Real-time risk assessment of explosion on offshore platform using Bayesian network and CFD

Combustion or explosion accident resulting from accidental hydrocarbon release poses a severe threat to the offshore platform's operational safety. Much attention has been paid to the risk of an accident occurring over a long period, while the real-time risk that escalates from a primary accident to a serious one was ignored. In this study, a real-time risk assessment model is presented for risk analysis of release accidents, which may escalate into a combustion or explosion. The proposed model takes advantage of Fault Tree-Event Tree (FT-ET) to describe the accident scenario, and Bayesian network (BN) to obtain the initial probability of each consequence and describe the dependencies among safety barriers. Besides, Computational Fluid Dynamics (CFD) is applied to handle the relationship between gas dispersion and time-dependent risk. Ignition probability model that considering potential ignition sources, gas cloud, and time series are also integrated into this framework to explain the likelihood of accident evolution. A case of release accidents on a production platform is used to test the availability and effectiveness of the proposed methodology, which can be adopted for facilities layout optimization and ignition sources control.     

CFD modelling of hydrogen release, dispersion and combustion for automotive scenarios

The paper describes the analysis of the potential effects of releases from compressed gaseous hydrogen systems on commercial vehicles in urban and tunnel environments using computational fluid dynamics (CFD). Comparative releases from compressed natural gas systems are also included in the analysis.

This study is restricted to typical non-articulated single deck city buses. Hydrogen releases are considered from storage systems with nominal working pressures of 20, 35 and 70 MPa, and a comparative natural gas release (20 MPa). The cases investigated are based on the assumptions that either fire causes a release via a thermally activated pressure relief device(s) (PRD) and that the released gas vents without immediately igniting, or that a PRD fails. Various release strategies were taken into account. For each configuration some worst-case scenarios are considered.

By far the most critical case investigated in the urban environment, is a rapid release of the entire hydrogen or natural gas storage system such as the simultaneous opening of all PRDs. If ignition occurs, the effects could be expected to be similar to the 1983 Stockholm hydrogen accident [Venetsanos, A. G., Huld, T., Adams, P., & Bartzis, J. G. (2003). Source, dispersion and combustion modelling of an accidental release of hydrogen in an urban environment. Journal of Hazardous Materials, A105, 1–25]. In the cases where the hydrogen release is restricted, for example, by venting through a single PRD, the effects are relatively minor and localised close to the area of the flammable cloud. With increasing hydrogen storage pressure, the maximum energy available in a flammable cloud after a release increases, as do the predicted overpressures resulting from combustion. Even in the relatively confined environment considered, the effects on the combustion regime are closer to what would be expected in a more open environment, i.e. a slow deflagration should be expected.

Among the cases studied the most severe one was a rapid release of the entire hydrogen (40 kg) or natural gas (168 kg) storage system within the confines of a tunnel. In this case there was minimal difference between a release from a 20 MPa natural gas system or a 20 MPa hydrogen system, however, a similar release from a 35 MPa hydrogen system was significantly more severe and particularly in terms of predicted overpressures. The present study has also highlighted that the ignition point significantly affects the combustion regime in confined environments. The results have indicated that critical cases in tunnels may tend towards a fast deflagration, or where there are turbulence generating features, e.g. multiple obstacles, there is the possibility that the combustion regime could progress to a detonation.

When comparing the urban and tunnel environments, a similar release of hydrogen is significantly more severe in a tunnel, and the energy available in the flammable cloud is greater and remains for a longer period in tunnels. When comparing hydrogen and natural gas releases, for the cases and environments investigated and within the limits of the assumptions, it appears that hydrogen requires different mitigation measures in order that the potential effects are similar to those of natural gas in case of an accident. With respect to a PRD opening strategy, hydrogen storage systems should be designed to avoid simultaneous opening of all PRD, and that for the consequences of the released energy to be mitigated, either the number of PRDs opening should be limited or their vents to atmosphere should be restricted (the latter point would require validation by a comprehensive risk assessment).     

An integrated quantitative hazard analysis method for natural gas jet release from underground gas storage caverns in salt rock. I: Models and validation

It is very important and necessary to perform quantitative hazard analysis for possible accidental leakage from an underground gas storage cavern in salt rock. An integrated quantitative hazard analysis method for natural gas jet release from salt caverns is presented in this paper, which was constituted by a revised model for gas leakage rate calculation, a consequence analysis and a model of probability assessment for harm. The presented method was validated by comparing the analytical results with the data collected from the real accidents (including the leakage, jet fire, fireball and vapor cloud explosion). It is indicated that the proposed method was more accurate than the TNT equivalence method for vapor cloud explosion and gave more reasonable results when applied to the consequence analysis for the thermal radiation from jet fire and fireball.     

