The material in this paper comprise a compendium of more detailed information and analyses of the 14 large-scale explosions described the paper Mechanisms and Occurrence of Detonations in Vapor Cloud Explosions by Oran, Chamberlain, and Pekalski [1]. Each section begins with a general description of the incident and then, where possible, more analysis (based on the work of GC) and useful information about the incident. The events all have certain physical similarities, but vary greatly in terms of the level of information available for analysis. Taken together, however, they present a powerful picture of what a detonation can do as well as indications of what might be done in the future to avoid such an incident. 1 S1. Introduction These additional materials are a compendium of more detailed information and analyses of the 14 large-scale Vapor Cloud Explosions (VCEs) described in [1]. The speci c explosions and the conclusions concerning them are: 1. Bunce eld, UK, 2005. Detonation occurred. 2. Jaipur, india, 2009 Detonation occurred. 3. CAPECO, Puerto Rico, 2009. Detonation probable . 4. Amuay, Venezuela, 2012. Detonation occurred. 5. Skikda, Algeria, 2004. Detonation occurred. 6. Brenham, TX, US, 1992. Might have occurred. 7. Ufa, Russia, 1989. Detonation occurred. 8. Port Hudson, LA, US, 1970. Detonation occurred. 9. Newark, NJ, US, 1983. Detonation might have occurred. 10. Flixborough, UK, 1974. Detonation occurred. 11. Pasadena, CA, US, 1989. Detonation occurred. 12. Decatur, IL, US, 1975. Detonation most likely occurred, evidence lacking. 13. Beek, Netherlands, 1975. Detonation might have occurred. 14. Donnellson, IO, US, 1979. Detonation did not occur. No mechanism for ame acceleration. Each section begins with a general description of the incident and then, where possible, more analysis (based on the work of GC) and useful information about the incident. The events all have certain physical similarities, but vary greatly in terms of the level of information available for analysis. Taken together, however, they present a powerful picture of what a detonation can do as well as indications of what might be done in the future to avoid such an incident. More detailed conclusions from assessing all of the information are given in the discussion in [1]. The levels of description in the materials presented here vary considerably. There is minimal description for incidents such as Bunce eld, UK and Jaipur, India, where there has been extensive recent analysis that can be summarized and referenced, and for which relatively little detail exists, such as Ufa, Russia. On the other hand, the CAPECO incident is discussed in considerable detail because of a lack of a prior report and evidence gathered and analysis carried out by one of the authors of this paper (GAC). 2 1. Bunce eld Accident A full description of this incident and follow up tests are available in [2] of the main text. It is generally accepted that ignition occurred in the pump house when the electric powered pumps were turned on. A vented explosion then occurred which ignited the external cloud. Flame acceleration along the tree line to the north of the site then led to a DDT in the region of the crossroads at Bunce eld Lane and Three Cherry Trees Road. There is considerable evidence from subsequent tests that a DDT occurred and that the detonation propagated through the remaining cloud. The layout of the site and location of some damaged objects are shown in Figure-S 1.1 and Figure- S 1.2. Figure-S 1.3 shows the direction of net drag on objects inside and just outside the detonation. The deformation exhibited by such objects provided information on the net drag impulse sustained by these objects. Other directional evidence was provided by the directional abrasion found on one side or one part of painted surfaces and tree bark. There was also evidence of large objects such as cars and skips being displaced in a particular direction. Figure-S 1.1Location of vehicles, drums and other enclosures which exhibited damage consistent with overpressures in excess of 200 kPa [2, 3]. For images of selected objects identi ed by numerical tags, see Figure-S 1.2. 3 Figure-S 1.2Objects identi ed by numerical tags Figure-S 1.1[2, 3]. Figure-S 1.3Direction of net drag impulse across the Bunce eld site [2, 3]. 4 2. Jaipur Accident Description of the Event During the evening of the 29th October 2009, preparations were being made at the Indian Oil Corporation (IOC) depot for the transfer of kerosene and motor spirit (MS)1to a neighboring terminal operated by Bharat Petroleum Corporation Limited. At approximately 6:10 pm, during the process of preparing the MS tank for pumping, a large leak occurred from a hammer blind valve on the tank outlet.2The leak resulted in a jet of MS directed upwards from the valve. The leak continued for 75 minutes in the background calm, low-speed wind. The nature of the release, with an upwards jet of MS, is likely to have assisted in the production of vapor. Post-incident analysis indicates that a ammable vapor cloud covered much of the IOC site. The Independent Investigation Committee (IIC) estimated that of the order of 1000 MT of MS were released from the tank. The vapor cloud was not visible, unlike the vapor cloud at Bunce eld, where the vapor appeared as a by mist in the low-temperature weather conditions. Personnel on site were aware of the presence of the vapor by its odor. A full and concise description of this incident are available in [4, 5] and in Section 5.2. Discussion The directional indicators and damaged items in this incident are all consistent with a detonation of the vapor cloud over a large portion of the site. The northeast corner of the site, however, shows no evidence of high overpressure and only a random set of directional indicators, as indicated by the red arrows in Figure-S 2.1. The area that shows evidence of the ammable cloud, but no signi cant pressure generation, has been marked in light yellow. Approximate directional indicators interpreted from pictures and video are shown as dark yellow arrows. Similarly, approximate directional indicators corresponding to two boxes that appear to have been exposed to a directional pressure loading are shown as blue arrows. Also shown are the positions of the tree to the north of the control room that does not appear to have su ered severe pressure or ow damage. Figure 4.8a,b in the main report show damaged tanks, and Figure-S 2.2 shows various kinds of damage caused by overpressure. Figure 4.10 (right) in the main report shows directional indicators, and others are shown in Figure-S 2.3. These gures are taken from the detailed paper by [4] and in the additional technical report [5]. 1Motor spirit volatile liquid used as a fuel in internal-combustion engines. It is usually gasoline or petrol. 2The valve at Jaipur had two positions, one providing full ow through the tank outlet pipe and the other completely blocking the ow. It was an `on/o ' valve that was manually moved and had no other control over the liquid rate. 5 Figure-S 2.1Jaipur directional indicators in the Pipeline Division area and area without apparent pressure generation. Figure taken from [4]. The partial transparency through the yellow area shows the green tank, and the wall that is not damaged is the northern edge of the blue area. The main rewater pump house is just south of the rewater ponds. The main evidence for the lack of pressure generation in this yellow shaded area is: { The lack of damage to the wall. It remained standing in this area, but collapsed further to the west on the north wall and to the south on the east wall. { The limited damage to the trees to the east of the main rewater pump house. { The lack of scouring on the lamppost by a green rewater tank on the western side of the shaded area (not visible due to the shading). { The lack of signi cant pressure damage in the area of the green rewater tank. { The blue arrow marked in the shaded area is consistent with a pressure wave traveling into the area from an explosion event to the south of the shaded area. It is postulated here that a ammable cloud was present in the shaded area at the time of the explosion, as the trees along the section of the north wall to the east of the green rewater tank were all re damaged. In addition, there was no particular change in the ground level in the vicinity of the green rewater tank that would have prevented the vapor cloud from spreading into this area. Nonetheless, it is possible that the re damage occurred as a result of re propagating along the tree line after the explosion. In this scenario, it is possible that the vapor cloud in this area was below the lower ammable limit, which thus explains the absence of pressure damage. 6 Another mechanism that would prevent pressure generation in this area is if it had been burned prior to the main vapor cloud explosion. Alternatively, it is possible that the vapor cloud was so thin in this area that it could not sustain a detonation. This hypothesis, however, is inconsistent with the ground level in this area and the persistence of the detonation throughout the rest of the site. The most likely explanation is that the vapor cloud burned out before the main explosion. This also suggests that the ignition point was located in this area. Figure-S 2.2Examples of pressure damage on the IOC site [4]. Figure-S 2.3Examples of directional indicators: bent trees and scoured poles [4]. 7 3. CAPECO Accident Description of the Event On the night of 23 October, 2009, a large explosion occurred at the Caribbean Petroleum Corpora- tion (CAPECO) facility in Bayam on, Puerto Rico, as gasoline was being ooaded from the tanker ship, Cape Bruny, to the onshore CAPECO tank farm. A 5-million gallon ( 19,000 m3) above- ground storage tank over owed into a secondary containment dike. The gasoline spray evaporated, forming a large vapor cloud, which ignited after reaching an ignition source in the wastewater treat- ment (WWT) area of the facility. The blast and re from multiple secondary explosions resulted in signi cant damage to 17 of the 48 petroleum storage tanks and other equipment onsite, as well as to neighborhoods and businesses o site. The res burned for almost 60 hours. What follows now is a summary of the forensic study carried out by one of the authors (GAC) to determine the sequence of events that led to the VCE. Discussion One of the authors (GAC) visited the site in March of 2010 at the invitation of the Chemical Safety Board (CSB). Most of the items damaged in the explosion were still in place, but the barren area of vegetation at the east end of the site had begun to grow back. Many photographs of the site were taken after the explosion, and the videos of the actual incident taken from xed CCTV cameras were supplied to GAC by the CSB. The locations of the cameras are shown in Figure- S 3.1. Selected frames from the videos are used in the analysis given below. A selection of frames pertinent to the discussion is shown here in Figure-S 3.11 to Figure-S 3.15. The video frames showed that the ame traveled through the site as a ash re with occasional small explosions in con ned and congested regions. Severe explosions occurred towards the far eastern end of the site in dense vegetation (the \bamboo area") and between tanks 404 and 405 at the western end of the site at the time when most of the fuel had been consumed (see Figure- S 3.2). This overall sequence of events is in some ways opposite to the sequence at Bunce eld, where the main DDT explosion occurred early in the sequence of events, and a detonation was able to propagate through the rest of the cloud. The three main videos were taken from the top of the tower of the uidized catalytic cracker (FCC), the waste water treatment plant (WWT), and the gatehouse (see Figure-S 3.2). The timers on each video were all synchronized to the same time base, thereby allowing events to be cross correlated. The time between ignition and the main explosion was about 7 s, during which there were several explosions and ash res. Nearly all of the blast e ects can be explained by conventional understanding of ame acceleration. There are examples of ame acceleration through repeated LPG tank obstacles (see Figure-S 3.3), ame acceleration down a 240-m-long buried water duct, an external explosion at the far end (see Figure-S 3.4), and ame acceleration in highly congested regions of dense vegetation (see Figure-S 3.5). 8 Figure-S 3.1Location of CCTV cameras [6]. Figure-S 3.2Overview of site showing burn area [6]. 9 Figure-S 3.3View from the waste water treatment CCTV camera (also called the NW camera). Note crushed white tank [6]. Figure-S 3.4Damage to road above water culvert [6]. 10 Figure-S 3.5The damage to the area that once contained dense vegetation is shown inside the superimposed black oval ring in the middle of the left side of the photograph [6]. There are several anomalies. One is the increase in brightness of an area on the left viewed from the WWT (see Figure-S 3.3). This was at the same time as the rst main ash from the right. This brightness could be a re ection of light from trees in the eld of view. A more likely explanation is a street lamp (Figure-S 3.3) that was recorded when the camera was triggered by the light of the ignition. The second anomaly concerns the damage to tanks 406 and 407 (Figure-S 3.6). These tanks did not catch re, and their damage is typical of a blast impact. Yet they are, respectively, 300 m and 350 m from the location of the explosion in the bamboo area at the east end of the site. Equation (4{2) (from Section 4 of main report) gives 0.08 atm overpressure at tank 407 and 0.07 atm at tank 406. CAM [7] gives approximately the same values, using a highly congested region of 100 m x 40 m. These overpressures are not sucient to cause the level of damage sustained. The situation is further complicated by the fact that another tank containing fuel oil, number 603, was only about 60 m from the edge of the bamboo explosion, and yet it sustained only super cial damage (Figure-S 3.7). One possibility here is that tanks 406 and 407 were hit to their southwest position by a blast from fast ames propagating through the line of 11 empty LPG bullets located to their southwest. If the ame speed through this region were 180 m/s, as observed from the northwest video, then an overpressure of around 0.6 atm would be generated. CAM then predicts overpressures of 0.15 11 Figure-S 3.6Blast damage to empty tanks 406 and 407. Figure Figure-S 3.3 was taken from left side of this gure [6]. Figure-S 3.7Tank 607 is the smaller tank near the center of the picture [6]. 12 and 0.09 atm at tanks 406 and 407, respectively. which is insucient to account for the damage. Additionally, the blast at the gatehouse, 450 m away, is then 0.026 atm, but nothing appears on the gatehouse video at the elapsed time of 23.06 frame 7. The questions, then are: Why is there no evidence in the video? How do we explain the level of damage? The third anomaly concerns a slightly damaged switch box (Figure-S 3.8) located 100 m from the bamboo area. Tests following the Bunce eld explosion have shown that the incident overpressure must be greater than 1 atm to cause distortion of the box. Yet, assuming a detonation in the bamboo area, the overpressure estimated from Equation (4{2) is only 0.16 atm. Nonetheless, it is still possible that a DDT occurred in the bamboo area. The region is devastated and some items have been drawn inwards, e.g., tree trunks, railings, wire netting, and roadside barriers (see, e.g., Figure-S 3.8). The overpressure at the gatehouse, some 720 m distant, is estimated to be 0.03 atm, which is sucient to shatter large windows as observed. Figure-S 3.8Damaged switch box, front face (left gure) and side view (right gure) [6]. Another feature that supports the hypothesis of DDT is an analysis of bent railings 60 m from the bamboo area (Figure-S 3.10). The minimum overpressure required to produce the measured distortion of 483 mm is about 0.6 atm (static). For a 1-ms pulse, the required overpressure is about 10 atm. The overpressure estimated from Equation (4{2) (Section 4 of main report) is 0.4 atm. The lack of agreement between the two values indicates that the railing was much closer to, or even on the edge of, a detonating cloud. Following a meeting at the UK Health and Safety Laboratory (HSL) between author GAC and G. Atkinson, on 3 February 2016, it was decided that further analyses were necessary to try to resolve at least some of the anomalies. It was agreed that the rst blast to reach the gatehouse at 23.03 frame 21 originated from the south end of the culvert (at 23.02 frame 17) shown in Figure-S 3.2. Figure-S 3.11 shows the explosion. It was also agreed that a major explosion also took place in the bamboo area, probably around the same time as the other explosion identi ed in frames 23.07.21 to 23.08.08 (Figure-S 3.15). 13 Figure-S 3.9Trees, railings and posts drawn inwards towards bamboo area [6]. Figure-S 3.10 Pressure-impulse diagram for bent railings. Left: Pulse durations are shown as- suming a triangular shaped pulse. (The pressure-impulse calculation was performed by B. Burgan of the Steel Construction Institute, private communication, 2016). Right: From [6] showing bent railing. 14 Figure-S 3.11 Explosion from the south end of the culvert along 4th Street [6, 8]. Figure-S 3.12 Flash re has begun to propagate from the culvert, and also into both LPG bullet areas at 23.04 frame 25 [6, 8]. Figure-S 3.13 First light at east end of site, 23.06 frame 24 [8, 6]. 15 Figure-S 3.14 First frame of a major explosion at 23.07 frame 21 referenced to site photo (lower). The frames on the left are from FCC camera, and the frames on the right are from the WTT camera [8, 6]. 16 Figure-S 3.15 Progression of the main explosion event [8, 6]. 17 A ttention turned to the severe explosion in the region of tanks 404, 405, 406 and 407. Triangulation from frames taken from FCC and northwest videos (Figure-S 3.16) indicate that the explosion originated in a region of congested pipework between tanks 404 and 405 (Figure-S 3.17). A blast in this area could account for damage to tanks 406 and 407 and to the fence line and trees to the north of the site (see [6]). Further evidence supporting a major explosion in this area came from the arrival of blast at the FCC camera and at the gatehouse. The FCC camera lost its imaging 1.24 s after the video image of the explosion, indicating a distance of 430 m from the blast origin, assuming a sound speed of 350 m/s. This is the exact distance between FCC camera and congested pipework. The correspondence of distance between the gatehouse and pipework, 560 m, compared with 630 m derived from glass breakage and sound speed, is not as close, but perhaps still within reasonable error. The question, then, is: Was there enough congestion in this pipework area to create such a major explosion? At rst, it appears that there was not. The author (GAC) then asked the CSB to send diagrams of the drainage systems around the site. It appears that there are two 4-inch drains and a clean-out pipe to the sides and underneath the pipework in question (Figure-S 3.18 and Figure- S 3.19). We know that ammable vapors entered the drain system, and it likely that vapors entered others, also, as manhole 13 showed signs of blast. This suggests that high-velocity jet ames from the two drains or the clean out pipe induced a detonation in the vapor cloud surrounding the pipework. Then this detonation propagated through the remaining cloud. There could also have been a detonation in the drain system itself, and then this detonation could have exited into the existing vapor cloud. The distance from manhole 13 to the 4-inch open drains is approximately 44 m, consisting of a 6-inch pipe for about 40 m then branching into the three 4-inch pipes of about 3 m, 8 m and 2 m. There is most likely enough distance and pipe interconnections, leading to more shocks, for DDT to occur in this con guration. Another possibility is that a detonation propagated all the way to the bamboo area from the tank area, as opposed to a DDT that starts in the bamboo area after the remaining fuel had burnt out. The second case, in which DDT occurred in the bamboo area after the remaining fuel burned out, would have created the positive pulse to cause the directional indicators in Figure-S 3.20. 18 Figure-S 3.16 Triangulation of the explosion shown in frames 23.07.21 to 23.08.8 [6]. . Figure-S 3.17 Congested pipework in the region between tanks 404 and 405 (triangulated area) (left) and crushed steel box (right) 70 m to the east of the triangulated region [6]. . Note uniformly crashed tank. 19 Figure-S 3.18 Drain system in the region of tanks 405 and 406. Note the two 4-inch drains between the tanks, and the clean out. It would seem that the clean out pipe exits underneath the gasoline pipework [8]. Figure-S 3.19 Close up of clean-out section of drain system [8].20 Figure-S 3.20 Directional indicators on the eastern side of the site [8, 6]. Thus the current hypothesis is that a detonation of the vapor cloud occurred by ignition from a high-velocity jet ame emerging from the drain system between tanks 404 and 405. The vapor cloud in this region was close to the original leaking tank and occupied a large volume, possibly 10-m high. The detonation propagated through the vapors that remained after a partial burnout in the northern part of the site, and it then continued into the bamboo area at the east end. 21 4.Am uay Accident Description of the Event An operational decision was made to continue using a leaking ole n pump and to rely on the wind to disperse the vapors. The leak started days before the explosion, and the wind conditions reinforced the decision. At about midnight on the night of the incident, however, the wind slowed, became NW to SE, and vapors started to accumulate in the re nery. The ow rate of the leak also increased. The leak rate has been reported in the PDVSA report [9] as 8 m3/min. The cloud drifted around