Will floating systems bring in a new wave for offshore wind?
Image credit: Ideol & V Joncheray
Floating wind systems are on the cusp of a huge transformation as they transition from pilot projects to commercial-sized wind farms.
Offshore wind farms are becoming a familiar site in prime shallow-water locations around the world, from South Korea to the North Sea. Most of these are anchored in the seabed, but winds are stronger and more consistent further offshore, so energy companies are turning their attention to deeper waters unsuitable for anchored turbines.
The final 9.5MW turbine was towed into position in what is currently the world’s largest floating offshore wind farm in August 2021, and the Kincardine Offshore Wind Farm was completed. The array of six Vestas turbines (five at 9.5MW and one 2MW) sits 15km off the coast of Aberdeen, Scotland – Europe’s windiest country – in a location with a water depth of 60-80m. The larger turbines have a rotor diameter of 164m, and each blade weighs 35 tonnes.
The 50MW site will not hold the world title for long, as recent Scottish and Celtic Sea seabed auctions attracted multiple bids for gigawatt-scale operations from consortia including Shell/Scottish Power and Norway’s Equinor. Floating offshore wind (FOW) costs are currently considerably higher than offshore fixed bottom systems, but costs have already dropped by 30 per cent in the last four years for fixed systems, and as the floating sector begins to scale up similar economies of scale are expected.
If wind power is to deliver its contribution to a zero-carbon future, floating offshore wind must become a reality. The tough deep water environment presents the industry with a set of technical challenges that must be overcome before commercialisation. Much of the technical groundwork has been done in the oil and gas sector but installation, monitoring, and the ongoing essential maintenance of multiple individually low-value sites (turbines in an array) comes with a quite different basket of economics from a single high-value constantly producing asset like an oil rig.
According to the International Energy Agency, the world’s offshore wind resource is capable of meeting estimated global electricity needs in 2040 by a factor of 11. Harnessing that energy will be a key component in meeting global net-zero carbon emissions targets by 2050. Eighty per cent of the windiest parts of the ocean’s real estate is too deep to install farms that have their foundations in the seabed. The solution is to float the giant turbines and tether them to the sea floor, which will extend wind power generation capability to countries with deeper coastal waters, or where the seabed is unsuitable for fixed structures. Norway, France, Spain, South Korea, Japan and much of the US coast will benefit.
Floating offshore structures are in constant motion, exposed to vibrations and shocks from waves up to 15m in height; 365 days a year, for their whole lifetime – estimated at 30-35 years. The job of designing structures to permit the tall, top-heavy windmills to float upright is largely done. Borrowing heavily from deep-water oil and gas experience, the most common design is a semi-submersible triangular frame that holds water-ballasted vertical tanks at two corners counter-balancing the third point, which holds the windmill itself. Seawater can be moved around the semi-submersible structure to maintain stability.
Other approaches to keeping the turbines upright and relatively stable include the spar design developed by Equinor, which puts the turbine into a long, deep ballasted tube like a flower in a vase. Norwegian BW Ideol’s Damping Pool is a concrete or steel barge-like structure like a picture frame, where sloshing water movement inside the floating frame stabilises the movement of the turbine above. This design has already survived three typhoons in trials off Japan.
The final basic design type is a tension-leg platform. These are anchored to the seabed via adjustable mooring cables. Sensors identify incoming wind and swell activity and adjust the cable lengths in real time, to give the gigantic turbine platform a smooth ride on rough offshore waves. General Electric in the US is pursuing this strategy.
Floating substations will also need to be developed, because the voltage losses along a long cable taking the energy to shore would be unacceptable. Small subsea transformers already exist to service oil and gas installations but are too limited in capacity for floating offshore wind.
César Saiz, head of innovations and solutions for renewables at Hitachi ABB Power Grids, is planning for the next generation of FOW, fields of 250MW and above planned for completion in 2025/6. He explains: “We expect to have operational floating substations by the middle of the decade. The technology is already developed. But building these floating substations will require a whole new supply chain and close collaboration and coordination with both the energy companies and the platform designers.”
Hitachi has a long history in FOW, having manufactured an early proof of concept for the 2013 Fukushima Floating Offshore Wind demonstration project. The new generation of floating substations will be modular, assembled, and mounted on a substructure in port, then towed to their final positions. “Anything out in deep water for up to 35 years will be subject to constant day-to-day environmental stress, in perpetual motion from the action of waves and currents. The movements generated by each different platform design will need to be accommodated in any final substation design. We are working closely with the turbine manufacturers on these aspects,” he says.
Wind power in numbers
180GW is the annual rate that the world must build new wind installations in order to meet IPCC targets for limited global warming by 2050.
93GW commissioned in 2020, of which 6.1GW was offshore.
743GW global total wind power in 2020.
35GW total offshore wind capacity.
74MW is floating offshore (2021).
244.5MW coming online 2022/23.
Source: GWEC 2021
With the intended lifespan, maximum reliability with minimal on-site intervention will be key to economic operations. In common with the turbines, substations will have multiple sensors to monitor performance constantly, and AI and machine learning will be used to give early warning of fatigue that might lead to costly downtime. Saiz added that looking much further out, into the next decade, hybrid underwater and floating solutions might be in use.
If the offshore structures and substations are areas where companies are confident that solutions are in place for the near future, there are other technologies that lag farther behind. The Carbon Trust is an international consultancy working to accelerate the delivery of a sustainable, low-carbon future.
