Floating Offshore Wind
World’s first floating wind farm - Hywind Scotland. Image source
Why floating wind turbines?
Floating offshore wind is currently much more expensive than fixed bottom offshore wind, and that in turn is more expensive than onshore wind.
As I explained in more detail in my previous YouTube video about offshore wind, the wind resource at sea is much better than on land. There are higher wind speeds because on the ocean there are no trees, hills or buildings to slow down winds close to the ground. And winds tend to be more consistent. In combination that adds up to significantly increased energy output for offshore wind compared to onshore.
And then, perhaps counter-intuitively, offshore wind farms can often be located closer to coastal cities than onshore wind can, reducing the need for expensive and not always popular overland transmission infrastructure.
Winds are better at sea because there is no resistance such as buildings and hills.
But these advantages come at a financial cost compared to onshore wind. Floating offshore has the same advantages and cost penalty, only more so. Fixed bottom offshore wind is only suitable in waters up to around 60 m deep, whereas floating offshore can go much deeper. This opens many more options that might have better wind speed, and for areas with no shallow waters suitable for fixed bottom offshore. Another potential advantage is that wind farms can be located far from shore. That means that people who don’t like how wind turbines look or sound don’t have to look at or hear them.
How do they float?
A regular wind turbine is not exactly made to float with its high centre of gravity. The stability of a floating wind turbine is crucial, it needs to stay upright with the rotor facing the wind directly to capture wind energy effectively. That means controlling six types of motion: surge, sway and heave displacements, plus roll, pitch and yaw rotations.
So, how do we make these towering structures stay upright out at sea? Engineers have come up with some ingenious solutions, mostly borrowing from designs used in the offshore oil and gas industry and other marine applications.
Methods of Stabilisation
Floating foundations should withstand wind turbine forces and minimize pitch motions for optimal efficiency exist:
Gravity-Stabilised: This method makes the platform stable by lowering its center of gravity. Think of it like a toy with a heavy bottom that always stays upright, similar to a punching bag that bounces back up when you hit it.
Waterplane-Stabilised: This technique increases stability by making the base of the platform wide and flat. Think of how much easier it is to capsize a canoe compared to a catamaran. The wide base of the catamaran spreads out over the water, making it more stable and less likely to tip over when waves or wind push against it.
Moor-Stabilised: This approach uses multiple strong anchors and cables attached to the seabed to keep the platform in place. Imagine a tent held steady with several ropes and stakes all around it. If one rope gets loose, the others still hold the tent securely. Similarly, multiple mooring lines keep the floating platform stable in the water.
Types of Floating Platforms
To make the turbine float, a simple option is a barge or buoyant substructure. It's like a giant raft that supports the turbine, using air or lightweight materials to stay afloat. It's a simple design, that works best in calmer waters.
For rougher seas, we have the semi-submersible platform. This one's got a floating platform with underwater pontoons or columns that keep it stable, even in choppy conditions. This design is borrowed from the offshore oil and gas industry, where it's been used for decades to support drilling rigs and other structures. The five floating wind turbines in the Kincardine wind farm located 15 km off the coast of Scotland use this type of substructure.
Then there's the tension leg platform (TLP). Imagine a giant water balloon anchored to the seabed with taut cables. That's essentially how a TLP works, providing excellent stability in deep waters. This design also has its roots in the oil and gas industry, where it's been used for floating production platforms. The world’s first floating wind turbine used this tech, and last year three 8.4 MW turbines were installed on TLP floaters in 100 m deep waters off the coast of France.
Finally, we have the spar buoy. This is a long, cylindrical structure that floats upright, like a giant buoy. Its weight is evenly distributed, making it incredibly stable, even in the deepest waters. Spar buoys were originally used to moor naval vessels but have been adapted for use with floating wind turbines due to their excellent stability characteristics. This is probably the most common that we are seeing in floating windfarms currently - Hywind Scotland and Hywind Tampen both use spar-buoy structures.
Kincardine Offshore Windfarm has a total capacity of 50 MW and is capable of powering nearly 35,000 homes in Scotland. Source
In addition to these traditional types of substructures, another possibility is to integrate active control systems into floating wind turbines to enhance stability. Sensors, actuators, and control algorithms enable real-time monitoring and adjustment of the turbine's behaviour. This can minimize structural loads, optimize power generation as the turbine is always facing exactly into the wind instead of dipping and turning a little every time there’s a gust or a wave.
