Release Scenario Assumptions for Modeling Risk From Underground Gaseous Pipelines

Underground pipelines transport toxic and/or flammable pressurized gases in close proximity to residential and commercial areas and highways.  The quantified risk assessment of these pipelines is dependent on release scenarios, assumed frequencies, weather probabilities, neighboring populations, etc.

One key learning while re-evaluating methods for modeling flammable and toxic pipeline risk was the importance of the assumed release rate or rates.  Models, such as PHAST-Risk, only model a single release rate or a series of stepped rates for dispersion or radiation calculations.  Release rates from pressurized vapor pipelines is a function of the hole size and pipeline pressure and inventory.  Therefore, careful consideration needs to be given to the flow averaging assumptions that are used.  Optimal release rate assumptions depend on depressurization time and on the nature of the hazard.  Radiation hazards from hydrogen pipelines are assessed using initial flow rates to determine the hazard zone.  Toxic hazards from carbon dioxide and other toxic materials are assessed either using a multistep or peak flow rate to give the best representation of risk given the pipeline depressurization time. 

A second key learning was the importance of release momentum and direction.  Releases from underground pipelines have the potential to be high momentum angled or vertical releases from a crater or low momentum releases diffusing through the soil.

Examples will be shared to demonstrate the impact of release rate(s), release direction and momentum on calculated hazards.  Lessons learned from the pipeline risk methodology can be applied to vapor releases from storage vessels.