Risk Analysis

The functions of risk analysis include:

• Possible failure modes for the asset
• Fault analysis of failed components
• Actual probability of events and the subsequent consequences, and the distribution of the probability from minor consequences to catastrophic consequences
• Costs of the consequences of the failure for the organization.

There are many well-established techniques for risk analysis, for example:

• FMECA (Failure modes, effects and criticality analysis)
• Threat and vulnerability
• Energy damage
• Time sequence models
• HAZOP.

Some of these are qualitative risk analyses while others are quantitative.

Except for FMECA, these techniques have been developed for major/catastrophic events.

Although these events are important to most service authorities, they do not have a high probability and therefore do not constitute a great business risk.

There is a larger group of failures related to individual assets involving medium and small risks, which constitute the bulk of the total risk.

It is essential that the analysis technique deals with all asset delivery failures.

Expert risk assessors should be involved in risk assessments involving catastrophic events in specialist areas.

For large service authorities with like assets, the temptation to deal with them as though they were all identical must be resisted. The original construction materials, the quality of the construction, and varying maintenance and operation history of assets will result in a distribution of failure at very different effective lives.

For example, sewers constructed to the same standards and in the same ground conditions at the same time have been found to have a distribution of effective life from 45 years to 85 years. For economic purposes, a mean effective life of 70 years may be adopted. However, to be able to effectively manage the sewerage system in an urban area, each manhole length should be considered individually, and its life assessed more accurately.

Variation in effective life is most obvious for long-lived passive civil assets. Shorter-lived dynamic assets exhibit the same effect, although the deviations are not usually as great.

Assets don't usually fail as a whole. It is the components of the assets that fail, for example: joints in a water main or control units in an automatically actuated valve etc. Such faults or failures of components do not mean that the asset has reached the end of its effective life. However, if it happens often, then it may be economically sensible to replace the asset as a whole. Most passive civil assets will be repaired many times before their replacement can be economically justified.

The probability of failure will therefore be influenced greatly by factors such as:

• The asset condition
• The way in which it has been treated throughout its life.

The severity of failure will depend on:

• The position or location and surrounding environment
• The size of the asset and its potential to cause a cost or loss to the organization (destructive potential)
• The ability of the organization to control or affect the failure
• The outcomes or consequences of the failure
• Influences that will affect the magnitude of the failure.

A fault tree example is shown below:

Examples of events for reticulation systems are listed below: