Abstract:
According to some embodiments, a subsea fluid processing system is described that includes a subsea electric motor that rotates a motor shaft about a central axis. A subsea fluid processing machine driven by a second shaft being rotated about the central axis. A subsea gear train system includes a plurality of gears positioned in one or more at least partially or fully oil-filled volumes. The plurality of gears are configured and arranged to transmit power from the first shaft to the second shaft wherein one revolution of the first shaft causes greater than one revolution of the second shaft. In some examples the gear form an epicyclic gear train arrangement.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to using subsea electric motors to drive subsea fluid processing equipment. More particularly, the present disclosure relates to electric motor driven gear train systems for subsea fluid processing equipment. 
       BACKGROUND 
       [0002]    In subsea fluid-processing applications such as subsea pumps and subsea compressors, the fluid processing equipment is typically directly driven by a subsea electric motor. In some cases a coupling is provided between the drive shaft of the electric motor and the shaft of the pump or compressor, so as to provide the ability to tolerate axial elongation and/or shrinkage. However, even in such cases the rotational speed of the load shaft of the pump or compressor is the same as the rotational speed of the electric motor drive shaft. 
         [0003]    It is desirable to provide increasingly higher power and higher capacity subsea pumps and subsea compressors. It is also desirable to provide increasingly higher differential pressure for such rotating equipment. When working with a given speed range of a subsea electric motor, higher differential pressures and/or higher capacities can be obtained by increasing the diameter of the impeller elements of the pump or compressor. However, this can cause undesirable effects, such as increased loads on various components. 
         [0004]    While subsea electric motors can be designed with higher speed capabilities, in many cases this is undesirable. For example, higher speed motor designs can suffer from greater viscous losses, as typical motors for subsea use are liquid filled and liquid cooled. Furthermore, in some cases there are significant efficiency losses in transmitting electric power at higher frequencies used to drive the motor at higher rpms. 
       SUMMARY 
       [0005]    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. 
         [0006]    According to some embodiments, a subsea gear train system is described. The system includes a plurality of gears arranged and configured to transmit power from a first shaft driven by a subsea electric motor to a second shaft that drives a subsea fluid processing machine. The gearing is configured such that one revolution of the first shaft causes greater than one revolution of the second shaft. 
         [0007]    According to some embodiments, the gears are positioned in one or more volumes each of which is partially filled with an oil. According to some embodiments the volume or volumes are completely filled with an oil. According to some embodiments, the oil has a viscosity grade of at most 32 centistokes at 40° C. According to some embodiments, the oil has a viscosity grade of at most 10 centistokes at 40° C. 
         [0008]    According to some embodiments, the gears form an epicyclical gear train that includes: a sun gear having a plurality of teeth about an outer periphery; a plurality of (e.g. three, four, or more) planetary gears supported by a carrier, each having teeth about an outer periphery and positioned such that the teeth of each planetary teeth mesh with the teeth of the sun gear; and a non-rotating annular gear having a plurality of teeth about an inner periphery and positioned such that the teeth of the annular gear mesh with the teeth of each planetary gear. According to some embodiments, the plurality of gears are arranged and configured such that one revolution of the first shaft causes at least 1.5, 2 or 3 revolutions of the second shaft. 
         [0009]    According to some embodiments, the subsea fluid processing machine is a subsea pump, such as a helico axial impeller pump or a centrifugal impeller pump. According to some other embodiments, the subsea processing machine is a subsea compressor. According to some embodiments the subsea processing machine is an electrical submersible pump. 
         [0010]    According to some embodiments the sun gear, planetary gears and annular gear are straight-cut gears. According to some other embodiments the sun gear, planetary gears and annular gear are helical gears. The gears can be configured such that they generate an axial force that at least partially counteracts an axial force on the second shaft generated during operation of the fluid processing machine. According to some embodiments, the gear train includes pairs of helical gears configured such that axial forces generated by the helical gears tend to counteract each other. According to some embodiments, first shaft and the carrier include one or more conduits configured to carry cooled lubricating barrier fluid towards bearing surfaces for each of the plurality of planetary gears. 
