Pressure vessels, pipelines, and structures develop flaws during service β€” cracks, corrosion, wall thinning. When an inspection finds such a flaw, engineers face one key question:

Can this equipment keep running? If so, for how long?

This is exactly what Fitness-for-Service (FFS) assessment answers.

The Limit of Traditional Rules

Traditional design codes (such as ASME or GB 150) are written for new, flaw-free equipment. When a flaw is found, the codes often say: “out of tolerance β€” repair or retire.”

But in practice, not every flaw that breaks a manufacturing tolerance will cause a failure. An overly conservative call creates two problems:

  1. Unnecessary shutdowns β€” lost production time and repair costs;
  2. False confidence β€” the repair itself may introduce new flaws.

FFS assessment replaces rule-of-thumb conservatism with fracture mechanics and materials science, giving engineers a documented, defensible verdict on whether a flawed component is still safe.

The Core Tool: The Failure Assessment Diagram (FAD)

The most widely used FFS tool is the Failure Assessment Diagram (FAD).

The FAD has two axes, each measuring a different failure mode:

  • Vertical axis $K_r$ (fracture ratio): crack driving force divided by material fracture toughness β€” measures the risk of brittle fracture;
  • Horizontal axis $L_r$ (load ratio): applied load divided by the limit load β€” measures the risk of plastic collapse.

Plot the assessment point $(L_r, K_r)$ for the flawed structure on the diagram:

  • Point inside the Failure Assessment Line (FAL): flaw is acceptable β€” the component can keep running;
  • Point on or outside the FAL: flaw is not acceptable β€” repair or retire.
$$ K_r = \frac{K_I^P + K_I^S}{K_{mat}} + \rho \leq f(L_r) $$

Here $K_I^P$ is the primary stress intensity factor, $K_I^S$ is the secondary stress contribution (e.g. residual stress), $K_{mat}$ is the material fracture toughness, and $\rho$ is the plasticity correction term.

Main Assessment Standards

Three standards are widely used around the world for FFS:

Standard Typical use
BS 7910 Welded structures, pressure vessels (UK / international)
API 579-1 / ASME FFS-1 Refinery and process pressure equipment (USA / global)
ASME XI Nuclear power plant pressure boundaries (USA nuclear)

All three share the same FAD framework in principle, but differ in option levels, formula details, and scope.