基于CRITIC-有限区间云模型的边坡稳定性评价

针对岩质边坡危险性分级的不确定性,选取坡高、坡角岩体结构特征、岩石单轴抗压强度等12项定量评价指标构建评价体系。根据有限区间云模型的相关概念和计算模型,求出露天采场边坡实测数据的云模型特征参数,利用正向云发生器生成云滴图进行量性概念的转化,通过改进的CRITIC法确定指标权重,进而求出不同采场隶属于不同危险等级的综合隶属度,实现边坡稳定性等级的划分。研究结果表明:评价结果与实际相符,利用CRITIC算法求取的权重值,可以考虑各评价指标的综合信息量以及指标间的相关性系数,使评判结果更具有准确性;同时可以快速准确判断出边坡稳定性分级,进行安全预测,为评价边坡的稳定性提供了新思路。     

A stochastic approach for risk analysis in vapor cloud explosion

A stochastic approach for evaluating the risk of vapor cloud explosions is proposed in this work. The proposed methodology aims to incorporate the effect of uncertainty into the risk analysis to produce a better overall view for the risk. Some stochastic variables are used to estimate the probability of vapor cloud explosions: frequency of the release, the probability of not having an immediate ignition, the probability of delayed ignition and the probability of a vapor cloud explosion given a delayed ignition, as well as different possible meteorological conditions. These stochastic variables are represented with probability distribution curves. Different curves for the frequencies of releases from process equipment types (steel process pipes, flanges, manual valves, actuated valves, etc.), different equipment diameters and different leak sizes are also used in this analysis. Monte Carlo simulation is performed to obtain the risk as a probability distribution using the Analytic Solver Platform. Then the risk distribution curve obtained by Monte Carlo simulation is used to estimate the probability of satisfying the risk tolerance criterion.     

燃气管网定量风险分析方法综述

以城市燃气管网的风险为研究对象,分析并提出一种可用于城市燃气管网定量风险分析的新思路,包含了不同事故后果及其物理模型的分析即事故可能性分析、后果分析和风险评价,分为失效事故假定、泄漏率计算、物理效应计算、致死率计算、风险值计算、风险评价等环节;整理、研究城市燃气管网定量风险分析所涉及的多种物理模型,并通过比较不同模型的特点,分析各个模型的不足之处;最后针对国内外研究现状及燃气管网风险的特点,指出研究发展方向:研究风险在燃气管网内的传播,提出燃气管网相继失效的风险分析方法。所提出的分析思路、计算方法可与工程应用相结合。     

Risk assessment methodology for high-pressure CO2 pipelines incorporating topography

This paper presents a risk assessment methodology for high-pressure CO₂ pipelines developed at the Health and Safety Laboratory as part of the EU FP7 project CO2Pipehaz.Traditionally, consequence modelling of dense gas releases from pipelines at major hazard impact levels is performed using integral models with limited or no consideration being given to weather bias or topographical features of the surrounding terrain. Whilst dispersion modelling of CO₂ releases from pipelines using three-dimensional CFD models may provide higher levels of confidence in the predicted behaviour of the cloud, the use of such models is resource-intensive and usually impracticable. An alternative is to use more computationally efficient shallow layer or Lagrangian dispersion models that are able to account for the effects of topography whilst generating results within a reasonably short time frame.In the present work, the proposed risk assessment methodology for CO₂ pipelines is demonstrated using a shallow-layer dispersion model to generate contours from a sequence of release points along the pipeline. The simulations use realistic terrain taken from UK topographical data. Individual and societal risk levels in the vicinity of the pipeline are calculated using the Health and Safety Laboratory's risk assessment tool QuickRisk.Currently, the source term for a CO₂ release is not well understood because of its complex thermodynamic properties and its tendency to form solid particles under specific pressure and temperature conditions. This is a key knowledge gap and any subsequent dispersion modelling, particularly when including topography, may be affected by the accuracy of the source term.     

Accident modeling of toxic gas-containing flammable gas release and explosion on an offshore platform

Toxic gas-containing flammable gas leak can lead to poisoning accidents as well as explosion accidents once the ignition source appears. Many attempts have been made to evaluate and mitigate the adverse effects of these accidents. All these efforts are instructive and valuable for risk assessment and risk management towards the poisoning effect and explosion effect. However, these analyses assessed the poisoning effect and explosion effect separately, ignoring that these two kinds of hazard effects may happen simultaneously. Accordingly, an integrated methodology is proposed to evaluate the consequences of toxic gas-containing flammable gas leakage and explosion accident, in which a risk-based concept and the grid-based concept are adopted to combine the effects. The approach is applied to a hypothetical accident scenario concerning an H₂S-containing natural gas leakage and explosion accident on an offshore platform. The dispersion behavior and accumulation characteristics of released gas as well as the subsequent vapor cloud explosion (VCE) are modeled by Computational Fluid Dynamics (CFD) code Flame Acceleration Simulator (FLACS). This approach is concise and efficient for practical engineering applications. And it helps to develop safety measures and improve the emergency response plan.     

