What does it take to make a 178-metre-high tower with a 480-tonne generator on top, float? A whole lot of offshore engineering and software wizardry, in fact.
Meet the Equinor expert who cracked the code for keeping Hywind stable and upright — and who can rightly be called the brains behind its brain: professor in cybernetics, Bjørn Skaare.
And we ask: what next for floating offshore wind?
Equinor’s offshore wind turbine design — Hywind — has become synonymous with floating wind power, and after more than 20 years of testing, it has proved that its vertical floating cylinder design — or spar — is viable.
Hywind has proven itself through extensive real-world testing in the harshest of conditions, in multiple locations. Recently, Hywind Tampen’s performance figures were announced, showing a world-leading capacity factor. And before it, Hywind Scotland and the Hywind Demo paved the way.
But how did we turn that drawing on a paper napkin into a real world prototype, and how did we solve the software engineering challenges? Read on to find out.
Back in 2001, two of our engineers had a ‘eureka’ moment — not in the bath, like Archimedes, but in a boat. Dag Christensen and Knut Solberg were the sailors and colleagues who ‘imagineered’ a floating spar buoy concept on a paper napkin.
Photo: Øyvind Hagen
Three years after Dag Christensen and Knut Solberg’s brainwave in a boat, Equinor researchers Finn Gunnar Nielsen, Tor David Hanson and Dagfinn Sveen in Bergen had started work on turning their concept into a working prototype.
Now Finn Gunnar and his team were facing how to solve the monumental mathematical modelling tasks for the floating spar design.
It was very challenging. We had to think how the floating structure would behave dynamically under load from waves, winds and currents. It might look simple, but it’s a simple structure with complicated dynamics.
They knew they’d need new software, because the turbine would be ‘dynamically unstable’ with a conventional turbine controller. That’s engineer-speak for saying it would roll and sway uncontrollably, putting huge strains on the towers and anchoring systems.
The trouble was no-one had ever attempted to model the kinds of dynamics you get with a slender floating cylinder with a heavy wind turbine on top.
What the team needed was a computer whizzkid and expert in marine dynamics. People like that don’t grow on trees, but Finn Gunnar knew just who to ask.
Fresh out of university with a doctorate in marine cybernetics under his belt, Bjørn Skaare was 30 at the time and was expecting to begin his career in oil and gas. Instead, he was snapped up by the Hywind team. Although surprised, he rose to the challenge.
Bjørn’s modest manner belies a razor-sharp mind (“a real wolf in sheep’s clothing, one of the brightest minds in the company,” one of his colleagues confided).
I’d like to thank Equinor for entrusting me with that responsibility early in my career. Finn Gunnar and Tor David were immensely experienced, and willing to share their knowledge. I was given a lot of responsibility in a small team, so I wanted to do my best.
A regular land-based wind turbine contains a software controller to regulate its performance, but a floating, swaying tower adds to the complexity by several orders of magnitude.
Finn Gunnar and his team knew that conventional wind turbine software wasn’t up to the job for a floater. That’s because when a floating turbine heeled backwards, a conventional controller would interpret this as a drop in the wind, pitching the blades to catch more wind, amplifying the backward tilt (increasing the pitching) — making the problem worse.
And conversely, when the turbine tower heeled forward again, a standard controller would interpret this as a surge in the wind, pitching the blades to catch less wind, thereby exaggerating the forward movement.
The result would be a turbine tower rocking like a pendulum, tearing itself apart with material stresses and requiring massive anchors to tether it to the seabed.
Bjørn Skaare was given the task of inventing, developing and programming a new motion controller to solve this stability problem. He didn’t have much to play with. With nothing else than pitch-controllable turbine blades and his own custom code, he needed to ensure that the turbine could work dynamically to keep itself stable and upright in strong winds and giant waves. In other words: Hywind’s ‘brain’ would need to be super-smart compared with other turbines. Bjørn set to work.
Because there were no analytical tools for such a task, Bjørn first had to develop a simulator so that they could run so-called fully coupled analyses of floating structures.
He then had to consider dozens of parameters – including thrust forces on the blades, pitching and rolling movements, bending moments and material fatigue – before creating the code that went on to be called the Hywind Motion Controller: a software ‘black box’ that sits on top of the controller from the turbine supplier, adding the algorithms needed for floating operation.
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Bjørn had to consider dozens of parameters – including thrust forces on the blades, pitching and rolling movements, bending moments and material fatigue – before creating the code that went on to be called the Hywind Motion Controller: a software ‘black box’ that sits on top of the controller from the turbine supplier, adding the algorithms needed for floating operation.
After a series of successful experiments in a test tank in Trondheim in November 2005, the team went on to commission a full-scale prototype from Finland. It was an exciting day for Bjørn when the Hywind Demo arrived in Norway.
I was full of anticipation! We’d carried out model experiments and I had confidence in our simulation tools, but it wasn’t until I came to the quayside and saw the rotor lying there that it really struck me how big this thing was.
“We hoped it would work, but you don’t get the final answer until it’s actually working with a real wind turbine on top.” The Hywind Motion Controller worked perfectly from the outset.
“I felt a tremendous sense of relief when it worked as we had hoped,” says Bjørn. “And our simulations correlated well with the measurements from the real turbine,” he adds.
