Questions & Answers

How, what, why?

Here are the most frequent questions we get at SeaTwirl. Don’t hesitate to contact us with what you´re wondering! Send an email to


What is the biggest challenge of scaling up from S1 to S2x?
When scaling up the wind turbine you also increase the forces on the structure. The dynamics of a significantly larger structure is more challenging than for a smaller. It does not scale linearly with size. Remember that S2x is approximately 30 times bigger than S1. We are mitigating this through performing thorough in-house analyses. S2x is also being designed to become a commercial product with all challenges that follow being compliant with rules and regulations to a higher extent than for a prototype as S1.

How high waves can S2x handle?
The advantage of a spar solution is that it is relatively insensitive to waves, horizontal motions are limited as the wave “passes” by the slender spar. The wind turbine is being designed site specific, the first unit will be withstanding the actual wave height at site in Norway, but newer WTs will be able to withstand higher waves.

What are the dimensions/diameter of a 10MW rotor? 
A 10MW VAWT wind turbine would have a diameter of 135m and a blade height of 110m (approximately).

How large is the clearance of the sea surface? Can it meet requirements of 25-30 meters clearance to the sea, also with the max angle? 
The air gap between blades and water surface is appr. 15 meters at calm sea with 0 deg heel.

Has the capacity factor been proved with S1? Or is the capacity factor number taken from HAWT’s?
The capacity factors we use are calculated for each specific design and site, based on our VAWT technology.

Why can VAWT reach 30MW when HAWT can’t? What is the physical rational behind that?
Reference is given to the following documents: – F. Ottermo, H. Bernhoff. An upper size of vertical axis wind turbines DOI: 10.1002/we.1655 (2013) – S. Eriksson, H. Bernhoff, M. Leijon. Evaluation of different turbine concepts for wind power DOI:10.1016/j.rser.2006.05.017 (2006)

What are the losses of spar buoy rotating against the water?
This depends on spar buoy diameter, surface area and rpm. Weighing this together with our large advantages we expect our LCOE (Levelized Cost of Energy) to be very competitive.

What are the most challenging structural problems?
Since we have a rotating structure and we want it to last over a long lifetime, fatigue becomes an issue to address. It is important to work with materials with high stiffness and low weight. At the same time, we ensure we are well below the fatigue limit by performing extensive simulations and analysis. One advantage is that during the rotation of the rotor, a VAWT does not have additional fatigue load due to gravity, the gravity load is constant during operation.

What is the nominal healing angle at rated wind speed?
Maximum healing angle at operation is about 5 degrees and at storm condition 10 degrees.

What protects the turbine during storm conditions? And how do you handle overwind shutdown?
If the experienced or forecasted environmental conditions exceeds the operational limits the control system of the wind turbine will perform a controlled shutdown of the turbine utilizing the mechanical brake if needed. Once the turbine has come to a complete stop a mechanical rotor lock is engaged which will prevent the turbine from rotating.

What’s the reason for three blades on a VAWT?
Three blades is a compromise between reducing fatigue loading and keeping the overall weight of the rotor low. Two blades increase torque variation, four blades mean additional weight. HAWT’s face the same challenge.

What happens if mooring cables break, if you have a cluster of turbines?
Mooring design is performed according to existing DNV GL rules and regulations. To have “backup” lines for the mooring is basically a risk/economy evaluation. This evaluation will depend on each park configuration and environmental conditions.

Will sea movement be used as source of power?
No, we are only utilizing the wind.

How do you intend to install it, given that deep harbors are few and far between?
We haven’t disclosed our installation method yet, but we are not planning on using deep water harbors for installation.

What is the capacity factor of S1?
The S1 turbine has a rated power of 30 kW. Operating all the time, that is equivalent to 10-13 normal households (5000 kWh/year/household).

What is larger? Does it mean larger in all dimensions or could it mean wider but not necessarily higher or could it mean a greater number of blades with a narrower spacing or different in any other fashion?
Length, area and volume scale differently, so depending on what dimensions are scaled, scaling is made differently.

