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. The motor housing may have turbulators that increase the turbulence of the production fluid to increase the rate of heat transfer. The turbulators are designed for manufacturability and maintenance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to provisional application 61/138,060, filed Dec. 16, 2008. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to well pumps, and in particular to a well pump housing varying geometry to increase heat transfer. 
     BACKGROUND 
     Referring to  FIG. 1 , a well contains a casing  10 . The casing  10  lines a wellbore (not shown) and is cemented in place. A pump  12  is located inside the casing  10 , frequently at great depths below the surface of the earth. The pump is used to pump production fluid from the depths of the well up to the surface. A shaft (not shown) connects pump  12  to motor  16 . Production fluid enters the pump inlet  17  and is pumped out through tubing  18 . 
     The motor tends to produce heat that must be removed to prolong the life of the motor. External devices used to decrease heat create additional costs. External cooling devices, for example, use a coolant pump above the well and coolant lines running through the wellbore to the pump. These cooling devices cool the pump by circulating the coolant through the pump and transferring the coolant back to the surface. The coolant pump, coolant lines, and coolant all create additional costs. Furthermore, the coolant lines may interfere with well operations. 
     The motor-pump assembly is located inside a wellbore so it is desirable to transfer heat to the production fluid that is flowing past the motor. It is common to arrange the pump and motor such that the production fluid flows past the motor on its way to the pump. Heat is transferred to the production fluid and carried away as the production fluid moves to the surface. It is desirable to increase the rate of heat transfer from the motor to the production fluid. 
     One method to increase the rate of heat transfer is to increase the surface area of the pump that is in contact with the production fluid. This can be done by elongating the motor housing or attaching a shroud to the pump or motor. The production fluid flows between the motor and the shroud so that heat can move from both the motor and the shroud into the production fluid. Other devices, such as fins, may be used to increase surface area of the motor. All of these methods of increasing surface area are limited by the small space available inside the wellbore. Furthermore, there is a problem with fins breaking off and creating blockages within the production fluid flow. 
     Fins may be used to create vortices within the production fluid. The vortices in the production fluid increase the rate of heat transfer between the motor and the production fluid. Unfortunately, the vortice-inducing fins, like fins used to increase the surface area, can break off and obstruct fluid flow. Fins also make pump manufacture and maintenance more difficult because they interfere with the assembly, disassembly, and the movement within the wellbore of the pump assembly. 
     Assembly is more difficult because the fins must be installed on the motor before the motor is inserted into the cylindrical shroud. The difficulty arises because the fins tend to interfere with the fit between the motor and the shroud. The height of the fins must be limited to allow for insertion, but even with a limited height they can still catch on other fins, the sides of the motor, or the wellbore. If the fin is attached to the motor, for example, there must be a gap between the outer edge of the fin and the shroud to allow clearance during assembly. Clearance issues also make it extremely difficult to attach fins to both the motor and the shroud in the same assembly because the fins interfere with each other during assembly and disassembly. Furthermore, fin clearance issues prevent the fin from spanning the entire gap between the shroud and the motor. 
     It is also difficult to perform maintenance on the motor when fins are attached directly to the motor housing because the fins make it more difficult to put the motor on a flat surface or hold it in a vice. In addition to increased assembly and maintenance costs, there is a cost associated with manufacturing and attaching the fins to the shroud and pump. It is desirable to increase the rate of heat transfer without incurring the disadvantages of fins. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of prior art pump assembly in a wellbore. 
         FIG. 2  is a sectional view of the pump assembly of  FIG. 1  with a shroud having an irregular-shaped side wall. 
         FIG. 3  is a sectional view of a pump assembly with a “stair-step” shroud attached. 
         FIG. 4  is a sectional view of a pump assembly with dimples on the shroud. 
         FIG. 5  is a sectional view of a pump assembly with dimples on the pump motor housing. 
         FIG. 6  is a sectional view of a pump assembly with a wire coil attached to the inside of the shroud. 
         FIG. 7  is a sectional view of a pump assembly with a wire coil attached to the motor housing. 
