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As a part of zero emission ambitions within the maritime, aviation and elsewhere, liquid hydrogen (LH 2) will increasingly be used as an energy carrier and fuel in the coming years. It is therefore important to understand safety aspects of LH 2 , including gas dispersion and explosion properties. In the process of building the world's first ferry using LH 2 as fuel the Norwegian Public Roads Administration commissioned large-scale LH 2 release tests to study dispersion and explosion properties relevant for the design and approval processes. Experiments with LH 2 releases both indoor and outdoor were performed. In this article these experiments are described and compared to simulations with the CFD-model FLACS. The aim of the modelling is both to better understand the experiments performed, and to demonstrate that important phenomena and physics related to LH 2 safety assessments for approval processes can be simulated and predicted with confidence. [ABSTRACT FROM AUTHOR]

• Hydrogen risk assessments and safety assessments. • Consequence tools – screening models and CFD. • Liquid and compressed hydrogen. • Assessment of explosion incident. • DDT and detonation. With the ambition to cut emissions from transport hydrogen fuelled vehicles and marine vessels are now being introduced several places in the society. To support this development there is a need for infrastructure to produce and transport gaseous and liquid hydrogen. The properties and safety challenges related to the use of hydrogen are very different from those of conventional fuels, thus safe design may require unconventional solutions. Hydrogen has extreme properties in many ways. It is buoyant when in gas phase while a liquid hydrogen spray will develop a dense plume. The reactivity is higher, flammable range wider and the ignition energy lower than for conventional fuels. Flames may be invisible, and radiation is low. When performing risk assessments for land planning purposes, bunkering assessments or passenger and crew safety these aspects must be reflected. Properties like the positive buoyancy, strong dilution for sonic releases into air, and a low reactivity and energy content for concentrations below 10% must be exploited during design to ensure acceptable risk levels. In this article a two-level risk assessment and design optimization approach is presented in which risk screening with rapid consequence calculations and frequency assessments for release, dispersion, fire and explosion can be performed during concept selection phase with indicative hazard distances estimated. Possible risks of concern are in this way identified, and design can be adjusted, or mitigation measures introduced. For final design risk assessment CFD calculations can be performed for more precise consequence estimates. The risk assessment approach is described with illustrating examples. The focus is not only to ensure safety, but to do so in a cost-efficient and practical way. [ABSTRACT FROM AUTHOR]

The number of maritime initiatives with hydrogen as alternative fuel is increasing. While most of the early projects aim at using compressed hydrogen the use of liquid hydrogen (LH2) is more practical and is expected to become more attractive for implementation on larger vessels due to more efficient storage, bunkering and handling of the fuel. In the industry there seems to be some confusion regarding the behavior of LH2 releases, in particular whether a release into air will behave like a dense gas or a buoyant gas. The understanding of this aspect is critical to optimize design with regard to safety. This article will explain the expected behavior of LH2 releases and discuss expected hazard distances from LH2 releases relative to gaseous hydrogen releases and LNG. Some other safety concerns of LH2, like indoor releases, releases from vent masts, potential BLEVEs and RPTs are also discussed. The article explains why a higher safety standard may be required when designing hydrogen fueled vessels than for existing LNG fueled vessels. • The paper describes under what conditions LH2 releases show dense gas behavior. • Hazard distances for LH2 bunkering extends further than for LNG. • For releases indoor the potential for strong explosions is much higher for hydrogen than LNG. • Hydrogen fueled vessels must be designed to higher safety standard than LNG vessels. [ABSTRACT FROM AUTHOR]

A concept risk assessment of a hydrogen and fuel cell driven high speed passenger ferry has been performed. The study focused on fatality risk related to the hydrogen systems on the vessel, both during operation and while moored in harbour overnight. The main objective with the study was to evaluate whether the risk related to the hydrogen systems is equivalent to that of conventionally fuelled vessels and can be considered acceptable according to the requirements of the IGF-code (International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels). Since hydrogen behaves differently than other flammable gases, some adjustments to existing models and vulnerability criteria have been proposed. The conclusion of the study is that the estimated risk related to hydrogen systems is relatively low, and much lower than the expected acceptable risk tolerance level of 0.5–1.0 fatalities per 109 passenger km. Furthermore, for the overnight mooring in harbour the estimated risks are well within acceptable limits. The work presented is part of a maritime case study performed within MoZEES, a Norwegian research centre for environmentally friendly technology and zero emission transport. • Risk assessment of hydrogen ship according to IGF-code Alternative Design Approach. • Risks associated with hydrogen ferry are well within expected tolerance criteria. • New hydrogen ignition probability model and vulnerability thresholds proposed. • Vessel design recommendations and risk reducing measures proposed. [ABSTRACT FROM AUTHOR]

