Innovative Condition Assessment of Strategic Aqueducts

Large diameter cast iron pipe remaining life analyses have historically relied upon the concept of undertaking fieldwork to measure internal and external pit depths. Knowing the age of the pipeline, and assuming a linear corrosion rate, a "time to failure" is calculated, based on a worst case of the largest internal pit aligning with the largest external pit. This is clearly a flawed approach for a pipe material that does not tend to fail by through-wall corrosion. In reality, other failure mechanisms tend to occur before pitting corrosion penetrates the pipe wall. United Utilities Water (UUW) challenged MWH to come up with a new approach to evaluate the remaining life of the cast iron Vyrnwy Large Diameter Trunk Main (LDTM). This approach consisted of evaluating the actual loads placed upon the pipeline from internal pressure, backfill load and traffic load, and the reduced strength of the pipeline due to corrosion. The most likely failure scenario is through circumferential (ring) bending. MWH found that the flexural strength of the pipe material has a reasonably predictable deterioration rate based the depth of corrosion pits, and critically, their circumferential location around the pipe wall. Using this information, together with the actual applied loads, allowed a more reliable estimation to be made of the remaining life of the LDTM.