2006 Water Comprehensive Plan - City of Bellevue

PIPELINE RENEWAL AND REPLACEMENTBellevue’s program for pipe replacement is based on a plan that balances the cost ofreplacement against corrective action after failure and anticipated preventative maintenance forthe residual life of the pipe. To accomplish the that objective, a predicted useful life for thevarious pipe materials is combined with actual field experience for all pipe in the system todetermine the likelihood of failure. The likelihood of failure is combined with an assessment ofthe consequence of failure to determine business risk. The renewal and replacement (R&R) planthen recommends pipe replacement in the most cost-effective manner, while minimizing thenumber of pipeline failures that would result in significant customer inconvenience andproblems.The City’s program recognizes that the entire piping system will be replaced over a time periodequivalent to the useful life of the pipe. This analysis refines replacement criteria and estimatesresulting projected expenditures over time so that appropriate water rate revenue adjustmentscan be made to provide the necessary funding for the R&R program.The City preliminary estimate for pipeline replacement costs and timing (to predict the cashflow requirements) is based on the age, diameter, pipe material, length of pipe in the system,projected time of pipe replacement, and estimated present value cost of the replacement. Thiswork is used during rate analyses to forecast required adjustments to water rates.Pipeline Replacement ProgramBellevue has a capital investment program to increase fire flow protection by replacing smalldiameter (4-inch and some 6-inch) AC pipe that restricts available fire flow to less than 1000gpm by the year 2020. This fire flow improvement program involves annual replacement ofapproximately 2.3 miles of pipe. This small diameter pipe is typically replaced with 8-inch DIpipe regardless of the size of the existing pipe. Coincidentally, the small diameter AC pipe isgenerally the oldest pipe in Bellevue’s system.Figure 7-1 includes anticipated pipeline R&R expenditures. These expenditures are based oneffective useful life estimates and incorporate the use of survival curves.Identifying Specific Replacement ProjectsThe development of the effective useful life data and projected expenditures does not identifyindividual pipelines is to be replaced at any particular time. The City must determine andprioritize the specific pipe replacement schedule. City staff develop the annual capitalimprovement schedule based on predicted life, maintenance and failure history, business riskexposure (criticality), and other considerations such as street resurfacing and related projects.Table 7-1 lists various criteria that the City could use to assist in prioritizing specific R&Rprojects as well as a suggested method for obtaining the information.7-6