12.10.2022 / Nikolas Ironside
The Floating Offshore Wind (FOW) industry has had a monumental year in 2022, and things are just getting started. Until now, offshore wind has been confined to areas of shallow water, where structures can be fixed to the seabed. The rise of FOW is set to be the great democratiser of the green transition, unlocking access to offshore wind power for vast swathes of the world’s coastal regions.
The FOW market today shares many parallels with the bottom-fixed offshore wind market of the early 2010’s - ambitions are high, yet there are numerous obstacles to overcome before the cost of electricity falls to competitive levels. There is nothing like a challenge to inspire engineers, and many of the sharpest minds in the offshore engineering field are dedicated to bringing this revolutionary technology into the mainstream. Two such experts from COWI's Wind Energy & Renewables department, Business Development Manager Volkert Oosterlaak, and Structural Specialist Stian Fiskvik, shared their insights on the floating wind frontier, outlining where we are and what needs to happen next.
The sea demands respect, and the forces at play in the deep ocean are as wild as it gets. For floating turbines, it is the floating substructure, or floater, which holds them upright and anchors them in place no matter the conditions. There is no universally dominant floater design, however, four main typologies have attracted the most interest: Semi-submersible, pontoon/barge, spar buoy and Tension Leg Platform (TLP). Beyond these are a myriad of novel approaches which, while less popular, continue to fuel innovation in the industry.
The four main FOW typologies: (L-R) pontoon/barge, semi-submersible, spar buoy and tension leg platform
An explanation of each of the four main typologies can be found here, but in a nutshell:
Semi-submersible floaters consist of large, partially submerged columnar tubes, often anchored with catenary mooring systems. An example of a semi-submersible floater is Principle Power’s WindFloat units, which have been used in Scotland to establish the Kincardine floating wind farm. This five-turbine farm has a capacity of 50 MW, making it the largest operational floating wind farm in the world.
Pontoon/barge floaters are steel or concrete hulls anchored to the seabed via catenary chains. In the French Mediterranean, the Eolmed project is using pontoon/barge floaters developed by BW Ideol, called the Damping Pool. Eolmed features three turbines with a total capacity of 30 MW and is expected to be commissioned in mid-2024.
Spar buoy floaters utilise a long, weighted cylindrical draught, moored by catenary chains and drag embedment or suction anchors. The Hywind Tampen floating wind farm in the Norwegian North Sea is using Equinor’s Hywind spar buoy technology. Hywind Tampen consists of 11 turbines, and with a total capacity of 88 MW will take the title of world’s largest floating wind farm once operational.
Tension Leg Platform (TLP) floaters feature a submerged central column, anchored to the seabed by taut, vertical tendons rather than catenary mooring. EDF Renewables are leading the Provence Grand Large project, the first French FOW project in the Mediterranean. A TLP design from SBM Offshore, Float4WindTM, will be used for the project’s three turbines, which will have a total capacity of 25 MW.
Each of the four main FOW typologies are represented in different projects across Europe
One of the main reasons why developers have yet to coalesce around a single typology is due to the extremely variable conditions of FOW sites. Water depth, seafloor conditions and wave profiles can fluctuate wildly across even small areas.
“At the moment there are many technology providers demonstrating and selling their specific concepts,” Volkert explains. “In the future there will be more project specific requirements that need to be met, meaning designs that are tailored to a specific site and supply chain will be required. It is not yet clear if any of the existing concepts can meet all the site requirements out there, which is why COWI has chosen a technology agnostic approach. We work with all typologies in all conditions, depending on what is the best solution for our clients and partners.”
New FOW designs are constantly emerging, from hybrid designs of the four main floaters to completely exotic concepts that push the boundaries of what is possible. As Stian explains, continuous innovation is essential for the industry:
“There are exotic designs aiming to tackle the issues facing the more conventional concepts, whether it be how the floater is manufactured and installed, or how the turbine generates power. It is fascinating to see, and the research being done on these designs drives the technology and industry forward.”
Some examples of these new concepts are markedly different from anything seen before. French company Eolink have designed a blade held aloft by a pyramid structure, while Norwegian company Wind Catching Systems have a concept consisting of 117 rotors stacked vertically within a 300 m frame and mounted on a semi-submerged trimaran boat.
The novel concepts from Wind Catching Systems (left) and Eolink (right) showcase the diversity of designs present in the FOW market
“There are indeed many different concepts out there,” says Volkert. “Some with vertical access turbines, some with two bladed rotors, some have eliminated the tower altogether and use only the generator to support the blades. However, the biggest companies are investing heavily in the use of conventional turbines, and we have gained so much knowledge on them that they will likely continue to be the main type used going forward.”
