Floating technology

Major types

The guide uses a single reference design of floating substructure to provide a narrative that can be followed easily, this is a three-column, steel, semi-submersible substructure. It is selected because it has already been demonstrated at two pre-commercial floating wind farms and could be used widely elsewhere. It was not selected to represent the best future solution.

There are more than 50 other designs for floating substructures currently being proposed by technology innovators, but only a handful of these have been tested at full scale. These substructure designs have a wide range of different characteristics and performance, and all belong to one of four major substructure types that have already been successfully used within the offshore oil and gas industry.

An overview of each main type is presented in the sections which follow to complement the more detailed narrative for the reference design.

Semi-submersible substructures

Overview and description

  • Semi-submersible substructures typically consisting of typically three or four buoyant columns or other floating elements at the periphery that are connected using pontoons and/or trusses. They are typically ballasted to provide additional stability.
  • At present, only Principle Power’s WindFloat has been installed at full scale demonstration.
  • Suitable for water depths greater than 40 m.
  • Design variables include: the number of columns, placement of tower (eccentric vs central), construction material (steel vs concrete) and ballast system, with some designs opting to use a suspended submerged counterweight to lower the centre of gravity.
  • Can be used with a wide range of mooring and anchor configurations.
Example of a semi-submersible floating substructure. Photo of the WindFloat Atlantic project courtesy of Principle Power/Ocean Winds.
Example of a semi-submersible floating substructure. Photo of the WindFloat Atlantic project courtesy of Principle Power/Ocean Winds.

Characteristics

  • Smaller draft than spar substructures. This enables quayside turbine installation and adjustable ballasting can make the complete structure stable for tow-out and installation.
  • Tugs and anchor-handling vessels (AHVs) can be used in broad weather windows, reducing the need for specialist vessels.
  • The largest floating substructure type, in terms of length and width.
  • Higher mass than tension leg platforms.
  • Large sea and land areas are required for the storage and marshalling of substructures during construction.
  • Semi-submersibles experience higher wave-induced motions than spars, but lower than barges and experience large heave motions in extreme weather conditions when the wave period is close to their heave natural period.

Barge substructures

Overview and description

  • Barge substructures have a single hull that pierces the waterline. They have a large surface area in contact with the water which provides stability, however this can make it more susceptible to wave loading.
  • The overall dimensions are less than the equivalent semi-submersible.
  • The barge-type floating substructures that have been installed to date are BW Ideol’s Damping Pool and Saitec Offshore Technologies’ SATH.
  • Suitable for water depths greater than 40 m.
  • Design variables include: construction material (concrete or steel) and the shape of the single hull which may be square or cylindrical.
  • Another key variable between designs is the presence and size of a moonpool to improve the stability of the substructure in rough sea states.
  • Can be used with a wide range of mooring and anchor configurations.
Example of a barge floating substructure. Image courtesy of BW Ideol. All rights reserved.
Example of a barge floating substructure. Image courtesy of BW Ideol. All rights reserved.

Characteristics

  • The turbine can be erected onto the barge substructure in a sheltered harbour then towed to the installation site because the combined structure is stable in transport.
  • Reduced transport and installation cost associated with building floating projects using barge substructures compared to spars, which can only use specialist marshalling ports because of their depth, or tension leg platforms (TLPs), which need specialist solutions for transport and installation because they have low stability until installed.
  • Barges may experience large heave motions in extreme weather conditions when the wave period is close to its heave natural period. This may require turbines installed on barge-type substructures to be engineered for larger tower motions than for other substructure types.

Spar substructures

Overview and description

  • Spar substructures use ballast-stabilised designs. They consist of a tall cylinder housing dense ballast in its lower part to lower the centre of gravity below the centre of buoyancy, creating a self-righting motion.
  • Spar substructures have a large draft.
  • This substructure type has been used by Equinor at its first three projects in the in the North Sea, which have used concrete and steel-based designs.
  • Suitable for water depths above 100 m.
  • Design variables include: construction material, ballast material, and the size of the cylinder.
  • Can be used with a wide range of mooring and anchor configurations.
Example of a spar floating substructure. Image courtesy of ORE Catapult. All rights reserved.
Example of a spar floating substructure. Image courtesy of ORE Catapult. All rights reserved.

