Local Thin Areas Adjacent to Other Local Thin Areas in API-579/ASME FFS-1

When a corroded pressure vessel is inspected, a potential finding is multiple local thin areas (LTA’s) adjacent to one another. API-579-1/ASME FFS-1 (hereafter referred to as FFS-1) contains rules to determine if adjacent LTA’s should be combined and analyzed as a single flaw based on spacing and sizing. This paper reports the results of a parametric study to determine if the FFS-1 rules can be overly conservative and require the analyst to combine LTA’s that in fact have no or minimal interaction. Results are presented based on collapse pressure using elastic-plastic FEA. It is found that the collapse mechanism of very wide flaws in the circumferential direction versus narrow flaws is quite different. Narrow longitudinally oriented flaws experience earlier failure due to localized bending stresses resulting from the discontinuity of the side-walls. Diameter is found to control the relationship between RSF and flaw length for axi-symmetric flaws, while thickness is more controlling for narrow longitudinally oriented flaws. Due to the lower collapse pressure, an axi-symmetric representation of a narrow width LTA can be significantly non-conservative when performing a Level 3 analysis. This paper is limited to the evaluation of internal flaws where the flaw depth is approximately 50% of the wall thickness and where pressure acts on the side walls of the flaw. We recommend that a similar study be done for external flaws as well as for other flaw depths.