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OFFSHORE STRUCTURES

Concept, design and construction of offshore structures related to the oil, gas, petrochemical and renewable energy sectors

  • Offshore Structures
    Once again, VSL was entrusted with the prestressing works on a Offshore structure. The gigantic Troll platform.
    Norway - 1994 read more

    Offshore Structures

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  • Nkossa Barge
    VSL is the PT contractor of the world’s largest floating barge.
    France - 1996 read more

    Nkossa Barge

                     DOWNLOAD PROJECT REFERENCE(S)

The use of concrete for large marine structures is recognized as a proven solution. Compared to steel solutions, prestressed concrete offers distinct advantages for offshore floating and gravity based structures in terms of durability, low maintenance requirements, fatigue resistance, stability and economical aspects.

VSL Offshore develops and builds offshore concrete structures related to:

  • Oil and gas, including off-shore floating and gravity based structures such as FPSO, and on-shore structures with LNG/LPG containment tanks.
  • Marine and harbor civil structures
  • Renewable and sustainable energy projects, with pre stressed concrete base or space frame foundations for the offshore wind farm market.

VSL has been involved in offshore structures for many years, in particular during the 70’s and 80’s when there was a boom in the construction of oil production platforms for the North Sea. Many of these structures were built as post tensioned concrete structures that were cast in the fjords of Norway and then towed to their respective locations in the North Sea.

At that time, VSL carried out the post tensioning works for these large structures, mostly working for Norwegian Contractors, and consequently has built an impressive reference list with these projects. Apart from post tensioning, VSL also offered other specialist services such as slip forming civil works construction, as well as heavy lifting applications for these massive structures.

Since that period, more projects have been carried out. These included:

  • Post-tensioning for the Troll Oil production platform built in Norway in 1994,
  • Post-tensioning for the N’Kossa Barge for Bouygues (1995),
  • Post-tensioning for the Adriatic LNG terminal for Aker Kvaerner (2008),
  • Tjuvholmen in Norway for Spennarmering Norge AS (2008),
  • And a few other heavy lift projects for various platforms.

As the world demand for oil and gas continues to grow, particularly with the rapid development of emerging countries such as Brazil, Russia, India and China, there will be a need to continue developing the oil and gas pools that are offshore. Data from various sources are pointing to declining rates of oil production from existing facilities, yet increasing world energy demand at the same time. This impending shortfall in supply suggests a need for further investment into deeper water and harsher environments to raise production levels. At the moment, offshore oil accounts for 33% of global output, the majority of which is derived from shallow water activity. The strong demand for oil and other resources will support strong oil and gas prices in the coming years providing an excellent backdrop for exploration and production activities.

Following the Gulf of Mexico oil spill in 2010, a rig replacement cycle is currently underway, given that a large proportion of the global jack up and semi-submersible rig fleet are old and the experience learned from the Gulf of Mexico incident has placed renewed focus on safety issues. The replacement of existing rigs, combined with the imbalance of increasing demand and declining production rates, will encourage greater investment in deep water and harsh environment offshore drilling to increase production, thus reinforcing a sustainable offshore and marine sector.

Gas is also considered to be a cleaner source of energy compared to coal and it is believed these environmental concerns will drive the development of more gas reservoirs. Many of these potential oil and gas reservoirs are located in remote harsh environments, cold climates and deep water locations. The capitalization of smaller stranded gas reserves is also being explored utilizing floating production units which can be relocated from one field to another.

Post tensioned concrete is a proven material for these harsh environments. Furthermore – the deep water locations will require floating solutions.

With the increasing demand for new jack ups and semi submersibles, existing ship yards are seeing favorable demand-supply dynamics which may have an impact on the availability of the yards and for global supply of new vessels. Floating LNG re gasification capacity is forecast to grow at an annual average growth rate of 36.4% between 2009 and 2015. Due to rapid domestic energy supply requirements, Indonesia in particular is looking to capitalize its expansive gas fields, many of which are offshore.

It is also forecast that floating storage capacity for oil and other hydro carbons will continue to expand due to limited available land or the intention to utilize available land for more profitable activities such as refining and processing.

As environmental concerns become more evident, and the prevalence of conventional carbon fuels is reducing, governments are looking for alternative sources of energy to supplement their existing supply chains, such as offshore windfarms.

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VSL Services

VSL Offshore Construction services include:

Post-tensioning

Post-tensioning has a major role in extending the use of concrete in marine structures. Post-tensioned concrete protect the steel reinforcement (passive and active) against corrosion since the prestressing keeps the concrete in compression, thus eliminating cracking. It also enhances water and air tightness of the structures and makes them suited for withstanding heavy wave and ice loading.

Site establishment

The sourcing of existing marine facilities or setting of new suitable facilities is carrying utilising our local network. VSL offers local site investigation and preparation, coordination with local authorities, design of suitable production facilities and establishment/ operation of production facility.