         FIG. 8  is a sectional view of a pump assembly and shroud with screws protruding from the inside of the shroud. 
         FIG. 9  is an orthogonal view of a clamshell shroud in which two halves of the clamshell are shown in the closed position. 
         FIG. 10  is an orthogonal view of one half of a two-part clamshell shroud and pins in the clamshell. 
         FIG. 11  is an orthogonal view of one half of a two-part clamshell shroud with fins. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , the casing  10  is shown in a vertical orientation, but it could be inclined. A pump  12  is suspended inside casing  10  and is used to pump fluid up from the well. The pump  12  may be centrifugal or any other type of pump and may have an oil-water separator or a gas separator. The pump  12  is driven by a shaft (not shown), operably connected to a motor  16 . A seal section  14  is mounted between the motor  16  and pump  21 . The seal section reduces a pressure differential between lubricant in the motor and well fluid. The motor  16  is encased in a housing  19 . Preferably, the fluid produced by the well (“production fluid”) flows past the motor  16 , enters an intake  17  of pump  12 , and is pumped up through a tubing  18 . Preferably, the 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  19  of the motor  16 , and into the pump inlet  17 . 
     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 (in this case, T is the difference in temperature between the motor housing  19  and the production fluid). 
     Referring to  FIG. 2 , a shroud  22  is mounted around motor  16  to increase the velocity of fluid flowing past the motor housing  19 . The shroud  22  has an open lower end  24  and an upper end  26  sealingly secured around pump  12  above intake  17 . The shroud  22  may be secured by other means and in other locations. The shroud  22  reduces the cross sectional area of the path of fluid flow and thus increases velocity. Increased velocity, or changing velocity, or both, will generally increase turbulence, which in turn increases the heat transfer coefficient (h) of the production fluid flow across the surface of the motor housing  19 . A device that increases turbulence in the fluid flow is referred to herein as a “turbulator.” 
     A turbulator may be a feature on a shroud, on the motor housing, or any other part of the motor. As shown in  FIG. 2 , the turbulator comprises shroud  22 , which may have an irregular sidewall  28  shape, and thus creates pockets of increased velocity and turbulence as the production fluid flows within shroud  22 . In  FIG. 2 , the sidewall  28  of the shroud  22  is formed into a pattern that is sinusoidal when viewed in cross section. The period of each rounded peak and valley may vary considerably. For example, the length of each curve could be much shorter than the length of the motor. The annular flow area varies along the length of the motor  16  as a result. 
     Referring to  FIG. 3 , turbulence is increased by using a “stair-step” shaped shroud  23  as the turbulator. The production fluid develops a higher velocity, and thus more turbulence, as the inner diameter (“ID”) of the shroud  23  decreases. The laminar flow is further disrupted as the fluid flows past the corners  30  of the indentations in the shroud  23 . In one example embodiment, the motor housing  19  has a 7.25″ diameter and the shroud  22  has a 10.75″ diameter, leaving a 1.75″ maximum gap between the motor housing  19  and the shroud  23 . The shroud  23  could constrict to allow, for example, a 0.5″ clearance between the motor housing  19  and shroud  23 , thus increasing the velocity. The steps of the shroud  23  may be various lengths measured in the direction of the shroud  23  axis, including, for example, 0.5″ or 1″. For example, section  30   a  has a smaller inner diameter and shorter axial length than section  30   b . Steps also could have a uniform, corrugated appearance such that, for example, every other step has the same inner diameter. 
     Another embodiment of the stair-step shroud  23  is an asymmetrical stair step (not shown) in which the inner diameter varies in one or more quadrants of the shroud  23 . This asymmetrical shape further disrupts laminar flow by creating pockets of higher and lower pressure from side-to-side across the motor housing  19  thus promoting lateral flow of the production fluid. 
