Civil Components and Credible Scenarios (Steps 1 & 2)

Find below additional details for Step 1 and Step 2 of the Civil RBI Methodology.

Components and Sub-Components and Asset Functional Failure

Analysis of a sub-Component or part of the asset is practical when the asset is made of different materials for which specific maintenance tasks and frequencies may govern the lifecycle costs of the entire asset.Decomposition of an Asset into Components, Sub-Components and Parts.Each Component may result in a different evidence of functional failure (Failure Mode) for the asset. Not all Failure Modes result in the same consequence of failure (the robust design that included safety factors and a built-in redundancy by means of secondary load paths, diminishes the effects of the Component Failure Mode), however a Failure Mode that prevents the asset from safely fulfilling its function is the functional failure of that asset.

The load that eventually causes functional failure usually has a random occurrence, e.g. an environmental load such as the wind or wave action. With reference to the above figure, assets are usually designed based on probabilistic load occurrence. Functional failure of the overall asset can occur where the probabilistic design load is exceeded. The following figure gives a simplified view of the decrease of the load-bearing capacity due to aging (black line), together with the random load that acts on the asset (blue line). For age-related failures it has been assumed that failure can be expected in the same year that the load-bearing capacity become less than the minimum required capacity. It has hence been assumed that the design load would occur in that particular year. When the failure consequences are not significant it may be decided to take the actual occurrence probability of the collapsing load into account.

Typical decrease of load-bearing capacity leading to failure.

Many existing Civil assets Components have unrevealed and revealed Failure Modes.

Revealed failures

Example: 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: Concrete table top

A concrete table top is subjected to carbonation and at some point concrete pieces fall down from the ceiling on top of process equipment located underneath.

 

Unrevealed failures

Example: 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

Example: Staircase

A stair tread is severely corroded, but only fails when somebody tries to climb it.

Dominant Failure Mode – Most credible failure scenario

The key question here is: “In what way will the asset fail?”

The figure below indicates that from a range of possible degradation mechanisms, Failure Modes and consequences for an asset, the practitioner determines which one(s) is the most likely and credible for his case (indicated in blue). The other non-dominant degradation mechanisms, Failure Modes and consequences can be briefly described for auditing/record keeping purposes, but are in subsequent steps of the study disregarded. The credible failure scenario should be reviewed as part of the Living Program and updated if needed.

Selection of Credible Failure Scenario from a Spectrum of Failure Scenarios.

Sometimes more than one Failure Mode can be dominant, and the PM plan should have those tasks that are effective to address all dominant Failure Modes.