Rehabilitation generally prolongs the life of an asset by reducing failure probability. This can be modeled as either a reduction in effective age (i.e., the asset is effectively younger after rehabilitation) or as a new, lower failure probability curve. In some cases, rehabilitation may correct an existing flaw, improving the Health Index of the asset and thereby reducing failure probability.

One commonly assessed rehabilitation intervention is injection of non tree-retardant cables. The cable asset tool calculates the net benefit of injection year-by-year to determine the net benefit of injection at a given age. The optimal timing of injection is the age at which the net benefit is greatest. The benefit of injection is quantified as the avoided risk between the case where the cable is injected and the alternate case – usually taken to be replacement of the cable at the end of its economic life. In other words, the benefit of injection is calculated as the difference between the Replacement Cost Stream without injection and the Replacement Cost Stream with injection. If the net benefit of injection (benefit of injection minus the cost of the injection) is positive, then it is cost-effective to inject the cable before ultimate replacement.

The figure below shows the calculation of benefit of injection at a particular age. Without injection, the cable would continue up its failure probability curve until it reached the EAC (i.e., dashed blue line). However, if instead the cable is injected at age 34, failure probability and risk cost drop immediately before resuming their climb. This example is based on the assumption that injection “rejuvenates” the cable by 20 years – that is, it takes 20 years off the effective age. The net present value of the pink shaded area represents the benefit due to avoided risk from injecting at age 34; the net benefit is this value minus the cost of injection.

The asset tool repeats the calculation of net benefit of injection year-by-year for each age to determine the optimal injection timing. Qualitatively speaking, the net benefit of injection starts low for young cables, you would not think of injecting a cable that was only five years old, and then rises to a maximum after which it falls until the cable becomes so old that it is better to simply replace it rather than inject.

The figure blow shows the economic life calculation for a sample cable segment. The black dots represent the net benefit of injection, so in this example injection at age 36 is the optimum strategy. Before age 20 and after age 48 the net benefit of injection is negative, so once the cable is 49 years old injection is no longer a feasible option. In some cases the net benefit of injection is always negative, meaning that injection is not cost effective for that cable at any time.

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