This project is funded by the European Union
Projekat finansira Evropska Unija
ACCIDENT SCENARIOS
AND TOP EVENTS
Antony Thanos
Ph.D. Chem. Eng.
antony.thanos@gmail.com
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
Project implemented by Human Dynamics Consortium
Projekat realizuje Human Dynamics Konzorcijum
• Risk Analysis Framework
Hazard
Identification
Accident
Scenarios
Risk reduction
measures
Consequence
Analysis
Accident
Probability
NO
END
YES
This Project is funded
by the European Union
Accepted
Risk
Risk
Assessment
Project implemented by Human
Dynamics Consortium
• Scenarios selection
 No unique approach within EU, as for rest of
Risk Assessment Methodology
 Nevertheless, worst case scenarios are almost
always required for :
o Emergency Response
o Land Use Planning reason
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, UK case :
• “representative” set of major accident scenario
required
• minimum scenarios list examples referred in
Assessment Guides (SRAG) for certain types of
establishments (although probabilistic
approach)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, UK case : (cont.)
 Example of scenarios list in HSE SRAG for LPGs
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, Netherlands case
 Detailed guidance in Reference Manual Bevi
Risk Assessments, as part of QRA
 Step 1 : Sub-selection method (based on TNO
selection method)
 Screening method (relative ranking)
 Basic principle: Identification of “Containment
Systems” which contribute most to external risk
 Not scenario selection, but Containment
systems selection
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, Netherlands case (cont.)
 Main criteria :
o Effects (1% lethality) extent out of fence
(calculation of consequence required for
worst case scenario for Containment)
and/or
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, Netherlands case (cont.)
 Main criteria : (cont.)
o Estimation of effects based on selection
method :
 Indication number A (intrinsic hazard)
Q, quantity
Q  O1  O2  O3 O1, O1, O1 factors for
A
process conditions
G
G, limit value (10000 kg for
flammables)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, Netherlands case (cont.)
o Selection method : (cont.)
 Selection number S: (hazard level at location fence)
n : 2 for toxics,
3 for flammables and explosives
n
 100
S 
 A
 L 
L : distance from fence (at least 8 points
examined)
S>1, candidate Containment Systems for
inclusion in QRA
Comparison of S values for various
Containment Systems provides final selection
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, Netherlands case (cont.)
Sub-selection
method
overview
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, Netherlands case (cont.)
 Step 2 : Definition of Releases
o Releases to be included for selected
Containment System based on tables per
equipment type referring also frequency
o Example for gas containers
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, Netherlands case (cont.)
 Step 3 : Top events (scenarios) defined on
event trees for releases
o Event trees included for main cases in
Manual
 Releases general cut-off limit :
o probability > 10-9 per year
o 1% lethality distance extending outside
fence
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, Cyprus case
 Deterministic approach
 Generic minimum list of scenarios
o Catastrophic failure of vessels, tanks, pipes
o Rupture of vessel/tank (hole with diameter
equal to max pipe connected to
tank/vessel), hole 20% of pipe diameter
o Small leak in vessel tank, pipe (hole
diameter 25mm or 50 mm)
 Selection method for critical equipment (TNO
Purple Book)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Scenarios selection, a few other EU Member States
cases
 Italy (Hybrid approach)
o Decree for LPGs : Certain scenarios are
excluded based on available measures
 France (Hybrid approach)
o High consequence scenarios must be
included in consequence analysis, even for
low probability
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Typical release scenarios per equipment type
failure :
 Pipes
o Catastrophic failure (Full Bore Rupture –
FBR- or guillotine break)
o Partial failure (hole diameter equivalent to a
fraction of pipe diameter, e.g. 20%)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Typical release scenarios per equipment type
failure (cont.) :
 Pressure vessel (process vessel, tank, tanker)
o Catastrophic failure: “instantaneous”
rupture (complete release of content within
short time e.g. 3-5 min)
o Mechanical failure : equivalent hole set to
e.g. 50 mm
o Small leakage (e.g. corrosion), smaller hole
with equivalent diameter of e.g. 20 mm
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Typical release scenarios per equipment type
failure (cont.) :
 Pressure vessel connected equipment
o Release from PSV
o Failure of connecting pipes (as for pipes
above)
 Pumps/compressors
o Release from PSV
o Leakage from seal (equivalent small hole
diameter set, e.g. 20 mm)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Typical release scenarios per equipment type
failure (cont.) :
 Atmospheric liquid fuel tanks
o Ignition in floating roof tank (tank fire)
o Ignition of constant roof tank (tank fire)
o Failure of tank with release to dike (bund) of
tank and subsequent fire in dike (dike fire)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Worst case scenarios
 Although low probability expected, indispensible
for Land Use Planning and Emergency Planning
 Worst case releases/scenarios to be provided
for the different sections of Plant (type of
activities) :
o Each Production Unit
o Tank-farm
o Movement facilities (road/rail tanker
stations, ports)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Worst case scenarios (cont.)
