Nonlinear Harmonic Monitoring of Gouged Dents in Pipeline Specimens Under Cyclic Loading

Abstract

The only in-line inspection technology commercially available for quantitative evaluation of gouged dents is the geometry pig which cannot discriminate between gouged and smooth dents and has no sensitivity to re-rounded dents. Southwest Research Institute® (SwRI®), has been funded by the US Pipeline and Hazardous Materials Safety Administration (PHMSA) and the Gas Research Institute (GRI) through the Pipeline Research Council International (PRCI), to determine the capability of the nonlinear harmonic (NLH) method to characterize the severity of gouged dents, including those that have been re-rounded by internal pressure. This paper describes the NLH method and presents a summary of results from previous work involving burst tests of gouged dents in 24” pipe as a precursor to the current work that involves experiments with four pressure chambers made from 12-inch line pipe under cyclic pressure changes. In each case, internal scanner hardware, driven from outside the pipe, deployed NLH probes against the pipe inner surface, the gouges being on the outer surface. Analysis of the mapped NLH signals on the inner pipe surface revealed residual strain patterns in the pipe and the strain anomalies produced by gouging. The strain anomalies clearly indicated the presence of the gouges on the outside surface, even when they had re-rounded. The signal maps also indicated the length and width of the gouges whereas the signal strength indicated the residual depth. Data are presented showing that the NLH method is capable of ranking the severity of pipeline gouged dents and their propensity for failure under cyclic loading.