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LNG/LPG containment structures

VSL’s post-tensioning solutions for the storage of LNG and LPG at very low through to cryogenic temperatures in above-ground concrete storage tanks

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  • LNG tanks, Ningbo
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  • Gwangyang LNG Tank
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The most efficient structural solution for LNG and LPG applications

When its temperature is lowered to -161°C, natural gas liquefies and takes up just 1/600th of the volume it does in its gaseous state. This makes liquefaction the ideal solution for transporting and storing natural gas located in areas far from pipeline infrastructure. LNG must be kept cold to remain a liquid, independent of pressure, and modern LNG storage tanks typically provide full containment and comply with extremely stringent safety regulations. They normally feature a prestressed concrete outer wall and a high-nickel steel inner tank, with extremely efficient insulation between the two walls.

Post-tensioned concrete is the solution that provides a safe outer containment structure as it reduces crack widths to provide a safeguard in case of failure of the inner tank.

VSL is the most experienced contractor in the tank construction industry and has a portfolio of more than 60 reference sites worldwide. Its construction of containment structures spans the last 30 years, with the scope varying from the supply of post-tensioning, through PT design and supply, to the provision of the forming systems required to implement the structure’s design.

This in-depth knowledge of containment structures and a strong R&D focus combine to allow VSL to achieve continuous improvement in the systems it provides, while always looking for better ways of tackling the particular challenges of each project.

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VSL products and services

VSL has an extensive array of related engineering services for LNG and LPG containment structures. In particular, VSL’s scope includes:

  • Design and detailing of the post-tensioning layout to provide compressive stresses in the concrete membrane in accordance with the designers’ requirements
  • Supply and installation of post-tensioning whose performance is maintained even at cryogenic temperatures
  • Design and supply of specialised formwork for construction of the walls, using VSL’s climb-form, slip-form, jump-form or guided gang-form systems
  • All related construction works, such as rebar and concreting, as part of a complete turnkey package

Associated services :

  • VSL’s Heavy lifting technology, using strand jacking to install the roof
  • VSL’s repair, strengthening and preservation services to improve the longevity of existing tank structures
  • Monitoring schemes to control the behaviour of the structure during construction or during the facility’s service life
  • Blast protection for critical infrastructure adjacent to the LNG/LPG tanks

Advantages of VSL’s services

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  • Extensive experience

VSL is recognised as an international leader in LNG and LPG tank construction

  • Compliance with international standards

VSL’s post-tensioning systems comply with ETAG requirements and have been tested under cryogenic conditions

  • Full encapsulation

Superior corrosion protection for tendons

  • Enhanced efficiency

VSL's PT-Plus® ducts offer improved force distribution along a tendon’s length and can also be used to provide a tendon with full encapsulation

  • Versatility

Five different types of anchorages to suit varying project requirements

  • Unique AF Anchorage

Allows the use of vertical tendons without the requirement for a stressing gallery

Contributing to sustainable solutions

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VSL post-tensioning systems enhance the safety of LNG/LPG containment structures, making the concrete walls airtight. Post-tensioning allows for a reduction in the thickness of structural elements and the amount of steel reinforcement, thus saving materials and therefore reducing the CO2 footprint of their construction.

How it works

Post-tensioned tendon layout

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The post-tensioning tendons are typically laid out in two directions:

  • Horizontal or circumferential tendons are used to contain the potential ‘hoop’ forces from the liquid in the unlikely case of a failure of the primary containment. These tendons, which typically pass through 180o or 240o, are stressed from buttresses or pilasters that are located at the quarter or third points around the tank.
  • Vertical tendons are used to control the potential bending loads up the tank’s wall. These tendons run the full height of the wall into the base slab and are stressed at anchorage points located at regular centres around the circumference of the roof.

Main components

Anchorages

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Potentially the most critical part of a post-tensioning system, the anchorage is designed to transfer the tendon’s prestressing force into the structure during stressing and then to secure the individual strands until the tendon has been grouted.

VSL has developed a series of different anchorages to suit every project’s needs.

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GC Anchorage
GC Anchorage

A compact and easy to handle stressing anchorage with a cast iron bearing-plate and machined steel anchor-head. It has been successfully tested and confirmed as suitable for cryogenic applications and represents VSL's new generation of anchorages. The GC anchorage replaces the EC anchorage, which had been widely used.

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E Anchorage
E Anchorage

This stressing anchorage comprises a machined steel bearing-plate, a steel trumpet and a machined steel anchor-head. The E anchorage was the first of VSL's anchorages to be certified for cryogenic applications.

