The ultimate aim of this project is to develop a measurement technique for detecting fatigue cracks in the threaded ends of NQ Drill Pipes. Guided wave non-destructive evaluation was used as the method of testing, with the ultrasonic waves being produced by piezoelectric transducers. A number of signal processing methods were examined, including acoustic emission and using trends in the signal peak amplitude.
Initially a contour bottom transducer holder was designed with the base curved to fit onto an NQ Drill Pipe. Preliminary experiments were conducted on unjoined and joined pipes to develop an understanding of ultrasonics and obtain typical signals for the pipe threads. Three drill pipes were then run in the fatigue test rig with a bending cylinder and tensile cylinder pressure of 250psi and 1000psi respectively. Ultrasonic signals were recorded for each pipe a number of times leading up to final fracture. The transducers were placed at four marked locations around the circumference of the pipe for each test. The final experiment examined the effectiveness of small diameter piezoceramic transducers on free male pipe threads.
It was found that a saw cut in a free pipe was detectable. A crack in a male thread produced significantly lower amplitude in the second half of the signal and this trend was used in signal processing. Some results were found to be repeatable but there were many factors that influenced this. This experimental error created a large degree of uncertainty in all results. In the fatigue test rig experiments only one of the pipes showed repeatability. Signal processing using the trend mentioned above was used to successfully detect a crack in one of the pipes, but this identification was not confident. Data from cracked joined pipes is required to develop signal processing methods which would provide a reliable way to detect cracks in other pipes. The detected crack was from a signal taken 2500 cycles before pipe failure, which is much closer than any other signals recorded for other pipes. The piezoceramic transducers were unsuccessful as the properties of the transducers coupled with those of the pipe, creating a low resolution result. It is recommended that a small diameter transducer be used in an accurately marked positioned to enable repeatability and a higher resolution signal.