Thursday, May 21 at 7 p.m. The Loft at the Red Building, Astoria Presented by Columbia-Pacific Common Sense and Columbia Riverkeeper |
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Offiical University of Arkansas Biography Jerry Havens After earning his Masters degree, Dr. Havens worked in the industry before serving as an officer in the U.S. Army Chemical Corps. He then returned to graduate school. Havens joined the faculty in 1970. While on sabbatical in 1976-77, Havens served as Technical Advisor to the Office of Merchant Marine Safety of the U.S. Coast Guard. In 1982, he was technical advisor to the British Health and Safety Executive during the Thorney Island Heavy Gas Trials in southern England. In 1994, Havens traveled to Bhopal, India, accompanying the International Medical Commission on Bhopal, in order to study the effects of a catastrophic release of methyl isocyanate that had occurred 10 years earlier. SANDIA REPORT: Guidance on Risk Analysis and Safety Implications of a Large Liquefied Natural Gas (LNG) Spill Over Water
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LNG TECHNOLOGY Fall 2002 • GasTIPS By Jerry Havens and Tom Spicer University of Arkansas Chemical Hazards Research Center and Kent Perry Gas Technology Institute Efforts at the University of Arkansas have resulted in computer models that accurately predict the behavior of high-density methane vapor releases from LNG tanks. Renewed global interest in LNG as a means of supplying growing gas markets has led to an increase in the number of proposed LNG terminals around the globe. Some of these include options (e.g. several types of offshore terminals) and locations not considered before. At the same time there is a heightened sense of concern over the potential consequences of an LNG leak at such facilities. The need for answers to contingency planning questions concerning possible accident scenarios or terrorist threats will require more specific and thorough consequence assessments. Project developers recognize that the economic burden of increased safety requirements will be considerable, and are eager to employ modeling tools that can provide insight as to the lowest cost options. In the US, the Code of Federal Regulations (CFR 49 Part 193) prescribes safety standards for LNG facilities subject to federal pipeline safety laws. This Code specifies requirements for thermal radiation protection and flammable vapor-gas dispersion protection, as well as for seismic activity, flooding, soil characteristics, wind forces, severe weather, adjacent activities, and separation between facilities and site boundaries. Beginning in the late 1970s and early 1980s, Gas Research Institute, now Gas Technology Institute (GTI), sponsored extensive research programs to develop and improve methods to specify the thermal radiation and gas dispersion protection zones at LNG terminals. The Chemical Hazards Research Center at the University of Arkansas (CHRC) remains at the focus of the GRI-GTI research program. This article presents a brief description of the history and current status of CHRC’s research on gas dispersion. Early Model Development In the mid-seventies and early eighties, in response to public concerns about proposed LNG importation projects on U. S. coasts, the Department of Transportation specified the use of a so-called “Gaussian” model (popularly referred to as the MTB or Material Transportation Bureau model) for calculation of the gas dispersion protection zones required by the new regulation. During the same period, the U.S. Coast Guard and GRI sponsored the development at the CHRC of the DEGADIS (DEnse GAs DISpersion) model to describe atmospheric dispersion of denser-than-air gases following accidental release. The principal component of LNG is methane, and although methane at ambient temperatures is lighter than air, the cold methane vapor formed from evaporating LNG is denser than air. DEGADIS is a general purpose dispersion model used worldwide to assess the consequences of accidental releases of hazardous, denser than air, gases and aerosols. The American Gas Association, under provisions of 49 CFR 193, petitioned the Department of Transportation to replace the MTB model with DEGADIS in the regulation, and this took place in 1992. The DEGADIS model, as compared to the MTB model accounts for • the effect of gravity on a denser-thanair cloud • the atmospheric “takeup” of gas by the wind • a realistic treatment of area, rather than point, sources, and, • a realistic treatment of time varying releases. DEGADIS treatment of the effects of gravity and the “takeup” of gas by the wind on the dispersion of gas or aerosol clouds were the principal scientific advances in the model. DEGADIS Limitations Since its introduction, DEGADIS has been demonstrated to accurately describe gravity spreading and decreased turbulent mixing observed in dense gas clouds. However, as with the MTB model, DEGADIS is limited to the prediction of dispersion of gas clouds released from a flat surface and dispersing in the atmosphere over smooth, obstruction-free terrain. Consequently, the method does not account for effects of terrain and flow alteration by obstacles such as buildings, tanks, and dikes, all of which would be expected to decrease the gas concentrations locally and reduce exclusion distances. Although this limitation should result in predictions that are conservative (greater dispersion distances), there are