Improvements in the Accurate Estimation of Top of the Line Internal Corrosion of Subsea Pipelines on the Basis of In-Line Inspection Data

The cost for repair or replacement of subsea pipelines is much higher than for onshore pipelines. To a large extent, the repair or replacement decision hinges on the outcome of fitness-for-service analyses that are in turn based on the results of in-line inspections. It is therefore of utmost importance to obtain in-line inspection data that are as accurate as possible. It has been reported in the literature that MFL tools may significantly exaggerate the localized wall loss for wet gas lines subject to top of the line corrosion. This paper reports the results of a study on a Chevron asset that was initiated to compare the performance of various inspection methods. Upon completion of the in-line inspections, a section of the pipeline was recovered off the ocean floor and subsequently replaced. The defect population of the recovered pipeline section together with the high-definition automated ultrasonic testing (AUT) results built the reference of the performance test of several inline inspection techniques like magnetic flux leakage (MFL), ultrasonic (UT) and a recently developed technology for accurate measurement of shallow internal corrosion (SIC) that is based on eddy current (EC) technology. The improvements in defect sizing that resulted from this investigation are reported.