A grant agreement for a pilot project funded by the EU to test the OO-Star Wind Floater foundation in the North Sea is expected to be signed in the next month. Olav Olsen, the company behind the concept, expects the pilot project to demonstrate its many advantages
Completion of the agreement will pave the way for the companies behind the project to sign a consortium agreement and begin engineering work on the innovative semi-submersible foundation in September 2020. Fabrication of the floating platform could start in Q2 2021, with installation in Q1 2022.
The ‘FLAGSHIP’ project will see the design, fabrication, installation and operation of a demonstration floating offshore wind turbine using a 10+ MW turbine installed on Olav Olsen’s semi-submersible concrete structure.
The floater will be tested in the North Sea, at the Met Centre located in Norway. In addition to Olav Olsen, the originator of the OO-Star Wind Floater concept, and the Met Centre, the consortium will be led by Iberdrola and also includes Core-Marine, Cener, IHC, Zabala Innovation Consulting in Spain, Kvaerner, UnitechSubsea, EDF in France, DTU in Denmark and class society DNV GL.
FLAGSHIP is being developed as part of the European Commission’s Horizon 2020 programme. The main objective of the project is to help reduce the levelised cost of energy of floating offshore wind to a range of between €40-60 (US$44-66) MWh by 2030, driven by economies of scale, competitive supply chains and innovations.
Speaking exclusively to OWJ, Olav Olsen manager renewable energy Trond Landbø highlighted some of the many advantages of the OO-Star Wind Floater compared to other concepts.
In addition to its ability to provide a foundation for new-generation, large-scale offshore turbines, Mr Landbø highlighted the scalability of the structure. He explained that Olav Olsen had worked closely on other floating wind foundations, such as spars, which he believes have inherent limitations with very large turbines. In contrast, he believes, the concrete semi-submersible is proven and has few limitations.
Mr Landbø said that, most importantly, spar-type foundations are good concepts for turbines of up to 8-10 MW in size, but not as scalable as the OO-Star Wind Floater is for larger turbines.
“We have had a good view of spar foundations,” he told OWJ. “Apart from the water depth required to build a spar, the weight and cost of a spar capable of operating with a very large turbine is questionable.”
In a spar, the centre of gravity of the structure is lower than its centre of buoyancy, which provides stability. Its deep-draft reduces the effect of wind, wave and currents. However, the design also means the weight of the tower and turbine have a significant influence the size of the floater. A spar also requires sufficient water depth for it to be built and floated out and may not be suitable for locations without deep water such as Norway’s fjords.
In contrast, a semi-submersible platform consists of a number of columns linked by connecting bracings and submerged pontoons. The columns provide hydrostatic stability, and pontoons provide additional buoyancy and damping. As the name suggests, the structure is semi-submerged, ensuring the overall stability of the system. One of its greatest advantages is its low draft, which makes construction and assembly simpler. Another is that it can be installed where the water depth is less than that required by a spar.
Mr Landbø said an important advantage with a semi-submersible like the OO-Star Wind Floater is it is possible to adjust parameters to take account of larger turbines and of conditions.
“With a semi-submersible, the waterplane area, which affects stability, can be adjusted in a number of ways. You can adjust the dimensions of the columns, their shape and the distance between and adjust the pontoons to suit the size of the turbine and the conditions where it is installed,” he said.
Then there is cost. Mr Landbø told OWJ that work Olav Olsen has conducted suggests that as the size of a turbine grows from 8 MW to 12 MW, the size of a spar on which it is mounted could grow by as much as 60%. In contrast, he said, the OO-Star Wind Floater grew less than 20% .
“We believe that a spar has limitations when it comes to construction and installation that a semi-submersible does not,” he said, noting that work undertaken by the company indicates the cost of the OO-Star Wind Floater falls by 30% per MW from 8 MW to 16 MW, from scaling effects alone.
Other well-known semi-submersible floating wind concepts are fabricated out of steel, and versions of Ideol’s ‘Damping Pool’ concept have been proposed in steel and concrete. Mr Landbø believes using concrete is another advantage for the OO-Star Wind Floater, because yards or fabricators capable of building large-scale steel structures are not available in every market, and concrete as a material is a mature technology that can be produced anywhere.
“Concrete has a long design life and a high future value, with potential for re-use,” Mr Landbø said. “It isn’t sensitive to fatigue and is maintenance-free, which reduces the cost of operations and maintenance. It can be constructed everywhere and provide local content.”
The consortium of which Olav Olsen is a part intends to build the OO-Star Wind Floater on Norway’s west coast. A sophisticated shipyard would not be required for the demonstrator nor for large-scale future projects.
“There are a number of ways that cost reduction could be achieved through industrialisation of the OO-Star Floater concept,” Mr Landbø concluded. “It can be easily assembled at the quayside and, in the longer term, we believe that horizontal assembly of the turbine and tower with the use of an upending tool we have developed and patented will be possible too.”
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