Sam Strivens is a maritime technologist and head of floating wind at the Carbon Trust. He explains: “Carbon Trust has been active in offshore wind for over 13 years, and in 2016 set up the Floating Wind Joint Industry Project to focus collaborative research between offshore wind developers. Our aim is to help de-risk commercial-scale floating offshore wind, identifying and removing the barriers to development by encouraging cooperative research.”
Early on, the CT identified dynamic export cables as key to supporting 500MW-plus operations and is helping five cable manufacturers to accelerate cable design to that stage. The cables have not been manufactured yet, but the designs are there and ready for deployment.
In other areas, there’s still more to do. “Heavy-lift offshore operations for key component exchange like blades or gearboxes currently do not exist, and a cost-effective solution for ongoing monitoring and inspection of components, especially subsea still has to be found,” Strivens adds.
When it comes to wind, bigger is always better. Turbine size is increasing rapidly, with 13-15MW turbines expected to be on the market by 2025. Bigger turbines are more efficient, but the current batch of windmill designs are reaching the limit of the capability of existing vessel-mounted cranes to reach up, maintain them, and replace key heavy components such as blades and turbines. Move these giants offshore and float them, and you have a compounded difficulty: one floating, moving object trying to service another.
The industry is taking two approaches to this very real future problem. Tow to port means disconnecting the turbine from its moorings and export cables, then slowly towing the whole thing into a nearby port for required maintenance. The disadvantage of this is twofold: excessive downtime detracts from the economics of floating offshore wind, and effective disconnect/reconnect couplings for power and mooring cables will need to be devised. There are also concerns among some of the energy companies that the towing process itself could induce metal fatigue and shorten the working life of the asset. Moreover, ports with the infrastructure and water depth suitable to handle the turbines are few and far between.
The second approach is to deal with operations and maintenance on site, in deep water and with harsh wind and sea conditions. Some heavy-lift crane vessels might have the required reach, but will be unable to meet the horizontal accuracy to avoid damaging either the crane tip or the turbine due to their relative movements at sea. Novel cranes will be needed for major maintenance of the next generation of turbines.
Joop de Fouw is technical director at Conbit, a Dutch engineering consultancy with 20 years of experience in oil and gas and offshore wind. “No current solution exists for the coming 15MW turbines,” he explains. “Our concept is to install a temporary lifting device in the nacelle, using pre-installed light winches to run up the tower and start building a crane in situ. But that will need standardised attachment points and needs the cooperation of the OEMs, perhaps driven by the field developers.” Conbit is looking for a partner to take that design to a fabricated prototype stage. “At a certain point, field developers will insist that there is a solution to this problem. There just has to be if the industry is to progress,” de Fouw adds.
There are no quick-fix solutions to monitoring and inspection of assets, but there are techniques in development. The use of the digital-twin approach is likely, but handling and analysis of the data flow from the very many sensors required above and below the waterline will definitely require some new ideas.
Global subsurface surveying and monitoring business Fugro has a keen interest in the potential for FOW. Dan Jones, Fugro’s service line director of inspection, repair and maintenance for Europe and Africa, says the company is planning to introduce a fleet of uncrewed surface vessels that can launch underwater remotely operated vehicles. “We will be testing a 12-metre vessel close to Aberdeen in the coming months at an offshore wind farm,” he says. “It’s clear to us that there will be a huge requirement for ongoing monitoring of floating wind because of the high-energy environment, and the sheer numbers of turbines and cables.”
In the near future, remotely operated vehicles will upload data from asset-mounted sensors and scan the moorings, inter-array cables and subsurface structures with laser scanners – the digital equivalent of callipers – to measure thinning, corrosion, wear, and fatigue of metal parts at the sub-millimetre level. Further out, these uncrewed vehicles could dock within the wind farm to recharge and upload data for processing. Fugro is also developing machine-learning algorithms to take the heavy lifting out of the sheer volume of data processing that will be required. “It’s a rapidly evolving space,” says Jones.
Floating offshore wind is an industry in its infancy with enormous potential, but the prize is great – access to an energy resource that is more consistent than nearshore wind and delivers up to 50 per cent more energy.
A radical idea
Ole Heggheim and his co-founders at Wind Catching Systems (pictured above) started from the premise that existing turbine designs based on a Dutch corn milling windmill were not necessarily the best for generating energy offshore. The team, based in Oslo, Norway, have been developing their multi-turbine design for several years in conjunction with the Norwegian Institute for Energy Technology.
Heggheim says: “Most turbines max out at [wind speeds of] 11-12m/s. At this speed the wind has an energy of 300W/m2, but Wind Catcher can operate up to 17m/s, equivalent to 1,300W/m2, taking advantage of the exponential energy levels at higher wind speeds.” At the pre-pilot stage now, Abiel have been contracted to design and engineer the Wind Catcher.
Wind Catching Systems claims that its design is five times more efficient than conventional turbines because it can combine operation at higher wind speeds, with the larger swept area of its multi-turbine design. In tests the company has also demonstrated a synergistic multi-rotor effect that more than compensates for any drag effect of the novel design.
The Wind Catcher is a 300m by 350m framework holding 117 dual-blade 1MW turbines above a deck sitting near sea level which accommodates crew and component exchange areas. The deck is big enough to house a substation to export the energy from the field to shore. Wind Catcher sits atop a hybrid subsurface structure that is a cross between a trimaran hull and a semi-submersible rig. It all rotates naturally and faces into wind. Necessary maintenance and inspections crews will take elevators up the vertical struts of the framework to work. There are also seabed usage benefits to the design, which occupies 20 per cent of the space of a floating turbine farm for the equivalent energy output.
“In addition, an ease of maintenance which does not require large vessels or cranes should make our system competitive in the future,” Heggheim adds.
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