Gas explosion analysis of safety gap effect on the innovating FLNG vessel with a cylindrical platform

After investigating gas dispersion on a cylindrical Floating Liquefied Natural Gas (FLNG) platform (Li et al, 2016), this second article focuses on assessment of gas explosion by using Computational Fluid Dynamics (CFD). Gas explosion simulations are carried out to evaluate the explosion overpressure mitigating effect of safety gap. The Data-dump technique, which is an effective tool in resetting turbulence length scale in gas explosion overpressure calculation, is applied to ensure simulation accuracy for the congestion scenario with safety gap. Two sets of different safety gaps are designed to investigate the safety gap on the cylindrical FLNG platform, the overall results indicate that the safety gap is effective in reducing overpressure in two adjacent congestions. However, for the explosion scenario where the flame is propagating through several safety gaps to the far field congestion, the safety gap mitigates overpressure only in certain explosion protecting targets. Two series of artificial configurations are modeled to further investigate the explosion scenarios with more than two safety gaps in one direction. It is concluded that the optimal safety gap design in overpressure mitigation for the cylindrical FLNG platform is to balance the safety gap distance ratio in the congested regions.     

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.     

Accident consequence calculation of ammonia dispersion in factory area

With the widespread use of ammonia in the process industry, more and more accidents were caused by ammonia leakage and dispersion. The dispersion of ammonia is determined by its physical properties, release source conditions and atmospheric environment. Full-scale numerical simulation based on CFD theory was carried out to study the dispersion law of ammonia in a food factory. It was found that ammonia concentrated on the symmetric plane and showed an upward movement near the source. Moreover, the effect of pressure on the dispersion of ammonia was explored showing that the concentration of ammonia near the source increased with the increase of pressure, while the dispersion of ammonia far from the source is mainly influenced by wind field. Last but not the least, the dangerous area completely covers the obstacle region according to the harmful concentration, but the lethal concentration range and explosion range both only existed near the release source. Correspondingly, the concentration of ammonia in the region far from the symmetric plane can be regarded as a safe area. When the accident happens, one should stay away from the release source and evacuate towards the sides in a timely manner. We hope that this work can provide an effective method in predicting the impact of ammonia dispersion and can arouse concerns over the public safety.     

Analyzing effective factors on leakage-induced hydrogen fires

Currently, novel energy resources are receiving increasing attention as a response to the limitation in fossil fuels as well as their adverse effects on human health. Hydrogen, one of the most abundant elements on the earth, can be regarded as a new energy source to replace fossil fuels. Therefore, safety assessment of the relating processes is very crucial by increasing use of hydrogen as a fuel source. In this regard, consequence analysis for risk assessment and power reduction is very important. The present study aims at modeling hydrogen dispersion along with consequence analyses for such events as jet fire and flash fire. The model was validated by using the data derived from a study on hydrogen leakage in supply pipelines in the laboratory of the University of Pisa. Modeling results reveal that ambient conditions will impose a milder impact on leakage consequences if internal pressure is high in release source. The safe distance was also estimated to be 14 m. Dispersion consequence modeling was performed, followed by the evaluation of the effect of environmental (i.e., stability, ambient temperature, surface roughness, wind speed, and humidity) and process (i.e., vessel temperature and pressure, leakage diameter, and releasing point height) parameters on maximum size flammable vapor cloud and maximum level jet fire radiation on the ground. The size of flammable vapor cloud (consequence dispersion index) and the maximum flux of radiation were affected by process parameters more than ambient parameters. Leakage diameter and the vessel pressure were found to have the highest impact on the operational parameters.     

柱形压力容器开口泄爆过程数值模拟研究

为研究柱形压力容器泄爆规律,采用经典流体力学软件FLUENT对典型的柱形压力容器泄爆过程进行数值模拟,分析从泄爆口开启到泄压结束时间段压力发展、火焰传播、气体流动及可燃气体浓度变化特性。结果表明:不同泄爆压力下容器内压力发展变化呈现不同特点,在较小泄爆压力情况下会出现压力再度上升的双峰现象。泄爆过程中产生的湍流沿泄爆口附近容器壁拉长火焰面,并加快燃烧速率。同时就容器内不同点火位置对爆炸强度影响进行研究,得出在泄爆压力为0.04 MPa时,底面点火对本柱形压力容器产生的最大升压速率约为中心点火最大升压速率的1.4倍。     

