**Abstract:**
Based on extensive investigation and analysis of field failure incidents, the primary causes of electric submersible pump (ESP) failures have been identified. The main reason for ESP submersibles failing is insufficient torque, which leads to buckling. To address this issue, several preventive measures have been proposed.
Electric submersible centrifugal pumps are high-efficiency devices used in deep wells for oil recovery. On average, these pumps can produce more than twice the volume of fluid compared to the pump itself. However, in real-world operations, ESPs often fail. According to data from an oil field survey, up to 10% to 15% of ESPs fall into the well, causing a significant drop in efficiency and crude oil output. Once an ESP falls, it is difficult and costly to retrieve, leading to substantial economic losses for operators.
From the investigation of falling incidents, two direct causes were identified: one is the failure of the inverted block due to reverse rotation at the threaded connections between the pump section, cylinder, or the system and the tubing; the second is the fracture of bolts connecting the pump or motor sections. This paper primarily focuses on analyzing the undercutting problem.
**1. Causes of Undercutting**
Field investigations revealed three main reasons for undercutting in ESP systems. First, when the ESP starts and operates normally, there is a large back-up torque. This occurs because the pipe joints are right-handed threads (M90 × 1.75), and the motor rotates clockwise. The motor’s torque is transmitted through the pump shaft, impeller, and guide wheel to the pump cylinder, creating a torque that tends to loosen the connection. Especially during startup, the higher starting torque and impact may cause the thread connection to undercut.
Second, the pre-tightening force on the pipe thread is insufficient. Pre-tightening force plays a crucial role in ensuring the reliability of threaded connections. If the pre-tightening force is too low, it may not resist the axial load and horizontal undercut torque, leading to loosening. Conversely, if it's too high, it may cause plastic deformation in the pump cylinder thread, resulting in relaxation. This can lead to failure if reverse torque is present, especially if the anti-pouring block is weak.
Third, the anti-down mechanism is unreliable. Anti-pouring blocks are installed at threaded joints to prevent falling, but their small thickness and poor welding quality make them prone to breaking off or shearing during operation. This further increases the risk of ESPs falling into the well.
**2. Preventive Measures**
**2.1 Determine the Pre-Tightening Torque Properly**
To ensure the threaded connection does not loosen under combined axial load and back-up torque, the actual pre-tightening torque must be calculated. For triangular threads, the tightening torque should be greater than the loosening torque. For ESPs, the maximum back-up torque is the starting torque, which can be 1.4 to 1.8 times the rated torque. Therefore, the pre-tightening torque should be set between 1.54 to 1.98 times the rated torque.
However, the pre-tightening force must not be excessive, as it could exceed the yield point of the material. A formula is provided to ensure the preload stays within safe limits.
**2.2 Change Thread Rotation Direction**
Switching the thread from right-handed to left-handed can prevent loosening caused by the torque generated during normal operation. While other factors like overload or underload can still cause occasional torque, they are much smaller than the starting torque and less likely to result in failure. Additionally, improving the welding process of anti-pouring blocks enhances their reliability.
Although these measures cannot completely eliminate the risk of undercutting, they significantly reduce it. Using a movable joint at the top of the ESP and the oil pipe can also help control the torque within a safe range. When the torque exceeds a certain threshold, the joint allows relative rotation, preventing damage to the threads.
In conclusion, understanding the causes of undercutting and implementing proper design and maintenance practices are essential to reducing ESP failures and increasing operational efficiency.
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