The first floating offshore wind (FOW) turbine built in France was launched at Saint-Nazaire on 13 October just 15 months after construction began. The project, beginning with the turbine, tower and floating foundation, are being funded by the European Commission. This coming winter, the demonstrator will be towed to the SEM-REV test site off Le Croisic between Nantes and Lorient in Brittany. There, it will then be connected to the French power grid for a two-year test period to validate the design concept and technologies and measure performance under real-world conditions.
(© Mer et Marine - Vincent Groizeleau)
Solo round-the-world sailor Catherine Chabot launches the Floatgen demonstrator (© Floatgen)
(© Mer et Marine - Vincent Groizeleau)
Advantages of FOW over BFOW
“Bottom-fixed offshore wind (BFOW) farms count some 3,600 turbines now in service worldwide, nearly all of them in northern European waters. The technology is, however, limited to sites with a water depth of 40 metres or less. Today, a large proportion of the most suitable sites are operational or in the development pipeline. Floating offshore wind farms are the new frontier in that they can be set up in waters of any depth. FOW opens up the market for offshore wind energy to all maritime countries instead of the few, mostly in northern Europe, with large expanses of shallow water. FOW technologies also make better use of wind resources, the best being typically farther out to sea. Sites for FOW farms that are farther from the coast also overcome issues like visual impact and conflict of use with the fishing industry and others. On the engineering front, one of the chief advantages of FOW over BFOW is that nearly all the construction work can be performed ashore. FOW turbines can be installed without the need for costly dedicated installation vessels that have to be booked well in advance and only operate in fair weather. FOW technologies offer lower costs and risks while at the same time improving offshore wind energy’s social acceptability,” says Paul de la Guérivière, CEO of Floatgen partner Ideol.
Paul de la Guérivière, CEO of Ideol (© Ideol)
Ideol’s innovative floating foundation
Ideol designed the Floatgen project’s floating foundation. The company, set up in 2010 and currently employing 65 people, is based at La Ciotat near Marseille. The foundation is a square ring-shaped platform enclosing a ‘damping pool’, a breakthrough concept patented by Ideol engineers. The concrete or steel foundation supports a conventional turbine and tower without significant design changes. In this respect, the Floatgen demonstrator is an excellent example in that it uses a Vestas turbine initially intended for an onshore wind farm project that was subsequently cancelled. While the turbine module had to be modified slightly to work with the floating foundation, this only required minor changes to the control and command system to accommodate sea conditions and wave motion. The tower, on the other hand, had to be strengthened to handle significantly higher loads.
According to the design team, the Damping Pool® and anchoring system can, thanks to their hydrodynamic properties, maintain the amplitude of platform motion within tight limits irrespective not only of sea conditions, but also of tower and turbine size.
Summer 2017: Tower transition piece being lowered onto the demonstrator’s foundation (© Ideol - BYTP)
The Floatgen demonstrator during rotor assembly (© ECN)
Simple and economical
Beyond the engineering aspects, the main challenge is, of course, economic: “The Damping Pool® concept makes Ideol floating foundations compact and simple. Moreover, it is currently the only technology to offer a viable future for FOW projects thanks to its low cost and scalability”, says Paul de la Guérivière.
Ideol foundations are simple to build and maintain. “All tasks performed offshore are both costly and dependent on fair weather. Indeed, this is precisely why we set out to develop a low-cost, low-risk concept. Our self-stabilising foundations include neither pumps nor electromechanical systems, making them inherently maintenance free. Concrete types can remain at sea for 30, 40 or even 50 years without maintenance”, says Bruno Geschier, Ideol’s chief sales & marketing officer.
Bruno Geschier, Ideol’s chief sales & marketing officer, on site (© Mer et Marine - Vincent Groizeleau)
More business for local contractors
Ideol developed the floating foundation/Damping Pool® concept specifically to facilitate local construction in steel or concrete depending on local skills and production costs. In Europe, concrete is relatively inexpensive while the level of expertise is high. In Asia, many contractors have high-level expertise in working with steel, which may make it preferable to concrete. This is important since, compared with BFOW projects, FOW offers local contractors better business opportunities. Given their size, foundations are typically built at a port close to the mooring site. The construction of the concrete Floatgen foundation called for a peak of 70 workers on site along with contributions, on various scales, by some 70 subcontractors throughout the region.
