This page provides supplementary material to the post on the final report of the Citizens’ Risk Assessment.
It is important to acknowledge that the Citizens’ Risk Assessment has its limitations, and most of them resulted in an underestimation of total risk. There were good reasons for the limitations (lack of statistical data, software limitations, and financial constraints), and we will probably never know exactly how the limitations affected the results, but we need to understand them as part of the context for the report. Here are the main ones that I am aware of.
- Individual risk vs. population risk. This study only addressed the risk of death associated with being at a given spot. It did not address the question of how many people are at that spot, so it gives the same risk value to a single fatality (for a farmer in a field, for example) as it does to a school playground where a hundred children might die. A far more expensive study, taking into account the distribution of population around the pipeline route, would have been required to assess this “population risk”. This is a major limitation, but funding a study that addressed population risk would not have been possible.
- “Black swan” events. It is important to recognize that the study does not attempt to deal with very rare events. It only deals with events for which there is good statistical data about frequency of occurrence. Quantitative risk assessments, like this one, calculate the risks in part based on how frequently a given event can be expected to happen. But how can you estimate the frequency of something that almost never happens?
As it turns out, the most severe accidents are always ones that result from rare events. These accidents will never be accounted for in a quantitative risk assessment. For that reason, it is important to learn about what could happen in a worst-case accident, and how to prevent it from happening, even though it is not listed as a possibility in any quantitative risk assessment.
- Death vs. Injury. The study addresses only the risk of fatalities, because there are good statistics available for fatalities but not for injuries. But where there are fatalities, there are likely to be far more injuries, and accidents with no fatalities can still cause many injuries.
- Flat terrain vs. terrain where gas is channeled. The equations in the software used to model the movements of escaping gas can only deal with flat terrain. This is a good first approximation for many situations, but much important detail is lost. In particular, if there is a leak that is not immediately followed by ignitions, these gases will flow downhill, staying near to the ground, and may pool in low-lying areas. If that happens, the consequences can be far worse than in flat terrain. Some of the worst NGL accidents have happened because of pooling.
- No information on one pipeline triggering a leak in another. In many areas, the various Mariner East pipelines run alongside one another. It is possible that an explosion cause by one of them could cause another to fail, resulting in far greater consequences. There are no statistics about this type of event, so it was not considered in this study.
- Low wind speeds. Low wind speeds increase consequences, because the gas does not disburse as readily. However, the software used in this study does not handle very low wind speeds (below one or two mph). The largest area of flammability might occur in a dead calm, but that possibility could not be tested.
- Toxic gasses from combustion. The study does not consider toxic or irritating gasses that could be generated by burning materials in structures. This is often a problem in major fires, but making predictions about it would require extensive analysis of all buildings in the vicinity of the pipeline. This was not possible financially.
- Dizziness or asphyxiation from leaking gasses. The study does not consider the effect of a cloud of unignited gas on nearby residents. Because they would displace the air’s oxygen, they could cause dizziness (which might limit mobility) or even asphyxiation.