Fabio Amico, Structural Project Manager at Iv-Consult, continues: “The tendons are placed in the mast of the wind turbine and are released by the floats, causing the ‘gravity base’ to submerge and sink to the seabed. When the ‘gravity base’ is positioned on the seabed, the tendons are tensioned, and the float is pulled underwater. Because the tendons are in the mast, the height thereof determines the maximum water depth. However, we wanted a system that would be suitable for greater water depths, and it has taken a great deal of brainstorming to suitably adapt the concept for this.” Our concept involves a so-called ‘jacking platform’ at the top of the mast and directly below this a rotating platform from which the tendons are hung. A securing platform is at the bottom of the mast. By means of ‘strand jacks’ the tendons are successively lifted, lowered (causing the ‘gravity base’ to descend) and subsequently fixed to the lower platform. The rotating platform then ensures that subsequent tendons are suitably placed. The procedure is repeated until the ‘gravity base’ is positioned on the seabed and the tensioning of the tendons can commence. Eventually, the wind turbine is partially submerged. The concept is suitable for water depths of approximately seventy to five hundred metres and, as an alternative to the ‘gravity base’, the system is easily adaptable for applying so-called ‘suction anchors’.

The offshore wind industry is experiencing major developments, and in the Netherlands these developments, among other things, are being stimulated by the renewable energy sector. Heavier wind turbines are needed to generate a higher energy yield in areas with constantly high wind speeds. Wind turbines with a capacity of 5 to 8 megawatt (MW) are currently the most commonly used in North Sea waters, but soon this may rise to perhaps 20 MW. Such large wind turbines, far above sea level, are preferably placed further away from the coast. However, this makes offshore work activities expensive. Especially for greater water depths (more than sixty metres deep) alternatives have been devised for existing foundation structures, such as monopiles and jackets. It is often assumed that the structure is floating in combination with anchoring by means of suction anchors or piles. Of course, it is also possible to secure the structure via a so-called ‘gravity-based construction’, on which Iv-Consult’s concept is based.

Iv-Consult is in consultation with a supplier to find out the practical issues in relation to the tendons. Ad explains: “We have completed a global feasibility study into the dimensions of the system. Our starting point for this was a 12 MW state-of-the-art wind turbine. Of course, the patented system can also be integrated into the mast of a smaller wind turbine and at variable water depths. Together with this partner, we are now investigating the integration of the tendons in the construction, considering the various installation phases and the connecting of the tendons. The concept is now evolving from an idea to a detailed plan.” Fabio adds: “Because the concept is based on an integral system where all components are influenced by each other, the parts cannot be engineered separately as the entire system must be considered. This also applies to the installation equipment, which is fully integrated into the mast. Splitting the system, into a turbine, transition piece and foundation, will not work.”   

And now? Ad continues: “There are many opportunities internationally for floating wind farms in greater water depths. Floating wind turbines are nothing new. Many concepts have already been devised and developed in the market. There are currently several functional prototypes and the first floating wind farm is operational. However, we are not focussing on the total system, but instead on the mechanism that allows the wind turbine to self-install. This is also completely in our comfort zone. Our goal is to present a working concept. It would, of course, be fantastic if this was to be taken on by the market and actually applied.” The concept is modular and can be adjusted to size. This makes it suitable for many different turbines. “We are busy creating a 3D digital model of a mast with the platforms and tendons”, says Fabio enthusiastically. “But, for example, we can also work this out with LEGO®, to make it clear how the system works.” Ad adds: “Or we could 3D print it.” 

Ad and Fabio are proud of what they have achieved with the team so far. “The challenge of coming up with the concept was the best part: ‘How can you make a wind turbine self-installing? How do we make this possible?’ There has been much sketching, designing, brainstorming and discussion for this project. Piece by piece, we finally arrived at a solution and the fact that what we came up with is to be patented is fantastic. I am really proud of the team!”, says Ad. Fabio fully agrees with him and adds: “Since 2018, we have been working part-time on the development of this concept. Due to other work, we often had to temporarily halt our research, and it was often challenging to again restart it. Creating the text for the patent application was complicated but luckily, we had help with this. We finally completed the text, with comments, with further comments and comments about the comments! It was a rainbow of colour due to all the adjustments, comments and the adding of extra sentences. It was a whole process; the patent really is the icing on the cake.”