     Referring to  FIG. 4 , the turbulator comprises multiple dimples  32  on the shroud  25 . The dimples  32  are indentations or protrusions in the interior face of the shroud  25 . The size of the indentations  32  may vary and could be, for example, made from a ¼″ or ½″ diameter round punch driven to a ⅛″ depth. Dimples  32  could also have a significantly larger or smaller diameter and be driven to a greater or lesser depth. Furthermore, the dimples  32  may have different shapes such as round, oval, square, and the like. The dimples  32  may be distributed about the surface in a symmetric pattern or they may be placed randomly. The dimples  32  may be concave or convex in relation to the interior of the shroud  25 . The dimples  32  increase the turbulence of the production fluid and thus increase the rate of heat transfer from the motor housing  19  to the production fluid. The dimples give the shroud a textured surface. Other kinds of textured surfaces may also be used to increase turbulence. Furthermore, the dimples  32  are an inexpensive design modification and are not detrimental to the maintenance, handling, and installation of the motor  16 . The dimples  32  may be used alone or in combination with other devices that increase production fluid turbulence. 
     Referring to  FIG. 5 , the turbulator comprises multiple dimples  33  on the motor housing  16 . The dimples  33  are indentations or protrusions in the exterior surface of the motor housing  27 . The size of the indentations  33  may vary and could be, for example, made from a ¼″ or ½″ diameter round punch driven to a ⅛″ depth. Dimples  33  could also have a significantly larger or smaller diameter and be driven to a greater or lesser depth. Furthermore, the dimples  33  may have different shapes such as round, oval, square, and the like. The dimples  33  may be distributed about the surface in a symmetric pattern or they may be placed randomly. The dimples  33  may be concave or convex in relation to the exterior of the motor housing  27  and may be used regardless of whether a shroud is used. The dimples  33  increase the turbulence of the production fluid and thus increase the rate of heat transfer from the motor housing  27  to the production fluid. The dimples give the housing a textured surface. Other kinds of textured surfaces may also be used to increase turbulence. Furthermore, the dimples  33  are an inexpensive design modification and are not detrimental to the maintenance, handling, and installation of the motor  16 . The dimples  33  may be used alone or in combination with other devices that increase production fluid turbulence. 
     Referring to  FIG. 6 , a wire coil  34  may be attached to the inside of a shroud  35  to form a turbulator. The presence of the helical coil  34  serves to disrupt the laminar flow of the production fluid and thus increase the rate of heat transfer. The coil  34  can be installed in any variety of positions. For example, it could be attached to the shroud  35  in one or more places as it loops around the motor housing  19 , or it could use spacers to hold the wire in the gap between the motor housing  19  and the shroud  35 . In other embodiments, more than one wire could be attached to the inside of the shroud  35 . The wire may have, for example, twists or coils to further disrupt laminar flow. In still other embodiments, the wire may be attached in two places near the inlet such that the wire forms a “horseshoe” shape inside the shroud. The wire may be used by itself or in conjunction with other means of flow disruption such as dimples  32  ( FIG. 4 ) or irregularly shaped shrouds. 
     Referring to  FIG. 7 , the turbulator may be a wire coil  37  attached in helical fashion to the outside surface of the motor  39 . The presence of the coil  37  serves to disrupt the laminar flow of the production fluid and thus increase the rate of heat transfer. The coil  37  can be installed in any variety of positions. For example, it could be looped around the motor  16  and attached directly to the motor housing  39 , or it could use spacers to hold the wire at a distance from the motor housing  39 . The wire may have, for example, twists or coils to further disrupt laminar flow. The wire may be used by itself without a shroud, or in conjunction with other means of flow disruption such as dimples  33  ( FIG. 5 ) or irregularly shaped shrouds. 
     Referring to  FIG. 8 , the turbulator comprises pins or screws  36  attached to the shroud  41  and extending radially inward to disrupt flow. The pins  36  may be, for example, ¼″ diameter studs that could be installed by inserting them through holes drilled shroud  41  such that they protrude from the interior of the shroud  41 . In other embodiments, screws  36  or bolts could be installed by screwing them through threaded holes tapped in the shroud  41 . The pins or screws  36  may be held in place by a variety of means, including, for example, their own threads, bolts, welding, and the like. The pins or screws  36  may be distributed around the entire circumference and along the entire length of the shroud  41 . The pins or screws  36  may be arranged in a symmetrical or in a random pattern. Furthermore, the pins or screws  36  may be used to disrupt flow in straight cylindrical shrouds or in irregularly shaped shrouds, as shown in  FIGS. 2 and 3 . 
