Step 5 Determine Probability or Likelihood of Failure (Civil)

As part of the Civil RBI Analysis you must determine the Probability of Failure (for Age-Related DMs) or Likelihood of Failure (for Non-Age-Related DMs).

Methodology

The overall question here is: “What is the probability/likelihood that the consequences as described in the Failure Scenario will be realized in the coming years by the asset in its present condition given what was assumed during its design?”

The probability/likelihood level should be judged from experience, by asking the question: ‘How often in the past has a failure resulted in a Consequence similar to the one that we are considering?’ The approach is one of applying history to predict the future. The estimate of Likelihood should be based on the Likelihood that the Consequence under consideration occurs. This is not the Likelihood that an incident occurs. For example, one should consider the likelihood that a partial collapse of a pipe rack results in a hydrocarbon release, not just that the pipe rack will collapse.

Probability of Failure for Age-Related Failures

The susceptibility to degradation is a measure of the rate of degradation and the probability that the Component will develop damage. For age-related failures, the damage level will progress over time and as a result the probability of failure will increase therefore the risk will increase. This is determined by comparing the actual exposure of the Component to the degradation mechanisms as compared to the degradation that was considered (or reasonably expected) during the design of the asset. The susceptibility rating is only applicable for AR DMs, due to the predictive character of these types of DMs.

Likelihood of Failure for Non-Age-Related DMs

For event-related DMs, the likelihood is based on historical evidence or experience that such consequences have materialized within the industry, the company, or a smaller unit. In those cases, the likelihood generally is constant over time. The ratings are given in the table below, in accordance with the Shell Risk Assessment Matrix (version April 2006).

Likelihood ratings:

Software

Determine Probability Rating for Age-Related Failures

• Select the Component – DM combination that you want to analyze.
• Select the Criticality Assessment tab.
• Determine the susceptibility to failure by answering the questionnaire (click Civil StF); or

Click Civil StF and answer the questionnaire to determine the susceptibility to failure.

• Alternatively click the Probability category along the vertical axis (to see the matrix, Overview must be selected).

Alternatively, select a probability category for the susceptibility to failure. 

Determine Likelihood Rating for Non-Age-Related Failures

For NAR DMs the selection of the likelihood of failure is done on the vertical axis of the Risk Matrix.

• Select the new Component – NAR DM combination that you want to analyze.
• Select the Risk tab.
• Determine the likelihood of failure by answering the questionnaire (click Civil Likelihood); or

Click Civil Likelihood to determine likelihood of failure.

• Alternatively click the Likelihood category along the vertical axis (to see the matrix, Overview must be selected).

Alternatively, select a Likelihood category.