Abstract:
A submersible pumping system for use downhole, wherein the system includes a pump, a pump motor, a seal section, a shaft coupling the pump motor to the pump, and bearing assemblies for radially retaining the shaft in place that are offset with respect to an axis of the shaft. The offset bearing assemblies produce side loads in the shaft that reduce shaft vibration during use. The bearing assemblies can be a combination of symmetric and asymmetric assemblies set in an alternating pattern along the length of the shaft.

Description:
BACKGROUND 
     1. Field of Invention 
     The present disclosure relates to downhole electric submersible pump (ESP) systems that are submersible in wellbore fluids. More specifically, the present disclosure involves a method for controlling the loading applied to the radial bearings in an ESP to control the dynamic characteristics of the bearings in operation. 
     2. Description of Prior Art 
     Submersible pumping systems are often used in hydrocarbon producing wells for pumping fluids from within the wellbore to the surface. These fluids are generally liquids and include produced liquid hydrocarbon as well as water. One type of system used employs an electrical submersible pump (ESP). ESPs are typically disposed at the end of a length of production tubing and have an electrically powered motor. Often, electrical power may be supplied to the pump motor via a cable. The pumping unit is usually disposed within the well bore just above where perforations are made into a hydrocarbon producing zone. This placement thereby allows the produced fluids to flow past the outer surface of the pumping motor and provide a cooling effect. 
     With reference now to  FIG. 1 , shown in a partial sectional view is a cased wellbore  8  having an ESP system  10  disposed therein. The ESP system  10  is made up of a motor  12 , a seal  14 , and a pump  16  and is disposed within the wellbore  8  on production tubing  18 . Energizing the motor  12  drives a shaft coupled between the motor  12  and the pump section  16 . The source of the fluid drawn into the pump comprises perforations  20  formed through the casing of the wellbore  10 ; the fluid is represented by arrows extending from the perforations  20  to the pump inlet. The perforations  20  extend into a surrounding hydrocarbon producing formation  22 . Thus the fluid flows from the formation  22 , past the motor  12  on its way to the inlets. 
     Traditionally, ESP systems  10  include bearing assemblies along the shafts in the motor section, seal section, and pump. Often, the bearings are plain sleeve bearings that provide radial support. One example of a bearing assembly provided in a motor section is provided in a cross sectional view in  FIG. 2 . Shown is a shaft  24  with an outer sleeve  26  that is circumscribed by a stator stack  28 . The sleeve  26  couples to the shaft  24 , such as by a key, and rotates along with the shaft  24 . A housing  30  encases the outer circumference of the stator stack  28 . A bearing assembly  32  is set between the outer sleeve  26  and stator stack  28  that radially encompasses a portion of the sleeve  26 . The motor bearing assembly  32  may have an insert  34  mounted on the outer circumference of the sleeve  26 ; a bearing carrier  36  encircles the insert  34  and in the absence of an insert directly mounts on the shaft sleeve. A T-ring  38  may be included that mounts to the inner surface of the stator stack  28  for preventing bearing rotation. The sleeve  26 , and therefore the shaft  24 , is radially supported by the insert  34  or the bearing carrier  36 . A lubricant film (not shown) allows for sleeve  26  rotation within the insert  34  or the bearing carrier  36 . 
     Referring to  FIG. 3 , shown in a side sectional view is a prior art example of bearings in a pump section of an ESP system. Diffusers  40  are typically coaxially stacked in close contact within a housing  30 . An impeller  42  is stacked between each successive diffuser  40 , where each impeller  42  is coupled to and rotates with the shaft  24 . Passages  44  curve radially and lengthwise throughout the diffusers  40  that register with passages  46  that similarly curve radially and lengthwise through the impellers  42 . Rotating the shaft  24 , and thus the impellers  42 , forces fluid through the passages  44 ,  46  to pressurize the fluid as it passes along the stack of diffusers  40  and impellers  42 . A sleeve bearing  48  couples around the shaft  24  to provide a bearing surface between the shaft  24  and inner circumference of the diffusers  40 . As the shaft  24  rotates, a film of lubricating fluid is maintained between the bearing  48  and diffuser  40 . 
