Step 2 Start Civil RBI Analysis, assign DMs & define credible Failure Scenario

Updated on 09 Aug 2024
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After your Civil Asset has been defined in IMS, you can start the RBI Analysis.

Methodology

The key question here is: “What would happen if we would do nothing?”

This means, we only react to the equipment when it triggers our attention. In this case, it is assumed that there is no inspection and maintenance plan for the asset under consideration that enables us to proactively prevent or pick up failures. The failure consequences are the benchmark against which the costs of a PM plan are evaluated. It can be quite difficult to describe the failure scenario and its consequences, since there is no abundant history of structural failures in the industry that can be referred to. Although the situation of no maintenance and inspection may seem unrealistic, it is a useful exercise to try to imagine that no tasks are carried out anymore, this is an essential step to get the level of maintenance pitched right. Only those tasks may be taken into account that are done for other equipment or assets and that may incidentally give information about the degradation mechanism and/or failures of the asset at hand.

Scenario steps and questions:

Scenario Step	Supporting Questions
Degradation Mechanism (DM)	What is causing the asset to degrade? How quickly does the degradation mechanism affects the asset?
Failure Mode	What is the most likely way the asset will fail, i.e. how does the failure event look like (e.g. partial collapse, full collapse, etc.) How can the failure be observed (by operators, camera’s, etc.) What are the likely actions that will be undertaken on site when the failure mode has occurred (e.g. shutdown, action of fire brigade)?
Effects	Once the failure has occurred, what happens as a result of it?
Consequences	What are likely production losses? Describe in physical units defining what would happen when the asset fails, covering economic, people, environment and assets.

Dominant Failure Mode

The Failure Mode describes the way in which the asset fails. It is important to only consider realistic and credible scenarios and not to focus on scenarios with unlikely high consequences or unrealistic probabilities of occurrence. Since most assets can fail in numerous ways, it is essential to focus on the likely Failure Modes to reduce the number of assessments. Worst-case scenarios that are characterized by an unlikely probability of occurrence and/or unrealistic consequences due to a series of knock-on effects (domino effect) should not be considered. Information about typical failures may be found through historical records on site, engineering textbooks, etc.

Examples: Failure Modes

Impact damage from vehicle collision or dropped objects causing cracks and/or deformation to structure.
General or localized corrosion on steel structures.
Cyclic loading that causes cracks at welded connections.
Installation damage.
Fabrication flaws.
Overload of the structure due to an extreme event such as hurricane or earthquake of greater magnitude than used for the design of the asset.

It helps when writing the scenario to imagine what would be shown on a film at the run-up to the failure and the actual failure. It is important also to be unambiguous when describing the failure scenario, i.e. do not use words such as ‘…may occur…’, ‘…perhaps…’, ‘…could happen…’.

Degradation Mechanism: Age-Related versus Non-Age-Related Failure Modes

A distinction is made between Age-Related (AR) degradation and Non-Age-Related (NAR) Degradation Mechanisms.

An Age-Related DM is a physical process (“mother nature”) that acts on the Component or asset and over time leads to failure. Hence, the probability of failure increases with time and signs of distress or (small) local failures can be noticed. If the rate of this aging process is known, it may be possible to estimate the time of failure.

Examples: Age-Related DMs

Atmospheric corrosion of a steel member.
Chloride-induced corrosion of the reinforcement bars in concrete.

Non-Age-Related DMs are, by definition, events that result in instant failure. During such an event the asset is loaded outside the design window, e.g. a bridge that is accidentally overloaded by a crane or a structure loaded by a wind event, greater than what it was design for.

Failure Mode: Revealed versus Hidden Failures

A Failure Mode defined as revealed when the consequences of failure occur immediately after failure has occurred. Staff will notice the failure instantaneously due to, for instance, visual observations and/or alarms. In contrast, a hidden failure requires the occurrence of an additional event before any adverse consequences are experienced. Many existing Civil assets have hidden Failure Modes, since the ageing process continues unnoticed until the design load cannot be withstood anymore. Due to the random occurrence of the design load (usually an environmental load, such as the wind), it may take time before this design load occurs. This combination of random loads with the aging process requires that inspections be conducted to avoid unwanted and unexpected failures.

Example Revealed failure: Concrete vessel support

A concrete vessel supports suffers from rebar corrosion, causing concrete spalling and loss of rebar section. At a certain moment the support fails, and the vessels falls down.

Example Hidden failures: Passive Fireproofing

Passive fireproofing on a pipe rack looks good from the outside, but when a fire occurs it becomes apparent that it cannot withstand the heat radiation for which it was designed, and failure of the pipe rack occurs much quicker than expected.