Liquefied Natural Gas Spill and Dispersion – Safety Assessment a report by Subramanya Nayak,1 Tejas Bhatelia,1 Tomasz Olewski2 and Simon Waldram3
1. Post-doctoral Research Associate; 2. Assistant Research Engineer; 3. Retired Interim Programme Chair, Chemical Engineering Programme, Texas A&M University at Qatar
In recent years, the demand for natural gas has risen significantly, leading to new natural gas processing units and liquefied natural gas (LNG) storage terminals. Increased production and storage of LNG has also increased the concern regarding the risks associated with LNG spills. To further enhance the safe operation of these facilities important questions need to be answered:
• How and where should we site LNG facilities for production, storage, export, receipt and re-evaporation?
• •
How can we ensure that site operatives, the local community and the environment are adequately protected?
What guidance and international standards are available? • How can we predict the outcomes of a major loss of LNG containment?
An improved understanding regarding the nature and the behaviour of the LNG spills in the complex geometries encountered in such facilities will help to develop better safety guidelines. International guidance for design and operation of LNG facilities recommends that validated models are used to predict the consequences of potential releases of LNG.1
At the
same time, government reports highlight that current knowledge of LNG behaviour is inadequate and prioritise specific areas for further research.2 Accordingly, this study focuses on modelling LNG spill and dispersion in a 3D environment. A theoretical model that considers methane vapour dispersion and warming following the loss of LNG containment was developed and solved numerically using commercial computational fluid dynamics (CFD) codes such as FLUENT (ANSYS Inc.) and FLACS (GexCon). The Immersive Visualization Facility at Texas A&M at Qatar was utilised to display stereoscopically the results of simulations.
Anatomy of an Liquefied Natural Gas Spill and Dispersion LNG is stored essentially at atmospheric pressure and as a cryogenic liquid at its normal boiling temperature of -162 °C. Spills of LNG over land in production facilities or storage locations, or over water during tanker loading, unloading or sea transportation, generates an LNG vapour cloud. The magnitude of the source term for this depends on the heat transfer rate to the cold, liquid phase LNG spill.3
Vapour produced is initially also
cold and therefore more dense than air. It clings closely to the ground or sea as it disperses, but as it mixes with the much warmer air the density of the resulting mixture decreases until eventually and at low methane concentrations, it approaches the density of air. On the periphery of an LNG cloud, the local gas concentrations will be below their lower flammable limit in air. In the centre of the cloud, at least initially, gas concentrations will be well above the upper flammability limit. However, in some region between these two locations, the gas concentrations will be flammable and an encounter with a sufficiently energetic source of ignition will result in fire. Depending on the conditions of the spill, this may burn back to the liquid LNG that was the original source for the vapour. Back radiation from the fire then enhances the LNG vaporisation rate and consequently, the fire intensity. Ignition of LNG vapour clouds does not normally lead to an
© TOUCH BRIEFINGS 2011
explosion as flame velocities are relatively low. However, as local turbulence is increased (e.g. as a consequence of burning around the complex geometry of, for instance, process equipment that may be present) the combustion can become much more rapid.
Subramanya Nayak graduated from Washington University in St Louis with a PhD in Energy, Environmental and Chemical Engineering in 2009. As a graduate student, his research focused on multiphase reaction engineering and heterogonous catalysis. After his graduation, he joined Texas A&M at Qatar as a Post-doctoral Research Associate in the Chemical Engineering Programme. During this time, he helped in establishing a process safety laboratory and developed methods for 3D
visualisation of computational fluid dynamics (CFD) simulations. In 2011, he joined the Mary Kay O’Connor Process Safety Centre at Texas A&M as an Assistant Research Scientist.
E:
subramanya.v.nayak@
gmail.com
Tejas Bhatelia is presently working as a Post-doctoral Research Associate and is actively involved in the development of a comprehensive kinetic model of Fischer–Tropsch synthesis. Before joining Texas A&M University at Qatar, he worked as a Research Associate at the Curtin University of Technology, Perth. His PhD work was focused on investigating novel reactors for multiphase processes using CFD. His areas of interest include chemical reaction engineering, CFD modelling, catalysis
and reactor engineering applied to gas to liquids (GTL) technologies and process safety.
Tomasz Olewski was appointed to the position of Post-doctoral Research Associate at Texas A&M University at Qatar at the start of 2009 and was promoted in 2010 to Assistant Research Engineer. His appointment at Texas A&M in Qatar is funded by the BP- and Qatar Petroleum-sponsored, five-year project on ‘Liquefied natural gas (LNG) safety: advancing the science and technology’. He is on an extended leave of absence from the Process and Occupational Safety
Department of the Technical University of Lodz, Poland. He graduated from the Faculty of Process and Environmental Protection, Technical University of Lodz in 2008, receiving a PhD degree in Chemical Engineering.
Simon Waldram was a Student Apprentice with ICI and studied Chemical Engineering at Loughborough University, graduating at the top of his year in 1968. After reading for a Masters’ degree in Chemical Engineering at Illinois Institute of Technology, Chicago, and a PhD at the University of London, he followed an academic career for 21 years in the department of Chemical Engineering at University College London (UCL). In 1992, he left UCL and was appointed
Technical Director at Hazard Evaluation Laboratory (later to be renamed HEL), where he also become Director for Business Development. During the next 15 years, and under his direct supervision, HEL completed more than 900 industrial and governmental projects. Most of these were consultancy projects in the general areas of process safety, process development, education and training, or involved the provision of bespoke research equipment. In July 2007, he joined the Chemical Engineering Department at Texas A&M University at Qatar as Professor of Chemical Engineering where he helped set up teaching laboratories, taught a variety of core Chemical Engineering courses and established the Process Safety Research group. He retired as Interim Programme Chair at the end of 2010.
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