A Convex Phased-Array Ultrasonic Inspection Gauge for Small-Diameter Pipelines With Enhanced Resolution and Sensitivity

Accurate measurement of wall thickness and defect detection is essential for pipeline integrity. Small-diameter pipelines (4-6 inches), common in distribution networks, flowlines, and process systems, are difficult to inspect because they are often buried, insulated, or otherwise inaccessible. Ultrasonic pipeline inspection gauges (PIGs) provide a solution, but to navigate the tight bends found in traditional ”unpiggable lines” a short form factor unibody PIG is required. Conventional unibody PIGs rely on multiple carriages and large transducer elements that achieve high signal-to-noise ratio (SNR) with fewer channels at the expense of spatial resolution. Conversely, reducing element size increases resolution but also raises the inactive fraction of the aperture, weakening signals and reducing beam uniformity. To address these limitations, this paper presents a proof-of-concept inspection instrument based on a convex phased array with small elements. Compensation is applied through coherent transmit sub-apertures and multi-element receive beamforming. Simulations using Huygens modelling show that a 128-element convex array achieves full circumferential coverage in 4-6′′ pipes. Importantly, the 128-element design provides approximately four times the resolution of a 32-element array while maintaining comparable SNR, thereby overcoming the traditional tradeoff. Two industrial-focused experiments confirmed the benefits of compensation. In the 4′′ pipe, compensation achieved an SNR improvement of 15.34 dB compared with uncompensated operation, along with a reduction in wall-thickness measurement error by 0.59 mm and in measurement uncertainty by 1.43 mm. In the 6′′ pipe, tests further demonstrated reliable detection of 2 mm flat-bottom holes, which equivalent to 0.8% of the pipe’s circumference. Together, these findings demonstrate that phased-array compensation overcomes the traditional trade-off between resolution and sensitivity, enabling compact ultrasonic PIG tools to inspect 4–6′′ pipelines with high reliability.