An Engineering Assessment Methodology to Evaluate Arc Burns

Abstract

Arc burns, also known as arc strikes, are caused by momentary interaction of an electric arc, e.g., welding electrode or welding ground clamp, and a pipe or fitting, upon which a minimal or no amount of weld metal is deposited. Arc burns typically correspond with localized alteration of microstructures, shallow pitting, sharp surface contours, re-melting, and/or cracking. The damaged microstructures manifest in the form of a locally harder material due to accelerated cooling rates. Arc burns mainly form during the pipeline construction and are typically located adjacent to manually installed girth welds. The hard microstructures associated with arc burns are susceptible to hydrogen-induced cracking (HIC) in the presence of atomic hydrogen. Pipeline maintenance codes consider arc burns as defects and require their complete removal by grinding. Due to the relatively small dimension of arc burns, removal by grinding followed by etching contrast test is often the simplest and most reliable permanent repair for such defects. However, in some circumstances grinding to the maximum allowable depth may not completely remove the affected microstructures. Also, removal of arc burns often requires grinding near girth welds and significant grinding depths may require through-thickness inspection of the welds to ensure safety. Type B pressure containing steel sleeves are another permanent repair method that can be used to repair arc burns or partially removed arc burns within grinding metal loss features. Installation of permanent repairs over an arc burn is costly and may introduce additional or higher risks to the integrity of pipeline when scarce industry studies are available that conclusively demonstrate the dangers of leaving arc burns or partially removed arc burns in pipes. Despite the need, there is no validated engineering assessment method for the evaluation of arc burns. This paper will summarize an engineering assessment methodology and the findings of the evaluation of crack-free arc burns and partially removed arc burn features for two scenarios on vintage liquid pipelines. A combination of one- and three-dimensional finite element models was utilized to investigate the effect of arc burns and/or partially removed arc burns on the integrity of the pipeline based on plastic collapse, local yielding, and fatigue failure modes. The effect of the buried pipeline profile and soil was considered in the assessment of the axial load capacity of the pipeline. The geometrical and metallurgical stress concentrations of the features were considered in the engineering assessment. The engineering assessment determined if the pipeline with the arc burns and/or partially removed arc burns can survive rupture, brittle fracture, and fatigue damage mechanisms during its operation and if reinforcement of the area or cut-out is required.