Patent Publication Number: US-2009232664-A1

Title: Permanent magnet motor for subsea pump drive

Description:
BACKGROUND 
     The invention relates generally to subsea pumping systems and methods, and more specifically a canned permanent magnet motor for a subsea pump drive. 
     The need for subsea pumps in the oil and gas industry has been increasing due to increasing energy requirements, and because onshore energy sources are becoming more scarce. These industries must now look for energy sources offshore; and the distance between shore and subsea fields continues to increase. 
     Electrical motors have been selected as a standard to drive the subsea pumps due to ease of power transfer over long distances when compared to other drive systems and methods, including, for example, hydraulic driven pumps. Conventional systems and methods employ induction motors for driving the aforesaid subsea pumps. Use of induction type motors has been problematic however, since induction motors are low efficiency and low power factor motors. This low efficiency and low power factor undesirably require an oversized umbilical connection and variable frequency converter on the topside in order to provide a large amount of VAR power to the subsea motor. Both, the oversized umbilical connection and variable frequency converter undesirably increase the cost to the subsea pumping system. 
     It would be both advantageous and beneficial to provide a subsea pumping system that overcomes the problems generally associated with subsea pumping systems that employ induction motors. The subsea pumping system should have an overall efficiency that is greater than known subsea pumping systems utilizing induction motors, such that the subsea pumping system could function using a low power rating umbilical. It would be further advantageous if the subsea pumping system had a higher power factor than known subsea pumping systems utilizing induction motors, such that the subsea pumping system could function using a low power rating topside variable frequency converter. 
     BRIEF DESCRIPTION 
     Briefly, in accordance with one embodiment, a subsea pump drive motor comprises a stator, a rotor comprising a plurality of permanent magnet pole pieces, and a non-magnetic can configured to affix the pole pieces to the rotor. 
     According to another embodiment, a subsea pump drive system comprises a permanent magnet subsea pump drive motor having a rotor configured with a plurality of permanent magnet pole pieces, the rotor and plurality of pole pieces disposed within a non-magnetic can configured to prevent corrosion of the rotor and plurality of pole pieces. 
     According to yet another embodiment, a method of controlling a subsea pump comprises: 
     providing a permanent magnet (PM) subsea pump drive motor; and 
     controlling the PM drive motor such that the PM drive motor drives a subsea pump in response to variable frequency converter signals received by the PM drive motor. 
    
    
     
       DRAWINGS 
       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 in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates a permanent magnet motor subsea pump drive according to one embodiment of the invention; 
         FIG. 2  illustrates the permanent magnet motor subsea pump drive depicted in  FIG. 1 , but that does not have a wireless transmitter such as depicted in  FIG. 1 ; 
         FIG. 3  illustrates in more detail, the rotor portion of the permanent magnet motor depicted in  FIGS. 1 and 2 , according to one embodiment; 
         FIG. 4  is a cross-sectional view of the permanent magnet motor depicted in  FIGS. 1 and 2 , according to one embodiment; 
         FIG. 5  illustrates a permanent magnet motor subsea pump drive according to another embodiment of the invention; and 
         FIG. 6  the permanent magnet motor subsea pump drive depicted in  FIG. 5 , but that does not have a wireless transmitter such as depicted in  FIG. 5 . 
     
    
    
