INFRASTRUCTURE once the pipe strength cannot resist the hoop stresses developed by vertical, frost and internal pressures. Since internal pressure can fluctuate along the pipe lifespan, it is assumed that the remaining economic life corresponds to the original pressure rating. While this provides a conservative value for a newly constructed pipeline, one could argue that the pipe is operating at a lower pressure than the original pressure rating, and the corresponding hoop stresses will be much lower. To address this issue, there is a need to distinguish between the remaining service life and the remaining economic life. While remaining service life corresponds to fluctuating pressure between working pressure and transient pressure, together with other vertical loads, the remaining economic life corresponds to the pressure rating and other vertical loads. This approach aligns with asset value depreciation with time, as an asset may lose its economic value but still operate below strength capacity. Once the

To project the wall thickness reduction due to corrosion, the two-phase empirical corrosion model is used to predict pit depth increase with time. FOS reaches 1.0, based on stresses that include fluctuations between working pressure and transient pressure, the pipe is at the end of its service life and can no longer stay in service. As mentioned earlier, the ability of pit CI to withstand in-service loading conditions is reduced by external surface corrosion that can occur in an aggressive soil environment. The end of economic life criterion for buried pit CI can be defined when the residual FOS reaches 1.0. To estimate hoop stresses, the residual wall thickness is required. The remaining thickness is only true to the point of time when the pipe was

inspected. To project the wall thickness reduction due to corrosion, the twophase empirical corrosion model developed by Rajani et al. (2000) is used to predict pit depth increase with time. There are other available internal and/ or external pit depth corrosion models based on empirical relationships and/or based on Weibull engineering statistics. However, since we are using the fracture mechanics equation developed by B. Rajani, Rajani’s corrosion model was considered for this article. Although the reduction in CI strength as a result of corrosion was analyzed by using fracture mechanics, pit CI pipes have a wide variation in behaviour across identical pipes. Tests conducted by Rajani showed that the fracture toughness of Pit CI varies between 5.7 and 13.7 Mpa√m. Given the material variability, in addition to changes in the local environment that influence corrosion rates, the predicted economic lifetime equation will exhibit uncertainty. A relatively straightforward technique is to use Monte Carlo Simulation (MCS) in conjunction with the physical failure criterion for pit CI pipes described above. Once the probability for each condition is generated using MCS, a cumulative probability curve can be developed to estimate the remaining economic life of the pipe. The pipe age corresponding to a high cumulative probability (>90%), represents the economic end of pipe life. CASE STUDY In the Municipality of Thames Centre, Ontario, an acoustic-based inspection tool, ePulse by Echologics, a Mueller brand, was used to assess existing pit cast iron pipe, 150 mm in diameter and 131 m in length. The pipe was constructed in 1956. The acoustic tool can assess ferrous pipe segments (i.e., using existing appurtenances such as fire hydrants and/or valves)

12  |  June 2019

Environmental Science & Engineering Magazine