rate may be warranted to compensate for this uncertainty in soil conditions.”1 The bureau provided no specific guidance on how to adjust the data, but it did state that, regarding an average annual failure rate of 0.000044 failures per mile per year, “we believe this analysis supports Reclamation’s original response to the Committee’s question of what we define as reliably providing a minimum service life of 50 years for our pipelines.”2 This statement indicates that the threshold value that Reclamation provided (0.000044) is within its risk-tolerance range.

The committee believes that a threshold value must be defined by Reclamation, and whether this information is based on pipe failures of other pipe systems or on an economic risk-based procedure, some value is necessary to allow a comparative analysis to be performed. Without discussing whether the analysis for determining the threshold value used by Reclamation is appropriate, but realizing that Reclamation accepts the 0.000044 value as within its risk tolerance, the committee first uses the threshold value of 0.000044 failures per mile per year as suggested by Reclamation. This value is identified as Threshold 1. This threshold value can change based on other assumptions, so the committee examined two additional possible threshold values derived from the gas pipeline data set for comparison with the performance of DIP with PE and CP in soils with soil resistivity values less than 2,000 ohm-cm. The two additional gas pipeline-based thresholds are listed below with Threshold 1:

  • Threshold 1: Gas pipeline failure data from 2002 to 2008.

  • Threshold 2: Gas pipeline failure data from 2002 to 2008 (data for Threshold 1) that include four additional leaks associated with external corrosion of welds, elimination of leaks for pipeline in service for more than 50 years, and the use of all mileage for bonded dielectric steel gas pipeline.

  • Threshold 3: Gas pipeline failure data from 1974 to 1983, a time period during which regular inspections were not required by law.

The threshold provided by Reclamation of 0.000044 (Threshold 1) articulates the bureau’s risk tolerance. 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 representative of Reclamation’s risk tolerance. However, the committee believed that it was necessary to develop additional threshold values to identify the potential variation

1

Michael Gabaldon, Bureau of Reclamation, letter to Emily Ann Meyer, National Research Council, 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.

2

Gabaldon, letter to Meyer, August 21, 2008.



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