This project is funded by the European Union Projekat finansira Evropska Unija

1. This project is funded by the European Union Projekat finansira Evropska Unija ACCIDENT SCENARIOS AND CONSEQUENCE ANALYSIS Antony ThanosPh.D. Chem. Eng.antony.thanos@gmail.com Project implemented by Human Dynamics Consortium Projekat realizuje Human Dynamics Konzorcijum

2. Risk Analysis Framework Hazard Identification Accident Scenarios Consequence Analysis Accident Probability Risk reduction measures NO YES Accepted Risk Risk Assessment END

3. Hazard identification usually specify release expected and not final accident (top event) • Typical release scenarios per equipment type failure : • Pipes • Catastrophic failure (Full Bore Rupture –FBR- or guillotine break) • Partial failure (hole diameter equivalent to a fraction of pipe diameter, e.g. 20%)

4. Typical release scenarios per equipment type failure (cont.) : • Pressure vessel (process vessel, tank, tanker) • Catastrophic failure: “instantaneous” rupture (complete release of content within short time e.g. 3-5 min) • Mechanical failure : equivalent hole set to e.g. 50 mm • Small leakage (e.g. corrosion), smaller hole with equivalent diameter of e.g. 20 mm

5. Typical release scenarios per equipment type failure (cont.) : • Pressure vessel connected equipment • Release from PSV • Failure of connecting pipes (as for pipes above) • Pumps/compressors • Release from PSV • Leakage from seal (equivalent small hole diameter set, e.g. 20 mm)

6. Typical release scenarios per equipment type failure (cont.) : • Atmospheric liquid fuel tanks • Ignition in floating roof tank (tank fire) • Ignition of constant roof tank (tank fire) • Failure of tank with release to dike (bund) of tank and subsequent fire in dike (dike fire)

7. 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) : • Each Production Unit • Tank-farm • Movement facilities (road/rail tanker stations, ports)

8. Worst case scenarios (cont.) • Worst case releases/scenarios within sections : • Catastrophic failure of vessel (process vessel, tank, tanker) with maximum inventory size • Catastrophic failure of pipe :Full Bore Rupture (FBR)/Guillotine Break) for pipes, especially for movement facilities (import/export pipelines, hoses/loading arms)

9. Worst case scenarios (cont.) • Worst case releases/scenarios within sections : • For liquid fuels tanks, fire in : • Largest diameter tank • Dike with largest equivalent diameter

10. Worst case scenarios (cont.) • Worst case releases/scenarios must take into account : • 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)

11. Worst case scenarios (cont.) • Worst case scenarios selection criteria (cont.) : • Different operating conditions (P/T/phase) e.g. (cont.) : • Smaller tank of pressurized ammonia can produce more extended consequences than larger refrigerated ammonia tank

12. Worst case scenarios (cont.) • Worst case releases/scenarios must take into account : • Different substances, e.g. smaller tank of a very toxic substance can produce more extended consequence than a larger tank of a toxic substance • Proximity to site boundaries, especially if vulnerable objects are close

13. Worst case scenarios (cont.) • Worst case scenarios usual convention : Only one failure can happen at a certain time • No simultaneous accidents expression, e.g. only single tank BLEVE in LPG tank farm at a time • 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

14. Release rates models from vessels • Release of liquids (Bernoulli equation) • Release of gases (adiabatic expansion at hole) • Release of liquefied gases : • Gas phase release, as for usual gases • 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.

