This project is funded by the European Union
Projekat finansira Evropska Unija
ACCIDENT SCENARIOS
AND CONSEQUENCE
ANALYSIS
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
• Hazard identification usually specify release
expected and not final accident (top event)
• 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
• Release rates models from vessels
 Release of liquids (Bernoulli equation)
 Release of gases (adiabatic expansion at hole)
 Release of liquefied gases :
o Gas phase release, as for usual gases
o Liquid phase release, special two-phase
release models to be used, taking into
account equilibrium (or not) at release point
 Evaporation from pools : complex models,
taking into account : substrate type, substance
properties, atmospheric conditions etc.
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Hazard identification usually specify release
expected and not top event (final accident)
 Example : Release of LPG (gas phase) from
tank identified in a HAZOP. Various types of top
events can be evolved (Jet flame, flash fire,
UVCE)
• Consequence analysis requires top events to be
specified
• Gap closed by techniques such as “Event tree”
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Event tree
 Logic evolution of initial release identified, as
far as its outcome type (top event)
 Top events identified per initial release event
(e.g. jet flame after failure of pipeline due to
corrosion)
 Technique in the borderline of hazard
identification and consequence analysis
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Event tree (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
• 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
• 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
• Pool fire characteristics
 Confined (liquid fuels tank/dike fire) /
Unconfined (LPG pool from LPG tank failure –no
dike present)
 Pool dimensions (diameter, depth)
 Flame height, inclination
 Medium to low emissive power (thermal
radiation flux, up to 60 kW/m2 for liquid fuels)
 Long duration (hours to days)
 Combustion rate
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Pool fire models
 Combustion rate per pool surface based on
empirical equations (Burges, Mudan etc.)
 Flame dimension from empirical equations
(Thomas, Pritchard etc.)
 Radiation models :
o Point source
No flame shape taken into account
Fraction of combustion energy
considered to be transmitted by point in
pool center
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Pool fire models
 Solid flame, radiation emitted via flame
surface, calculation based on : flame shape,
distance (View Factor), emissive power
Pool diameter
Flame height
Pool depth
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
• BLEVE characteristics and models
 Fireball radius
 Duration (up to appr. 30 sec, even for very large
tanks)
 Very high emissive power (in the order or 200350 kW/m2)
 Radius and duration from correlations with tank
content
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• BLEVE characteristics and models (cont.)
 Solid flame radiation model, radiation emitted
via fireball surface, calculation based on :
sphere shape at contact with ground, distance
(View Factor), fireball emissive power
Evolution of BLEVE
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
• Jet flame characteristics and models
 Cone shape, dimensions from empirical
equations
 Long duration (minutes to hours, depends on
source isolation)
 Very high emissive power (in the order or 200
kW/m2)
 Combustion rate determined by release rate
 Solid flame model, radiation emitted via flame
surface, calculation based on : shape (cylinder),
distance (View Factor), emissive power
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion
 Neutral dispersion (stack type)
 Heavy gas dispersion, e.g. liquefied under
pressure gas releases as for LPG. 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.)
 Extent : dimensions, downwind/crosswind till
specific endpoints (concentration)
 Endpoints:
o Flammables : LFL, ½ LFL
Deaths expected within cloud limits where
ignition is possible (Flash fire) due to
thermal radiation and clothes ignition
o Toxics : several toxicity endpoints (e.g. IDLH,
LC50)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion (cont.)
 Affecting parameters:
o Release conditions : substance properties,
flowrate, hole diameter, pressure,
temperature, release point height, release
direction (upwards –PSV-, horizontal)
o Meteorological conditions : atmospheric
stability class (A-F), wind speed,
temperature, humidity
o Type of area : rural/industrial/urban,
roughness factor
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour cloud (gas) dispersion models
 Passive (neutral) dispersion : Gauss model
 Heavy gas dispersion : special complex models
 Flue gases : Gauss model modified for plume
rise effects
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
• Vapour Cloud Explosion (VCE)
 Very short duration (sec)
 Models (several assumptions used in every
model)
o TNT equivalency :
Simple, based on explosives effects
Fraction of combustion energy attributed
to overpressure development
High uncertainty in both fraction value
and assumed quantity of flammables to
be used
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour Cloud Explosion (VCE) (cont.)
 Models (cont.)
o TNO Multi-energy :
Only confined areas of cloud considered
Complex empirical rules for definition of
confined areas and blast strength
Overpressure from Berg graph using
Sachs distance
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Vapour Cloud Explosion (VCE) (cont.)
 Models (cont.)
o Baker-Strehlow-Tang
Similar principles as TNO Multi-Energy
model
Gas type reactivity taken also into
account along with obstacle density
Overpressure from graph using Sachs
distance
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Impacts
 Probit functions
o Relation of probability for a certain damage
level (e.g. 2rd degree burn, death) and cause
value (e.g. thermal dose value)
P = (Pr), Pr = A + B ln(D),
P : probability value
Pr : probit value
 : standard function of probability with probit value
A, B : probit constants for a specific harm
D : cause value
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Impacts (cont.)
