Accurate Short Period Corrosion Growth Rates From Combined MFL-UT Inline Inspection: A Case Study

Abstract

Inline inspection (ILI) identifies most significant anomalies - at the time of inspection. To recognize those individual features, which have the highest potential to enter these ranks, perhaps soon, is a challenging question, which is not necessarily covered with established ILI approaches. Whether the circumstances permit the identification and reliable description of individual corrosion hot spots depend on the ILI quality, the specific feature shape-dependent sizing, the comparability of the data and the time-dependent development of the anomaly. This case study demonstrates the enormous benefits of the combination of two technologies. The robust, indirect Magnetic Flux Leakage (MFL) methodology and the direct Ultrasonic Testing (UT) ILI tool. The intelligent interpretative combination of the two ILI information sources exceeds significantly the straight forward statistical combination benefit. The presented approach demonstrates how to: • identify features not suitable to UT stand alone sizing. • compensate systematic errors from MFL sizing models and calibration assumptions. • introduce UT shape information as major improvement to MFL sizing. • apply MFL amplitude based depth sizing. • reduce the subsequent sizing tolerance to values, which allow corrosion prediction based on even short term repeated ILI intervals. Applied to the identified group of pinholes and embedded pinholes with an average maximum depth of more than 50% the method delivered a depth sizing accuracy of ±6.0% wall thickness at 80% certainty. The fact, that even smaller corrosion growth rate values become resolvable, allows for the equivalent shortening of the re-inspection intervals and for the prolongation of the validity of prediction, whatever may be advisable in specific cases. Also it is shown, that this approach does not require to rerun similar tools, because systematic errors can be compensated absolutely with this approach. The optimization of inspection strategies and methods with regard to specific pipeline problems can have monetary benefit even at higher inspection costs. More sophisticated ILI systems requiring specialist data analysis become justified for equivalent pipeline use cases. The presented combination of the two independent ILI methodologies bears significant useful potential to approach aggressive corrosion mechanisms, e.g. microbial corrosion or CP shielding.