oil & gas vapour cloud explosions - how the buncefield and...
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DNV GL © 11 December 2019 SAFER, SMARTER, GREENERDNV GL ©
Mike Johnson
11 December 2019
OIL & GAS
Vapour Cloud Explosions - How the Buncefield and Jaipur Incidents Changed our Understanding
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Overview
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▪ Background on early research
▪ Accepted assessment methodology
▪ Buncefield and Jaipur incidents
▪ Implications
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‘Unconfined’ Vapour Cloud Explosions
▪ Major explosions in the 2nd half of the 20th century where the gas/vapour cloud was not confined
▪ No understanding of the cause of damaging pressures
▪ A key incident for the UK was in Flixborough in 1974
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Effect of Process Congestion
▪ One characteristic was that clouds usually
engulfed congested process areas
▪ Research examined the effect of pipework in the
gas cloud
– Conducted ~1980-1986
– No computer models
– Simple regular obstacle arrangement
– Parameter variations easily specified
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Experimental Arrangement
Ignition
45m
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Natural Gas – Half full of Pipes
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Extended Pipework Region
Ignition
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Cyclohexane – Full Pipes
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Natural Gas with Initial Confinement
Ignition
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Maximises flow through pipes giving rapid flame acceleration
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Natural Gas – With Initial Confinement
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Confinement and Congestion
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Cyclohexane and Propane
Ignition
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Cyclohexane
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Slow Motion
▪ Deflagration to Detonation Transition
(DDT)
– Flame speed ~Mach 2
▪ Detonation sustained through
remainder of cloud
– 1.8 km/s (~Mach 5.5)
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Deflagration to Detonation Transition
Private and confidential
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Summary – Late 20th Century
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▪ Supersonic Deflagrations with natural gas
▪ Dependant on congestion
▪ DDT with Cyclohexane and Propane
▪ Required only 15m of flame propagation
▪ Results published in 1988
▪ Industry adopted assessment based on
deflagrations in process regions
▪ DDT ignored or considered unrealistic
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Buncefield – December 2005
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Vapour Cloud
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▪ Overfilling of petrol tank in calm conditions
▪ Petrol broke up into droplets as it fell from tank roof, generating vapour
▪ Vapour cloud extended offsite
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Jaipur – October 2009
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▪ 1000Te of petrol spilled as a ‘geyser’ from
the tank outlet pipe
▪ Break-up of liquid into droplets enhanced
vapour generation
▪ In calm conditions, vapour cloud spread to
cover most of the site (an area 3 times that
of the Buncefield cloud)
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Characteristics of Buncefield and Jaipur Incidents
▪ Very little process congestion on sites
▪ Dense vapour cloud covering large area
▪ Widespread severe blast damage through most of the vapour cloud
▪ Does this indicate a detonation of the cloud?
▪ More on pressure damage from Bassam Burgan tomorrow
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Directional Indicators
▪ Observed throughout clouds in
Buncefield and Jaipur incidents
– Bent or leaning lampposts
– Trees scorched on one side
– Branches on trees snapped
and bent over in one
direction
– Scoured paintwork on one
side of posts
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Initial Investigation
▪ Early Buncefield report gave initial
assessment of the directional indicators
▪ Suggested three explosion events!! (Indicated
by the red and blue arrows)
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Assumed
direction of
explosion
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Directional Indicators
▪ Experimental work showed significant reverse
flow
▪ Modelling confirmed net force in reverse
direction
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Re-interpret as
opposite direction
of explosion
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Directional Indicators - Buncefield
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Red inside cloud, Yellow outside cloud
Red arrows point to location of DDT
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Directional Indicators - Jaipur
▪ Large red arrows show summary of
many directional indicator measurements
▪ Point towards a single source, as in
Buncefield
▪ Indicates location of DDT
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Other Vapour Cloud Explosion Incidents
▪ Recent publication of a review
of VCE incidents*
▪ Evidence consistent with DDT
in most major VCEs
– Pressure damage
– Directional indicators
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* G. Chamberlain, E. Oran, A. Pekalski, Detonations in industrial vapour cloud explosions,
Journal of Loss Prevention in the Process Industries, Volume 62, November 2019, 103918
Flixborough 1974
Skikda, Algeria, 2004
Paraguaná Refinery Complex, 2012
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So is this Very Bad News?
▪ First reaction can be ‘I can’t design against for a 20bar detonation pressure’
▪ So it looks like very bad news
▪ However, current good practice will minimise the risk:
– Prevention or minimising release or spill is even more important
– Separation of occupied buildings from process area (minimises effect on design strength)
– Reducing potential for flame acceleration
– Maintaining safety critical systems to original design intent
▪ In the end, this is reality, we need to deal with it
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Summary
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▪ All the elements of the Buncefield and Jaipur VCEs
were understood before the events
▪ VCE assessment methods avoided this ‘uncomfortable
truth’
▪ What has changed is our willingness to accept DDT as a
reality in VCE incidents
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