The most widely used material for FOW systems are steel and concrete, while designs featuring more novel materials have yet to come to the fore. This is largely due to the cost pressures associated with experimentation.
“Steel is the most popular material for FOW concepts, but concrete is a fine substitute when there are supply chain constraints,” Volkert explains. “For instance, California doesn’t have a steel industry but is well equipped for construction with concrete, so we can expect to see concrete concepts arising in California.”
Stian notes that there are different trade-offs with the materials. “Concrete doesn’t need as highly skilled labour as steel, which requires specialists like welders and bar benders,” he says.
Designers and engineers are also exploring the use of super-durable and versatile materials, well equipped for the harsh marine environment. One such candidate is Fibre Reinforced Polymer (FRP), which is so strong and low maintenance that it is used in the hulls of minesweeper ships, and can be exposed to incredibly harsh marine environments for decades. Volkert sees merit in such materials eventually becoming more widely employed.
“At the moment they are too expensive,” he says, “but we should never rule these materials out, nor 3D print components. 3D printing is maturing very quickly, and is being used for everything from houses to ships propellers. We may not see 3D printed floating wind farms this decade, but why not in the future?”
As with all emerging markets it is rare for everything to go exactly according to plan, and delays are rife in the FOW industry. There are many reasons for this, from immature regulatory frameworks to inexperienced government agencies, however, above all else one aspect of the industry remains particularly vulnerable: the supply chain.
Turbines and floaters require large amounts of raw materials, and rare earth elements form essential components for the nacelles and generators. Access to these materials is disrupted when global supply chains are squeezed by external shocks. According to Volkert, two actions are key to overcoming these challenges.
“Firstly, the FOW supply chain is immature and needs much greater investment. We need investments to boost the capacity of production with both steel and concrete. We need harbour facilities able to assemble and launch the turbines and floaters, as well as the mechanisms to transport them to the site. In addition, we need investments in the mooring systems, which require a lot of space and production capacity,” he says.
“The second action is that we need to adapt designs for serial production. This means designs that are customised for certain yards - they fit the slipway at a specific yard, or the maximum weight it can handle. If, say, the transport vessel is the limiting factor, we would make sections that fit a specific vessel. We need to merge the production side with the design side so that they work hand in glove.”
The FOW supply chain requires a significant amount of raw materials and processing steps to produce the end product
This is a particularly knotty challenge, as on the production side FOW competes with other high value products. In South Korea, for instance, there are three large steel yards which regularly receive orders for products considered much higher value than FOW, such as LNG carriers.
“With FOW it’s not just a question of finding the right production capacity, we need to produce the steel as cheaply as possible to keep the overall CAPEX low, and we are competing with well-established industries,” says Volkert. “There are large shipyards in countries such as Finland and Germany well suited to FOW production, but it is difficult to compete with the demand from high-value products like cruise ships.”
Stian notes that converging towards similar floater designs will improve the efficiency of the supply chain and help the industry reach maturity faster.
“If everyone pursues their own concepts, the knowledge about each concept will be limited. If we have more universally agreed upon procedures in the industry, we gain deeper insight as a whole and we are able to come up with better solutions,” he shares. “It is a difficult and ongoing process, to try and converge, hone and refine the divergent designs into a more universal solution.”
A precedent can be seen in the bottom-fixed offshore wind industry, which is now dominated by two design typologies – the monopile and the jacket. The development of a FOW typology that fits well in an average shipyard and uses components that can be sized up or down depending on project constraints could be a game changer for the industry’s supply chain woes.
Stretched supply chains aside, FOW is a very hot topic globally. At these early stages a lively competitive atmosphere is palpable, with developers and countries vying for the title of ‘The World’s First’, or ‘The World’s Largest’. According to Volkert, the tail end of the 2020’s is when we can expect to see the first commercial floating wind farms come online.
A market outlook from environmental consultancy 4C Offshore states that 16 GW of FOW is forecast to be installed or underway globally by 2030, reaching 48 GW by 2035. The US and South Korea are leading the pack with targets of 10 GW each by 2035, while Europe is expected to have capacity of 18 GW by 2035.Looking even further ahead to 2050, the tendering activities and targets for installed capacity globally are booming, especially following the outbreak of war in Ukraine as European nations look to become more self-sufficient in their energy production.
“Belgium, Germany, the Netherlands and Denmark have announced they want to install a collective 150 GW of offshore wind – including FOW – by 2050. This is a significant increase on their previous targets, and really highlights how many countries are rapidly trying to secure an independent domestic energy supply,” Volkert notes.
Such targets are essential signals of support for the industry’s growth, but the simple fact remains that the cost of electricity from FOW at present is uncompetitive with other renewables. Remedying this will require continued growth in the global project pipeline, significant investments to retrofit and equip ports, and constant innovation into all components of a FOW system.
FOW assembly and maintenance requires dedicated port facilities, with many ports needing to undergo retrofits to support the industry
For countries serious about establishing commercial FOW farms, many of their port facilities will need significant retrofits to handle the components. At present, only a select few ports in Europe are geared up for this. The Kincardine farm in Scotland, for example, must send its turbines back to the Netherlands for maintenance, until ports in the UK are able to perform these tasks.
“I expect most investments will involve elongating key sites, installations to launch components that are constructed onshore and increasing the water depth through dredging - but these processes take time,” Volkert shares.
Floater design is complex and computationally demanding, particularly now as the industry trends towards the development of ultra-large turbines (15-20 MW). Accurately modelling the behaviour of the floaters that accommodate these giant structures can be a real challenge with existing tools.
Floater design is complex and computationally demanding
One project which aims to assist developers and floater designers is known as ‘efficient numerical methods for ultra large floating wind turbines’ – EMULF for short! EMULF is a collaboration between COWI, the Norwegian University of Science and Technology (NTNU), the Technical University of Denmark (DTU) and classification society DNV, to develop highly specific and efficient methodologies, to support the economical design of floaters. Stian, who is COWI’s lead representative on the EMULF project, explains further:
“We do have quite advanced tools for FOW, however, they require a lot of computer power to obtain a high-precision analysis. With EMULF we make simplified methods for analysis, with the trade-off of losing some accuracy to gain computational speed,” he says. “We then assess how much accuracy we can live with losing compared to how much computational effort we also gain - it’s a give and take. This is important for any project because we always work from a standpoint of not needing full accuracy in the initial stages through to the detailed design phase where we need to be meticulously precise.”
The next steps for EMULF will see the findings incorporated into COWI’s work, and the results and the methodologies made publicly available, to ensure the initiative benefits the entire industry.
A general rule in the wind industry is the larger the turbine the higher the energy output, and FOW turbines are becoming very large indeed! It is important to prevent periods of high electricity production from either going to waste or overwhelming the power grid. Solutions like the North Sea Energy Islands, which will use electrolysis to store surplus wind energy as green hydrogen, are being increasingly considered for new offshore wind farms.
“Converting wind energy into storable green fuels fits very well with COWI's ambition to advance the green transition,” Stian notes. “We can’t control when the wind blows, so taking the raw energy and producing something we can store and use at a later stage is a key challenge we need to tackle.”Some emerging concepts show electrolysers situated directly on the floater, while Siemens Gamesa is developing a design featuring an electrolyser within the turbine, allowing for localised hydrogen production. According to Volkert, the best method for aligning green fuel production with FOW will be very site specific.
“Is it cheaper to put a complex electrical cable down on the seabed, or lay a gas tube instead? I expect in general it is cheaper to lay electrical cabling, but there are some site-specific advantages that may make exporting the energy as hydrogen gas more attractive – for instance if the electrical grid connection onshore is distant or non-existent,” he remarks.
It may come as a surprise that there are vast amounts of engineering knowledge from the offshore oil and gas industry that can be adapted for FOW - after all, the Perdido oil rig in the Gulf of Mexico floats at an incredible depth of 2450 m! FOW installations at up to 800 m have been proposed, which raises the question – does a natural limit exist?
“There is a natural limit with regards to the cost of mooring lines,” says Stian. “If we have the same wind conditions at two different sites, one at 100m depth and one at 800m, then we will not gain any more energy from the deeper site, but we will have to pay significantly more for the mooring. There is a sweet spot where the mooring lines are most effective and the cost is lowest, between 100 to 500m."
Shallower water is not always cheaper, as shallow waters require heavy, powerful systems to keep the floater in place. Deeper waters can allow for more motion in the horizontal plane.
“I’m not sure there is a limit on maximum depth, but it is about finding the right concept,” says Volkert. “For the deepest waters we would look at TLP’s rather than semi-submersibles, as a long catenary mooring would get very soft, and it would be difficult to avoid collisions or the crossing of lines. It’s difficult to say what limits exist since there is so much research into materials occurring. TLP’s are traditionally moored with steel tubes, but maybe in the future it will be dyneema wires.”The bathymetry map of Europe highlights the sheer potential for FOW, as the offshore wind industry moves beyond areas of shallow continental shelf
From Volkert and Stian’s insights, one point in particular stands out. New developments and technological advances are occurring so fast that we cannot know exactly how the floating wind industry will look in the future, but one thing is for certain – it is here to stay. A 2019 report from the International Energy Agency concluded that bottom-fixed and floating wind together have the potential to produce 11 times more energy than the world will consume in 2040. As we exit the era of fossil fuels, and build a cleaner future, it is difficult to imagine a stronger pillar for the green transition than floating offshore wind.