Abstract: Computational fluid dynamics (CFD) tools are increasingly employed for quantifying incident consequences in quantitative risk analysis (QRA) calculations in the process industry. However, these tools must be validated against representative experimental data, involving combined release and ignition scenarios, in order to have a real predictive capability. Forschungszentrum Karlsruhe (FZK) has recently carried out experiments involving vertically upwards hydrogen releases with different release rates and velocities impinging on a plate in two different geometrical configurations. The dispersed cloud was subsequently ignited and resulting explosion overpressures recorded. Blind CFD simulations were carried out prior to the experiments to predict the results. The simulated gas concentrations are found to correlate reasonably well with observations. The overpressures subsequent to ignition obtained in the blind predictions could not be compared directly as the ignition points chosen in the experiments were somewhat different from those used in the blind simulations, but the pressure levels were similar. Simulations carried out subsequently with the same ignition position as those in the experiments compared reasonably well with the observations. This agreement points to the ability of the CFD tool FLACS to model such complex scenarios even with hydrogen as a fuel. Nevertheless, the experimental set-up can be considered to be small-scale. Future large-scale data of this type will be valuable to confirm ability to predict large-scale accident scenarios. [Copyright &y& Elsevier]

Abstract: When introducing hydrogen-fuelled vehicles, an evaluation of the potential change in risk level should be performed. It is widely accepted that outdoor accidental releases of hydrogen from single vehicles will disperse quickly, and not lead to any significant explosion hazard. The situation may be different for more confined situations such as parking garages, workshops, or tunnels. Experiments and computer modelling are both important for understanding the situation better. This article reports a simulation study to examine what, if any, is the explosion risk associated with hydrogen vehicles in tunnels. Its aim was to further our understanding of the phenomena surrounding hydrogen releases and combustion inside road tunnels, and furthermore to demonstrate how a risk assessment methodology developed for the offshore industry could be applied to the current task. This work is contributing to the EU Sixth Framework (Network of Excellence) project HySafe, aiding the overall understanding that is also being collected from previous studies, new experiments and other modelling activities. Releases from hydrogen cars (containing 700bar gas tanks releasing either upwards or downwards or liquid hydrogen tanks releasing only upwards) and buses (containing 350bar gas tanks releasing upwards) for two different tunnel layouts and a range of longitudinal ventilation conditions have been studied. The largest release modelled was 20kg H2 from four cylinders in a bus (via one vent) in 50s, with an initial release rate around 1000g/s. Comparisons with natural gas (CNG) fuelled vehicles have also been performed. The study suggests that for hydrogen vehicles a typical worst-case risk assessment approach assuming the full gas inventory being mixed homogeneously at stoichiometry could lead to severe explosion loads. However, a more extensive study with more realistic release scenarios reduced the predicted hazard significantly. The flammable gas cloud sizes were still large for some of the scenarios, but if the actual reactivity of the predicted clouds is taken into account, moderate worst-case explosion pressures are predicted. As a final step of the risk assessment approach, a probabilistic QRA study is performed in which probabilities are assigned to different scenarios, time dependent ignition modelling is applied, and equivalent stoichiometric gas clouds are used to translate reactivity of dispersed non-homogeneous clouds. The probabilistic risk assessment study is based on over 200 dispersion and explosion CFD calculations using the commercially available tool FLACS. The risk assessment suggested a maximum likely pressure level of 0.1–0.3barg at the pressure sensors that were used in the study. Somewhat higher pressures are seen elsewhere due to reflections (e.g. under the vehicles). Several other interesting observations were found in the study. For example, the study suggests that for hydrogen releases the level of longitudinal tunnel ventilation has only a marginal impact on the predicted risk, since the momentum of the releases and buoyancy of hydrogen dominates the mixing and dilution processes. [Copyright &y& Elsevier]

