Financial jury still out over Floating v. Fixed offshore wind farms – despite subsidised Hywind Scotland launch

A Hywind Scotland wind turbine being assembled prior to towing to its anchorage site off Peterhead.
A Hywind Scotland wind turbine being assembled prior to towing to its anchorage site off Peterhead.

The world’s first floating windfarm – anchored off the coast of Peterhead – began generating electricity last week.

These structures could have a number of advantages over conventional offshore windfarms, but with the technology still in its infancy and the costs relatively high, the question remains when, or if, they will be able to compete against fixed offshore wind turbines or other sources of energy. 

This Carbon Trust analysis a look at the status of floating windfarms and their potential to provide renewable energy in the future.

Norway’s Statoil, which developed the pilot Hywind windfarm, says the project aims to demonstrate the feasibility of future commercial, utility-scale floating windfarms.

Floating windfarms could have a number of advantages over conventional offshore windfarms. They can be placed in deeper waters where the wind is stronger and less variable. The turbines can be fully assembled close to shore before being towed out to sea and they could have less impact on wildlife than other types of wind turbine.

How do floating turbines work?

Just as oil and gas started with easier-to-access onshore developments, before moving to near offshore and, finally, to deeper locations, floating turbines represent a next phase in the location of wind turbines.

A floating wind turbine mounts a standard offshore model on a floating structure, rather than the fixed-bottom towers typically used for conventional offshore schemes. This allows floating windfarms to be located in depths far greater than the 50m-or-so limit of conventional offshore wind.

There are three dominant designs for floating wind structures: spar, tension leg platform (TLP), and semi-submersible. Each of these turbines gain their stability in different ways.

Spar-buoys rely on having the centre of gravity lower in the water than the centre of buoyancy. This is done by adding a heavy weight to the lower part of the structure.

Tension leg platforms – currently, the least advanced design – are anchored to the seabed and stabilised using tensioned mooring lines.

Semi-submersible platforms meanwhile are stabilised by buoyancy. They have three to five cylindrical platforms connected by tubes and float half submerged on the surface of the ocean whilst anchored to the seabed with mooring lines.

Rhodri James, a manager in Carbon Trust’s policy and innovation team, said these designs have been adapted from oil and gas technologies.

Hywind floating turbine at Buchan Deep, off Peterhead
Hywind floating turbine at Buchan Deep, off Peterhead

“The key difference is that, whereas oil and gas consists of a small number of very large platforms, floating offshore wind requires a larger number of smaller platforms,” he said.

(It’s worth noting the relative use of the word “small” here. The structures used for floating turbines still reach up to around 3,000 tonnes for steel designs and roughly 12,000 tonnes for concrete designs.)

The wind turbines used for floating windfarms are nearly identical to fixed-bottom structures.

What are the advantages of floating windfarms?

The promise of floating offshore wind turbines lies in their ability to be tethered in deep waters of 50-1,000 metres. These are not suitable for conventional fixed-turbine offshore wind structures, which are constrained to water depths of less than 50 or 60 metres because of the need to embed foundations in the sea floor.

Therefore, they open up areas of sea not previously suitable for offshore wind power, including areas where the continental shelf drops off too fast for fixed turbines to be viable, such as off the US west coast, Japan and, in this case, Aberdeen.

Floating turbines could become increasingly important as the world exhausts the “low hanging fruit” of shallow near-shore sites and moves outwards from the coast.

The deeper waters where floating wind turbines can be located also have higher average wind speeds than closer to shore. This could lead to floating windfarms producing more electricity per gigawatt (GW) of installed capacity, increasing revenues. Higher capacity factors could also benefit onshore transmission networks by reducing variability.

They can also be almost completely put together close to shore before being towed out to their destination by simple, low-cost ships. This avoids the need to use expensive, heavy vessels to construct the wind turbines out at sea on top of permanent foundations.

The five turbines of the Scottish Hywind project, for instance, were assembled in Norway and towed to the Scottish coast off Peterhead. This mobility could also be an advantage for heavy maintenance operations, where the turbines could be towed back to port.

Finally, their location further out to sea could offer a remedy for some public opposition to windfarms. Visually they would be even less prominent, while, according to RSPB, they could also be less harmful to birds than farms placed closer to the coast and, thus, seabird nesting sites.

In addition, they avoid the need to pile-drive a large foundation into the seabed, as well as lengthy offshore construction activity, both of which can be temporarily disruptive to sea life, including whales and birds.

The Buchan Deep offshore Hywind floating wind farm location
The Buchan Deep offshore Hywind floating wind farm location

Are there any disadvantages?

