Corrosion-induced defects in long and inaccessible pipelines are a concern for the Department of Defense (DoD) because of the potential for leaks and oil spills on land and underwater caused by metal corrosion. Current practices of inspection techniques allow seriously deficient items to be repaired or removed from service, sometimes at inconvenient times and high cost as a result of emergencies. However, none of these techniques provides sufficient information to predict the growth rate of small defects. The objective of this project was to demonstrate the capabilities of commercially available ultrasonic guided wave technology for the detection, sizing, and growth monitoring of corrosion-induced defects in fuel piping.
Contrary to conventional ultrasound, which by equipment design excites wave propagation through the thickness of a material, guided wave ultrasound in a pipe propagates along the axis of the pipe over a long distance. By analyzing the reflections from defects at a long distance, the location and the severity of the defects can be determined by suitable calibration techniques. This project demonstrated that ultrasonic guided wave technology can be a noninvasive, cost-effective technology and that this can be achieved by simply recording the changes in ultrasonic guided wave signals periodically.
In Phase I, a pipeline was established in the facilities at the Naval Surface Warfare Center, Carderock Division (NSWCCD), incorporating welds, elbows, and hidden corrosion-induced defects to serve as a test bed for ultrasonic guided wave technology. Results demonstrated that a defect growth rate could be established based on ultrasonic signal characteristics and established the viability of this technology to monitor defect growth in pipelines in the field. This technology was demonstrated in Phase II in a steel pipeline at Norfolk Naval Station, Norfolk, VA. Transducers were installed on above and below ground pipe sections of mixed 10-inch, 8-inch, and 6-inch outside diameter piping with numerous bends, welds, and reducers. Access points above ground allowed convenient monitoring of pipeline conditions over a period of 20 months. This project monitored the achievable inspection distance, the stability of signals in variable environments, defect detection, and defect growth. It was concluded that there was not sufficient corrosion to produce a wall cross section loss exceeding 30%, an indirect indication of the efficacy of the cathodic protection system for the pipeline. Compared to the Phase I results, the signal-to-noise ratio in the old pipeline in the field was larger by a factor of four in most locations. Several locations suspected of having corrosion have not yet produced consistently increasing ultrasonic signals to warrant excavation and physical examination. It is recommended that monitoring be continued to further demonstrate this technology for future DoD and commercial use.
Some of the performance objectives were not met in the monitoring duration of 20 months because natural corrosion occurs slowly and because sufficient information was unavailable on the physical construction of the different sections of the pipeline, which could have served as location and defect calibration markers. It is recommended that a method to select promising locations for condition monitoring using permanent sensors in a pipeline should be preceded by a preliminary evaluation for weld signal response using remountable transducers. Additionally, locations where weld signals are well above noise level in the baseline should be identified first to enhance the success of defect growth monitoring later on. Extensive planning for the selection of measurement points is necessary to achieve good results.