 Thermal radiation
o Impacts depend on both thermal radiation
flux and exposure duration, e.g.
o Thermal radiation flux 37,5 kW/m2 :
damage to equipment after 20 minutes
100% lethality in 1 minute
1% lethality for 10 seconds
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Impacts (cont.)
 Thermal radiation (cont.)
o Best practice the use of Thermal Dose :
TDU = Q4/3 t
Q (W/m2), emissive power (thermal
radiation flux) at flame/fireball surface
t (sec), exposure time :
 BLEVE event : BLEVE duration
 other events : escape time, usually 0,5-1 minutes
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Impacts (cont.)
 Thermal radiation (cont.)
o Probit constants available in literature for
several levels of harm from thermal
radiation
o Endpoints for thermal radiation defined
usually for effects (e.g. lethal effects,
irreversible damage) to humans
o Effects to structures usually useful only for
Domino effects
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Impacts (cont.)
 Toxic effects
o Dose concept : Dose = Cn t
C, concentration
t, exposure time (in the order of 30-60
minutes)
n, exponent depending on substance
(available on literature for several toxics,
usually 1-2)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Effects (cont.)
 Toxic effects (cont.)
o Probit constants available in literature for
several toxics
o Toxic endpoints definitions must include
exposure time, e.g. LC50 (30 min)
o Literature toxicity data must be adjusted to
humans and for the required exposure time,
e.g. literature data for LC1 (2 hours) on rats
must be adjusted to LC50 (30 min) for
humans
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Impacts (cont.)
 Overpressure
o Usual endpoints defined on constant values
for expected effects to structures (light
damage, severe damage etc.)
o Effects to humans are present at similar or
higher overpressures than for effects to
structures
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Effects (cont.)
 Environment
o No mature and wide-used quantitative
models for estimation of effects to
environment
o Qualitative models applied some times
o No unique approach in EU members in
relevant requirements
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Risk : The probability of cause of harm from
accident
 The probability of dead from fall of lightning is
10-7 per year (1 person per 10.000.000
persons will die from lightning per year)
• Individual Risk : Risk of harm from accident, at
specific location, independent of affected subjects
 Example : The risk of lethal effects from thermal
radiation at distance of 100 m from a specific
gasoline tank is 10-6 per year from fire in the
gasoline tank
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Societal Risk :
 Relationship between frequency and the
number of people suffering from a specified
level of harm in a given population from the
realisation of specified accidents
 Concerns estimation of the chances of more
than one individual being harmed
simultaneously by an incident
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Consequence/Risk acceptance in EU
 Probabilistic approach
o Limits usually set for individual risk
o Strong dependency on quality of data
o Differences in data from different sources
(e.g. failure rates in UK and Netherlands, or
for probit function of toxics)
o Usually requires large set of scenarios
o Specialized software required for efficient
implementation
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Consequence/Risk acceptance in EU (cont.)
 Deterministic approach
o Simpler to implementation
o No probabilities of accidents used
o Smaller set of scenarios required
o More conservative
o Worst case scenarios included
o Safety Zones usually set in-line with Zones
for emergency planning
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Consequence/Risk acceptance in EU (cont.)
 Hybrid approach
o Probability band use
o Results not so strongly related to probability
value quality
o Acceptance criteria defined by Risk Matrix
o Closer to Rulebook approach
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Consequence/Risk acceptance in EU (cont.)
 No unique methodology in determination of risk
values
 No unique approach in perception of risk (only
vulnerable objects taken into account in
Netherlands)
 Diversity in limit values for same approach
 Not always unique approaches for permitting,
Land Use Planning and Emergency Planning
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Literature for Accident Scenarios and Consequence
Analysis
 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
 Methods for the Calculation of Physical Effects due to Releases of
Hazardous Materials (Liquids and Gases), Yellow Book, CPR 14E,
VROM, 2005
 Guidelines for Quantitative Risk Assessment, Purple Book, CPR 18E,
VROM, 2005
 Methods for Determining and Processing Probabilities, Red Book,
CPR12E, VROM, 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 Consequence
Analysis (cont.)
 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 Process Equipment Reliability Data with Data Tables,
CCPS-AICHE, 1989
 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
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Literature for Accident Scenarios and Consequence
Analysis (cont.)
 Taylor J., Risk Analysis for Process Plant, Pipelines and Transport, E&FN
SPON, 1994
 Drysdale D., Fire Dynamics, J. Wiley and Sons, 2 nd Edition, 1999
 Beychok M., Fundamentals of Stack Gas Dispersion, 3 rd Edition, 1994
 Yaws C., Handbook of Chemical Compound Data for Process Safety,
Elsevier Science & Technology Books, 1997
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium