In December 2005 a very severe explosion took place at the Buncefield tank farm north of London, UK. As a tank farm is both unconfined and has low congestion, experts had difficulties explaining what caused the very strong explosion. Total, who was the operator of the facility at the time of the accident, contacted GexCon for assistance in the investigation.

At the site, the GexCon team observed that there were arrays of trees and dense vegetation surrounding the facility and suspected that the congestion of the trees could be causing the strong flame acceleration observed. Detailed FLACS modeling was performed to demonstrate that the actual vegetation density could cause sufficient flame acceleration to explain the accident. In April 2007 this theory was communicated to the Buncefield investigation board, and gradually their experts adopted the theory that the trees caused the flame acceleration. GexCon later performed experiments to demonstrate how trees and vegetation could cause strong flame acceleration.

A video summarizing the GexCon work can be found by clicking here.

The report from the experimental study showing the influence of trees on flame acceleration can be downloaded from here. 

One main conclusion after the accident is that operators at facilities which can cause major releases of flammable material should not only consider the actual congestion level on their plant in a risk assessment, but may also need to consider congestion caused by surrounding vegetation (actual and future).