Experimental research of LNG accidental underwater release and combustion behavior

Evaluating potential hazards caused by accidental LNG release from underwater pipelines or vessels is a significant consideration in marine transportation safety. The aim of this study was to capture the dynamic behavior of LNG jet released under water and to analyze its vapor dispersion characteristics and combustion characteristics on the water surface during different release scenarios. Controlled experiments were conducted where LNG was jet released from a cryogenic storage tank. The dynamic process of LNG being jet released from orifices of different sizes and shapes, as well as the rising plume structure, were captured by a high-speed camera. The leakage flow rate and pipeline pressure were recorded by a flow meter and pressure gauge, respectively. The concentration distribution that emanated from the water surface was measured utilizing methane sensors in different positions with various wind speeds. The flame combustion characteristics of LNG vapor clouds, which immediately ignited upon the enclosed water tank, were also recorded. Additionally, the mass burning rate of the flame on the water surface was evaluated, and a new correlation between the ratio of flame length and width was established. The results indicated a large dimensionless heat release rate (Q*) and a continuous release flow rate in a limited burning area. This study could provide greater understanding of the mechanisms of LNG release and combustion behavior under water.     

The effect of orifice plates with different shapes on explosion propagation of premixed methane–air in a semi-confined pipeline

The effect of internal shape of obstacles on the deflagration of premixed methane–air (concentration of 10%) was experimentally investigated in a semi-confined steel pipeline (with a square cross section size of 80 mm × 80 mm and 4 m long). The obstacles used in this study were circular, square, triangular and gear-shaped (4-teeth, 6-teeth and 8-teeth) orifice plates with a blockage ratio of 75%, and the perimeter of the orifice was regarded as a criterion for determining the sharpness of the orifice plate. The overpressure history, flame intensity histories, flame front propagation speed, maximum flame intensity and peak explosion overpressure were analyzed. The explosion in the pipeline can be divided into two stages: initial explosion and secondary explosion. The secondary explosion is caused by recoiled flame. The perimeter is positively related to the intensity of the recoiled flame and the ability of orifice plate to suppress the explosion propagation. In addition, the increase in the perimeter will cause the acceleration of the flame passing through the orifice plate, while after the perimeter of the orifice reaches a certain value, the effect of the increase in perimeter on explosion excitation becomes no obvious. The overpressure (static pressure) downstream of the orifice plate is the result of the combined effect of explosion intensity and turbulence. The increase in perimeter leads to the increase in turbulence downstream of the orifice plate which in turn causes more explosion pressure to be converted into dynamic pressure.     

A mechanistic model for hydrocarbon plumes rising through water

For releases of hydrocarbons from a subsea pipeline, riser, or production facility, the shape of the plume rising through the water must be predicted prior to any assessment of gas dispersion, liquid pools, or fire above the water surface. The location and size of the plume at the water surface are key parameters for subsequent consequence modeling. A mechanistic model has been developed to predict the plume trajectory and size, based on mass and momentum balances and an empirical water entrainment ratio from the literature. With suitable physical property values available, the model is applicable to releases of gas and/or liquid hydrocarbons, predicting the vaporization and vapor expansion due to decreasing hydrostatic pressure as the plume rises through the water. Some validation of the model was obtained with 16 tests in a small-scale transparent tank. The data cover a wide range of flow rates, including both choked and unchoked flow. The predicted and measured trajectories (centerline displacement) agreed reasonably well. Predictions of the model are presented for three fluids. The model is valuable for assessing the consequences of underwater hydrocarbon releases, providing input for subsequent modeling of gas dispersion or liquid pools and pool fires.     

城市综合管廊燃气舱室输气管道泄漏扩散规律研究

为了掌握输气管道在城市综合管廊舱室泄漏扩散的基本规律,采用FLUENT软件,针对管廊正常通风—泄漏报警—事故通风—警报解除的全过程进行动态分析。首先在正常通风速度建立的稳态风场中,模拟天然气在不同管输压力下发生小孔泄漏后的报警时间,根据首个响应的报警器的位置判断泄漏源位置。结果表明,当泄漏孔径为20 mm,通风速度为1.92 m/s,且泄漏源处于2个报警器中间时,管输压力为200,400,800 kPa时对应的报警时间分别为10.4,6.7,4.5 s。事故通风速度下,对不同管输压力的天然气扩散进行分析,当天然气朝逆风侧扩散时,随动量逐渐减小而到达不同的边界坐标。同时,环境大气压的降低不仅会缩短报警器的首次报警时间,还能延长总扩散距离。预测所得的天然气爆炸上下限浓度区移动速度有助于动态了解处于爆炸上下限浓度之间气体的实时位置。解除报警时间与进风口风速呈近似线性关系,可为现场救援队伍选择经济通风量提供理论指导。     

