Review of the Bureau of Reclamation's Corrosion Prevention Standards for Ductile Iron Pipe (2009) / Chapter Skim
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4 Failure Criteria
4 Failure Criteria
Pages 99-115
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From page 99... ...
The objective of these questions was simply to obtain from Reclamation a threshold value other than zero so that a comparative analysis could be performed by the committee. In responding to the committee, Reclamation noted that an average annual failure rate of 0.000044 failures per mile per year was computed from gas pipeline performance data from the Department of Transportation's (DOT's)
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From page 100... ...
The two additional threshold values derived from gas pipeline data are necessarily estimates, owing to the lack of information on the corrosivity of the soils where the failures occurred; they may not be representa tive of Reclamation's risk tolerance. However, the committee believed that it was necessary to develop additional threshold values to identify the potential variation Michael Gabaldon, Bureau of Reclamation, letter to Emily Ann Meyer, National Research Coun cil, re: National Academies Review of the Bureau of Reclamation's Corrosion Prevention Standards for Ductile Iron Pipe -- A Response to the Committee's Request for Clarification on Project Scope, August 21, 2008.
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From page 101... ...
Another approach to determining a rational threshold for comparison with DIP with PE and CP is to use the failure rates of pipelines for conditions as reported in Table 2 entitled, "Corrosion Prevention Criteria and Minimum Requirements," in the Bureau of Reclamation's Technical Memorandum 8140-CC-2004-1, "Corrosion Considerations for Buried Metallic Water Pipe." If the failure rates for Bureau of Reclamation, U.S. Department of the Interior, Technical Memorandum 8140-CC-2004-1, "Corrosion Considerations for Buried Metallic Water Pipes," Washington, D.C., July 2004.
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From page 102... ...
If this information were available, it could constitute a fourth threshold value. VARIATIONS OF GAS PIPELINE THRESHOLD VALUES Threshold values are necessary to establish the probability of pipeline failure.
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From page 103... ...
However, this information was not readily available to the committee, and because the objective is to assess the potential variability in the threshold value, the committee believes that this is a reasonable estimate. In addition, Reclamation noted: The DOT database does not include information on the soil conditions in which the pipelines are installed, so we are unable to further screen the data to include only pipe installed in severely corrosive soils.
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From page 104... ...
Some of the steel gas pipelines, depending on company policy, may not have been adequately cathodically protected for the first 30 or 40 years of their life, until this was required by DOT regulations. The Eiber study eliminated duplicate and clearly invalid reports and reported nearly 400 service failures caused by external corrosion out of 5,872 incidents.
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From page 105... ...
was determined to be 22.5 years. Using the best-fit curve shown in Figure 4-1, the equivalent-age failure rate for the older steel pipes with an average age of 22.5 years was determined to be 0.000041 failures per mile per Thismileage-weighted average can be calculated using the following equation: Σ(mi ai ) mileage-weighted average age = , where mi is the mileage of pipe age ai.
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From page 106... ...
WEB 24 12 150 3,600 0 0 Southwest 25 20.5 75.8 1,895 1 0.000528 Mid-Dakota 12 9 49.6 595.2 0 0 Montrose 27 27 27 729 0 0 Hanna 20 20 7 140 0 0 Santa Margarita 23 23 7.5 172.5 0 0 Vacaville 24 24 3 72 0 0 Akron 7 7 0.25 1.75 1 0.571429 Trinidad 29 27.5 10 290 0 0 California City 25 16.5 1.5 37.5 1 0.026667 Denver 24 24 9 216 0 0 Sheridan 16 15 12 192 0 0 TOTALS 352.65 7,940.95 SOURCE: In Chapter 3 of this report, see Table 3-8, "Partial List of Cathodically Protected Polyethylene Encased Ductile Iron Pipelines." year. This threshold value is only slightly lower than that provided by Reclamation (Threshold 1)
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From page 107... ...
Alternatively, if the probability is lower than the values shown in Table 4-3, it will be concluded that the DIP with PE and CP meets Reclamation's performance requirements. Although the committee evaluated alternative methods for assessing failure rates, it does not know whether these failure rates fall within Reclamation's risk tolerance.
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From page 108... ...
However, because of other factors, there have been -- and will continue to be -- corrosion failures on dielectrically bonded steel pipelines with CP. Therefore, the committee elected to treat the DIP failure data in a manner consistent with the treatment of the DOT gas pipeline failure data.
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From page 109... ...
Using this information, the annual probability of failure for a given mile of DIP with PE and CP embedded in highly corrosive soils can be determined as follows: 11 Graham E.C. Bell, Schiff Associates, "Measurements of Performance of Corrosion Control Mecha nisms on DIP," presentation to the committee, Washington, D.C., July 29, 2008.
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From page 110... ...
4-4) The probability of having zero failures in a 50-year period for a given mile of DIP with PE and CP embedded in highly corrosive soils can then be determined as follows: P (0 failures)
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From page 111... ...
4-11) Using these data, Equation 4-11 was evaluated to obtain a length-timeweighted average failure rate for DIP with PE and CP in corrosive soils: 1, 895 Failure Rate′ = × Failure Rate1 + 7, 941 Average DIP with PE and CP 1.75 37.5 6, 607 × Failure Rate 2 + × Failure Rate 3 + × Failure Rate 4 (Eq.
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From page 112... ...
. Because 1.8 percent is much greater than all alternate values reported in Table 4-3, the probability of having at least one failure on a given mile of DIP with PE and CP is likely significantly higher than the benchmark threshold values reported in Table 4-3 for steel pipe with bonded dielectric coatings and CP.
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From page 113... ...
shown in Table 4-3. The threshold values from 12 As an example of how to use Figure 4-3: assume that an additional 353 miles of DIP with PE and CP embedded in highly corrosive soils were identified and that this pipe exhibited no failures; this would increase the pipe length from 353 to 706 miles, a 100 percent increase in pipe length; from this 100 percent change in variable point on the abscissa, draw a line up to the curve that represents "length of pipe"; at this intersection draw a horizontal line to the ordinate, indicating that the original probability of failure of 0.018 will be reduced by 50 percent to 0.009.
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From page 114... ...
Or, stated otherwise, reasonable changes in the input variables (three failures, 353 miles of DIP with PE and CP, and a 23-year reporting period) will not reduce the probability of failure of DIP with PE and CP below the threshold values shown in Table 4-3, or, more importantly, below the threshold values desired by Reclamation.
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From page 115... ...
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.
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