Offshore Renewable Energy: Wind Construction - Innovating a Cost Effective Approach

Ed Dudson, Technical Director, BMT Nigel Gee

Based on UK Government climate change commitments, a six-fold increase in output from renewable energy sources will be required by 2020.

With offshore wind set to deliver the lion's share of the new renewable energy portfolio, a significant upturn in the rate of construction of offshore wind farms is required. Traditional transportation, construction and installation methods for offshore wind turbines are intricate, expensive and involve multiple offshore operations in an environment that is prone to inclement weather. The current methodology also relies on using very costly offshore jack-up barges, installation vessels and floating cranes - some of which are now in short supply.

In a bid to tackle this complex challenge, Ramboll (previously Gifford), BMT Nigel Gee and Freyssinet (GBF) have come together to develop a unique and innovative solution to provide foundations for offshore wind turbines without the need for specialist offshore construction plant. GBF's concrete gravity foundations can be deployed without the need for offshore piling operations, costly specialist vessels or heavy lifting at sea. The potential of the scheme is so great that GBF was selected by the Carbon Trust for its Offshore Wind Accelerator - Foundations programme.

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As a key enabling technology, the Transportation and Installation Barge (TIB) allows the entire wind turbine structure to be transported and installed in one operation. Ed Dudson, Technical Director at BMT Nigel Gee, a subsidiary of BMT Group highlights the potential cost advantages of this patented technology and why such an approach could revolutionise the development of future offshore wind farms.

The Carbon Trust believes mass deployment of offshore wind is critical to bridge the UK's energy gap and to meet the UK's targets for security of supply, carbon reduction and renewable energy. To meet the EU's 15% renewable energy target for the UK, as much as 40% of electricity must come from renewables in 2020. In 2008, the figure was just 5%, therefore an eight-fold increase is required. Offshore wind is the only technology that can be deployed at sufficient scale to achieve this, and has the potential to supply as much as 25% of the UK's electricity by 2020.

Wind1The Carbon Trust's Offshore Wind Accelerator R&D programme was set up to not only drive down costs and accelerate deployment, but to provide an opportunity for collaboration with industry experts so that innovative and cost effective solutions could be developed for the next generation wind farm foundations in deeper water, located further from shore. From over 100 initial proposals the GBF® solution was one of seven design concepts which received funding for a Phase 1 Study. Four of these designs proceeded to be developed and independently scrutinised by the Carbon Trust in a further de-risking Phase 1.5 Study and the GBF concept became the only gravity based concrete design.

Traditionally, wind turbines situated in shallow waters just off the coast would utilise monopile foundations, a simple design in which the wind tower is supported by a steel pipe and typically driven into the seabed by either large impact or vibratory hammers, or the piles are grounded into the sockets drilled into rock. Although reasonably cost effective, these monopiles are not suited to water depths of more than about 20m and are therefore unable to address many of the Crown Estate's Round 3 locations. Other challenges that developers face with this approach include:

  • High transportation costs- expensive  jack-up vessels and other specialist vessels are required in order to transport the foundation and turbine structure to the specified location. Hiring such bespoke vessels can incur daily costs of £200k, as well as numerous trips back and forth to the wind farm site to deliver all the components.
  • Disturbance to marine life- the large impact or vibratory hammers used to construct and install the monopile foundations and other foundations that require piling create significant noise levels which can impact on local marine life
  • Steel versus concrete- traditional methods use steel structures which can be costly and subject to supply constraints
  • Short supply- Demand for vessels and jack-up barges is outstripping potential supply due to increasing efforts in the development of offshore wind farms in northern Europe. Vessels that are used to transport the structure are also multi-purpose and will therefore be utilised for other construction activities in other sectors such as offshore oil and gas.

It's important to note that concrete gravity foundations will only be suitable for certain locations but where they are appropriate they will help to address the challenge of locations further offshore by providing a highly cost solution.

The GBF® Integrated Solution for Offshore Wind Turbines seeks to avoid many of the supply chain hot-spots and inefficiencies of the traditional methods. Current methods involve the construction and installation of the foundations and turbines in several steps. After fabrication, the foundation is transported to sea on a specialist vessel and installed using a floating or jack-up marine crane and possibly marine piling equipment. Then the tower segments, nacelle and blades are transported to the site and erected, again using a specialist vessel. These specialist vessels such as Jack-up barges and floating cranes are in short supply due to the increasing demand and as a result the developer becomes tied to a specific delivery window early on in the process. The GBF® solution has been designed to overcome these disadvantages by minimising offshore operations and
maximising onshore assembly works. The solution involves construction of gravity based concrete foundations in a production line facility on land and assembly of the entire wind turbine structure (including tower, nacelle and blades) onshore. This methodology allows land-based construction plant to be used with associated benefits of greater availability and lower risks and therefore lower costs. Furthermore safety and quality during the construction phase will undoubtedly improve and, as less of the process is weather dependant, productivity and surety of delivery can be enhanced. In addition the turbine can be pre-commissioned prior to installation. 

Wind2

The GBF® solution secures these advantages through three distinct strategies, one of which is the transportation and positioning on the sea bed of the completed turbine using a purpose built Transport and Installation Barge (TIB).

The TIB enables the loading and transportation of the complete wind turbine to the site and its subsequent lowering onto the prepared sea bed foundation. The TIB is U-shaped in plan and it engages with a lifting collar cast into the gravity base. The TIB has three buoyancy towers which enable it to be ballasted down to the level of the gravity base then, upon connection, to re-float to transportation depth. The capacity and position of the towers relative to the wind turbine ensure the stability and seakeeping characteristics of the TIB and its load combination during the transportation and positioning process. The TIB is unmanned and non-powered so it is towed by conventional tugs to the windfarm and subsequently manoeuvred into the required position with the assistance of additional tugs.  Once in position, the TIB is then ballasted so that the wind turbine is lowered gently and precisely on the prepared seabed. Once the correct positioning has been verified, the TIB releases the gravity base, moves away and is re-floated, ready for its return journey back to the quayside to collect the next unit. The scour protection can then be placed, the inter-array cables connected and final commissioning of the wind turbine can be completed. The same TIB can then be used for eventual decommissioning of the wind turbine at end of life. The TIB will have a wide operating window as sea keeping analysis has demonstrated that the motions and accelerations during transit are acceptable in conditions up to 3m significant wave height.

GBF is now working with VINCI Offshore Wind to bring this technology to the market and has completed some initial studies for developers and energy companies which have clearly shown the advantages. There is no doubt that the offshore wind market will experience accelerated growth in the next five to ten years. Although regarded by many as a well-established industry, the offshore wind market is still quite immature will continue to evolve as the sector better understands what technological innovations and solutions are required in order for the UK to meet its targets. The GBF® solution is one such concept that could potentially revolutionise the way in which wind turbine structures are constructed, transported and installed - the question is will the market realise this before it's too late.

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Contact Information

Ed Dudson

BMT Nigel Gee Ltd
Building 14, Shamrock Quay
William Street
Southampton
Hampshire 
SO14 5QL
 
United Kingdom 

Tel: +44 (0)23 8038 2528
Email: admin@ngal.co.uk

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