Offshore wind has gained traction and is one of the biggest movers in the energy industry due to a drastic fall in prices in the last few years, as the size of wind turbines grew, increasing output, and the technology of providing the footings and connections have improved . As a good example, only in 2020, global offshore wind installation has increased by 47%. In comparison to 6.9 GW in 2020, it has reached 21.1 GW capacity in annual installation in 2021. It means offshore wind witnessed more than 3 times increase .
Wind turbine generators as the main component of harnessing wind energy have developed extensively. The wind turbine generators of today are much more complicated than the prairie windmills of the past. A modern wind turbine generator has as many as 8,000 different components . Wind turbines are soaring to record sizes. In 2018, the average rotor diameter of turbines installed increased by 141% to 115.5 meters. There is also an increase in the average nameplate capacity of turbines, meaning they have stronger generators.
Why is foundation more important in the offshore wind than onshore?
From an investment per megawatt (MW) point of view, offshore wind is still some 50% more expensive than onshore wind . The cost of offshore wind turbines typically exceeds that of similar onshore turbines by 20% and towers and foundations by 350% . Generally, offshore wind foundations can be categorized into two primary categories: bottom-fixed and floating. For bottom-fixed offshore wind platforms, the turbine is the most expensive item, contributing about 31.8% to the overall cost, while the second and third most expensive parts are assembly and installation (19.3%), followed by substructure and foundations (14.7%) . For floating offshore wind platforms, the substructure and foundations are the most expensive components (36.2%). Turbines and their assembly and installation take up about 22.1% and 11.1% of the total cost . Therefore, the foundation is one of the main pillars for offshore wind installation as well as being a cost-effective platform.
The foundation structure provides support for a wind turbine generator by anchoring it to the seabed. Up to now, the majority of offshore wind turbine farms and projects have utilized bottom-fixed foundations to secure the wind turbines to the sea bed. However, as technology develops and more wind projects move into deeper waters, it is expected that floating offshore wind foundations will become increasingly used.
Here, we briefly introduce two types of bottom-fixed offshore wind foundations and three common floating ones.
Fixed-bottom offshore wind turbine Foundations: Monoplie and Jacket
Monopile and Jacket : (lattice structure) are two typical bottom-fixed offshore foundations . Typical monopile foundations consist of a steel tube pile with a diameter of 3–8 meters. A monopile is typically found in water with a depth between 20 and 40 meters. Thanks to its ease of manufacture, low cost, and manageable construction, the monopile foundation has been utilized worldwide for offshore wind turbine foundations. In seabeds with clay, sand, or chalk stratigraphy, monopiles can be installed with impact hammers or vibratory drives. Typically, drilling and bored pile methods are used for rocky seabeds . The jacket foundation comprises a space frame structure assembled from steel tubular members, which is usually fabricated in advance by welding on land. Afterward, the jacket is transported to the seabed and piled there. Despite being relatively economical in terms of steel consumption, jacket foundations can be expensive in terms of storage, logistics, and installation . In recent years, jacket foundations have been used extensively in intermediate depths between 5 and 50 meters.
Floating Offshore Foundations: TLP, Semi-Submersible, and Spar Buoy
Floating offshore foundations with mooring systems and anchor foundations have been proposed and tested, benefiting from advances in the floating oil and gas platforms such as tension leg platforms (TLPs), semi-submersibles, and spars. A floating wind foundation can be made from any of these three types of floating structures. The first test floating wind turbine (with TLP platform type) was installed by Blue H Technologies off the Italian coast with a rated capacity of 80 kW in 2008 . The world’s first MW-scale wind farm with a floating foundation, Hywind, with a capacity of 2.3 MW and a spar foundation, was installed by Statoil in the North Sea near Norway in 2009 .
Tension Leg Platform is a vertically moored compliant platform. The floating platform with its excess buoyancy is vertically moored by taut mooring lines called tendons (or tethers). The structure is vertically restrained precluding motions vertically (heave) and rotationally (pitch and roll).
Among the 15 floating offshore wind turbine farms that are currently in operation in the world, the floating foundations include eight spar buoys and five semi-submersibles . A spar buoy is a ballast-stabilized, simple structure that has a great deal of stability and a large draft, limiting its deployment in shallow waters. The semi-submersible is a complex free-surface stabilized structure with a relatively small draft, which allows for high site flexibility . In 2011, the 2 MW WindFloat 1 became the first full-scale semi-submersible wind turbine installed off the coast of Portugal . A water depth of 60 m is considered as a cut-off level for bottom-fixed structures and the entry point of the floating platforms in the offshore wind industry due to economic reasons . Therefore, almost 80% of the world’s offshore wind resource potential is currently profitable only for floating offshore wind turbines .
Did You Know?
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 SolarEdition, “Global Offshore Wind Installation Has Increased by 47% during 2020”.
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 Department of Energy, “Top 10 Things You Didn’t Know About Wind Power”.
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