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Concrete plans for floating offshore wind

IN DEPTH | Steel has taken floating wind power from flagship to fully fledged array in under eight years. But concrete might be key to speeding up industrialisation of the fast-evolving technology, writes Darius Snieckus

Concrete — though it seems conterintuitive — floats. And floats well. This summer, in the waters off western France, this rock-hard material — used for megaprojects from the ancient Roman dome of the Pantheon to the Hoover Dam — will be tested for its next industrial incarnation: floating wind power.

The flagship of a concrete platform-based concept being built by floating wind pioneer Ideol under the aegis of the EU’s €25m ($26.5m) FloatGen project is racing towards completion at Bouygues Travaux Publics’ yard in the French port of Saint-Nazaire, on track for tow-out this autumn for first trials in open sea.

As Recharge went to press, the concrete panels making up the prototype’s foundation — a barge-like platform measuring 36 metres square and 10.8 metres deep, with an open “damping pool” in the centre — were being finished. Mating with the unit’s turbine, a 2MW Vestas V80, is slated for the summer.

“We were one of the companies that was first to take to the idea [of concrete floating wind turbines] seriously and now we are seeing that others in the industry are coming around to it,” says Ideol chief technology officer Thomas Choisnet.

“But, of course, concrete structures have already been used for decades in the offshore environment [in the oil & gas industry] and built up a very good reputation for durability, resistance to impact and shocks, handling high and low temperature conditions. Concrete can be poured almost anywhere in the world by local civil engineers, it’s low-cost and it’s long-life, it is really what you want for floating wind.”

With 200MW of floating wind forecast to be moored offshore by the end of the decade, the industry is rapidly approaching commercialisation. Michael Guldbrandtsen, managing consultant at MAKE Consulting, expects floating wind — “in some combination of steel-based and concrete-based” — to grow into a global fleet of up 3.4GW by 2030, led by Japan, France and “potentially” the UK, turning at a levelised cost of €125 per MWh by 2025. “This,” he says, “would show that the technology will be a long-term major contributor to the green energy transition.”

The case for concrete floating wind substructures

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Others are still more optimistic: the UK Energy Technology Institute suggests that on current trajectories floating wind power could be down to less than €100/MWh by 2025. The International Renewable Energy Agency has spotlighted industrial-scale floating wind as a “key [cost-reduction] driver” in opening up new, deeper-water markets to offshore wind power production in the next decade, hand in hand with the upscaling of turbines towards nameplates of 15MW.

“With more supportive policies, development could be significantly accelerated towards 2030,” notes Guldbrandtsen.

Concrete’s record offshore — a go-to for the oil industry since the 1970s and already field-proven for near-shore wind on projects including Denmark’s Rødsand 2, Belgium’s Thornton Bank and Sweden’s Kårehamn — underpins its potential to help catalyse the industrialisation of floating wind, says Cian Conroy, sector head for offshore wind at UK technology innovation agency Offshore Renewable Energy Catapult.

“There has always been potential for concrete [as a material for floating wind turbines],” he says.

“The longevity and robustness of the material are two if its best features to my mind. It was always who would be the first mover and who gets a commercial turbine onto a concrete structure.

“Because once this happens, then it is only a question of scale-up to demonstrate the industrial and commercial opportunity. I feel concrete is approaching that critical mass.”

The steel floater fraternity is already well on its way to reaching this industrial breakthrough — through Statoil’s 30MW spar-centred Buchan Deep array off Scotland, which is set to begin turning this summer; the 25MW WindFloat Atlantic project off Portugal being developed by the WindPlus consortium; and the operational Marubeni-led 14MW Fukushima Forward project off Japan.

Ideol believes it can help concrete catch up almost single-handedly. In January it inked a deal with Atlantis Energy, the “strategic diversification” ocean-energy spin-off of tidal-power outfit Atlantis Resources, to develop a pipeline of as much as 1.5GW of concrete floating wind off the UK, kick-started by a 100MW “pre-commercial project” by 2021.

This will add huge impetus to Ideol’s global aspirations, today focused on the single FloatGen turbine and coming array pilots in the French Mediterranean with developer Quadran; off Japan with government R&D institute Nedo; and with China Steel Corporation in the waters off Taiwan.

But it will also have a sector-wide influence in de-risking concrete, a material not yet embraced by investors despite its offshore industrial pedigree.

“Concrete has struggled in the fixed-bottom offshore wind market,” says Rhodri James, manager for policy and innovations at UK government-industry body the Carbon Trust. “Of course, you now have [the UK’s] Blyth [project, which is using concrete gravity base (CGB) foundations] and other projects that are helping to change this, but steel has dominated as a material to this point — and I think this has translated across to floating wind in many ways.” The Carbon Trust has produced landmark reports on the global floating wind market and on CGBs as a next-generation fixed-bottom foundation technology.

“Project pipeline is what is needed to justify the big upfront costs of developing new construction infrastructure — orders for 1GW-plus,” he adds.

“This is what concrete floating wind needs — and it now looks like we might see this sooner rather than later.

“There are challenges faced by concrete for floating wind and the case for concrete versus steel is not clear-cut.

“Yet, if this is scaled up properly and approached on a region-by-region basis [aligned with local construction infrastructure], there is the argument that concrete could potentially effect a game-change.

