Onshore wind turbines can be found everywhere from the tropics to the Arctic. Three decades ago, developers started putting them on fixed foundations out at sea, sparking the rise of the offshore wind market, which added 6.1 gigawatts of new capacity in 2019.

More recently, the wind industry embarked on an even more ambitious endeavor: putting turbines on floating platforms in the water, rather than fixed foundations. Now on the verge of commercial maturity, floating wind has the potential to become one of the most important new renewable energy markets.

So, what is floating offshore wind?

It’s pretty much exactly what it sounds like. Instead of putting a wind turbine on a fixed foundation in the sea, you attach it to a structure that floats in the water. The structure is tethered to the seabed to stop it from drifting off into a beach or shipping lane.

Today’s floating wind designs envision using standard offshore turbines, export cables and balance-of-plant materials. The key difference between floating and fixed-foundation offshore wind is that the latter is limited to water depths of up to around 165 feet.

How does floating offshore wind work?

To keep turbines upright, floating foundations rely on the iceberg principle: Most of the mass is underwater.

At Hywind Scotland, the world’s only commercial floating wind farm today, each Siemens SWT-6.0-154 turbine has a towerhead mass of around 350 tons and sits on a foundation with roughly 6,060 tons of solid ballast and a displacement of some 13,230 tons.

Provided the water underneath the turbine is deep enough, the shape of the foundation may not matter much. In practice, though, floating foundation developers have focused on designs that are going to be cheap to build and easy to work with from the perspective of operations and maintenance.

That still leaves plenty of room for imagination: Every developer has a different concept and a compelling argument for why it is the best. Four basic designs are leading the market today.

Using its experience in the oil and gas industry, Equinor (formerly Statoil) has based its pioneering Hywind floating platform on a spar buoy design that relies on gravity for stability. The spar buoy is assembled in sections and extends down to around 260 feet beneath the sea surface, making it appropriate for water depths of between roughly 310 and 390 feet.    

Other developers, such as Principle Power and Hexicon, favor a semi-submersible platform design that relies on buoyancy for stability and is suitable for shallower drafts. Critics note that the large size of the structures could restrict maneuverability in ports.

A third design, championed by the French foundation maker Ideol, is a square barge that contains a damping pool to maintain turbine stability. Like the semi-submersible model, this is suitable for shallower waters, and Ideol touts the fact that its concrete fabrication comes in handy from a local content perspective.

Finally, a concept called the tension-leg platform relies on a taut mooring system to provide stability. This allows the structure to have a smaller physical footprint and potentially be cheaper than competing models. Danish firm Stiesdal Offshore Technologies is leading the development of this concept with a product called TetraSpar.

Which of these designs will win out?

Equinor was the first company to build commercial-scale floating wind farm, and to date, its Hywind design is the only one that has a significant operational track record.

The Norwegian energy giant claims it was able to cut costs by up to 70 percent between its first demo project, off the coast of Scotland, and its 30-megawatt commercial wind farm. It's expecting to cut foundation costs by up to an additional 50 percent for Tampen, an 88 MW project scheduled to enter operation off the Norwegian coast in 2022.

In February, Sebastian Bringsværd, Equinor’s head of floating wind development, cited a cost target of €40 to €60 ($44 to $66) per megawatt-hour by 2030. These rapid cost reductions could give Equinor the upper hand in forthcoming floating offshore wind tenders.

But a May 2019 analysis of upcoming project figures collated by IHS Markit found that 90 percent of floating offshore wind capacity is likely to be installed on semi-submersible platforms, with Principle Power leading the market.

How are floating offshore wind turbines installed?

Equinor’s spar buoy foundation is essentially a hollow steel cylinder that can be towed out to the site before being partially filled with water and ballast to force it to remain upright. The latest iteration of the foundation needs at least 345 feet of water when upright, meaning turbine installation would most likely happen out at sea, as with traditional foundations.  

With other foundation designs, the minimum depth requirement is much lower — so turbine installation could happen onshore with the fully assembled turbines and foundations then towed to site, significantly cutting costs.

Why would anyone want to float a wind turbine?

Putting turbines onto floaters gives a developer access deeper waters, which means more potential project sites and lots more potential capacity.

Some 60 percent of available offshore wind resource in the U.S. is beyond the reach of fixed-bottom foundation turbines, including practically the whole of the West Coast, according to a 2017 statement from industry body WindEurope, 

In Europe, floating offshore wind could deliver an extra 4 terawatts over and above the continent’s already leading level of bottom-fixed capacity. And in Japan, floating foundations will be critical for the development of an offshore wind sector that could offer 500 gigawatts of capacity.

Beyond the ability to capture vast untapped energy resources, floating offshore wind also carries significant industrial promise. For the U.S., it could be a way to get into a renewables sector that the country has so far barely been able to qualify for. And Europe’s oil and gas companies, which seem increasingly committed to joining the energy transition, see floating wind as an area where their existing offshore experience can pay handsome dividends.

Who’s getting into the game?

European oil and gas companies, for starters. Equinor is the most notable example, but Royal Dutch Shell is emerging as a major player, and Italian contractor Saipem unveiled a platform last year. 

France's Total bought into the market in March and earlier this month acquired a 20 percent stake in the Eolmed project in the Mediterranean that will use Ideol's foundation and MHI Vestas turbines. 

These players seem keen to compete or partner with a host of independent floating platform developers, such as Ideol, Principle Power and Stiesdal Offshore. At the same time, the oil majors may take on project development and asset ownership roles. In this respect, they could compete with established offshore wind farm developers such as Ørsted and Iberdrola.

Ørsted hasn’t revealed any floating offshore wind plans yet, but EDP Renewables and Engie have joined forces on the WindFloat Atlantic project (alongside Spanish oil and gas firm Repsol), and Iberdrola announced two pilot projects in March.

Finally, there are the wind turbine manufacturers. Offshore wind turbine leaders such as Siemens Gamesa, MHI Vestas and GE have stayed away from the intricacies of floating foundation design, but they hardly need to worry. The massive turbines they are launching are increasingly designed to operate far offshore on floating platforms, and the market potential they are looking at is impressive.

Where's the market going?

Regardless of whether it takes off in the U.S., there's no doubt that floating offshore wind is going places at the global level. Even in the short term, that could lead to some interesting developments in the offshore wind sector.

American company Principle Power could play a leading role in the development of the industry, for example. Japan could finally develop offshore wind. And European oil and gas majors might really come into their own as wind energy players.