The Hywind Demo captivated people’s imaginations with its gravity-defying design. It was simple, elegant, and it borrowed the floating spar concept from Equinor’s industrial roots: offshore oil and gas. (And Hywind Demo is still running today, by the way.)
But the Equinor engineers weren’t satisfied yet. There were more efficiency and cost gains to be made from further refinements.
The team turned their attention to Hywind Scotland – the world’s first floating wind farm that would become a test facility off the coast of Peterhead, with five turbines. Bjørn, Finn Gunnar and the team wanted to develop the controller further, to solve another issue they’d identified.
They had the pitching and rolling of the turbine tower under control, but they were keen to resolve yaw — the rotational movements around the tower’s vertical axis that would put great strains on the moorings. If the controller could mitigate yaw as well, they would be able to reduce the size – and thereby the cost – of the anchors, anchor lines and bridles.
Finn Gunnar worked on the basic physical principles, while Bjørn translated this into a real-life solution in software.
Bjørn now hatched up the Active Yaw Individual Pitch Control (AYIC) system to control the yaw motion. He used a complex 3D model of the wind forces acting on a turbine rotor across its entire sweep area, adjusting each blade’s pitch individually in real time based on the input from dozens of sensors across the entire turbine.
He walks us through the maths, using a diagram on his computer. If you thought this story was complicated already, buckle up now.
“If you give a turbine blade a positive pitch, you reduce the thrust forces on that blade,” says Bjørn. So, if the turbine tower rotates around its own vertical axis, call this angle theta, θ, you can create a positive restoring moment in one part of the sweep, and a negative pitch in the other part, and if you do this cyclically for all the blades, the controller can create a restoring moment to correct the yaw angle,” he says, clearly assuming I’m still with him. My mind boggles, but he goes on.
“Then we measure the yaw angle and give signals to each blade individually. We ran simulations where we limited the yaw angle to three degrees using the controller and saw a significant reduction in the tension and forces in the mooring lines,” he says.
After successful simulations, the team incorporated the new algorithms into an updated motion controller and installed it on four of the five Hywind Scotland turbines, keeping the fifth turbine as a control reference unit. The yaw control wizardry also performed as expected.
“The motion controller, the brain that controls the blade pitching to keep the floater stable, has been one of the main innovations behind Hywind,” says Finn Gunnar Nielsen today. Bjørn Skaare has been the key person here. It’s the most important single element we have developed, and it has been an incredibly exciting development journey,” he says.
Project director for Hywind Demo and Hywind Scotland, Leif Delp, agrees: “The controller was one of the main innovations in the entire Hywind project,” he says.
“Throughout our Hywind concept development, we’ve been using our floating motion controller. We’ve been front runners all the way, and not least after Finn Gunnar brought Bjørn Skaare in, in 2005. Without his controller, it wouldn’t have worked as well as it does, and we wouldn’t be seeing the success we’re seeing today,” he says.
Furthermore, the motion controller solution is applicable to any kind of floating wind platform design and not tied purely to the Hywind concept, for instance, in currently ongoing early-phase studies for steel semi-submersible floating turbines.
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Hywind Tampen is the world’s largest floating wind farm and the first built specifically to power offshore oil and gas installations, and is now supplying electricity to Equinor’s oil and gas fields Snorre and Gullfaks in the Norwegian North Sea.
We announced world-leading results for Hywind Tampen offshore wind farm recently, with a capacity factor of 58.4% and production-based availability of 95.2%, and production of 115.7 GWh in the first quarter of 2025 alone.
But the question on everyone’s lips these days is: what next for floating offshore wind?
In a challenging economic climate, offshore wind developers have become more cautious about committing to major projects. In Equinor, the focus is on making floating offshore wind economically viable:
New technology always comes at a premium. Testing a prototype is expensive, but industrial development eventually brings the costs down. That’s something we know well from oil and gas.
“For example, we see that for the Hywind spar concept applied for the North Sea area, concrete has a lot of advantages,” he explains. “Not only is it cheaper than a steel spar solution used for Hywind Demo and Hywind Scotland, but we can free ourselves from the constraints of yard capacity, we reduce transportation, we eliminate the need to upend the spar substructures and fill them with solid ballast, and we can use local suppliers and a local workforce.”
“Until now we have worked on technology development and de-risking of project execution, and we have qualified the solution. But we don’t have profitable industry, and to make it profitable, we will have to increase the size of the turbines and build larger windfarms,” he says. The key to cost savings is scale.
“The technology scales very well, and now we’re talking about next-generation turbines that could be 15, 20, or 25 MW, with wingspans up to 280 metres, and nacelles that weigh 1000 tonnes or more.”
He’s one of the early pioneers in the Hywind team and has been involved in its development since 2006. He’s convinced that floating wind will be competitive in the longer term.
“14 of the roughly 20 areas around the Norwegian coast that have been earmarked for offshore wind, will have to be floating wind, or a combination of floating and bottom fixed. We could mass-produce standardised floating turbines in concrete for all these projects.”
Floating wind turbines can open up new markets like these, enabling us to go into more windy areas, and deeper waters,” says Leif Delp, with conviction.
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