Author: Bernadette Ballantyne

Partner: Fugro

Until a few years ago the offshore wind industry was constrained by the seabed characteristics of countries seeking to harness the generating capacity of the wind. Only those with shallow waters of around 40m or less could implement fixed offshore arrays. But in just a few years the game has changed. Floating offshore turbines are entering commercial production and opening up vast swathes of ocean, where the seabed is deep and the wind resources are powerful, to generate low carbon energy.

No-one seems more excited than UK Prime Minister Boris Johnson. “We believe that in ten years-time offshore wind will be powering every home in the country, with our target rising from 30 gigawatts to 40 gigawatts,” Johnson told the Conservative Party Conference in October 2020. “We will not only build fixed arrays in the sea; we will build windmills that float on the sea – enough to deliver one gigawatt of energy by 2030, fifteen times as much as the rest of the world put together.”

Other countries around the world are also seeking to exploit wind resources in deep waters, but the UK is the first to set a hard target. Alastair Dutton is chair of the Global Wind Energy Council global offshore wind taskforce and lead consultant to the World Bank’s offshore wind programme. He says that  there are currently 72 projects being developed in 12 countries. “The USA, Norway, Greece, Ireland, Portugal, Italy, France, United Kingdom, Spain, South Korea, Japan, and China. That’s quite a compelling scale of market,” he says.

Global growth

These projects are becomingincreasingly larger from arrays of just a handful of turbines up to hundreds of megawatts of generating capacity. “And then we have in development now some well over a gigawatt. And that’s when we’ll get to the sweet spot,” says Alastair.

The sweet spot is the point at which floating wind becomes so large that economies of scale really kick in and prices plummet. This has already happened for fixed offshore wind.

Confidence then is high, that floating technology can emulate the success of fixed turbines and perhaps one day go even further but it wasn’t always the case. “Two years ago, when we discussed this with the global offshore wind task force as to where we should focus, developers were saying, just stick to fixed foundations,” says Alastair.

And then something changed. “And then I raised it again a year ago, and there was a completely different response. All the developers that have pipelines forward of 10 to 15 years, are having to include it as a possibility. And therefore, my view is it’s not a matter of if but when this technology will come through at scale, at a much lower cost. So that’s the game in town at the moment.”

So what happened to change the game in just a couple of years? Cian Conroy of Principle Power knows the answer. His technology is being used on a floating wind project that is already providing power to the grid. His company makes a semi-submersible floating wind turbine foundation called WindFloat, which was one of the first in the world to prove that floating offshore wind is commercially and technically viable. “We deployed our first unit off the coast of Portugal in 2011. So that was a two-megawatt demonstration,” says Cian. “So that was really to prove the concept of a few different elements. At the time, we commissioned the turbine, we installed we hooked up, we ran it for a five-year cycle. And that was really just to show the whole lifetime.”

Proving the technology

This became the second successful floating wind turbine in the world behind Equinor’s Hywind project in Norway which was also a single turbine. “We learned an awful lot, we survived 41 metre per second wind speeds,” says Cian.

Optimal wind speed is around 8m/s so 41m/s was massive, and of course high winds mean high waves of up to 17m, but the turbine kept on generating. “And that gave us a lot of comfort to then go from two megawatts to the big units. So we went from sort of two megawatt turbines to the 8.3, four times the capacity output.”

Four times greater capacity meant that diameter of the turbine blades doubled from 80m to 164m requiring a taller tower, and these considerations of scale are crucial when it comes to building large arrays with 10, 12 or even 14MW turbines.

The next project for Principle Power was the 25MW, three turbine, Wind Float Atlantic, off the west coast of Portugal. It became the first project to attract private finance. “Every project before then was either paid for off the balance sheet or heavily subsidised by public sector funding. This one, we were able to secure project finance, which again, show the evolution from a demonstration technology to bankable and commercial technology.”

So far Principle Power have had good production output, and there have been no negative effects in terms of reliability from being in deep water, but on the positive side they have found that their floating wind farms have benefitted from the greater wind speeds that can accompany deeper waters, which is good news for performance. And it is this proving out of the technology, demonstrating consistency and strong generation figures that has changed the game.

Floating wind scales up

Moving forward for Principle Power means a 50MW project at Kincardine in Scotland which will use 9.5MW turbines taking the scale up another notch. But their ambitions are much larger. “We have a project in Korea, which is 500 megawatts, we’re working on 1000 megawatts elsewhere in the world.”

1000MW is 1GW. That is a floating offshore wind farm so vast that it would meet Boris Johnson’s target in a single project with 80 to 100 turbines. This is another feature that we can expect to see with floating wind farms of the future. As the wind farms grow so will the turbines as developers seek to maximise power generating potential. The good news here is that wind turbines are well used to operating offshore and the industry has decades of development in this area with longer and more reliable blades being key features of offshore turbines.

For floating offshore the more critical development has been into the foundations, the floaters that will support the turbines. For Cian and Principle Power their three-column asymmetric semi-submersible design actually came from the offshore oil and gas industry.

“We have the turbine mounted on one of the columns. The three columns are braced together and are connected and in terms of dimension, the three-column approach and the technology we use enables us to have a relatively shallow draft. And that’s a key issue when you think about these as how you commissioned them.”

In this case the depth of the unit is from 10m, but it varies depending on the project. Cian says that there are three main elements that enabled WindFloat to have a shallow draft and maintaining its horizontal position, a process known as station keeping. First is a ballast system. “So we have a sealed water system, which gives the mass to the structure. So where the turbine only mounted on column one, there’s a sea of water system, and that which adds the ballast to that column, there’s a different amount of ballasts in columns two and three, that keeps the platform level,”

Secondly there is the active balance system, which is basically a set of pumps used after high energy events like storms. Finally water entrapment plates are also used for maintaining position. “So the three of those working as a system enables the platform just to keep its station and to stop moving to one server or tipping over.”

Positioning challenge

As well as the station keeping aspect of floating turbines there is the need to position them out in deep water in the first place and this is something that Fugro’s Alistair McKie knows all about as Fugro helped position the WindFloat Atlantic project. “When we got to the floaters in Portugal they were being assembled. And then once they’re assembled, we installed our equipment onto it. And then we assisted in the tow from field to the site, much in the same way we would do with a typical semi-submersible drill rig move that we’ve got 20-30 years’ experience of moving.”

But just because the technology was tried and tested it didn’t mean that there weren’t unique challenges for floating offshore wind. “Actually, one of the challenges on the tow from the Quayside to the WindFloat site was the tow was actually quite long, relative to some of the other projects that we’ve done,” says Alistair. “So we needed our survey system to stay up and running for at least five days, with no power. So we actually sort of challenged our workshop to think outside the box. And one of the solutions they came up with is reuse the small wind turbine generator on the large floater to power our survey systems. We could power the system and charge the system as we went.”

Even though the WindFloat Atlantic Project used a semi-submersible foundation type, this is not the only platform system finding favour in the market at the moment. “I think we identified 37 distinct platform types that were medium to high technology readiness level,” says Sam Strivens, a senior associate at the Carbon Trust.

The Carbon Trust is very active in the offshore wind sector running a number of joint industry R & D programmes from the offshore wind accelerator to the floating wind joint industry project, which Sam says is the world leading R & D forum for floating offshore wind. “So our involvement there is to is to reduce the cost and de risk technology for floating offshore wind to the realisation of commercial scale, floating offshore wind farms.”

Finding favoured foundations

Over the past five years the Carbon Trust has run 23 projects to explore and develop technical capability in offshore wind as well as providing regular market reports to update industry on progress. For floating offshore wind there is a lot to report, including the whittling down of foundation type from 37 to just four; semi-submersible; the Spar Buoy; the Tension Platform (TLP) and the Barge.

Semi-submersible is that used by WindFloat in Portugal; the Spa Buoy is the technology favoured by pioneering developer Equinor on its Hywind Project which has moved from demonstrator in Norway to a commercial 30MW project in Scotland; the TLP platform is a buoyant platform that is anchored to the seabed using taught tendons that provide hydrodynamic stability. The most well-known barge system is the Floatgen platform from French firm Ideol.

A particularly critical part of the technology puzzle for floating offshore wind is the need to convey power back to the mainland. Typically for fixed offshore wind inter-array cables are 33 or 66kV whereas export cables are 220 of 230KV. To date the Hywind and WindFloat projects have used the 66 kilovolt dynamic cables for export, as well as for linking the platforms together. Larger wind farms won’t do this. “So one of the challenges for going towards commercial scale floating offshore wind, so say 500 megawatts plus will be a dynamic cable capable of transmitting up to say 230 kilovolts, which is what we’ve been supporting with our dynamic export cable competition,” says Sam. “So we’ve supported the development of a core for export voltage cables from five cable manufacturers around the world.”

The main difference between a static cable and a dynamic cable is the shielding that surrounds the transmission cables. Static cables traditionally use lead but for dynamic cables companies are investigating materials with better performance characteristics such as aluminium, steel or copper.

Other important areas of innovation that the Carbon Trust is supporting concern the operation and maintenance activities of new floating offshore wind farms. This could be vessels with cranes capable of reaching 120m hub heights, or new software that can better monitor performance and the regime for this will be very different to oil and gas where there tends to be single units. “With floating offshore wind, you’d have up to say 50 units with a revenue generation potential. So any technical intervention that can reduce the requirement, automate, that that process will go a long way to reducing the cost of floating offshore wind.”

Seeking innovation

What the industry needs is technical innovations that can help bring down costs.

“What we found in oil and gas is that the mooring lines that fix these floating structures to the seabed do encounter problems,” says Stuart Killbourn who works in the structural instrumentation division of Fugro. “And there have been cases where these mooring lines have failed – they’ve basically snapped. And that that’s a big concern.”

Traditionally in the oil and gas industry mooring lines would be periodically inspected using remotely operated vehicles.  But using ROVs could become uneconomic when it comes to the large arrays of floating wind turbines planned for the future. “When we move to a wind farm, a floating wind farm, each structure might have three mooring lines you know in each direction, it’s like a three-legged stool. But there could easily be 100 floating wind turbines so that there are going to be 300 mooring lines.”

So Stuart, along with colleagues and a team from the University of Strathclyde, are investigating a new approach, thanks to funding from the Carbon Trust. They measure the position of the floating structure in different wind conditions and how it bobs up and down in the water, in response to the wave conditions to a computer simulation model of that floating structure and the mooring lines. “We can take all that information. And we can then calculate what the tension is in each of the mooring line, and how different waves give you cycles of tension so pulling it taut and letting it slack.”

And it is the response of the mooring lines to the cyclical loading that is really important.

“We can then begin to predict how long it would take for these cycles to produce what’s called a fatigue crack, it’s a microscopic crack in the material. But over time, and over repetitive cycles, it would then begin to grow and they would link together until they form an actual, what you might call a visible crack that would then propagate and grow.”

Of course the power of this technology is that it would mean that there wouldn’t be any cracks because they would be predicted and prevented. So sensors and software will tell the windfarm operator not only what the loading conditions are at any time but what the effect of those will be on the mooring lines, and specifically where and when any fatigue cracking could occur. They also have the advantage of providing real time information on condition via the fibre optic cables that run back to shore along with the export cables. Stuart’s vision is that all floating wind farms would use this technology. But how many floating wind farms are there going to be? Boris Johnson has set out 1GW for the UK by 2030, what about the rest of the world? Alastair Dutton from GWEC says that there are a range of estimates. “GWEC’s outlooks takes us to six and a half gigawatts by 2030. I have seen a developer projection that’s double that. And I’ve seen some more conservative figures,” he says.

Opening deeper waters

One of the reasons that the industry is so excited about floating offshore wind is the vast amount of territory that floating platforms opens up. To date fixed offshore wind has been built in waters of 40m depth or shallower because of the constraints of construction such as the need to drill into the seabed. Floating wind opens deeper waters all over the world from Northern France to the West Coast of the US and Japan. But Great Britain is the first to set a hard target.

“In terms of water depths floating offshore wind is definitely most compatible with the Celtic sea and some of the Scottish waters. If you look at the Scottish marine plan, there are certain zones that on water depths alone are only compatible with floating offshore wind,” says Julia Roope Global Offshore Wind Business Development Manager at Fugro , and co-chair of the newly formed Floating Offshore Wind Group for industry organisation Deep Wind. Deep Wind was formed as part of the delivery programme for the UK’s offshore wind sector deal announced in March of 2019. “We’ve got the political backing, the technology and the experience to make floating offshore wind succeed. And it’s an exciting time for the industry.”

But that doesn’t mean there aren’t challenges. And the biggest –is getting to the scale of delivery that brings down costs and meets the need for cleaner, greener power. Just as floating wind is ramping up, so are the fixed arrays meaning that the entire offshore wind industry is growing fast. “Some of the biggest challenges that could come in the supply chain are linked to the assembly side of things and the marshalling ports. So you need quite deep water, depending on the types of substructure that you’re going to be using. And we have very limited capacity in terms of deep-water ports in the UK. And they may be competing for space with the fixed foundation turbine structures as well,” says Julia.

This is one of the reasons that when Boris Johnson made his offshore wind announcements in October he committed £160m to portside development specifically targeted at Scotland, Teesside, Humberside and Wales. But what we need is a plan. “As a whole there needs to be a strategic and coordinated plan of how we develop our port infrastructure to accommodate the needs of the offshore wind industry to make sure that we use our ports and play to their advantages and maximise what they can do to support the offshore wind industry,” says Julia.

We also need to learn from experience in offshore oil and gas, where deep water operations have been carried out for decades. The good news is that technology transfer is happening naturally as we transition to low carbon forms of power generation, from the semi-submersible rigs becoming floating wind platforms to positioning technology and updating instrumentation for improved monitoring and maintenance. “It’s important the floating wind industry doesn’t look to then reinvent the wheel of what’s already been around over 30 to 40 years from the oil and gas industry,” says Fugro’s Alistair McKie. “There are a huge amount of lessons learned. And that can go in to ensure that floating wind continues at the pace that it’s required to furnish the energy transition, but do it in a sustainable and sensible manner.”

Understanding that the technology exists and is adaptable is one of the reasons that developers are getting excited about floating offshore wind. Early projects are delivering fantastic results and perhaps most importantly political support is growing. These factors are changing the game for the industry. 

Looking to the future one of the most exciting things about all of this is that floating offshore wind could also create opportunities for another avenue for clean energy.

“My vision is that we’ll move to places where the wind is really spectacular. 14 to 15 metres a second, generate the hydrogen there and then frequently we’ll be using floating foundations and take the hydrogen to market on ships like LNG ships. So this market will turn upside down. Instead of finding places with power demand will be going to just the very best wind in the world,” says Alastair Dutton.

Carbon Trust, Floating offshore wind, Equinor, Fugro, Global Wind Energy Council, GEWC, Hywind, offshore wind, Principle Power, renewable energy, turbines, wind energy, wind power, WindFloat, WindFloat Atlantic, World Bank, Vestas