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.     

Numerical simulation of LNG dispersion under two-phase release conditions

The use of LNG (liquefied natural gas) as fuel brings up issues regarding safety and acceptable risk. The potential hazards associated with an accidental LNG spill should be evaluated, and a useful tool in LNG safety assessment is computational fluid dynamics (CFD) simulation. In this paper, the ADREA-HF code has been applied to simulate LNG dispersion in open-obstructed environment based on Falcon Series Experiments. During these experiments LNG was released and dispersed over water surface. The spill area is confined with a billboard upwind of the water pond. FA1 trial was chosen to be simulated, because its release and weather conditions (high total spill volume and release rate, low wind speed) allow the gravitational force to influence the cold, dense vapor cloud and can be considered as a benchmark for LNG dispersion in fenced area. The source was modeled with two different approaches: as vapor pool and as two phase jet and the predicted methane concentration at sensors' location was compared with the experimental one. It is verified that the source model affect to a great extent the LNG dispersion and the best case was the one modeling the source as two phase jet. However, the numerical results in the case of two phase jet source underestimate the methane concentration for most of the sensors. Finally, the paper discusses the effect of neglecting the ?9.3° experimental wind direction, which leads to the symmetry assumption with respect to wind and therefore less computational costs. It was found that this effect is small in case of a jet source but large in the case of a pool source.     

城镇燃气管道动火作业安全评价

针对城镇燃气管道动火作业的高危险性,为保障生命财产安全,提出一种基于未确知测度的安全评价方法。首先,从动火施工作业的前、中、后3个阶段对整个作业过程进行安全分析,并基于作业的前阶段建立安全评价体系。然后,引入未确知测度模型,将因素的量化值与测度函数结合得出测度评价矩阵,采用变异系数法处理评价矩阵,确定各危害因素的客观权重。最后,根据测度评价矩阵与因素权重得出动火作业的安全等级,并对危害因素进行排序。实例结果表明:应用该方法能有效地确定出城镇燃气管道动火作业的安全等级,对于不安全的施工作业,可根据排序确定高危险因素,并采取措施降低其危险性,提高整个动火作业过程的安全性。     

城镇燃气管道第三方施工损伤风险评价方法研究

针对第三方施工对城镇燃气管道的不利影响,提出一种半定量风险评价方法。首先,基于“人—机—环境—管理”系统确定失效因素,利用G2赋权法对因素赋权,并引入物元模型分析失效可能性;然后,建立喷射火焰、闪火和蒸气云爆炸的数学模型计算管道泄漏的伤害面积,确定失效后果等级;最后,根据API 581中的风险矩阵得出燃气管道第三方施工破坏风险等级。实例分析表明,该方法兼具定性评价与定量评价的优点,能更加准确有效地进行风险分析,确定风险等级。     

Societal risk acceptance criteria for gas distribution pipelines based on incident data from the United States

A significant number of pipeline operators use pipeline integrity management (PIM) to improve pipeline safety and reliability. Risk assessment is a critical step in PIM, because it determines the necessity of conducting the following steps in PIM for certain pipelines. Risk acceptance criteria are required in the process of risk assessment. Individual risk and societal risk are most frequently adopted as the two indicators of the risk acceptance criteria. To the best of the authors’ knowledge, quantitative societal risk acceptance criteria, especially for gas distribution pipelines, do not exit. The aim of this paper is to establish the societal risk acceptance criteria for gas distribution pipelines. Hence, FN curves were established using historical incident data from 2002 to 2017 provided by the U.S. Department of Transportation (DOT). Linear regression and the ALARP principle are used in evaluating the limits of the negligible line and intolerable line to obtain a graphical societal risk acceptance criterion for gas distribution pipelines. A line having a slope of −1.224, and an anchor point of (1, 8.413 × 10⁻⁷) is proposed as the negligible line. Further, the intolerable line has a slope of −1.224, and an anchor point of (1, 2.524 × 10⁻⁶). Both the negligible risk and the intolerable risk for the gas distribution pipeline are lower than the current societal risk acceptance criteria for hazardous installations. The reasons for these relatively lower risk acceptance criteria are discussed.     

