What are the three API 579 assessment levels?

Level 1 is a conservative screening using simple charts and limits, doable by a plant inspector. Level 2 is a more detailed hand/spreadsheet assessment that relaxes the conservatism with more input data. Level 3 is advanced analysis — typically finite element — for geometries or loads the lower levels do not cover. You escalate only if a lower level fails or does not apply.

What is the allowable Remaining Strength Factor?

The default allowable RSF (RSFa) in API 579 is 0.90. If the computed RSF for a metal-loss flaw is at least 0.90 the component is acceptable at its current MAWP; if it is below, the pressure is derated in proportion (MAWP_reduced = MAWP × RSF / RSFa).

When do I need the Failure Assessment Diagram?

The FAD is for crack-like flaws, where both brittle fracture and plastic collapse must be checked together. Volumetric metal loss (general or local thinning, pitting) is handled by thickness/RSF methods instead.

Integrity · guide

API 579 fitness-for-service, in practice

Fitness-for-service answers a blunt question: a vessel or pipe has damage — can it keep running, and at what pressure? API 579-1/ASME FFS-1 is the dominant standard. This is the working engineer's map of its assessment levels and the two calculations you will reach for most.

Three levels of effort

API 579 is organised by damage mechanism (metal loss, pitting, blisters and HIC, crack-like flaws, creep, fire damage, and more), and each is assessed at one of three levels. Level 1 is a conservative screen; Level 2 trades more input data for less conservatism; Level 3 is advanced analysis, usually finite element, for cases the simpler levels cannot bound. You climb the ladder only when you have to.

Metal loss and the Remaining Strength Factor

For general or local thinning, the central quantity is the Remaining Strength Factor — the ratio of the damaged component's plastic-collapse strength to that of the undamaged component:

RSF = (collapse strength, damaged) / (collapse strength, undamaged)

Compare it to the allowable, RSF_a = 0.90 by default. The acceptance and derating rule is simply:

RSF ≥ RSF_a → acceptable at MAWP; otherwise MAWP_reduced = MAWP · RSF / RSF_a

Level 1 metal-loss screening also enforces a minimum remaining thickness and a groove/flaw-profile check. A single thin reading does not automatically fail the component — the averaged profile over the critical length is what the RSF uses.

Crack-like flaws and the FAD

A crack can fail by brittle fracture, by plastic collapse, or by an interaction of the two. The Failure Assessment Diagram plots both at once. The vertical axis is the toughness ratio and the horizontal axis the load (collapse) ratio:

K_r = K_I / K_mat L_r = σ_ref / σ_yield

The assessment point (L_r, K_r) is plotted against the FAD envelope. Inside the curve and below the L_r cut-off, the flaw is acceptable; on or outside it, it is not. The same diagram makes the failure mode obvious — low L_r means fracture-driven, high L_r means collapse-driven.

Open the calculatorFracture mechanics / FAD calculator →Build the Level 2 FAD for a crack-like flaw — K_r vs L_r with the assessment point, envelope and margins — plus probabilistic fracture if inputs are uncertain.Open the calculatorAPI 579 general metal-loss (Part 4) →Screen a thinned region against the minimum required thickness and the RSF, with automatic MAWP derating.

Getting a defensible result

Fitness-for-service stands or falls on its inputs: accurate thickness profiles, the right material toughness (measured or a conservative lower-bound), and honest stresses including secondary and residual terms. Where any of these is uncertain, a probabilistic assessment that carries the scatter through to a probability of failure is far more useful than a single deterministic pass/fail.

Frequently asked

What are the three API 579 assessment levels?: Level 1 is a conservative screening using simple charts and limits, doable by a plant inspector. Level 2 is a more detailed hand/spreadsheet assessment that relaxes the conservatism with more input data. Level 3 is advanced analysis — typically finite element — for geometries or loads the lower levels do not cover. You escalate only if a lower level fails or does not apply.
What is the allowable Remaining Strength Factor?: The default allowable RSF (RSFa) in API 579 is 0.90. If the computed RSF for a metal-loss flaw is at least 0.90 the component is acceptable at its current MAWP; if it is below, the pressure is derated in proportion (MAWP_reduced = MAWP × RSF / RSFa).
When do I need the Failure Assessment Diagram?: The FAD is for crack-like flaws, where both brittle fracture and plastic collapse must be checked together. Volumetric metal loss (general or local thinning, pitting) is handled by thickness/RSF methods instead.

References

API 579-1/ASME FFS-1, "Fitness-For-Service," American Petroleum Institute / ASME.
R6, "Assessment of the Integrity of Structures Containing Defects," EDF Energy (FAD methodology).
BS 7910, "Guide to methods for assessing the acceptability of flaws in metallic structures," BSI.
T.L. Anderson, "Fracture Mechanics: Fundamentals and Applications," CRC Press.

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