Accident investigations. Kees van Wingerden Gexcon

Transcription

1 Accident investigations Kees van Wingerden Gexcon

2 Contents Introduction Buncefield explosion Sløvåg explosion Other examples 2

3 Introduction Gexcon has been involved in accident investigations numerous times and used FLACS in many of these FLACS describes the processes of an explosion as they occur. The same accounts for release and dispersion processes Hence, FLACS can be a welcome additional element in accident investigations to understand the course of events and by that determining the cause of the accident Examples are given to underline the beauty of using FLACS in accident investigations 3

4 Buncefield Explosion Series of explosions and followed by large fire on large fuel storage site (Buncefield depot, Hemel Hempstead, Hertfordshire, UK) on 11 December 2005, Destroyed large parts of the depot and caused widespread damage to homes and businesses surrounding the site, Overfilling of fuel tank resulted in large cloud of petrol vapour and air, Cloud covered approximately m 2, Most likely ignition source in emergency pump house Very strong explosion (estimate of pressure in cloud > 2 bar locally) Nobody was killed, but 43 people suffered minor injuries Far field damage suggests 5 kpa at m from site Damage (windows broken) up to 8 km from site 4

5 Plan view of Buncefield showing assumed cloud extent 5

6 Photo of Buncefield site and Fuji building before accident 6

7 Buncefield: Explosion damage to Fuji Building 7

8 Buncefield: Explosion damage to north side of Northgate building 8

9 Emergency pump house damage 9

10 GexCon s involvement in investigation Buncefield explosion 11 December 2005 January 2006 official appointment Buncefield Major Incident Investigation Board overviewing investigation by HSE and EA May 2006: Buncefield Investigation, Third Progress Report: The magnitude of the overpressures generated. is not consistent with current understanding of vapour cloud explosions... The investigation has, so far, been unable to establish why the ignition of the vapour cloud and the explosion propagation. caused significant overpressures that produced the severe damage to property June 2006, Plant owner approached GexCon to support them in the investigation 29 June 2006: site survey by GexCon showing that congestion of tank farm indeed looked too limited to cause overpressure levels needed for causing the damage seen. 10

11 GexCon s involvement in investigation At the same time the survey showed that the road sides along the outside of the perimeter of the tank farm were densely planted with coppiced trees and undergrowth The congestion (metres of branches per cubic metre of volume) was at least as high as that of pipework in a densely congested process area A detailed FLACS model of the tank farm and its immediate environment was constructed, including congestion by trees The simulations with tree congestion included show explosion overpressures are in order of magnitude required to explain the observed damage in the car parks and buildings to the west May 2007: presentation of results to HSE 11

12 Picture of trees in Buncefield Lane 12

13 FLACS model of Buncefield 13

14 Normal, coppiced with undergrowth and enhanced density tree 14

15 FLACS simulations: scenario parameters Horizontal cloud extent was based on HFRS estimate which showed that the cloud extended far outside the site perimeter (based on CCTV footage) The cloud is taken as a homogeneous, stoichiometric mixture of butane and air which in experiments has been found to represent reasonably well heavy hydrocarbon fuels Ignition was generally assumed to take place in the pump house, but scenarios where ignition was effected in the emergency generator room were also investigated 15

16 FLACS simulations: results Simulations show that the flame burns in all directions but particularly along Cherry Tree Lane and Buncefield Lane where it acquires significantly higher velocities, generating higher overpressures than elsewhere The simulations show explosion overpressures in order of magnitude required to explain the observed damage in the car parks and buildings to the west with tree congestion included Simulations without trees gave very low overpressures This supports the main conclusion that trees can provide congestion sufficient to accelerate a flame burning through a large cloud to high speeds, thus giving rise to significant overpressures GexCon findings confirmed during FABIG meeting

17 Video no trees 17

18 Video with trees 18

19 Overpressure with and without trees, for 7 m and 5 m cloud heights Max Fuji Northgate Trees Trees No trees No trees 7 m 5 m 7 m 5 m 19

20 Experimental investigation effect of vegetation Comprehensive data sets of experimental investigations into effects of congestion (obstructions and confinement) on flame propagation exist The influence of vegetation on the course of flame propagation in clouds encompassing vegetation has however never been addressed experimentally A preliminary experimental investigation was performed in a 20 m long semi-circular tent (diameter 3.2 m) supported by arch profiles Experiments concerned effect of single and double rows of small and larger bushes. Blockage ratio 3-4% and 20-40% respectively Gas mixture: % v/v propane-air Ignition was a high voltage spark effected on one end of tent 20