15. 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”

16. 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

17. Event tree (cont.) • Example: Gas phase release from LPG tank

18. Consequence analysis framework Release scenarios Accident type Hazard Identification Event trees Dispersion models Release models Consequence results Release quantification Fire, Explosion Models Domino effects Limits of consequence analysis

19. Main top event categories Initial event Top event Consequences Fire Fire Fire Thermal Radiation Thermal Radiation Thermal Radiation Hazardous substance release Explosion Overpressure Toxic dispersion Toxic dispersion Toxic effects Toxic effects

20. Pool fire • Ignition of flammable liquid phase Main consequence Thermal radiation Liquid fuel tank fire

21. 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

22. 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 : • Point source • No flame shape taken into account • Fraction of combustion energy considered to be transmitted by point in pool center

23. Pool diameter Flame height Pool depth • Pool fire models • Solid flame, radiation emitted via flame surface, calculation based on : flame shape, distance (View Factor), emissive power

24. 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: • Fragments (missiles) • Overpressure LPG BLEVE (Crescent City)

25. 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 200-350 kW/m2) • Radius and duration from correlations with tank content

26. 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

27. Jet flame • Ignition of gas or two-phase release from pressure vessel Main consequence Thermal radiation Propane jet flame test

28. 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

29. 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 Propane cloud

30. Vapour cloud (gas) dispersion (cont.) • Extent : dimensions, downwind/crosswind till specific endpoints (concentration) • Endpoints: • Flammables : LFL, ½ LFL • Deaths expected within cloud limits where ignition is possible (Flash fire) due to thermal radiation and clothes ignition • Toxics : several toxicity endpoints (e.g. IDLH, LC50)

31. Vapour cloud (gas) dispersion (cont.) • Affecting parameters: • Release conditions : substance properties, flowrate, hole diameter, pressure, temperature, release point height, release direction (upwards –PSV-, horizontal) • Meteorological conditions : atmospheric stability class (A-F), wind speed, temperature, humidity • Type of area : rural/industrial/urban, roughness factor

32. 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

33. 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 • Secondary consequences: • Fragments (e.g. broken glasses) VCE results (Flixborough)

34. Vapour Cloud Explosion (VCE) • Very short duration (sec) • Models (several assumptions used in every model) • 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

35. Vapour Cloud Explosion (VCE) (cont.) • Models (cont.) • 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

36. Vapour Cloud Explosion (VCE) (cont.) • Models (cont.) • 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

37. Impacts • Probit functions • 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

38. Impacts (cont.) • Thermal radiation • Impacts depend on both thermal radiation flux and exposure duration, e.g. • Thermal radiation flux 37,5 kW/m2 : • damage to equipment after 20 minutes • 100% lethality in 1 minute • 1% lethality for 10 seconds

39. Impacts (cont.) • Thermal radiation (cont.) • 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

40. Impacts (cont.) • Thermal radiation (cont.) • Probit constants available in literature for several levels of harm from thermal radiation • Endpoints for thermal radiation defined usually for effects (e.g. lethal effects, irreversible damage) to humans • Effects to structures usually useful only for Domino effects

41. Impacts (cont.) • Toxic effects • 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)

42. Effects (cont.) • Toxic effects (cont.) • Probit constants available in literature for several toxics • Toxic endpoints definitions must include exposure time, e.g. LC50 (30 min) • 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

43. Impacts (cont.) • Overpressure • Usual endpoints defined on constant values for expected effects to structures (light damage, severe damage etc.) • Effects to humans are present at similar or higher overpressures than for effects to structures

44. Effects (cont.) • Environment • No mature and wide-used quantitative models for estimation of effects to environment • Qualitative models applied some times • No unique approach in EU members in relevant requirements

45. 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

46. 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

47. Consequence/Risk acceptance in EU • Probabilistic approach • Limits usually set for individual risk • Strong dependency on quality of data • Differences in data from different sources (e.g. failure rates in UK and Netherlands, or for probit function of toxics) • Usually requires large set of scenarios • Specialized software required for efficient implementation

48. Consequence/Risk acceptance in EU (cont.) • Deterministic approach • Simpler to implementation • No probabilities of accidents used • Smaller set of scenarios required • More conservative • Worst case scenarios included • Safety Zones usually set in-line with Zones for emergency planning

49. Consequence/Risk acceptance in EU (cont.) • Hybrid approach • Probability band use • Results not so strongly related to probability value quality • Acceptance criteria defined by Risk Matrix • Closer to Rulebook approach

50. 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