Abstract
Dents in pipelines can seriously reduce the design life of a pipeline. Dents cause stress concentrations to develop which make dents susceptible to fatigue failures. Acceptance criteria of dents are currently only based on dent depth. To understand the behavior of dents in pipelines, a finite element parametric study was performed. Three dimensional shell element models were given elasto-plastic behavior for modeling dents in pipelines. Parameters modeled include dent depth, dent type, dent restraint, pipe diameter, thickness, grade, longitudinal stress, pipe support, and pressure at indentation. For the given parameters, the rebound behavior and stress behavior of dents were investigated. The displacement data were studied to understand the rebound characteristics of dents. Stress data were studied to determine failure modes of different combinations of dents and pipes. Dents reduce in size, or rebound, with removal of the force at indentation and pressurization. The amount of rebound is influenced by several parameters such as dent type and pipe wall thickness. Larger dents in terms of surface area of the dented region experience more rebound than smaller dents. Thinner pipes experience more rebound than thicker pipes. From the finite element modeling, the rebound characteristics can be predicted for a broad range of values for the various parameters. The study of the distribution of stresses in dented regions shows locations of possible fatigue failures. Two different modes of failure were found to exist. Longer dents in terms of dent length are susceptible to fatigue failures in the contact region of the dent. Shorter dents are not susceptible to failures in the contact region, but can have failures away from the contact region in the dent periphery. Shorter dents have higher fatigue lives as compared to longer dents. The mode of failure of a dent is influenced by a variety of parameters. The influence of various parameters of the fatigue strength of dents is studied using rebound and stress behavior to develop better acceptance criteria of dents to improve the reliability of dents in pipelines.
Hoffmann, Roger Lynn (1997). Modeling fatique behavior of dents in petroleum pipelines. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1997 -THESIS -H643.