Improving the Accuracy of Traditional Dent Fatigue Analysis: A Method for Quantifying the Initial Damage Caused by Dent Formation

Abstract

The pipeline industry is currently taking several approaches to evaluate the integrity of dents, ovalities, or other geometric anomalies identified from in-line inspection (ILI). A primary threat associated with these features that operators should be concerned with is failure due to fatigue. In order to carry out a more accurate dent fatigue analysis, it is important to be able to quantify the amount of damage accumulated during the initial dent formation process and subsequent shakedown of the dent. Dents result from permanent deformation of the pipeline which leads to accumulation of plastic strain. Whether this permanent deformation was caused during initial construction (a backhoe striking the pipeline) or in service (changing underground soil conditions), the plastic strains that are observed will result in a decrease in the pipeline’s fatigue life. Pressure cycling has the potential to accumulate additional plastic stain, thus accumulating more fatigue damage. Eventually as the pipeline continues to be cycled, no additional deformation or accumulation of plastic strain will occur; this behavior is referred to as “shakedown.” Finite element analysis (FEA) can be utilized to quantify how much fatigue damage has been accumulated during the initial dent formation process and subsequent shakedown of the dent. When analyzing pipeline dents using FEA, importance should be placed on accurately simulating the dent forming process so that realistic plasticity effects can be captured. The process of calculating plastic stresses and strains during the dent forming process can be computationally expensive and result in numerical instabilities within the analysis. As a result, methods for simulating the formation and shakedown of a pipeline dent are continuously being refined. However, since it is difficult to determine exactly how these geometric pipeline anomalies were formed, the applicability and accuracy of such methods contains a great amount of uncertainty and is thus expensive (both from a cost and time standpoint) for an operator to validate. This paper will identify a new and innovative approach for using FEA to determine the amount of damage accumulated during the initial dent formation process and subsequent shakedown of the dent. This approach uses state-of-the-art FEA modeling techniques coupled with industry knowledge and experience to develop an accurate and efficient method for quantifying this damage. The knowledge gained during this analysis can be used in conjunction with a traditional rapid dent assessment methodology. A case study will be presented which demonstrates the impact that a direct calculation of this initial damage has on representative pipeline dent assessment analysis. By undertaking this additional analysis, operators will have the potential to eliminate unnecessary digs. Additionally, operators can be more confident that their resources are being applied to the highest priority features.