Electric submersible pumps (ESPs) are widely used for high volumetric oil and gas recovery from downhole wells. ESPs in oil fields can be equipped with multiple ac motors in tandem. To assist a motor start-up, a solid-state soft-starter can be deployed in such ESP wells. ESP shafts have a long axial length, and thus experience significant dynamic torsion when started from reduced voltage soft-starters. Premature shaft breakdowns had occurred in the field during the motor start-up for soft-starter fed ESPs. This paper investigates the shaft breakdown phenomenon in a 750-HP ESP system using a real-world case scenario. A lumped parameter electro-mechanical model for the ESP system is developed to observe its dynamics. The effects of static as well as fluctuating torsional stress on the interconnected pump shafts are analyzed to determine the root cause of the shaft breakdown. Criteria for successful start-up for soft-starter driven ESP systems are also presented to increase the equipment's operational life.

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