ACCIDENT SCENARIOS AND TOP EVENTS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail

1. This project is funded by the European Union Projekat finansira Evropska Unija ACCIDENT SCENARIOS AND TOP EVENTSAntony 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. Scenarios selection • No unique approach within EU, as for rest of Risk Assessment Methodology • Nevertheless, worst case scenarios are almost always required for : • Emergency Response • Land Use Planning reason

4. 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)

5. Scenarios selection, UK case : (cont.) • Example of scenarios list in HSE SRAG for LPGs

6. 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

7. Scenarios selection, Netherlands case (cont.) • Main criteria : • Effects (1% lethality) extent out of fence (calculation of consequence required for worst case scenario for Containment) and/or

8. Scenarios selection, Netherlands case (cont.) • Main criteria : (cont.) • Estimation of effects based on selection method : • Indication number A (intrinsic hazard) • Q, quantity • O1, O1, O1 factors for • process conditions • G, limit value (10000 kg for flammables)

9. Scenarios selection, Netherlands case (cont.) • Selection method : (cont.) • Selectionnumber S: (hazard level at location -fence) n : 2 for toxics, 3 for flammables and explosives • 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

10. Scenarios selection, Netherlands case (cont.) • Sub-selection method overview

11. Scenarios selection, Netherlands case (cont.) • Step 2 : Definition of Releases • Releases to be included for selected Containment System based on tables per equipment type referring also frequency • Example for gas containers

12. Scenarios selection, Netherlands case (cont.) • Step 3 : Top events (scenarios) defined on event trees for releases • Event trees included for main cases in Manual • Releases general cut-off limit : • probability > 10-9 per year • 1% lethality distance extending outside fence

13. Scenarios selection, Cyprus case • Deterministic approach • Generic minimum list of scenarios • Catastrophic failure of vessels, tanks, pipes • Rupture of vessel/tank (hole with diameter equal to max pipe connected to tank/vessel), hole 20% of pipe diameter • Small leak in vessel tank, pipe (hole diameter 25mm or 50 mm) • Selection method for critical equipment (TNO Purple Book)

14. Scenarios selection, a few other EU Member States cases • Italy (Hybrid approach) • Decree for LPGs : Certain scenarios are excluded based on available measures • France (Hybrid approach) • High consequence scenarios must be included in consequence analysis, even for low probability

15. 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%)

16. 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

17. 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)

18. 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)

19. 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)

20. 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)

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

22. 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)

23. 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

24. 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

25. 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

26. 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: • jet flame • vapour cloud explosion

27. 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

28. Event tree and Top Events (cont.) • Example: Gas phase release from LPG tank

29. Why Event Trees? • Consequence analysis requires top events to be identified • Technique in the borderline of hazard identification and consequence analysis

30. 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

31. 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

32. Top events related with thermal radiation • “Fire” categories: Pool fire FLEVE (fire ball) Flash fire

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

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

35. 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)

36. 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

37. Vapour cloud (gas) dispersion • Passive (neutral) dispersion (Gauss) : • Release of gas with density equal or higher than air

38. Vapour cloud (gas) dispersion (cont.) • “Positive” buoyant dispersion : • Release of gas at elevated temperature (e.g. flue gas at stack) • Treated as special case of Gauss models (plume rise)

39. 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

40. Vapour cloud (gas) dispersion (cont.) • Typical example of heavy gas dispersion, LPGs : • Propane BP = - 42 °C • Butane BP = - 0.5 °C • and • storage at ambient temperature (high pressure),propane case : T = 17 °C, P = 6,7 barg

41. Vapour cloud (gas) dispersion (cont.) • Other examples of heavy gas cases : • 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 • Propylene (BP = -47.7 °C) at semi refrigerated storage (T = 6°C, P = 6 barg)

42. Vapour cloud (gas) dispersion (cont.) • Heavy gas dispersion case - Released gas has lower density than air • Why heavy gas dispersion is different ? • At release point, pressure reduction occurs (from vesselpressure to atmospheric)

43. Vapour cloud (gas) dispersion (cont.) • General thermodynamics of heavy gas dispersion

44. Vapour cloud (gas) dispersion (cont.) • Why heavy gas dispersion is different ? (cont). • 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

45. Vapour cloud (gas) dispersion (cont.) • Why heavy gas dispersion is different ? (cont). • 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

46. Vapour cloud (gas) dispersion (cont.) • Heavy gas dispersion : Vapour cloud remains for long distance at ground level Heavy gas behaviour Propane cloud

47. Vapour cloud (gas) dispersion (cont.) • Refrigerated gases not considered in general as producing heavy gas dispersion : • Storage at cryogenic conditions (close to normal boiling point, atmospheric pressure) • Examples: • Liquefied Natural Gas (LNG) • Cryogenic ammonia

48. Vapour cloud (gas) dispersion (cont.) • Refrigerated gases (cont.)

49. 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) • deaths expected within cloud limits where ignition is possible (LFL-HFL), due to thermal radiation and clothes ignition • low flame front propagation velocity (as per wind speed) • insignificant overpressure

50. 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)