ERMUSR Misc 05-10-2007 - Laserfiche WebLink

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Failure of underground pipe becomes very evident when sinkholes form to swallow up cars, which happened in the city of Fort Lauderdale, Fla. a statistical distribution, such as abell-shaped curve. Within this dis- tribution, the average life of most pipes makes up the middle of the distri- bution and the ex- tremes-represented by the "tails" of the distribution curve-are either shorter or longer than the average. 'This bell-shaped curve of life ex- pectancy stretches out replacement investment needs over a longer period of time than would be the case if all pipes had exactly the same life expectancy. Stretching out reinvest- ment needs sounds good, but the total picture is a bit more complicated. Most water systems installed additional pipes as the utility expanded. Every installment of pipe assets will have its own bell-shaped curve of replacement needs. The suc- cession of bell-shaped curves will be staggered over time, and the distri- butions will overlap. To get an accu- rate picture of total replacement needs, the overlapping curves must be added together. The result is a replacement invest- ment forecast, or echo wave, that reflects the original waves of con- struction and expansion in a water system. In the past, the expansion rate of pipe networks was a result of population growth, which tended to ebb and flow as waves of growth swept over US cities. Combining the demographic patterns of original con- struction work with the bell-shaped curves of expected pipe life produces a graph like the one in Figure 2. This graph illustrates the relationship between the original waves of con- struction and the echo waves of replacement for each class of pipe assets. The resulting ramplike shape of reinvestment results because most system expansion occurred during the latter part of the previous century (i.e., most pipes are less than ~0 years old) and because even the oldest pipes have long lives (life expectancy for many of the oldest pipes is more than 100 years because of the thick-walled design of the early iron pipes). Two key variables determine the echo wave of pipe reinvestment needs; one is known, and the other is unknown. The original pipe instal- lation pattern is fixed and known; it can be inferred reasonably well from data on historic population growth trends even if there are no utility records. The second variable-the bell-shaped distribution of pipe life for each annual cohort group of pipe assets-is not known with precision. 1Vloreover, it is not fixed but rather evolving as individual pipes continue to wear out, and it is changeable because of maintenance interventions. Asset management focuses on this "pipe life" variable in an attempt to optimize the rate of reinvestment in terms of total life-cycle costs (flat- tening the ramp) while maintaining the level of asset performance required to deliver the desired level of service to customers. In addition to physical failures such as main breaks, pipe perfor- mance is affected by line pressure, water chemistry, and microbiology. CROMWELL ET AL °9:4 JOURNAL AWWA ~ APRIL 2007 113