**Abstract:**
Based on extensive investigation and analysis of field failure incidents, this paper identifies the main causes of electric submersible pump (ESP) failures. The primary reason for ESP submersibles to fail is insufficient torque, which can lead to buckling or disconnection. To address this issue, several preventive measures are proposed to reduce the risk of ESP failure in downhole operations.
Electric submersible centrifugal pumps are widely used in high-yield, deep-well oil recovery due to their high efficiency. On average, these pumps can lift more than twice their own volume of liquid. However, in practice, ESP failures are common, with failure rates reaching up to 10% to 15% in some oil fields. These failures not only reduce the efficiency of the system but also result in significant economic losses due to the difficulty in retrieving the pump from the well. According to accident investigations, two direct causes were identified: one related to thread connections failing due to reverse torque, and the other caused by bolt fractures between pump and motor sections. This paper focuses primarily on analyzing the issue of thread undercutting.
**1. Analysis of Downhole Failure Causes**
Field surveys have revealed three key reasons for thread undercutting in ESP systems. First, a large amount of back-torque is generated during the start-up and normal operation of the pump. Since the pump's pipe joints are right-handed threaded (M90 × 1.75), and the motor rotates clockwise, the torque is transmitted through the pump shaft, impeller, and guide wheel to the pump cylinder in the same direction. During start-up, the initial torque is higher, creating an impact that may cause the pipe threads to loosen.
Second, insufficient pre-tightening torque on the pipe threads can lead to loosening under combined axial and rotational loads. Proper pre-tightening force is crucial for maintaining the integrity of the connection. If it is too low, the joint may become loose; if too high, it could cause plastic deformation of the pump cylinder, leading to relaxation of the thread. This can result in the anti-pouring block being overloaded, potentially causing it to break off or be sheared, resulting in the pump falling into the well.
Third, the anti-dumping mechanism itself is often unreliable. The anti-pouring blocks are typically thin, with small weld areas and poor welding quality, especially when connected to medium carbon steel components. This leads to frequent failures, such as desoldering or shear-off, which further increases the risk of pump failure.
**2. Preventive Measures Against Undercutting**
**2.1 Determining Appropriate Pre-Tightening Torque**
To prevent thread loosening, the actual pre-tightening torque must be carefully calculated. For triangular threads, the tightening torque should be greater than the loosening torque. The relationship between the torque required to tighten and loosen the threads can be expressed using equations based on thread geometry and material properties. Additionally, the pre-tightening torque must be sufficient to withstand the maximum back-torque experienced during pump operation, which is typically the starting torque. It should be set at 1.54 to 1.98 times the rated torque to ensure reliability without overloading the system.
**2.2 Reversing Thread Direction**
Changing the thread direction from right-handed to left-handed can help prevent loosening caused by the rotation of the motor. While unexpected torque may still occur due to overload or power failure, it is generally much smaller than the starting torque and less likely to cause failure. Strengthening the existing anti-pouring mechanisms, along with improving the welding process, can further enhance the system’s resistance to undercutting.
In addition, special movable joints can be used at critical points to limit the range of torque and prevent excessive stress on the threads. These joints allow controlled relative rotation when torque exceeds a certain threshold, thus protecting the system from damage.
While these measures cannot completely eliminate the risk of undercutting, they significantly reduce the likelihood of failure and improve the overall reliability of ESP systems.
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