Fracture criterion and control plan on CO2 pipelines: Theory analysis and full-bore rupture (FBR) experimental study

Pressurized pipelines are the most reliable and cost-effective option for the long-distance transportation of CO₂ from an emitter to an onshore storage site. Propagating or unstable factures are considered catastrophic pipeline failures, resulting in a massive escape of inventory within a short period of time. The decompression curve for CO₂ exhibits a large drop in decompression wave speed at the phase transition pressure, leading to a higher driving force for crack propagation. The study of fracture control plans is very important for assessing the possibility of fracture propagation and preventing unstable fracturing along CO₂ pipelines. Three full-bore rupture (FBR) experiments were performed using an industrial-scale (258 m long, 233 mm inner diameter) CO2 pipeline with initial CO2 states of gaseous, dense and supercritical phases, respectively. The relation between the decompression velocity and the pipeline fracture propagation velocity was analyzed during the process of buried CO2 pipeline release. A fracture propagation criterion was established for the buried CO₂ pipeline. For the gaseous CO₂ leakage, the pressure plateau corresponding to the decompression wave velocity only appeared near the closed end of the pipeline. For the dense CO₂ leakage, the pressure plateau corresponding to the decompression wave velocity was observed near the saturation pressure after rapid decompression. For the supercritical CO₂ leakage, the pressure plateau corresponding to the decompression wave velocity was observed in the stage when the supercritical CO₂ transformed into the two phases of gas and liquid. Compared with the gaseous and dense CO₂, for the supercritical CO_2, the initial decompression wave velocity was the smallest, and the requirement of the pipeline safety factor was the highest.     

压力容器气体非稳态泄漏模型研究

为计算气体在非稳态泄漏过程中的泄漏率,提高危害后果评估的量化水平,对压力容器失效后气体泄漏过程进行了研究。基于现有的初始泄漏率模型,结合实际泄漏过程中压力容器内各项状态参数的动态变化规律,构建气体非稳态泄漏模型,并通过计算实例进行分析和验证。结果表明,该模型可计算压力容器气体非稳态泄漏过程中(包括音速泄漏阶段和亚音速泄漏阶段)任意时刻容器内的各项状态参数值和孔口处气体的平均泄漏率;同时,对于储存压力较高(大于3.0 MPa)的容器,提出近似计算总平均泄漏率的2种简化方法。     

海底管道泄漏油气扩散规律数值仿真与对比分析

为研究海底原油与天然气单相泄漏扩散规律的差异性,合理制定应急响应策略,减小事故损失,针对海底管道失效所致的原油与天然气泄漏问题,基于计算流体动力学CFD方法,建立海底油气管道泄漏事故后果预测与评估模型,对特定事故场景下的海底原油与天然气泄漏扩散过程进行模拟与分析,从泄漏扩散过程、工况因素影响、泄漏后果及应对策略4个方面对比原油与天然气的泄漏扩散特性。结果表明:相同工况下,海底原油与天然气在泄漏速率、扩散时间、扩散形态及水平最大扩散距离方面存在显著差别;与天然气相比,原油泄漏扩散行为对工况因素具有更高的敏感性;原油泄漏会引发严重的环境灾害,天然气泄漏则会影响海上结构物的稳定性及引发火灾爆炸事故,据此需合理制定具有针对性的应对策略。     

城区天然气管道泄漏数值模拟与爆炸危害分析

在人口密度为三级和四级的城区内,密集的高建筑物对天然气管道泄漏后的扩散和流场形成产生重要影响。本文以某城市的实际情况为例,建立多建筑物的空间几何模型,采用k-ε湍流方程,SIMPLE算法,模拟了在三种不同风流速度、三种不同压力条件下,城区天然气管道泄漏气体在多建筑物地形中的扩散情况。根据模拟结果,依据天然气的爆炸极限,对模拟结果及其火灾爆炸危害的范围进行了对比分析。结果表明,CH4气体的泄漏扩散同时受管道压力、风流速度和周围建筑物的影响;同时受当地风速的影响,泄漏气柱在风流作用下会发生偏折,造成阻挡风流的建筑物内侧危险气体浓度升高,大大增加建筑物周围环境的危险性。研究结果对城区天然气管道的建设具有一定的指导意义。     

基于元胞自动机数学模型的瓦斯抽采管道漏点定位研究

为了解决普通数学模型难以准确描述瓦斯抽采管道内流体的流动状态问题，提出了以元胞自动机模型为基础的瓦斯抽采管道漏点定位模型。根据元胞自动机在空间和时间上离散化的特性来演化管道流体在时空上的连续变化，将管径变化、管壁粗糙度、管构异件种类和数量以及温度等参数沿管道进行离散化，利用元胞自动机理论以及管道两端的信号对管道沿线压力和流量等参数变化进行预测，以判断泄漏的发生和漏点定位。通过实验验证，该方法能提高漏点定位精度。     

中压燃气管道泄漏工况下流量扰动幅度模拟分析*

为检测和定位燃气管道泄漏,基于泄漏定位公式,利用模拟软件Pipeline Studio构建等效中压管道模型,模拟不同工况条件下燃气管道泄漏动态。结果表明:泄漏发生后,一定时域内用户端流量将出现扰动,供气压力越高、供气量越大、泄漏孔径越小、管道长度越长,流量扰动持续时间越长;忽略管长影响,泄漏位置越接近气源,流量扰动幅度越易出现先增大后减小趋势,反之易出现单调递减趋势;泄漏端位置距离用户端越近,流量扰动幅度越大,反之越小。研究结果可为燃气管道泄漏检测和定位提供理论依据。     

Measurement of gas detonation blast loads in semiconfined geometry

The ignition and explosion of combustible vapor clouds represents a significant hazard across a range of industries. In this work, a new set of gas detonations experiments were performed to provide benchmark blast loading data for non-trivial geometry and explosion cases. The experiments were designed to represent two different accident scenarios: one where ignition of the vapor cloud occurs shortly after release and another where ignition is delayed and a fuel concentration gradient is allowed to develop. The experiments focused on hydrogen-air and methane-oxygen detonations in a semiconfined enclosure with TNT equivalencies ranging from 9 g to 1.81 kg. High-rate pressure transducers were used to record the blast loads imparted on the interior walls of a 1.8 m × 1.8 m × 1.8 m test fixture. Measurements included detonation wave velocity, peak overpressure, impulse, and positive phase duration. A comparison of the pressure and impulse measurements with several VCE models is provided. Results show that even for the most simplified experimental configuration, the simplified VCE models fail to provide predictions of the blast loading on the internal walls of the test fixture. It is shown that the confinement geometry of the experiment resulted in multiple blast wave reflections during the initial positive phase duration portion of the blast loading, and thus, significantly larger blast impulse values were measured than those predicted by analytical models. For the pressure sensors that experienced normally-reflect blast waves for the initial blast impulse, the Baker-Strehlow and TNT equivalency models still struggled to accurately capture the peak overpressure and reflected impulse. The TNO multi-energy model, however, performed better for the case of simple normally-reflected blast waves. The results presented here may be used as validation data for future model or simulation development.     

Numerical study on the horizontal stretching effect of ground on high-pressure vapor jets of LNG tank leakage

The objective of this work is to investigate the horizontal stretching effect of ground on high-pressure vapor jet of LNG tank leakage near the ground. A numerical model for leakage jet was developed and several series of leakage scenarios were theoretically analyzed for different heights of the tank orifice, inner pressures and outer temperatures. The results show that the near ground plays an important role in the horizontal transportation of LNG leakage vapor. The corresponding danger distance is surprisingly lengthened because of the horizontal stretching of leakage vapor cloud by the ground nearby, especially for those cases with a lower orifice on tank. It is illustrated that there is an obvious change for the central axis track, gas concentration and velocity of the jet in the far-field during the jet is touching the ground. In addition, the dimensionless analyses on the dependence of gas concentration, velocity and gas concentration on the transportation distance indicated that there were two stages of deflection behaviors of the jet. Finally, the enlarged danger distance by the horizontal stretching for the LNG tank leakage with a low orifice indicated the more dangerous scene of those leakage close on ground. The data and revelation here about the danger area prediction can be an important guide for the emergency management during the LNG tank leakage accidents.     

