Developing time-based probability of failure
The instantaneous or short-term probability of failure can be reasonably assessed by a general assessment of the asset conditions in terms of structural integrity.
In general the probability of failure can be assigned a relationship to the structural integrity in accordance with the following ranking schedule.
Asset Condition | Probability of Failure (in next 5 years) |
Excellent | 0.0 |
Very Good | 0.0 |
Good | 0.0 |
Average | 0.1 |
Below Average | 0.2 |
Poor | 0.3 |
Very Poor | 0.6 |
Unsatisfactory | 0.7 |
Critical | 0.9 |
In the actual data collection by field staff, only three categories of condition rating need to be used to simplify the human judgment involved, as per the following examples.
Structural Integrity | Probability of Failure (in next 5 years) |
| Example 1 | Example 2 |
Satisfactory | 0.01 | 0.001 |
Moderate | 0.1 | 0.01 |
Poor | 0.5 | 0.1 |
Very Poor | 0.9 | 0.4 |
However, to provide indication of the probability of failure for use in an economic risk assessment over a project period, a time based probability of failure should be develop.
The following sections relate to the time-based or long term probability of failure and they involved some statistical analysis of past failure information.