Growing demand for cleaner energy sources means offshore wind farms are being built all over the world. More than 5,000 turbines must be installed each year until 2050 to limit global warming to 1.5℃.

But in certain regions, like California, it is difficult to build wind turbines directly on the sea floor due to the steep drop-off of the continental shelf.

Even in areas with shallow coastal waters, such as the North Sea, congestion from shipping lanes, fishing activities, marine protected areas, tourism and existing energy infrastructure all impede new turbine construction.

So it’s hardly surprising that many of these new turbines will have to be located in deeper waters further out to sea. 

Floating wind turbines are emerging as a promising solution. But turbines are also getting bigger at a rapid rate – allowing electricity to be produced at a lower cost.

The blades of Hywind Scotland, the world’s first commercial floating wind farm, tower 175 metres above the sea surface – the same height as the London skyscraper known as 'the Gherkin'.

This represents a huge technical challenge. Located in deep waters, these large floating structures must withstand the relentless push and pull of the ocean while maintaining stability to ensure ongoing energy generation.

So, how do these colossal structures remain in place? 

The four types of floating wind farm platform. Image: ActeonCC BY-NC-ND 

The floating wind turbine

The mast of a floating wind turbine is connected to a platform, which is designed to provide stability. Several different types of floating platform exist, each with the dimensions of a football pitch.

Beneath the water, mooring lines keep the turbine stable and prevent it from drifting away. Mooring lines can be either very large steel chains or synthetic ropes. Each of the three steel chains used for Hywind Scotland, for example, are approximately 900 metres long and weigh 400 tonnes.

The mooring lines are attached to the seabed with a ground anchor. Most people will be familiar with anchoring a boat or securing the guy ropes of a tent with pegs.

In both cases, the anchor (or peg) is embedded into the ground, making it harder for the anchor to become dislodged as the weight and strength of the ground has to be overcome to pull the anchor out. The anchors used for floating wind turbines are based on the same principle, but at a far greater scale. 

Three main types of anchor are used to fix the floating platform to the seabed, each with unique characteristics.

  • Drag anchors are similar to traditional boat anchors, but can have a six-metre wingspan and weigh up to 50 tonnes. They are dragged into the seabed by an installation vessel and embed themselves into the ground until the required holding resistance is achieved.

  • Pile anchors are like very large (up to 60 metres in length) but hollow nails. These anchors are hammered into the ground using an extremely heavy hammer. If the turbine is being installed above very hard soils or a rocky seabed, then a hole can be drilled to facilitate the pile installation.

  • Suction pile anchors are also hollow cylindrical tubes, but a sealed top cap creates suction pressure when water is pumped from inside of the pile. This forces the pile into the seabed without the need for hammering (an effect similar to the use of a plunger to unclog a drain). This is the type of anchor used to secure Hywind Scotland.

The mooring line for a floating wind turbine at Polarbase, Hammerfest, Norway. Image: Øyvind Gravås and Even Kleppa/EquinorCC BY-NC-ND 

Choosing the right anchor

Floating wind farms are being planned for areas such as the Celtic Sea and coastal waters west of France. However, the presence of hard rock seabeds in both areas means drag anchors will be difficult to use.

Even in dense sand, a drag anchor may only partly enter the seabed, creating inadequate support for the largest turbines. Drilled piles are the best way to anchor floating turbines to hard rock, so in this case, a driven pile might be the only option.

But driving these piles into the ground generates significant underwater noise that can be harmful for marine species. Research has also found that the movement behaviour of Atlantic cod subtly changed in response to pile driving in the North Sea.

Even small changes in movement behaviour could affect individual growth and reproduction rates, potentially influencing the growth rate of entire populations.

Several techniques have now been devised to reduce noise. This includes air bubble curtains to limit the ecological impact of floating wind farms. But these techniques may result in additional costs that could make pile anchors too expensive. 

The installation of pile anchors generates significant underwater noise. Image: ATJA/Shutterstock

The world needs a lot more wind turbines, and technology now allows installation further out to sea. But, as identified in our recent review paper, these environmental and technical challenges for anchoring the structures in place must be addressed.

Without more investment in anchor technology to streamline installation, improve anchor performance and limit damage to the natural world, the potential of floating wind to help the energy transition will be greatly reduced. 

Is there any limit to how big wind turbines will get?

Separately, Simon Hogg, executive director of the Durham Energy Institute, Durham University, writes: This year, about 160km off the coast of northeast England, the world’s largest wind turbines will start generating electricity. This first phase of the Dogger Bank offshore wind farm development uses General Electric’s Haliade X, a turbine that stands more than a quarter of a kilometre high from the surface of the sea to the highest point of the blade tip.

If you placed one in London, it would be the third-tallest structure in the city, taller than One Canada Square in Canary Wharf and just 50 metres shorter than the Shard. Each of its three blades would be longer than Big Ben’s clock tower is tall. And Dogger Bank will eventually have nearly 300 of these giants. 

Just two decades have passed since the UK’s first proper offshore wind farm was built off the coast of north Wales. Its turbines were each able to produce two megawatts (MW) of electricity in ideal conditions – considered huge at the time. In contrast, the Haliade X is able to produce 13MW of electricity, and 15MW turbines are only another year or two away. 

Next up: an Eiffel-sized turbine? Image: GE Renewable Energy / Facebook

So why are turbines increasing in size at such a rapid rate, and is there a limit to how big they can go? In short, the first answer is to reduce the cost of energy and the second is that there must be a limit – but nobody has put a number on it yet. 

Big turbines, cheap electricity

Just five years ago, the offshore wind industry hoped to reduce its energy pricing to below £100 per megawatt-hour by 2020 from new projects in UK waters. Even at that level, projects would still have relied on government subsidies to make them economically viable, compared with other types of electricity generation.

But in fact, costs quickly reduced to the extent that offshore wind farm developers were soon committing to selling their electricity at much lower prices. Today, developers are building wind farms such as Dogger Bank where they have committed to prices below £50 per megawatt-hour. This makes offshore wind competitive with other forms of power generation, effectively removing the need for subsidy.

The major factor in reducing these costs was turbine size. Ever-larger turbines came to market faster than virtually everybody in the sector had expected. 

Dogger Bank is ideal for offshore wind as the water is very shallow. When complete, the project will power six million UK homes. Image: Dogger Bank Wind Farm.

Blades cannot spin too fast

In theory, turbines can keep getting bigger. After all, a bigger blade extracts energy from the wind over a greater area as it rotates, which generates more electricity.

But there are some engineering constraints. One concerns erosion of the blades caused by them colliding with raindrops and sea spray. For current designs, the speed of the blade tips must be limited to 90 metres per second (which works out at a little less than 320km/h) in order to avoid erosion. Therefore, as turbines get bigger and blades get longer, their rotors have to turn more slowly.

A consequence of having to slow the rotor down is that, to produce the same amount of power, the blades must deflect the wind to a greater extent. This results in greatly increased forces on the whole turbine.

We can address these high forces, but only by increasing both turbine weight and cost. And that means the point at which the turbine becomes unprofitable – the point at which the extra cost is no longer worth it for the value of extra electricity generated – is reached much sooner than if the blade tips were allowed to go faster.

Also, as blades get longer they become more flexible. This makes it more difficult to keep the aerodynamics of the wind flow around them fully under control, and harder to ensure the blades do not strike the turbine tower under extreme wind conditions. 

Logistical constraints

Engineering challenges like these can perhaps be solved in the longer term, though. This will mean that wind turbines are more likely to be limited in size by manufacturing, installation and operational issues, rather than any physical limit on the design of the turbine.

Just transporting blades and towers from factory to site and assembling the turbine when you get there presents huge challenges. Each of those Big Ben-sized blades must be shipped in one piece. This requires huge ports, giant vessels, and cranes that can operate safely and reliably far offshore. This is where the limit is most likely to come from. 

Needed: huge ships, ports and cranes. Image: DJ Mattaar/Shutterstock

You can see these limits in practice in the UK, which is surrounded by windy and shallow seas that are perfect for generating energy. Despite this, the UK is likely to miss its ambitious target to more than treble its offshore wind capacity by 2030.

This is not because of technology or lack of offshore sites. Rather, the industry will not be able to manufacture turbines quickly enough, and the port infrastructure and number of installation vessels, suitable cranes and workers with requisite skills is unlikely to be sufficient.

So if the UK is to maximise the benefit to its economy from what is, so far, a fantastic success story, the focus now needs to switch from pure cost reduction to developing workers’ skills and the offshore wind supply chain.

Turbines will get bigger, I am sure, but I suspect at a slower rate than we have seen in recent years. And if the turbines are deployed 160km offshore, will anybody care? After all, the public will not be there to see them.

Authors: , lecturer in geotechnical engineering, University of Southampton; , Royal Academy of Engineering Chair in Emerging Technologies – Intelligent & Resilient Ocean Engineering, University of Southampton. This article first appeared in The Conversation.