State of Technology Report for Force Main Rehabilitation, Final ...
State of Technology Report for Force Main Rehabilitation, Final ...
State of Technology Report for Force Main Rehabilitation, Final ...
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ferrous main, this can be an internal or external inspection using electromagnetic or ultrasonic tools to<br />
measure the remaining wall thickness. For PCCP, it might include acoustic monitoring to locate active<br />
wire breaks. If the PCCP main can be taken out <strong>of</strong> service, then an internal electromagnetic survey is<br />
possible where cumulative wire breaks are detected. Many other tools are available <strong>for</strong> inspecting a<br />
sewer <strong>for</strong>ce main (WERF, 2009).<br />
The results <strong>of</strong> the indirect and direct investigation can then be fed into a mechanistic model to determine<br />
the likelihood <strong>of</strong> failure. Figure 4-3 is an example <strong>of</strong> a belief chart <strong>for</strong> assessing the risk <strong>of</strong> a structural<br />
failure in PCCP pipe based on indirect evidence. This type <strong>of</strong> model is pipe material specific with<br />
predictions made <strong>of</strong> the remaining factor <strong>of</strong> safety or remaining life <strong>of</strong> the pipeline. The other aspect to<br />
carrying out a risk assessment and criticality ranking is to evaluate and quantify the consequences <strong>of</strong> a<br />
failure in a pipeline. Figure 4-4 illustrates a belief network <strong>for</strong> a PCCP pipe to estimate the consequence<br />
<strong>of</strong> failure, but would be equally valid <strong>for</strong> any pipe material.<br />
Often the consequences <strong>of</strong> a failure can be put in terms <strong>of</strong> costs. For example, it might be based on the<br />
direct cost to repair a broken pipe and provide temporary service during the repair if needed, the cost to<br />
clean up local flooding or repair adjoining property damaged as a result <strong>of</strong> a major rupture, and/or indirect<br />
costs associated with the socio-economic impact <strong>of</strong> a failure. The socio-economic costs can <strong>of</strong>ten exceed<br />
the cost expended in the immediate repair <strong>of</strong> the broken main. This is especially true <strong>for</strong> sewer <strong>for</strong>ce<br />
mains where raw sewerage can cause damage to the environment and adjacent property resulting in<br />
significant clean-up costs.<br />
Sewer <strong>for</strong>ce mains that convey a major portion <strong>of</strong> a municipality’s sewage, with no backup or<br />
redundancy, would have a high consequence <strong>of</strong> failure rating. Here, local knowledge <strong>of</strong> the system is<br />
extremely important in making these determinations. From the standpoint <strong>of</strong> what action may be<br />
warranted, certainly pipelines that have a high likelihood <strong>for</strong> failure combined with significant<br />
consequences associated with such a failure (e.g., a high criticality rating) deserve urgent attention. Plus,<br />
the higher the consequences associated with a failure, the more conservative will be the approach towards<br />
renewal <strong>of</strong> the main. A partially deteriorated <strong>for</strong>ce main that has an extremely high consequence <strong>of</strong><br />
failure would be one that would probably be treated as fully deteriorated from the perspective <strong>of</strong> the<br />
design <strong>of</strong> the rehabilitation system.<br />
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