 Worst case releases/scenarios within sections :
o Catastrophic failure of vessel (process
vessel, tank, tanker) with maximum
inventory size
o Catastrophic failure of pipe : Full Bore
Rupture (FBR)/Guillotine Break) for pipes,
especially for movement facilities
(import/export pipelines, hoses/loading
arms)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Worst case scenarios (cont.)
 Worst case releases/scenarios within sections :
o For liquid fuels tanks, fire in :
Largest diameter tank
Dike with largest equivalent diameter
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Worst case scenarios (cont.)
 Worst case releases/scenarios must take into
account :
o Different operating conditions (P/T/phase)
e.g. :
For liquefied gases piping, worst case is
usually expected from liquid phase pipe
failure
For LPGs, worst case is usually expected
from pure propane compared to butane
(due to higher pressure)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Worst case scenarios (cont.)
 Worst case scenarios selection criteria (cont.) :
o Different operating conditions (P/T/phase)
e.g. (cont.) :
Smaller tank of pressurized ammonia
can produce more extended
consequences than larger refrigerated
ammonia tank
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Worst case scenarios (cont.)
 Worst case releases/scenarios must take into
account :
o Different substances, e.g. smaller tank of a
very toxic substance can produce more
extended consequence than a larger tank of
a toxic substance
o Proximity to site boundaries, especially if
vulnerable objects are close
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Worst case scenarios (cont.)
 Worst case scenarios usual convention : Only
one failure can happen at a certain time
o No simultaneous accidents expression,
e.g. only single tank BLEVE in LPG tank
farm at a time
o No double containment failure, e.g. in
refrigerated tanks with secondary
containment only primary containment
failure is taken into account, if no special
reasons are present
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Hazard identification usually specify release
expected and not final accident (top event)
• Example :
 Initial event (release) : failure of LPG pipeline
due to corrosion
 Top events:
o jet flame
o vapour cloud explosion
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Event Tree and Top Events
 Logic evolution of potential outcomes (top
event) of an initial event (release) identified
 Usually used in categorisation of final accidents
(top events) per initial release identified
 Scenario evolution parameters (e.g. ignition)
produce differences in top events
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Event tree and Top Events (cont.)
 Example: Gas phase release from LPG tank
PHASE
IGNITION
CONFINEMENT
DIRECT
GAS
DELAYED
NO IGNITION
This Project is funded
by the European Union
TOP EVENT
JET FLAME
NO CONFINEMENT
FLASH FIRE
CONFINEMENT
UVCE
SAFE DISPERSION
Project implemented by Human
Dynamics Consortium
• Why Event Trees?
 Consequence analysis requires top events to be
identified
 Technique in the borderline of hazard
identification and consequence analysis
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Consequence analysis framework
Hazard
Identification
Release
scenarios
Event
trees
Dispersion models
Consequence
results
Accident
type
Release models
Release
quantification
Fire, Explosion Models
Domino effects
Limits of
consequence analysis
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Main top event categories
Initial event
Hazardous
substance
release
This Project is funded
by the European Union
Top event
Consequences
Fire
Thermal
Radiation
Explosion
Overpressure
Toxic
dispersion
Toxic effects
Project implemented by Human
Dynamics Consortium
• Top events related with thermal radiation
 “Fire” categories:
Pool fire
FLEVE (fire ball)
Flash fire
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Pool fire
 Ignition of flammable liquid phase
Main consequence
Thermal radiation
Liquid fuel tank fire
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Jet flame
 Ignition of gas or two-phase release from
pressure vessel
Main consequence
Thermal radiation
Propane jet flame test
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Fireball, BLEVE (Boiling Liquid Expanding Vapour Explosion)
 Rapid release and ignition of a flammable
under pressure at temperature higher than its
normal boiling point
Main consequence
Thermal radiation
Secondary consequences:
oFragments (missiles)
oOverpressure
LPG BLEVE (Crescent City)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Fireball, BLEVE Mechanism (exposure of tank to
fire)
GAS PHASE
LOW HEAT TRANSFER,
LOW HEAT CAPACITY,
RAPID INCREASE OF SHELL TEMPERATURE,
POSSIBLE FAILURE
LIQUID PHASE
HIGH HEAT TRANSFER RATE,
HIGH HEAT CAPACITY
RATHER LOW SHELL TEMPERATURE
Shell at gas phase collapses due to weakening and in combination
to pressure increase. Massive release of tank content. Rapid
evaporation and ignition of the whole tank content
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion
 Passive (neutral) dispersion (Gauss) :
o Release of gas with density equal or higher
than air
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 “Positive” buoyant dispersion :
o Release of gas at elevated temperature (e.g.
