Hywind Scotland - Øyvind Gravås

How a perfect storm of converging new technologies plus policy and economic incentives could unleash the full potential of floating wind. By Benjamin Cerfontaine and Susan Gourvenec.

Harvesting wind energy to generate electricity has become one of the pillars of the transition towards a green and carbon neutral economy.

This article was originally published in Power Engineering International Issue 5-2020. Read the mobile-friendly digimag or subscribe to receive a print copy.

While onshore wind farms represent 89% of the total installed wind capacity in Europe, the cumulative installed capacity of offshore wind – 22GW, 11% of total in 2019 – has increased by an average 30% yearly over the last decade.

To date, most wind turbines were built in relatively shallow waters of under 60m depth, where they are supported by bottom-fixed foundations, such as monopiles (large steel pipes up to 10m in diameter).

Many of the most accessible shallow water locations have already been populated by wind turbines, but the offshore wind sector still has an enormous growth potential.

However, 80% of the European wind potential is located in water depths deeper than 60m.

The cost of fixing offshore wind turbines in position in the ocean will increase non-linearly with water depth, as the dimensions of foundation, the amount of steel and the size of the lifting vessels will all increase.

Therefore, continuous upscaling of the current design is not sustainable to unlock the remaining offshore wind potential and innovative solutions must be brought to the market.

Floating wind turbines are composed of the turbine and mast, a floating platform – such as a semi-submersible or spar buoy – and a mooring system of lines plus anchors to maintain the system in position.

By freeing the requirement of a stiff connection between the turbine and the seabed, floating turbines can theoretically be installed at the windiest locations without any water depth constraint.

One of the main drivers for installing turbines further offshore is to harvest stronger and more reliable wind. For instance, the average capacity factor (the ratio of energy output to the maximum potential energy output) of onshore wind is approximately 24% and the average for bottom-fixed offshore wind is 38%, up to a maximum of 48%.

Hywind Scotland, the first commercially installed floating wind farm achieved 54%. As a corollary effect, the displacement of wind turbines further from shore reduces the risk of visual impact that could put off coastal communities.

Image: Øyvind Gravås

Floating turbines will be built nearshore or in dry docks and towed to their final location. This will decrease the need for heavy lifting and underwater operations, limited to the mooring system only, decreasing the cost and risk of the turbine installation. Similarly, decommissioning operations will be less costly and risky, limited to the reverse procedure.

Additionally, at the end of operational life, various anchor systems can be removed, for example, suction caissons and drag embedment anchors, while the widespread monopiles of fixed-bottom turbines are cut and left in place in the seabed.

Challenges and hurdles

Demonstration projects have proven that floating turbines are technically feasible, but the cost of this technology remains the main hurdle to be cleared before a large scale deployment is possible.

While the cost of bottom-fixed offshore wind has strongly decreased over the past decade to reach a competitive level ($48-60 per MWh), the cost of floating offshore wind is twice this amount.

Offshore operations are always costly, from site investigation to installation and operation/maintenance. Installing turbines further offshore and in deeper waters will increase the cost due to the remoteness of the farm, inaccessibility of the seabed and harshness of the environment.

Challenges of connecting the wind farm to the electric network will also grow as a function of the distance to shore.

Floating platforms are not new in the oil and gas industry. However, wind turbines are lighter and taller than those platforms, inducing different loading conditions and responses. In addition, the capital investment for oil or gas rigs is amortised over a very large energy output.

On the contrary, each wind turbine will produce a much smaller amount of energy output, requiring in turn more cost-effective engineering solutions, across site survey, fabrication, mooring systems, operation and maintenance. It is not enough to optimise existing solutions, new procedures and technologies must be brought to practice.

Europe at the forefront

Europe has already the largest installed offshore wind capacity – 22GW out of a world total of around 28GW – although offshore wind only represents 2.3% of the total energy production (0.3% worldwide).

China has the fastest growing rate of its offshore wind capacity, but Europe is now leading the development of floating wind turbines, with several world firsts and ongoing projects.

Hywind Scotland was the first commercial floating wind farm in the world, installed 25km from the Scottish shores in 130m water depth. The five 6MW turbines (154m in diameter) were built on a spar platform.

Three massive suction anchors of 16m length, 5m diameter and weighing 300 tonnes were used to anchor each turbine, demonstrating their applicability in sand. It currently powers 20,000 UK homes.

The Kincardine floating offshore windfarm is currently the largest project under construction, off the coast of Scotland, in 60m-80m water depths. The 50MW project will use some 9.5MW turbines mounted on the semi-submersible Windfloat platforms, making use of an active ballasting solution to maintain the stability of the platform.

Further projects are in development, such as the 100MW Atlantis pre-commercial farm based on the Ideol semi-submersible damping pool platform. Hywind Tampen will be one of the deepest (260-300m water depths) offshore wind farms, with construction set to start in 2022.

It will be based on the same platform as Hywind Scotland, demonstrating the capacity of floating wind turbines to be free of water depth constraints. Shared anchors will be used for the first time to reduce the foundation costs.

Ultimately, the cost of floating wind energy will decrease due to a reduction of uncertainties, including demonstrated survivability of concepts during a storm, and economy of scale such as standardisation of fabrication.

But to decrease enough will require development of new practices based on the most recent research.

What’s next?

The digital revolution is upon us. Robots and autonomous systems can be used to control offshore floating wind farms, overcoming the risks of sending staff to a remote and harsh environment and enabling optimal operation of a complex system.

Seabed investigation, inspection and maintenance of the deployed structures could be undertaken in a more costeffective, yet thorough and reliable, way.

Similarly, the development of smart sensors embedded in all parts of the floating wind farms offer a fast and comprehensive insight into the state of a wind farm, or detect damage after a storm. Besides, it would be impractical to undertake in-person inspections on the very large wind farms under development, and those proposed and needed for the future.

Smart sensors and autonomous robots will generate a huge amount of data.

Machine learning has become a game changer in the analysis of big data sets and can already control the wind turbine blade position to maximise the energy production or avoid damage during a storm, based on weather data.

Algorithms could be trained to detect damage before it becomes critical, triggering maintenance or repair. Design algorithms could be developed to optimise anchor design or interpret site investigation data, combining different sources of information, ultimately reducing the necessary amount of data and increasing the reliability of predictions.

The potential of floating wind energy has been recognised in Europe and worldwide.

While the technical feasibility of some technologies has already been proven, unleashing the full potential of floating wind now requires the convergence of new technologies, adequate policies and economic incentives.

ABOUT THE AUTHORS

Benjamin Cerfontaine obtained his PhD at the University of Liège in 2014. He was awarded an MSCA fellowship at the University of Dundee, then became a lecturer at the University of Southampton in 2020. He is specialised in physical and numerical modelling of innovative foundations for offshore renewable energy devices.

Susan Gourvenec is Royal Academy of Engineering Chair in Emerging Technologies in Intelligent & Resilient Ocean Engineering and Deputy Director of the Southampton Marine & Maritime Institute. She is a specialist in offshore geotechnical engineering.

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