Abstract:
The motor of an electrical submersible pump generates a significant amount of heat that can be removed by transferring it to the well production fluid. Grooves in the stator and motor housing facilitate more rapid heat transfer from the rotor and stator, through the motor lubricant, to the motor housing. Increased heat transfer to the motor housing facilitates increased heat transfer to the production fluid on the outside of the motor housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to provisional application 61/165,339, filed Mar. 31, 2009. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates in general to well pumps, and in particular to an electrical submersible pump motor using internal oil circulation to increase heat transfer. 
       BACKGROUND 
       [0003]    Electrical submersible pumps (“ESP”) can be used to pump fluid from a wellbore towards the surface of the earth. The ESP is inserted inside the wellbore, generally at great depths below the surface of the earth. The ESP includes a pump assembly, a motor, and a seal section between the pump and the motor. The motor includes a rotor that rotates within a stator. The rotor rotates on bearings which are connected to the stator. The bearings can generate a significant amount of heat that must be removed. Heat may also be generated by other heat sources, such as, for example, electrical resistance in the windings of the stator, rotor, and in the laminations of the motor. Failure to remove the heat can significantly shorten the life of the motor. To remove the heat, it is desirable to move the heat from the rotor and stator to the motor housing. The heat is then conducted through the motor housing to wellbore fluid located outside of the motor housing. There is a problem, however, in transferring the heat from the stator to the housing. 
         [0004]    In a typical motor, there is a slight gap between the stator and the motor housing. The gap is necessary to be able to install and remove the stator from the housing. Unfortunately, the gap is generally filled with air, which is a poor heat conductor. 
         [0005]    It is desirable to efficiently transfer heat from the stator to the motor housing. 
       SUMMARY OF THE INVENTION 
       [0006]    In this invention, internal grooves are used to facilitate lubricant flow between the stator and the motor housing in an electrical submersible pump (“ESP”) motor. The lubricant flow between the stator and the housing increases the rate of heat transfer from the stator to the housing, and therefore increases the rate of heat transfer from the housing to production fluid in contact with the exterior of the housing. 
         [0007]    In some embodiments, grooves are formed on the interior of the motor housing. The grooves may extend longitudinally past each end of the stator, from an oil reservoir at one end of the housing to an oil reservoir at the other end of the housing. In various embodiments, the grooves may be longitudinal, circumferential, or helical. Furthermore, a plurality of groove types may be used in a single embodiment. In some embodiments, grooves on the interior of the housing create a corresponding ridge on the exterior of the housing. 
         [0008]    In some embodiments, grooves are formed on the exterior of the stator. The grooves may extend from one end of the stator to the other. Like the housing grooves, the stator grooves may be longitudinal, circumferential, or helical. A plurality of groove types may be used. Stator grooves may be used in the same embodiment as housing grooves. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic view of a pump assembly in accordance with an embodiment of the invention in a wellbore. 
           [0010]      FIG. 2  is a sectional view of a motor housing of the motor in  FIG. 1  with internal oil grooves. 
           [0011]      FIG. 3  is a cross-sectional view of the motor housing from  FIG. 2 , taken along the line  3 - 3  of  FIG. 2  to illustrate longitudinal grooves. 
           [0012]      FIG. 4  is a cross-sectional view of an alternative embodiment of a motor housing having circumferential grooves. 
           [0013]      FIG. 5  is a cross-sectional view of another alternative embodiment of a motor housing having longitudinal and helical grooves. 
           [0014]      FIG. 6  is a sectional view of another alternative embodiment of a motor housing with internal longitudinal lubricant grooves and external ridges. 
           [0015]      FIG. 7  is a cross-sectional view of the motor housing of  FIG. 6 , taken along the line  7 - 7  of  FIG. 6 . 
           [0016]      FIG. 8  is a side-view of an embodiment of a stator having longitudinal lubricant grooves. 
           [0017]      FIG. 9  is a side view of another embodiment of a stator having helical lubricant grooves. 
           [0018]      FIG. 10  is a side view of another embodiment of a stator having circumferential lubricant grooves. 
           [0019]      FIG. 11  is a sectional view of an embodiment of the pump assembly of  FIG. 1 , having dimples on the pump motor housing. 
           [0020]      FIG. 12  is an orthogonal view of another embodiment of the shroud of  FIG. 1 , showing one half of a two-part clamshell shroud with fins. 
           [0021]      FIG. 13  is a side view of an alternative embodiment of the pump of  FIG. 1 , having external oil circulation tubes. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings which illustrate embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the prime notation, if used, indicates similar elements in alternative embodiments. 
         [0023]    Referring to  FIG. 1 , wellbore casing  10  is shown in a vertical orientation, but it could be inclined. Pump  12  is suspended inside casing  10  and is used to pump wellbore fluid up from the well. Wellbore fluid may be any kind of fluid including, for example, crude oil, water, gas, liquids, other downhole fluids, or fluids such as water that may be injected into a rock formation for secondary recovery operations. Indeed, wellbore fluid can include desired fluids produced from a well or by-product fluids that an operator desires to remove from a well. Pump  12  may be centrifugal or any other type of pump and may have an oil-water separator or a gas separator. Pump  12  is driven by a shaft  14 , operably connected to a motor  16 . Seal section  18  is mounted between the motor  16  and pump  12 . The seal section reduces a pressure differential between lubricant in the motor and well fluid. Motor  16  comprises housing  20 . Housing  20  can be a cylindrical housing, and typically encases the other components of motor  16 . Preferably, the fluid produced by the well (“production fluid”) flows past motor  16 , enters an intake  22  of pump  12 , and is pumped up through tubing  24 . Normally, motor  16  is located below the pump  12  in the wellbore. The production fluid may enter the pump  12  at a point above the motor  16 , such that the fluid is drawn up, past the motor housing  20  of the motor  16 , and into the pump inlet  22 . 
         [0024]    Stator  30  is stationarily mounted in housing  20 . Stator  30  comprises a large number of stator disks (laminations) having slots through them which are interlaced with three-phase copper windings. Stator  30  has an axial passage that extends through it. The clearance between the outer diameter (“OD”) of stator  30  and inner diameter (“ID”) of the housing  20  may be quite small. 
         [0025]    Rotor  32  is located within the stator  30  passage and is rotably mounted on a plurality of bearings, the bearings being located between the rotor and the stator. Rotor  32  is mounted to shaft  14 . Motor  16  has at least one rotor  32  and, in some embodiments, may have a plurality of rotors  32 . Each of the rotors  32  are mounted on bearings (not shown). Alternating current supplied to windings cause rotor  32  to rotate. Motor  16  may generate heat in a variety of ways. For example, friction caused by the rotation of rotor  32  can generate heat or electrical resistance in the windings of stator  30  and rotor  32  can generate heat. Indeed, a variety of electrical and mechanical components within motor  16  can generate heat. Lubricant within the motor  16  transfers heat from components of the motor  16  to motor housing  20 . Heat is then transferred from motor housing  20  to the production fluid on the outside of motor housing  20 . 
         [0026]    The rate of heat transfer is determined by the equation Q=h(A)(T); where Q=rate of heat transfer, h=the heat transfer coefficient, A=surface area, and T=the difference in temperature. The rate of heat transfer between the motor housing  20  and the production fluid may be increased by increasing (T), the difference in temperature between the motor housing and the production fluid. The difference in temperature may be increased by increasing the rate of heat transfer from the heat generating components of the motor  16 , such as the rotor  32  and stator  30 , to motor housing  20 . 
         [0027]    Motor  16  uses a lubricant to lubricate the moving parts such as rotor  32  and the bearings upon which rotor  32  is mounted. The lubricant could be, for example, a dielectric oil. In addition to lubricating the parts, the lubricant conducts heat from rotor  32  and stator  30  to the motor housing  20 . Motor  16  may be filled with lubricant, such that lubricant occupies any spaces within housing  20 . Lubricant pump  34  may be located in the lower end of housing  20 . Lubricant pump  34  pumps lubricant through motor  16 . 
         [0028]    Referring to  FIGS. 2 and 3 , in one embodiment, one or more longitudinal grooves  36  are formed in the ID of motor housing  20  by, for example, stamping or milling grooves parallel to the axis of the motor housing  20 . Longitudinal grooves  36  are parallel with the axis of housing  20 . The distance from the recessed surface  38 , which is the back of the grooved portion, to the axis of housing  20  is greater than the ID of the non-grooved portion  40 . Snap ring grooves  42  indicate the location of the ends of the stator  30 . The longitudinal grooves  36  intersect the circumferential snap ring grooves  42  and extend past the ends of the stator  30  so that oil may flow through the groove  36  from one end of the housing  20 , past the stator  30  to the other end of the housing  20 . 
         [0029]    In one embodiment, lower reservoir  44  may be a void, filled with lubricant, located at one end of motor housing  20 . Lubricant pump  34  ( FIG. 1 ) may be located within lower end space  44 . Upper reservoir  43  may be a void, filled with lubricant, located at the other end of motor housing  20 . Reservoirs  44  and  43  are typically located beyond the axial length of stator  30 . Lower reservoir  44  may be larger or smaller than upper reservoir  43 . Some embodiments may have just one reservoir  44 , or may have other voids, in different locations, that contain lubricant. 
         [0030]    In one embodiment, longitudinal grooves  36  are in communication with lower lubricant reservoir  44  and upper lubricant reservoir  43 . The number and spacing of longitudinal grooves  36  may vary. In the example there are four longitudinal grooves  36  equally spaced around the ID of housing  20 . 
         [0031]    Grooves  36  increase the surface area of the ID of the motor housing  20 . The increased surface area increases the rate of heat transfer between the lubricant and the motor housing  20 . A stator such as stator  30  in  FIG. 1  closely fits within housing  20 . Thus, a passage is defined by recessed surface  38 , sidewalls  39  of groove  36 , and an exterior surface of stator  30 . 
         [0032]    Grooves  36  thus provide a flow channel between stator  30  and housing  20 , allowing lubricant to flow between the stator  30  and the housing, and thus flow in and out of reservoirs  43 ,  44 . Lubricant pump  34  may cause the lubricant to flow through the passage associated with groove  36 , thus transferring heat from hotter regions of motor  16  to cooler regions of motor  16 . For example, heat can be transferred from stator  30  to housing  20 . Furthermore, the lubricant can be located within the annular gap between stator  30  and housing  20 , both within groove  36  and in the smaller gap outside of groove  36 . 
         [0033]    Furthermore, the irregular shape of the grooved ID on the motor housing  20  may create turbulence within the lubricant. The increased turbulence can increase the heat transfer coefficient (h) and thus increase the rate of heat transfer. In an exemplary embodiment (not shown), a series of longitudinal grooves is uniformly spaced around the circumference of the interior of the motor housing  20 , each groove having the same depth, thus creating a profile that is corrugated in appearance. Alternatively, the depths of the grooves or the depth within a groove may vary. 
         [0034]    Referring to  FIG. 4 , in another embodiment, circumferential grooves  45  are formed around the circumference of the ID of the motor housing. The circumferential grooves  45  follow a line around the circumference of the motor housing  20 , and may be used in combination with other grooves such as longitudinal grooves  36 . Circumferential grooves  45  may be located between the upper and lower ends of the stator so that they are intersected by longitudinal grooves  36 . The number and spacing of circumferential grooves  45  may vary. 
         [0035]    Referring to  FIG. 5 , in this embodiment helical grooves  46  extend in helical fashion around the circumference along the length of the ID of the motor housing  20 . The helical grooves  46  may be used with longitudinal grooves  36 . Furthermore, a single embodiment could use grooves running in multiple directions, such that some could be longitudinal, some could be circumferential, and some could be at an angle in relation to the axis of the motor housing  20 . Grooves such as circumferential grooves  45  and helical grooves  46  do not contain seals or snap rings; rather they comprise a void filled with lubricant. 
         [0036]    Referring to  FIGS. 6 and 7 , an internal groove  50  may also change the shape of the outer diameter (“OD”) of the motor housing  52 . The result would be a raised surface or rib  54  on the OD, such that the OD of the raised surface  54  is greater than the OD  56  of other portions of the motor housing  52 . Like the grooves  36  ( FIGS. 2 and 3 ), the raised surface  54  may be longitudinal as shown, circumferential, helical, or a combination thereof. The raised surfaces  54  can be used to increase the surface area of the exterior of the motor housing  52 , increase turbulence of the production fluid flowing past the housing, or both. The wall thickness of housing  52  radially outward from groove  50  may be substantially the same thickness as between grooves  50  due to raised surface  54 . 
         [0037]    Referring to  FIG. 8 , a stator  60  is a cylindrical component inside motor housing  20  ( FIG. 1 ). The outer diameter of the stator  60  is slightly smaller than the inner diameter of motor housing  20 . Stator  60  is made up of a large number of thin, flat metal discs (laminations) with windings passing through aligned slots in the discs. Stator  60  extends substantially the length of the motor housing  20 . The stator  60  defines a generally cylindrical outer diameter and central bore. The rotor  32  rotates inside the bore of fixed stator  60 , spinning the motor shaft  14 . In some embodiments, a plurality of rotors  32  rotate inside the bore of stator  60 . Each rotor  32  is made of thin, metal discs also grouped in segments. Longitudinal grooves  64  may be formed in the OD of the stator  60 . The groove or grooves  64  could be generally straight and extend from one end of the stator  60  to the other, parallel to the axis of stator  60 . The depth of the grooves  64  may be shallow, such as less than ⅛″, or it may be deeper. The width of the grooves  64  may vary from less than ⅛″ to greater. Stator  60  could be located in a housing that has a cylindrical ID free of any oil grooves such as those shown in  FIGS. 2-7 . Alternatively, stator  60  could be located in one of the housings having grooves, as shown in  FIGS. 2-7 . Each groove  64  defines a passage bounded on three sides by the three surfaces of groove  64 , and on the fourth side by in interior surface of housing  20 . 
         [0038]    Referring to  FIG. 9 , in this embodiment an internal helical groove  66  could extend about the cylindrical OD of stator  68  in helical fashion from one end to the other. Referring to  FIG. 10 , in this embodiment, the stator stack  70  may have circumferential grooves  72  on its OD that are circumferential about the OD of the stator  70  and may promote lateral lubricant flow. Any combination of longitudinal, circumferential, and helical grooves may be used. 
         [0039]    Grooves in the OD of stator stack define passages between the stator and housing. The passages promote lateral and linear lubricant movement to transfer heat to the motor housing more effectively. The grooves may also increase turbulence in the lubricant, increase the surface area that is exposed to the lubricant, and increase the volume of lubricant between the stator and the motor housing. 
         [0040]    An ESP motor comprising passages on the ID of the motor housing, OD of the stator, or both may be enhanced with other devices that increase the rate of heat transfer between the motor housing and the production fluid. A turbulator, for example, can be used to increase the turbulence of the wellbore fluid that is in contact with motor  16 . Turbulators are fully described in U.S. patent application Ser. No. 12/416,808, which is incorporated herein by reference. In one embodiment, the turbulator, can comprise shroud  80  ( FIG. 1 ). Passages on the ID of housing and/or the OD of the stator, for example, can increase the heat transfer from stator  30  to housing  20 , and then a turbulator can increase the heat transfer from housing  20  to the wellbore fluid. 
         [0041]    Referring to  FIG. 1 , the turbulator, shroud  80 , can have an open lower end  82  and an upper end sealingly secured around pump  12  above intake  22 . The shroud may be secured by other means and in other locations. The shroud  80  reduces the cross sectional area of the path of fluid flow and thus increases velocity. The higher velocity increases turbulence, which in turn increases the heat transfer coefficient (h) of the production fluid flow across the surface of the motor housing  20 . The shroud  80  may have an irregular sidewall shape  84  to create pockets of turbulence between the shroud  80  and the motor housing  20 . Furthermore, the motor housing  20  may have an irregular shape, such as dimples, to promote turbulence in the wellbore fluid as the wellbore fluid passes over the exterior of the motor housing. 
         [0042]    Referring to  FIG. 11 , the turbulator comprises multiple dimples  86  on motor housing  88  of motor  90 . The dimples  86  are indentations or protrusions in the exterior surface of motor housing  88 . The size of the indentations  86  may vary and could be, for example, made from a ¼″ or ½″ diameter round punch driven to a ⅛″ depth. Dimples  86  could also have a significantly larger or smaller diameter and be driven to a greater or lesser depth. Furthermore, the dimples  86  may have different shapes such as round, oval, square, and the like. The dimples  86  may be distributed about the surface in a symmetric pattern or they may be placed randomly. The dimples  86  may be concave or convex in relation to the exterior of the motor housing  88  and may be used regardless of whether a shroud is used. The dimples  86  increase the turbulence of the production fluid and thus increase the rate of heat transfer from the motor housing  88  to the production fluid. The dimples give the housing a textured surface. Other kinds of textured surfaces may also be used to increase turbulence. The dimples  86  may be used alone or in combination with other devices that increase production fluid turbulence. 