Methodology for global sensitivity analysis of consequence models

A methodology is presented for global sensitivity analysis of consequence models used in process safety applications. It involves running a consequence model around a hundred times and using the results to construct a statistical emulator, which is essentially a sophisticated curve fit to the data. The emulator is then used to undertake the sensitivity analysis and identify which input parameters (e.g. operating temperature and pressure, wind speed) have a significant effect on the chosen output (e.g. vapour cloud size). Performing the sensitivity analysis using the emulator rather than the consequence model itself leads to significant savings in computing time.To demonstrate the methodology, a global sensitivity analysis is performed on the Phast consequence model for discharge and dispersion. The scenarios studied consist of above-ground, horizontal, steady-state discharges of dense-phase carbon dioxide (CO₂), with orifices ranging in diameter from ½ to 2 inch and the liquid CO₂ stagnation conditions maintained at between 100 and 150 bar. These scenarios are relevant in scale to leaks from large diameter above-ground pipes or vessels.Seven model input parameters are varied: the vessel temperature and pressure, orifice size, wind speed, humidity, ground surface roughness and height of the release. The input parameters that have a dominant effect on the dispersion distance of the CO₂ cloud are identified, both in terms of their direct effect on the dispersion distance and their indirect effect, through interactions with other varying input parameters.The analysis, including the Phast simulations, runs on a standard office laptop computer in less than 30 min. Tests are performed to confirm that a hundred Phast runs are sufficient to produce an emulator with an acceptable degree of accuracy. Increasing the number of Phast runs is shown to have no effect on the conclusions of the sensitivity analysis.The study demonstrates that Bayesian analysis of model sensitivity can be conducted rapidly and easily on consequence models such as Phast. There is the potential for this to become a routine part of consequence modelling.     

石化行业控制室承爆风险评估方法研究

针对传统的气体爆炸风险评估方法的不足之处,提出采用一种基于CFD技术的气体爆炸风险评估方法,对某煤气化厂区氢气爆炸对控制室造成的风险进行模拟计算与预测分析。并把研究结果与传统的TNT当量法、Multi-Energy方法预测结果进行比较。结果表明,该方法能考虑到密集管道与复杂装置布局、气云大小等因素对爆炸超压的影响,且能用于超压波的近场预测,以及确定空间不同位置处的爆炸超压,更适用于石化行业控制室的承爆风险评估。     

高压天然气非恒定速率泄漏扩散数值模拟研究

为保障天然气工业安全生产与运营,以某天然气储配厂为例,采用等效喷嘴和过程模型,利用FLACS软件对罐区高压天然气非恒定速率泄漏扩散进行数值模拟,考察环境风速及泄漏时间对气体泄漏扩散的影响.结果表明:储存压力为1.05 MPa的天然气储罐发生泄漏会产生欠膨胀射流,泄漏初期具有447.44 kJ的高动能,并在近场扩散起主导...     

Estimation of safety distances in the vicinity of fuel gas pipelines

In this paper, safety distances around pipelines transmitting liquefied petroleum gas and pressurized natural gas are determined considering the possible outcomes of an accidental event associated with fuel gas release from pressurized transmission systems. Possible outcomes of an accidental fuel gas release were determined by performing the Event Tree Analysis approach. Safety distances were computed for two pipeline transmission systems of pressurized natural gas and liquefied petroleum gas existing in Greece using real data given by Greek Refineries and the Greek Public Gas Enterprise. The software packages chetah and breeze were used for thermochemical mixture properties estimation and quantitative consequence assessment, respectively. Safety distance determination was performed considering jet fire and gas dispersion to the lower flammable limit as the worst-case scenarios corresponding to immediate and delayed cloud ignition. The results showed that the jet fire scenario should be considered as the limiter for safety distances determination in the vicinity of natural and petroleum gas pipelines. Based on this conclusion, the obtained results were further treated to yield functional diagrams for prompt safety distance estimation. In addition, qualitative conclusions were made regarding the effect of atmospheric conditions on possible events. Thus, wind velocity was found to dominate during a jet fire event suppressing the thermal radiation effect, whereas gas dispersion was found to be affected mainly by solar radiation that favors the faster dissolution of fuel gas below the lower flammable limit.     

