Research: concrete cravity foundations
CGFs are more economical and have lower whole-life costs than steel foundations. They are also less susceptible to the market price fluctuations caused by the price volatility of marine-grade steel.
The BVG Associates report commissioned by The Crown Estates, “Offshore wind cost reduction pathways: Technology work stream” (May 2012), concluded that monopiles are not cost-effective for typical sites in the Round 3 development programme for offshore wind, and that CGFs are more economical.
The report also acknowledged the cost benefits from the serial production of CGFs and stated that “increasing the production level (of concrete gravity bases) from 50 to 100 or more per year or increasing the pipeline from 5 to 10 years reduces costs per MW by up to 10%”.
The report also identified an additional 1% saving for foundation structures manufactured in the UK, rather than in mainland Europe. This saving would be achieved with CGFs.
CGFs are the lowest-carbon foundation solution. The carbon footprint of a concrete gravity base was found to be 57% less than that of a steel jacket.
A comparative study for a 5MW turbine in 42m-deep water between a steel jacket and a range of concrete solutions corroborates previous findings that concrete solutions have a lower carbon impact. The comparison included raw materials and other resources to be used in the construction of the foundation, but excluded the manufacturing facility, bespoke vessels, installation, maintenance and deconstruction.
The average carbon footprint for CGFs was estimated to be 1190 tonnes CO2e per 5MW unit (in a range of 708 to 1597 tonnes CO2e per 5MW unit). This compared with an estimated carbon footprint for a steel jacket solution of 2770 tonnes of CO2e per 5MW unit. A summary of findings was published in Concrete, February 2013, pp. 32-33.
CGFs avoid the need for piling operations. Piling has been shown to cause significant displacement of marine mammals.
A review of marine environmental considerations was carried out by Marine Space, and it found that CGFs offer a considerable environmental benefit over steel foundations due to the lack of significant noise emissions generated during their installation and emplacement.
With regard to surface area footprint at the seabed, this was similar to other deeper water foundation solutions. The blockage area for all offshore wind farm foundations for deeper water is typically small. Therefore the environmental impacts associated with waves, sediment transfer and blockage, while needed for the environment impact assessment, can be considered of minimal risk.
Both summary report and full report can be downloaded: