Abstract:
A vibration monitoring system for an electrical submersible pump (ESP) that includes a fiber optic cable coupled with the ESP and an optical sensor coupled with the fiber optic cable. The optical sensor emits electromagnetic radiation into the fiber optic cable, some of which reflects back from optical discontinuities provided in the fiber optic cable. By receiving the reflected electromagnetic radiation, the optical sensor estimates the location of the optical discontinuities. Displacement and rate of displacement of the optical discontinuities can be estimated when the reflections are received over a period of time. Because the fiber optic cable, and the optical discontinuities, move with the ESP, displacement measurements of the optical discontinuities correlate to ESP movement, including ESP vibration.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/222,268 filed Jul. 1, 2009, the full disclosure of which is hereby incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to fiber optic sensors, and in particular, to a system using fiber optic sensors to measure vibration in downhole electric submersible pump systems. 
         [0004]    2. Description of Related Art 
         [0005]    Electrical submersible pump (ESP) systems may be deployed within a wellbore to pump downhole fluid to the surface. An ESP system may typically include an electric motor and a pump that is used to pump oil or other fluids within a wellbore. The electric motors have a rotatable rotor that is contained within a stationary stator. During operation in a wellbore, ESP systems may vibrate for multiple reasons. For example, when fluid film bearings are used to protect between a rotor shaft and a bearing sleeve, the bearings may become destabilized due to lack of an applied side load to cause excessive motor vibration. Motor vibration induces the bearings to vibrate, that in turn can cause the bearing sleeve to break through the oil film. When the oil film is breached, metal to metal contact occurs that can lead to premature wear and motor failure. Motor vibration can also occur when unexpectedly high levels of gas are being pumped by the ESP. While vibration monitoring systems may be employed, vibration measurements are typically limited to average overall levels. 
       SUMMARY OF INVENTION 
       [0006]    In an example embodiment, a fiber optic cable or cables may be utilized in conjunction with an electric submersible pump assembly to measure and monitor variables throughout the assembly including vibration and temperature. Fiber optic cables are typically a glass fiber that has been drawn, and then modified at precise locations using a laser or other means, and then enclosed within a sheath. The modifications allow an optical time domain reflectometer to measure reflected laser light from these locations. The time difference between the laser pulse and the reflection is used to determine the location of the modified fiber in the cable. The sheath is typically stainless steel but may be made of other material as well. The fiber optic cable may be installed on the outside of the ESP or built into (installed inside) the ESP components (motor, seal, pump, etc.). By properly placing the fiber optic cables so that axial deflections or strain on opposite sides of the ESP may be measured, lateral bending and vibration may also be measured. 
         [0007]    By attaching a fiber optic cable that has been prepared to measure the location of manufactured anomalies in the fiber to a motor, the deflection of an ESP assembly, and as a result, the vibration of the ESP assembly may be measured. Deflections along the length of the fiber optic cable can be measured using optical time domain reflectometery or swept wavelength interferometry techniques. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is an elevational view of a well within which an electric submersible pump is disposed. 
           [0009]      FIG. 2  is a longitudinal sectional view of the electric submersible pump of the present invention taken along the line  2 - 2  of  FIG. 1 . 
           [0010]      FIG. 3  is a horizontal sectional view of the electric submersible pump of  FIG. 2 . 
           [0011]      FIG. 4  is a longitudinal sectional view of the electric submersible pump of  FIG. 2  while undergoing a bending force. 
           [0012]      FIG. 5  is a longitudinal sectional view of the electric submersible pump of  FIG. 2  while undergoing a bending force in the opposite direction of  FIG. 5 . 
           [0013]      FIG. 6  is an elevational view of a motor portion of an alternate embodiment electric submersible pump. 
           [0014]      FIG. 7  is an elevational view of a motor portion of an alternate embodiment electric submersible pump. 
           [0015]      FIG. 8  is sectional view of a stator of an alternate embodiment electric submersible pump. 
           [0016]      FIG. 9  is a longitudinal sectional view of a pump portion of an alternate embodiment electric submersible pump. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1  is an elevational section view of a well  10  with an ESP disposed therein, the embodiment of the ESP includes an electric submersible pump  12  mounted to a string of tubing  14 . Pump  12  includes an electric motor  16  and a pump section comprising a centrifugal pump assembly  18  within housing  19 . A cable  20  extends downhole, terminating in a motor lead to provide power to the electric motor  16 . A pothead connector  22  is mounted to the motor lead of cable  20 , and electrically connects and secures the motor lead of cable  20  to a housing  24  outside of motor  16 . Pump  12  is used to pump well fluids from within the well  10  to the surface. 