Characteristics

  • Its large draft and small waterplane area mean it is less affected by wind, wave and current compared to other designs.
  • The large draft means that assembling turbines onto substructures from the quayside requires deep-water locations, which may not be available in some areas. This may also be done using floating installation vessels in sheltered deep-water areas, such as the Norwegian fjords, but this adds cost.
  • The large draft also limits site locations to allow tow-out for installation and tow-back for major-component replacement.
  • It has the highest tilt during normal operations of all technology types, and active ballasting is not an option to address this.

Tension leg platforms

Overview and description

  • TLPs achieve stability through the mooring system. They typically use mooring lines that connect to anchors either vertically or predominantly vertically. The upwards buoyancy force acting on the hull needs to be sufficient so that the tendons are continuously under tension under all operating loads.
  • TLPs are well established in the oil and gas industry but have not been used with wind turbines, up to now, on any commercial-scale demonstration projects.
  • A star-pontoon arrangement is expected to be used for floating offshore wind turbine applications with minimal structure piercing the waterline and minimal steel mass.
  • The first full-scale TLP demonstrator was SBM Offshore’s design at Provence Grand Large, France, which was installed in October 2023.
  • Suitable for water depths above 80 m.
  • Design variables include: construction material, the shape of the hull and whether there is any active adjustment of the tendon load.
  • The high loading in the mooring system, and their vertical, or near vertical, configuration, requires an anchor type that can withstand a strong vertical pull, such as a driven pile or a suction anchor.
Example of a tension leg platform floating substructure. Image courtesy of ORE Catapult. All rights reserved.
Example of a tension leg platform floating substructure. Image courtesy of ORE Catapult. All rights reserved.

Characteristics

  • Installation is complex as the hull is less stable than other technology types. This means that final assembly of the turbine onto a TLP in port followed by tow out to site is not possible.
  • It is expected that either turbines will be assembled onto installed TLPs at site, requiring a weather sensitive floating-to-floating lift, or pre-assembled on a vessel capable of installing the turbine and TLP together. This also makes tow-to shore options for maintenance harder than for other floating foundation types.
  • The mooring system and anchors are expected to be more expensive than for other technology types as they are subjected to higher loads and they require sufficient redundancy to counter the consequence of failure.
  • It has the lowest structural mass of all floating substructure types once installed, although this has to be set against the higher costs of transport and installation, and the mooring system and anchors.
  • The lowest substructure motions of all floating substructure types other than spars once installed. This reduces the structural loadings on the turbine and array cables compared to other types.
  • As the mooring system is critical to stability there could be reluctance to use it in areas prone to seismic activity.

Other floating offshore substructure concepts

The other floating offshore wind substructure concepts included here are variants of the four substructure types previously described, but sufficiently novel to describe further. Many offer the potential for significant mass reduction but often increase the complexity of design. Lessons from the oil and gas industry has shown the benefits of simplicity over complexity. It is important, however, that the industry properly examines other concepts.

The non-exhaustive list of example concepts included here are intended to show the spread of potentially disruptive solutions.