         [0043]    Referring to  FIG. 12 , in one embodiment, shroud  92  is a clamshell configuration, wherein the shroud can be separated into two or more components. Fins  94  may be installed on motor housing  20  ( FIG. 1 ) or shroud  92 . A fin  94  could, for example, be welded to the shroud  92  and contact or nearly contact the motor housing  20  when the motor  16  is installed. This embodiment overcomes the inherent manufacturing and maintenance difficulties associated with attaching fins  94  directly to the motor housing  20 , yet still creates turbulent flow immediately adjacent to the motor. 
         [0044]    The fins  94  may be oriented in a variety of positions. In one embodiment, the fins  94  are attached at a 90 degree angle or normal in relation to the wall of the shroud  92 . Fins  94  may be slanted in relation to the axis of the shroud  92 , such as at a 45 degree angle. As illustrated by group  96  of fins  94 , adjacent fins  94  may incline at the same inclination relative to the axis of shroud  92 . Also, some of the adjacent fins  94  may slant at alternating angles to each other. For example, one fin  94  is slanted at a 45 degree angle in one direction, and the adjacent fin is slanted at an opposing 45 degree angle in the opposite direction, such that the bottom most edges  98  of the fins  94  are nearest each other and the fins diverge as they go up along the axis of the shroud. Other fins  94  may have the same 90 degree opposed orientation, but with the top most part  100  of the fins  94  nearest each other. The angle between opposed sets of fins  98  could be any angle. The fins  94  may be set at any variety of angles, and the fins need not be uniform in layout or in angles. In some embodiments, the fins join shroud  92  at an angle other than 90 degrees or normal relative to the surface of the shroud. 
         [0045]    The various fin  94  configurations serve to disrupt the laminar flow of the production fluid as it flows past the motor housing  20  ( FIG. 1 ) and shroud  92 . In some embodiments, the flow develops swirling or vortexes. The fins  94  may be various lengths, including, for example, 1 to 3 inches long. The fins  94  may be attached to the clamshell shroud  92  by, for example, welding or adhesives before the halves of the clamshell  92  are joined. 
         [0046]    Other techniques for increasing the rate of heat transfer from motor  16  to the wellbore fluid may also be used in conjunction with grooves on the ID of housing  20  and the OD of stator  30 . For example, the motor lubricant may be circulated through external oil tubes. Apparatus and techniques for external oil circulation are illustrated in U.S. patent application Ser. No. 12/632,883, incorporated herein by reference. 
         [0047]    Referring to  FIG. 12 , lubricant may circulate through circulation tubes  102  located on the exterior of pump motor  104 . Each circulation tube  102  is a passage that is in fluid communication with interior portions of motor  104  in at least two locations. Circulation tubes  102  may attach to oil ports  106 ,  108  at any point on motor  104 . Tubes  102  may, for example, attach to oil port  108  at the head of the motor  104 , which is the end nearest the pump, and, for example, to oil port  106  at the base of motor  104 . The circulation tubes  102  may connect to the oil ports  106 ,  108  by a variety of techniques, including, for example, pipe thread connections, welding, or quick disconnect fittings, and the like. Lubricant may circulate by, for example, entering each tube  102  at port  106 , flowing up through tube  102 , reentering motor  102  at port  108 , and then passing through the interior of motor  102 . When passing through motor  102 , the lubricant may pass through, for example, grooves  36  located on the ID of housing  20  ( FIG. 2 ) or grooves  64  on the OD of stator  60  ( FIG. 8 ). 
         [0048]    As the lubricant circulates through motor  104  and circulation tubes  102 , the lubricant carries absorbed heat to circulation tubes  102 . The exterior surfaces of circulation tubes  102  are submerged in and exposed to production fluid inside the wellbore. Thus heat is transferred from the circulating lubricant to circulation tubes  102  and then conducted through the surface of circulation tubes  102  and transferred to the production fluid. The production fluid carries the heat away as it is drawn past tubes  102 , into intake  110  of pump  112 , and subsequently pumped to the surface. Lubricant pump  114  may assist the flow of lubricant through motor  104  and circulation tubes  102 . The lubricant may flow through circulation tubes  102  from the head towards the base, or from the base towards the head. 
         [0049]    While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.