This paper presents a feasibility assessment of eliminating the protector of an electric submersible pump (ESP) by transferring the torque from the motor to the pump using a magnetic coupling, thereby allowing the motor to be hermetically sealed from the wellbore fluids in the operating environment. The paper also briefly discusses means of handling the motor oil expansion as well as the pump axial thrust.
The industry seeks to enhance ESP reliability by designing protectorless ESPs. This requires new techniques to handle torque transfer, motor oil volume change and pump axial thrust, critical functions that a typical protector section performs. Magnetic coupling is a potential method to handle motor/pump torque transfer. For ESP applications, a radial magnetic flux type of coupling is considered. Finite Element software is used to identify the appropriate magnet materials, estimate the coupling dimensions, and predict the coupling performance under transient loads. A sensitivity study is also included to evaluate dimensional variations for different power requirements under different pump rotational speeds.
The analysis identifies two permanent magnet (PM) materials to be appropriate for the magnetic coupling. The length of the coupling is determined for three values of the transmitted power (100, 200 and 300 HP) when operating at three different speeds (3500, 6000 and 10000 rpm). The results show that the magnetic coupling length ranges from 0.95 m to 3.7 m for the considered power range (100 to 300 HP) when operating at a speed of 3500 rpm. The length of the coupling is found to be in the range from 0.65 m to 2.55 m for the speed of 6000 rpm and from 0.4 m to 1.8 m for the speed of 10000 rpm. Mechanical integrity analysis shows that only the case of 100 HP/10000 rpm operates at a subcritical condition, where the predicted total eccentricity has a permissible value. All other operating conditions are supercritical and the whirling deflections of the shaft are likely to exceed the available space within the air-gap. To make the coupling design feasible for these operating conditions, the use of intermediate radial bearings are necessary, which can be challenging in the mechanical implementation and assembly.
This study represents the first systematic and comprehensive attempt in the ESP industry to investigate and assess the feasibility of using a magnetic coupling to replace the conventional protector section of an ESP system. It shows that under low power requirement and high operational speeds, a magnetic coupling is feasible. For other conditions, additional challenges need to be addressed to stabilize the coupling and achieve longevity with the coupling design.