     SUMMARY OF INVENTION 
     The present disclosure describes a method of controlling the loading of bearings in a submersible pumping system. In an example embodiment the method includes providing a submersible pumping system that has a pump section, a motor section, a shaft extending between the pump and motor sections, and a housing around the shaft and the pump and motor sections. Bearing assemblies are further provided that provide a bearing surface that allows rotation of the shaft and supports that mount the shaft in the pumping system. The bearing assemblies include a substantially symmetric bearing assembly and an asymmetric bearing assembly. The symmetric bearing assembly is disposed in an annular space between the housing and the shaft and substantially coaxial with the shaft. The asymmetric bearing assembly is disposed in the annular space and axially spaced from the substantially symmetric bearing assembly and with an axis of the asymmetric bearing assembly offset from an axis of the shaft. In this embodiment, when the shaft rotates within the symmetric and asymmetric bearing assemblies, a force between the shaft and the substantially symmetric bearing assembly in a direction divergent to an axis of the shaft to reduce vibration of the shaft. In an example embodiment, the substantially symmetric bearing assembly includes a sleeve having a bore that is coaxial with the sleeve. The asymmetric bearing assembly, in an example embodiment, is a sleeve having a bore with an axis that is offset from an axis of the sleeve. A rotor stack can be included with the submersible pumping system that mounts on the shaft, further included can be a stator stack set in the housing; the rotor and stator stacks can form the motor section. In an alternative embodiment, impellers are included with the submersible pumping system that are mounted on the shaft; in this alternative embodiment, diffusers can be set in the housing. The impellers and diffusers can form the pump section. The method may further include energizing the motor section so that the shaft and impellers rotate to pump fluid through the pump section. In another alternate embodiment, further provided are a multiplicity of substantially symmetric bearing assemblies and asymmetric bearing assemblies that are disposed on the shaft and in the housing. When the shaft rotates, the multiplicity of bearing assemblies exert a force onto a surface of the shaft and in a direction divergent from the axis of the shaft and wherein the direction of the force on adjacent bearing assemblies is substantially opposite. Optionally, when more than one substantially symmetric bearing assembly is provided, they can be disposed on opposite sides of the asymmetric bearing assembly. 
     Also described herein is a method of pumping fluid from a borehole. This method can include providing a submersible pumping system that has a pump section, a motor section, a shaft extending between the pump and motor sections, and a housing around the shaft and the pump and motor sections. The method further includes disposing the pumping system into a borehole with fluid and pumping the fluid from the borehole. Pumping includes energizing the motor section to rotate the shaft and drive the pump. In this example embodiment, bearing assemblies are provided at locations along an axis of the shaft and in an annular space between the shaft and the housing. Dynamic forces exerted by the bearing, as well as vibration in the shaft of the pumping system, can be reduced by generating a force between the shaft and each bearing assembly. Moreover, the force is in a direction divergent to an axis of the shaft; and in a direction divergent to a direction of the force generated by an adjacent bearing assembly. In an example embodiment, the bearing assemblies include substantially symmetric bearing assemblies that are made up of a sleeve with a coaxial bore. The bearing assemblies also include asymmetric bearing assemblies that include a sleeve with a bore having an axis offset from an axis of the sleeve. In an example embodiment, the bearing assemblies can be arranged so that a substantially symmetric bearing assembly is adjacent each asymmetric bearing assembly. Alternatively, the bearing assemblies can be arranged so that forces on the shaft from the bearing assemblies are applied at one of two locations on the outer surface of the shaft that are separated by approximately 180°. The submersible pumping system may have a rotor stack mounted on the shaft and a stator stack set in the housing; this arrangement forms the motor section. Optionally, impellers may be mounted on the shaft and diffusers can be set in the housing; this forms the pump section. In an example embodiment, the motor can be energized so that the shaft rotates and rotates the impellers to pump fluid through the pump section. 
