Analysis of Causes for Fracture of N80 TBG Tubing

To find out the causes for the fracture at the factory-end thread root of the Φ73.02 mm×5.51 mm N80 TBG tubing as occurred during the well workover, its fracture morphology, SEM morphology, and the macro/micro structures are analyzed, and its mechanical properties are tested so as to identify the mechanism of the said failure.The test results demonstrate that the physiochemical properties of the failed tubing are up to the requirements for N80 steel as specified under API Spec 5CT-2011;the metallographic structure of the tubing has little relevance with the fracture;and that the fracture is to some extent associated with such elements as making-up with over torque, declination of the tubingstring, and thecombined loading and offset stress as imposed upon the tubing that is not supported with a centralizer.