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.

大型储罐在石油和天然气行业中大量用于盛装液体。在停用检查期间，使用各种无损检测方法定期从储罐内部进行储罐底板的厚度测绘。从外部环形圈筛选储罐底部以确定储罐是否需要进一步检查或维修将节省大量时间。目前，低频超声导波通常用于筛选长段管道。在第一高阶模式的截止频率下，超声导波能够以最小的衰减长距离传播。然而，罐底存在多种衰减源：(1) 与罐内的流体接触，(2) 与罐外的土壤接触，(3) 地板总成。实际上，储罐的底板通常使用许多重叠的焊接钢板建造。在最大的储罐中，直径上的一条线可能由多达 15 个焊接接头组成。因此，最大限度地减少每个关节处的超声导波振幅损失对于成功筛选至关重要。在本文中，通过焊接搭接接头的振幅损失研究了基本剪切水平（SH₀ ) 和基本对称兰姆波 (S ₀ ) 模式。使用有限元模拟和实验，研究了 SH ₀和 S ₀在以搭接配置焊接的 12.7 mm 钢板中的传播。结果表明，SH ₀模式可以比焊接搭接接头下游的S ₀模式高6 dB 。因此，仔细选择模式和频率可能会导致较长的传播距离，并且可以使用超声波导波对地板进行筛选。