Discrete depth sizing beyond 4 mm for crack-like anomalies has been made available recently for large-diameter ILI tools. For small-diameter pipelines (OD ? 12?), additional challenges to the measurement performance have to be considered and addressed by the inspection tool design before extended depth sizing can be provided in a robust and reliable manner.

The sharp circumferential curvature of small-diameter pipes has a focusing effect on the ultrasonic beam, thus altering the characteristics of the sound pressure distribution and signal strength. For small-diameter pipelines, the ultrasound propagation angle in the pipe wall can significantly differ for internal and external axial anomalies because of the difference between the curvature radius of the outer and inner pipe surface (OD vs. ID).

Small-diameter pipelines generally have a limited tolerance for variations of the incidence angle. Therefore, UT-C inspections of small-diameter pipes are generally more sensitive to the operating conditions and in particular to sound velocity variations of the coupling medium (e.g., due temperature variations during the run).

The limited space available for the ILI tool in the small-diameter pipe poses an additional challenge for the mechanical design and housing of all components required for the high-resolution UTCD tool.

This article discusses the aforementioned limitations in detail and presents technical solutions to realize high-performance UT-C tools for small-diameter pipelines. Furthermore, it presents the performance qualification results of a new generation of extra-high-resolution UTCD tools for 6?, 8? and 10? pipeline diameters.