Failure analysis and structure optimization of the connecting thread of driving shaft in positive displacement motor

Xiaohua Zhu,Jianan Li,Dian Lin

doi:10.1177/1687814016652313

Xiaohua Zhu, Jianan Li + Show 1 more

Open Access

https://doi.org/10.1177/1687814016652313

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Abstract

Due to high efficiency in helping breaking rocks, positive displacement motor has a widespread application in the field of oil and gas drilling engineering. However, service life of positive displacement motor becomes shorter resulting from failure of the driving shaft thread. The three-dimensional simulation model of driving shaft’s connecting thread has been developed based on the principle of virtual work, Von Mises yield criterion, and contact nonlinear theory. This article has put forward the plan to optimize structure of driving shaft with straight thread based on the design principle of double-shouldered thread; besides, fatigue life of driving shaft with straight thread before and after optimization has been assessed by resorting to the multiaxial fatigue principle and Brown–Miller critical plane rule. In line with research results, mechanical properties of driving shaft with straight thread are superior to those of driving shaft with taper thread. Under the load of axial drill pressure with fluctuation and alternating torque, fatigue life of driving shaft with straight thread after optimization is greatly improved. This article has not only provided basis for optimizing connecting thread of driving shaft but also boasts of high engineering value for improving service life of positive displacement motor.

Highlights

With constant improvement on drilling technology, and popularization of direction well technology, horizontal well technology, and sidetracking well technology (STH), market demand of positive displacement motor (PDM) is on increase
It has been an important factor to affect service life of PDM whether the thread of driving shaft is subject to fatigue destruction under high impact and alternating torque
The results show that the driving shaft with straight thread has higher fatigue life, and stress level is one of the indicators which affect the fatigue life, subject to influence by the loading form, stress concentration, and so on

Summary

Introduction

With constant improvement on drilling technology, and popularization of direction well technology, horizontal well technology, and sidetracking well technology (STH), market demand of positive displacement motor (PDM) is on increase. Keywords Positive displacement motor, driving shaft, thread, failure analysis, double-shouldered thread, fatigue life Based on studying the static mechanical properties of the upper thread of driving shaft, this article analyzed and compared the fatigue life of different types of thread under variable load.

Results

Conclusion