|
BBR Technology meets demand for LNG storage
Over the past few decades, world consumption of LNG - Liquefied Natural Gas - has increased more than five-fold and it is predicted that growth will continue to be very strong. The rising demand from large markets such as China and India, combined with the increasing popularity in a large number of other smaller markets has resulted in the development of many new LNG facilities throughout the world.
There are significant natural gas reserves globally and exploration companies are rapidly developing facilities for exporting the natural gas, with the corresponding receiving facilities being planned and built in emerging markets. With a timeframe of some 5-10 years for planning and construction, there is currently much activity underway in the LNG supply chain in preparation for current and predicted demands.
The growth in this sector has seen the development of significant LNG storage tank facilities for LNG exporters and importers. These massive storage tanks are essential for reception and safe storage of the liquid gas.
The storage temperature of LNG is -162ºC and is described as "cryogenic" conditions. The liquid occupies 600 times less space than natural gas in its gaseous state, making it practical to ship by ocean tanker. In addition, it is stable and safe because, even though compressed in volume, the liquid remains at normal atmospheric pressure. On land, LNG is stored in specially engineered and constructed double-walled storage tanks. At these temperatures, the requirements for the containment structures are very stringent and post-tensioned concrete tanks are ideally suited to the task.
The large concrete tank structures are extremely robust with significant amounts of prestressing required - all being designed and installed under tightly controlled quality conditions, with hardware requiring special certifications.
LNG tanks are generally constructed under design and build arrangements, with the principal contractor being responsible for determining the specific design requirements for the prestressed concrete. The post-tensioning specialist examines the required force profile and details the spacing and tendon size for the post-tensioning.
Design and construction techniques have been specially formulated for LNG tank construction. The outer walls of the tank are most commonly constructed from post-tensioned concrete. The void between the tank double walls is filled with insulation.
Tanks are around 80-90m in diameter and 50m high, with a wall thickness of some 750mm.
The post-tensioning tendons are very large and typically run both vertically and horizontally.
- Vertical tendons
These can either be single directional tendons from the top of the tank, terminating in a recess or socket at the bottom - or "U" tendons starting at the top, coming vertically down through the tank, curving around through 180º degrees and returning to the top.
- Horizontal tendons
Typically, these start at a buttress and travel half way around the tank, terminating at the opposite buttress. Another tendon commences from the same buttress and travels back through the remaining half of the tank, terminating at the original buttress - thus creating a complete "hoop" with the two tendons.
For efficient use of post-tensioning, adjacent tendons are anchored at alternate buttresses, 90º from the actual buttress.
There is no official standard for the design of these tanks and the first guidelines published were based on pioneering work in cryogenic applications. According to fip SR 88/2, testing is required to be carried out on:
- Prestressing steel - at room temperature and at cryogenic temperature
- Tendon anchorage assembly - at room temperature and cryogenic temperature
- Load transfer - at cryogenic temperature
Tests according to these guidelines were completed for the BBR Technology. Subsequently, a new guideline has been published to cover prestressing in cryogenic applications - ETAG 013.
The testing and quality control of prestressing materials used in cryogenic applications is critical to the successful performance of the containment systems.
The BBR CONA post-tensioning system is in full compliance with the testing regime under cryogenic conditions.
LNG storage tanks are ideally suited to construction methods employing slipformed or climbing insitu concrete construction combined with post-tensioning. The design and installation techniques are very specialised and require specially certified and tested materials and highly experienced contractors.
A large database of information has been developed during construction of these massive concrete structures and many innovative techniques have streamlined activities associated with the supply and installation of post-tensioning materials and other construction-related engineering. The nature of the typical design and build project delivery method has seen the formation of some strong design and construction relationships and this has seen the rapid development and optimisation of design and installation techniques.
|
