Dedicated hardware solutions now include floaters with single or multiple turbines and upwind and downwind configurations, each requiring specific dedicated solutions. Ongoing technical challenges being addressed and advanced in parallel include single and multi-point mooring systems, anchors, cabling and chains, and flexible feed-in electricity transport cables.

A key ongoing challenge is to further drive down the lifecycle-based generating cost (LCoE) of floating wind, with industrialization of floater designs a primary overall wind industry objective.

One example of such industrialized floater design is Henrik Stiesdal’s TetraSpar, a lightweight tetrahedral structure composed of triangles and assembled from (mainly) factory pre-manufactured tubular steel components. The recently completed prototype with 3.6MW Siemens Gamesa direct drive turbine atop will soon be tested off the Norwegian coast in 200-metre deep water.

The Department of Wind Energy at DTU in Denmark has built comprehensive know-how with the design and evaluation of floaters says Head of DTU’s division for Wind Turbine Design Kenneth Thomsen: ”We have used our HAWC2 simulation tool for almost all floating concepts from Hywind to TetraSpar and downwind self-orienting designs. The dynamics of each floater and turbine is always an interplay between rotor loads, wave forcing and control – under the site-specific conditions. Damping is also key and still requires model tests to become precise.”

He further explains about five model test campaigns conducted with DTU’s 1:60 scale model of the DTU 10MW reference turbine on various floaters in collaboration with DHI. This modeling has thaught the researchers a great deal on model calibration as an input to further design studies.

“Floating wind is the new frontier of offshore wind and our industry standard simulation software suite has been tailored for accurate design of floater configurations through use on industry demonstration projects over the past 15 years. WindEnergy Hamburg 2022 will be an important platform for DTU Wind Energy to present our knowhow, test facilities and commercial solutions”, Thomsen concludes.

Demonstration projects

Two main suppliers, Siemens Gamesa and Vestas, have so far supplied the bulk of turbines for prototype demonstration and the first semi-commercial offshore floating wind projects. Turbine size thereby grew from initially small-scale 2-2.3MW single-unit prototype projects, towards larger-scale semi-commercial ventures with 3-5 turbines of 6-10MW each and increasingly larger fully commercial projects the next major envisaged step.

Siemens Gamesa made its floating wind segment entry with the 2009 Hywind Demo project of the Norwegian coast, involving a spar floater and 2.3MW SWT-2.3-82 VS (Variable Speed) turbine. The second operational project in 2017 became Hywind Scotland, now incorporating 5x 6MW SWT-6.0-154 direct drive turbines, was again put atop spar-type floaters. Two additional ventures involving in total 14 SG 8.0-167 turbines are Hywind Tampen (11) in Norway and a 3 turbine-project in France.

Head of Offshore BoP (Balance of Plant) Jesper Möller says that the overall company strategy is to use comparable turbine hardware for floating wind projects, with adapted software suiting different floater designs: “The floating wind market remains likely also in the short and medium-term limited in volume, and our product-market focus is to continue stretching turbine sizes designed for mass production. It remains further difficult to predict what will happen in future and at what pace. Key for us is to stay away from wishful thinking and always keep in mind the practical fact that small and big projects are equally time-consuming. We try in the meantime to sell as many turbines as possible for floating and stay in a leading position with these developments, using the fleet of turbines we have already.”

Pace of production

He adds that a crucial aspect for floating foundations is to get them industrialized in line with the pace of turbine production, and Siemens Gamesa actively supports floater concept owners in understanding industrialization. Another topic Möller touches on is which wind technology complexity level is acceptable for floating applications regarding (main) components repair/exchange as integral part of life-cycle asset upkeep. He highlights three interlinked main solutions and strategies:

“The first is developing dedicated tools allowing on-site repairs, the second to develop dedicated 3D-compensated floating cranes for more complex remedying operations, and the third towing defect installations onshore for full exchange. Murphy’s law is further always present. This could happen for instance when a defect unit is located in the worst possible position within a turbine-string and/or having to deal with parallel in-field flexible cable and/or anchoring issues. This could lead to several turbines being out of service to repair work on a single turbine. ”

Möller further expects that there could be floating electricity collection substations within 8-10 years and that these will then operate at 132kV feed-in level instead of the current 66kV semi-standard voltage for inter-array cabling. The cables interlinking individual turbines should ideally become a combination of short expensive highly flexible cable sections when needed and non-flexible less-expensive cabling whenever possible. “There remains a lot of work to be done, and it is crucial to continuously look into smarter ways of doing things. This could in the long run lead for instance to Siemens Gamesa developing hybrid turbine solutions for floating. However, dedicated turbines for floating applications is not on the table yet from a short to medium-term perspective. Highly encouraging on the future of floating wind is that the combination increasingly larger turbines and bigger floaters does scale well. This will together with ongoing industrialization become key in further driving down floater CAPEX and floating wind LCOE”, he concludes.

Various structures

Vestas pioneered its floating wind involvement by supplying a V80-2.0 MW turbine for Principle Power’s semi-sub WindFloat prototype installed off Aguçadoura, Portugal, in late 2011. The V164-8.0 MW made its debut last year with three units for WindFloat Atlantic in Portugal. 14 more V164 turbines at various floating structures are planned within four projects between this year (5) and 2023.

Pablo Necochea is Vestas lead engineer floating segment. He explains that with floating projects incorporating V164-9.5/10.0 MW turbines both tower and controller are adapted, while the nacelle and rotor remain unchanged: “Key part of our involvement is to support individual floater designers in optimizing their designs, aimed at optimal integrated solutions at turbine-floater systems level. Main tower adaptation is increasing wall thickness with the extra mass primary benefit offering more freedom and flexibility in floater designs, especially regarding operational performance improvement and mass and cost reduction. Changes to the controller focus at optimizing system dynamic behaviour and include the support of active floater solutions.”

He adds that a common misperception is to consider concepts with limited motion behaviour as superior, and larger inclinations instead results in lighter cheaper overall solutions. Essential thereby is the cooperation between turbine OEM, floater designer, and project developer being ‘the party that pays!’

Necochea further observes that floater’ CAPEX is the main contributor to total system investment cost. “Crucial factor is to push industrialization of floater designs, for increasing volumes together with massive cost reduction. Ship-building and other industries has already huge experience with high-volume steel fabrication including for its replicability. However, concrete also has a role to play like in those market circumstances where local production is demanded. Current developer preference for our V164 turbine series instead of the latest V174-9.5 MW sister model can be explained by the offshore track record and thus wind industry familiarity, together with the lengthy timeline of current floating wind projects”, he concludes.

Big and biggest

US-based General Electric (GE) in another development develops a 12MW floating solution, which merges GE’s 12-14MW Haliade -X turbine platform with the PelaStar tension-leg floater developed by its US Glosten.

Finally, again another development angle that builds at a near-century old multi-rotor idea is presented by a 15MW+ prototype of Aerodyn-engineering’s Nezzy2 currently being built in China. The radical design has two closely interspaced counter-rotating downwind turbines of at least 7.5MW each with corresponding 160m+ rotors. Other product-specific advanced features include a single-point mooring system platform, elimination of the yaw system, and an unusual hollow concrete Y-shape floater.