January 2017: Foundation formwork on barges (© Ideol - BYTP)
The Foundation in March 2017 (© Ideol - BYTP)
Foundation after return to construction dock (© Ideol - BYTP)
Early October 2017: Rotor assembly (© Mer et Marine)
One of the chief advantages of Ideol floating foundations is their relatively small size. The foundation for the Floatgen demonstrator and its 2-MW turbine is just 36m on a side for a total height of 9.5m. Its 7.5-metre draught is far less than that of competing FOW concepts, making construction compatible with a larger number of ports. Note also that foundation size is not proportional to the turbine’s power rating. For example, the four Ideol foundations for the EolMed windfarm in the Mediterranean will measure just 50m on a side despite the fact that the 6.2-MW Senvion turbines will be more than three times more powerful than the Vestas turbine for the Floatgen project.
An innovative, 5000-tonne concrete foundation
Viewed from afar, the Floatgen foundation looks like it is made of steel. But no; it is made of concrete that, like the steel transition piece at the base of the tower, has been painted bright yellow in compliance with the relevant maritime regulations. It was built by Bouygues Travaux Publics, one of France’s leading specialists in concrete structures. The foundation weighs over 5000 tonnes. “One of the main aims of the Floatgen project is R&D with a view to innovation at every step. One early step was digital modelling, another the development of a special low-density self-compacting concrete that both minimises the structure’s weight and can be poured without having to vibrate it. Yet another was an innovative construction process on three interconnected barges. When the foundation was complete, the barges and foundation were towed into the Joubert drydock at the entrance to the port of Saint-Nazaire. The barges were then sunk in the flooded drydock allowing the foundation to float free,” explained Nicolas Jestin, BYTP’s sales manager.
The low-density self-compacting concrete developed by BYTP weighs 2 t/cubic metre, compared with 2.4 t/cubic metre for conventional concretes. This was achieved by adding lightweight granules made of an expanded material to the mix. “This engineering concrete has special characteristics; it can withstand high structural loads, but is unaffected by years of exposure to seawater.”
Foundation with yellow paint (© Mer et Marine - Vincent Groizeleau)
Floatgen approaching completion on a foggy day (© Mer et Marine - Vincent Groizeleau)
Like a ship’s hull
The foundation comprises 16 cells, the volume of air in each being calculated to provide the necessary floatation. “The foundation behaves like a ship’s hull, complete with watertight compartments. It will remain afloat even if a cell is breached. We tested this by filling the cells with water one at a time while checking that the foundation continued to float normally.”
BYTP’s contribution drew heavily on its proven expertise in floating concrete structures, including the seawall (352m x 28m, 160,000 tonnes) built in the early 2000s to extend the port of Monaco and, before that, the concrete hull of the Nkossa barge, a massive floating offshore gas production facility measuring 220m by 46m and weighing 71,000t. The barge was built in 1995 and towed to its production location off the coast of Congo. “We know how to design and build long-lasting floating structures that can withstand extreme loads and operating conditions. The Nkossa barge has now operated at sea for over 20 years without any trouble despite its constant exposure to pounding swells.” The concrete of the Floatgen foundation is 35cm thick on average and the structure fully protected against corrosion. In addition, the outer coating protecting the first steel is sufficiently thick to prevent any contact between steel and seawater. The concrete’s special properties include a low risk of cracking and very low porosity to prevent the infiltration of seawater.
Nylon mooring lines (© ECN)
Nylon mooring lines, a world first
The mooring lines selected for the Floatgen project are also innovative. The foundation will be held in place by six semi-taut lines, each connected to a 16-tonne anchor on the seabed. Two lines will be connected to the front of the foundation and four to the rear. Each line is 400 to 600 metres long and will function as a catenary mooring. While the end portions (attached to the anchors and the foundation’s anchorage points) are made of traditional mooring chain, the rest are made of nylon. Brittany-based LeBéon Manufacturing won the contract to supply this innovative mooring system with the help of two partners with extensive experience in the offshore sector, namely Belgian specialist Bexco for the nylon rope and South Korean supplier Dai-Han for the chain parts. This design — a world first for a permanent mooring system of this size — is both lighter and less expensive than more conventional approaches. “These 220-mm nylon mooring lines will hold the demonstrator at precisely the required location, even in the biggest storms,” says Bertrand Alessandrini, head of development at the Centrale Nantes engineering school which manages the SEM-REV test site where the mooring system was pre-installed in August.
Anchorage points on foundation (© Mer et Marine - Vincent Groizeleau)
Anchorage points on foundation (© Mer et Marine - Vincent Groizeleau)
Anchor (© ECN)
Summer 2017: Pre-installation of mooring lines (© ECN)
Tow to test site
The SEM-REV test site is located 22km off Le Croisic in a restricted maritime area. The Floatgen demonstrator will be moored in water 33m deep and connected to the onshore medium-voltage grid via the site’s dedicated power cable.
