Interpretation of External Cracking on Underground Pipelines

When external cracks are discovered on underground pipelines, it is sometimes difficult to identify the cracking mechanism. This is especially true when the cracking coincides with local stress discontinuities such as dents, where significant stress fluctuations can occur. Additionally, cracking may initiate by one mechanism and another mechanism may subsequently dominate. For purposes of developing integrity programs for the affected pipeline segment, proper interpretation of the mechanism for crack formation is important. This paper describes the characteristics of, and methods to distinguish between, the common types of time-dependent cracking found on underground pipelines. It also discusses the relationships between the mechanisms for the environmentally assisted cracking phenomena.