Impact of leakage location and downwind storage tank on the gas dispersion in a typical chemical tank storage area

Toxic gas leakage in a tank area can have catastrophic consequences. Storage tank leakage location (particularly for high leakage) and downwind storage tanks potentially influence gas diffusion in tank areas. In this study, we developed a numerical and experimental method to investigate the impact of a high leakage location and downwind storage tank on gas diffusion based on three (1.05H, 0.90H, and 0.77H, H was the tank height, 22m) leakage field experiments on the leeward side of storage tank, which have been not conducted before. The experiments revealed an unexpected phenomenon: the maximum ground concentration first decreased and then increased with increasing leakage height. The simulations illustrated that the differences in micrometeorological conditions caused the maximum ground concentration of gas emitted from the roof to be higher than that emitted from the tank wall near the storage tank height. The downwind storage tank 1) had little influence on the entire diffusion direction but altered the local diffusion pattern; 2) reduced the maximum ground concentration (∼18.7%) and the distance from the emission source (approximately a storage tank diameter); and 3) had strong influences on the concentration, velocity, turbulence, and pressure on the leeward side. The concentration negatively correlated with the velocity, pressure, and turbulence in the middle of the two storage tanks on wind centerline. Our results can improve understanding of gas dispersion in tank areas and provide references for mitigating loss and protecting lives during emergency response processes.     

The evolution of flame length for the oil tank fire with different top cover widths and lip heights

This study presents a quantitative analysis and interpretation of the variation in oil tank fire flame lengths for different oil tank sizes, top cover widths, and horizontal air flow velocities. The experimental results show that, at first, the flame length rises slowly with an increase in air flow speed. Then, once over a critical speed (0.6 m/s), the flame length decreases significantly with a further increase in air flow speed. Based on the characteristic length, a new dimensionless heat release rate is obtained, allowing the correlation between flame length, air flow speed, and dimensionless heat release rate to be calculated, which can be used to predict the flame length of an oil tank fire under different air flow speeds, lip heights, and cover widths.     

Electrostatic charges during liquid leakage

Leakage accidents of pressurized flammable liquids often occur in chemical plants. To investigate electrostatic hazards due to liquid leakage, the amounts of electrostatic charge during the leakage were observed. The electric field of the clouds generated by liquid leakage was also examined. Various types of pipeline have been designed in consideration of potential leakage that might lead to accidents during industrial processes. Leakage pressure in the range of 0.1–0.3 MPa was used. With regard to the materials, water and kerosene were used. The results obtained from the experiments show that electrostatic charges depend on leakage parameters, such as the type of liquid, the gasket material between flanges, and the pipeline pressure. In all tests, the amount of the electrostatic charges of water, 0.12–0.83 μC/kg, was larger than that of kerosene, ?0.04 to 0.16 μC/kg. The maximum value of the electric field, generated from the leakage liquid in this study, 40.9 v/cm, is a safe level. No incendiary electrostatic sparks, such as brush and/or spark discharges, were detected in our tests.     

输气管道泄漏压力波的频谱特性模拟

为了研究输气管道泄漏压力波的频谱特性,基于大涡模拟对管道泄漏进行了非稳态分析。模拟了不同管道压力、不同泄漏孔径以及不同管道气体流速下的泄漏压力波频谱分布,探讨了不同条件对压力波频谱特性的影响。所得结论对各类管道泄漏压力波的频谱分析具有一定的借鉴作用。     

CFD analysis of the dispersion of toxic materials in road tunnels

The present work is aimed at analyzing the evolution of accidental scenarios deriving from the release of toxic materials inside a tunnel. This scenario, compared to the more frequently investigated cases of fire, followed by smoke dispersion, may involve a large variety of common products characterized by widely differing physical properties; nonetheless it has been analysed in the literature less than expected. The present study compares the dispersion of two common toxic chemicals (chlorine and ammonia), in order to derive some preliminary information about the influence of the physical properties and the release rate. A reference road tunnel geometry is assumed, while the release occurs from ground level, at the centre of one lane and in the middle of the tunnel. Two study cases involving a road tanker, transporting the product as liquefied gas under pressure, were considered: a catastrophic release, from a 220 mm hole, emptying the tanker in a few tens seconds (case A), and a continuous release, from a much smaller hole (15 mm), lasting 5 min (case B). For the sake of simplicity, the release is assumed to be in gaseous phase; the dispersion of the toxic is simulated for the 5 min period following the start of the release using a CFD (Computational Fluid Dynamics) analysis, according to an RANS (Reynolds-Averaged Navier–Stokes) approach with the standard k–ε turbulence model, assuming no ventilation conditions. Structured curvilinear grids with hexaedric cells, refined according to the local concentration gradient, are used. For case A scenarios, especially for the whole release duration, dispersion is mainly governed by the “plug-flow” effect caused by the large volume of toxic entering the tunnel in a rather short time; then, the role of diffusivity and gravity becomes more important. Chlorine, heavier than air and with lower diffusivity than ammonia, progressively accumulates towards the floor; the dispersion of ammonia, which is lighter than air, appears more influenced by diffusivity than by gravity, since a limited stratification is observed. These trends are more evident for case B scenarios, where the toxic flow rates are much lower. It is expected the results will give some useful insight into the dispersion phenomenon within highly confined spaces and maybe also provide some suggestion about ventilation systems design and emergency procedures.     

