The Floating wind turbine sector requires advancement in the development of foundations and mooring systems

02.11.2018

Europe remains the world leader in floating wind energy installations with the North Sea staying the main region for deployment, accounting for 78% of the European total, followed by the Baltic Sea, accounting for 14.1%. Based on the number of offshore wind projects in construction, WindEurope estimates total European offshore wind capacity will be 24.6 GW by 2020.

Furthermore, Europe is continuously investing in innovative projects and designs aiming at improving the performance, maintenance and installation of the offshore floating wind turbines. There is Hywind and the simulations models, developed by Nielson (2006, 25th International Conference on Offshore Mechanics and Arctic Engineering, Volume 1: Offshore Technology; Offshore Wind Energy; Ocean Research Technology; LNG Specialty Symposium, Hamburg, Germany, June 4–9, 2006) for integrated dynamic analysis and the role of the effect of pitch-angle control of blades. There are many researches done as well into the pontoon-type floating wind turbine (Jonkman and Buhl, 2007, Jonkman, J.M. and Buhl, M.L. Jr, (2007). Loads analysis of a floating offshore wind turbine using fully coupled simulation. Proc. of Wind Power 2007 Conference and Exhibition, Los Angeles, California.), aiming at characterizing the dynamic response and at identifying potential loads and instabilities, resulting into significant design modifications.

It is important to mention that besides leading European industries, there are a couple of serious emerging markets, interested in the development and deployment of offshore floating wind power as China, North America and Mexico.

The Sector’s Biggest Challenges and Ambitious Promises

Even though progress is clear, all professionals know that the industry is still facing significant challenges as the need for appropriate vessels for installation and maintenance, innovative design solutions and the lack of industry standardization. Weather fluctuations are another significant hurdle and to fight these, offshore wind installations still need some improvement and installation vessels – better navigation, higher crane capacity and more space.

Since the floating offshore wind turbine is a relatively recent invention, there is still a lot of space for new ideas concerning the way the turbine can be hold in place and the entire design of the configuration. All known for now  mooring architecture which has been used with success in the oil and gas industry includes the catenary mooring or the taunt leg. For the platform the assessment is the same with the development of semi-submersible or spar platform which are well known in the offshore industries. These solutions could feature some imperfection and  their choice strictly depends on the project’s priorities including environmental conditions and operating parameters.

Another problem originates from the power grid connection. Integrating large amounts of offshore wind generation to the power system requires solid knowledge and better infrastructure.

Technical advancements in the overall design and manufacture of floating wind turbines

Both areas where the most progress was achieved include larger, more effective turbines and improved foundations.

Here are some examples:

  • Equinor  (Ex Statoil) has deployed in 2009 the first full scale spar buoy off Karmoy Island. The 5300 Tons spar buoy was equipped with a 2.3 Mw wind turbine. In 2017 Equinor has deployed Hywind Scotland, first floating pilot farm. This project features an important scaling with a 11 200 tons spar buoy and a 6 MW Wind turbines.
  • Ideol has deployed earlier this year a prototype based on its damping pool platform, a platform different from all known oil and gas standards such as spar, semi sub or TLP. Moreover, its mooring system is equipped with innovative synthetic fiber mooring lines, an innovative choice of material for a permanent mooring system.
  • At a lower stage of deployment Sway has chosen the horizontal axis for its wind turbine. This technological choice allows, according to Sway, to lower the gravity center, eases the maintenance and reduce the spacing between wind turbines.
  • Still in a concept phase Hexicon expects to reduce CAPEX & Installation costs by developing a floating multi-turbine platforms allowing to increase the power generated per platforms.

There are of course some hybrid types, comprising two or three of the mentioned turbines, for example the spar floater and tension leg mooring system mix. However, much more advancement is expected in this area.

Despite the different types and configurations, there are in general three basic floater types of floating offshore wind turbines and these are the semi-submersible type, moored by catenary lines, the TLP (Tension Leg Platform) type, moored by vertical tendons, using buoyancy and the spar one, formed as a single deep-draft cylindrical and vertical column. 

The biggest challenge concerning the foundation type is that cost can quickly increase and therefore the high demand for innovative, cost-efficient and high-quality solutions. Mooring methods feature a close relationship with the construction performance and for a successful turbine performance should be carefully studied. Many tests and observations have confirmed that the mooring method using weights is highly effective in reducing cost.

In general, a floating platform can achieve stability through ballast, mooring lines and buoyancy. It is important to mention that the performance of the turbine is impacted by the choice of platform and every existing concept right now uses one or a combination of these three primary stability methods.

Business Case: Floatgen Has Choosen Clump Weights

Floatgen is the first French floating offshore wind turbine system for power generation in Atlantic waters. The three main objectives of this project are to prove the technical, economic and environmental feasibility of an EU technology floating system in deep waters, bringing wind energy applications closer to market in diverse European deep offshore areas and assessing the expected global generation cost per MWh in a 15-year perspective.

The turbine was submerged this September 22 km off Le Croisic (Loire-Atlantique) and is expected to provide electricity to the 5000 habitants of the city.

Floatgen is kept in place by mooring design including clump weights, designed and manufactured by the European leader in ballast solutions, the French company FMGC. These are the 1st clump weights in France that have been ever used in the installation of a floating offshore wind turbine. The solution contributes to the optimization and cost effectiveness of the entire mooring system.

The standard offer of FMGC includes clump weights, designed to offset the vertical forces against the anchor and restrict the movement of the floating structure, available in two models:

  • The “distributed” configuration is a set of medium-sized clump weights, distributed over a segment of the anchor line. This configuration optimizes the effectiveness and the cost of the solution.
  • The “mutualized” configuration consists of one clump weight, attached to one specific point of the anchor line. This configuration neutralizes the impact of wind & wave on the anchoring line.

However, in the case of Floatgen, the foundry has provided a customized solution, based on the target weight and the available installation means.

The mutualized clump weights, made with EN GJL 200 cast iron have been connected to the mooring lines by using a forged steel insert designed for this purpose. To reach the targeted weight and ensure an easier installation the clump weights were composed of an assembly of several smaller pieces positioned on the steel inserts. Connected with shackle the clump weights are hanging to the mooring lines and contribute to the limitation of the tensions on the lines. With this dynamic effect the FMGC solutions contribute to a reliable and cost effective station keeping system. 

In Europe – and around the world – all professionals are trying to invest knowledge and resources into technological developments in order to cut costs and make offshore floating wind power function better, faster and easier. As technology is continuously advancing, costs are becoming to drop down and this trend is hope to continue, pushing future projects to expand.

Europe is for now the world leader in the investment and development of offshore floating wind power with greater transparency and understanding of the key factors. Other regions in Asia and North America are expected to follow this trend and take an active stance on shift to renewable and clean energy sources.