Methodology Step 7

Step 7 Determine Susceptibility to Failure (StF):

Below the Step 7 Methodology is described. See also Software Step 7.

The Susceptibility to Failure (StF) is the probability (likelihood) side of the S-RBI risk matrix (see Risk Based Inspection (RBI) - Theory). The StF should be assessed per DM per Component. Only if all Equipment and piping in a CL are in non-corrosive service, the assessment may be done once at CL level and an S-RBI Reassessment Schedule can then be created for the Equipment in the loop.

StF Categories

The significance of the StF categories should be interpreted as:

• Negligible: Under no foreseeable conditions this DM may lead to Equipment damage. However, damage is possible in case of IOW excursions;
• Low: Equipment degradation is per design and Equipment/piping will reach its intended design life;
• Medium: Degradation rate is higher than the intended rate and a need for repair/replacement is likely within the design life of the Equipment;
• High: Degradation rate is very high (over 4 times design), and loss of in-service integrity is likely.

The StF of a Design should never exceed Low. A Medium or High StF indicates that a Component will not reach its intended design life. From exiting Equipment this may result from an operational change (operating structurally outside IOW).

StF Criteria

The criteria to assess StF should be kept specific to the DM. Refer to Guidance per Special Emphasis and Guidance per DM for specific guidance per Special Emphasis Component / DM (see the table below) and use the built-in probability questionnaires. These vary depending on the DM (AR, CUI, HTHA, etc.). In general:

Age-Related DMs

• For corrosion mechanisms, the actual or expected CR (i.e., SCR) should be compared to the design CR (CR_d) (see Step 6)
• For other AR DMs, the degradation rate should be compared to the design values (RL is based on usage): e.g., for creep the numbers of actual hours to design hours, and for fatigue by cyclic service the number of cycles compared to the permissible number of cycles.

Susceptibility to Failure for Corrosion Mechanisms in the Risk Matrix.

 

Non-Age-Related DMs

• For NAR DMs, the assessment methods vary, e.g., tables, graphs, decision trees.
• The guidance per Special Emphasis Component or Guidance per DM should be followed.

For Corrosion Mechanisms the difference between Potential Corrosion Rate (PCR) of the Special Emphasis Component and the Design Corrosion Rate (DCR) of the main flowing pipe should be used to determine the StF (see Corrosive Deadlegs and Corrosive Injection and Mixing Points). Susceptibility to Fail is the vulnerability to actual degradation as compared to the degradation expected per design.

Specific Guidance Summary

Specific guidance summary - StF: