After a multi-year design & development phase and extensive simulation studies, Naval Energies (formerly DCNS Energies) recently completed the first prototype of a new family of floating offshore wind turbines, or FOWTs. These semi-submersible structures can be made of steel, concrete, or a combination of the two. “Our design concept offers optimal stability while simplifying construction and equipment integration in any port as well as transfer to the windfarm and on-site installation. Using the same basic architecture, we tailor the design and choose the materials according to local skills and resources,” says Naval Energies sales manager Frédéric Chino. The company is a joint subsidiary of Naval Group (formerly DCNS) (55%), French sovereign fund Bpifrance, and offshore engineering group Technip.

The benefits of steel and/or concrete are evident once it is realised that while many contractors in Asia have proven expertise in steel, others in regions like Europe and more particularly France, have even more advanced expertise in concrete. “It’s important to examine the cost of materials at ports near the proposed site. Do local contractors have good track records in steel or concrete construction? Which material is more competitively priced? What are local resources and infrastructure like?” The choice also depends on environmental conditions at the proposed site and the likely stresses on the machines and structures.

Although Naval Energies favours steel foundations for projects in eastern Asia, its first two contracts are for hybrid and concrete foundations.

First contracts in Brittany and the US

In April 2016, the US Department of Energy chose a consortium comprising Emera, Cianbro Corporation and the University of Maine to develop the Maine AquaVentus project and a design concept using concrete foundations by Naval Energies. This pilot array will comprise two 6-MW turbines scheduled for installation in 2019, and for commissioning in 2020.

Through the Eolfi consortium, Naval Energies is also working on the Groix-Belle-Île project off Brittany, one of four pilot arrays using floating foundations planned for French waters. This project calls for four 6-MW Haliade 150 turbines developed by GE Renewable Energy (France) to be installed in 2020. The three-blade rotors are 150m in diameter.

A hybrid FOWT foundation with a concrete base (© Naval Energies)

Hybrid version for Groix-Belle-Île

Despite the prime contractor’s early preference for an all-steel solution, the Groix-Belle-Île project team finally decided in favour of a hybrid solution, with combined steel and concrete solutions. “This solution is better suited to the conditions at sea off the Brittany coast. The base will be made of concrete because it can better handle the long-term stresses produced by Atlantic weather conditions. A steel structure would have required extra reinforcement.” In addition to the transition pieces between the tower and the base, each foundation for this site will include three steel columns to improve weight distribution. To this end, both the towers and the bases will be filled with ballast.

A hybrid FOWT foundation with a concrete base (© Naval Energies)​​​​​​​

Each hybrid foundation for the 6-MW turbines will weigh between 7000 and 8000 tonnes (two-thirds for the concrete base), compared with almost 3000t for an all-steel solution. The arms of the Y-shaped base will be between 60 and 70m long. When ready to tow out to sea, the structure will have a draught of less than 8.5m, which can be accommodated at most ports. Once on site, the base will be ballasted and sunk to leave an emerged height of 18m.

Construction in Drydock 3 at Brest?

While it has already been decided that the foundations will be built in Brest, it has not yet been determined where. One idea is to use the commercial port’s Drydock 3 which could accommodate all four foundations at once. The Vinci group, Naval Energies’ partner for foundation construction, will build the concrete bases while Naval Energies will fabricate the steel transition pieces. For cost reasons, the three large steel columns of each foundation will probably be entrusted to a subcontractor. If all goes to plan, construction work should begin in 2019 with a view to on-site installation in 2020. Turbine production will take from 12 to 18 months. This project will be used to fine tune the production process in preparation for the ramp-up to longer runs and shorter lead times. The target is to install a 70-turbine windfarm in less than two years.

(© : Naval Energies)

Moorings

Several mooring solutions are under consideration. The most conventional, and the most widely used in the offshore industry, uses metal chains. Several newer technologies are also available, including synthetic materials. One is the nylon mooring lines selected for the Floatgen demonstrator (see Floatgen FOW demonstrator launched) which offer significant weight savings. “It’s an interesting idea, but the technology has yet to be qualified and there is no guarantee that it would meet the array’s 20-year design lifetime,” says Frédéric Chino. Naval Energies is also keeping a close eye on other emerging technologies, particularly tension-leg platforms, or TLPs. This concept is used in the offshore oil & gas industry and has been selected for the Provence Grand Large pilot array in the Mediterranean, one of three projects under development in French waters. “Although expensive during the installation phase, TLPs offer excellent stability.”

Frédéric Chino, Naval Energies sales manager (© Mer et Marine - Vincent Groizeleau)

After considering technologies, innovations and costs, the best solution for any given project inevitably depends on the windfarm’s location, the environmental conditions, including swell characteristics and wind speeds, and the depth of water. “Each solution — from the architecture to the materials used, the moorings and the turbine proper — must be tailored to local conditions. The SeaReed project with GE Renewable Energy (France) to integrate a Haliade 150 turbine with one of our foundations proved to all and sundry that you can’t just plonk any turbine on any foundation. Quite a few criteria have to be met first to ensure the structure’s stability and then to maximise the turbine’s output.”

Following an extensive R&D phase that began with the Winflo project in 2008, Naval Energies considers that it has now acquired solid expertise in floating offshore wind turbines, or FOWTs, and is ready to progress to full-scale programmes. “We’ve assimilated the lessons learned from working with 6-MW turbines and have the data to quickly develop a solution for 8-MW types before moving on to turbines rated at 10MW or more, but for which detailed design specifications are not yet available. The foundations for higher power turbines will be larger, but the relationship is not linear.” The difference between the size of a Naval Energies foundation for a 6-MW turbine and an 8-MW one is much smaller than one would expect.

Original by Vincent Groizeleau published on 16 November 2017. Translated and adapted by Steve Dyson.