The use of risk criteria in comparing transport modalities

For the transportation of dangerous substances, several transport modalities are possible. The choice for a specific transportation mode is often based on practical assumptions, like "we have a railroad available" or "we have a shipping contract". But suppose you have a choice for different types of transport, which one would be the safest for your substance? TNO has developed a method to compare the different transport modalities by using standard risk criteria like cumulated Individual Risk (a.k.a. Locational Risk) and Societal Risk. The evaluation was done for two typical substances: ammonia and propane, which are both transported often in the Netherlands. By using a standard QRA (Quantitative Risk Assessment) tool, the individual risk contours, the societal risk (FN) curve, and societal risk maps were generated and compared. A number of important assumptions were made in the study: 1. Since loading / unloading are essential steps in the distribution process, the risk of these activities were included for all batch based transportation methods (so not for pipeline transport) 2. The currently prevailing legislative failure rates and accident distances have been applied (calculated for the number of required ships/cars/wagons and typical dimensions). Although there is no consensus on "correct" failure rates or consequence distances, the statutory rules and values were applied. For the Individual Risk contour, one tends to use the "distance to 10-6 risk value" in comparing risks of transport. Due to the important contribution of loading and unloading activities at the beginning and endpoints of the route, one really should evaluate the cumulated contours for interference with vulnerable objects. For Societal Risk, the Dutch prescribed approach would be to compare the FN-curve for the highest kilometer section of the route. The study pointed out that this approach may give misleading results in comparing situations for different route tracks. Furthermore, because stationary equipment is subjected to stricter allowance rules (a lower guide value of societal risk curve), comparison of the combined effect of the transport and stationary loading & unloading activities is complicated. The use of societal risk maps, where (cumulated) risk is observed from the point of view of the receiver, appeared a very valuable addition in ranking risks