Abstract
Crude oil is often transported by pipelines from remote production sites to refineries. Sometimes oil releases take place, caused by sabotage, corrosion or external impact. In a safety consequence study the main hazards considered are often pool, jet or flash fire. The integral models applied may give somewhat misleading hazard estimates, as a flat terrain surrounding the pipeline is assumed, and the models lack the ability to predict dense gas dispersion for calm and low wind scenarios. In a varied terrain an unrealistic large pool and following pool/flash fire, is often predicted. A major hazard often ignored is vapour cloud explosion. The fractions of volatile components in crude oil are limited. Still, as demonstrated by cascade flow experiments after the Buncefield accident, certain release mechanisms may lead to an enhanced evaporation and the likely generation of flammable vapour clouds. For an oil release it can be assumed that a pressurized oil release generating a vertical oil spray into the air, can lead to the evaporation of significant fractions of flammable vapours of propane, butane and pentane. For major pipeline releses during calm wind conditions (0-0.5 m/s) in a varied terrain, a CFD simulation study demonstrated that very large flammable vapour clouds could be generated and could fill lower lying areas of the terrain. The simulated clouds could be 5-10m deep with likely ignition sources as homes, farm houses and roads inside for 20-30 minutes. Areas with trees and dense vegetation were also inside the gas cloud, and could have led to significant flame acceleration, potentially deflagration to detonation transition (DDT) and severe consequences as seen for the Buncefield accident. For pipelines buried below the ground one possible way to prevent such a scenario will be to avoid the possibility for oil sprays upwards, for instance by covering the pipeline with plates which will remain in place during a major release. If this is not possible, vapour fences may be used to limit the gas cloud migration towards likely ignition sources or areas with dense vegetation. While such scenarios may be rare for unintended releases, the severity of the potential consequences should justify their evaluation as part of a safety study.
