In risk assessment, “business as usual” means our day-to-day practice, an unchanging state of affairs.
That is, despite the occurrence of non-divergent hazards of any kind.
The variability of any parameter considered and specified in the design of a system falls under “business as usual,” meaning that variability does not represent a hazard. For example, the variation of the oil price by ±10% could be considered as “business as usual.” However, a +30% increase in price would be a non-divergent hazard as it represents a “usual extreme”.
“Usual extremes” are those that have been observed to reoccur in a timeframe similar to or shorter than the expected duration of the system’s life. At a mine with an expected life of 20 years, any extreme phenomenon exhibiting a long-term average frequency of 1/20 or higher is a “usual extreme.” Of course, the hazard magnitude and frequency, and its consequences are always subject to uncertainties.
Divergent hazards, on the other hand, differ from long-term averages and usual extremes both in terms of frequency and intensity. For example, a 1/100 rain event occuring three times in a short period, say five years, is a divergent hazard. The likelihood goes from 1 in 100, 1%, and climbs to 3/105, nearly 3%. Even if the consequences remain constant over the period, the risk is tripled. Additionally, if repairs are not immediate and therefore the system is weaker at the second and third event, consequences may also increase. Thus, the risk could increase exponentially, and other parameters may change, such as land use, population, and the environment.
Let’s consider another example. One can consider a refinery explosion, even one of significant proportion, as business as usual. That is because we know these events do occur with a certain frequency. Their damages are foreseeable with a certain degree of uncertainty, especially if one gives due regard to contributing factors. This event could become a divergent event if it is unpredictable due to ripple effects on interdependent consequences, or if it unexpectedly overcomes all lines of defense. Suppose that divergent event hit several operations simultaneously. Then we would enter in the realm of a force majeure.
