Ultrasonic instrumentation for accurate real time corrosion monitoring using permanently installed transducers

Corrosion assessment helps to ensure that a ship can be safe to use without catastrophic failure. Periodic human-conducted ultrasonic inspection is commonly used for the assessment of structural integrity of ships using time-of-flight (ToF) measurements. Unfortunately, the accuracy of the ultrasonic thickness estimate is reduced when the measurement is confronted by rough surfaces. In addition, such inspections require a lot of time and cost a lot of money to perform. However, continuous in service structural health monitoring of the hull could potentially translate into a reduction of maintenance costs by extending time intervals between costly dry dock periodic inspection and/or their durations. This thesis presents the development of a prototype measurement system for accurate real-time corrosion monitoring of wall thickness in sea vessels using permanently attached transducers. The first objective attempts to understand forms of corrosion and their impact on the stability of ToF wall thickness estimation methods by modelling. The second objective attempts to establish a method for accelerated corrosion to make testing possible in lab timescales to confirm the modelling and tracking the algorithm performance. The third objective is the development of custom cost-effective transducers. The last objective is experimental verification of the complete prototype system. The research has established five principal findings which are detailed below. Firstly, the modelling gave an insight into the effect of the underlying surface morphology on the ultrasonic thickness estimate. It was found that the adaptive cross-correlation (AXC) ToF estimation method gave most robust delay estimates to the simulated recorded echo signals from rough surfaces. The AXC reduced the standard deviation of error to within +/-0.7% whereas the other algorithms resulted in getting errors within +/-3.2%. Secondly, a test rig for accelerated corrosion of a steel sample for ultrasonic mon- itoring was designed based on the reversed electroplating process. An ultrasonic transducer was permanently attached to the other end of the corroding sample, and experimental echo waveforms were recorded throughout several hours of accelerated corrosion which corresponds to simulating six months of corrosion at sea. An analog front end for multi-channel instrument was designed to enable using the high accuracy ultrasonic instrumentation together with many transducers (fully prototyped for four independent channels). The ultrasonic monitoring of several channels could ultimately be used to serve many transducers permanently attached to a ship hull and record their pulse responses continuously while the ship is operating. An important part of the research was related to development of a fabrication technique to produce low cost ultrasonic transducers which can be attached to the surface of the structure being monitored. The developed transducer roughly cost around £12 in materials, and their operation was tested with the developed electronic prototype. During processing of the recorded experimental waveforms, it was found that two ToF estimation algorithms, the conventional cross correlation (XC) and the AXC should be used together. The XC performs well with uniform corrosion and mod- erate pitting corrosion. The AXC prevents unreasonable wall thickness estimates when the pitting corrosion is severe. The joint use of these two ToF estimation algorithms achieves a high degree of reliability for extracting wall thickness loss rate consistently and accurately