To assist in emergency response decisions and planning in case of releases of pressurized liquefied chlorine from railroad tank cars in industrial sites and cities, the FLACS Computational Fluid Dynamics (CFD) model has been used to simulate the transport and dispersion of the dense chlorine cloud. Two accident locations are studied: an actual railcar accident at an industrial site in Festus, MO, and a hypothetical railcar accident at a rail junction in the Chicago urban area. The results show that transport of a large dense gas release at ground level in an industrial site or large city could initially extend a hundred meters or more in the upwind and crosswind directions. The dense cloud may follow terrain drainage, such as river channels. Near the source, the obstacles tend to slow down the dense gas cloud and may constrain it and cause increased concentrations. Farther downwind, the obstacles may cause enhanced mixing and dilution once the cloud has grown larger. In some cases, significant amounts of cloud mass may become “trapped” in obstacle wakes for many minutes after the main cloud has passed. Although the CFD model can account for the details of the flow and dispersion much better than standard widely-used simple dense gas models, many similarities are found among the various models in their simulated variations with downwind distance of the maximum cloud centerline concentration. [Copyright &y& Elsevier]

The FLACS CFD model, which can be used to estimate the flow and dispersion around buildings and other large roughness obstacles, has been evaluated with tracer data from several field experiments involving obstacle arrays. The following experiments were used in the evaluation exercise: Kit Fox (52 trials with puff and plume releases of slightly dense CO2 gas in arrays of billboard-shaped obstacles), MUST (37 trials with puff releases of neutrally buoyant tracer gas in an array of 120 shipping containers), Prairie Grass (43 trials with continuous plume releases of neutrally buoyant tracer gas over a flat agricultural field), and the EMU L-shaped building (a wind tunnel experiment involving a release from an open door in the courtyard area of an L-shaped building). The primary focus is on the maximum concentration on the monitoring arcs. The performance statistics are consistently fairly good, with a median of 86% of the predictions within a factor of two of the observations, a median relative bias suggesting a 20% underprediction, a median relative scatter of about 50%, and a median 20% underprediction of the overall experiment maximum. These results are all well within the criteria of acceptance for dispersion models. Evaluations with the EMU L-shaped building data show that 72% of FLACS predictions are within a factor of two of observations, and that the model can predict the dimensions of the recirculating cavity behind the building within a factor of two. It is suggested that these extensive data sets involving tracer releases in obstacle arrays be used to evaluate other CFD models. [Copyright &y& Elsevier]

Abstract: One of the main benefits sought by including hydrogen in the alternative fuels mix is emissions reduction – eventually by 100%. However, in the near term, there is a very significant cost differential between fossil fuels and hydrogen. Hythane (a blend of hydrogen and natural gas) can act as a viable next step on the path to an ultimate hydrogen economy as a fuel blend consisting of 8–30% hydrogen in methane can reduce emissions while not requiring significant changes in existing infrastructure. This work seeks to evaluate whether hythane may be safer than both hydrogen and methane under certain conditions. This is due to the fact hythane combines the positive safety properties of hydrogen (strong buoyancy, high diffusivity) and methane (much lower flame speeds and narrower flammability limits as compared to hydrogen). For this purpose, several different mixture compositions (e.g. 8%, 20% and 30% hydrogen) are considered. The evaluation of (a) dispersion characteristics (which are more positive than for methane), (b) combustion characteristics (which are closer to methane than hydrogen), and (c) Combined dispersion + explosion risk is performed. This risk is expected to be comparable to that of pure methane, possibly lower in some situations, and definitely lower than for pure hydrogen. The work is performed using the CFD software FLACS that has been well-validated for safety studies of both natural gas/methane and hydrogen systems. The first part of the work will involve validating the flame speeds and flammability limits predicted by FLACS against values available in literature. The next part of the work involves validating the overpressures predicted by the CFD tool for combustion of premixed mixtures of methane and hydrogen with air against available experimental data. In the end, practical systems such as vehicular tunnels, garages, etc. is used to demonstrate positive safety benefits of hythane with comparisons to similar simulations for both hydrogen and methane. [Copyright &y& Elsevier]

