electric motor by outputting a series of control signals corresponding to a selected drive frequency; a communication system configured to transmit the series of control signals from the surface control system to the subsea location; a plurality of gate drivers deployed at the subsea location and configured to receive the series of control signals and output corresponding gate voltages; and a plurality of semiconductor active switches deployed at the subsea location and configured to receive the gate voltages and in combination convert a source power into pulse-width modulated quasi-sinusoidal alternating currents (AC) at the selected drive frequency thereby driving the motor at the selected drive frequency. 
     According to some embodiments, a subsea high-speed de-multiplexer is deployed at the subsea location and configured to distribute the series of control signals to the plurality of gate drivers. The communication system can use one or more optical fibers to transmit the series of control signals. In some cases the semiconductor active switches are gate-controlled semiconductor switching devices, including for example, insulated-gate bipolar transistor (IGBT) switching devices, integrated gate-commutated thyristor (IGCT) switching devices or gate turn-off thyristor (GTO) switches. 
     According to some embodiments, the source power is transmitted as three-phase electric power from the surface to the subsea location, and a diode bridge rectifier system converting the three-phase electric power into direct current (DC) electric power for use by the switches. In some cases, a subsea step-down transformer is used to reduce voltage levels of the transmitted three-phase electric power for use by the diode bridge rectifier system. In some cases, the source power is transmitted as DC electric power from the surface to the subsea location. In some cases, the rectifier system can be designed with active switches, thereby providing an active front end that is controlled by a surface system. 
     According some cases, the parts of the system that are located in the subsea location are free from digital control electronics and/or are free from any gas-filled sealed housings. According to some embodiments, the electric motor is used to drive fluid processing equipment such as a subsea pump, a subsea compressor, or a subsea separator. The fluid being processed can be produced from a subterranean hydrocarbon-bearing reservoir. According to some embodiments, the surface control system is capable of providing continuously variable control over the frequency of the motor. 
     According to some embodiments, a system is described for controlling a subsea electric motor. The system includes: a surface control system configured to adjust operating speed of a subsea electric motor by outputting a series of control signals corresponding to a selected drive frequency; a communication system configured to transmit the series of control signals from the surface control system to the subsea location; a plurality of gate drivers deployed at the subsea location and configured to receive the series of control signals and output corresponding gate voltages; and a plurality of semiconductor active switches deployed at the subsea location and configured to receive the gate voltages, and in combination convert a source power into pulse-width modulated quasi-sinusoidal alternating current (AC) at the selected drive frequency thereby driving the motor at the selected drive frequency, wherein the portion of the system in the subsea location is free from digital control electronics. 
     According to some embodiments, a method is described for controlling a subsea electric motor. The method includes: at a surface location, selecting a drive frequency; using a surface control system, generating a series of pulse-width modulated gate control signals which correspond to the selected drive frequency; transmitting the series of gate control signals to a subsea location where a subsea motor is deployed; in the subsea location, distributing the series of gate control signals to a plurality of gate drivers; using the gate drivers, converting the gate control signals into gate control voltages; and driving the electric motor by inputting the gate control voltages to a plurality of semiconductor active switches deployed at the subsea location, and converting a source power into three pulse width modulated quasi-sinusoidal alternative currents at the selected drive frequency. 
     According to some embodiments, one or more of the described systems and/or methods can be used in topside or subsea fluid processing equipment in an analogous fashion. 
     According to some embodiments, the techniques described herein are applicable to any subsea converters where gate controlled semiconducting switches are used. Examples of subsea settings where topside control signals can be used to control subsea switching include: high-voltage, direct current (HVDC) electric power transmission systems; subsea DC-DC converters; subsea uninterruptible power supply (UPS) systems; subsea semiconducting circuit breakers; and subsea VAR compensator systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the subject disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIG. 1  is a diagram illustrating topside and subsea environments in which a subsea motor driven by a VSD having topside control and subsea switching can be deployed, according to some embodiments; and 
         FIG. 2  is a block diagram illustrating further aspects a subsea motor driven by a VSD having topside control and subsea switching, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The particulars shown herein are by way of example, and for purposes of illustrative discussion of the embodiments of the subject disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details of the subject disclosure in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Further, like reference numbers and designations in the various drawings indicate like elements. 
