Minimising Uncertainty in Vapour Cloud Explosion Modelling

There have been significant advances in consequence modelling of releases of hazardous materials in the last decade, but the use of simple models for predicting overpressures from vapour cloud explosions (VCEs) continues to elude safety practitioners. The TNO multi-energy model has generally been regarded as the best model available to date, for a rapid assessment of explosion overpressures and positive phase durations. This model requires two major assumptions to be made the level of congestion in the plant that provides obstacles for flame front acceleration, and the explosion efficiency. The former enables the selection of an appropriate charge strength from the family of parametric curves in the model. While there has been some guidance available for making relevant assumptions, there is still a high level of uncertainty in the model results, and the drag load obtained cannot be confidently applied as the basis for structural design. Flame acceleration models based on Computational Fluid Dynamics (CFD) have been developed for gas explosion modelling in offshore oil and gas facilities. Sensitivity analysis is conducted on these models, using various cloud sizes and ignition point locations, and the results are processed into an exceedence curve based on event frequencies, in order to obtain the best estimate of peak overpressures and drag loads for design purposes. In this paper, the TNO multi-energy model has been applied to two different configurations of topsides of offshore installations. In these applications the assumptions regarding explosion efficiency and charge strength selections have been calibrated against the results obtained through the CFD modelling, by obtaining a match of the explosion over-pressures. The paper comments on the findings from this exercise in order to give guidance on the application of the TNO multi-energy method and on the selection of model parameters with relation to the equipment lay-out and level of equipment congestion.