Margins Against Plastic Collapse for FFS Assessments of Pipelines Based on Elastic-Plastic Stress Analysis

Abstract API 579-1/ASME FFS-1 includes three levels of assessment for each type of flaw or damage mechanism. The Level 3 assessment procedures are intended to provide the most detailed evaluation that produces results that are more precise than those in a Level 1 and a Level 2 assessment. The Level 3 evaluation is based on numerical methods such as Finite Element Analysis (FEA) or experimental techniques when appropriate. The requirements for volumetric flaws assessments may include protection against the following failure modes: Plastic Collapse; Local Failure; Collapse from Buckling; Creep, Fatigue or Creep-Fatigue Damage. When evaluating the protection against plastic collapse, three distinct methods of analysis are provided: Elastic Stress Analysis; Limit Load Analysis; and Elastic-Plastic Stress Analysis in which the allowable load on the component is established by applying assessment margins to the plastic collapse load. The assessment margins are in general based on the construction code of the component being assessed. However, this approach is challenging when the component design is not based on the minimum specified ultimate tensile strength of the material of construction. This paper presents a comparison of the 2021 API 579 assessment factors for pipelines with alternative assessment factors. The calculated burst pressures from elastic-plastic FEA for different materials and geometries are used to compare the different margins. The FEA results were also compared with the burst pressure solution from API 579. Finally, a proposal for modifications to the assessment margins for pipelines in Table 2D.4 of API 579 is presented offering an approach more aligned with ASME B31G by considering the pipeline material strength.