Compressed natural gas (CNG) storage well is a kind of pressure vessel buried underground. The detection of corrosion, which may be induced by the surrounding soil and different medium, is important for the safety security of the well, and protects it from CNG leakage or casing explosion. Among non-destructive examination techniques for corrosion detecting, the ultrasonic techniques are popularly utilized, in which the phased array approach can offer distinct advantages. To investigate both reliability and applicability of the phased array technique in the storage well, a complex design with a 512-element ultrasonic phased array that covered the entire cross-section was discussed, and phased array parameters were determined, including array elements, array element size, ultrasonic frequency and so on. An ultrasonic testing system was designed based on the above design, including a frame for holding phased array probes, a specific vessel for storing ultrasonic cards and other components, and an in-pipe robot designed for instruments’ moving in the storage well. The general corrosion condition described by thickness images were captured by the ultrasonic testing system with B, C and D-scan functions, in which circumferential electronic linear scanning was performed by the phased array probes and axial scanning was done by a mechanical scanning device. A method for minimizing the external pressure from water column necessary for coupling was put forward in the scanning and detection process. The sample for CNG storage well with artificial defects was built in laboratory environments and experiments were conducted to validate detection effects, and the phased array technique provided good sensitivity and efficiency, which may lead to a successful application in CNG storage well examination.

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