Enhancement of guided wave tomography for pipelines via improved transduction and signal deconvolution

• Main Author: Mohamed Mahadhir Mohamed Hamzah Caffoor
• Other Authors: Fan Zheng, David
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Aeronautical engineering
• Online Access: https://hdl.handle.net/10356/146141
• Institution: Nanyang Technological University

Structural health monitoring (SHM) as opposed to conventional NDT methods involve the placement of permanent sensors and improve the reliability, cost efficiency and allows for continuous monitoring of non-uniform degradation. SHM monitoring can be catergorised into local or area monitoring but area monitoring has the benefit of providing a better baseline to assess the structural integrity of the pipeline in cases where critical parts are unknown because local monitoring does not provide enough of a representation to assess the integrity of the pipeline in its entirety. In area monitoring SHM, methods such as isual or thermal cameras are limited to surface breaking defects which are unable to quantify thickness from the inner pipe wall. Vibration techniques are a possible method although they are insensitive. This leaves acoustic emission (AE) or guided waves (GWs). However, AE requires continuous monitoring without interruption due to the nature of the defect propogation that is random and irreversible. This has implications on battery life and therefore the use of GWs provide a more suitable approach. Guided Waves (GWs) are bulk ultrasonic waves that interact with the boundaries of the structural wall and travel along the length of the structure. Most cases of GWs are confined to the low frequency range, where attenuation is low, allowing long sections of structures, e.g. pipelines to be screened. Although this method allows a quick assessment of a large area with defects, it is difficult to obtain the information on the size of the defect. Alternatively, a pitch-catch arrangement between two arrays of transducers, can be combined with tomographic principles, over a shorter distance, to provide more accurate sizing of defects. This concept, referred to as GW tomography (GWT), and still poses challenges for implementation in industry. The strong attenuation of fundamental modes in liquid loaded pipelines and the need to use voltage due to the intrinsic safety of operation around flammable pipelines have significant implications on the need for an improve Signal to Noise Ratio (SNR). To increase the Signal to Noise Ratio (SNR) and increase the effective distance between arrays, stacking of PZT discs are investigated and the parameters investigated to optimize the design are the mode amplitude, bandwidth, and mode purity. It was observed that the designed transducer with 10 PZTs stacked provides the optimum central frequency, nearest to "0.5 MHz∙mm" for GWT applications. However, the bandwidth decreases with increasing height of the stack. This drop in bandwidth with increasing number of PZTs stacked is compensated for by the addition of an Alumina composite backing mass. While increasing mode amplitude provides the possibility of using a lower voltage of excitation, a deconvolution technique using a Linear Frequency Modulated (LFM) signal is investigated to improve the SNR. The technique involves excitation of an LFM signal and deconvolution of the response to a toneburst. While this method has been previously used to optimize the number of cycles and frequency of tonebursts used, the application here is to reduce the voltage. During this study however, a trend of increasing input SNR with a plateauing output SNR is discovered. This is achieved by using the inherent convolution of a rectangular filter with the chirp frequency response. It was found that as the input SNR of the excited LFM signal increases, the noise after deconvolution (attributed only to the ringing effect) increases with the increased signal amplitude, resulting in an unchanging SNR. This area can be used to lower voltage while maintaining SNR. Finally, this thesis discusses the reliability and integrity of the application of the designed stacked transducer and lowering of voltage through deconvolution in an on-site application to validate the findings. Full Waveform Inversion (FWI) based GWT is used to provide a reconstruction of measured signals and defects on the gas pipeline were monitored for a period of 5 months. A simple statistical model is then incorporated to monitor the growth of selected defects beyond the 5-month period to predict the possible failure or when intervention techniques such as lowering of pipeline pressure is needed to extend the life of the pipeline.