Structural Analysis of Large Diameter Cast Iron Pipes under Cyclic Loading and Fatigue Failure

The majority of cast iron pipelines were installed in the United States in the early to mid-20th century. Most of these pipes are in a sustainable working condition; however, any failure of these pipelines could be catastrophic due to the social, political, and environmental impacts of the failure. Utility engineers have reported failures on large diameter cast iron pipes that occurred suddenly and without warning, with no signs of prior damage, cracks, leaks, and no visual evidence of corrosion on the fracture surfaces. To facilitate better decision making for capital expenditures and to address the risk associated with these assets, the fatigue failure mechanisms of cast iron pipe must be understood. In addition, for planning and capital budgeting, it is important to determine the current condition of a deteriorated pipeline and its risk of failure due to cyclic loading. In this study, structural analysis of fatigue failures for buried cast iron pipes is performed. This paper explores the level of stress on corroded pits and crack growth when subjected to internal fatigue loading such as operating pressure variations and transient pressure events. Computational modeling is utilized to inform utility owners that there is a threshold for size of initial crack and number of cycles that significantly change level of stress for cast iron pipes. It is very important to know that number of cycles before any failure happens. This will provide decision makers and utility owners a tool to enhance their asset management strategy based on inspection results combined with advanced computational fatigue modeling.