According to statistics compiled by the U.S. Office of Pipeline Safety, mechanical damage is one of the primary causes of pipeline failures in the United States. For more than 30 years a significant body of research has been collected in an effort to understand the failure mechanisms and mechanics associated with pipeline defects that include plain dents, wrinkle bends, and mechanical damage involving dents with gouges. In the U.S. organizations such as the Pipeline Research Council International, Gas Technology Institute, and the American Petroleum Institute have led the change in funding these research efforts, as well as other efforts from research organizations around the world.
While some guidance is provided by the ASME B31.4 and B31.8 pipeline codes in assessing pipeline damage, there is no single document that captures the lessons learned from the extensive body of research and experience that currently exists. To a large extent this is related to the complexity of the subject; however, there is a significant need to develop for industry a method for ranking the severity of pipeline damage. At the present time there is no single method for doing this. This paper will provide insights on a proposed three-tiered system to help operators determine which defects represent the most serious threat to the mechanical integrity of their systems. The intent is to provide operators with a grading tool based on research testing, material characteristics, experience, and dent mechanics in order for repairs to be made in a manner that ensures the safe operation of pipeline systems.
One of the most critical elements when assessing pipeline damage is classification of defects. There is a significant amount of information available in the open literature; however, one of the challenges is putting everything together in a manner that can be used to assess damage severity. This is one of the main purposes of this paper. The second purpose is to provide a systematic methodology for operators and pipeline service companies who are tasked with making decisions about what to do when pipeline damage occurs. Because of the extensive research that has been conducted world-wide relating to dented pipelines, it is possible to draw information required on a range of defect types. The driving motivation for many research programs is to develop a better understanding of damaged pipelines in an effort to characterize their behavior. As with many areas of engineering, the ability to accurately predict the response behavior of structures is important to ensure adequate safety and consistent performance. The complexities associated with damaged pipelines make this a challenging task.
Material issues, corrosion, cyclic pressure conditions, soil-pipe interactions and complicated stress fields are but a few examples.
Provided below are the major defect classifications that typically arise when assessing pipeline damage.
Plain dents
Constrained dents
Gouges
Mechanical damage
Wrinkles
