Evaluation of Standard API Casing Connections and Parametric API Buttress Improvement by Finite Element Analysis

Threaded and coupled connections generally present a non-uniform stress distribution, which is related to the higher stiffness of the box when compared to the pin. The non-uniform stress distribution can contribute to fatigue cracks and other failure modes in high pressure oil wells. An API 5CT P110 steel obtained from a seamless pipe was mechanically characterized. FEA models were carried out to investigate and compare two API casing connections under make-up torque and tensile efforts. A parametric study was performed using numerical models to determine the influence of some geometric features on the behavior of the API Buttress threaded connection. The API Buttress connection supported higher tensile loads than the API Short Round, however both standard connections showed high stress concentration in the last engaged thread and a non-uniform stress distribution. The manufacturing of grooves at the first and the last engaged threads proved to be an efficient way to reduce the stress concentration of Buttress casing connection and could be an alternative to the development of new products.