Sucker Rod Couplings: Fatigue Failures and Stress Cracking Under Corrosive Environment

The sucker rod joint is designed to be made up to a given preload stress level in order to prevent separation between the pin shoulder and the coupling face during operation. As long as the connection is correctly made up, non-failures are expected to occur working within the fatigue limit of the design. Nevertheless, sucker rods joint failures occur constantly in oil fields around the world. It has been observed these failures both in loose joints and in properly made up connections. When the connection is loose, additional loads appear on the threads, not only adding more tensile stresses but also some flexion components that increase the risk of fatigue both in the pin and the coupling (in both cases in the last engaged thread, the most stressed). These failures can occur with equal probability in either the pin or the coupling. Once initiated, the failure growths by fatigue mechanism. But in several cases, coupling failures with the same apparent pattern as in a loose connection have been observed in properly made up ones at the failure moment. A common element detected in all these failures was the operation in corrosive environments. The goal of this work is to assess the performance of different threads and materials having the same manufacturing method (cold rolled threads) under stress corrosion conditions. Comparative tests were performed in different sucker rod coupling grades. Modified NACE TM0177 Method A Tensile Tests (Proof Ring) as well as full scale fatigue tests under corrosive environments were performed. The results show that the performance of a Sour Service steel (widely use in OCTG), cold rolled, is better than an 8630M rolled material and even better than the same steel with cut threaded element.