Floating wind turbines replace fixed underwater foundations with floating foundations. Different floating foundations go with different mooring systems. In the following paragraphs, InfoLink introduces two types of mooring systems, their components, and three mainstream floating platform technologies, and compares their cumulative capacities and number of wind farms.
Mooring systems set floating platforms at specific locations, transferring environmental forces to the seabed to prevent displacements and loss of generation efficiency. A mooring system consists of mooring lines and anchors. Each mooring system has mooring lines of various materials and anchors of different kinds.
There are generally two kinds of mooring systems, and one of them is taut-leg configurations, which are mostly integrated with tension leg platforms (TLP). Taut-leg configurations comprises iron mooring lines made of synthetic fiber rope or wire. Taut-leg configurations have a 30-to-40-degree angle between each mooring line and the seabed, and thus smaller footprint and less impacts on the seabed. Doubled with vertical loading and the restoring force of elastic mooring lines, taut-leg systems provide exceptional stability and resistance to horizontal and vertical forces. However, such configurations acquire greater tension, and thus have latent risk of metal fatigue and can only be installed with special vessels.
Taut-leg configurations need to be capable of withstanding vertical and horizontal forces. Therefore, driven piles, gravity anchors, and suction anchors are better fitted with them. Driven piles and pin piles of fixed foundations are similar in terms of their concepts and installation process, which saw costs surge as water depth increases. Additionally, pile driving produces noises. Gravity anchor consists of concrete and reinforcing steel, providing sufficient holding power with tonnes of weight. However, driven piles and gravity anchors make it difficult to install and decommission wind turbines.
Hence, suction anchors are the mainstream for taut-leg configurations. Suction anchors pump out water in the hawsepipe, creating pressure difference and forcing anchors to sink. To dismantle foundations, simply fill water into the hawsepipe, and the pressure will push anchors to the surface.
Another mooring system is catenary configurations, which are usually used with spar-buoy and semi-submersible foundations and mostly use mooring lines made up of iron chains or wires with enough of weight. Catenary configurations make more impacts on the seabed than taut-leg configurations and may interfere fishing activities, for some mooring lines are placed upon the seabed, and thus have larger footprint. The weight of mooring lines on the seabed provides horizontal restoring force for foundations, whilst anchors withstand horizontal force. Therefore, for catenary configurations, buoys or clump weight provide vertical restoring force, but with stability slightly inferior to taut-leg configurations. With catenary configurations, it’s easier to install mooring lines, thereby cheaper maritime engineering costs. However, considering vessel payload, iron chains are gradually replaced by synthetic ropes for deeper waters.
Catenary configurations are best fitted with drag-embedded anchor (DEA). DEA has been widely applied and is applicable in cohesive soil, which is not too hard for anchors to penetrate. Such anchor is easy to remove when decommissioning wind turbines. However, DEA may accidentally damage submarine communications cables or transportation pipelines.
Floating platform technologies
Presently, mainstream foundations include spar-buoy foundation, tension leg platform, and semi-submersible foundation. The following paragraphs shed light on their maturity, optimal water depths, costs, pros and cons.
Spar-buoy floating foundations stabilize the entire platform with spar buoys and ballast in the bottom half of the structure. As a result, with lower and steadier center of gravity, there is no need for dynamic stability systems. The structure is rather simple, with only some easy-to-make components. Therefore, costs are expected to drop rapidly. Spar-buoy foundations must be installed in waters with at least 100m of depth. Installation and transportation are not easy, owing to its massiveness. Wind turbines installation must be conducted offshore, which acquires lifting vessels and other maritime engineering capacities. Greater draft makes it difficult to drag turbines back onshore for maintenance.
Spar-buoy floating foundation is one of the most mature technologies by far. Hywind Scotland, the world’s first MW-scale commercial wind farm, adopted this technology. Led by Norwegian developer Equinor, Hywind Scotland had demonstration turbine installed in 2009 and completed the entire wind farm in 2017, with capacity factor reaching 53.1% in the recent three years. This highlights the richness and stability of wind sources offshore. Besides Norway, wind farms in Goto City of Japan and Les Éoliennes Flottantes du Golfe du Lion in France use spar-buoy foundations.
Tension Leg platform, TLP
Tension-leg platform (TLP) is a type of fixed platform that maintains stability with tension created by mooring lines, and thus fix the entire structure. Therefore, TLP is the most stable kind of foundations. With smaller sizes and thus lighter weights, TLP type foundations have the least difficulties in manufacturing, assembling, installation, and maintenance.
