Assessment of an accidental vapour cloud explosion: Lessons from the Indian Oil Corporation Ltd. accident at Jaipur,India

On 29 October 2009, at 19:30 IST, a devastating vapour cloud explosion occurred in a large fuel storage area at the Indian Oil Corporation (IOC) Depot in Jaipur, India, generating significant blast pressure. As a consequence of this explosion, the entire installation was destroyed, buildings in the immediate vicinity were heavily damaged, and windowpane breakages were found up to 2 km from the terminal. The IOC estimated that the total loss from the fire and explosion was approximately INR 2800 million.Ironically, as a storage site, the Jaipur terminal was not highly congested, and thus was not considered to have adequate potential for a vapour cloud explosion (VCE). Nevertheless, the prima facie evidences indicate that this was a case of VCE. Therefore, the main objective of this study is to quantify the potential overpressures due to vapour cloud explosions (VCEs) using the Process Hazard Analysis DNV Norway based PHAST 6.51 Software. The results are validated by the extent of the damage that had occurred. The estimation of the VCE shows that a maximum 1.0 bar overpressure was generated in the surrounding area. The initial assessment of the accident data roughly estimates the release mode, time, and amount of vaporized fuel. A more accurate estimate has been obtained by modelling the dispersion of vapour clouds in the surrounding atmosphere, which reveals trends and relationships for the occurrence of vapour cloud explosions.     

The potential for vapour cloud explosions – Lessons from the Buncefield accident

At around 06.00 on Sunday 11th December 2005, a vapour cloud explosion occurred at Buncefield Oil Storage Depot, Hemel Hempstead, Hertfordshire, UK, generating significant blast pressures. However, as a storage site, the Buncefield terminal had very little pipework congestion and at first sight would not have been considered as having much potential for a vapour cloud explosion. As a consequence, one of the actions of the Buncefield Major Incident Investigation Board (BMIIB) was to initiate a review of the possible causes of the severe explosion on the site. This review was then extended to a Joint Industry Project, Phase 1 of which has offered an explanation of the cause of the explosion. The conclusions are summarized along with reference to relevant experimental studies, illustrating how the elements of the explanation were already known. The implications of the incident for the assessment of vapour cloud explosion hazards will be discussed, both in terms of understanding worst case consequences and the use of risk based approaches.     

A violent,episodic vapour cloud explosion assessment: Deflagration-to-detonation transition

A large vapour cloud explosion (VCE) followed by a fire is one of the most dangerous and high consequence events that can occur in petrochemical facilities. The current process of safety practice in the industry in VCE assessment is to assume that all VCEs are deflagration. This assumption has been considered for nearly three decades. In recent years, major fire and VCE incidents in fuel storage depots gained considerable attention in extreme high explosion overpressure due to the transition from Deflagration to Detonation (DDT). Though the possibility of DDTs is lower than deflagrations, they have been identified in some of the most recent large-scale VCE incidents, including Buncefield (UK), 2005, San Juan explosion (US), 2009, and IOCL Jaipur (India), 2009 event. Such an incident established the need to understand not only VCE but also the importance of avoiding the escalation of minor incidents into much more devastating consequences.Despite decades of research, understanding of the fundamental physical mechanisms and governing factors of deflagration-to detonation transition (DDT) transition remains mostly elusive. An extreme multi-scale, multi-physics nature of this process uncertainly makes DDT one of the “Grand Challenge” problems of typical physics, and any significant developments toward its assured insistence would require revolutionary step forward in experiments, theory, and numerical modelling. Under certain circumstances, nevertheless, it is possible for DDT to occur, and this can be followed by a propagating detonation that quickly consumes the remaining detonable cloud. In a detonable cloud, a detonation creates the worst accident that can happen. Because detonation overpressures are much higher than those in a deflagration and continue through the entire detonable cloud, the damage from a DDT event is more severe. The consideration of detonation in hazard and risk assessment would identify new escalation potentials and recognize critical buildings impacted. This knowledge will allow more effective management of this hazard.The main conclusion from this paper is that detonations did occur in Jaipur accident at least part of the VCE accidents. The vapour cloud explosion could not have been caused by a deflagration alone, given the widespread occurrence of high overpressures and directional indicators in open uncongested areas containing the cloud. Additionally, the major incident has left many safety issues behind, which must be repeatedly addressed. It reveals that adequate safety measures were either underestimated or not accounted for seriously. This article highlights the aftermath of the IOCL Jaipur incident and addresses challenges put forward by it.     

Buncefield: A violent, episodic vapour cloud explosion

Damage caused by the 2005 Buncefield explosion indicates pressures in excess of 2000 mbar over all of the area covered by the vapour cloud. Such high overpressures are normally associated with high (super-sonic) rates of flame spread. On the other hand, evidence from witnesses, building damage analysis and CCTV cameras all suggest the average rate of progress of the explosion flame front was only around 150 m/s.The high overpressures in the cloud and low average rate of flame advance can be reconciled if the rate of flame advance was episodic, with periods of very rapid combustion being punctuated by pauses when the flame advanced very slowly. The widespread high overpressures were caused by the rapid phases of combustion; the low average speed of advance was caused by the pauses.Mechanisms of flame spread through radiative ignition of particulates ahead of the flame front provide possible explanations for such unusual episodic behaviour.The first part of this paper reviews a wide range of empirical evidence on average flame speed and rate of blast pressure increase.The second part explores the theoretical consequences of forward radiation and how the new theory might be developed into a practical means of assessment.     

The multi-energy critical separation distance

Devastating vapour cloud explosions can only develop under appropriate (boundary) conditions. The record of vapour cloud explosion incidents from the past demonstrates that these conditions are readily met by the congestion by process equipment at (petro-) chemical plant sites. Therefore, the possibility of an accidental release of a flammable and a subsequent vapour cloud explosion is a major hazardous scenario considered in any risk assessment with regard to the process industries.If an extended flammable vapour cloud at a chemical plant site extends over more than one process unit, which are separated by lanes of sufficient width, the vapour cloud explosion on ignition develops the same number of separate blasts. If, on the other hand, the separation between the units is insufficient, the vapour cloud explosion develops one big blast. The critical separation distance (SD) is the criterion that allows discriminating in this matter for blast modelling purposes.This paper summarises some major results of an experimental research programme with the objective to develop practical guidelines with regard to the critical SD. To this end, a series of small-scale explosion experiments have been performed with vapour clouds containing two separate configurations of obstacles. Blast overpressures at various stations around have been recorded while the SD between the two configurations of obstacles was varied.The experimental programme resulted in some clear indications for the extent of the critical SD between separate areas of congestion. On the basis of safety and conservatism, these indications have been rendered into a concrete guideline. Application of this guideline would allow a greater accuracy in the modelling of blast from vapour cloud explosions.     

Effect of separation distance on gas dispersion and vapor cloud explosion in a storage tank farm determined using computational fluid dynamics

Storage tank separation distance, which considerably affects forestalling and mitigating accident consequences, is principally determined by thermal radiation modeling and meeting industry safety requirements. However, little is known about the influence of separation distance on gas dispersion or gas explosion, which are the most destructive types of accidents in industrial settings. This study evaluated the effect of separation distance on gas dispersion and vapor cloud explosion in a storage tank farm. Experiments were conducted using Flame Acceleration Simulator, an advanced computational fluid dynamics software program. Codes governing the design of separation distances in China and the United States were compared. A series of geometrical models of storage tanks with various separation distances were established. Overall, increasing separation distance led to a substantial reduction in vapor cloud volume and size in most cases. Notably, a 1.0 storage diameter separation distance appeared to be optimal. In terms of vapor cloud explosion, a greater separation distance had a marked effect on mitigating overpressure in gas explosions. Therefore, separation distance merited consideration in the design of storage tanks to prevent gas dispersion and explosion.     