Challenges faced by floating wind turbines
All that sounds complicated right, compared to simply putting a wind turbine tower into the seafloor? Yes, floating wind is complicated. Complexity usually means two things: higher costs and lower reliability. So far floating offshore wind is both of those.
Floating wind currently costs much more than fixed bottom offshore, usually in the range of two to four times more expensive. To give an example, the next UK auction for renewable energy has maximum prices set to £73 pounds for fixed bottom and £176 pounds for floating.
In terms of reliability, the Hywind Scotland project, the world's first commercial scale floating offshore wind farm, is currently undergoing heavy maintenance, with its 5 turbines taken offline for three to four months over the northern summer, and the turbines towed back to Norway, to perform the work. This maintenance is necessary due to the operational data indicating the need for extensive repairs and exchange of components including defective bearings.
But we are still early days for floating offshore wind. Both cost and reliability tend to improve as technologies mature. Floating offshore is still very young and is moving in the right direction on both these measures. Projects so far have been small first-of-a-kinds, so costs will reduce naturally as we develop more and larger projects. Other opportunities to reduce costs will involve reducing materials requirements - floating offshore should be able to use less steel than fixed bottom. We would also expect to see a reduction in complexity of designs and improvements in manufacturing, also better reliability and less onerous maintenance requirements as we learn more about the operating environment.
History of floating offshore wind
The challenges facing floating offshore wind are substantial, but the potential rewards are too great to ignore. While it's true that we are still in the early days of floating offshore wind, with just a handful of wind farms installed today, the industry is rapidly evolving.
Floating wind got started back in 2007 when the world’s first floating wind turbine, the 80 kW Blue H prototype was commissioned off the coast of Italy. Designers chose a submerged Tension Leg Platform (TLP) technology adapted from the oil and gas industry.
Blue H Prototype is a small scale TLP turbine with a capacity of 80 kW and it was decommissioned at the end of 2010. Image source
A couple of years later, we saw the first megawatt scale floating wind turbine, which was 2.3MW in Norway. After that It took nearly another decade before a whole floating wind farm was installed and commissioned in 2017, this was Hywind Scotland, developed by Equinor and Masdar. It features five 6 MW turbines installed at depths of 95-120 meters, supported by a spar-buoy substructure technology.
Next came the 25 MW WindFloat Atlantic wind farm off the coast of Portugal, and the nearly 50 MW Kincardine wind farm in Scotland, which both use WindFloat semi-submersible platforms at depths of 60-100 metres depths.
Last but not least, we arrive at Hywind Tampen, currently the world's largest floating wind farm, commissioned in 2023. Located off the coast of Norway, Hywind Tampen powers nearby oil and gas platforms with its 11 turbines using floating concrete substructures in waters 260-300 m deep. It produces 88 MW of electricity, which covers about 35% of the annual electricity of the platforms it supplies.
Current projects around the world
So far, these past projects have accumulated to about 200 MW commissioned in the first 16 years of floating offshore wind, or about 0.02% of the world’s wind energy, which is not very impressive.
So far, projects initiated since 2021 have a total capacity of only 200 MW. Source
Under construction are projects that will quadruple the amount installed and additionally over 200 GW of potential projects have been announced. To put that in context that’s about 20% of today’s installed wind power. If they all went ahead, and let’s be real, they won’t, but something like 60 GW is reasonably likely to go ahead.
That is a tremendous number of projects for a technology that is still very young with many design kinks remaining to be worked out. Sounds like a big risk.
Despite the infancy of offshore wind technology and its complexities, developers are betting big on its potential. Let’s look at some of the countries with floating wind projects planned and why they think it’s worth the risk.
Europe has been the hub for floating offshore wind, with Norway leading the charge, leveraging its offshore expertise. This trend continues with Norway's Utsira Nord zone, poised to accommodate 1.5 GW of floating wind across three projects. France has several smaller projects under construction, and the UK leads with ambitious plans for up to 13 GW of floating wind.
Countries with limited clean energy options are also exploring floating wind. Japan, with its extensive coastline and vast exclusive economic zone (EEZ), sees offshore wind as vital for its energy transition and has plans to use this industry to reinvigorate local manufacturing by mass-producing floating platforms. They are currently tentatively dipping a toe into their floating wind potential with just a small 16.8 MW project currently in construction, for completion hopefully in 2026.
Marine Power Systems' (MPS) floating wind platform technology will help deliver floating offshore wind projects in Japan. Image source
South Korea, another country with limited domestic renewable energy resources, is not just dipping a toe but instead diving straight in with several projects of around 1GW currently in planning, and the first to begin construction expected to be the 504 MW stage 1 of Gray Whale 3 wind farm expected to begin construction in late 2024.
