Pre-event Influences

The mode of failure of an asset can be affected by many physical and environmental factors. They can cause identical assets to fail in different ways over a variable period.

This deviation from the average life can be quite substantial e.g. + or - 30%. For example the first failures may occur at 50 years and the last at 95 years. The figure below shows a typical, or Weibull distribution:

For a watermain, the factors that affect the ultimate life might be:

• Defects in the original manufacture of the pipe
• Stresses and damage that occurs during the cartage and loading and stacking of pipes
• Inadequate physical characteristics for the duties or stresses expected in its normal working conditions
• Poor construction techniques used in the laying of the main (inadequate bedding or foundations)
• Damage during construction (pipes dropped, backfill impacts etc.)
• Operating pressures that exceed the specified working environment at times throughout the life of the pipeline (water hammer from valves or major breaks)
• External stresses placed on the pipeline during its working life (construction traffic)
• Peculiar ageing characteristics of the pipeline during its effective life due to aggressive water or ground conditions.

In most cases an asset manager cannot predict the reduced life of an asset from these factors. However it is possible to:

• Monitor the performance of the asset
• By analysis of failures, attempt to isolate the cause
• Review the effective life of each asset.

Another pre-event influence is the time available between the asset failure and the time that the system or service fails as a result. These can depend on weather and operating conditions.

Factors affecting the failure of an asset

Example

In some coastal regions, climatic conditions, topography and the prevailing winds can combine to make what is known as a "salt dump location or area". The salt charged atmosphere is drawn in from the sea to a point where it is dumped as salty rain.

This highly corrosive environment has a dramatic effect on the life of assets exposed to it, such as overhead electricity distribution systems, buildings, above ground water mains.

If these salt dump areas are overlaid on a digital asset map, asset managers can quickly identify those assets that are likely to experience a shorter effective life and require more intensive condition monitoring than their other similar assets.

External factors affecting the costs of consequences of failure

Example

For a typical drainage or sewerage failure such as a broken joint or a collapsed pipe, the immediate consequences relate to the failure of the service.

These faults may not reduce service level if the pipe is laid through impervious material that has adequate structural integrity, such as rock, sandstone or clays, to maintain the flow path.

However, this would not be the case if the sewer is laid through sand, saturated silts or other unstable ground. In these circumstances the surrounding material will collapse into the void and may be carried away by the flows. In some cases the void will continue to grow until the cavity reaches the surface, often resulting in substantial collapses and severe consequences of failure.

Ground conditions and previous failure events thus need to be considered when ranking the consequences of failure. This primarily relates to underground sewer and drainage conduits, however it will also impact on the failure of water mains. It will substantially affect the cost of repair in all cases because of the difficulty of construction work in unstable ground.

The following issues will impact on the cost of the failure and the renewal options:

• Location, particularly accessibility to repair equipment, disruption during repair etc
• Voids created outside sewers and drains due to inflow washing away parent, bedding, or refill material
• Washing out of bedding support by leaking water mains
• The geology involved, groundwater depth and movements, and other local conditions, which determine the degree of difficulty of repair
• The extent of asset failure.

Factors Affecting the Failure of The Asset