Abstract
For a positive displacement motor (PDM), the threaded joint connecting drive shaft shell (DSS) and universal shaft shell that is close to the bit is inclined to fracture. In this paper, elastic-plastic fracture performance of the threaded connection is simulated under both make-up torque and bending moment. Firstly, an FE model, which includes a cracked external thread of the DSS and an engaging internal thread of the universal shaft shell, is established and validated. Secondly, influences of both plastic deformation and the helix angle on fracture properties of the cracked thread are evaluated quantitatively. Meanwhile interactions between two cracks are also discussed. Finally, under the two kinds of loading conditions, i.e. loaded by pre-load only and loaded by both pre-load and bending moment, explicit relationships between characteristic J-integrals and the crack depth are obtained for the DSS.
Highlights
Positive displacement motor (PDM) is a new type of downhole motor that drives drill bit rotating to break rocks
PDM is composed of five assemblies of by-pass valve, anti-drop, power section, universal shaft and drive shaft, as shown in Fig. 1 [1, 2]
Due to the complexity of the problem, elastic fracture analysis is often conducted without considering the helix angle, while the error generated by this simplified model is rarely mentioned
Summary
Positive displacement motor (PDM) is a new type of downhole motor that drives drill bit rotating to break rocks. Luo & Wu [7] calculated the stress concentration factor (SCF) of a threaded joint of drill pipe under the action of both tensile and bending load They found the maximum stress concentration occurred at the root of the first tooth next to the external thread shoulder. Unlike improving the thread structure, Algimantas et al [13] presented a new procedural type method, named as duplex positioning method, to increase the operating lifetime of the threaded joint under eccentric loading These analyses all aimed at threads without defect, whereas failure of the threaded joint is mostly due to crack propagation. Wang et al [16] reviewed 21 cases of the longitudinal fractures in tool joint box of drill pipe by means of scanning electron microscopy and energy dispersive X-ray spectrometer It revealed that crack initiation was caused by tong tooth bite marks, friction damage and frictional heat check cracking. Variation of J-integral with crack evolution is formulated
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
