Failure mechanism of sucker rod coupling

Failure of sucker rod coupling or tubing is a major problem in sucker rod pumping systems, which occurs constantly in oil fields around the world. Such failures can lead to heavy maintenance work and higher expenses in oil production. To date, no economical method has yet been proposed to address the problem. In this paper, three samples with different service times are selected from California oil fields and are analyzed in details. A comparison of the dimensions and macro-morphologies of the samples with different service times is used to reveal the failure process. Failure mechanism is discussed based on the analysis of the micro-morphologies, microhardness, microstructures, and chemical composition of the samples. The analytical results indicate that the damage is initially caused by abrasion between the coupling and the tubing; the contact load decreases as the coupling diameter decreases, and the synergistic action between wear and corrosion becomes the main damage cause. Large numbers of defects (pores) in the material can accelerate both corrosion and wear aspects of the damage. Alternating stress may affect the mechanical properties of the coupling, but it is not the main factor of failure.