         [0018]    Shown in  FIG. 2  is a side sectional view of an example embodiment of the motor  16  taken along lines  2 - 2  of  FIG. 1 . As seen in  FIG. 2 , the housing  24  surrounds the components within motor  16  and is sealed to protect components in the motor  16  from contact with well fluids. A drive shaft  26  is coupled to a shaft (not shown) within pump  18  ( FIG. 1 ) to transfer torque from motor  16  to pump  18 . Motor  16  creates a torque on shaft  26  to cause the shafts to rotate, providing power to drive pump  18 . In the example embodiment of  FIG. 2 , motor shaft  26  is cylindrical and extends coaxially through the motor  16 . A rotor  28  is shown mounted around the shaft  26  for rotation within a stationary stator  30 . Rotor  28  is comprised of a number of laminated disk-like plates (not shown) coaxially stacked along a portion of the shaft  26 . Stator  30  is an annular member anchored along a portion of an inner surface of the housing  24  and circumscribing the rotor  28 . An annular space separates the rotor  28  and stator  30 . 
         [0019]    Still referring to  FIG. 2 , a fiber optic cable  33  is shown included with the example embodiment of the motor  16 . In an example embodiment, the fiber optic cable  33  is made up of a drawn glass fiber and encased in a sheath; an example of sheath material is a substantially rigid material such as stainless steel or other suitable material. In the example embodiment of  FIG. 2 , the fiber optic cable  33  includes optical discontinuities  34  at discrete locations along the length of the fiber optic cable  33 . The optical discontinuities  34  may include locations where the index of refraction changes in the fiber optic cable  33 . Optionally, the optical discontinuities  34  may be formed in the fiber optic cable  33  such as by applying a laser beam to the fiber optic cable  33 . In another example embodiment, the optical discontinuities  34  include connection points, such as a splice or a cable connector. The fiber optic cable  33  is shown axially passing through the stator  30  at an angular position with respect to an axis A X , exiting an end of the stator  30  proximate the pothead connector  22 , looping to another angular position of the stator  30 , and passing back axially through the stator  30 . Looping back through the stator  30  defines a pair of legs  33   1 ,  33   2  in the fiber optic cable  33 . The legs  33   1 ,  33   2  of  FIG. 2  exit the stator  30  at the end distal from the pothead connector  22  and connect to a fiber optic termination point  36  shown provided in the housing beneath a lower end of the shaft  26 . 
         [0020]    In an example embodiment, the termination point  36  may include sensors, such as for measuring conditions at the pump  16 , such as temperature, pressure, vibration, and the like. As shown in  FIG. 2 , an optical time-domain reflectometer (OTDR)  37  may be included with the termination point  36  or be located on the surface using a fiber optic cable to transmit the signals between the ESP and the OTDR  37 . An OTDR  37  can send optical pulses into an optical fiber and collect electromagnetic radiation, such as visible light, that is scattered or otherwise reflected from within the optical fiber. Changes in an index of refraction in the optical fiber can define scatter or reflection points. Analyzing the collected return light can yield the distance to the changes in the index of refraction. Thus, in an example embodiment, the OTDR  37  can be used to monitor the relative location(s) of the optical discontinuities  34 . 
         [0021]    The termination point  36  may optionally include a processor  38  integrated or otherwise in communication with the OTDR  37 . The processor  38  can include signal processing software for generating a signal or data representative of the measured conditions. A communication link  39  can provide means to transmit the generated signal or data from the processor  38  back to the surface. The communication link  39  be hard wired, fiber optic cable, telemetry, or other transmission means. 
         [0022]    Referring now to  FIG. 3 , a sectional view is provided of a section of a stator  30  and illustrating a lamination  31  in a plan view. Coaxially stacking the laminations  31  form the stator  30 . Formed axially through each lamination  31  are slots  35 ; so that when the slots  35  are aligned, a passage extends axially through the stator  30 . In the example embodiment of  FIG. 3 , the slots  35  are arranged along an inner diameter of the stator  31 . Wire like electrical conductors  32  are shown inserted through the aligned slots  35  and extend along the length of the stator  30 . The conductors  32  form a winding through the slots  35  either by looping from one slot  35  to another, or by having ends that are connected to ends of other conductors  32  in other slots  35 . 
