A new concept for the improved inspection of corrosion under the annular plate region in large storage tanks using a medium-range ultrasonic guided wave technique that uses a collection of Higher Order Modes Clusters, called here as HOMC. This technique employs the piezo-electric mode of generation of ultrasonic guided waves modes with appropriate wedges. Ultrasonic guided waves (axial mode waves, in this case) once generated will be reflected from corrosion and other features on the annular plate. Inspection can be carried out from the outside of the tank even when the tank is in service.

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Corrosion detection in the annular plate region of above-ground storage tanks is a critical need for tank farm operators in the oil and gas industry. The critical zone between the tank shell wall and the first few inches of the annular plate is difficult to inspect with conventional floor scanning methods due to the presence of coatings, uneven surfaces and a lack of access due to the presence of the weld toe. This region, however, is prone to accelerated corrosion due to the additional stresses caused by the weight of the tank wall and the increased possibility of water entrapment under the annular plate. Repairs in this region demand replacement of the entire annular plate and this leads to long shutdown of the tank, and often failures occur without any warning. A new method using the Higher Order Modes Cluster Guided Waves (HOMC) to detect and screen defects in the critical inaccessible zones in the annular plate is discussed. This testing can be carried out online while the tank is in operation using HOMC guided waves generated on the outer exposed region of the annular plate. Robotic scanners allow for the inspection to be carried out in relatively short periods of time. Data collected in the laboratory as well as from the field trials are presented. The HOMC technique has potential for non-destructive inspection of inaccessible regions in structures for medium-range applications (2 to 3 m).