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PIPELINE INTEGRITY ASSESSMENT
comparison of ILI data from different inspection years. Available data therefore suggested that active corrosion in the order of 0.26 mm/yr, as estimated by the operator, was unlikely.
Significant increase in metal loss features
Figure 1: Locations of the 5 steepest inclined slopes.
Additionally, metal losses of 10.36 – 18.77% WT were reported in year 1 at locations 5, 6, 7 and 8; such operational losses appear unusually high given the subsequent inspection readings. The year 6 ILI contractor reported that many of the MLFs, and particularly those in the final 6 km of the pipeline (where the most corrosion damage was seen), had the appearance of microbiologically influenced corrosion (MIC). The deepest features from each of the year 1 and year 6 ILIs were in completely different locations and so could not be matched to estimate a corrosion rate. Additionally, the significant increase in internal MLFs in the year 6 ILI meant a simple arithmetic mean calculation could not be performed to determine the average increase in WT loss.
Pipeline Region (as per Fig. 1) Region 1 Region 2 Region 3 Region 4 Region 5
18 16 IMT August/September 2014
The alternative method was to consider a sub-set of the data points (specifically the largest ‘n’ values) for each ILI. Considering only the 100 largest features gave a mean feature depth of 1.16 mm for the year 1 ILI and 1.40 mm for the year 6 ILI, a difference of 0.24 mm. On this basis an average corrosion rate of 0.05 mm/yr was estimated for the 5 year inspection interval.
Step 3 – Flow modeling to assess potential for water hold-up
Flow modeling was performed to estimate the extent of water hold-up at various points along the pipeline. Examining the pipeline topography, the steepest inclined slopes were found to all be present in the final 6 km. The angles and locations of these regions are presented in Table 3 and Figure 1
Inclination Angle (degrees) 4.28 6.72 6.08 6.24 8.66
Table 3: Angles of 5 steepest pipeline inclines. respectively.
The water hold-up was estimated using the inclination angles shown for each of these worst case (steepest) regions. The results of the flow modeling exercise, shown in Table 4, suggested possible water wetting of up to 55% of the internal pipeline perimeter, which was in reasonably close agreement with the spread of the feature orientations shown by the year 6 ILI data.
Step 4 – Overall review of findings to assess corrosion threat
Corrosion rates from inspection data
Based on the alternative approaches applied when examining the inspection data, corrosion rates in the range of 0.05 to 0.16 mm/yr were estimated. Analysis of the UT data did not yield any corrosion rates in excess of 0.10 mm/yr. The maximum estimation of 0.16 mm/yr was based on a MLF match between the year 1 and year 6 ILIs, and did not take into account possible ILI tool measurement error. Direct comparison of UT examination results should be considerably more reliable than direct
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The significant increase in recorded MLFs during the year 6 ILI could be considered in the context of ILI tool reporting thresholds. If assuming the lowest estimated ongoing corrosion rate of 0.05 mm/ yr between years 1 and 6, this would give a WT loss of 0.25 mm, equivalent to a 3.5% WT increase (nominal WT of 7.14 mm). Many of the ‘new’ features from the year 6 ILI were ~10% WT in depth. Therefore, it is possible that many of these features were present at the time of the year 1 ILI (but were below the reporting threshold) and had increased in depth sufficiently so as to be reported in year 6, without necessarily requiring a significant corrosion rate.
Pre-existing corrosion at pipeline start-up
Considerable corrosion damage was reported in the year 1 ILI suggesting corrosion rates as high as 4.14 mm/yr (face-value 29% WT deepest MLF and 6 months operation). The maximum localised (shallow pitting) corrosion rate demonstrated to be caused by severe MIC (the suggested corrosion mechanism) is ~2 mm/yr [1] and more usually closer to 1 mm/yr [2].
More significantly, locations 5, 6, 7 and 8 in Table 2 reported UT measured feature depths in year 1 of between 10.36 – 18.77% WT; equivalent to corrosion rates over the 12 month interval between pipeline start-up and inspection of 0.74 – 1.34 mm/yr. These rates appear far too great for operational losses given by subsequent inspection readings. It could be concluded that pre-existing corrosion at start-up
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