Gas dispersion and explosion dynamics can be very complex. Blast effects in the far field are less sensitive to the local dynamics of an explosion event, and simplified techniques have been developed to roughly estimate the amount of fuel involved (i.e., energy released) in an explosion based on the observed damage in the far field. However, these simple techniques are not suitable to predict the complex explosion dynamics that occur in the near field. The phenomena that drive explosions and resulting damage in the near field are complex in nature, as they depend on non-linear interactions between multiple variables (e.g., ignition location, flame accelerations due to object interaction, fuel reactivity, geometry of the facility regarding confinement and interconnected rooms, vent areas, etc.). Furthermore, the damage created in the near field by overpressure development, blast wave reflections and focusing, pressure impulse, pressure piling, blast wind and ensuing drag forces, may be difficult to interpret using simple methods. The origin of an explosion may, in fact, differ from the intuitive epicenter determined from oversimplified assumptions and may lead to incorrect conclusions regarding the cause and origin or initiating events of the accident. State-of-the-art 3D modeling tools, such as FLACS, are typically required to understand and evaluate complex explosions. FLACS was specifically developed to predict the consequences associated with complex explosions, especially in the near field, and has been extensively validated against hundreds of full-scale experiments. Therefore, FLACS simulations can be used to investigate the chain of events of the explosion and provide a more complete understanding of the evidence, including near-field blast damage. This paper presents the findings of how FLACS was used to help determine the explosion origin in the explosion that occurred at an ink and paint manufacturing facility in Danvers, Massachusetts, on November 22, 2006. This paper is not intended to provide complete investigation findings and will only provide the necessary background material to follow the explosion origin analysis. [ABSTRACT FROM AUTHOR]

Abstract: A benchmarking exercise on quantitative risk assessment (QRA) methodologies for hydrogen safety has been conducted within the project HyQRA, under the framework of the European Network of Excellence (NoE), HySafe. The aim of the exercise was twofold: (i) to identify the differences and similarities in approaches in a QRA and their results for a hydrogen installation and (ii) to identify knowledge gaps in the various steps and parameters underlying the risk quantification of hydrogen safety. First, a reference case was defined for the benchmark: a virtual hydrogen refuelling station (HRS) in virtual surroundings comprising housing, school, shops and other vulnerable objects. For the study, a two phase approach was followed. In phase 1, all nine partners were requested to conduct a QRA according to their usual approach and experience. Basically, participants were free to define representative release cases, to apply models and frequency assessments according their own methodology, and to present risk according to their usual format. To enable inter-comparison, a required set of results data was prescribed, like distances to specific thermal radiation levels from fires and distances to specific overpressure levels. Moreover, complete documentation of assumptions, base data and references was to be reported. It was not surprising that a wide range of results was obtained, both in the applied approaches as well as in the quantitative outcomes and conclusions. This made it difficult to identify exactly which assumptions and parameters were responsible for the differences in results. These results provided the basis for a more guided QRA, the second phase. This phase 2 was defined in which the QRA was determined by a more limited number of release cases (scenarios). The partners in the project agreed to assess specific scenarios in order to identify the differences in consequence assessment approaches. The results of this phase provide a better understanding of the influence of modelling assumptions and limitations on the eventual conclusions with regard to risk to on-site people and to the off-site public. [Copyright &y& Elsevier]

The article discusses the detailed simulations of atmospheric flow and dispersion in downtown Manhattan. The study aims to contribute for planning purposes and for real-time emergency response, which decision makers want to know whether either evacuation or shelter-in-place is required and what areas are affected and for how long a time. It states that the study is a part of a sequence of intensive urban field experiments that have taken place over the past five years, sponsored collaboratively by several agencies.

Reports that low-density radiolarian claystone initiates the plate-boundary decollement, which are shown in northern Barbados accretionary prism borehole logs. Information on decollements; In-depth look at the decollement zone's evolution and initiation; Details on the decollement zone's fluid content and seismic reflections.