WRC 591 ✓ Most Recent

API 579-1/ASME FFS-1 Fitness-For-Service (FFS-1) is a consensus, industry-written standard comprised of recognized engineering methods to evaluate various forms of damage found in service-exposed pressurized equipment. The methods employed in the FFS-1 standard are typically based on a combination of historic derivations from first principles and advanced analyses which are simplified to enable inspectors and engineers with limited expertise to perform reliably conservative assessments to ensure the structural integrity of the equipment. As such, many procedural limitations are used throughout the FFS-1 standard to limit the scope of a given Level 1 and 2 Assessment and prevent a user from applying a calculation when more detailed analysis is required to capture complex responses; these procedural limitations are often necessary to maintain the balance between conservatism and simplicity of implementation.

The assessment of Local Thin Areas (LTAs) within Part 5 of the FFS-1 standard relies on several engineering studies, all of which are founded on the assumption that the LTA flaw is sufficiently far from major disruptions in geometry; this allows for a uniform membrane stress field distribution in the region. Many studies reference Timoshenko’s Theory of Plates and Shells, which presented a characteristic distance for a thin cylindrical body, beyond which no effect of local perturbation is expected to be present. This distance, 1.8√Dt, was carefully derived and can be re-validated numerically using Finite Element Analysis (FEA) models with small displacement theory, or geometric linearity assumption.

The FFS-1 standard adopts the 1.8√Dt distance as a procedural limitation to verify the separation between a major structural discontinuity (LMSD) and a locally thinned area (LTA). However, it does not account for the specific category of structural discontinuity or whether it interacts adversely with the LTA. The objective of the study presented in this bulletin is to: (1) propose a method for evaluating the influence of a discontinuity on an LTA, and (2) demonstrate this method at the junction between cylinders and 2:1 ellipsoidal heads. It should be noted that the results of this study apply only to damage in the cylinder portion of the vessel resulting from the LTA. The current investigation is limited to local thin areas (LTAs) located in the cylindrical shell region near the junction with a 2:1 elliptical head. The analysis considers internal pressure loading, including pressure end loads and hydrostatic pressure. As such, only damage within the cylindrical shell is addressed. A detailed assessment of the effects of additional loading conditions is deferred to future phases of the study.