State of Technology Report for Force Main Rehabilitation, Final ...

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