SOLUTION

WJE performed a FFS evaluation based on API 579-1/ASME FFS-1 to determine whether the Amine Splitter Tower was safe to continue operating using Part 9, Assessment of Crack-Like Flaws. The assessment used a Level 2 methodology to establish limiting flaw or crack sizes under applicable design loading.

Surface-breaking flaws of prescribed length and depth were modeled using linear elastic fracture mechanics principals to evaluate the tendency of material failure due to both fracture and plastic collapse on a failure assessment diagram. The evaluation included the influence of stress concentration effects at discontinuities (e.g., tray supports) and residual stresses at the welded joint. Material toughness values were estimated using guidance in API 579 for austenitic stainless steel materials.

The assessment predicted relatively large maximum allowable crack sizes based on anticipated loading conditions. Long surface cracks, well in excess of those identified in the inspection, were acceptable at depths up to half-wall, with shorter flaws acceptable to through-wall. The FFS assessment demonstrated that the weld undercut defects could be safely left in place without repairs. Although significant cyclic loading was not anticipated, WJE recommended periodic inspection for sub-critical crack growth.