“Look at French offshore, where you have hundreds of megawatts in prospect for floating wind and a well-developed civil engineering sector that is keen to diversify into new markets. It could work very well.”

The case for steel floating wind substructures

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The Carbon Trust’s next market study on floating technology is now in the works, but James says early calculations suggest costs have been dropping “dramatically” since 2015, when the organisation reported that concrete foundation concepts would cost £15m-18m ($18.4m-22.1m) per unit, roughly £2.5m-3m/MW.

“From a cost perspective, concrete is competitive [with steel],” says James.

“A great deal comes down to [manufacturing] location and existing infrastructure. But this is only measured on a single unit or indeed 100 units in terms of capital cost. Once you factor in opex [operating expenditure], which hasn’t been done in great depth or detail, concrete might have an edge — given its expected longevity of 80-plus years. But right now the jury is still out.”

Conroy agrees: “At the single unit level, and maybe array, the economics are currently skewed towards steel. But when you factor in scaleability, the cost reduction potential is really there for concrete — from supply chain to fabrication to operational life at sea.”

Ideol is the standard bearer for concrete floating wind, but a frontline is forming behind it. The economic case for the material over steel — boiled down to three arguments: capital cost, mass manufacturability and field life — has never been in question at Dr Techn Olav Olsen. The Norwegian offshore oil engineering outfit has a semi-submersible floating wind concept, the OO-Star, being built for a 10MW turbine as part of the EU’s Life50+ technology demonstration project.

“There are a number of reasons to believe that floating offshore wind will be able to compete with bottom-fixed in the future, starting with the increased energy potential far offshore and the fact that most offshore areas close to energy consumers have water depths of 60 metres or more,” says Trond Landbø, renewable-energy business unit manager at Olav Olsen

“Concrete can, without much extra cost, be designed for a very long design life, 100-plus years, and it has excellent fatigue resistance.”

Managing director Olav Weider adds: “This should make concrete floaters very attractive in the future when methods for standardisation of design and mass fabrication are further developed.” Both Weider and Landbø were involved in the 1.2-million-tonne Troll A Condeep concrete oil platform — the heaviest structure ever moved by man — and early engineering for the world’s first industrial-scale floating wind turbine, the Hywind 1, off Norway.

“Our [OO Star] design is very much related to our knowledge of offshore oil & gas structures — we saw there were many synergies in competence with marine renewables, including floating wind,” says Landbø.

“We started with the [then-Norsk Hydro-owned] Hywind project in 2003, doing conceptual development for them on what, at the time, was a concrete design and remained so

until the bidding stage in 2006, when it became clear there was no contractor willing to take on the risk for this project, so then Technip [which fabricated the Hywind prototype spar] switched to steel.”

Weider adds: “For larger structures, we feel that concrete is much better suited for floating wind both in terms of design robustness and serial production.”

Last month, Olav Olsen tied up with two-bladed wind turbine designer Seawind in a strategic partnership that aims to deploy an innovative 6.2MW model mated to a CGB foundation next year off the west coast of Norway, before moving on to a first floating unit based on the OO Star.

“Concrete would have been much further along as a technology for floating wind if Hywind had stuck with it,” notes Landbø. “But it may catch up as we look to the next generation of turbines, the 10MW-plus class, which will require floating foundations that are very big indeed — and concrete will help solve many strength and fatigue issues, while keeping costs low.”

Concrete would have been much further along as a technology for floating wind if Hywind had stuck with it

The 2MW FloatGen project taking shape in Saint-Nazaire is a historic milestone for the rapidly emerging concrete floating wind sector — and a showcase for a technology that needs to step up to array scale in short order. As with floating wind more generally, the longer-term view as turbines continue to scale up in size is tantalising.

The choice between steel and concrete is not an “either/or”, says Conroy. “Different materials and designs have their merits for different sites — because of metocean or seabed conditions and local construction infrastructure. There may be more development of hybrid steel-and-concrete concepts, such as we have seen at the project off the Goto Islands in Japan [where Toda Corporation moored a Hitachi turbine mated with a steel-and-concrete spar].”

Johan Sandberg, energy consultancy DNV GL’s floating wind segment leader, feels concrete’s prospects hinge on “future-proofed” thinking.

“Concrete foundations have the particular advantage that they have a longer life expectancy than their steel counterparts,” he says.

“If we can only minimise the risk of the design being completely incompatible with the wind turbines of the 2040s, it could be possible to retrofit new turbines on these foundations — which would improve the technology’s overall economics.

“While it is a challenge to make technology predictions that far in advance, it could make the difference for the concrete floaters.”

However the markets shake out, floating wind has come a long way from the technology many dismissed a short few years ago as “experimental”. And it has the wind at its back.

As Choisnet notes: “The units so far have all been around 2MW, but for floating concrete structures, the bigger and bigger turbines the industry is developing will be even better: the motion characteristics will be better because the [foundation] structures will be bigger, but also the construction, transport and maintenance will be no more difficult than with [smaller units such as] FloatGen.

“After so much preparatory work, FloatGen is happening very fast now,” he concludes. The same might soon be true for the concrete floating wind industry as a whole.

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