RBI 581 Methodology

The API 581 methodology is covered under 3 parts:

• Part 1 – Inspection planning methodology

• Part 2 – PoF methodology

• Part 3 – CoF methodology

Part 1 – Inspection planning methodology

Part 1 provides methods used to develop an inspection plan for fixed equipment, including pressure vessels, piping, atmospheric storage tanks (ASTs), PRDs, and heat exchanger tube bundles. The user must specify the target risk criteria, since this is not specified in API 581.

The calculation of the risk involves:

• Determination of Probability of failure (PoF)

• Determination of Consequence of failure (CoF)

• Risk = PoF x CoF

Part 2 – PoF methodology

API 581 specify two possible methods for calculating the PoF:

• GFF method - used to predict loss of containment PoF from pressure boundary equipment.

• Two-parameter Weibull distribution method - used to predict PoF for PRDs and heat exchanger bundles.

The GFF method is implemented in IMS.

Part 3 – CoF methodology

API 518 provide two methods:

• Level 1 – Finite variables for common fluids.

• Level 2 – More rigorous method that can be used for any fluid stream composition.

Level 1 is implemented in IMS.

Probability of failure (PoF) - GFF method

Using the GFF method the PoF is calculated by:

P_f(t) =GFF * FMS * D_f(t)

P_f(t) = PoF as function of time

GFF = Generic Failure Frequency

FMS = Management systems factor

D_f(t) = Damage Factor (DF) as a function of time

Generic Failure Frequency (GFF)

The Generic Failure Frequency (GFF) is set at a value representative of refining & petrochemical industry’s failure data. This is tabulated in API 581, Part-2 Table 3.1. Four-hole sizes are used to model release scenarios (from a small leak to a rupture) and the error rate is set between 3% to 10%. This is implemented as a lookup table in IMS.

Management systems factor (FMS)

This factor accounts for the probability that accumulating damage, which results in loss of containment, will be discovered in time. It is directly proportional to the quality of a facility’s mechanical integrity program and applies equally to all components as part of the facility. Factors such as inspection quality, corrosion management effectiveness, and adherence to appropriate materials and design codes are taken into consideration. Determining the FMS usually involves a structured assessment, which may include a questionnaire or scoring system.

This score is therefore calculated outside of IMS, and needs to be inputted in the IMS Settings on Site / Plant / Unit level. From this the FMS is calculated.

pscore = Score / 1000 × 100%

FMS = 10^{(-0.02*pscore+1)}

Damage Factor (DF)

The basic function of the Damage Factor (DF) is to statistically evaluate the amount of damage that may be present as a function of time in service and the effectiveness of the inspection activity to quantify that damage. The DF is determined based on things like:

• Applicable damage mechanisms.

• Materials of construction.

• Process service.

• Physical condition of the component.

• Inspection techniques (and frequency) used to quantify the damage.

API 581 provides DFs for the following eight mechanisms:

1. Thinning (both general and local).

2. Component lining damage.

3. External damage (thinning and cracking).

4. Stress corrosion cracking (SCC).

5. High temperature hydrogen attack (HTHA).

6. Mechanical fatigue (piping only).

7. Brittle fracture, including low-temperature brittle fracture, low alloy embrittlement, 885 °F embrittlement.

8. Sigma phase embrittlement.

Consequence of failure (PoF) - Level 1 method

Reference fluids are provided within API 581. The reference fluid closest in boiling point and molecular weight to the substance in question should be selected for assessment.

API 581 quantifies the consequences of containment loss in terms of:

• Affected impact area.

• Financial terms.

Affected impact area consequence

The affected impact area assesses flammable, toxic and non-flammable & non-toxic consequence, and accounts for both the component damage consequence area and the personnel injury consequence area.

Financial consequence

The Financial consequence accounts for cost due to: Component damage; Lost production; Personnel injury (or) fatalities; and Environmental damage.

Level 1 COF calculation steps

Implementing the Level 1 Consequence of Failure (COF) in API 581 involves the following steps:

1. Estimate the release rate.

2. Calculate inventory release.

3. Identify release type (continuous or instantaneous).

4. Assess leak detection and isolation systems.

5. Adjust release rate and mass.

6. Estimate consequence area.

7. Assess financial consequences.

RBI 581 Outcome

The output of an API 581 RBI assessment is the inspection frequencies or intervals; the scope of work; and priorities.

API 581 utilizes risk matrices to visualize and prioritize the risks linked to various equipment pieces. It is a plot with the Probability of Failure (PoF) on one side and the Consequence of Failure (CoF) on the other. Each cell in the matrix represents a distinct combination of PoF and CoF, signifying a unique risk level. This matrix aids in identifying equipment with high-risk levels needing urgent attention and those with low-risk levels that can undergo less frequent inspections.

The PoF and CoF calculations result in a PoF category (1, 2, 3, 4, or 5) and a CoF category (A, B, C, D, or E), based on predetermined ranges outlined in the API 581 standard. These two categories are plotted on the risk matrix (as depicted with the blue “R” in the image below).

In addition, a Criticality Target, such as “Medium” (as illustrated in the image below), needs to be set. Subsequent iterative calculations (by increasing the Age) are performed to determine when the target risk is exceeded (indicated by the blue arrow in the image below). From this the Maximum Inspection Interval (MII) and the Next Inspection Date (NID) is identified.

In IMS either a balanced or unbalanced RBI 581 matrix can be selected (on Site level).

Criticality Target

The target risk criterion is not defined in API 581, allowing you to define this independently. In IMS there are three possible Criticality Targets:

1. Risk Target Criteria (set at the Site, Plant, or Unit level in the RBI 581 Specific Data settings).

2. PoF Target (set by the user in the RBI 581 Details Page on the Risk tab).

3. LII Target (Legal/Local Inspection Interval, also set by the user in the RBI 581 Details Page on the Risk tab).

Age

The Probability of Failure calculation varies depending on the type of Damage Factor (DF) and sometimes also by Equipment Group or Component Type. Each PoF calculation involves a series of steps to arrive at the final PoF value. A key parameter for this calculation is the Age.

The Age parameter is calculated by subtracting the Last Inspection Date (LID) from today's date. If there is no LID, the Start Date is used instead. Since DFs differ, the calculations for Age also vary.

The Age parameter is also crucial for the Maximum Inspection Interval (MII) calculation. To determine the MII, we iterate through the Age until we reach a Criticality Target limit.