     While the above-identified drawing figures set forth alternative 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 
       FIG. 1  illustrates a permanent magnet motor subsea pump drive  10  according to one embodiment of the invention. Subsea pump drive  10  includes a permanent magnet motor  12  comprising a stator  14  and a rotor  16 . Windings  20  are disposed in stator slots. The rotor  16  comprises a plurality of permanent magnet (PM) poles described herein below with reference to  FIGS. 3 and 4 . The rotor  16  also includes a non-magnetic can configured to fix the permanent magnets to the rotor  16 , also described further herein below with reference to  FIG. 3 . 
     Four sets of blades  22  are disposed on the rotor shaft  24 . These blades  22  are configured to pump cooling fluid  26  flowing through a motor sealing can  28  that encapsulates both the stator  14  and the rotor  16  according to one aspect of the invention illustrated in  FIGS. 5 and 6 , where both the stator  14  and the rotor  16  are canned for corrosion protection when processed fluid is used for cooling. The cooling fluid  26  works to provide cooling of the stator  14 , rotor  16 , and the associated bearings. In another embodiment illustrated in  FIGS. 1 and 2 , the stator  14  is not canned, and the machine cavity  30  is filled with a clean cooling fluid having a suitable thermal conductivity while also possessing workable electrical insulation characteristics. A heat exchanger  34  operates to transfer heat from the motor  12  to outside seawater. 
     A rotor  16  position signal generated via an encoder  32  is transferred to a variable frequency converter (VFD)  35  via a wireless signal transmitter  36  according to one embodiment. In another embodiment depicted in  FIG. 2 , the rotor position signal is transferred to a VFD via a suitable communication cable ( 40 ). 
     The encoder  32  is connected to one end of rotor shaft  24  to detector rotor position for proper speed/torque control of the permanent magnet motor  12 . Traditional control approaches utilizing communication cables are difficult to employ when the VFD  35  is far away from the motor  12  due to signal attenuation along cables between the motor  12  and the VFD  35 . Further, traditional sensorless control approaches also face challenges due to difficulties associated with accurate measurement of motor terminal voltages through such long distances. 
     The foregoing challenges associated with traditional control approaches utilizing communication cables are overcome using a wireless signal transmitter  36 , discussed herein above. The rotor position signals are sent to the wireless signal transmitter  36 , which then transmits the rotor position signals to a topside controller/VFD  35  that is used to drive the PM motor  12 . 
       FIG. 2  illustrates the permanent magnet motor subsea pump drive depicted in  FIG. 1 , but that does not have a wireless transmitter such as depicted in  FIG. 1 . The rotor position signal is transferred through suitable communication wires  40 . This topology is especially useful when a long cable is not required, i.e. a subsea VFD  38  is employed and is located in close proximity to the PM motor  12 . 
     The end of the rotor shaft  24  opposite the end connected to the encoder  32  is connected to a subsea pump  40 , such as a multiphase pump. There is a seal  42  between the motor  12  and pump  40  to block motor cooling fluid  26  from flowing into the pump  40 . The fluid pressure inside the motor  12  is normally maintained higher than the fluid pressure inside the pump  40  via a pressurizer typically located subsea beside the motor  12 , such as described below with reference to  FIGS. 5 and 6 , to prevent any processed fluid  44  flowing into the motor side from the pump side. Any motor cooling fluid leakage that may pass from the motor side into the pump side that occurs during motor-pump set rotation is replenished via a topside fluid tank  46  that is connected to the subsea motor  12  through an umbilical supply line  48  to provide cooling fluid as needed. 
       FIG. 3  illustrates in more detail, the rotor  16  portion of the permanent magnet motor  12  depicted in  FIGS. 1 and 2 , according to one embodiment. A nonmagnetic can  50  that is constructed from a suitable nonmagnetic material such as, without limitation, inconel or aluminum, is configured to attach a plurality of magnets  52  to the rotor core or back iron portion  54  of the rotor  16 , and to protect each magnet from corrosion. The back iron portion  54  is constructed from a suitable ferromagnetic material. 
       FIG. 4  is a cross-sectional view of the permanent magnet motor  12  depicted in  FIGS. 1 and 2 , according to one embodiment. One portion of the motor shaft  24  is encapsulated via the rotor core  54 . The permanent magnets  52  having north and south poles, are attached to the rotor core  54  via the rotor can  50 . Stator laminations  56  having slots  58  surround the rotor can  50 . 
       FIG. 5  illustrates a permanent magnet motor subsea pump drive  100  according to another embodiment of the invention. Pump drive  100  includes a permanent magnet motor  102  that is cooled using the fluid  44  processed by the subsea pump  40 . Subsea pump drive  100  does not require a topside storage tank or associated umbilical cooling fluid supply line such as employed by pump drive  10  described above with reference to  FIGS. 1 and 2 . 
     A pressurizer  104  is employed to maintain a positive pressure from the motor  12  to the subsea pump  40  under all conditions. An optional liquid storage tank  106  can be used to store processed fluid  44  for motor cooling purposes when the processed fluid is purely gas. 
     The stator  14  is also encapsulated via a can  108  to prevent any process fluid  44  or gas from entering the stator  14  portion of the permanent magnet motor  102 . This stator can  108  is filled with a clean cooling fluid  26 , such as a suitable oil, to cool the stator  14 . A heat exchanger  34  can be employed to exchange heat from the motor  102  to outside seawater. 
     Subsea pump drive  100  also employs an encoder  32  that is connected to one end of rotor shaft  24  to detector rotor position for proper speed/torque control of the permanent magnet motor  102 . A rotor  16  position signal generated via the encoder  32  is transferred to a variable frequency converter (VFD)  35  via a wireless signal transmitter  36  according to one embodiment. In another embodiment depicted in  FIG. 6 , the rotor position signal is transferred to a VFD  38  via a suitable communication cable ( 40 ) and does not have a wireless transmitter such as depicted in  FIG. 5 . 
     In summary explanation, a subsea pump drive employs a permanent magnet (PM) motor to drive a subsea pump. The PM motor rotor in one embodiment is canned with a non-magnetic material such as inconel, that can provide a desired level of corrosion protection. The PM motor provides a subsea pump drive that is smaller and more efficient, having a high power factor than a subsea pump drive utilizing a conventional induction motor. The PM motor subsea pump drive eliminates the necessity for a topside storage tank and associated fluid transfer lines when the motor rotor is cooled with processed fluid. 
     The PM subsea pump drive motor achieves its high efficiency due to the permanent magnetic flux on the rotor linking the stator so that the PM motor can achieve higher efficiency due to absence of rotor current. 
     The PM subsea pump drive motor further has an increased power factor due to the absence of exciting current. 
     The PM subsea pump drive motor employs lower power umbilical features due to the aforesaid high power factor and high motor efficiency. 
     The PM subsea pump drive motor employs a lower power topside variable frequency converter due to the aforesaid high power factor and high motor efficiency. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.