fredag den 12. september 2008

Using the ALARP principle

The ALARP-principle stands for "As Low As Reasonable Practible".

It means, that if it is practible, with reasonable effort, to reduce the risk from the "Tolerable" (Yellow) hazards in the risk table below, it should be done.
In all cases an "observation" should be foreseen in the sense that indicators of this special kind of risk should be more frequently and detailed supervised than others.

This goes particularly for the yellow hazards in the right-bottom corner.


In risk analysis you might end up with a risk matrix looking like above.

In a major project some years ago, it was necessary to decide, at a meeting, what to do with the "yellow" hazards in the ALARP region. The Project Manager looked at the table and said: "It is easy, - we have no money, and the customer has no time".

The Project Manager was interpreting "Reasonable Practible" in the ALARP-definition and came to the logic conclusion, that we didn't have to do anything.

The definition needs to be coupled with the general dislike of large accidents in society, also named: ("Differential Risk Aversion" (DRA)).
This principle concerns the "yellow" hazards in the bottom-right corner in the risk table above. This field contains all the controversial and unpleasant hazards; they are almost improbable, but the outcome is catastrophic.

For these hazards, the ALARP-principle should be implemented.

Driving through the tunnel

For example, let's say that hazard 1, in the risk table above, concerns the catastrophic scenario where a train is caught with fire while driving in a tunnel.
The risk analysis has revealed, that it is possible to reduce the frequency from 10 occasions pr. 1 Million year to 2 occasions pr. Million year by implementing an "Escape-button" on the Train driver panel.
The Escape-button allows the driver to speed up, even if the traction power has failures, in order to the help the driver escaping the tunnel.
The button is expected to cost 100,000 Euro's and taking 12 weeks to install.

The Project Manager, from above, might say: "It is not practible because of the budget and the time schedule. Furthermore, the reduction from 10 to 2 occasions pr. 1 Million years is so small that it's not worth the effort."

Acoording to the ALARP-principle, the button shall be implemented because the budget and time schedule is inside reasonable practible and because of the dislike of large accidents.

In order help the Project Manager, it would be reasonable to give the train a running permit for service for a limited time period of 12 weeks, until the button has been implemented; because the risk reduction, after all, constitutes a minor risk reducing contribution.

In any case the risk management should also ensure that indicators of hazards in the yellow area are supervised more frequently and detailed than others in the FRACAS system.
For the tunnel hazard, it would therefore be necessary to supervise activated smoke and fire alarms on the train fleet.

Next chapter >> 4.4 Quantitative Risk analysis

Focus on the Source (/EN 50126/)

See the quick guide.

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