Detonations and vapor cloud explosions: Why it matters

The methods used to evaluate the consequences of a vapor cloud explosion assume deflagrations within congested process pipework regions and consequently a significant effort has been invested in developing models to estimate the severity of these deflagrations. Models range from the simpler screening approaches to detailed Computational Fluid Dynamics. There is clear evidence from large scale experiments and incidents that transition from deflagration to detonation is credible and has occurred and it is the contention of this paper that deflagration is only the first stage in many major vapor cloud explosions and that detonation is readily foreseeable. Why does this matter? The methods currently used in the design and location of buildings on and around process sites are based on an incomplete picture of vapor cloud explosions. Whilst this might not have a significant effect in some cases, it is shown that there is the potential to significantly underestimate the explosion hazard. This will result in occupied buildings either being placed in the wrong location or under-designed for the explosion threat, increasing the risks to personnel on these sites.     

液化石油气站储罐区火灾爆炸危险性分析与安全措施

选择具体的液化石油气储配站,分析了该站的危险特性、危险产生的途径及可能造成的后果。在没有任何防护措施的情况下,采用蒸气云爆炸和沸腾液体扩展蒸气云爆炸模型,对该站一个50m3储罐发生泄漏造成的火灾爆炸事故后果进行预测,得出火灾爆炸后的安全距离为大于211．0m。在储配站不能满足此安全距离的基础之上,从防止产生爆炸性气体环境、消除点火源和抑制事故扩大三方面来提出有效的安全措施,降低事故发生的概率及事故造成的损失。其中,站址选在全年最小频率风向的上风侧且周围空旷的地区,罐上设置液位计、压力表、温度计及可燃气体报警器可防止产生爆炸性气体环境;罐及管道设静电接地,法兰用铜线跨接,站内设警示标志可消除点火源;生产区与辅助区间设置隔离墙,罐区周围设置砖混围堤,罐上设安全阀可抑制火灾爆炸事故扩大。     

Causative factors for vapour cloud explosions determined from past-accident analysis

Past-accident analysis shows that most dangerous incidents are related to process operations. Often these operations are carried out under high pressures and/or high temperatures. The consequences, therefore, are significant. A scientific analysis of past accidents which led to vapour cloud explosion has been performed. The analysis has provided vital information for most probable accident scenarios for a new situation. Factors such as chemical characteristics, its release mode, time etc. show trends and relationships for the occurrence of vapour cloud explosions.     

A study on the effect of trees on gas explosions

The downstream as well as the upstream oil and gas industry has for a number of years been aware of the potential for flame acceleration and overpressure generation due to obstacles in gas clouds caused by leaks of flammable substances. To a large extent the obstacles were mainly considered to be equipment, piping, structure etc. typically found in many installations. For landbased installations there may however also be a potential for flame acceleration in regions of vegetation, like trees and bushes. This is likely to have been the case for the Buncefield explosion that occurred in 2005 (Buncefield Major Incident Investigation Board, 2008), which led to the work described in the present paper. The study contains both a numerical and an experimental part and was performed in the period 2006–2008 (Bakke and Brewerton, 2008, Van Wingerden and Wilkins, 2008).The numerical analysis consisted of modelling the Buncefield tank farm and the surrounding area with FLACS. The site itself was not significantly congested and it was not expected to give rise to high overpressures in case of a hydrocarbon leak. However, alongside the roads surrounding the site (Buncefield Lane and Cherry Tree Lane), dense vegetation in the form of trees and bushes was included in the model. This was based on a site survey (which was documented by video) performed in the summer of 2006.A large, shallow, heavier-than-air gas cloud was defined to cover part of the site and surroundings. Upon ignition a flame was established in the gas cloud. This flame accelerated through the trees along the surrounding roads, and resulted in high overpressures of several barg being generated by FLACS. This is to the authors’ knowledge the first time a possible effect of vegetation on explosions has been demonstrated by 3D analyses.As a consequence of these results, and since the software had been validated against typical industrial congestion rather than dense vegetation, a set of experiments to try to demonstrate if these effects were physical was carried out as well. The test volume consisted of a plastic tunnel, 20 m long with a semi-circular cross-section 3.2 m in diameter allowing for representing lanes of vegetation. The total volume of the tent was approximately 80.4 m³. The experimental programme involved different degrees of vegetation size, vegetation density (blocking ratio) and number of vegetation lanes (over the full length of the tunnel). The experiments were performed with stoichiometric propane–air mixtures resulting in continuously accelerating flames over the full length of the tunnel for some of the scenarios investigated.The main conclusions of the study are that trees can have an influence on flame acceleration in gas–air clouds, and that advanced models such as FLACS can be used to study such influence. More research is needed, however, because even if FLACS predicts flame acceleration in dense vegetation, no evidence exists that applying the code to trees rather than rigid obstacles provides results of acceptable accuracy.     