     According to some embodiments, a subsea motor can be controlled using subsea deployed gate drivers and semiconductor switches, but with the control electronics located in topside surface facility. Such an arrangement avoids many of the drawbacks associated with deploying the entire variable speed drive (VSD) system in the subsea location. The control electronics, or “the brain” of the VSD which includes a CPU and various other frequency converter circuitry include many of the components that are most prone to either failure or obsolesce over time. Repair, replacement and/or updating hardware of the control electronics is easily performed on the topside. Furthermore, according to some embodiments, locating the control electronics and CPU on the topside can avoid the use of a subsea air-filled atmospheric enclosure for protecting those components subsea. The gate drivers and semiconductor switches are easier to make pressure tolerant than the control electronics. 
       FIG. 1  is a diagram illustrating topside and subsea environments in which a subsea motor driven by a VSD having topside control and subsea switching can be deployed, according to some embodiments. On sea floor  100  a station  120  is shown which is downstream of several wellheads being used, for example, to produce hydrocarbon-bearing fluid from a subterranean rock formation. Station  120  includes a subsea pumping module  140 , which is powered by an electric induction motor. The station  120  is connected to one or more umbilical cables, such as umbilical  132 . The umbilicals in this case are being run from a platform  112  through seawater  102 , along sea floor  100  and to station  120 . In other cases, the umbilicals may be run from some other surface facility such as a floating production, storage and offloading unit (FPSO), or a shore-based facility. In many cases to reduce energy losses, it is desirable to transmit energy through the umbilicals at higher voltages than is used by the electric motors in pump module  140 . Station  120  thus also includes a step-down transformer  130 , which converts the higher-voltage three-phase power being transmitted over the umbilical  132  to lower-voltage three-phase power for use by pump module  140 . The motor within pumping module  140  is driven with a variable speed drive (VSD) that includes topside control, within platform  112  and a subsea switching module  150 . Umbilical  132  also has one or more data lines for transmission of control signals for switching module  150 . According to some embodiments, the one or more data lines include one or more optical fibers. The umbilical  132  can also be used to supply barrier and other fluids, and control and other data lines for use with the subsea equipment in station  120 . Although a pumping module  140  is shown in  FIG. 1 , according to some embodiments the VSD having topside control and subsea switching is used to drive electric motors for other applications, including subsea compressors and/or subsea separators. 
       FIG. 2  is a block diagram illustrating further aspects of a subsea motor driven by a VSD having topside control and subsea switching, according to some embodiments. According to some embodiments, the functionality of a conventional VSD is “split” with the CPU with frequency converter control being located topside and the low-level switching hardware being located subsea with the subsea motor. Normally the “brain” of the converter with CPU, IO, and Communication etc., is located in the same “cabinet” or enclosure as the entire frequency converter. For subsea applications this means a number of circuit boards and other electronic components need to be subsea qualified. Furthermore, any failures tend to be rather significant since an intervention is likely to be used to replace the circuitry. A conventional frequency converter is multipurpose, where some applications rely on high-speed control. In such applications it is important to have low time-delay between the control system doing the calculations and the switching elements in the inverter. However, in many subsea applications, most or all of the loads are motors for pumps and the like, which do not rely on such high speed. Therefore, it has been found that it is in many cases acceptable to put the control system with the calculations and control algorithms topside, leaving only the gate-drivers and a multiplexer subsea. 
     According to some embodiments, at the surface platform  112 , the “brain” of the VSD is located, namely the topside frequency converter control system with CPU  210 . According to some embodiments, control system  210  includes a CPU, I/O, and communications, and a user interface (e.g. a keypad interface and display). The control system  210  sets (or receives instructions for) the operating speed of the electric motor  202  that drives subsea pump  204  in subsea pump module  140 . According to some embodiments, the control system  210  generates pulse-width modulated control signals that cause subsea switching module  150  to generate sine-like voltage waveforms for each of three-phases in order to drive motor  202  at the selected frequency. According to some embodiments, switching patterns other than pulse-width modulation are used, such as for example: customized pulse patterns, hysteresis switching patterns and other switching patterns. By locating control system  210  (including the brain/CPU) topside, the system is flexible in that control signals generated using various methods and different manufacturers/vendors can be paired with the general subsea switching module  150 . 
     According to some embodiments, the control signals generated by control system  210  are multiplexed using high-speed multiplexer  222  on the surface such that the signals can be sent via a single fiber optic communication link  220  through the seawater surface  200  to the subsea station (e.g. station  120  in  FIG. 1 ) where switching module  150  and pump module  140  are located. The multiplexed control signals are received by subsea high-speed de-multiplexer  224  which distributes the control signals via optical fiber links  230 ,  240 ,  250  and  260  to gate drivers  232 ,  242 ,  252  and  262 . The gate drivers in response to the control signals generate the appropriate gate voltages and currents to switch (turn on or off) the semiconductor power switches  234 ,  244 ,  254  and  264 . The power switches, operated according to the gate driver signals, generate the three phase pseudo-sine wave AC voltages driving motor  202  in pump module  140 . Note that although four gate driver/switches are shown, in general there will be different numbers depending on the particular application. Generally, there will be at least 6 gate drivers and semiconductor switches although in some cases there can be much greater numbers. Also, according to some embodiments, various types of gate-controlled semiconducting switches can be used such as: insulated-gate bipolar transistor (IGBT); integrated gate-commutated thyristor (IGCT); and/or gate turn-off thyristor (GTO) switching devices. 
     Also shown in  FIG. 2  according to some embodiments is supply power system  270  that includes a topside three-phase power supply  272  and topside step-up transformer  274 . The high-voltage power is transmitted to the subsea station via electrical power transmission lines  276  within an umbilical (such as umbilical  132  shown in  FIG. 1 ). System  270  also includes subsea step-down transformer  130  and rectifier system  278 , which converts the three-phase power to DC current for use by semiconductor power switches  234 ,  244 ,  254  and  264 . According to some embodiments, rectifier system  274  is a diode bridge rectifier system that is configured to convert the three-phase electric power into direct current (DC). According to some other embodiments, rectifier system  278  is configured with active switching capability using gate drivers and semiconductor switches that are similar or identical to those used in switching module  150 . In such cases, the “active front end” rectifier system  278  can be controlled using a topside CPU/brain that is similar or identical to control system  210  in the manner described herein. Such active front end system may be desirable in some situations such as, for example, where regenerative braking is used or in cases where current source converters (e.g. LCI-converters) are used. 
     The components of switching module  150 , namely the demultiplexer  224 , gate drivers  232 ,  242 ,  252  and  262  and semiconductor power switches  234 ,  244 ,  254  and  264  are easier to make pressure tolerant than the components in control system  210 . In some cases the components of switching module  150  are pressure compensated in an oil-filled enclosure. In such cases the use of a subsea gas-filled enclosure can be avoided altogether. Even in cases where some or all of the components of switching module  150  are contained in a gas-filled enclosure, there are far fewer components in the enclosure in comparison to designs that locate the control system electronics subsea. 
     According to some embodiments the transformers  274  and  130  are not used. According to some other embodiments, multiplexer  222  and demultiplexer  224  are not used. In such cases, the communication link  220  includes one or two optical fibers for each gate driver. 
     Although the embodiments described in  FIGS. 1 and 2  thus far are in the context of a subsea pumping module for developing subsea oil and gas, according to some embodiments, the techniques described herein are applicable to any subsea converters where gate-controlled semiconducting switches are used. Examples of subsea settings where topside control signals can be used to control subsea switching include: high-voltage, direct current (HVDC) electric power transmission systems; subsea DC-DC converters; subsea uninterruptible power supply (UPS) systems; subsea semiconducting circuit breakers; and subsea VAR compensator systems. 
     While the subject disclosure is described through the above embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while some embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the subject disclosure should not be viewed as limited except by the scope and spirit of the appended claims.