Release and dispersion behaviour of carbon dioxide released from a small-scale underground pipeline

The development of carbon capture and storage (CCS) brings challenges for safety issues regarding carbon dioxide (CO₂) transmission pipelines. Once a pipeline is punctured or full-bore ruptured, the leaked CO₂ is hazardous to personnel and the environment. Small-scale devices were established with the aim of studying the release and dispersion behaviour of gas and liquid CO₂ from a punctured underground pipeline. A sandbox was built to simulate the underground conditions. The parameters of the sand used in the experiments were tested. CO₂ concentrations on the ground and temperatures around the release orifice in the sand were analysed. The results indicate that in the CO₂ gas release experiments, the CO₂ concentration on the sand surface decreases with increasing horizontal distance in the form of a power function. CO₂ concentrations in upward release are slightly larger than those in horizontal release at the same location but are obviously bigger than values in downward release. The temperature-drop region is much smaller than that in air. A frozen ice ball can be generated near the release orifice during the gas phase of the CO₂-release process. In the liquid phase of CO₂-release experiments, a large amount of dry ice is generated near the release orifice. Dry ice can only be generated in the area close to the release orifice, especially in the near-field area.     

Evaluation of multi-phase atmospheric dispersion models for application to Carbon Capture and Storage

A dispersion model validation study is presented for atmospheric releases of dense-phase carbon dioxide (CO₂). Predictions from an integral model and two different Computational Fluid Dynamics (CFD) models are compared to data from field-scale experiments conducted by INERIS, as part of the EU-funded CO₂PipeHaz project.The experiments studied consist of a 2 m³ vessel fitted with a short pipe, from which CO₂ was discharged into the atmosphere through either a 6 mm or 25 mm diameter orifice. Comparisons are made to measured temperatures and concentrations in the multi-phase CO₂ jets.The integral dispersion model tested is DNV Phast and the two CFD models are ANSYS-CFX and a research and development version of FLACS, both of which adopt a Lagrangian particle-tracking approach to simulate the sublimating solid CO₂ particles in the jet. Source conditions for the CFD models are taken from a sophisticated near-field CFD model developed by the University of Leeds that simulates the multi-phase, compressible flow in the expansion region of the CO₂ jet, close to the orifice.Overall, the predicted concentrations from the various models are found to be in reasonable agreement with the measurements, but generally in poorer agreement than has been reported previously for similar dispersion models in other dense-phase CO₂ release experiments. The ANSYS-CFX model is shown to be sensitive to the way in which the source conditions are prescribed, while FLACS shows some sensitivity to the solid CO₂ particle size. Difficulties in interpreting the results from one of the tests, which featured some time-varying phenomena, are also discussed.The study provides useful insight into the coupling of near- and far-field dispersion models, and the strengths and weaknesses of different modelling approaches. These findings contribute to the assessment of potential hazards presented by Carbon Capture and Storage (CCS) infrastructure.     

Impact assessment of traffic-induced vibration on natural gas transmission pipeline

The purpose of this paper is to present a study of impact assessment of the traffic-induced vibration on a buried natural gas transmission pipeline. The basic assumption in this study is that the traffic on pipeline-transportation route crossing might have a significant impact on natural gas pipeline structural integrity due to the traffic-induced vibration which propagates from the road surface through the soil and excites the buried natural gas pipeline. The resulting dynamic stress causes pipeline material fatigue loading which consequently may cause pipeline failure with the gas release into the environment exposing the population and the buildings in pipeline vicinity to a significant threat. The experiment on operating buried natural gas pipeline was conducted where measurements were performed on the road surface, the two operating buried natural gas pipelines of external diameter 500 mm and 250 mm and on corresponding casing pipes. The measurement data analysis was performed and the results were used for determination of pipeline lifetime period in the model for theoretical estimation of pipeline lifetime which has been exposed to traffic-induced vibration. The findings of the study in this paper show that the traffic-induced vibration on given buried natural gas pipeline is detectable, however this vibration, compared to the other factors which are influencing pipeline's structural integrity, does not have a significant impact on pipeline lifetime period.