The application of pressure–impulse curves in a blast exceedance analysis

The magnitude of damage due to a vapor cloud explosion can be estimated in many ways, ranging from look-up tables to quantitative risk analysis. An explosion overpressure analysis is a routine part of compliance with the American Petroleum Institute (API) Recommended Practice (RP) 752 when evaluating occupied buildings in a facility that processes flammable or reactive materials. In many cases, a risk-based approach is useful because consequence modeling studies often indicate major problems for buildings at existing facilities. One of the most common risk-based methods, overpressure exceedance, incorporates a wide range of potential explosion scenarios coupled with the probability of each event to develop the probability of exceeding a given overpressure at specific locations. But this and other methods that only use overpressure may not represent an accurate building response. By combining the risk-based methodology of the exceedance analysis with pressure and impulse data in the form of pressure–impulse (P–I) curves, a better measure of building damage can be generated. P–I curves for blast loading determination have been in use for decades, and allow the user to determine levels of damage based on a predicted overpressure and its corresponding impulse. Curves have been published for entire buildings, individual structural members, window breakage, and even consequences to humans. This paper will explore application of P–I curves for building damage, and will highlight some of the benefits, as well as some of the potential problems, of using P–I curves.     

Evaluating the potential for overpressures from the ignition of an LNG vapor cloud during offloading

Ignition of natural gas (composed primarily of methane) is generally not considered to pose explosion hazards when in unconfined and low- or medium-congested areas, as most of the areas within LNG regasification facilities can typically be classified. However, as the degrees of confinement and/or congestion increase, the potential exists for the ignition of a methane cloud to result in damaging overpressures (as demonstrated by the recurring residential explosions due to natural gas leaks). Therefore, it is prudent to examine a proposed facility’s design to identify areas where vapor cloud explosions (VCEs) may cause damage, particularly if the damage may extend off site.An area of potential interest for VCEs is the dock, while an LNG carrier is being offloaded: the vessel hull provides one degree of confinement and the shoreline may provide another; some degree of congestion is provided by the dock and associated equipment.In this paper, the computational fluid dynamics (CFD) software FLACS is used to evaluate the consequences of the ignition of a flammable vapor cloud from an LNG spill during the LNG carrier offloading process. The simulations will demonstrate different approaches that can be taken to evaluate a vapor cloud explosion scenario in a partially confined and partially congested geometry.     

Dispersion of the vapour cloud in the Buncefield Incident

Dispersion of the flammable vapour cloud in the 2005 Buncefield Incident is examined. Footage from security cameras around the site is analysed and the results from Computational Fluid Dynamics (CFD) simulations of the vapour dispersion are presented. It is shown that the shape of the terrain and the presence of obstacles significantly affected the dispersion of vapour from the overflowing tank. The CFD model is shown to produce similar qualitative behaviour to that observed in the incident, both in terms of the arrival time of the vapour cloud and its final depth.     