Floatgen will be the first demonstrator to use the SEM-REV site, the only offshore facility in France for marine renewable energy research. The site’s leading promotors include the Centrale Nantes engineering school and national science research agency CNRS. It took ten years of negotiations, administrative procedures and preparatory work before the site was ready for business. The site’s electrical hub can accommodate the simultaneous connection of up to three turbines to an 8-MW submarine power cable to the onshore grid. Other SEM-REV facilities include an onshore research centre and substation, as well as a data cable comprising 24 optical fibres and a full suite of instruments to measure sea and weather conditions, including wind, swell and local parameters. The 1-sq.km site is clearly delimited by buoys and has all the necessary administrative approvals.
A two-year test campaign
The Floatgen test campaign at the SEM-REV site will gather data to validate the demonstrator’s operation, performance and behaviour over time and under varying environmental conditions, including powerful winter storms when the Atlantic Ocean produces sustained winds of over 100kph and swells of up to 16m off Le Croisic. These conditions combined with the relatively shallow water are particularly challenging. Indeed, the project teams believe that the test conditions will be among the most severe to be faced by future FOW farms, whether pilot projects or fully commercial.
A pilot project for future FOW farms
While the Floatgen demonstrator may look large, it is, in fact, small compared with the machines planned for the next generation of pilot projects and commercial FOW farms. Floatgen’s 60-m tower (including the transition piece), 2-MW turbine and 80-m-diameter three-blade rotor will look small compared with 6-MW machines with 150-m rotors. And that may only be the beginning. “We are the only design team to have foreseen from the outset the potential rapid growth in offshore turbine power ratings, hence also the only one to have developed a concept that can readily accommodate turbines generating up to 15 or 20 MW,” says Paul de la Guérivière.
A €25m European project
The Floatgen project is expected to cost 25 million euros with the European Commission contributing €10m and France €9.5m through a PIA future programme. The Floatgen consortium comprises seven partners; three French, two German, one British and one Spanish. The partners are: first, the École Centrale de Nantes engineering school, Ideol, and Bouygues Travaux Publics in France; in Germany, Stuttgart university for coupled loads simulations and measurement campaign evaluation and Fraunhofer-Iwes to benchmark the Floatgen demonstrator relative to other floating solutions; in the United Kingdom, the RSK Group for floating system environmental impact analysis; and, in Spain, Zabala for consortium management and internal communication.
Unfortunately, the project is currently two years behind schedule. Despite this delay, Ideol signed a contract to produce two demonstrators in Japan — one with a steel foundation, the other with a concrete foundation — before the Floatgen demonstrator had even reached its test site. The first foundation for Japan is under construction and scheduled to be moored in mid-2018. Events are also moving quickly in France. In 2016, the government awarded contracts for four pilot FOW farms, one to be located off the Brittany coast, the others in the Mediterranean. One of these Mediterranean projects, led by Quadrans, calls for Ideol Damping Pool® foundations carrying 6.2-MW Senvion turbines.
Nicolas Jestin, Bouygues Travaux Publics sales manager (© Mer et Marine - Vincent Groizeleau)
Demonstrators yield lessons learned
Given the above, won’t the results of the Floatgen experiment come in too late to be of much use? “No. We don’t think so because it has already resulted in a range of lessons learned during the design and construction phases as well as during the pre-installation of the mooring system. Moreover, some have been incorporated into the other projects now under development. Also, once the Floatgen demonstrator is generating power, we will receive additional data in good time to draw useful conclusions for pilot projects that will, by then, be under construction. Together, the Floatgen demonstrator and the one now under construction in Japan will help us to quickly accumulate data and experience to further optimise our design and reduce costs,” replies Paul de la Guérivière. The BYTP team also says that the Floatgen demonstrator will prove a useful testing ground for the construction and installation methodologies. “It’s the beginning of an adventure, not a goal”, says Nicolas Jestin.
Ideol Damping Pool® foundations (© Ideol)
Next step, pilot farms
In France, the next step, the EolMed farm, is already under way. EolMed calls for four FOW turbines to be moored 15km off Gruissan near Narbonne on France’s Mediterranean coast. The application for the administrative permit to set up and operate this pilot farm is scheduled to be filed towards the end of the year. The contractors hope to start work in 2020 following the completion of the permit investigation phase and the expiry of the appeal period. The proposed concrete foundations will be built at Port-la-Nouvelle. The project calls for an installed power rating of 24.8 MW producing 100 million kWh per year, or enough electricity to meet the needs of 50,000 people. EolMed is one of four pilot FOW projects being funded by the French government. The other three will test different turbine and foundation concepts. One will be set up between Belle Île and Groix off the Brittany coast and the other two in the Mediterranean off Faraman and Leucate.
Original by Vincent Groizeleau, translated and adapted by Steve Dyson.