Risk — API 580

In API 580, Risk is defined as:

Probability of the failure (POF) × Consequence associated with the failure (COF).

However, the more detailed calculation steps to calculate POF & COF is elaborated in API 581.

For POF Calculation, 581 recommends the GFF method & Weibull distribution method.
The GFF method is used to predict loss of containment POF from pressure boundary equipment. The Weibull distribution method is used to predict POF for PRDs & heat exchanger bundles.

Using GFF Method, The POF is calculated from Pf (t) = gff × FMS × Df (t)
· gff is the generic failure frequency (Failures/Year)
· FMS is the management system factor.
· Df(t) is the overall damage factor

In simple words, the generic industry failure data (gff) is adjusted to equipment specific basis using DF and FMS factors.

To elaborate, The GFFs are tabulated in API 581, Part-2 Table 3.1 as representative of the refining and petrochemical industry’s failure data. Therefore, these generic values represent an industry in general rather than the true failure frequencies for a specific component subject to a specific damage mechanism.
The GFFs are associated with four hole sizes to model the release scenarios covering a full range of events (i.e. small leak to rupture)

Then, the Adjustment factors are applied to the industry data based GFF to account for active / susceptible damage mechanism(s) & damage rates specific to the component’s operating environment and for reliability management practices within a plant.

The DF also takes into consideration historical inspection data and the effectiveness of both past and future inspections. The management systems factor adjusts for the influence of the facility’s management system on the mechanical integrity of the plant.

DF estimates are currently provided for the following damage mechanisms.
a) Thinning
b) SCC
c) External damage
d) HTHA
e) Mechanical fatigue (piping only)
f) Brittle fracture

DFs are determined as a function of inspection effectiveness. The effectiveness of each inspection performed within the designated time period is characterized for each damage mechanism. The number of inspections and effectiveness of each inspection is used to calculate the DF.

When inspection effectiveness is introduced into risk equation becomes,
R(t , IE ) = Pf (t , IE ) × CAf for area-based risk

R(t,IE) is the risk as a function of time and inspection effectiveness, m2/year (ft2/year) or $/year
Pf (t,IE) is the POF as a function of time and inspection effectiveness, failures/year
CAf is the flammable consequence impact area, m2 (ft2)

Further, the calculated risk level that requires risk reduction against the set target criteria qualifies for inspection and other mitigation planning process.

References: API 580 & API 581