Inspection of Double-Shelled Storage Tank Bottoms using Ultrasonic Guided Waves from Side Wall

Storage tank inspection is performed commercially using multiple techniques. For tank bottom inspection, the vessel is typically emptied to allow access from the tank interior, or a submersible system is used. Although these approaches are practical in many industries, there are conditions where emptying the tank is too costly or dangerous, and submerging a robotic system is impractical. One example is inspection of the inner vessel bottoms of double-shelled storage tanks (DSTs) located at the Hanford site. These large tanks, built as early as the 1960s, are used to store hazardous nuclear waste. The tank bottoms were not inspected originally because access was not possible given the vessel contents. A leak that originated from the inner tank bottom was recently detected in one of the oldest DSTs, creating a need to develop new inspection approaches. This paper describes a guided wave inspection solution with the sensor located on the inner vessel sidewall. The electromagnetic acoustic transducer (EMAT) used propagates waves from the sidewall to the tank bottom; data is collected at multiple locations and combined to create an image of the bottom condition. One advantage of this approach is that nothing is inserted into narrow air slots that exist in a concrete pad between the inner and outer vessels, reducing the likelihood of the sensor getting stuck and allowing for a larger sensor system. Experimental results are shown from DST mockup tests being used to develop the sensor system.

References

1. Washington River Protection Solutions, “Request for Expression of Interest: Double-Shell Tank Integrity Project Primary Tank Bottom Non-Destructive Examination System,” Testing Protocol Document, 12 September 2016.

2. Hirao, M. and H. Ogi, EMATs for Science and Industry: Noncontacting Ultrasonic Measurements, Kluwer Academic Publishers, Boston, 2003.

3. Cobb, A. and J. Fisher, “Nuclear Containment Inspection Using an Array of Guided Wave Sensors for Damage Localization,” Review of Progress in Quantitative Nondestructive Evaluation Vol. 29, AIP Conference Proceedings 1211, D. Thompson and D. Chimenti (Eds.), America Institute of Physics, Melville, NY, pp. 1713-1720, 2010.

4. Rose, J., Ultrasonic Waves in Solid Media, Cambridge University Press, Cambridge, UK, 1999.

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