important cases where terrain features and or flow obstacles could significantly decrease, or even increase, the dispersion distance. In such instances, DEGADIS predictions could be unrealistic. Although the CHRC has continued DEGADIS development and evaluation for application to other dispersion scenarios (such as jet releases), at present, the model remains applicable only to dispersion over smooth obstaclefree terrain. Challenges of Physical Modeling GRI initiated a research project at the CHRC in the mid 1980s, concurrently with the DEGADIS development, to evaluate dispersion models that could account for the effects on dispersion of terrain features and obstacles. Physical (wind tunnel) modeling methods were evaluated, as were the rapidly developing computational fluid dynamic (CFD) modeling methods. During the last decade, the CHRC has thoroughly evaluated the methods which are potentially applicable to the more complex dispersion problems to which DEGADIS is not applicable. Evaluation of physical and CFD modeling methods has resulted in the definition of several important challenges. First, it was recognized that physical modeling of dense gas dispersion requires wind tunnel operation at very low speeds. Such low speeds introduce fundamental problems in reproducing the desired turbulent flow properties, in a laboratory gas cloud, that are observed in the atmosphere. Second, CFD limitations include the requirement for demonstrated turbulence closure models, and, particularly for application to complex terrain and obstacle fields, CFD models require very large computer resources. Fortunately, economical computer resources continue to grow at a rate which seem to insure that the required resources become available by the time the more fundamental requirements, such as adequate descriptions of fluid turbulence affected by density gradients, have been demonstrated. Third, demonstration of a predictive model requires experimental data, and while many attempts have been made to perform field experiments to obtain such data, the resulting experience is mixed, primarily because of the difficulties in control of the field experiment conditions (Figure 1). It is also very expensive, perhaps prohibitively so, if one wants to demonstrate a model’s performance over a range of conditions that match its intended applications. Development of ULS Wind Tunnel CHRC, with support from GRI, constructed an ultra-low-speed (ULS) wind tunnel specifically designed to study dense gas dispersion. This wind tunnel is the largest of its kind in the world. The tunnel is used to conduct dense gas dispersion experiments at reduced scale (e.g., 150/1). Although the facility can physically model many LNG and other gas release scenarios with great accuracy, its principal use has been for conducting model experiments that can be simulated directly with CFD models. This method allows the mathematical models to be verified by direct comparison with accurate data at the reduced scale, increasing confidence that the model will accurately describe the physical phenomena expected in the field. The combined use of CFD tools and the wind tunnel model to validate computer models avoids many of the uncertainties inherent in earlier model validation efforts which relied primarily on difficult and costly field experimentation. Figure 1: Field Test of Effect of Vapor Fence on LNG Vapor Cloud CFD Model Verification Using Wind Tunnel In 2001 the GTI-CHRC research program completed a five-volume report describing the effort to verify the FEM3A (CFD) model. This model was also developed beginning in the mid 1970s, in response to the same public concerns that drove DEGADIS development. Subsequently, the Department of Transportation revised 49 CFR 193 to allow the use of the FEM3A model to account for the effects of terrain or obstacles on the vapor dispersion distance. The limits of the vapor cloud extending downwind from a “design” spill into the annular space between a model LNG tank and its dike are illustrated by flow visualization experiments in the ULS tunnel and compared with FEM3A model output (Figure 2). The limiting concentration used to define the cloud corresponds to the 2.5 percent carbon dioxide concentration (carbon dioxide density, at ambient temperature, is essentially identical to LNG vapor density at its temperature of release), were calculated with the FEM3A model. The model output shows vapor dispersion protection zones predicted with FEM3A for LNG spills corresponding to the wind tunnel scenarios shown in Figure 2, including: • an area gas source, with tank and dike, with flat, smooth terrain, • an equivalent area source without a tank or dike and flat terrain. The FEM3A predictions reveal important reductions in the vapor dispersion exclusions resulting separately from surface roughness and from the presence of the tank and dike, as well as from their combination. Extensive measurements in the CHRC wind tunnel of the gas concentration fields for these experiments confirmed the FEM3A predictions. Such predictions were extremely important to the acceptance by the Department of Transportation of the FEM3A model for inclusion as an alternate to the DEGADIS model for those cases where the DEGADIS model is not applicable. Figure 2: Wind Tunnel Dispersion (A) With and (B) Without Tank and Dike Compared with FEM3A Simulations of the Associated Hazard Zones B. A. |
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