Executive Summary : | Small damage in an industrial pipeline may progress over time and cause leakage leading to monetary losses and sometimes a catastrophic event. Thus, damage detection in such pipes is essential. The traditional point-by-point ultrasonic inspection method is a time consuming one as the transducer probe has to be moved along the length of the pipe, either manually or using an automated mechanism. Many a times, it is not possible for the operator to access full length of the pipe for inspection, especially when the pipes are insulated. Ultrasonic guided wave (UGW) testing may overcome this issue as they interrogate a larger area when excited at a point using an appropriate transducer. Thus, the UGW technique is fast and cost-effective. The UGW testing is performed by placing transducers around a pipe. The ultrasonic waves emitted from the transducers travel down both directions of the pipe. If they come in contact with a damage, they are reflected back towards the transducers that collect the data automatically. Such a method wherein actuating and sensing transducers are mounted at a place and uses reflected signal for detecting a damage is called pulse-echo method. Whereas, the method in which the actuating and sensing transducers are mounted at a distance apart and uses variation in the linear characteristics such as time-of-flight, amplitude, phase, etc. of the transmitted signal for detecting damage in a pipe is called as pitch-catch method. Most of the commercially available UGW testing methods use pulse-echo approach. The accuracy of the pulse echo method is dependent on the sensitivity of the transducer to sense the reflected signal coming from a damage. A smaller damage may cause weak reflection that may get further attenuated because of material damping and thus, the damage may remain undetected. There is always a threshold size of the damage below which the damage may not be detected in the case of pulse-echo technique. This problem is relatively more severe in the case of insulated pipes because there is even no scope for visual inspection. The pipes that are insulated are highly susceptible to corrosion. The corrosion is generally spread over a larger area and in the initial stage, its depth is very small compared to the pipe thickness. The reflection from such corrosion damage is feeble and thus, corrosion often remains undetected in its initial stage in the insulated pipes. Thus, the proposed research will focus on the pitch-catch approach to detect smaller damages in the insulated pipes. The UGWs exhibit changes in their linear characteristics after passing through a damage. The formulation of an effective damage index based on the minute changes in the linear characteristics of the transmitted wave signal may help significantly to detect a shallow damage like corrosion in an insulated pipe. Additionally, development of a suitable algorithm may further help locating such a damage in an insulated pipe with higher accuracy. |