# Some Buzz-words and their meaning. Reliability

Dec 5th, 2013

## Some Buzz-words and their meaning. Reliability

A little bit of history and a couple definitions.

There was a time where professors in good engineering schools around the world used to state that a chain is only as strong as its weakest link. Actually, most likely the Romans and many military engineers like Marechal de Vauban (who were also the first “serious” geotechnical engineers around ‘600-‘700, by the way) were also designing following that principle. The idea was: in a system the weakest component will drive the failure.

Then, when the Germans were trying to develop the V1 bomb during WWII, they observed that “a system of components has a probability of failure higher than the highest probability of one single component” and they actually formalized this into an equation (probability of failure of a series system). Reliability engineering was born! One can model even extremely complex systems by de-convoluting them into sets of linked series and parallel systems.

Adding parallel components (like two or three tailings pumps, where one would be sufficient; having twin engines in a plane, two independent hydraulic systems, etc.) is a very common tactic to increase reliability. Reliability is the chance of NOT having a failure, so it is the complement to one of the probability of failure.

A reliable system is a system that has little chance of failure.

Robust (Merriam Webster) means successful or impressive and also “not likely to fail or weaken”. Alternatively it also means capable of performing without failure under a wide range of conditions.

• To comply with the first definition of robust, good engineering will dictate to take a good margin of safety, and traditionally one would reach that result by increasing the factor of safety (FoS) above the usually accepted values. By how much? Well as FoS is not a linear function with respect to probability of failure (PoF), the simple answer is: you do not know unless you evaluate the PoF.
• To comply with the second definition of robust, you have to perform a hazard assessment where you will first identify the “wide range of conditions”..i.e all the hazards potentially impinging on your system, then evaluate the PoF under stochastic combinations of hazards.

Only then you will know if your system is robust! Now suppose you have different design alternatives (for the whole system or for sub-components) characterized by different PoF and different consequences of the failure(s), then a risk assessment is the only way to truly prioritize them in term of risk and opportunity. This was done for the environmental rehabilitation of an asbestos mine in Italy (Environmental Restoration of a 60Mm3 Dry Asbestos Tailings Dump Using Risk Based Decision Making), within the framework of an international competitive bid, and the fact that the Risk Based Design won illustrates that risk assessment gives a competitive edge to those who can use it.

If a system is robust, it is also reliable….the two terms can be considered synonyms.

Now let’s talk about system’s fragility: fragile applies to objects that are not made of strong or sturdy material and that require great care when handled: fragile porcelain plates.The term fragile implies “extreme delicacy of material or construction and need for careful handling“.

Can a robust/reliable system be fragile? Well not really: the poor material that requires great care when handled, requiring careful handling are not compatible with the definition of robust/reliable.”

Category: Mitigations, Probabilities, Risk management

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