         [0011]    According to some embodiments, a subsea fluid processing system is described that includes: a subsea electric motor configured to rotate a first shaft about a central axis; a subsea fluid processing machine driven by a second shaft being rotated about the central axis; and a subsea gear train system including a plurality of gears positioned in one or more at least partially oil-filled volumes, the plurality of gears being configured and arranged to transmit power from the first shaft to the second shaft wherein one revolution of the first shaft causes greater than one revolution of the second shaft. According to some embodiments, the one or more at least partially oil-filled volumes are completely filled with oil. 
         [0012]    According to some embodiments an electrical submersible pump system is described that includes: a submersible electric motor configured to rotate a first shaft about a central axis; a submersible pump driven by a second shaft being rotated about the central axis; and a submersible gear train system including a plurality of gears positioned in one or more at least partially oil-filled volumes, the plurality of gears being configured and arranged to transmit power from the first shaft to the second shaft wherein one revolution of the first shaft causes greater than one revolution of the second shaft. According to some embodiments, the one or more at least partially oil-filled volumes are completely filled with oil. 
         [0013]    According to some embodiments, a method of driving a subsea fluid processing machine is described that includes: powering a subsea electric motor that applies torque to and thereby rotates a first shaft; and transmitting power using a gear train system from the first shaft to a second shaft that drives the subsea fluid processing machine. The gear train system includes a plurality of gears arranged and configured such that one revolution of the first shaft causes greater than one revolution of the second shaft. 
         [0014]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    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: 
           [0016]      FIG. 1  is a diagram illustrating a subsea environment in which a subsea gear train system can be deployed, according to some embodiments; 
           [0017]      FIG. 2  is a diagram illustrating a subsea pumping or compressor module in which a subsea gear train system can be deployed, according to some embodiments; 
           [0018]      FIG. 3  is a diagram illustrating further details of a subsea pumping or compressor module in which a subsea gear train system can be deployed, according to some embodiments; 
           [0019]      FIG. 4  is a diagram illustrating further details of a subsea gear train system, according to some embodiments; 
           [0020]      FIG. 5  is a diagram illustrating aspects of an epicyclic gear train arrangement used in a subsea gear train system, according to some embodiments; and 
           [0021]      FIG. 6  is a diagram illustrating aspects of a helical gear train arrangement used in a subsea gear train system, according to some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    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. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “subsea” and “submersible” shall be considered alike and intended to mean either under the sea surface or downhole. As such, for example, “a subsea fluid processing” machine can be installed on any location between the sea floor and the sea surface or on any subterranean location, for example inside an oil or gas well. 
         [0023]    Pump and compressors comprise a motor and a pump/compressor portion. In some embodiments, the motor and the overall lubrication system might be 100% liquid filled. The oil is circulated around the motor internals, and the pump or compressor bearings and seals to lubricate and cool vital parts of the machine. This cooling and lubrication system is often described as the barrier fluid system. In typical rotating equipment such as pumps and compressors, an overpressure is often applied in the barrier fluid system versus to keep the rotating equipment internals clean at all times and to avoid any processing fluid intrusion. 
         [0024]    In the current commercial setting, it is desirable to provide higher power and higher capacity in pumps and compressors. It is also desirable to provide rotating equipment (such as pumps and compressors) that has higher differential pressures. According to some embodiments, a gear train system such as a planetary gearbox is provided between the electric motor and the pump and/or compressor. The gear train system can be configured to achieve higher capacity and higher efficiencies especially for a helico axial impeller pumps, such as used in multiphase pumps/compressors, as centrifugal impeller pumps, such as used in single phase pumps/compressors. It is also desirable to provide a subsea electric motor that has lower rates of barrier fluid viscous losses, which can be achieved by tuning the motor and power supply system to provide better efficiency at lower rpms. 