Aside from the cost due to being a nascent technology, there are several technical challenges floating windfarms are faced with.

Several bespoke elements will need to be developed before they are used at large scale. These include dynamic electrical cables, mooring and anchoring systems, and floating substations.

According to James, while these are not necessarily going to be “showstoppers”, they will still require further research and development before large scale deployment can be achieved.


How much will they cost?

As a young technology not yet commercially deployed, the costs of floating wind currently remain high. Early projects will need to be subsidised, meaning the rate of progression will depend to a large extent on political support.

The Hywind pilot is currently heavily subsidised by the Scottish government.

If this support is given, costs are expected by several experts to fall in line with conventional offshore wind, as designs are optimised and the technology is deployed at scale.

A 2015 Energy Technologies Institute (ETI) report found that large-scale floating windfarms could deliver a levelised cost of electricity of around £85 per megawatt hour (MWh) by the mid 2020s.

The Carbon Trust study similarly concluded leading floating wind concepts could reach a levelised cost of electricity of £85-95/MWh in large-scale commercial projects in the 2020s, with further innovation potentially seeing costs fall still further.

For comparison, the Hinkley Point strike price was set at £92.50 MWh in 2012 prices, a figure that is index linked so will rise with inflation over its 35-year contract.

Statoil aims to reduce the costs of energy from the Hywind floating wind farm to €40-€60 MWh by 2030.

James said that floating wind could be competitive with other energy technologies within the next decade if it is deployed at scale, particularly in markets with high energy demand in coastal areas with deep continental shelves.

Comparing conventional and floating windfarms, while the cost of the substructures for floating turbines – the spar buoy, for example – will likely remain higher than for fixed foundations, this may be negated to some extent by the advantages of floating windfarms listed above.

The IEA has said the cost of floating turbines today is the same as fixed-bottom ones a decade ago.

Any attempts to estimate the costs of future commercial projects will inevitably contain a great deal of uncertainty, since the technology is still in its infancy with few demonstrations actually in the water. 

The Hywind Scotland project is expected to deliver 135 gigawatt-hours (GWh) of electric power per year. 

How does Hywind work?

Statoil, the developers of the 30 megawatt (MW) Hywind pilot project claim it is the world’s first commercial floating windfarm.

The £190 million pilot follows six years of testing by Statoil of a 2.3MW Hywind prototype installed off the island of Karmøy in Norway in 2009.  During this time, Statoil’s design optimisation allowed it to tripled the power output of the turbine.

Statoil says the project aims to show cost efficiency and feasibility of multiple floating wind turbines in a region with optimal wind conditions, with its end goal being large scale floating offshore wind parks of 500-1,000MW.

James said: “Hywind Scotland is a major step forward to the industry as it will demonstrate the technology in an array layout for the first time (as opposed to the single prototype demonstrations to date). This will deliver important learnings in the construction and installation of multiple units and the interactions between turbines in a single windfarm.”

Hywind is subsidised under the UK’s Renewable Obligation Certificate (ROC) scheme, through which it will receive £160/MWh on top of the wholesale price of electricity

Statoil claims the costs of projects such as Hywind can be reduced by 40-50% by 2030. This could bring the cost down to around £100/MWh.


How many floating offshore projects are there?

According to the Carbon Trust, there are over 30 concepts currently under development, although only five have been demonstrated at full scale in an offshore environment.

US firm Principle Power installed a prototype of its WindFloat semi-submersible design in Portugal in 2011 and plans to mount pilots in Portugal in 2019 and France in 2020-21. 

SBM Offshore also aims to install a pilot farm of its tension-leg platform design in France in 2020-21.

Will we see floating wind turbines all over the sea?

While a nascent technology, floating offshore windfarms could open up areas of deeper sea not previously suitable for wind energy and show promise of some advantages over conventional offshore wind.

Scotland’s considerable natural wind resources, along with its well-developed supply chain and infrastructure developed for the offshore oil and gas sector, offer potential as a market for floating wind technology.

This could also open up further markets for the UK: another Carbon Trust paper said Europe’s more developed offshore wind industry could be leveraged to speed up deployment in Japan, highlighting floating wind as a key area.

According to James, current market signals suggest that the first large-scale floating windfarms could be installed by 2025. This could pave the way for considerable growth over the coming decades.

The development of floating wind could see wind power expanded to areas in the northern part of the North Sea, the Mediterranean and Atlantic coastlines of Western Europe, as well as locations outside Europe with deep continental shelves, such as parts of Japan, Taiwan, South Korea and the US.

23 Oct 2017

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