Review of the Bureau of Reclamation's Corrosion Prevention Standards for Ductile Iron Pipe (2009) / Chapter Skim
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6 Findings, Conclusions, and Recommendations
6 Findings, Conclusions, and Recommendations
Pages 131-142
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From page 131... ...
The charge of the committee included a responsibility to reply to two specific questions: • Question 1 -- Does polyethylene encasement with cathodic protection work on ductile iron pipe installed in highly corrosive soils? • Question 2 -- Will polyethylene encasement and cathodic protection reliably provide a minimum service life of 50 years?
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From page 132... ...
. In spite of the improvement obtained by the addition of PE and CP to DIP, the pipeline failures seen for DIP with PE and CP have occurred with maximum linearized pitting rates Particularly troubling to the committee were the use of the weighted average or mean maximum pitting depths without maximum and minimum pitting depths, or the distribution of pitting depths reported, the combination of pitting data from various sites with very different corrosion behavior, the lack of reported time-dependent pitting depths (when such results are available)
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From page 133... ...
against which to measure the performance of ductile iron pipe installed in severely corrosive soils with polyethylene encasement and cathodic protection." From the OPS data set, Reclamation calculated a failure rate of 0.000044 failures per mile per year. The most important aspect of this failure rate is that despite many shortcomings considered and discussed earlier in this report and mentioned below, it did set a threshold level of risk tolerance for Reclamation.
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From page 134... ...
Attempts were made to estimate the fraction of soils that are highly corrosive and the fraction of failures that would have occurred in such highly corrosive soils where gas pipelines are located, but these attempts ended with the realization that these estimates were highly speculative and not well supported by any engineering data available to the committee. Therefore, the committee under stands that the failure rate for pipelines buried in highly corrosive soils will likely be higher than the rate for the entire system, but it has no means of quantifying the degree of that likely increase.
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From page 135... ...
The committee then sought to find failure data on DIP with PE and CP to compare to the benchmark. Only about 350 miles of such water pipeline that was buried in highly corrosive soils could be found.
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From page 136... ...
DIPRA studies involved burying 4- to 8-foot sections of bare DIP, as-manufactured DIP, DIP with intentionally damaged PE, and DIP with PE in corrosive soils in various locations and excavating sets of the pipes at regular intervals to evaluate maximum pit depth at each burial time. Mean maximum pitting rates were calculated on the basis of aggregate data for all burial times, and in many cases by aggregating information for multiple sites.
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From page 137... ...
The committee also collected data on maximum observed pitting rates of DIP with PE from various other field studies, as summarized in Tables 3-7 and 3-9 in Chapter 3. The data in Table 3-7 for maximum observed pitting rates under PE frequently give linear pitting depth rates significantly greater than 5 mpy, indicating that by conventional industry extrapolations, these pipes could fail in much less than 50 years and will not meet the Reclamation threshold in corrosive soils.
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From page 138... ...
However, others who recognize these principles would argue that experience has shown them to be unimportant or not contributing to significant corrosion. With respect to the alternative responsibility in the committee's charge, since the committee answered question 2 in the negative, it is asked by the Bureau of Reclamation to "provide recommendations for alternative standards that would provide a service life of 50 years." After considerable study and deliberation, the committee finds that using the performance of bonded dielectric coatings on steel pipe with cathodic protection as a benchmark for reliability, and based on available information, it is unable to identify any corrosion control method for DIP that would provide reliable 50-year service in highly corrosive soils.
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From page 139... ...
To the committee's knowledge, none of these pipelines has failed, but the length of pipe involved and the length of service for the pipeline prevent the committee from endorsing this as a method leading to the desired level of reliability. Beyond the lack of data for DIP with bonded dielectric coatings and CP, the principles used to understand the mechanisms that may prevent DIP with PE and CP from meeting the needed pipeline reliability criteria can in some cases be used to question the ability of DIP with bonded dielectric coatings and CP to meet the reliability benchmark.
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From page 140... ...
However, the committee does not believe that DIP with PE and CP is assured to provide the level of reliability expected by Reclamation over the 50-year pipeline service life in highly corrosive soils. The committee con sidered alternative corrosion control methods that would provide the desired level of reliability of DIP in highly corrosive soils.
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From page 141... ...
Studies should be initiated to evaluate some of the promising corrosion control systems considered in this study for water pipelines in order to determine whether the adoption of any of these systems would permit the system to meet the desired level of pipeline reliability. • The committee recommends that data on pipeline reliability be assem bled for all types of pipe specified by the Bureau of Reclamation in Table 2, entitled "Corrosion Protection Criteria and Minimum Requirements," in Technical Memorandum 8140-CC-2004-1 along with the specified cor rosion protection applied in the various soil types.
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