the low points surrounding the tank and traveled at least 600 m towards the SE, engul ng the lube oil plant and houses in the town to the east. It is thought that a car traveling down the Avenida Bolivar Road, to the east of the re nery, ignited the cloud (Figure-S 4.1). Figure-S 4.1Rough extent of the ammable cloud. Note location of car reported to have ignited the cloud and location of pedestrian overpass (footbridge) [9]. Discussion The damage pattern is confusing. The car that is supposed to have ignited the cloud shows damage signs equivalent to at least 5 atm of overpressure (Figure-S 4.2). The fence posts near this car point towards the re nery and away from the car, yet posts further along point in the opposite direction, and the car itself is pushed away from the re nery. 22 Figure-S 4.2Fence poles bent inwards towards re nery. Car close to, or responsible for, point of ignition. (Figure taken from [10].) The ole n storage spheres to the north, close to the leaking pump and storage tanks further to the north, were largely undamaged. Yet oil storage tanks to the south were severely damaged. The lube oil area was totally destroyed, and there was signi cant damage to buildings just over the road from the lube oil plant (Figure-S 4.3). Figure-S 4.4, Figure-S 4.5, and Figure-S 4.6 show various crushed storage tanks. Figure-S 4.3Houses adjacent to road running east of the lube oil area. Notice displacements of reinforced concrete poles. (Figure taken from [10].) 23 Figure-S 4.4Crushed storage tank. (Appeared on website of Christian Science Monitor , 16 September 2013. ) Figure-S 4.5Crushed storage tanks. (Appeared on website of Christian Science Monitor, 16 September 2013. ) Figure-S 4.6Crushed storage tanks. (Appeared on website of Christian Science Monitor, 16 September 2013. ) The footbridge (Figure-S 4.7 and Figure-S 4.8) was damaged, and it appears that the supports and railings were drawn towards the lube oil area. 24 Figure-S 4.7Note footbridge drawn inwards towards re nery, suggesting footbridge inside deto- nation. (Figure taken from [10].) Figure-S 4.8Close up of footbridge showing damaged railings and supports drawn towards the re nery. (Figure taken from [10].) Figure-S 4.9 and Figure-S 4.10 show damage to trucks inside the lube oil area, representative of overpressures greater than 5 atm. Car and tree damage indicate detonation (Figure-S 4.11). The oil drums in Figure-S 4.12 indicate overpressures of at least 3 atm. 25 Figure-S 4.9Damage to truck inside the lube oil plant. Damage suggests detonation. Figure-S 4.10 Lube oil plant damage to truck and oil drums. Note building destroyed, but poles not bent. 26 Figure-S 4.11 Car and tree damage suggest detonation. Figure-S 4.12 Top: Drums and car damage. Bottom: Enlargement of Figure-S 4.10showing creased drums. (Figure from CTV News, August -September, 2012.) If a detonation did occur in the lube oil area, the overpressure at the buildings in Figure-S 4.3 can be estimated using the correlation in Equation (4{2) from the main text. Assuming H= 3 m and R= 40 m, we have P0:6 atm. This is sucient to create the devastation seen in Figure-S 4.3. 27 Note also that there is signi cant distortion and failure of reinforced concrete supports. It seems unlikely that a fast de agration in the lube oil area would lead to a strong enough blast at these buildings. The distance from the lube oil area is around 350 m. Assuming a source pressure of 5 atm and a radius of 27 m, and using CAM [7], the overpressure at the buildings is less than 0.1 atm. Figure-S 4.13 shows tank damage due to blast impact (suggesting detonation) with a subsequent phenomenon called top tank re that destroys the impact evidence because it melts walls. The overpressure at the nearest fuel tank ( 10 m from the edge of the cloud) would have been around 0.23 atm, sucient to deform and open up the tank. Additionally, a large storage tank 470 m to the west remained largely undamaged. The overpressure there would have been around 0.05 atm. Other storage tanks inside the cloud show de nite uniform crushing (Figure-S 4.14 to Figure-S 4.15), suggesting detonation impact. Figure-S 4.13 Crushed storage tank with tank top re that melts and deforms the tank walls. Figure-S 4.14 Crushed storage tank. Notice damaged cars. 28 Figure-S 4.15 Crushed storage tank suggesting detonation impact. (From website Runrun.es, by permission [11].) A likely hypothesis on the course of events is as follows: The car on Avenida Bolivar road ignited the ole n cloud. A cloud re ensued which propagated through the cloud covering lower ground between ole n spheres and fuel storage tanks and eventually reached the lube oil plant (Figure- S 4.1). The ame speeds at this stage were not great, and little blast damage was caused to the nearby ole n spheres. Plant congestion and vented explosions caused a DDT in the lube oil area which propagated through the remaining cloud over a largely uncongested region towards the footbridge and buildings to the east. The hypothesis does not explain the bent posts near the ignition point towards the re nery, nor does it explain the lack of bent posts in the lube oil plant itself (Figure-S 4.10). The displacement of the car to the other side of the road and its signi cant damage also remain an anomaly. Unless, and here is another hypothesis, there was another fast ame or detonation near this location. Building and tree damage suggest a considerable blast in the vicinity (Figure-S 4.16), which was strong enough to be a detonation. Figure-S 4.16 Building damage in the vicinity of the car allegedly responsible for ignition of the cloud [10]. 