A CFD based explosion risk analysis methodology using time varying release rates in dispersion simulations

A full probabilistic Explosion Risk Analysis (ERA) is commonly used to establish overpressure exceedance curves for offshore facilities. This involves modelling a large number of gas dispersion and explosion scenarios. Capturing the time dependant build up and decay of a flammable gas cloud size along with its shape and location are important parameters that can govern the results of an ERA. Dispersion simulations using Computational Fluid Dynamics (CFD) are generally carried out in detailed ERA studies to obtain these pieces of information. However, these dispersion simulations are typically modelled with constant release rates leading to steady state results. The basic assumption used here is that the flammable gas cloud build up rate from these constant release rate dispersion simulations would mimic the actual transient cloud build up rate from a time varying release rate. This assumption does not correctly capture the physical phenomena of transient gas releases and their subsequent dispersion and may lead to very conservative results. This in turn results in potential over design of facilities with implications on time, materials and cost of a project.In the current work, an ERA methodology is proposed that uses time varying release rates as an input in the CFD dispersion simulations to obtain the fully transient flammable gas cloud build-up and decay, while ensuring the total time required to perform the ERA study is also reduced. It was found that the proposed ERA methodology leads to improved accuracy in dispersion results, steeper overpressure exceedance curves and a significant reduction in the Design Accidental Load (DAL) values whilst still maintaining some conservatism and also reducing the total time required to perform an ERA study.     

Accuracy improvement in evaluation of gas explosion overpressures in congestions with safety gaps

This paper reports a comparison of simulations and published data from experiments carried out by TNO Prins Maurits Laboratory on geometric configurations that involved safety gaps of various separation distances. The Computational Fluid Dynamics (CFD) based software – FLACS is utilized to conduct the numerical simulations. In the majority of cases, good agreement is found between the simulated results and those obtained by experiment in both the donor and acceptor modules. However, a large discrepancy in the overpressures in the acceptor module is seen when the size of the separation gap approaches one or two times of the module size. A Data-dump technique is used in this study to reset the turbulence length scale for these cases with different separation distances, five sets of explosion scenarios are then numerically simulated and the overpressures are compared with experimentally measured explosion overpressures. The overall results indicate that the software with the Data-dump technique is still an extremely effective tool when it comes to the evaluation of gas explosion overpressures in areas with large separation gaps.     

高压管道天然气泄漏扩散过程的数值模拟

采用CFD模型的方法对高压管道内的天然气泄漏和扩散过程进行了数值模拟。其结果表明,从高压管道泄出的天然气在大气中主要表现为高速射流的泄漏过程和随后的扩散过程。在泄漏过程中,天然气在泄漏口附近为欠膨胀射流,整个泄漏过程具有一定的高度;在扩散过程中,天然气在浮力作用下以向上扩散的形式发展。研究了不同环境风速对扩散过程的影响,较大的风速可以使天然气向下风方向更远的距离扩散,从而增大了天然气爆炸危险浓度的范围。研究结果可     

Forced ventilation effect by Air-Fin-Cooler on flammable gas cloud dispersion

This paper is the second installment of a paper published on Process Safety and Environment Protection in 2013, which evaluates the Air-Fin-Cooler (AFC) forced ventilation effect over natural ventilation inside congested LNG process train, i.e., modularized LNG, considering the Air Change per Hour (ACH) using Computational Fluid Dynamics (CFD) analysis. This second paper evaluates the effect of forced ventilation on gas cloud dispersion using CFD in order to evaluate possible design measures, such as safety distance in trains and whether to shut down the AFC in case of releases. The results of this evaluation show that gas cloud accumulation is reduced by AFC induced air flow in the case of shorter separation distances between modules. Based on the results, two design measures are proposed, i.e., keep AFC running during emergency and train orientation against prevailing wind direction.     

Influence of dustiness on small-scale vented dust explosions

A new safety characteristic the “dustiness” according to VDI 2263 – part 9 (Verein Deutscher Ingenieure, 2008) is investigated. Dustiness means the tendency of a dust to form clouds. The paper deals with the physical reasons for the different behavior of dusts, even if they have similar properties such as particle size and density and the influence of the dustiness on dust explosions. In order to study the effects of the dustiness on dust cloud formation for different dispersion methods experiments in a vertical dust dispersion glass tube apparatus were carried out. Furthermore vented dust explosion experiments were done for two different dispersion methods and two static activation pressures.Experiments show that particle size and density are not the only factors which influence dispersibility. Particle shape, specific surface area, flow and dispersion method have an influence which can outweigh size and density. Preliminary explosion experiments showed that the dustiness has an influence on the reduced explosion pressure and flame speed in a vented 75 L test apparatus. In order to verify the results for applications in the process industries further tests with industrial scale experiments are planned.     