21 No vegetation: reference test 21

22 Effect of vegetation: bushes blocking 40 % of cross section 22

23 Conclusion The strength of the Buncefield vapour cloud explosion was most likely due to strong flame accelerations along the lanes surrounding the site which were congested by trees and undergrowth Strong indications of DDT and detonation are also confirmed by FLACS simulations 23

24 Contents Buncefield explosion Sløvåg explosion Other examples 24

25 Tank yard II: used for purification of gasoline 25

26 Process description Purification of coker gasoline Gasoline is transported to tank yard by ship and pumped into tanks in tank yard Extraction of sulphur containing compounds (mercaptanes) Mixing with sodium hydroxide-water solution (30 % w/w) during pumping Sulphur containing compounds solve in NaOH-water solution Separation by gravity Pumping back to ship 26

27 Accident Purification process results in waste (NaOH solution with sulphur containing compounds) Cleaning process based on mixing with hydrochloric acid Process tested on small scale (20 l): Generation of vapor and development of thin layer of flammable liquid on top First tank to be cleaned: tank T3 (4000 m 3 ) containing ca. 50 m 3 of waste First step: addition of 205 m 3 of contaminated water 27

28 Accident 23 May 2007 start of adding hydrochloric acid (30-36 % w/w) (approximately 1 m 3 per hour) 24 May 2007 approximately 10:00 hours: explosion followed by fire in tank T3 Shortly later Tank T4 (neighboring tank) explodes as well followed by fire No fatalities, some lighter injuries, all tanks in tank yard II, 2 trucks in neighborhood as well office building (wooden barracks) are destroyed People in direct surroundings of accident location suffer from release of toxic/poisonous gases 28

29 Accident 29

30 Accident 30

31 Accident 31

32 Tank T3 32

33 Investigation: gas cloud generation Due to adding of hydrochloric acid reduction of phvalue resulting in generation of mercaptanes (and perhaps hydrogen sulfide) Estimate: total amount of mercaptanes in tank T3 at start of cleaning process: approximately kg Release rate: estimated from laboratory tests to be approximately 1 kg/s from total volume of waste in tank 33

34 Investigation: gas cloud generation Tank is gradually filled with pure gas with explosive mixture on top (concentration gradient) gradually from the bottom and up (due to density of mercaptanes) There is sufficient fuel in the tank to allow for this 1.E+00 Molfraksjon brensel 1.E-01 1.E-02 1.E-03 P1 P20 P30 P40 P50 P60 P70 1.E-04 1.E Tid (s) 34

35 Investigation: gas cloud generation Outside of tank: considering ignition sources around tank 35

36 Investigation: gas cloud generation Effect of release rate: 1 kg/s and 9 kg/s (pure gas) (6 m/s wind speed) 36

37 Investigation: gas cloud generation Effect of wind speed: 6 m/s and 3 m/s (1 kg/s release rate) 37

38 Investigation ignition source: self-ignition in active coal filter Adsorption of organic hydrocarbon vapours by active coal filter results in heat release, especially for vapours such as ketones, aldehydes and mercaptanes At high adsorption rates the heat release can be that high that selfignition in the coal may develop 38

39 Self-ignition in active coal filter 39

40 40

41 Conclusion The investigation performed showed that the explosion most likely was caused by the following chain of events The adding of hydrochloric acid to the waste in tank T3 led to the release of mercaptanes from the waste due to a reduction of the ph-value of the mixture The tank was gradually filled with a layer of mercaptanes, with a flammable region on top In the active coal filter a self-ignition occurs due to heat release caused by adsorption processes causing hot surfaces/ burning material When the flammable part of the cloud in the tank reaches the hot surfaces/ burning material ignition is effected 41

42 Contents Buncefield explosion Sløvåg explosion Other examples 42

43 Other examples Danvers, USA: Ink and paint manufacturing facility TWA-800: Explosion in central wing tank Boeing 747 Abu Dhabi, UAE: Oil tank explosion ADCO West, USA: Ammonium nitrate explosion Charleston, USA: UBB mine explosion Deep Water Horizon: explosion drilling rig Norway: several non-ignited releases offshore 43

44 Thanks for your attention!