Corrosion Growth Analysis - Case Study of MFL-UT Combined inspection

ROSEN offers a combined inline inspection (ILI) service for Magnetic Flux Leakage (MFL) methodology and the Ultrasonic Testing (UT) with one inspection tool. These two methodologically independent measurements with one ILI run enable to supply a combined inspection service with many operational and qualitative benefits. The case study presented is based on a representative amount of high quality field verifications, covering a well defined time interval with repeated ILI measurements. This allows to quantitatively analyze the advantages of the combination also in regards to corrosion growth. The paper will show the enormous benefits of the combination of the robust, indirect MFL and direct UT technology 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 and how to compensate systematic errors from MFL sizing models. The most efficient method combines for depth sizing the UT shape information with the MFL amplitude, because of its high repeatability. This approach reduces the subsequent sizing tolerance to values, which allow the identification of hot spots and the corrosion prediction based on even short term repeated ILI intervals. Also the MFL/UT combination makes obsolete to rerun similar tools, because systematic errors can be compensated absolutely. 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 in the order of the field verification itself. Such accuracy allows for both the equivalent shortening of the re-inspection intervals and also for the prolongation of the validity of prediction. 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.