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AF Anchorage
AF Anchorage

This is designed for the lower non-stressing anchorage of vertical tendons where access to the underside of the tank is not possible. It removes the need for a stressing access gallery beneath the structure. It is made of cast iron and is another of VSL's new generation of anchorages. The strands, equipped with compression fittings, are installed one-by-one, then first-stage grouting of the anchorage is carried out, using a high-performance grout with a compressive strength in excess of 100MPa. The tendon is then stressed to the final stressing force and the free length of the tendon is grouted.

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CS Anchorage
CS Anchorage

This high performance stressing anchorage was developed by VSL to provide increased fatigue and corrosion resistance. It comprises a cast iron bearing-plate and a machined steel anchor-head. The anchorage can be supplied with a full encapsulation system should even greater corrosion protection be required.

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L Anchorage
L Anchorage

Also referred to as the 'U' tube, this non-stressing anchorage is used for vertical tendons where the dead-end anchorage is not accessible. The bent rigid pipe allows the strands to be installed after concreting the structure and stressing is carried out from stressing anchorages located at the opposite extremity of the tendon.

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Enhanced durability suitable for cryogenic applications

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VSL’s Research & Development department has tested anchorages under cryogenic conditions (circa -170°C) to ensure that the materials perform as required at extreme temperatures in the unlikely event of a failure of the primary containment. This has involved the full-scale testing of various tendon assemblies at extreme temperature conditions to assess the potential failure modes.

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Anchorage Cryogenic Tests

VSL’s E anchorage and the new-generation GC anchorage have both been successfully tested and confirmed as suitable for cryogenic applications.

Ducting

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The duct provides a void within the concrete section that enables the individual strands to be threaded. It also allows the necessary freedom of movement during stressing. For tendons where PT-Plus® is used, the encasing plastic duct provides an additional barrier to enhance the tendon’s corrosion protection. Strands are locked into place by injecting a cementitious grout into the annulus of the duct that remains after threading of the strands and stressing of the tendon. The grout also provides critical corrosion protection for the tendon.

VSL has two ducting systems that are suitable for use in LNG and LPG tanks.

PT-Plus® duct
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PT-Plus® duct

This high-resistance polypropylene (PP) or high-density polyethylene (HDPE) plastic duct provides improved corrosion protection. It also offers superior force distribution along the tendon’s length, due to the reduced coefficient of friction (µ = 0.12-0.14/rad).

Steel spiral duct
Steel spiral duct

The most common form of ducting is the steel spiral duct, made from thin (0.4mm-0.6mm) steel strips that are cold-formed and joined using specialist equipment. Depending upon the surface treatment of the ducting material, the coefficient of friction is typically higher that of plastic ducts (µ = 0.20-0.24/rad).

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Grouting - the critical final step

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VSL has many years of experience in the sector and recognises the importance of high-quality grouting to ensure the long-term durability of a post-tensioned tendon. VSL has developed a full process dedicated to guaranteeing the quality of grouting activities on site. Grouting materials are checked for their compatibility and the quantities are optimised to produce stable, low-bleed grouts that ensure the complete filling of the ducts and provide an alkaline environment.

VSL has developed a pre-bagged high-performance grout under the trademark VSL-HPI® (High Performance Injection). This satisfies all standard tests for bleed, flow time, strength and volume change as well as the more stringent requirements of the inclined tube test, wick induced bleed test, mud balance test and stability of flow time. VSL-HPI® ensures a consistently high standard of grout, allaying risks due to possible fluctuations in local grout quality.

Cost-effective and easy to maintain

Safe storage of the liquefied gas relies on two elements:

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    The primary or inner containment of the tank must meet the extreme demands of cryogenic conditions (in the case of LNG) and so is typically constructed of high-nickel-content steel, aluminium, stainless steel or in some cases post-tensioned concrete. It must be able to withstand the hydrostatic load of the liquid in its own right.
  • The secondary or outer containment is most commonly constructed from post-tensioned concrete. Its role is to withstand abnormal loads of external origin as well to retain the liquid and vapours should there be a fracture of the primary containment. It is designed and constructed to satisfy both requirements.

Post-tensioned concrete

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Post-tensioned concrete is typically chosen for the secondary containment of LNG/LPG tanks to allow a thinner wall than conventionally reinforced concrete. The reduced crack width achieved with post-tensioning leads to an improvement in the leaktightness, enabling the secondary containment to confine the liquefied gas in case of failure of the primary containment. It also offers improved durability against corrosion of the structure.

VSL’s technology is based upon the principle of post-tensioning, where the prestressing force is permanently introduced into the structure via the stressing of high-strength tendons once the concrete has hardened. VSL has more than 50 years’ experience within the industry and its technology has been constantly refined throughout.VSL’s post-tensioning systems comply with the requirements of national and international standards as well as the specific needs of each individual project.

Example of a full scope construction of a LNG containment structure

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