China, a renewable manufacturing giant, is also entering the floating wind arena, aiming to drive down costs and accelerate global adoption similar to what they’ve done for solar panels and lithium-ion batteries.
The US, despite challenges with fixed-bottom installations, is moving ahead with floating wind efforts, recognizing its vast deep-water potential in critical markets like California, the Gulf of Maine and the Gulf of Mexico, and the federal government have launched the “Floating Offshore Wind Energy Shot,” to try to take out about three quarters of the cost of floating wind by 2035, to USD 45/MWh.
And even here in Australia, where we don’t have a single fixed bottom offshore wind farm yet, we’re forging ahead with floating wind projects being planned in a couple of regions that don’t suit fixed bottom.
Future designs
So far, all the floating wind farms have just stuck a standard offshore wind turbine onto one of those structures and called it a day, but there are other design options emerging too.
There are vertical axis wind turbines that can locate the generator at the bottom and so get a lower centre of gravity. And any number of changes to tower configurations to make installation easier or make them easier to float.
There are multi-rotor designs that can get more power from each electrical connection without needing to build massive rotors and hopefully avoiding the supply chain, installation and maintenance headaches that go along with that.
The new Vestas wind turbine design offers a 1.5% power gain in annual energy production thanks to the interaction between the rotors. Image source
With the right design, there is potential for floating wind to solve several of the big challenges of fixed bottom offshore - like needing massive amounts of steel for substructures and challenges related to assembling, installing, maintaining massive turbines offshore. It is not outside the realms of possibility that floating could end up being the simpler, cheaper technology for offshore wind in any depth of water some day in the future.
There are currently more than 40 floating wind concepts are under development, trying to do just that. The reason that there are so many is that floating wind is such a different design problem with different design constraints than what led to the three-blade rotor on a stick design that totally dominates the environment that it evolved in, onshore wind. For floating wind, the floating structure and mooring is as important as the rotor, maintenance is massively more expensive and the operating environment much harsher.
If we were starting from scratch, if there was no onshore wind and we went straight to floating, there is no way that we would end up with the standard “Danish design”. But we aren’t starting from scratch. Legacy wind turbine manufacturers have decades of experience at making their turbines, they have been through all the little unexpected ways that these designs failed, and they’ve had time to refine their designs over dozens of iterations.
In contrast, a brand-new design like the vertical axis Sea Twirl, or multirotor Wind Catching have brand new aerodynamics to learn about, brand new structural requirements to satisfy, etc. All the surprising ways that these designs can fail lie ahead of them. If they experience failures while floating a hundred kilometres out to sea, they might not have the resources to refine their designs based on those learnings. So even if these concepts are superior to simply placing a standard wind turbine onto a floating platform, that doesn't guarantee their success.
SeaTwirl's new S3 models have a capacity of 4-6 MW for larger electrification applications, such as wind farms or the electrification of offshore assets. Image source
Can floating wind become the next disruptive force in renewable energy?
We’re still too early in floating wind’s journey to be able to say how the technology is going to pan out, but there are disruptive elements to the technology.
Yes, in terms of cost it currently underperforms compared to mainstream electricity generation technologies. For now, it is mostly appealing in quite niche markets. Likely a big deal for countries who don’t have a lot of other options for energy security, but many think that’s as far as it will go, with DNV for example projecting that floating offshore wind will make up 6% of the world’s wind energy, or 15% of offshore wind by 2050. That’s still 264 GW, about a thousand times more floating offshore than we have installed today, and not so far off the world’s total nuclear capacity at about 380 GW. But there is a chance for it to make a much bigger contribution.
Underperformance in Mainstream Markets and Appeal to Niche Markets are two of the hallmarks of a disruptive innovation according to Clayton Christensen’s classic book the Innovator’s Dilemma. The third is rapid improvement: that’s a tick, we are learning fast in this technology. Fourth is potential for disrupting business models. There is a possibility for floating wind to cut some of fixed bottom offshore wind’s grass if it lives up to its potential for lower materials costs, and cheaper and simpler installation and maintenance.
Hopefully there will be new applications that floating wind could open up that don’t exist today for traditional wind. Could we see floating windfarms powering refuelling stations on major shipping routes, either to recharge batteries or creating hydrogen or derivatives and thereby disrupt the shipping fuel industry also? We will have to wait and see.
Watch this content as a video on Engineering with Rosie on YouTube.
