Novel Techniques to Reveal Defects Hidden in Wall-covered Building Risers

Recently, guided ultrasonic waves (GW) based inspection technique has been proven effective in detecting risers’ defects. In this paper, it presents the results obtained by using GW to inspect the defects occurred in real gas risers that are commonly installed in tall buildings. Risers extend from the external wall of the building, penetrate through the concrete wall, into the kitchen or bathroom of each household unit. All metal-based in-service risers are prone to corrosion due to water leaks into the concrete wall. However, the defects occur in the section of riser, which is covered by the concrete wall, is difficult to be inspected by conventional techniques. Hence, GW technique was employed. The experimental results show that GW can partially penetrate through the riser’s section that is covered by wall. The integrity of the wall-covered section of a riser can be determined by the reflected wave signals generated by defective area that may exit inside the wall-covered section. Based on the reflected wave signal, one can determine the health of the wall-covered riser. Because different GW mode conversions and noise are inevitably generated by wall-covered sections of riser, the received signals that contain the defect information are often overwhelmed by these undesired disturbances. Hence, the received signals must be analyzed by effective signal processing methods so that the critical defect information can be extracted from the blurred received signals. Empirical Mode Decomposition (EMD) has been proven to be effective in analyzing non-stationary signals. Moreover, the employment of wavelet transform can be adopted in the sifting stage of EMD to improve its final outcome. In the sifting stage of EMD, wavelet de-noising was applied to eliminate unwanted frequency components from each decomposed signal. This combined method, which is named the Enhanced Empirical Mode Decomposition with Wavelet (EMDW) greatly enhances the performance of EMD in guided wave analysis for highly contaminated signals. With the help of this advanced method, the location and severity of the corrosion were successfully determined. Hence, the likelihood of riser ruptures that may trigger gas leakage and explosions can be avoided.