基于稳定风场的埋地天然气管道泄漏数值模拟

针对目前城镇埋地管道天然气泄漏研究模拟工况简单、可信性较低等问题,考虑障碍物对环境风场的影响,利用计算流体力学（CFD）软件建立天然气管道三维泄漏模型,将模拟过程分为环境风场的稳态模拟和管道泄漏扩散的瞬态模拟两步,分析天然气泄漏扩散规律。结果表明:在风场稳态模拟中,建筑物附近风场受干扰明显,上游形成小范围的低速滞留区,下游形成较长的尾迹。在天然气泄漏扩散瞬态模拟中,土壤层天然气受风速影响较小,气体在近地面及贴近建筑物侧积聚,扩散范围随时间逐渐趋于稳定,泄漏扩散达到稳定后表现出土壤层积聚、气云沉降、贴近建筑物积聚、气云扩散局限性的特征。风速主要影响天然气的扩散高度,对水平方向的扩散范围影响较小,风速与天然气扩散高度成反比。     

Heavy gas dispersion by water spray curtains: A research methodology

The mitigation of the consequences of accidental releases of dangerous toxic and/or flammable cloud is a serious concern in the petro-chemical and gas industries. Nowadays, the water-curtain is recognized as a useful technique to mitigate a heavy gas cloud. The paper presents a research methodology, which has been established and undertaken to quantify the forced dispersion factor provided by a water-curtain with respect to its configuration.

The method involves medium-scale field tests, Wind-Gallery tests and numerical simulations. These different approaches are discussed and exemplified by typical results emphasizing the observed concentration reduction due to the water-curtain.     

Dynamic risk analysis of offloading process in floating liquefied natural gas (FLNG) platform using Bayesian Network

The growing demand for natural gas has pushed oil and gas exploration to more isolated and previously untapped regions around the world where construction of LNG processing plants is not always a viable option. The development of FLNG will allow floating plants to be positioned in remote offshore areas and subsequently produce, liquefy, store and offload LNG in the one position. The offloading process from an FLNG platform to a gas tanker can be a high risk operation. It consists of LNG being transferred, in hostile environments, through loading arms or flexible cryogenic hoses into a carrier which then transports the LNG to onshore facilities. During the carrier's offloading process at onshore terminals, it again involves risk that may result in an accident such as collision, leakage and/or grounding. It is therefore critical to assess and monitor all risks associated with the offloading operation. This study is aimed at developing a novel methodology using Bayesian Network (BN) to conduct the dynamic safety analysis for the offloading process of an LNG carrier. It investigates different risk factors associated with LNG offloading procedures in order to predict the probability of undesirable accidents. Dynamic failure assessment using Bayesian theory can estimate the likelihood of the occurrence of an event. It can also estimate the failure probability of the safety system and thereby develop a dynamic failure assessment tool for the offloading process at a particular FLNG plant. The main objectives of this paper are: to understand the LNG offloading process, to identify hazardous events during offloading operation, and to perform failure analysis (modelling) of critical accidents and/or events. Most importantly, it is to evaluate and compare risks. A sensitivity analysis has been performed to validate the risk models and to study the behaviour of the most influential factors. The results have indicated that collision is the most probable accident to occur during the offloading process of an LNG carrier at berth, which may have catastrophic consequences.     

CFD simulations of dust dispersion in the 20 L vessel: Effect of nominal dust concentration

Measurements of flammability and explosion parameters for dust/air mixtures require uniform dispersion of the dust cloud inside the test vessel. In a previous work, we showed that, in the standard 20 L sphere, the dust injection system does not allow generation of a uniform cloud, but rather high gradients of dust concentration are established. In this work, we used a previously validated three-dimensional CFD model to simulate the dust dispersion inside the 20 L sphere at different dust nominal concentrations (and fixed dust diameter). Results of numerical simulations have shown that, as the dust nominal concentration is increased, sedimentation prevails and, thus, when ignition is provided, the dust is mainly concentrated at the vessel walls.     

Risk assessment of major hazards: the effect of ground slope

Liquefied gases, such as chlorine and ammonia, are stored in large quantities at industrial sites. If released accidentally, they form a heavy gas cloud that has the potential to kill or injure large numbers of people. The dispersion of such a cloud is thus of interest to the risk assessment community [Nussey, Pantony, & Smallwood, 1992. HSE’s risk assessment tool, RISKAT. In: Major Hazards: Onshore and Offshore. pp. 607–638].Little is understood about the effect of slope on risk. Here, the risk (probability) of being exposed to the gas cloud, given a release, is considered; probability language is needed because wind direction is assumed to be a random variable.This paper shows how the risk of being exposed to toxic gas released over a slope may be estimated using simple physical modelling.The physical model used is that of Tickle [J. Hazard. Mater. 49 (1996) 29], who showed that a finite-volume instantaneous release on an inclined plane can form a stable wedge-shaped cloud that moves down the line of greatest slope. Nonzero windspeeds are accounted for by following Tickle’s suggestion of vectorially adding windspeed to the advection induced by the slope.A range of windspeeds and slopes are considered. The slopes substantially affect the risk in the sense that the predicted risk contours are far from circularly symmetric.