Detailed Dent Assessment: Avoiding the Pitfalls
Lockey A., Turner T., Turner S.
Pipeline Pigging and Integrity Management Conference; Houston; February 2019.
Pipelines frequently contain large numbers of dents. In the last ten years the number of shallow dents reported by in-line inspections has increased, as tools have become more sensitive. These dents can fail at the time they are formed, or later under fatigue loading. However, the great majority of dents do not fail. How do we determine which dents need to be repaired?
For some pipeline defects, such as corrosion, we have accurate assessment methods based on simple parameters. Similarly, simple methods have historically been used to assess dents, based solely on dent depth. Indeed, the regulations in some parts of the world (including North America) mandate this approach. However, the reliability of these methods is poor, so large safety factors are used and assessments can be excessively conservative.
More recently, dent assessment methods based on strain and finite element analysis have been adopted by many pipeline operators. These methods can be more accurate, but they must be applied correctly to avoid non-conservative results. This paper discusses several potential pitfalls in applying these methods, including some that have appeared in published work. The important factors which need to be considered in a dent assessment are discussed, along with the relative conservatism of different assessment approaches.