17 Mar 2015
A number of independent reports have stated that there are significant benefits to be gained from using concrete gravity bases; in construction, installation and in use. Andrew Minson discusses if the latest installation in the English Channel will be the turning point for this technology in the UK.
Gravity foundations are concrete shells that are placed on the sea bed and then ballasted, and represent a simple and effective engineering solution for offshore wind farms. With the proven long term performance of concrete structures, in a wide range of marine environments, and over 300 installed concrete offshore wind foundations across the world, this foundation is extremely durable and provides a solution which is both economic to build and delivers considerably reduced maintenance over the life of the structure.
Concrete gravity foundations (CGFs) use locally sourced UK materials, the manufacturing facility can be developed and operational quickly and there is no need for costly heavy lift cranes and complex installation processes, as demonstrated by the latest installation in the English Channel.
English Channel installation
The Seatower installation of a concrete gravity foundation (CGF) located 13km off the coast of Normandy in deep waters is a project for Fecamp development partners EDF Energies Nouvelles, Dong Energy and wpd offshore. The installation only used standard towing vessels and was tested to prove that installation was possible in the challenging weather conditions typical of the English Channel and the North Sea.
Petter Karal, CEO of Seatower said: “Cranefree Gravity® foundations are generally more cost-efficient, quicker to install and less risky than the alternative steel foundation designs for equivalent depths of water. They also allow for local construction, which provides welcome economic activities to the coastal communities close to the wind farms. “Our technology will ease and speed up the construction of offshore wind farms at a time when cutting costs and risks is necessary to help the sector progress to more challenging project sites.”
The base weighed approximately 2,000 tonnes, with a 30m diameter and was in a water depth of 27m. The base on this occasion is being used to carry maritime measuring equipment.
As we can see from Seatower’s installation project, construction and installation were all achieved using established and efficient technologies and working practices.
Longer life with capacity for evolving turbine technology
Turbine manufacturers are already developing larger turbines with increased generation potential, but these are larger, heavier and will have an impact on tower and foundation requirements. Concrete gravity bases provide the robustness and performance required to meet the engineering demands placed on foundation solutions and as widely reported, also represents a step change in technology that can deliver a cost saving against alternatives.
UK materials – local, responsible sourcing
Concrete gravity bases will also meet the drivers for UK content, skills and competition. Current technology for foundations is monopiles but this technology is already at the limits of its performance capabilities. The need for a new technology gives an opportunity for a solution that does not use imported raw materials, maximises UK job creation and provides an alternative and hence competition. Concrete gravity bases is this solution .
For concrete, all the raw materials can be UK sourced, production also provides opportunity for jobs in design and construction as well as those related to the ports’ infrastructure, transportation and installation processes.
A typical manufacturing facility producing 50 bases per year will generate 500 to 600 jobs directly at the facility. The extraction and delivery of the concrete constituents will generate a further 100 UK jobs.
Less vibrations and marine disturbance
In UK waters, a significant impact associated with the installation of offshore wind farms is the significant level of underwater noise and sound pressure waves resulting from piling operations. The hammering of steel piles can produce underwater sound and pressure waves at levels high enough to cause death, damage and displacement of marine mammals and some fish species; including fish eggs and larvae. International legislation now reflects the serious consideration of these possible effects.
As wind farms move into deeper water these offshore environments may also provide larger potential for more extensive sound wave propagation, with less potential for attenuation than in near-shore environments. Both steel jacket and tripod foundations require piling to secure the ‘feet’ to the seabed; resulting in multiple noise emissions per foundation. Therefore increased noise impacts are a reality that will require closer scrutiny.
Less embodied carbon than a steel jacket
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 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.
Summary
To date there are over 300 installed concrete offshore wind foundations across the world, as well as similar concrete caissons being used as foundations for thousands more marine structures across the world. This demonstrates the confidence of engineers in designing and constructing these structures and the established expertise. The EDF project in the English Channel, provides the offshore wind industry with a recent demonstration of the installation of this foundation solution and can provide yet more confidence in this solution. There are so many benefits to the developer, the constructer, the UK and the environment offered by concrete gravity bases, this can be the turning point for the UK offshore wind market.
More information on Gravity Foundations.