In-line inspections have been part of the verification of pipeline integrity since the late seventies in N.V. Nederlandse Gasunie. The discovery of external microbial corrosion (MIC) in 1999 in one of the high pressure pipelines changed the inspection policy from inspection of a randomly selected pipeline once every 5 year to an inspection program for the whole high pressure grid (approximately 5.600 km) to be completed in 10–12 years. One of the MIC influenced lines is used to qualify MFL-pig from different suppliers. In the time period 1999–2004 4 different MFL qualification runs have taken place, resulting in 18 excavations. Rather simple calculation of corrosion rates after every pig run indicated values of 0.2–0.3 mm/yr as an average value for the MIC corrosion. After the fourth pig run (5 years after the first) Gasunie decided to determine the corrosion rate also in a more sophisticated way. In coorperation with the mathematical department of the Technical University of Delft statistical analyses were performed. The paper describes how the data from pig runs and excavations was used to extract a suitable subset of corrosiondefects for which the corrosion rate was determined. It was decided to use only these defects that were indicated by all suppliers as external corrosion thereby leaving out defects that were indicated as “mill defect but possibly corrosion”. A second criterion was that a defect has to be reported by at least three of the four suppliers. These criteria resulted in a subset of 52 defects to be analyzed. All of the reported defect depths were corrected for the bias that was determined from the excavation results. The corrected values were then used to calculate the corrosion rate using three approaches: calculation of the corrosion rate for every defect in two ways and calculation of the corrosion rate for the pool of defects assuming a corrosion growth that is linear in time. The average corrosion rate for the defects was in the range of 0.12 to 0.24 mm/yr. Dividing the data set into deep and shallow defects showed that the average corrosion rates for both subsets, 0.23 mm/yr and 0.25 mm/yr, are not significantly different. This underpins one of the assumptions that the corrosion rate is constant. Further discussion within Gasunie is envisaged to determine how these values can be used in the calculation of re-inspection intervals for this line. And at the same time another discussion will focus on whether these values can be used on other lines.

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