     Yet further described herein is a submersible pumping system. In an example embodiment the pumping system includes a pump section, a motor section, a shaft extending between the pump and motor sections, and a housing encircling the shaft and the pump and motor sections. Included with the pumping system of this embodiment is a substantially symmetric bearing assembly set in an annular space between the housing and the shaft and positioned substantially coaxial with the shaft. The pumping system of this embodiment also has an asymmetric bearing assembly axially spaced from the substantially symmetric bearing assembly and positioned in the annular space with an axis of the asymmetric bearing assembly offset from an axis of the shaft. When the shaft is rotated, a force is generated between the shaft and the bearing assemblies in a direction divergent to an axis of the shaft that adjusts dynamic forces exerted by the bearing and reduces vibration of the shaft. In an example embodiment, the substantially symmetric bearing assembly includes a sleeve having a bore that is coaxial with the sleeve and the asymmetric bearing assembly includes a sleeve having a bore with an axis that is offset from an axis of the sleeve. A rotor stack may optionally be mounted on the shaft and a stator stack set in the housing to form the pump section. Impellers may also mounted on the shaft with diffusers set in the housing to form the pump section. The pumping system, in an example embodiment, may further include a multiplicity of substantially symmetric bearing assemblies and asymmetric bearing assemblies disposed in the annular space and wherein when the shaft is rotating, the multiplicity of bearing assemblies exert a force onto a surface of the shaft and in a direction divergent from the axis of the shaft and wherein the direction of the force on adjacent bearing assemblies is substantially opposite. In an alternate example embodiment, the bearing assemblies may be arranged to generate a force that increases vibration of the shaft. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a side partial sectional view of a prior art submersible pumping system disposed in a wellbore. 
         FIGS. 2 and 3  are a side sectional views of prior art bearing systems for use in a submersible pumping system. 
         FIG. 4  is a side sectional view of an embodiment of bearing assemblies for use in a submersible pumping system in accordance with the present disclosure. 
         FIG. 5  is an axial sectional view of a centered bearing assembly of  FIG. 4 . 
         FIG. 6  is an axial sectional view of an offset bearing assembly of  FIG. 4 . 
         FIG. 7  is a side perspective view of a coaxially disposed shaft and bearing sleeve. 
         FIG. 8  is a side perspective view of a shaft set in an asymmetric bearing sleeve. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments of the invention are shown. 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. 
     Referring now to  FIG. 4 , an example embodiment of an ESP assembly  50  is shown in a side sectional view. ESP assembly  50  includes an outer housing  52  that closely circumscribes an outer equipment stack  54 . The outer equipment stack  54  is illustrated as an annular section and schematically represents equipment on the inner surface of the housing  52  that includes diffusers, such as illustrated in  FIG. 3  above, or motor stators, as described and illustrated in  FIG. 2  above. An elongate shaft  56  is shown within the ESP assembly  50  and substantially coaxial within the housing  52 . The shaft  56  couples with an internal equipment stack  58  that is encircled by the outer equipment stack  54 . The internal equipment stack  58  of  FIG. 4  schematically represents equipment that includes impellers, such as illustrated in  FIG. 3  above, or motor rotor sections, as shown in  FIG. 2  above. The outer and internal equipment stacks  54 ,  58  define an annular space  59  between these two stacks  54 ,  58 . 
     Example embodiments of bearing assemblies  60 ,  62 ,  64  are illustrated mounted within the internal equipment stack  58  that provide a bearing surface between the shaft  56  and mounting structure for retaining the shaft  56  within the ESP assembly  50 . Bearing assembly  60  has a bore  65  through the assembly  60 , an axis A B  of the bore  65  is substantially coaxially with the axis A X . The shaft  56  inserts through the bore  65  and defines an annular space  66  between the shaft  56  and outer periphery of the bore  65 . The example embodiment of the bearing assembly  60  of  FIG. 4  is shown with its bore  65  substantially coaxial with the remaining portion of the bearing assembly  60 ; and for the purposes of discussion herein, is referred to as a substantially symmetric bearing assembly. As such, the annular space  66  between the shaft  56  and outer periphery of the bore  65  has a substantially consistent clearance C ( FIG. 5 ) for all angular values along the circumference of the shaft  56 . 