         [0023]    In the example embodiment of  FIG. 3 , the fiber optic cable  33  is shown inserted within slots  35   1 ,  35   2  that are disposed on substantially opposing sides of the lamination  31 . In an example, one leg  33   1  of the fiber optic cable  33  travels through a passage aligned with slot  35   1 , i.e. one side of the motor  16 ; loops to the passage aligned with slot  35   2 , then travels through the passage (leg  33   2 ) and into slot  35   2 . This arrangement forms the two legs  33   1 ,  33   2  of the fiber optic cable  33 ; that while in the stator  30  are parallel to one another ( FIG. 2 ). 
         [0024]      FIG. 4  schematically represents an embodiment of the motor  16  bending or otherwise deforming in response to an applied side load L. In the example of  FIG. 4 , the load L compresses a portion of the motor  16  on one side of the axis A X  and tensions a portion of the motor  16  on the other side of the axis A X . In  FIG. 5  a load L is applied to an opposite side of the motor  16  to reverse the sides of the motor  16  in compression and tension. Thus in the example of  FIG. 4  leg  33   1  is in compression and leg  33   2  is in tension, whereas in the example of  FIG. 5  leg  33   1  is in tension and leg  33   2  is in compression. The legs  33   1 ,  33   2  elongate when in tension and shorten when in compression, thereby axially moving the optical discontinuities  34  with respect to the OTDR  37 . Accordingly, the amount of deflection or deformation in the legs  33   1 ,  33   2  can be measured by activating the OTDR  37  and analyzing the relative distance shift(s) of the optical discontinuities  34 . For the purposes of discussion herein, deflection of the legs  33   1 ,  33   2  includes lateral (radial) as well as axial deformation. 
         [0025]    Monitoring displacements of the optical discontinuities  34  with the OTDR  37  can be correlated to time span and time frequency to indicate when the displacements constitute vibratory motion and the vibration mode. The vibratory motion can include lateral vibrations in the motor  16  or a specific portion of the motor  16 . Moreover, the vibratory motion can include lateral deflection of the motor  16  oscillating between the configurations of  FIG. 4  and  FIG. 5 , wherein the deformation can be compression or tension. The OTDR  37  can communicate signals to the processor  38  representative of the vibrations monitored by the OTDR  37 . The processor  38  can in turn process the monitored vibrations into quantified values that can be used for analysis. The quantified values can be conveyed to the surface via the communication link  39 . Optionally, the processor  38  can be disposed at the surface and receive signals directly from the OTDR  37 . 
         [0026]    In an alternate embodiment, the temperature of the motor  16  may be measured and monitored by communicating a temperature sensor to the fiber optic cable  33 . In an alternate embodiment, multiple fiber optic cables  33  may be installed inside the slots  35  and would allow for vibration to be measured in multiple planes. 
         [0027]    Another method of measuring vibration is to measure the strain from deformations during vibratory oscillation. Strain measurements may be taken along a deforming surface and along a path oblique or normal to a path of vibratory oscillation. A fiber optic sensor, such as a Fabry-Perot interferometer acoustic emissions sensor, may be employed. The Fabry-Perot interferometer acoustic emissions sensor has a high sensitivity to detect low levels of strain caused by vibration. The signal from the fiber optic sensor may be transmitted to the surface using a fiber optic wire, and analyzed in the same manner using a fast Fourier transform or other data reduction technique that is presently being used. 
         [0028]    In an oscillatory vibration, the motor  16  may vibrate similar to a rod or guitar string thereby producing nodes (no lateral oscillation) separated by portions of the motor  16  that are oscillating and defining an anti-node at the location of maximum lateral deflection. As illustrated in  FIG. 6 , in an additional alternate embodiment, a fiber optic cable  43  is shown coupled to an exterior of a housing  44  of a motor portion  45  of an ESP assembly  47 . Optical discontinuities  48  are illustrated in the fiber optic cable  43 , and are optionally set a known discrete locations. The fiber optic cable  43  may be housed in a sheath (not shown) that is rigidly attached to the ESP assembly  47  so that a deflection (stretching or compression) of the ESP assembly  47  where it couples with the fiber optic cable  43  will correspondingly deflect the fiber optic cable  43 . As discussed above, monitoring relative motion of the optical discontinuities  48  with respect to an optical sensor, such as an OTDR, can yield information regarding vibration of the ESP assembly  47 . In this example embodiment, legs  43   1 ,  43   2  of the fiber optic cable  43  are disposed from one another at substantially opposite sides of the housing  44 . Optionally, an additional fiber optic cable (not shown) with legs disposed at substantially opposite locations around the ESP assembly  47  can be provided and offset from the fiber optic cable  43  by an angular amount. 