     The pins or screws  36  serve to disrupt the laminar flow of the production fluid and thus increase the rate of heat transfer. In a preferred embodiment, the pins or screws  36  are inserted to a depth such that they contact or nearly contact the motor housing  19 . By contacting or nearly contacting the motor housing  19 , the pins or screws  36  create turbulence close to the motor and thus increase the rate of heat transfer. The user may insert the screws  36  or pins through the shroud  41  after the motor  16  is already installed in the shroud  41 . This embodiment allows easy insertion of the motor  16 , followed by installation of screws  36  that nearly contact the motor and the shroud  41 . The screws  36  may be removed prior to removal of the motor  16  from the shroud  41 , thus providing the heat transfer benefits of the screws  36  while still allowing for easy maintenance access. The pins or screws  36  may be used in combination with any other embodiment of invention, including irregularly shaped shrouds and dimples  32 . 
     Referring to  FIG. 9 , the shroud  44  may be split into two or more halves or pieces  46  that may be joined together around the motor  16  in a “clamshell” configuration. The joint  48  may be any variety of joint types, including flange, tongue-and-groove, dowel pin, and the like. The pieces  46  may be held together with bolts, quick release latches, interlocking pieces, and the like. The clamshell may divide the shroud  44  into two, three, or more segments or pieces  46 . Each piece  46  may be a segment of a cylinder. One or more joints between the components may have a hinge. The clamshell design may be used to facilitate easier installation of the turbulators. 
     Referring to  FIG. 10 , the clamshell shroud  44  overcomes the difficulty, for example, of installing and removing the motor  16  when other devices, such as pins  50 , screws, fins  52 , and the like are present between the motor and shroud  44 . Separating the clamshell segments facilitates installation of objects located between the shroud  44  and the motor  16  by giving better access to the inside surface of the shroud  44 . Furthermore, it is easier to manufacture irregularly shaped shrouds when the shroud  44  is split. It is easier, for example, because the pieces can be produced by metal-stamping rather than requiring extrusion, turning, or otherwise shaping a cylindrical object. 
     Referring to  FIG. 11 , in one embodiment of the clamshell configuration, fins  52  may be installed on the motor housing  19  or the shroud  54 , and the fins  52  may be so long in radial dimensions that they contact both components. A fin  52  could, for example, be welded to the shroud  54  and contact or nearly contact the motor housing  19  when the motor  16  is installed. This embodiment overcomes the inherent manufacturing and maintenance difficulties associated with attaching fins  52  directly to the motor housing  19 , yet still creates turbulent flow immediately adjacent to the motor. 
     The fins  52  may be oriented in a variety of positions. In one embodiment, the fins  52  are attached at a 90 degree angle or normal in relation to the wall of the shroud  54 . Fins  52  may be slanted in relation to the axis of the shroud  54 , such as at a 45 degree angle. As illustrated by group  56  of fins  52 , adjacent fins  52  may incline at the same inclination relative to the axis of shroud  54 . Also, some of the adjacent fins  52  may slant at alternating angles to each other. For example, one fin  52  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  58  of the fins  52  are nearest each other and the fins diverge as they go up along the axis of the shroud. Other fins  52  may have the same 90 degree opposed orientation, but with the top most part  60  of the fins  52  nearest each other. The angle between opposed sets of fins  58  could be any angle. The fins  52  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  54  at an angle other than 90 degrees or normal relative to the surface of the shroud. 
     The various fin  52  configurations serve to disrupt the laminar flow of the production fluid as it flows past the motor housing  19  and shroud  54 . In some embodiments, the flow develops swirling or vortexes. The fins  52  may be various lengths, including, for example, 1 to 3 inches long. The fins  52  may be attached to the clamshell shroud  54  by, for example, welding or adhesives before the halves of the clamshell  54  are joined. 
     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.