What will be the height of S2x? 
The overall height of the S2x is appr. 55 meters from the still water line.

How big will the “risk zone” be around the turbine cluster?  
The risk zone may vary depending on the requirements and legislation governing each specific installation site.

What is the estimated lifetime for a S2x turbine? How long time do you expect it to be working offshore? How often do you need to perform service and which parts are expected to be replaced most often (and how often is that)? 
The intended testing period of the S2x pilot is 5 years, based on the permit we are given at the site. A commercial unit should have an expected lifetime of 25-30 years. We are not intending on replacing any significant components.

How does the risk analysis look like regarding ships crashing into a turbine cluster? 
 A HAZID will be performed for each site prior to installation.

In your simulations, how do you take sea state into account? 
Sea state is included in many of the simulations we perform of the S2x.

How you do handle marine growth on the spar buoy? 
Marine growth is something that we are taking into consideration. However, we are not ready to disclose how yet.

Having the gearbox close to the water surface may not be positive in all aspects since it also ends up in a more corrosive environment. Even if you think a structure is safe regarding corrosion that is not always the case. What are your thoughts on this? 
All sensitive components are located within the generator house and hence protected from the surrounding environmental conditions. Humidity and temperature will also be regulated inside the generator house which further reduces the risk of corrosion.


When will you have the 1MW prototype in the water? And when will we see the 10MW unit spin?
The S2x 1MW pilot will be installed in 2023. No firm date is set for a 10 MW or other size unit yet.

Do you see physical damage and business interruption scenarios being reduced or increased versus other floating turbine designs?
Both being reduced due to simpler design and fever moving parts.

What is the major difference in ecological impact between S1, S2x and your competitor’s solution? 
Compared to a bottom-fixed structure we have less impact on the seabed. S1 and S2x use traditional anchors, 3 off for each turbine. There are indications that VAWT’s are more friendly for seabirds due to that our blades rotate with the same speed throughout the complete blade, however this remains to be verified.

When do you see the commercialization of the S2x? And the upscaling? 
The commercialization of the S2x has already begun and something we work with continuously. Of course we will learn a lot during our first year of operating the S2x. The S2x size of floating wind turbines fits really well into our customer category we call the Local grid. Remote islands, industrial applications etc. We are already looking at scaling up our design and the intention is to intensify this work in the seconds part of 2021. Scalability had been a key component from the start.

What are the expected/planned sizes of the units in terms of MW? 
We are looking into larger sizes, and evaluating a SX in the range of 6-10 MW.

Do you collaborate with any wave power companies that can supplement your products at sea today or can your product also generate electricity via wave power? The same applies to solar cell. 
We see other renewable energy sources, such as solar and wave power as possible partners in future energy mix applications. Our design uses wind as the sole source of energy.

Where do you see your first market being? 
We have several interesting markets on our radar. The first market will probably be a combination of where we get advantageous project conditions, a suitable location, some sort of project financing and incentive plans for renewable energy.

What are the plans of recycling hardware? 
Steel parts recycled as steel. Carbon fiber components, as we are using, are commercially recyclable today, in contrast to glass fiber blades which today are being used as landfill at end-of-life.

Many of your competitors are in one way or another backed by deep pockets. Competitors are also already on their way to scaling up to handle very large turbines. How will you be able to compete given that your larger designs are estimated to reach market at the earliest 2025? 
We are designing something unique, that no-one has done before. Our competitors are at different readiness levels and we are monitoring their ventures of course. However, we are focusing on our task, which is to design the most competitive and scalable solution for the future of floating offshore wind. We will be able to offer a game-changing solution for clean energy at a competitive cost level (LCOE, Levelized Cost of Energy).

How far off the coast is it profitable to install the turbines?
This is completely dependent on the size of the turbines, the environmental conditions in the specific site and of the configuration of the offshore wind farm. As costs come down over time, we will be able to reach sites further away from the coastline. Today, anything less than 60 km offshore is considered to be “near-shore” following IEA (International Energy Agency).

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