Evaluation of fatigue crack propagation in a threaded pipe connection using an optical dynamic 3D displacement analysis technique

摘要

In the oil and gas industry threaded pipe couplings are commonly used to connect pipelines, risers, drill pipes, casings etc. When subjected to dynamic loads, the threads of the connection act like stress raisers that can initiate fatigue cracks. To increase the fatigue life, connection designs are continuously improved [1,2]. However, one of the main problems remains the online detection and monitoring of fatigue cracks even in laboratory conditions. Cracks tend to initiate in the contact interface at the thread roots away from the outer surface of the pipes. This makes it hard to have a clear distinction between crack initiation and propagation [3]. The most common techniques for crack sizing in threaded fasteners are alternating current field measurements (ACFM) [4] and magnetic flux leakage (MFL) [5]. Both techniques require the connection to be disassembled for the measurement, which makes continuous online monitoring impossible. When ultrasonic inspection is used for crack measurements in threaded connections, they can remain coupled. However, with conventional ultrasonic testing methods it is difficult to distinguish flaws from the signals reflected by the different threads. Therefore, advanced signal processing is necessary to make ultrasonic testing applicable for online monitoring of fatigue crack growth [6].