Dents can occur during pipeline construction or in-service causing a local stress and strain concentration and a local reduction in the pipe diameter. If failure as a result of a dent is not immediate, it is possible that the damage can deteriorate in service and cause failure at some time after the initial impact. The challenge to the pipeline operator is the identification of the dents that may threaten the future integrity of the pipeline from those that are dormant and require no further action.
In recent years there has been a shift from simple depth-based assessment of plain dents to the use of strain-based assessment that uses the dent local radius of curvature to define severity. Furthermore, as pipeline dents subject to cyclic pressure loading can also develop fatigue cracks it is necessary to assess dent fatigue life. Up until recently, the options for fatigue assessment were essentially a simple, conservative dent depth based assessment or the use of finite element analysis (FEA). A new approach developed via a PRCI research project1 uses measurements from the dent axial and circumferential profiles to predict dent fatigue severity and the cyclic pressure spectrum to derive the dent remaining life.
This paper discusses the benefits and limitations of the new approaches that can be used to evaluate dent severity and fatigue life. How the dent attribute input measurements are extracted from smoothed ILI dent profiles and used in the equations is demonstrated. Real case studies are provided to illustrate the process and to compare the findings to the older depth-based methodologies used previously.
1. Improved Pipeline Dent Integrity Management, Tiku et al (BMT Fleet Technologies) et al, IPC2016, Calgary.