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LNG Storage Technology
For many decades DYWIDAG has been in the forefront of the development, design and construction of prestressed concrete protective containers for the storage of liquefied gases. The current safety standards for container systems were to a large extent influenced by DYWIDAG's work. Gases requiring storage range from natural gas (LNG) and ethylene to propane and butane, which are by-products of oil refining.
LNG is the abbreviation for Liquefied Natural Gas. Natural gas is liquefied by cooling it to below boiling point (approx. -162°C). This reduces its volume by a factor of about 600. The space required for transport and storage is accordingly reduced. Liquefaction is therefore a technically safe and economical solution to the problem of storing large quantities of gas at atmospheric pressure and transporting them by ship over large distances.
The beginning and end of such transport chains are formed in each case by a base load terminal with storage tanks. Up to date the largest above-ground tanks have a capacity of 200,000 m³. Future tanks are planned with a capacity of 250,000 m³.
A further reason for storing liquefied natural gas is to cover peaks in demand in a regional or local supply network. This takes place in a peak-shaving facility, where the tanks are, however, generally smaller than in liquefaction plants.
Storage System
By reason of the low temperatures and the high energy potential of liquefied gases, planning LNG tanks is a complex task. The technical development of the storage systems began about 40 years ago with steel tanks, taking existing oil tanks as models. Thanks to the emergence of a logical and desirable safety concept for liquefaction plants, they eventually developed into the full containment systems, common standard worldwide today. Here the storage tank resembles a thermos container.
The outer tank in the form of a prestressed concrete containment represents the protective component in the container system. The inner tank as an independent component forms the primary storage tank, and the thermal insulation between the inner and outer tanks prevents cold loss, thus limiting the vaporisation rate. The inner and outer tanks each possess separate hydrostatic stability.
In addition, the design of the tank system aims at restricting all the effects of any conceivable accident to the inner storage system. The outer tank protects the primary tank from potential operational disturbances from outside sources and, conversely, also protects people and the environment from an explosive gas-air mixture forming as a result of a leak in the inner tank. In this system the outer tank has to fulfil the following protective functions:
- physical protection e.g. against pressure waves from explosions, impact caused by parts of the plant flying through the air, helicopter collision and liquid impact
- thermal protection e.g. against fire in adjacent tanks, fire on the tank roof, cold shock
- collecting and retaining fluids or gases e.g. escaping from a leak in the inner tank.
These requirements mean that the outer tank must form a self-contained prestressed concrete containment. In addition a multilayer liner system is required on the inner faces of the containment.
Protection from earthquakes
Locations with high levels of seismic activity pose special problems. Large storage tanks have a fundamental frequency of approx. 2 to 10 Hz and thus lie more or less within the resonance range of typical earthquakes, i.e. they will be accelerated 3 to 4 times more than the ground on which the tanks are built.
This critical condition can be largely avoided if the vibrational behaviour of the tank can be disassociated from that of the ground. One step in this direction is to isolate the base, a technique also applied to machine foundations. To effect this, the container is placed on isolators. If the isolation is correctly designed to take into account the dynamic properties of the ground and the structure, the stresses on the vulnerable steel inner tank can be reduced by 80 - 90 %. For these calculations a dynamic model of the tank structure is prepared. The complex interaction between foundation, isolators, outer tank, inner tank and the liquid can be reproduced realistically with a Lump-Mass model.
DYWIDAG pioneered the worldwide systematic use of base isolation for large-scale LNG tanks in the design of the Inchon LNG Receiving Terminal, using round elastomer bearings made of natural rubber.
DYWIDAG played a leading part in the planning and construction of the following tanks for liquefied gases:
1968 LNG Tank Stuttgart, Germany, Volume 30,000 m³
1971 LNG Tank Nürnberg, Germany, Volume 1,600 m³
1990 LNG Tank Mosselbay, South Africa, Volume 10,000 m³
1991 LNG Tanks Lumut, Brunei Darussalam, Volume 2x 65,000 m³
1993 LNG Tanks Inchon, South Korea, Volume 10x 100,000 m³
1995 LPG Tanks Ruwais, UAE, Volume 2x 43,000 m³
1997 LNG Tanks Qalhat, Sultanate of Oman, Volume 2x 120,000 m³
2000 LNG Tank No. 3 Tongyeong, South Korea, Volume 140,000 m³
2002 LNG Tanks Nos. 4 - 10 Tongyeong, South Korea, Volume 7 x 140,000 m³
2002 LNG Tank Point Fortin, Trinidad, Volume 160,000 m³
2003 LNG Tanks POSCO Gwangyang, South Korea, Volume 2x 100,000 m³
2003 LNG Terminal, Long Beach, California, Volume 2x 160,000 m³
2003 LNG Tanks Nos. 1 + 2 Sagunto, Spain, Volume 2x 150,000 m³
2004 LNG Tanks Nos. 11 - 14 Pyeongtaek, South Korea, Volume 4x 140,000 m³
2005 LNG Tanks Bal Haf, Yemen, Volume 2x 140,000 m³
2006 LNG Tanks Nos. 11 + 12 Tongyeong, South Korea, Volume 2x 140,000 m³
2006 LNG Tank No. 3 Sagunto, Spain, Volume 150,000 m³
2007 LNG Tanks Nos. 13 + 14 Tongyeong, South Korea, Volume 2x 140,000 m³
2007 LNG Tanks Design for South Korean Client, Volume 3x 140,000 m³
2007 LPG Tanks Design for South Korean Client, Volume 2x 30,000 m³
2008 LNG Tanks Nos. 19 + 20 Pyeongtaek, South Korea, Volume 2x 200,000 m³
2008 LNG Tanks Nos. 15 + 16 Tongyeong, South Korea, Volume 2x 200,000 m³
2008 LNG Tank Nynäshamn, Sweden, Volume 20,000 m³
2009 LNG Tank No. 4 Sagunto, Spain, Volume 150,000 m³
2009 LNG Tank No. 23 Pyeongtaek, South Korea, Volume 200,000 m³
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