Using Multifrequency Near-Field Testing to Assess PCCP Mains with Multiple Pipe Classes

Near-Field Testing (NFT) is a well-established electromagnetic inspection technique for assessing Prestressed Concrete Cylinder Pipe (PCCP) with thin internal steel cylinders. As the thickness of the internal steel cylinder increases, however, NFT's accuracy in identifying wire breaks is known to decrease. This paper demonstrates that for every cylinder design, an optimal Near-Field Testing inspection frequency exists. This means that the observed sensitivity reduction in thick cylinder PCCP can be countered by selecting the optimal inspection frequency with an appropriate power level. For thick-cylinder PCCP designs, this typically means a much lower inspection frequency. Now, a PCCP pipeline often consists of a variety of steel cylinder thicknesses due to changes in pressure, soil loads, the presence of bends and valves, and other considerations. In addition, construction records can be incomplete, and it is not uncommon to find different pipe classes intermixed. This changing cylinder thickness presents a challenge to analysts as the selected inspection frequency can fall outside of the optimal zone, resulting in reduced inspection sensitivities—especially for PCCP constructed with thicker cylinders. However, further investigative work suggests that the simultaneous use of multiple inspection frequencies provides a near-optimal inspection frequency that is available to assess changing PCCP sections. The efficacy of this approach is described first using theoretical models and yard pulls, then using field data from a pilot project. The pilot project consisted of a 66-in. main constructed using a variety of pipe classes, with the lower pressure classes employing 16Ga cylinders and the higher ones employing PCCP with a 3/16-in. (4.7-mm) thick cylinder. By deploying a multifrequency tool, analysts had a clearer picture of the pipeline's true condition.