About
Controversy over LNG facilities siting has focused attention on LNG safety issues, particularly the potential impact of large fires on adjacent areas. Participation in LNG projects over the past thirty years sparked the interest in updating LNG process safety R&D for several MKOPSC staff. The reason for continued research is that the massive size of LNG storage tanks and LNG tankers means that any containment releases may have the potential for a very large incident. The hazards posed by LNG can be minimized by good engineering design and an understanding of the physical properties of LNG.
Beginning with research in computational fluid dynamics in source modeling, vapor dispersion, and fire, MKOPSC has signed a contract with BP Global Gas SPU for “LNG Vapor Cloud Control and Mitigation Research”. This project will focus on improved detection, suppression, protection and vapor control methods. The results from this research will help in developing definitive guidelines on the engineering design criteria for mitigating the consequences of LNG spill and/or fire. In addition, results of this R&D will be used to improve the BP – Texas A&M LNG Fire Fighting School curriculum and training methods. And lastly, the results may assist in alleviating some of the causes of public concern about LNG safety and emergency preparedness. There is also research pertaining to LNG Reliability Data and discovering ways to further increase safety.
LNG Vapor Dispersion Modeling
One of the major hazards from accidental LNG release is the formation of flammable vapor cloud. Medium-scale experimental tests have been conducted at BFTF (Brayton Fire Training Field) since 2005 in College Station, Texas to study the nature of LNG vapor cloud for release onto land, water and underwater. The results of the tests were used to determine the exclusion zones using consequence models as it plays a vital role in the risk assessment of LNG operations to determine potential hazardous impact of worst credible accidents. Federal safety regulations require the use of validated consequence models to determine the vapor cloud dispersion exclusion zones for accidental liquefied natural gas (LNG) releases. One tool that is being developed in industry for exclusion zone determination and LNG vapor dispersion modeling is computational fluid dynamics (CFD).
LNG industry had been heavily relying on the comprehensive-integral models to predict the vapor exclusion zone. However, recent concerns had been focused on underprediction from the integral model due to the limitation in describing the complex behavior of LNG spills during its initial stage. A CFD code is capable of handling obstacles in a three-dimensional environment and can provide a detailed description of physical processes. They are efficient in handling complex geometries and can thus be used to predict the behavior of LNG vapor cloud dispersion in a site-specific risk analysis. The atmospheric dispersion of LNG vapor is a type of buoyant, multicomponent fluid flow. Hence the different key parameters involved in the vapor cloud phenomenon and its variation was computed by calibrating the CFD model with experimental data obtained at BFTF. The modeling phase is studied in two parts as the source term model and a dispersion model due to the different key parameters used to describe the physical process at each stage.
Expansion Foam Application for LNG Hazard Mitigation
High expansion foam has an expansion ratio of more than 200, which could be applied to provide blanketing effect to the surface of most hydrocarbon fuels and also have made it possible to be used as a mitigation measure against a boiling and evaporating pool of flammable gases and subsequent pool fires. Because of this fire suppression characteristic, the liquefied natural gas (LNG) industry has identified expansion foam as one of its safety provisions for pool fire suppression.
The effects of foam application on LNG and LNG pool fires through outdoor spill experiments at the Brayton Fire Training Field are being investigated. The study is aimed at obtaining key parameters such as temperature changes of methane and foam and the extent reduction of vapor concentration for evaluating the use of foam to control vapor hazard from LNG. The foam effectiveness in suppressing LNG pool fires is also studied to determine the thermal exclusion zone, by investigating temperature changes of foam and fire, profiles of radiant heat flux, and fire height changes with foam. Additionally, a schematic model of a LNG-foam system with fire for theoretical modeling was also developed. It is seen that expansion foam has positive effects on reducing flame height and radiant heat flux by decreasing heat release and radiant heat feedback from the LNG pool fire, ultimately reducing the safe separation distance. Through extensive data analysis, several key parameters, such as the minimum effective foam depth and the mass-burning rate of LNG with applied foam are identified. The CFD modeling is applied to understand the effects of expansion foam on LNG behavior and to the overall consequence of the release. This study can be used to design an effective expansion foam system as well as to develop defensive measures and emergency response plans for mitigating the consequences of LNG releases.
Effective Use of Water Curtains in Dispersing LNG Vapor Cloud
One of the major hazards is the formation of a flammable vapor cloud from an inadvertent LNG release, which may lead to a massive fire. Therefore, the mitigation of accidental release consequences of LNG is a serious concern. Federal safety regulations and standards require a “dispersion exclusion zone” for LNG facilities so that vapor generated during releases of LNG will not propagate beyond the plant boundaries. This exclusion zone begins at the LNG spill site and extends to the predicted distance at which the average vapor concentration in air is 2.5% volume. An effective technical approach to create this safety zone is forced dispersion of LNG vapor through control and mitigation measures. Water curtain is recognized as an efficient engineering method to mitigate various types of hazards in the petrochemical and gas industries because of its availability, simplicity of use, efficiency, and adaptability for various hazards such as gas dispersion, absorption, and fire inhibition. Today, water curtain is recognized as a promising technique to suppress LNG vapor clouds.
Even though extensive theoretical and experimental research has been carried out to determine the effectiveness of water curtain in dispersing LNG vapor, there is no comprehensive and substantiated engineering guideline for water curtain design. The aim of this research is to provide comprehensive guidance towards the development of engineering design criteria of an effective water curtains to disperse LNG vapor and establish an effective safety zone for LNG storage facilities. The research had been focused to determine the effectiveness of water curtain through comprehensive theoretical and experimental analysis of its LNG dispersal mechanisms. Medium –scale experimental works had been conducted at BFTF using various commercially available water nozzles to evaluate the effectiveness in dispersing the LNG vapors. The CFD modeling had been used to understand the fundamentals of complex interaction between the droplet and air-vapor mixture. Finally, the results from experimental measurements using the industrial water curtain information and the droplet-vapor system modeling will be used to provide an engineering analysis which can serve as fundamentals to support formulating guidelines for water curtain applicable for LNG industry.
