A Failsafe Method of Preventing Voltage Generation During Installation and Pulling of ESPs with Permanent Magnet Motors

Abstract

With the increased use of permanent magnet motors (PMM) versus less efficient induction motors (IM) comes an increased safety risk. While both the IM and PMM can generate power themselves if their shaft is turned by external forces, the induction motor cannot generate enough power to require additional safety procedures. The PMM when spinning at 2,000 rpm can generate up to 240 volts to create a tremendous safety risk when one considers voltage over 50 volts can be lethal. The objectives/scope of this paper is to present an alternative method of preventing the shaft of a Permanent Magnet Motor (PMM) from rotating during the install and pull process. The result of implementing this method is the creation of safer working conditions for the field technician. The alternative method presented here utilizes a mechanical lock to physically secure the shaft thereby preventing it from rotating while the lock is in place. The mechanical lock mechanism is secured to the top end of the equipment string with the lock engaged during the installation of the Electric Submersible Pump (ESP) string. Once the motor is powered, the force generated automatically unlocks the shaft to allow for the normal operation of the ESP. To engage the lock for the pulling of the ESP string, a heavy weight is lowered on top of the shaft lock. The two present methods to prevent the pump shaft from rotating while installing or pulling are shunting of the cable and plugging the production tubing. Shunting involves tying all three phases of the cable together to short circuit the electrical system. However, the potential to rotate still exists, but requires an extremely large force. Plugging the production tubing contains the fluid column's potential energy from flowing back through the ESP, but this too has its completion and operational issues. The mechanical shaft lock method described in this paper uses shear pins that physically lock the shaft to prevent it from rotating. Only the mechanical motor horsepower has enough force to shear the pins loose; downhole pressure is not enough. Utilizing a PMM mechanical lock mechanism will physically prevent the shaft from rotating while installing or pulling a PMM-ESP string to provide a failsafe method to prevent potentially fatal conditions.