In the AS 2885 risk matrix the five frequency levels are:
- Frequent – expected to occur once per year or more
- Occasional – may occur occasionally in the life of the pipeline
- Unlikely – unlikely to occur within the life of the pipeline, but possible
- Remote – not anticipated for this pipeline at this location
- Hypothetical – theoretically possible but has never occurred on a similar pipeline
I get the impression that a lot of people struggle to interpret some of those descriptions, particularly those lower on the scale. I bypass that problem by using supplementary numerical guidelines, as mentioned in this post with additional supporting information here.
However not everyone is comfortable with departing from a strictly literal interpretation of the standard (notwithstanding the inherent contradiction in literal interpretation of something as ambiguous and subjective as the frequency descriptions). The main problem is with the Hypothetical description which includes the words “… has never occurred on a similar pipeline”. Many people seem to interpret that as “has never occurred” without further qualification, so if a similar type of failure occurred on a pipeline somewhere in the world they feel unable to adopt the Hypothetical frequency. However the bit about “a similar pipeline” is critical.
A “similar pipeline” is not just one of similar diameter and pressure rating but one which has similar characteristics in terms of its protection against the threat in question. In some cases even diameter and pressure rating might be irrelevant. For the commonly encountered external interference threats the relevant similarities are include particularly the physical protection and procedural protection and perhaps also the installation environment (population density, level of third party activity, etc).
The other consideration is that is it crucial to carefully identify the exact failure consequences for which one is estimating the frequency. A hit on the pipe does not equate to a full bore rupture and multiple fatalities. There is perhaps no type of external interference for which it can honestly be said that it has never occurred to a pipeline somewhere. So it might be correct to never accept the Hypothetical frequency rating for a strike on the pipe. But a strike is just a strike – in the great majority of cases it will result in a gouge or dent with no effect on safety or supply.
To get to the extreme worst case of full bore rupture and multiple fatalities you need:
- Initial strike on the pipe, AND
- Impact sufficient to penetrate the pipe, AND
- Resulting hole large enough to exceed critical defect length and rupture, AND
- Ignition of the escaping fluid, AND
- Sufficient people present and unable to escape or shelter
Each “AND” has a conditional probability that is quite small, of the order of 10% . By the time you combine all those probabilities the likelihood of multiple fatalities is less than 1000th of the likelihood of the initial strike. So to take the example of, say, damage by a power pole auger for a pipeline in a suburban street, it may be quite reasonable to adopt a frequency of Unlikely for a strike, but completely unreasonable to then apply that frequency to the case where it results in multiple fatalities.
The number of catastrophic pipeline failures that have genuinely resulted in multiple fatalities is very small – San Bruno, Ghislenghien, Carlsbad are the only transmission pipeline disasters I can find from the last 20 years, and only Ghislengien was an external interference failure (San Bruno was a construction defect, Carlsbad was corrosion).
Don’t feel inhibited about adopted the Hypothetical frequency rating.
 My statement about the conditional probabilities being of the order 10% is supported by evidence mainly from historical incident data but the details are a bit complex to include here.