Propagation of Low Frequency Ultrasonic Guided Waves Through Welded Lap Joint

Large storage tanks are heavily used to hold liquid in the oil and gas industry. The thickness mapping of the storage tank floor is carried out from inside the tank periodically during out-of-service inspection using a variety of non-destructive testing methods. Screening the storage tank bottom from the external annular ring to identify whether the tank requires further inspection or repair would save a significant amount of time. Currently, low frequency ultrasonic guided waves are routinely used to screen long sections of pipelines. Under the cut-off frequency of the first high-order mode, ultrasonic guided waves have the ability to propagate over long distances with minimal attenuation. However, multiple sources of attenuation are present in tank bottoms: (1) contact with fluid inside the tank, (2) contact with soil outside the tank, and (3) the floor assembly. Indeed, the floors of storage tanks are typically built using many overlapping welded steel plates. In the largest storage tanks, a line across the diameter may consist of up to 15 welded joints. Minimising the ultrasonic guided wave amplitude loss at each joint is therefore paramount to a successful screening. In this paper, the amplitude loss through welded lap joints is studied for the fundamental shear horizontal (SH0) and the fundamental symmetric Lamb wave (S0) modes. Using finite element simulations and experiments, the propagation of SH0 and S0 was studied in 12.7 mm steel plates welded in a lap joint configuration. The results show that the SH0 mode can be up to 6 dB higher than the S0 mode downstream of welded lap joints. Therefore, a careful selection of the mode and frequency may result in long propagation distances and may enable the screening of the floors using ultrasonic guided waves.