CONCRETE AND WATER
Concrete foundations for offshore wind turbines (Photos: Gifford.)
Energy policy in Britain and the European Union requires a substantial increase in renewable energy from wind turbines by 2020. But onshore windfarms are not popular with the public and the limitations of current production techniques mean capacity targets for windpower look increasingly unlikely to be met.
WILLIAM BROOK-HART, GIFFORD
T
he Government has set a target of 33GW capacity of renewable energy production by 2020 from the country’s installed base of offshore windfarms. Given that Britain’s current capacity is just 500MW, there is a long way to go. There will have to be a massive increase in construction capacity if the industry is to build sufficient wind turbines and their supporting structures. The turbines themselves will need to be larger (3–5MW each or more) and they will have to be installed in deeper, rougher waters further offshore. Depths of more than 20m will be commonplace. The present manufacturing and installation supply chain is not coping fully with even the current demand for wind turbines, let alone a substantial increase in build rate. We require adaptation and innovation of existing technologies and procurement methods to allow us to build foundations for a new generation of wind turbines cost-effectively and reliably. Early investment is essential so that affordable designs and construction solutions can be developed and the required manufacturing infrastructure built to handle a new round of windfarm construction. Gifford has been working for a number of years on concepts for alternative designs and construction solutions for offshore wind generator towers and foundations. It is the lead partner in a feasibility study jointly funded by the Department for Business Enterprise and Regulatory Reform (BERR) and E.ON, the energy group and a member of the design team. The team includes the BMT Group, specialising in naval architecture, offshore transport designs and offshore logistics; Bierrum International, a specialist contractor of concrete towers and structures; the University of Nottingham, providing geotechnical research; and DONG Energy, like E.ON, a windfarm developer and operator. The first offshore windfarms, built up to 2007, typically used steel tubular monopiles 4–6m in diameter to secure the foundations for the wind turbines in waters up to 15m deep. Embedment needs to be 20–30m and the structure projects above the sea surface as a transition piece of slightly larger diameter is fitted. A tubular tapered steel tower is then bolted to the foundation to provide a turbine height of 75–90m above low water. This construction technique suited both shallower sea locations and also the technology and equipment of the offshore oil and gas industry. Furthermore, developers favoured engineering, procurement and construction
(EPC) contracts that provide a substantial transfer of risk. This procurement method, short timescales for tendering and an uncertain flow of projects have all inhibited the development of alternative foundation designs and installation methods – until now. In future installation rounds, where turbines will be bigger and sited in deeper waters, steel monopile foundations are unlikely to be viable and so new construction and delivery methods must be found. The team determined that new types of foundations will be necessary and new construction methods must be developed, otherwise foundation costs will soar. Previous work by Gifford, for The Concrete Centre, established a number of potential advantages of concrete gravity foundations for offshore wind turbine towers and this forms the basis of the company’s proposed solution. The study team proposes a single structure base unit of prestressed concrete comprising a hollow conical stem and a circular raft footing with continuous reinforcement of passive and prestressed steel. The anchoring of the turbine to the tower determines the dimensions of the stem and the layout of the anchor bolts is designed to resist loads
Figure 1 top: Concrete gravity base foundation for offshore wind turbine. Figure 2 above: Model showing foundation with wind turbine.
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CONCRETE AND WATER
(Illustratiions: BMT Group.)
Figure 3 right: Base unit build and launch. Figure 4 far right: Offshore transport and installation.
“The use of concrete gravity base foundations offers a sustainable solution. Concrete foundations have a potential design life of well over 50 years, with minimal maintenance.” at the base of the tower from dead weight, wind and turbine vibrations. The dimensions of the raft footing are determined by the overturning moment at the foot of the stem and the allowable bearing pressures on the seabed and are therefore site-specific. The design of the foundations has been analysed for a range of water depths, sea states and wind regimes. These have confirmed the robustness of the design, while highlighting the need for proper preparation of the seabed as well as the implementation of scour protection around the footing after construction. The base unit is built onshore and has been designed to sink even under conditions of maximum buoyancy of the hollow conical stem. This allows the unit to be transported out to its offshore location using a bespoke delivery system which has also been developed by the team. A major obstacle to deepwater installations in the past has been the practicality of handling the large concrete structures weighing more than 2000 tonnes. Now Gifford has created an innovative solution using dedicated transport and installation barges along with conventional offshore support vessels.
Once built, the foundation is transferred to a submersible launch barge and sunk. The transport and installation barge is brought to the launch barge and engages the foundation. The barge is deballasted and can then be towed with the foundation to its deepwater location. The barge is positioned, flooded and disengaged from the foundation once the foundation has landed on the seabed. There is no need for expensive heavy-lift ships and jack-up barges. The use of concrete gravity base foundations offers a sustainable solution too. Concrete foundations have a potential design life of well over 50 years, with minimal maintenance. This enables continual reuse of the foundation with replacement turbines installed every 25 years or so. When eventually decommissioned, the entire foundation can be refloated and recycled with no piles left in the seabed.
Concluding remarks The study team has produced an integrated design solution for the construction, installation, operation and eventual complete removal of concrete gravity foundations for offshore wind turbines. It combines year-round onshore production and seasonal offshore delivery of units. The onshore construction process using slipformed concrete for speedy production enables conventional construction by mainstream civil engineering contractors. Prestressed concrete results in a structurally efficient design with good fatigue resistance and durability. Delivery using the innovative and relatively lowcost transport and installation barges avoids the need for expensive heavy-lift vessels and jack-up barges. If this alternative to piled foundations means greater diversity in the supply chain then it might just be possible to meet the Government’s very challenging offshore windpower tar■ gets after all.
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FEBRUARY 2009 CONCRETE