Using computational fluid dynamics (CFD) for blast wave predictions

Explosions will, in most cases, generate blast waves. While simple models (e.g., Multi Energy Method) are useful for simple explosion geometries, most practical explosions are far from trivial and require detailed analyses. For a reliable estimate of the blast from a gas explosion it is necessary to know the explosion strength. The source explosion may not be symmetric; the pressure waves will be reflected or deflected when hitting objects, or even worse, the blast waves may propagate inside buildings or tunnels with a very low rate of decay. The use of computational fluid dynamics (CFD) explosion models for near and far field blast wave predictions has many advantages. These include more precise estimates of the energy and resulting pressure of the blast wave, as well as the ability to evaluate non-symmetrical effects caused by realistic geometries, gas cloud variations and ignition locations. This is essential when evaluating the likelihood of a given leak source as cause of an explosion or equally when evaluating the potential risk associated with a given leak source for a consequence analysis.In addition, unlike simple methods, CFD explosion models can also evaluate detailed dynamic effects in the near and far field, which include time dependent pressure loads as well as reflection and focusing of the blast waves. This is particularly valuable when assessing actual near-field blast damage during an explosion investigation or potential near-field damage during a risk analysis for a facility. One main challenge in applying CFD, however, is that these models require more information about the actual facility, including geometry details and process information. Collecting the necessary geometry and process data may be quite time consuming. This paper will show some blast prediction validation examples for the CFD model FLACS. It will also provide examples of how directional effects or interaction with objects can significantly influence the dynamics of the blast wave. Finally, the challenge of obtaining useful predictions with insufficient details regarding the geometry will also be addressed.     

Acoustic analysis of blast waves produced by rapid phase transition of LNG released on water

Massive offshore and onshore storage of fuel have led the international community to raise questions about the hazards on the surrounding installations and people. Among the possible accidental scenarios when cryogenic gas as liquefied natural gas (LNG) is spilled on water at a very fast rate, the phenomenon of rapid phase transition (RPT) may occur: large amounts of energy are released during phase transition which can generate explosions. The related consequences should be added to the possible consequences of fire in terms of flash fire, fireball, pool fire, and vapour cloud explosion for confined and congested geometry surrounding the release point.In this paper, the analysis of RPT of LNG has been studied from the point of view of blast wave production, through ab initio acoustic analysis for monopole source. Maximum overpressures, as calculated at the source point and along the blast pathway are compared with results of large scale experiments. Safety distances are given for the sake of comparison with threshold distances reported in the open literature.     

Major accidents in process industries and an analysis of causes and consequences

This paper briefly recapitulates some of the major accidents in chemical process industries which occurred during 1926–1997. These case studies have been analysed with a view to understand the damage potential of various types of accidents, and the common causes or errors which have led to disasters. An analysis of different types of accidental events such as fire, explosion and toxic release has also been done to assess the damage potential of such events. It is revealed that vapour cloud explosion (VCE) poses the greatest risk of damage. The study highlights the need for risk assessment in chemical process industries.     

Coupling effects of the explosion shock wave and heat radiation on the dynamic response of a fixed-roof tank

The occurrence of leakage in large tank farms or oil deposits can lead to fire or explosion accidents. Coupling effects of fire and explosion loadings can cause considerably more damage to adjacent tanks or buildings than either loading individually does. In this study, the combined loadings of the explosion shock wave and heat radiation from a pool fire on a neighboring empty fixed-roof tank were numerically investigated. The effects of the explosion shock wave intensity and relative height of the explosion center [the ratio of the height of the explosion center to the height of the tank (h_r)] were analyzed. The results indicate that tanks damaged by explosion shock waves have decreased fire resistance and critical buckling temperature. Moreover, the thermal buckling deformation of the predamaged tank largely depends on the explosion shock wave. With an increase in the explosion shock wave intensity or a decrease in h_r, the explosion shock wave has greater influence on the fire resistance of the tank, and the critical buckling temperature decreases. This paper can provide an understanding of the dynamics of a tank under explosion shock wave loading, and of the critical failure criterion and failure modes of a target tank under the coupled loading of the explosion shock wave and an adjacent pool fire.