Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk. Continued

Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk. Continued

Feb 6th, 2015

Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk. Continued

Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk. Continued.

The mandate given to the expert panel forced them to consider all failures “equal”. Thus the risk equation became the simple expression of the probability of failure.  We have discussed how tolerance and risk engineering would apply to large portfolios of dams and other critical infrastructures. We also expressed our opinion in the aftermath of Mount Polley failure in a blogpost.

Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk. Continued

Mount Polley tailings dam failure

Using a similar reasoning we introduced in our 2013 paper we confirm the panel’s findings: 2 expected failures for a 10 years horizon, respectively 6 failures for a 30 years horizon, provided the standard of care, repairs, etc. is maintained and structures are not damaged by natural or man made events.

The statement “The Province’s portfolio risk is clearly excessive for ensuring that similar failures do not occur at other mine sites in B.C.” that concludes this Appendix section comes however as a shortcut: we do not believe anyone can call a risk clearly excessive without considering real holistic consequences and cost-benefits. We are not claiming the statement is wrong. But we are claiming that no one can consider a risk tolerable or intolerable without a transparent definition. Understanding social and governmental/corporate tolerance to risk is an essential skill for governance and effective leadership, one that effectively reduces knee-jerk reactions and crisis potential in the aftermath of an accident.

“Risk” Reduction Strategies

We remind the reader that an unusual definition of risk was imposed to the panel. Indeed the “risk” did not include consequences.

The panel reaches values ranging from 9*10-5 to 3*10-6 . That brings the probability of no failures to 0.9-0.99 at 10-30 years.
In our blogpost on Mount Polley we stated that many hazardous industries (chemical, electrical, for example) around the world consider the limit of credibility for an accident at a probability in the range of 10-5 to 10-6 . That is one in hundred thousand to one in a million which, by the way, corresponds to large hydro dams (non US, see prior post on this subject) historic record of failure.

We noted that major tailings dams breaches are above credibility now, and more so, in the longer run. That despite long term consolidation may help a bit. Thus we concluded that there are ways to build and manage such as to reduce major failures to the credibility threshold.

A more recent paper has shown how it is possible to evaluate the probability of failure of a dam on the very long run. In particular that paper shows that depending on the initial FoS, standard of care, etc. the failure may be unavoidable. Mount Polley, with an initial FoS of 1.3 was a perfect candidate for a failure spiral.

Finally one more word on poorly made risk assessments. Unfortunately, common practice approaches in the risk assessment/management arena use misleading tools such as Probability Impact Graphs (PIGs, FMEA, risk matrix) that mask the reality of tolerance thresholds behind binning exercises with multicolored (and arbitrary) matrices. Misleading risk assessments lead to wrong decisions/design, improper allotment of mitigative funds and anger the public.

Proper tools exist and many have laid out rational concepts.

It is possible to get out of the rut of obsolete and misleading methods, avoiding future routs!

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Category: Consequences, Mitigations, Probabilities, Risk analysis, Risk management, Tolerance/Acceptability, Uncategorized

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