Civil Works partner

Conception, design and construction of prestressed, concrete structures related to offshore structures and onshore containment tanks.

VSL Offshore offers a complete package for floating structures, including but not limited to: innovative construction methodology and planning, temporary works design, project management and execution.

Associated Services

Heavy lifting services

Using automatically-controlled hydraulic jacks and strands for lifting, lowering, tilting or sliding of very heavy loads

Formwork systems

Fixed or moveable specialized formwork to build large or special structures such as wind farm foundations, breakwater structures, precast elements.

Structural preservation

Monitoring and preservation of structures using well proven technologies.

Infrastructure protection

Physical security assessments, design and construction of resilient solutions for critical infrastructure at risk from extreme events.

Advantages of VSL’s offer

High technical expertise

VSL’s main asset is its technical expertise and experience in the design and construction of post tensioned concrete structures. VSL has credible design, methods, construction expertise and strong project management capabilities.

VSL Network

VSL also has a well developed network of local entities that can assist with identifying project opportunities and provide strong local support.

Locally made

Concrete structures are often built locally and require a high level of local content, especially for the materials and the labor forces required. As such, they allow countries to develop locally skills and capacities that will eventually benefit further to the country development and to other projects.

An integrated service from design to delivery

VSL has the capability to design and construct a project entirely from the early design concept and development to the full construction and commissioning.

Contributing to sustainable solutions

Durability with low maintenance

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Concrete is a material very stable and durable that naturally protects steel reinforcement embedded into its matrix. The addition of post-tensioning compresses the concrete and eliminates cracking. The resulting prestressed concrete structures are very durable, long lasting and require very low maintenance during their service life.

Sulfate and chlorides resistance

Concrete has been used in seawater exposures for decades with excellent performance. However, special care in mix design and material selection is necessary for these severe environments. A structure exposed to seawater or seawater spray is most vulnerable in the tidal or splash zone where there are repeated cycles of wetting and drying and/or freezing and thawing. Sulfates and chlorides in seawater require the use of low permeability concrete to minimize steel corrosion and sulfate attack. Special mix designs have been developed for these applications, and attention must be paid to details in the structure of such as rebar cover and tendon corrosion protection.

Abrasion Resistance

Concrete is resistant to the abrasive effects of ordinary weather. Examples of severe abrasion and erosion are particles in rapidly moving water, floating ice, or areas where steel studs are allowed on tires. Abrasion resistance is directly related to the strength of the concrete. For areas with severe abrasion, studies show that concrete with compressive strengths of 80 to 130 MPa work well.

Locally sourced

Concrete is a widely used materials that is generally composed of materials locally available, thus reducing the CO2 from transportation.

Concrete, a green material

The predominant raw material for the cement in concrete is limestone, the most abundant mineral on earth. Concrete can also be made with fly ash, and silica fume, all waste byproducts from power plants, steel mills, and other manufacturing facilities. These materials are not only recycled materials but will also enhance the durability of the concrete.

How it works

Description of the different structure approach

Main components and equipment

Concrete

Advantages of concrete in a marine environment

  • Long design life
  • High strength/ high performance concrete is utilised (50MPa or greater, low permeability, suitable workability and stability as well as proven and adequate durability)
  • Resistance to harsh conditions
  • Extreme loads
  • Low maintenance
  • Can be sourced anywhere
  • Excellent cryogenic properties well suited for LNG applications
  • Short term fire protection properties

Rebar

  • Hot rolled ribbed bars of weldable quality and high ductility (yield stress of 500MPa or more)
  • High reinforcement ratios in certain locations (up to 500kg/m³)
  • Large diameter bars used
  • Use of couplers at construction joints/ heavily reinforced zones

Post-tensioning

  • Resisting tensile strength
  • Reduce the reinforcement ratios

Floating structures

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N’kossa barge, France, PT supply and installation

Deep water operating conditions may sometimes exclude the use of conventional jack up or gravity based structures (GBS).

Alternative concrete solutions may be more economical (even compared to a converted supertanker) than steel when considering the capital expenditure and maintenance costs. Increase operation life is also to be considered. The cost of a barge may be the same as the jacket solution, but it can bear a payload capacity superior than the jacket version.

Gravity base solutions

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Troll condeep platform, Norway

VSL earned a large experience in the North Atlantic sea during the construction of gravity oil platform base. 21 structures were completed over a 20 year involvement. The future now lays on LNG offshore tank or shallow water concrete installations, while deep water are better suited for floating structures.

