Analytical Approach to Determine Hydrotest Intervals

Many gas pipeline operators use hydrotesting as a means of managing stress corrosion cracking in pipelines. Historically the interval at which the pipelines are hydrotested is largely empirical. If for instance a failure occurred on a pipeline section four years after a hydrotest, then four years would become the baseline interval. As failures are rare, these worst-case intervals are often applied across the system with little consideration to the many factors that would affect the relevant interval for different pipelines. This can result in an overly conservative frequency of hydrotests. In IPC 2006 an analytical method of determining hydrotest intervals, based on a time vs. pressure plot, was developed and published by Fessler et al. The study conducted herein examines this method, its assumptions, applicability, and limitations, with regards to an in-service pipeline system. It also discusses how this method was adapted to account for variable crack growth rates and failures. Application of this adapted method to the pipeline system and its results are discussed. It was found that this method could be used to predict hydrotest intervals for in-service pipelines. An additional method of using crack assessments with certain growth characteristics to develop hydrotest intervals is also presented. This method incorporated crack failure prediction calculations, and estimated crack growth rates, to predict hydrotest intervals. The results and the limitations are critically examined. Practical ways of improving the methods and ongoing work to improve the method are also explained.