     Still referring to  FIG. 4 , the bearing assembly  62  is illustrated axially disposed distance from the bearing assembly  60  and within the housing  52  and outer equipment stack  54  of the ESP assembly  50 . The bearing assembly  62  is shown provided with a bore  67  having an axis A B  substantially parallel to the axis A X  and having the shaft  56  extending through the bore  67 . The axis A B  of the bore  67  is offset from the axis A X  of the shaft  56 . As such, an annular space  68  between the shaft  56  and outer periphery of the bore  67  has a clearance C (e) that varies with respect to the angular location on the outer circumference of the shaft  56  ( FIG. 6 ). Moreover, in circumferential locations where the clearance of the annular space  68  is reduced, a resultant force F 62  is exerted onto the shaft  56  from the bearing assembly  62  and acts as a loading mechanism on adjacent bearings. The reduced clearance can reduce the amount of fluid film between the shaft  56  and periphery of the bore  67  to thereby form a side load onto the shaft  56  that is divergent from the axis A X  of the shaft. In an example embodiment, the force F 62  is substantially perpendicular to the axis A X . 
     The bearing assembly  64  illustrated in  FIG. 4  has substantially the same dimensions and configuration as bearing assembly  60  and has a bore  69  formed to receive the shaft  56  therein and define the annular space  70  between the shaft  56  and outer periphery of the bore  69 . The radius of the annular space  70  is substantially consistent around the circumference of the shaft  56 . As noted above, a side load represented by F 62  is produced on the shaft  56  where it interacts with the bearing assembly  62  when rotated. Fluid dynamics of lubricating fluid within bearing assembly  60  and  64 , in combination with the bearing assembly  60 ,  64 , produce resultant forces F 60 , F 64  to counter the side load of F 62 . The applied side loads along the length of the axis  56 , applied at varying angular positions on the outer circumference of the shaft  56 , produce a more stable rotation of the shaft  56  and prevent excessive lateral movement within the respective bore  65 ,  67 ,  69  of the bearing assembly  60 ,  62 ,  64 . As such, vibration during use of the ESP assembly  50  of  FIG. 4  is substantially reduced by the disclosed configuration. 
     Referring now to  FIG. 5 , a sectional view of the ESP assembly  50  of  FIG. 4  is shown in a sectional view taken along line  5 - 5  of  FIG. 4 . In the example embodiment of the bearing assembly  60  of  FIG. 5 , an annular sleeve  72  is shown within the bearing assembly  60  through which the bore  65  is formed. As illustrated in  FIG. 5 , the shaft  56  is generally centered within the bore  65  so that the axis A X  and A B  are substantially collinear. Further provided in the example of  FIG. 5 , are mount members  74  that extend radially inward from an outer ring  76  to the outer circumference of the sleeve  72 . 
     Referring now to  FIG. 6 , an example embodiment of the asymmetric bearing assembly  62  is shown in a sectional view taken along line  6 - 6  of  FIG. 4 . As can be seen in this embodiment, the axes A X  and A B  are offset from one another. By being offset, the radius of the annular space  68  can vary depending on where on the circumference of the shaft  56  the radius of the annular space  68  is measured. Moreover, the radius of the annular space  68  can further vary depending on the particular design conditions of the ESP assembly  50 . In an exemplary embodiment, the “offset” location  71  for each asymmetric bearing assembly  62 , which corresponds to where the radius of the annular space  68  is at a minimum value, can be at the same angle with respect to the axis A X . Optionally, the offset location  71  can alternate along the length of the shaft  56  and may be placed at designated angular locations. As noted above, in regions where the radius of the annular space  68  is reduced can generate a lateral side force F 62  and directed against the shaft  56 . 
       FIGS. 7 and 8  respectively depict perspective sectional views of the bearing assembly  60  and bearing assembly  62 . In each of  FIGS. 7 and 8 , the shaft  56  extends through the respective bores  65 ,  67  of bearing assembly  60  and bearing assembly  62 . Referring now to  FIG. 7 , the bore  65  is formed coaxial to the sleeve  72  with the bore axis A B  coincident with the sleeve axis A S ; thereby providing a substantially even wall thickness around the circumference of the sleeve  72 . In contrast and as illustrated in  FIG. 8 , the bore axis A B , which is offset from the sleeve axis A S , forms an asymmetric wall thickness of the sleeve  72 A. In an alternative embodiment, the bore  67  may have a diameter that is greater than the diameter of the bore  65  in the symmetric bearing assembly  60 . 
     It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Example alternative embodiments include configurations where the symmetric and asymmetric bearings sequentially alternate. In another embodiment, patterns of symmetric and asymmetric bearing assemblies placement are repeated; exemplary patterns can include one (or more) asymmetric bearing assembly(ies) between two symmetric bearing assemblies.