Projects include :

  • Adriatic LNG Terminal, Spain
  • Troll GBS
  • Draugen platform
  • Troll condeep platform, Norway
  • Gulfaks C, Norway
  • Gulfacs A, Norway
  • Statfjord B&C, Norway
  • Brent C&D, Norway
  • Maureen Alpha Platform, Norway

Wind farm solutions

Gravity base structures

The use of gravity base structures in concrete for wind farm foundations is a real option when one or more of the following conditions / requirements are present:

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Project under development Patent
with Olav Olsen and Vici Ventus
applicable in Korea, China, Hong Kong, India
  • long design life
  • stationary structures
  • extreme loading
  • difficult geo-technical conditions or
  • high local content required
  • water depth 20-50 m

Wind farm space frame solution

The patented Space Frame Tower is a triangular shaped structure built-up from uniform standard elements for wind farm foundations.

  • Optimal strength and dynamic properties for support of wind turbines
  • A standardized solution for specific turbines makes independent of variable site conditions
  • Simple and efficient assembly close to wind farms
  • Optimal structure for cost effective mass production.

Foundation types for SFT:

  • Gravity base structure
  • Suction buckets
  • Piles pre-installed directly under legs
  • Piled foundations frame
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courtesy Dr. Techn. Olav. Olsen design

FSO Concept

The patented FSO (Floating Storage and Offloading – capacity varies from 1 mil bbls (160,000m³) to 2 mil bbls (320,000m³)) vessel provides a concrete oil storage facility for deep water areas. It is suitable for in loading of crude oil from tanker vessels, intermediate storage in the concrete terminal oil tanks and offloading to export tanker vessels. VSL Offshore and Dr. techn Olav Olsen have been developing the concept together.

Offshore civil structures

Immersed structures

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Immersed construction held in gravel basin, led VSL to optimise the construction with specially designed formwork. Immersed structures require VSL’s expertise in heavy lifting operations to design the suitable system to meet the client’s challenges. VSL can provide an all integrated operation comprising the following services:

  • Post-tensioning
  • Formwork design, supply and operation
  • Skidding and heavy lifting technical solution
  • Supervision

Projects include:

  • HKMZ (Hong Kong Macao) immersed tunnel, heavy lifting and sliding works, 2012-2013. To be verified with CVE
  • Tunnel Metro C, Czech Republic, Technical solution, sliding,
  • Thu Thiem Immersed Tunnel, Vietnam, Design and supply formworks, post-tensioning, sliding, 2005-2009.
  • Busan-Geoje Fixed Link, South Korea, Post tensioning, Formwork operation, construction of segments
  • Øresund bridge foundation caissons , Denmark/Sweden, Lifting and lowering of two 20,000 t caissons 1997
  • Rostock underpass, Germany

Breakwater / Jetty

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Tangier Med 2, Morocco, lifting & skidding caisson 3400t, 2011-2013

The construction of breakwater jetties shall be carried out using optimized long line production methods to meet project milestones.

VSL provides:

  • Sourcing exisiting/new facilities for production line establishment
  • Coordinate with local authorities for site implementation
  • Design optimization for form works system (climbform, slipform, concrete and rebar pieces
  • Heavy Lifting
  • Post-tensioning
  • Production and supervision

Projects include:

  • Datford Bridge caisson, England, 45.000m² slipforming, 1988-1989
  • Monaco harbour, Monaco, 1999-2002
  • Tjuvholmen, Norway, post tensioning, 2007-2008
  • Tangier Med 2, Morocco, lifting & skidding caisson 3400t, 2011-2013

LNG containment structures

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VSL has been involved for the construction of over 900 LNG tanks around the world, from Post tensioning service to full tank civil works construction.

Scope of works proposed includes:

  • Foundation and ground improvement
  • Post-tensioning, design detailing, supply and installation
  • Formworks design, supply and operation
  • Full construction services for the outer containment prestressed concrete works includes the supply of materials (concrete, rebar, post-tensioning), and the supply of management of the resources required for the full construction works.
  • AF anchorage unique VSL design for the lower non-stressing anchorage of the vertical tendons where access to the underside of the tank is not possible. AF anchorage has proven to be most suited for LNG construction

Some of these optimizations on Singapore LNG project, contributed to reduce the construction cycle to 11 days per formworks climb.

Projects include:

  • SNLG, Singapore, civil works and Post tensioning, 3 tank 180.000m³, 2011-2014
  • LNG tanks, Quintero, Chile, 2No. Tank of 160.000m³, Post tensioning, 2009-2010
  • Dragon LNG Tanks, Wales – UK, 2No. tank 154.000m³, post tensioning, 2005-2007
  • Sines LNG Terminal – Portugal, 2No. Tank 82m diamx 43m height, post tensioning, 2003-2004
  • Damietta LNG Terminal, Egypt, 2No. Tank 150.000m³, post tensioning, 2002-2004
  • Damietta LNG Tank, Egypt, 2No. 150.000m³ tank, civil works , 2001-2004
  • Qatargas LNG Plant, Qatar,4No. 85.000m³, Post tensioning 1994-1996
  • Pyeong Taek LNG Tanks, South Korea, 5No. Tank 100.000m³, Post tensioning, 1993-1996
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SLNG Singapore
SLNG Singapore

For further information about LNG, please click here

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