flue gas at stack)
o Treated as special case of Gauss models
(plume rise)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Heavy gas dispersion, e.g. liquefied under
pressure gas releases
 Common characteristic of substances :
 Normal Boiling Point (BP) less than ambient
temperature and
 Pressure higher than ambient
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Typical example of heavy gas dispersion, LPGs :
o Propane BP = - 42 °C
o Butane BP = - 0.5 °C
and
o storage at ambient temperature (high
pressure), propane case : T = 17 °C, P = 6,7
barg
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Other examples of heavy gas cases :
o Ammonia
 BP= -33°C and
Storage at ambient temperature,
usually in bullets (high pressure : T =
15°C, P = 6,3 barg) or
Semi-refrigerated storage (T = 0 °C,
P = 3,2 barg), usually in spheres
o Propylene (BP = -47.7 °C) at semi
refrigerated storage (T = 6°C, P = 6 barg)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Heavy gas dispersion case - Released gas has
lower density than air
 Why heavy gas dispersion is different ?
o At release point, pressure reduction occurs
(from vessel pressure to atmospheric)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 General thermodynamics of heavy gas
dispersion
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Why heavy gas dispersion is different ? (cont).
o Gas phase release:
Lower pressure incurs lower temperature
of gas (adiabatic expansion,
Joule/Thomson effect). Colder gas has
density higher than surrounding air (fells
to ground).
Additional effect by entrainment of air in
expanding gas and condensation of
humidity
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Why heavy gas dispersion is different ? (cont).
o Liquid phase release:
Reduction of pressure causes evaporation of
liquid to gas
Evaporation causes lower temperature in
both gas and liquid (equal to normal boiling
point temperature, freezing effect)
Expanding liquid/gas entrains air who is
getting cold by boiling, condensing also
humidity
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Heavy gas dispersion : Vapour cloud remains
for long distance at ground level
Heavy gas behaviour
This Project is funded
by the European Union
Propane cloud
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Refrigerated gases not considered in general as
producing heavy gas dispersion :
o Storage at cryogenic conditions (close to
normal boiling point, atmospheric pressure)
o Examples:
Liquefied Natural Gas (LNG)
Cryogenic ammonia
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Refrigerated gases (cont.)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Effects:
 Toxic substances (e.g. HF) : toxic effect via
inhalation
 Flammables (Flash fire) : Ignition of cloud in
area with no confinement (obstacles)
o deaths expected within cloud limits where
ignition is possible (LFL-HFL), due to thermal
radiation and clothes ignition
o low flame front propagation velocity (as per
wind speed)
o insignificant overpressure
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour Cloud Explosion (VCE)
 Delayed ignition of flammable vapour cloud
under partial confinement (obstacles within
cloud) producing overpressure during flame
front propagation
Main consequence
Overpressure
VCE results (Flixborough)
This Project is funded
by the European Union
Secondary consequences:
oFragments (e.g. broken glasses)
Project implemented by Human
Dynamics Consortium
• Explosion
 Explosion : General term for vapour cloud
ignition event, in which turbulence (necessary
for combustion surface increase) leads to
significant flame front propagation velocity and
overpressure
o Turbulence sources:
Leaks (release with jet characteristics)
Obstacles :
Equipment, e.g. air-coolers
Walls, tanks, or other confinement in
space
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Explosion (cont.)
 Explosion feedback mechanism
Combustion
Expanding
flue gases
Turbulence
High combustion
rate
High combustion
products volume rate
Overpressure
High heat release rate
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Deflagration
 Rather fast combustion rate
 Molecular diffusion limitation
 Small ignition energy requirements (10-4 J for
hydrocarbons)
 Flame front propagation velocity : 5-30 m/sec
 Flash fire : deflagration with no flame front
acceleration
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Detonation
 High ignition energy (106 J)
 Compression in flame front exceeds
autoignition temperature
 Supersonic flame front propagation velocity.
 High overpressure produced
 Highly homogenous cloud required – not
feasible in real life
 Explosives case
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Literature for Accident Scenarios and Top Events
 Lees’ Loss Prevention in the Process Industries, Elsevier Butterworth
Heinemann, 3nd Edition, 2005
 Methods for the Determination of Possible Damage to People and
Objects Resulting from Releases of Hazardous Materials , Green Book,
CPR 16E, TNO, 1992
 Guidelines for Chemical Process Quantitative Risk Analysis, CCPSAICHE, 2000
 Guidelines for Consequence Analysis of Chemical Releases, CCPSAICHE, 1999
 Guidelines for Evaluating the Characteristics of Vapour Cloud
Explosions, Flash Fires and BLEVEs, CCPS-AICHE, 1994
 Guidelines for quantitative risk assessment, Purple Book, CPR 18E,
RVIM, 2005
 RIVM, Reference Manual Bevi Risk Assessments, 2009
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Literature for Accident Scenarios and Top Events
(cont.)
 HSE, Safety Report Assessment Guide : LPG
 Benchmark Exercise in Major Accident Hazard Analysis, JRC Ispra, 1991
 Methodology for Evaluation of Safety Reports, Cyprus Ministry of Labour
and Social Security, 2007 (in Greek)
 Assael M., Kakosimos K., Fires, Explosions, and Toxic Gas Dispersions,
CRC Press, 2010
 Crowl D., Louvar J., Chemical Process Safety Fundamentals with
Applications, Prentice Hall, 2nd Edition, 2002
 Taylor J., Risk Analysis for Process Plant, Pipelines and Transport, E&FN
SPON, 1994
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium