Condition assessment modeling for distribution systems using shared frailty analysis

Condition assessment modeling is drawing increasing interest as a technique that can assist in managing drinking water infrastructure. This article develops a Cox proportional hazard (PH)/ shared frailty model and applies it to the problem of investment in the repair and replacement of drinking water distribution system components. The model has been applied to a pipe break database collected by the city of Laramie (Wyo.) Utility Division to show how such models can help managers understand the factors affecting the survival of drinking water pipe sections or entire pipelines or pipe runs. Shared frailty represents unobserved external factors, known to be important, which vary randomly and are more consistent among sections in the same pipe run than among sections in different pipe runs. Results from the model developed in this article indicate that metal pipe has fewer breaks on average, than polyvinyl chloride pipe but that it is more subject to undefined random factors. The Cox PH model is used to develop expected piperun break curves. In addition the Cox PH/shared frailty model is used within the inspection value method to assist in making improved repair, replacement, and rehabilitation decisions for selected drinking water distribution system pipes.