29 5. Skikda Accident Description of the Event Skikda LNG facility was operated by Sonatrach, a government-owned Algerian corporation formed explicitly to explore the hydrocarbon resources of the country. Sonatrach operated the LNG plant at Skikda, Algeria, a modern city on Mediterranean Sea coast. A layout of the plant is given in Figure-S 5.1 [12]. Figure-S 5.1Layout of the Skikda LNG plant (private communication to GAC, 2004. Source: Sonatrach, Poten and Partners). At 18:34 pm on 19 January 2004, an operator noticed the steam pressure in the boiler drum was rising to the point where the pressure safety valve was activated. The operator cut the fuel gas supply, but the pressure continued to rise. At 18:40 pm, the outside operator reported to the control room that he could see a large vapor cloud developing in Train 40, pole 1. At this time in Train 40, there was a small explosion that was quickly followed by a larger explosion and reball. Contributing factors include semi-con ned conditions (con nement on three sides) and no wind. Sonatrach's best interpretation of these events is as follows: 1. A cold hydrocarbon leak occurred near the boiler of Train 40. 2. A mixture of hydrocarbons and air was introduced by the fan to the boiler. 3. The pressure in the boiler drum rose quickly. 4. The gas in an entrance to the boiler developed into explosive mixture. 5. An explosion inside boiler destroyed the boiler water wall and ignited the vapor cloud outside. 6. There was a massive VCE. 30 Sonatrac h's investigation reported that the following inventory and equipment were in the vicinity of the explosion in Train 40: { Cold Fluids { LPG at a temperature of -40 C. { High pressure ash gas* (47% inert gas and 53% fuel, temperature = -148 C) [12]. { LNG at temperature = -161 C { Equipment { Fuel gas exchanger { Fuel gas compressor { Fuel gas ash drum { LNG pumps { Piping that transfers these gases and uids. Discussion An outside report (private communication, S. Davis) con rmed that Sonatrach con rmed that Train 40, where the leak occurred, used the proprietary PRICO Liquefaction Process. Exten- sive e orts were required to con rm the composition of the mixed refrigerant used to liquefy the methane, and which was the likely source of leaked volatile liquid. The presence of a considerable amount of ethylene makes the vapor cloud more susceptible to detonation. The timing of the leak, as reported on the radio by an eye witness who was later killed by the explosion, suggests that the leak duration was 2 min. This is consistent with a leak of liquid refrigerant from a 1" pipe break. Simulations using CAM2 [7] predict that the release rate is about 15.75 kg/s and that the dispersion distance to the lower ammable limit of the mixture is 85 m. An eye witnesses reported that the visible cloud, i.e., at least the extent of the ammable cloud, traveled around 80 m towards the Maintenance Building. At least ve eye witnesses reported a vapor cloud extending to the Maintenance Building and to Unit 30 (Figure-S 5.1). One witness said that there was a rst small explosion in unit 40 and immediately thought that the boiler had leaked, because a similar incident had occurred six months earlier. He looked towards the boiler and saw a cloud of white vapor. He then saw a second small explosion, followed by the third, much louder explosion that destroyed the plant. Some pictures show damaged cars, although it is not known where these cars were located at the time of the explosion. The damage sustained indicates overpressures of around 5 atm, but not necessarily detonation. The damage could be explained by a fast de agration in the highly congested region of the release. It might be possible that methane vaporized from LNG could have created such high overpressures. A more likely candidate would be LPG or mixed refrigerant. The equivalent fuel based on the * Flash gas is a spontaneous vapor produced from the heating or depressurizing the oil mixture extracted during various phases of oil production 31 PRICO mixed refrigerant composition is propylene. The results of CAM2 simulation suggests that overpressure greater than 3 atm were generated at source. This could be sucient to create the conditions for transition to detonation. Photographs of the damage reveal several interesting features. The Control Room (Figure-S 5.2) and Maintenance Building (Figure-S 5.3 and Figure-S 5.4) were totally destroyed. Some reinforced concrete supports from the remains of the Maintenance Building were displaced towards Unit 40. Several poles in the open space between the Maintenance Building and Unit 40 (Figure-S 5.5, Figure-S 5.6, and Figure-S 5.7) and a tower support (Figure-S 5.8) are bent inwards towards Unit 40. Figure-S 5.9 shows a considerably crushed heavy duty truck located away from congested space. The damage indicated overpressure signi cantly higher than 5 atm. This space also contains a few crushed oil drums, indicative of overpressures over 3 atm. The collapsed ash drum and railings in Figure-S 5.10 and Figure-S 5.11, and especially the bent poles in Figure-S 5.5, Figure-S 5.6, and Figure-S 5.7, have been drawn to the SW on Unit 40. When these directional indicators are plotted on a Google Earth snapshot of the site (Figure 5.3 in Main Text) they suggest that a detonation may have occurred. Consistent with this hypothesis is the fact that the vapor cloud was observed to have reached the Maintenance Building. Three separate explosions were heard. The rst was probably the furnace explosion, the second could be the de agration in the congested area of Unit 40, and the third, the largest, the transition to detonation. Figure-S 5.2Control Room at Skikda (private communication to GAC, 2004). 