         [0025]      FIG. 1  is a diagram illustrating a subsea environment in which a subsea gear train system can be deployed, according to some embodiments. On sea floor  100  a subsea 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 motor, such as an induction motor or permanent magnet 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. The umbilical  132  can also be used to supply barrier and other fluids, and control and 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 module  140  can be configured for other subsea fluid processing functions, such as a subsea compressor module and/or a subsea separator module. In all embodiments described herein, it is understood that references to subsea pumps and pumping modules can alternatively refer to subsea compressors and compressor modules. Furthermore, references herein to subsea pumps and subsea compressors should be understood to refer equally to subsea pumps and compressors for single phase liquids, single phase gases, or multiphase fluids. According to some embodiments, the subsea gear train system described herein is used in connection with a electrical submersible pump (ESP)  150  which can either be located downhole, as shown wellbore  154  in  FIG. 1  or it can be located in a subsea location such as on the sea floor in a christmas tree at wellhead  152  or other equipment. Thus in all embodiments described herein, it is understood that references to subsea pump and pumping modules can alternatively refer to ESPs whether deployed downhole or in a subsea location. 
         [0026]      FIG. 2  is a diagram illustrating a subsea pumping module in which a subsea gear train system can be deployed, according to some embodiments. Portions of pumping module  140  is shown, including subsea electric motor  200  and subsea pump  210 . Motor  200  is filled with an insulating lubricating oil, or other barrier fluid, that is supplied via an umbilical from the surface (as shown in  FIG. 1 ). According to some embodiments, motor  200  also includes a circumferentially arranged barrier fluid cooling coil, not shown. Motor  200  includes stator  204  and rotor  206 , which act to rotate the rotor  206  and motor shaft about central axis  202 . Subsea gear train system  220  transmits the rotation of motor shaft  230  to rotation of pump shaft  250 . According to some embodiments, the gear train system  220  is configured with a gear ratio of less than unity such that the pump shaft  250  revolves faster than the motor shaft  230 . In the subsea pump  210 , the pump shaft drives a plurality of vertically stacked impeller stages  216 . In a pump arrangement such as shown in  FIG. 2 , fluid enters from pump inlet  212  and travels upwards though each successive impeller stage that increases fluid pressure and exits via pump outlet  214 . 
         [0027]    It has been found that by providing a gear train system such as system  220  the motor  200  can be run at lower rpms. This results in lower barrier fluid viscous losses in motor  200 . Additionally, operating motor  200  at lower rpms allows for increased power transmission efficiency associated with lower frequency electric supply power that is transmitted via umbilical cabling such as umbilical  132  in  FIG. 1 . This can be significant especially for longer step out distances without using transformers at a given power cable cross section. Additionally, other benefits can be achieved such as increased power system flexibility. Finally, by using a gear train system such as system  200 , higher differential pressures and/or higher capacity flow rates can be obtained from the pump for a given pump diameter (e.g. pump diameter  218  shown in  FIG. 2 ). While higher pressures and/or higher capacities can be obtained by increasing the designed diameter of the impeller elements of the pump or compressor, larger diameter designs have other problems such as increased loads on bearings and seals. Thus, by providing a gear train system, the pump design can be optimized to run at higher rpm ranges while the motor and power transmission systems can be optimized to run at lower rpms ranges. 
         [0028]    While gearboxes designed for surface applications typically make use of relatively high viscosity oil, such as oils optimized for air-filled gear transmissions, a design goal in subsea applications is to provide a gearbox that is robust using relatively low viscosity oil. This is because in subsea applications the gear train is completely or nearly completely surrounded by oil and using high viscosity oil may increase losses to a point where other efficiency benefits of the gear train are outweighed. 