Assembling can be done at ports or onshore before dragged to wind farms. However, creating tension with mooring lines not only acquires special vessels, but also complicated maintenance and installation process. Additionally, their anchors are only applicable on certain kinds of seabed, whilst anchors chains bear heavier loads. As a result, costs of mooring systems and mooring lines are higher than those of other foundations. An entire TLP foundation relies on anchors for balance. Once anchors experience metal fatigue, the whole structure will collapse.
TLP is one of the most common types of offshore platforms, but less mature than spar-buoy and semi-submersible turbines, and still in the demonstration stage. The demonstration wind farm located in Germany is led by GICON SOP and equipped with a 2.3 MW turbine. Provence Grand Large, a pilot project scheduled to enter commercial operation by 2022-2023, is made up of floating foundations made by SBM offshore and turbines made by Siemens Gamesa. With a total capacity of 25.2 MW, Provence Grand Large is the first utility-scale wind farm with TLP type foundations. This project verified the feasibility of TLP technology.
Semi-submersible foundations connect numerous buoys and spars; the former provides buoyancy, and the latter stability, integrated with mooring lines that fix foundations at a specific place. Semi-submersible foundations are the most expensive to build. A wind farm with a capacity of 490 MW is estimated to see EUR 147/MWh of LCOE, slightly higher than TLP’s EUR 142/MWh and spar-buoy’s EUR 138/MWh.
The LCOE of WindFloat in Portugal is EUR 135.7/MWh, higher than pilot wind farms of other kinds. This can be attributed firstly to the difficulty in manufacturing. Semi-submersible foundation is a complex and huge structure that requires many connectors. Secondly, to ensure steadiness, it must be accommodated with dynamic stability system and ballast system. Still, semi-submersible foundation is easier to install.
Wind turbines can be assembled onto the foundation, and then dragged to desired locations. The process can be completed with traditional port lifting equipment and without turbine installation vessels. The structure can be dragged back onshore for maintenance. Additionally, semi-submersible foundations adapt well in waters with wide-ranging depths from 40m to thousands of meters, to which installation costs do not vary in accordance.
Semi-submersible technology has fully developed. The U.K.’s 50-MW Kincardine is the largest floating wind farm by far. Off the shore of northern Portugal, the 25-MW Windfloat Atlantic uses “WindFloat,” semi-submersible foundations made by Principle Power. The feasibility of semi-submersible foundation is undeniable, but challenges lie in the reduction of manufacturing costs.
The graph above shows that both semi-submersible and spar-buoy technologies have accumulated considerable capacities, each being expected to surpass 100 MW by 2022. TLP type foundations will not see marked increase in installed capacity until 2022.
The surge in spar-buoy during 2017 and 2019 is attributed by the 30-MW Hywind Scotland developed by Equinor in the U.K. Meanwhile, semi-submersible foundations saw no wind farms with capacity larger than 10 MW. Afterwards, installed capacity of semi-submersible foundations rose markedly, as Principle Power applied WindFloat technology to Portugal’s 25-MW WindFloat Atlantic 2 wind farm and the U.K.’s 48-MW Kincardine 2 wind farm. The amount of semi-submersible wind farms also increases from 2021 and will surpass spar-buoy after 2022, as indicated in the graph below.
These three types of foundations each has pros and cons and achieves mostly stability with ballasts, buoys, and mooring systems. Techniques of manufacturers differentiate in terms of the proportion of each source of stability. Besides structural differences, maritime engineering and infrastructure requirements also vary.
For instance, semi-submersible foundations need bigger storage space at ports, for they are larger in size and assembled there. Since turbine are hosted at ports, semi-submersible foundations no longer need large lifting vessels. No matter which type, the further floating wind farms are off the shore, the longer vessels will travel, posing greater challenges to the crew physically and mentally.
For now, semi-submersible wind farms outnumber those with spar-buoy foundation. InfoLink attributes this to the latter’s requirement for at least 100m of water depth.
The deeper the water, the further it is off the shore, pushing up costs of transportation, operation, maintenance, and submarine cables, since LCOE rises by EUR 1/MWh4 for every 10m increase in distance from the shore. Add to that the installation and maintenance of spar-buoy foundations are more difficult. On the contrary, the construction of semi-submersible foundations is more complex, but with vessel-like structure, technology merging, production capacity expansion, and cost reduction are easier to vessel manufacturers.
Heidari, S. (2017). Economic modelling of floating offshore wind power: Calculation of levelized cost of energy. Assuming a floating wind farm has 490 MW capacity and consists of 70 turbines.
Butterfield, S., Jonkman, J., Musial, W. & Sclavounos, P. (2005). Engineering Challenges for
Floating Offshore Wind Turbines. Washington D.C.: National Renewable Energy
Bjerkseter, C., & Ågotnes, A. (2013). Levelised costs of energy for offshore floating wind turbine concepts (Master's thesis, Norwegian University of Life Sciences, Ås).