To further examine the finding that DIP with PE and CP does not meet the Reclamation threshold, the committee considered the maximum observed linearized corrosion rates from Tables 3-2, 3-4, 3-7, and 3-9 in Chapter 3. Table 3-9 treats data for DIP protected with PE and CP. Most of the data available were for failed pipes, for which the corrosion rate was calculated by dividing the wall thickness by the years of service before failure. Table 3-9 includes data on the failed wastewater lines not used in the failure analysis earlier, but those data represent the lowest corrosion rates in the table.
When the maximum corrosion rate data available for bare and as-manufactured DIP (Table 3-2), DIP with damaged PE (Table 3-4), and DIP with undamaged PE (Table 3-7) are examined, it is found that in these categories as well as in the subject category of DIP with PE and CP above, the worst-case corrosion rate can well exceed the corrosion rate thought to be acceptable for reliable service. The committee recognizes that all of these data were selected as the maximum observed corrosion rates and therefore are not typical of the expected performance of most pipeline segments; they represent the tails of the failure distributions. All data in these tables suggest that there are conditions in which these pipes will corrode at undesirable rates and that these data reinforce the conclusion that it cannot be ensured that DIP with PE and CP will achieve the reliability expected by Reclamation.
The committee used various assumptions and methods to calculate different threshold values. These threshold values were then used to compare the performance of DIP with PE and CP and to determine whether DIP with PE and CP meet the various threshold requirements, but more specifically the threshold requirement provided by the Bureau of Reclamation. The probability analysis based on annual failure rates indicated that DIP with PE and CP does not meet any of the alternative threshold values as reported in Table 4-3 and that it is almost an order of magnitude higher than the threshold value requested by Reclamation. Two sensitivity analyses based on three valid failures supported the conclusion that DIP with PE and CP likely exhibits poorer performance than the threshold requirement provided by Reclamation. Comparison of maximum observed pitting rates for various DIP systems indicates that rates at the tail of the distribution can be similar for bare DIP pipe systems, as-manufactured DIP pipe systems, DIP with damaged PE, DIP with undamaged PE, and DIP with PE and CP. In summary, based on the information collected for this report, DIP with PE and CP will not “reliably” (quoting the committee’s statement of work) provide a minimum service life of 50 years and will not meet the threshold requirement set by Reclamation.