         [0029]      FIG. 3  is a diagram illustrating further details of a subsea pumping module in which a subsea gear train system can be deployed, according to some embodiments. As shown the example of  FIG. 3 , gear train system  220  can be an epicyclic gear train arrangement. Sun gear  310  is fixed to pump shaft  250 . Ring gear  314  is fixed to the housing of the pump module—in this case to the housing of pump  210 . Both sun gear  310  and ring gear  314  are concentric about the central axis  202 . A carrier  316  carries, via bearings, a plurality of planet gears, of which two gears  312  and  322  are visible in  FIG. 3 . The carrier  316  is fixed to the motor shaft  230  such that each of the planet gears rolls around the sun gear  310 . According to some embodiments, the epicyclic gear train arrangement shown in  FIG. 3  can replace an existing simple gear coupling that exists between a motor and pump or compressor, and can allow for the same tolerances of axial shaft expansion. 
         [0030]      FIG. 4  is a diagram illustrating further details of a subsea gear train system, according to some embodiments. Gear train system  220 , in the example shown in  FIG. 4  is also an epicyclic gear train arrangement with sun gear  310 , planet gear  312 , ring gear  314 , and carrier  316 . Note that while only one planet gear  312  is shown in  FIG. 4  for clarity, greater numbers of planet gears are provided according to some embodiments. In some embodiments three, four or more planet gears are provided. In the example shown in  FIG. 4 , the planet gears are fitted with bearings to which cooled and overpressurized oil is fed. Oil is supplied from a source of cooled higher pressure oil located near the top of the motor. The cooled pressurized oil travels through conduit  410  in shaft  230  and through conduit  412  in carrier  316 . The oil exits to the bearing via orifice  414 . 
         [0031]      FIG. 5  is a diagram illustrating aspects of an epicyclic gear train arrangement used in a subsea gear train system, according to some embodiments. In this example, the gear train system  220  includes four planet gears  312 ,  322 ,  512  and  522 . The planet gears roll around the central sun gear  310 . According to other embodiments, other numbers of planet gears, such as 3, 5, 6, 7 or 8 can be provided. Note that as carrier  316  is rotated by motor shaft  230  (not shown), in the direction of arrow  530 , each of the planet gears  312 ,  522 ,  322  and  512  will rotate about their own axes as shown by arrows  540 ,  542 ,  544  and  546 , respectively. Note that each of the planet gears also revolves about the central axis  202 , although this is not shown by the arrows. The sun gear will rotate about axis  202  as shown by arrow  550 . According to some embodiments, the gear train system  220  is configured with a gear ratio of less than unity such that the carrier  316  rotates about axis  202  at a slower rate than sun gear  310 . Thus the output speed to the pump is greater than the input speed provided by the motor. 
         [0032]    Subsea pumps and subsea compressors typically have strict tolerances for vibration levels. According to some embodiments, the sun, planet and ring gears in the gear train  220  are helical gears, which create less vibration in a gearbox when compared to straight gears. According to some embodiments, mechanical couplings can be provided on either side, or both sides, of the system  220  purposes. 
         [0033]    Using helical gears can generate axial forces that, according to some embodiments, can be designed to counter-balance other known axial forces in the system. For example, in  FIG. 2  the pump  210  will exert a downwards force on the pump shaft  250  as the impeller stages accelerate the fluid upwards. Helical gears in system  220  can be designed to partially counteract such downward force. According to some embodiments, other techniques such as the use of thrust bearings on the pump and/or motor shafts can be employed to accommodate axial forces generated by the helical gearing. Engineering to balance axial forces can be done several ways. 
         [0034]      FIG. 6  is a diagram illustrating aspects of a helical gear train arrangement used in a subsea gear train system, according to some embodiments. In this example, each helical gear is doubled. There are two sun gears  610  and  612 , and two of each planet gears (pairs  620 / 622  and  630 / 632  are visible in  FIG. 6 ). There are also two ring gears, not shown. Each pair of gears is fixed via carriers (such as carriers  614 ,  624  and  634 ). The doubling arrangement shown in  FIG. 6  allows for axial force balancing. The net axial force can be designed to be zero (or nearly zero) or, according to some embodiments, the net axial force can be designed to beneficially counteract the axial force generated by the pump. Another alternative to the arrangement shown in  FIG. 6  is to use double-helical or herringbone gears. 
         [0035]    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.