Methodology Step 4

Step 4 Determine the most Credible Failure Scenario:

Below the Step 4 Methodology is described. See also Software Step 4.

The credible failure scenario should be established by the RBI team. They should consider the expected DM, process conditions, the Equipment design, operating history, and inspection history.

A complete failure scenario should describe the failure itself (left-hand side of the bowtie), the top event, and the Barriers on the right-hand side of the bowtie. In general, this would include a description of:

• The failure location (on Equipment)
• The hole size
• The leak rate
• The products leaked (their flammability [Nf factor] and toxicity [Nh factor)
• The formation of liquid pool or gas cloud
• Ignition sources (e.g., auto ignition, no ignition sources, road presence, etc.)
• The location where leaked liquid will go to (e.g., oil containing sewer, surface water, soil)
• Detection of the leak
• Presence of people

An example of a Credible Failure Scenario is: “50 bar methane gas leak from 6 mm hole, containing < 1000 ppmw H2S at 60°C at approx. 650 kg/h forming a flammable gas cloud in semi-congested area without ignition sources. Unit down time for temporary repair 2 days.”

Specific Guidance Summary

Guidance per Special Emphasis gives additional information with regards to Special Emphasis Components (see the table below).

Specific guidance summary - Credible Failure Scenario: