Difference between fixed and floating offshore wind
At the end of 2021 cumulative installed global offshore wind capacity was approximately 57 GW. Only approximately 140 MW of this is floating, with the rest fixed to the sea bed. Fixed offshore wind projects generally use one of two basic designs for foundations. These are the:
Monopile
- A hollow steel tube driven into sea bed.
- Lowest-cost foundation available at depths up to 50 m, depending on ground conditions.
Examples of monopile foundations.
Jacket
- A lattice structure with three or four legs, typically fixed to the sea bed using pin piles or suction buckets.
- More expensive than monopiles to fabricate and install in waters generally less than 60 m, less expensive in deeper waters and where challenging ground and load conditions would otherwise make monopiles too expensive.
Examples of jacket foundations being transported.
Fixed offshore wind foundations are generally the cheapest way of building offshore wind farms, but they are limited to shallow water depths. Fixed foundations have been installed in waters up to 55 m, and future fixed projects are planned in waters up to 75 m deep. This depth limitation is being pushed as new technologies and manufacturing techniques are developed. As fixed wind foundations move into deeper waters, the demand for floating wind foundations may be affected.
Why use floating offshore wind?
Floating substructures allow the development of offshore wind in new areas with higher water depths where it is not possible to develop fixed offshore wind projects. This brings five main advantages:
Allowing offshore wind to be used in new markets
- Almost all constructed offshore wind capacity is fixed and located in markets where there are large, shallow continental shelves, such as in Northern Europe.
- Floating substructures allow projects to be developed in areas that don’t have as much shallow water available such as the west coast of the US, Japan and South Korea.
Increasing the offshore wind capacity of existing markets
- By opening new areas of the sea bed for development, floating offshore wind can reduce the need to develop sea bed that is already constrained by competition with other human use cases, such as defence, fisheries, the harvesting of marine aggregates, tourism and conservation.
- Projects in less constrained areas can be developed with more certainty than those that have clear conflicts of interest. Historically, legal challenges have slowed or prevented developers from realising projects in these areas.
- Floating offshore wind may also be preferable in areas where the sea bed conditions create challenges for installing fixed foundations, for example where hard sediments require foundations to be drilled into the sea bed, or in areas where soft sea bed conditions require deep piling to achieve stability.
Reducing emissions from oil and gas production
- Floating offshore wind farms can provide power to offshore oil and gas production facilities which might otherwise use large quantities of fossil fuels and are too deep to be partially powered by arrays of fixed offshore wind turbines.
- For example, Equinor’s 88 MW Hywind Tampen project will meet around 35% of the power needs of five Norwegian platforms in the Gullfaks and Snorre fields, which are in waters 300 m deep.
Capturing more energy
- Around 80% of the total ocean area is too deep for fixed foundations.
- Many deep-water areas are also located far offshore, where wind resources are stronger than fixed offshore wind locations.
- For example, the average capacity factor of a UK fixed offshore wind farm in 2020 was 45%.[1] This means the projects produced slightly less than half of the theoretical maximum amount that could have been produced if there were consistently strong winds offshore. For the UK’s first floating offshore wind farm the capacity factor over the same 12-month period was 57.1% – the highest in the UK[2].
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Many floating development areas will have different generation profiles to areas already developed with fixed offshore wind projects, producing power at different times of the year or day as they belong to different wind regimes. This could make floating projects complementary to fixed offshore wind projects within the broader energy system.
Providing local economic benefits
- Fixed offshore wind foundations are usually manufactured at specialist facilities around the world, providing few economic benefits to the area local to the offshore wind farm.
- Floating substructures are generally larger and heavier structures than fixed foundations making storing and transporting them over large distances difficult. This means that floating substructures will need to be produced, either from scratch or using prefabricated components, in ports close to future floating offshore wind farms.
- This will drive significant investment and create jobs in ports close to floating offshore wind farms.
[1]The Crown Estate, ‘Offshore Wind Report’, 2021, available online at https://www.thecrownestate.co.uk/media/4095/2021-offshore-wind-report.pdf , last accessed July 2022
[2]Equinor, ‘Hywind Scotland remains the UK’s best performing offshore wind farm, March 23 2021, available online at https://www.equinor.com/news/archive/20210323-hywind-scotland-uk-best-performing-offshore-wind-farm, last accessed July 2022
Barriers to floating offshore wind growth
The market for floating offshore wind will grow significantly over the next decade. The rate of growth for floating offshore wind projects will largely be determined by how quickly these three barriers can be overcome.
Higher costs
- At present the cost per MWh of floating offshore wind energy is higher than the costs for fixed offshore wind projects. This means floating offshore wind farms are reliant on price support schemes run by governments.
- One of the drivers of higher costs is the fact that floating offshore wind is a new and largely unproven technology which increases project finance and insurance costs for developers.
- We expect the cost of floating offshore wind to fall as developers, suppliers and the wider industry gain experience, and projects increase in size allowing economies of scale to be realised. While we expect the cost of floating wind to fall, it will likely always be higher than fixed wind due to the increased complexity and size of the foundations. This is discussed further in the Wind Farm Costs section.
Supply chain
- At present there are a limited number of ports with the right combination of quayside length, water depth (at least 10m deep) and storage space to support the simultaneous development of GW-scale floating offshore wind projects. In the UK, the port of Cromarty Firth has been awarded funding to develop it into a floating offshore wind construction and marshalling port. It has a quayside length of 372 meters, a minimum water depth of 12 meters, and over 90,000 square meters of open laydown area.
- Manufacturing facilities for the mass production of floating substructures need to be established to ensure the local supply chain has the capacity to support the buildout of projects at scale.
- A competitive and capable local supply chain can help to secure significant cost reductions and fully capture the economic benefits of floating offshore wind.
Technological challenge areas
- Solving component specific issues remains critical. As the industry scales up the design and fabrication of relatively new technologies such as floating substations, dynamic export cables and optimising existing technology such as mooring systems will need further refinement. For instance, floating substations are still at an early stage, with only one having been deployed globally to date, while dynamic cables capable of operating in deep waters and under constant movement are still undergoing testing and optimisation. There are a number of UK-led initiatives, such as the Floating Offshore Wind Centre of Excellence and ORE Catapult’s test programs supporting R&D in these areas, helping to develop robust, commercially viable solutions.
- Refining installation methodologies is critical. Installing large-scale floating wind farms poses logistical and operational challenges that go beyond those faced by fixed-bottom projects. At present the largest floating offshore wind project (Hywind Tampen) contains just 11 turbines, which were installed between May 2022 and August 2023. Installing a project that has 60 turbines in a single season represents a significant challenge. Current difficulties include storing constructed floating substructures and finding suitable weather windows to tow floating offshore wind turbines into position.
- Establishing ways of maintaining and servicing floating offshore wind turbines is essential. Unlike fixed-bottom turbines, floating turbines cannot easily be accessed by traditional jack-up vessels, and towing units back to port for repair is costly and time-consuming. New offshore maintenance methodologies will be critical to alleviating these challenges. For example, in 2022 a turbine in the Kincardine project in Scotland completed a world’s first in-situ major component exchange on a floating turbine, demonstrating the potential for offshore repairs and helping build operational expertise. Continued innovation in tools, technologies and process will be crucial to reduce cost and limit downtime when problems occur.