OptimalIntegrity Studio™ Wiki / FFS and inspection
Method
Fitness-for-service and inspection
When inspection finds damage, two questions follow: is the equipment fit to keep operating, and for how long. Fitness-for-service to API 579 answers the first, corrosion rate and remaining life answer the second.
Fitness-for-service, API 579
API 579-1 / ASME FFS-1 is the standard for assessing whether equipment with known damage is fit for continued service. It covers general and local metal loss, pitting, blisters and HIC and SOHIC, weld misalignment and shell distortion, crack-like flaws, creep and fire damage. It is worked at three levels of increasing rigour:
| Level 1 | Conservative screening with simple criteria, for a plant inspector or engineer. |
|---|---|
| Level 2 | A more detailed assessment with more data and less conservatism, for an engineer. |
| Level 3 | Advanced analysis, finite element and fracture mechanics, for a specialist, where levels 1 and 2 fail or the case is complex. |
A metal-loss assessment compares the remaining wall against what the code requires, often through a remaining strength factor, with an allowable RSF of 0.90; below it, the equipment is de-rated, repaired or run under a documented case.
Corrosion rate and remaining life
Corrosion rate = (tprevious − tactual) ÷ time between readings
Remaining life = (tactual − trequired) ÷ corrosion rate
Next inspection ≤ ½ remaining life
trequired is the minimum thickness the code needs for pressure and structural load. Worked example: a wall at 12.0 mm, a minimum of 8.0 mm, corroding 0.4 mm a year, has (12.0 − 8.0) ÷ 0.4 = 10 years of remaining life, so the next inspection is due within five. Both a long-term and a short-term rate are tracked, and the worse governs.
Thickness monitoring
Thickness is trended at fixed condition monitoring locations and thickness monitoring locations chosen where damage is expected. Repeated readings give the corrosion rate, the remaining life and the next inspection date, and feed the damage factor back into the RBI risk. The half-life rule, inspecting at no more than half the remaining life, is the same idea as the P-F monitoring interval in condition monitoring.
Inspection and NDE methods
| Ultrasonics (UT) | Thickness, and phased array and TOFD for cracks and flaw sizing. The workhorse. |
|---|---|
| Radiography (RT) | Profile thickness and volumetric flaws, including small-bore and corrosion under insulation. |
| Surface (MT, PT, ACFM) | Surface-breaking cracks, magnetic-particle, dye-penetrant and alternating-current field measurement. |
| Guided-wave UT | Rapid screening of long runs of pipe and buried or inaccessible lines. |
| Acoustic emission | Global listening for active flaws on vessels and tanks under load. |
Whatever the method, API 581 grades its effectiveness from A to E for the expected damage, and only an effective inspection actually lowers the risk.
Where OptimalIntegrity Studio™ fits
OptimalIntegrity Studio™ imports thickness data, computes long-term and short-term corrosion rates and remaining life, schedules the next inspection on the half-life rule, and runs level 1 and level 2 fitness-for-service so a damaged item gets a defensible stay-or-repair decision rather than a guess.