高含硫气田集气站设备定量风险评价

为预防高含硫天然气泄漏事故和制定设备检测策略,对集气站设备进行定量风险评价。利用DNV Leak数据库得到集气站关键设备的同类失效频率,利用API 581标准对同类失效频率修正得到失效频率。用PHAST软件计算4种典型泄漏孔尺寸的燃烧爆炸和毒性的影响区域范围,确定设备的燃烧爆炸风险和毒性风险。结果表明,利用该方法能够确定设备风险大小顺序。     

重大事故后果模拟法在天然气场站安全评价中的应用

场站是气田集输的枢纽,也是高风险存在和集中的场所。本论文采用重大事故后果模拟分析法建立天然气泄漏速率估算模型、蒸气云爆炸模型,并以陕北某一天然气场站为例,借助Risk System软件对天然气泄漏速率和泄漏量以及蒸气云爆炸最大事故进行数值模拟。模拟分析结果有助于对天然气场站可能发生的各种事故进行风险评价,也有助于对各类风险大的危险危害因素提出对应的安全对策措施,确保安全生产。     

工业危险源仿真预警技术

概述了应用数字仿真技术于液化天然气风险评价、蒸汽云爆炸后果预测以及液氨储罐泄漏监控预警系统等的国内外研究与发展现状，给出了计算机仿真结果和现场实测数据之间的对比曲线。     

基于云模型的海上交通系统风险蒙特卡罗仿真

为扩展海上交通系统(MTS)风险的数据样本,更好地开展海上交通系统风险评估研究,有必要研究系统风险仿真中的模型与算法问题。在讨论不确定性信息中随机性和模糊性的基础上,引入云模型来取代概率分布参数下的蒙特卡罗仿真模型。经对无确定度的原始云特征参数求解,获得海上交通系统风险的还原云滴,有效增加海上交通系统风险数据样本,进而分析出风险数值仿真下的海上交通系统风险特征。通过有效性检验,云模型下仿真的变异系数比基于对数正态分布下仿真的结果更稳定。云模型下的仿真结果更准确地逼近原始样本的特性。风险的模糊性和随机性信息量化问题得到解决。     

Dynamic risk assessment for underground gas storage facilities based on Bayesian network

Loss of the underground gas storage process can have significant effects, and risk analysis is critical for maintaining the integrity of the underground gas storage process and reducing potential accidents. This paper focuses on the dynamic risk assessment method for the underground gas storage process. First, the underground gas storage process data is combined to create a database, and the fault tree of the underground gas storage facility is built by identifying the risk factors of the underground gas storage facility and mapping them into a Bayesian network. To eliminate the subjectivity in the process of determining the failure probability level of basic events, fuzzy numbers are introduced to determine the prior probability of the Bayesian network. Then, causal and diagnostic reasoning is performed on the Bayesian network to determine the failure level of the underground gas storage facilities. Based on the rate of change of prior and posterior probabilities, sensitivity and impact analysis are combined to determine the significant risk factors and possible failure paths. In addition, the time factor is introduced to build a dynamic Bayesian network to perform dynamic assessment and analysis of underground gas storage facilities. Finally, the dynamic risk assessment method is applied to underground gas storage facilities in depleted oil and gas reservoirs. A dynamic risk evaluation model for underground gas storage facilities is built to simulate and validate the dynamic risk evaluation method based on the Bayesian network. The results show that the proposed method has practical value for improving underground gas storage process safety.     

Dynamic probability assessment of urban natural gas pipeline accidents considering integrated external activities

Urban gas pipelines usually have high structural vulnerability due to long service time. The locations across urban areas with high population density make the gas pipelines easily exposed to external activities. Recently, urban pipelines may also have been the target of terrorist attacks. Nevertheless, the intentional damage, i.e. terrorist attack, was seldom considered in previous risk analysis of urban gas pipelines. This work presents a dynamic risk analysis of external activities to urban gas pipelines, which integrates unintentional and intentional damage to pipelines in a unified framework. A Bayesian network mapping from the Bow-tie model is used to represent the evolution process of pipeline accidents initiating from intentional and unintentional hazards. The probabilities of basic events and safety barriers are estimated by adopting the Fuzzy set theory and hierarchical Bayesian analysis (HBA). The developed model enables assessment of the dynamic probabilities of consequences and identifies the most credible contributing factors to the risk, given observed evidence. It also captures both data and model uncertainties. Eventually, an industrial case is presented to illustrate the applicability and effectiveness of the developed methodology. It is observed that the proposed methodology helps to more accurately conduct risk assessment and management of urban natural gas pipelines.