Koyama.qxp 3/7/09 9:57 am Page 72
LNG
Computational Fluid Dynamics Simulation on Liquefied Natural Gas
Storage Tanks for Safety and Efficiency
a report by
Kazuo Koyama
Senior System Engineer, Tokyo Gas Co., Ltd.
The system for liquefied natural gas (LNG) transactions is changing may cause damage to the tank. To avoid stratification, different
from dedicated long-term transactions to more flexible short-term densities of LNG should not be stored in the same tank. However,
transactions with a wide range of densities. As a consequence, these days there is an increasing need for mixed LNG storage.
mixed LNG storage will be inevitable; this will require careful
observation due to the possibility of stratification followed by If mixed LNG storage is possible without sacrificing safety, LNG
rollover. Since its first reception of LNG in 1969, Tokyo Gas Co., Ltd. terminal operations can be run with greatly increased efficiency,
has been studying LNG storage tank operations with various including inventory management and cargo delivery schedule
densities of LNG. Computational fluid dynamics (CFD) simulation has arrangement, as well as reducing the costs of additional tanks.
helped maintain the current high safety level, as well as operational Mixed LNG storage is one of the key factors in achieving low-cost
efficiency and lower construction costs, making it an essential design and the effective operation of LNG receiving terminals.
technology for LNG receiving terminals.
Stratification and Rollover
What Is Liquefied Natural Gas? Where different densities of LNG are received to tanks with heel,
LNG is a super-cryogenic transparent liquid at -162ºC at atmospheric there is the possibility of stratification, with two or more layers
pressure. It consists mainly of methane and the other hydrocarbons forming because of their density difference. Once stratification
and nitrogen. All impurities, such as sulphur and carbon dioxide, are occurs, each layer has its own heat convection due to heat input
removed during the liquefaction process, making LNG one of the through the tank bottom and walls. The density difference between
cleanest fossil fuels. Its density and heating value differ widely, the upper and lower layers becomes smaller as time passes. When
mainly depending on the original natural gas components. This leads those layers are mixed together, the heat accumulated in the lower
to a wide density distribution, from 420 to 470kg/m
3
. layer is released in the form of large BOG generation over a short
period. This phenomenon is called rollover. If the amount of BOG
What Is Happening in the exceeds the BOG compressor capacity, safety valves are activated or
Liquefied Natural Gas Industry? the tank is damaged. A rollover incident occurred at La Spezia in Italy
New LNG receiving terminals are now attracting more attention in in 1971.
terms of environmental issues because of the growing demand for
LNG as a clean energy source. Many new LNG liquefaction plants, Prevention of Stratification
which produce a wide variety of LNG depending on the components When different densities of LNG are received, we have to be fully
of the original natural gas, have come on-stream around the world. aware of the stratification risk. Figure 2 shows reception facilities for
More players have appeared on both the supply and demand sides. typical LNG storage tanks. The density difference is caused by the
These developments have changed the face of LNG receiving different origins, heat transfers and vaporisations during
terminal operations. transportation and storage.
Due to huge investment in the whole LNG chain – liquefaction, Usually, two types of reception pipe – top and bottom feed – are
transportation and reception facilities – LNG has traditionally been produced to avoid stratification. The selection of which feed to use
sold on rigid long-term contracts. However, the system for LNG depends on the density of the LNG to be received: lighter and
transactions is now changing from dedicated transactions between heavier LNG is received through the bottom and top feed,
liquefaction plants and receiving terminals to more flexible spot or respectively, to increase the mixing effect by taking advantage of
short-term transactions with a wide range of LNG densities. As a their buoyancy. Factors that affect stratification are the tank
consequence, mixed LNG storage will be inevitable sooner or later; dimensions, the position, shape, and size of the feed and the inlet,
this requires careful observation due to the possibility of the density difference between the LNG, the heel level, the feed
stratification followed by rollover with a lot of boil-off gas (BOG) rate, heat transfer and so on. Avoiding the generation of
release over a short period. stratification is a priority at any LNG reception operation.
Mixed Liquefied Natural Gas Storage What Can We Do with Computational
For the purposes of this article, mixed LNG storage is defined as the Fluid Dynamics Simulation?
ability to receive LNG type A in the same tank that stored the heel of Since its first reception of LNG in 1969, Tokyo Gas has been studying
LNG type B with more than 1% lighter or heavier density (see Figure LNG tank reception procedures with different densities of LNG in order
1). Storing LNG with different densities may lead to stratification to improve safety and efficiency and reduce LNG storage costs. CFD
followed by rollover with a lot of BOG release in a short period, which simulation regarding stratification and rollover has helped maintain the
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© TOUCH BRIEFINGS 2009
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