Funds for gravity foundation design work

The award is part of the DECC/TSB innovation fund aimed at supporting development of next generation windfarm technology.

The concrete gravity foundation for large offshore wind turbines proposed by Gravitas has the potential for significant cost savings over more traditional foundation designs that have dominated the offshore windfarm scene up to now. Reducing the costs of offshore wind is seen as critical to securing the confidence of investors and this award is aimed at helping to demonstrate the cost effectiveness of the Gravitas project along with the development of reliability standards for concrete offshore windfarm foundations.

Maidenhead UK based Gravitas Offshore comprises a consortium of HOCHTIEF, Costain and Arup and draws on their long term relationships from previous projects to serve the growing energy sector. Turbine foundations currently make up around a quarter of an offshore windfarm’s total delivery cost and determined efforts are under way to reduce this cost.

Windfarm developers have a number of choices when selecting foundations, influenced by a number of variable factors including water depth, turbine type and size and conditions encountered at the windfarm’s location. With the trend for developments to move into deeper, more hostile waters, along with the steady growth in turbine size, the suitability of, for example, steel monopiles becomes limited.

Gravitas’ design involves a reinforced concrete, non-piled, ballasted gravity structure suitable for water depths up to 60m and turbine sizes up to an anticipated 8MW. Minimal seabed preparation is required through accommodation of existing seabed slopes and surface sediments. A particular feature is that the design presents the opportunity for the repowering of the turbine itself at the end of its design life without requiring replacement of the foundation. Assuming 35m water depth, hub height is 90m LAT, outer caisson diameter 31m, top of shaft diameter 6.5m, and concrete volume 2,800m ³ with 840 tons of steel reinforcement. The vertical component is achieved using an adaptor ring at the interface between the foundation and the tower.

Construction is carried out onshore, the foundation’s 10m draft negating the requirement for deepwater port facilities, in turn resulting in significant savings with the installation process. The foundation itself is self-buoyant with only standard suitable tugs required for the tow to site rather than specialist installation or heavy lift vessels. An increased installation season is claimed through a reduction in weather dependency. Other particular advantages include a reduction in the sea-borne noise and vibration typical with other piled foundation designs, low maintenance requirement, minimum scour protection requirement, and the ability to be re-deployed to other locations.

By Peter Barker