32 Figure-S 5.3The remains of the Maintenance Building. Note that some of the reinforced concrete supports have been drawn inwards (towards the center of the ammable cloud) (Figure taken from Reuters.). Figure-S 5.4Another view of the wrecked Maintenance building. Note also the crushed drums in the foreground left [12]. Figure-S 5.5Panorama of the west side of Unit 40 south end looking SE. The remains of the maintenance building are behind this photograph. Note the bent poles. (Private communication to GAC, 2004). 33 Figure-S 5.6Panorama of the south side of Unit 40, electrical building, looking SE. Note the bent lamppost 38 E 25 and fence drawn inwards. (Private communication to GAC, 2004). Figure-S 5.7Panorama of the west side of Unit 40 south end, electrical building. Note the bent lampposts all drawn inwards, i.e. towards the centre of the ammable cloud. (Private communication to GAC, 2004). Figure-S 5.8West side of Unit 40. Note that the tower by the yellow truck has been drawn 34 to wards Unit 40, similarly the lamppost (in front of the truck). (Private communication to GAC, 2004). Figure-S 5.9Unit 40 and the heavy duty car crushed; overpressure above 5 atm away from congestion. (Private communication to GAC, 2004). Figure-S 5.10 View of collapsed ash drum and bent railing Unit 40. (Private communication to GAC, 2004, also [12]). 35 Figure-S 5.11 Location of a ash drum and railings Unit 40 indicating direction of bend. (Private communication to GAC, 2004, and [12].). It is unlikely that the detonation reached the electrical substation to the south of Unit 40 or to the oces to the north-west of the site. Both were badly damaged, indicating high overpressures, but unlike the Maintenance Building or Security Hut to the west of Unit 40, they were not completely annihilated. Then, in summary, the following course of events can be proposed: An explosion rst occurred in the Unit 40 furnace after ingress of leak vapors from within the unit. The vapors consisted of mixed refrigerant, equivalent to propylene vapor. This vented into the vapor cloud which had become established over the 2 min of release. The external cloud ignited, thus creating a de agration in Unit 40. The ame speed increased rapidly in the congested unit, generating an 3 atm overpressure and thereby causing gross damage to northern end of Unit 40 and adjacent Unit 30. There was then a transition to detonation in the mid-western side of Unit 40, and then detonation consumed the remaining cloud in the open space between Unit 40, the Maintenance Building, the electrical sub-station, and the canal. Figure 5-4 in the main text. shows the proposed area over which the detonation took place. 36 6. Brenham Accident Description of the Event The explosion occurred when a large cloud of LPG escaped from a 1335 m3salt-dome storage facility that had been over- lled. A car passed into the cloud of gas accumulated in a gully. The explosion attened the car and blew it o the road. A 5-year-old child was thrown 46 m from his home. His mother was also thrown from the house. The trailer home was completely destroyed. The complete annihilation of houses and trailers within the cloud and the damage to trees and cars suggest overpressures greater than 5 atm. The blast registered 3.5 - 4 on the Richter scale. The explosion was most likely detonation of the hydrocarbon vapors. Discussion The location of ignition remains unknown, but evidence points towards the car entering the cloud, as shown in Figure-S 6.1. The level of damage resembles that of overpressures >5 atm, which could only have been generated by a fast de agration or detonation. Ignition of the cloud would not in itself create large overpressures and the ame would have to have accelerated. The car may have traveled some distance before being damaged to the extent shown in Figure-S 6.1. Figure-S 6.1The video shows a damaged car, shown in the captured image above. The crushing resembles damage caused overpressures >5 atm [13]. There is evidence that 3 separate blasts occurred. The rst blast was the strongest, and it was immediately followed by 2 other blasts. This suggests that there was a detonation followed by 2 de agrations. The spread of the cloud, the undulating terrain, and the distribution of trees, suggest that the detonation terminated, but was quickly followed by a ash re that propagated to 2 isolated, tree-covered areas as fast de agrations. Alternatively, the 2 weaker blasts may have arisen from explosions in houses, but this seems less likely given eye witness testimony. 37 Figure-S 6.2Sketch reproduced from [14]. Figure-S 6.3Brenham burn area. Sketch reproduced from [14]. Houses within 1.6 km radius of the explosion station were totally destroyed, but many of these were within only 800 m from the cloud edge where the overpressure may have been of the order 38 0.05 atm. This would have been sucient to destroy the mainly wooden houses, as observed in Figure-S 6.2 and Figure-S 6.4. Figure-S 6.4Area residences damaged by blasts. Sketch reproduced from [14]. Top right: a building about 800 m from the release point. 