Minimizing Uncertainty in Vapour Cloud Explosion Modelling
Process Safety and Environmental Protection, 83 (4):
298--306 (July 2005)DOI: http://dx.doi.org/10.1016/S0957-5820(05)71255-6
Abstract
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.