         [0029]    Illustrated in  FIG. 7  is yet another alternate embodiment, depicting a fiber optic cable  51  shaped into a helix that spirals around a portion of an ESP assembly  53 . In this particular alternate embodiment, the fiber optic cable  51  is spiraled around the exterior of housing  55  of the motor portion  57  of ESP assembly  53 . Referring to  FIG. 8 , in an additional alternate embodiment, a fiber optic cable  61  is run through a slot  63  in an outer periphery of a stator lamination  65  as found within a motor portion of an ESP assembly. Thus, forming a stack of laminations  65  and aligning the slot  63  forms a stator with an elongated groove in which the fiber optic cable  61  can be disposed. In an alternate embodiment, the fiber optic cable  61  may also be rigidly connected to the interior surface of the motor housing  24 . 
         [0030]    In additional alternate embodiments, slots could also be placed in various elements of an ESP assembly. For example, in a particular alternate embodiment illustrated in  FIG. 9 , a centrifugal pump  67  has a housing  69  that protects many of the pump  67  components. Pump  67  contains a shaft  71  that extends longitudinally through the pump  67 . Diffusers  73  have an inner portion with a bore  75  through which shaft  71  extends. Each diffuser  73  contains a plurality of passages  79  that extend through the diffuser  73 . Each passage  79  is defined by vanes (not shown) that extend helically outward from a central area. Diffuser  73  is a radial flow type, with passages  79  extending in a radial plane. An impeller  81  is placed within each diffuser  73 . Impeller  81  also includes a bore  83  that extends the length of impeller  81  for rotation relative to diffuser  73  and is engaged with shaft  71 . Impeller  81  also contains passages  85  that correspond to the openings in the diffuser  73 . Passages  85  are defined by vanes (not shown). Washers are placed between the upper and lower portions between the impeller  81  and diffuser  73 . 
         [0031]    Impellers  81  rotate with shaft  71 , which increases the velocity of the fluid being pumped as the fluid is discharged radially outward through passages  85 . The fluid flows inward through passages  79  of diffuser  73  and returns to the intake of the next stage impeller  81 , which increases the fluid pressure. Increasing the number of stages by adding more impellers  81  and diffusers  73  can increase the pressure of the fluid. In order to monitor the stress and strain in the pump  67 , legs  91   1 ,  91   2  of a fiber optic cable  91  may be run through corresponding passages  93   1 ,  93   2  located in the outer peripheries of the pump diffusers  73 . The legs  91   1 ,  91   2  are run through passages  93   1 ,  93   2  shown disposed at substantially opposite sides of the pump  67 . 
         [0032]    The invention has significant advantages. Fiber optic cables are utilized in an electric submersible pump assembly to measure and monitor variables throughout the assembly including vibration and temperature. Properly placed fiber optic cables permit axial deflections on opposite sides of the ESP to be measured, thereby enabling lateral bending and vibration to also be measured. In addition, the requirement for downhole electronics may be eliminated by using a fiber optic cable to return measurements to the surface for processing. 
         [0033]    While the invention has been shown in only one 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. For example, a fiber optic cable may be placed on the interior or exterior of a desired component of an ESP assembly. In an example embodiment, fiber optic cable can be rigidly attached to a component by soldering, brazing, gluing, combinations thereof, and other like techniques. In an example embodiment, distributed temperature and displacement sensing using optical fibers can include techniques based on Raman, Brillouin, and Rayleigh scattering as well as those involving multiplexed fiber Bragg gratings. The scattering techniques can employ optical time domain reflectometry. Alternatively, the Rayleigh backscatter can be measured using swept wavelength interferometry (SWI) as a function of length in optical fiber with high spatial resolution.