39 7. Ufa Accident Description of the Event The description in the next few paragraphs is essentially taken from [15]. (Additional information has been obtained from G.M. Makhviladze in private communication with one of the authors, AP). The incident occurred in the Ural Mountains area of Russia near the city of Ufa, 1710 km from Moscow. The damaged forest area, shown in Figure-S 7.1 and enclosed by the solid line, covered about 2.5 km2. The cause of the accident was the rupture of a large pipeline (total length 1852 km) that transported lique ed oil products from Siberia to large chemical plants in European Russia. The pipeline, of inner diameter 70 cm, lies at a depth of about 1.2{1.5 m below the surface and passes under the railway. It conveys about 10 MT/day (116120 kg/s) at a working pressure of 38 bar. Figure-S 7.1Ufa explosion event: (a) Sketch of demolished area (direction of tree fell is shown by arrows); (b) terrain pro le (not to scale). View of accident site from region of point A is shown in Figure-S 7.2. (Figure reproduced with permission from [15]). The pipeline was ruptured near the head of the valley (Figure-S 7.1, [15]) about 900 m from the railway track. The total area of this breach was 0.77 m2, twice the cross-section area of the pipe. 40 The original dent at the point of rupture may have occurred during the laying of the pipeline. The pipeline operates in severe winter conditions, under mechanical vibrations caused by possible hydraulics shocks, unavoidable in such a long pipeline over a hilly area. These factors could have contributed to the sudden fracture of the pipeline. The rupture of the pipeline at the head of the valley led to the release of hydrocarbons and the formation of a huge ammable vapor cloud that covered the dense forest area and part of the nearby rail line. Before the accident, at about midnight, the pressure in the pipeline dropped. The sta , unaware of the reason for this, powered the pumps to increase the pressure. At that time, inhabitants of the nearby villages, 4-7 km away, smelled the gas condensate. At 12:50 a.m., a goods train passed this site. The driver noticed the strong smell of gas and saw a \white cloud" along the railway tracks. Twenty minutes later, two passenger trains traveling in opposite directions approached the site and came into the cloud. At 1:14 a.m., a violent explosion occurred, followed by a very large re. There were 1284 people total in both trains, of whom 1224 died or were severely injured. The majority of the victims su ered from the strong heat radiation. In one train, 6 cars of out of 20 were thrown o the track. In the other, 5 cars out of 17 were thrown o . Seven cars of the 11 thrown o were completely destroyed; all other coaches of both trains were partly burned on the outside and burned up inside. Shock waves demolished the mound and consoles and dislodged 1800 m of overhead and contact wires. Figure-S 7.2Accident site viewed from region of point A in Figure-S 7.1. On the right of the railway is the remainder of one of the trains. (Reproduced from [15] by permission). Windows were broken in a local hospital, 15 km from the site. A photograph of the site (Figure- S 7.3) from Figure 10 in [15] was taken several days later. The tree fall in the valley below the railway track re ects the vortex character of the ow in this region (Figure-S 7.1). 41 Figure-S 7.3Site of accident (G.M. Makhviladze, private communication). According to the Center for Chemical Process Safety, two explosions took place in quick succession, and a ash re ran down the railroad track in both directions. A considerable part of each train was derailed. Discussion The rst step is to establish the facts surrounding this incident. There are several discrepancies between the accounts in [15] and [16], and the information provided in private communications (with G.M. Makhviladze to author AP), and these are listed and discussed below. { The total amount of fuel released between initial burst and ignition was estimated by various methods as 2 MT and 10 Mt [15] and [16]. { The composition of the lique ed product, quoted in [16] in mass percentages as: 0.02% methane, 2%,ethane , 36% propane, 15$ iso-butane, 27% n-butane, 7% iso-pentane, 6% n- pentane, and 7%hexane. This di ers from that quoted in [15] which has no methane or ethane, 41% propane, 26% n-butane, 15% iso-butane, and 16% heavier hydrocarbons. Since [16] is a later publication, we assume that the composition quoted there is correct. { The initial release rate calculated by FRED's [7] \pressurized release scenario" is 2700 kg/s. Using FRED's \2-phase blowdown scenario," and assuming propane fuel, the initial release rate is 2300 kg/s. FRED's \uni ed scenario" gives 2662 kg/s. These rates refer to release from one side, which is consistent with the break being near one end of the pipeline. { The volume of the pipeline is 713,000 m3and liquid density 569 kg/m3, giving a total mass of 4:05108kg in the pipeline before rupture. Using FRED's \uni ed scenario," this reduces to 3:9108kg after 1 hour, and a release rate of 420 kg/s. The mass lost after 1 hour (assuming no top-up of the pipeline by further pumping, which actually did take place) is therefore about 42 107kg or 10 MT tonnes. This is reasonably close to the upper estimate in [15]. { The spill occurred at an elevation of 40 m above and 900 m away from the railway track where the two passenger trains met. Figure-S 7.1, however, incorrectly indicates an elevation of 3,040 m above the railway track. Figure-S 7.4Sharp borderline between demolished and original forest, indicating sharp change of overpressure [15]. {