Fiber-Optic Monitoring in Electrical Submersible Pump Systems

Abstract

Abstract Fiber-optic technology provides operators the ability to monitor temperature and vibration at multiple locations in electrical submersible pump (ESP) components using a single optical fiber. Conventional accelerometers and thermocouples have limited use downhole because of their size and issues with electrical interference. Intrinsic fiber-optic sensors are small enough for installation inside the equipment at optimal sensing positions in protected areas and are immune to electrical interference. In addition, conventional accelerometers and thermocouples provide single- point measurements rather than spatially distributed measurements. The distributed sensing ability of fiber-optic technology enables better understanding of equipment performance downhole. This work consisted of an investigation of fiber-optic systems for obtaining distributed temperature and vibration measurements of ESP components. Prototype ESP motors and a seal were manufactured to enable fiber-optic monitoring of internal temperature throughout the equipment. Fiber Bragg Grating (FBG) sensors interrogated using Optical Frequency Domain Reflectometry (OFDR) enabled temperature measurements every 2 cm along the ESP assembly during operation as well as during epoxy cure. Separate prototype vibration sensors derived from FBG pairs were used to measure vibration of an ESP pump. Like the temperature measurements, these sensors were interrogated using OFDR. The prototype sensors were attached along the pump housing with accelerometers and measurements at multiple speeds and flow rates were taken. Vibration spectra and mode shapes were calculated from the fiber and accelerometer measurements and compared. Motor temperature test results showed that the FBG sensors are capable of monitoring winding temperatures throughout entire length of the motor as well as at the bearing. In addition, the sensors were able to detect the location of insulation failure before the motor shorted. During epoxy curing FBG sensors were used to monitor curing temperature across the stator to ensure the consistent temperature distribution across the stator, preventing uneven heating and curing issues. In the seal the sensors demonstrated the ability to measure internal temperature along the shaft and at the thrust bearing, enabling accurate calculation of oil volume expansion rates. The vibration results show that the fiber-optic vibration sensors successfully measured the vibration frequency and the vibration modes of the pump. Together, vibration and temperature measurements enable the monitoring of ESP equipment health as well as detection of abnormal operating conditions, providing improved reliability as well as scheduled interventions.