  • Counterweight concepts. One example is Saipem’s Hexafloat. These combine the benefits of a semi-submersible (shallow depth for transport) and a spar (stability from mass at depth). A challenge is the complexity of mechanisms to lower and raise the counterweight.
  • Pivoting about a single point, thus removing the need for a turbine yaw system and ability of the support structure to withstand loads from all directions. Examples include X1 WIND’s PivotBuoy, Aerodyn’s Nezzy2 and Saitec’s SATH. These use turret mooring/single point moorings, which is a proven technology for floating production storage and offloading solutions (FPSOs) in oil and gas. A challenge is how the floating offshore wind turbine behaves when strong waves or tide are not aligned with the wind direction.
  • Downwind rotor. Examples include X1 WIND’s PivotBuoy and Aerodyn’s Nezzy2. These are typically enabled by a pivoting substructure and allow unconventional tower concepts such as tower braces, guyed towers or inclined towers. A challenge is that the established wind turbine manufacturers are focused on turbine concepts relying on a yaw system that can be used onshore and on fixed offshore substructures.
  • Multiple rotors. Examples include Hexicon, Aerodyn’s Nezzy2, and Mingyang’s OceanX. These are typically enabled by a pivoting substructure and have the potential to reduce the cost of the floating substructure and array connection per MW, having double the installed capacity on a single floating substructure. A challenge is the impact of one turbine shutting down on the other(s).
  • Vertical axis floating wind turbines. Examples include SeaTwirl’s S1 and S2. A challenge is that vertical axis wind turbines on land have had higher levelised cost of energy (LCOE) than horizontal axis turbines due, in large part, to their rotors having lower coefficients of performance.
  • Combined wind and wave energy devices: examples include Floating Power Plant (FPP).
Floating substructures with counterweights: Saipem’s Hexafloat (image courtesy of Saipem, all rights reserved).
Floating substructures with counterweights. Saipem’s Hexafloat (image courtesy of Saipem, all rights reserved).
Floating substructures which pivot about a single point: X1 WIND’s PivotBuoy (image courtesy of X1 WIND, all rights reserved)
Floating substructures which pivot about a single point: Saitec’s SATH (image courtesy of Saitec, all rights reserved)
Floating substructures which pivot about a single point. From left to right: X1 WIND’s PivotBuoy (image courtesy of X1 WIND, all rights reserved) and Saitec’s SATH (image courtesy of Saitec, all rights reserved).
Floating substructures with multiple rotors: Hexicon’s TwinWind (image courtesy of Hexicon, all rights reserved)
Floating substructures with multiple rotors: Aerodyn’s Nezzy2 (image courtesy of Aerodyn, all rights reserved)
Floating substructures with multiple rotors. From left to right: Hexicon’s TwinWind (image courtesy of Hexicon, all rights reserved) and Aerodyn’s Nezzy2 (image courtesy of Aerodyn, all rights reserved).
Vertical axis floating substructure: SeaTwirl's S2 (image courtesy of SeaTwirl, all rights reserved)
Vertical axis floating substructure: SeaTwirl’s S2 (image courtesy of SeaTwirl, all rights reserved).
Combined wind and wave device: Floating Power Plant's substructure (image courtesy of Floating Power Plant, all rights reserved)
Combined wind and wave device: Floating Power Plant’s substructure (image courtesy of Floating Power Plant, all rights reserved).

Floating offshore substations and dynamic export cables

  • Early floating wind projects are likely to use fixed jacket foundations to support offshore substations.
  • Floating offshore substations will be required where the water is too deep for jackets, otherwise long array cables would increase electrical losses.
  • Dynamic export cables will be required to transfer power to shore from floating offshore substations.
  • These cables need to be able to withstand the same forces as the dynamic array cables used to connect floating turbines, but at higher voltages of 220 kV and above.
  • Cable manufacturers have developed and lab-tested designs for dynamic export cables but these have not yet been commercially developed.
  • They are expected to be used at depths greater than 100 m.

Concrete versus steel as the primary material

Floating substructure types can be designed using either steel, concrete or a hybrid of the two. The decision on what materials to use may be taken on a case-by-case basis considering a wide range of factors. For example, Equinor used steel spar substructures at its 30 MW Hywind Demo project in 2017 and concrete spar substructures at its 88 MW Hywind Tampen project.

There are four main factors which influence a developer’s choice of materials:

Cost

  • A developer’s decision to use steel concrete or hybrid substructures will involve careful consideration of the costs across all project phases.
  • Steel is expected to be less expensive per tonne than concrete but steel reinforcing bar is cheaper than steel plate.
  • Steel plate prices have been more volatile, with swings up to 50% recorded within a year. Concrete prices tend to be more stable. Reinforced concrete structures still use large amounts of steel, especially for reinforcement, but the overall volume is several times less than where steel is the primary material.

Supply chain

  • Where there is no existing steel fabrication supply chain locally, concrete fabrication may be more straightforward to establish as it requires less investment in new facilities.
  • The localisation of concrete fabrication provides a larger number of jobs compared to a steel fabrication yard. The jobs associated with steel and cement manufacture must also be considered.
  • Concrete substructures are heavier than steel so require more effort to lift or tow, and greater channel depth, if transporting them is needed.

Environmental impact

  • Environmental considerations are important for developers and may be included as decision criteria in competitive offtake auctions. The carbon footprint depends on how the steel and cement are made, the operational lifetime and any recycling or re-use at end of life.
  • Steel is frequently recycled. It is expected that concrete would be ground up. Lifecycle analysis is a useful tool for this sort of analysis.

Site conditions

  • Metocean conditions may influence the selection of material due to the performance of different materials through time. For example, concrete substructures are likely to be more vulnerable to freeze-thaw damage while steel foundations are likely to be more vulnerable to corrosion.

Guide to a Floating Offshore Wind Farm