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.

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