Overview of Floating Offshore Wind Turbine Concepts

During my specialisation curse, held at the National Technical University of Athens, I had to carry out an investigation on a free subject. Due to my previous background (naval architecture) I decided to look at floating offshore wind turbines.

The main topics of the report are described here below.

The concept of developing offshore floating wind turbines began in the early 70’s and Professor William Heronemus, from the University of Massachusetts, was one of the pioneers in investigating this kind of technology. The main research communities only appeared on the scene in the mid 90’s after the commercial boom of the wind energy industry.
The offshore wind industry will surely have a promising future in the North Sea and Baltic Sea off the coasts of Denmark, Netherland, Germany, Sweden, Belgium, UK and Ireland. However these kinds of installations are restricted to shallow waters. Except in some rare cases, most of the constructed or planned projects have a water depth up to approximately 20 m. In the case of water depth between 20 and approximately 50 m various fixed-bottom tripod and quadpod solutions have been proposed (e.g. Betrice Wind Farm in north eastern UK – jacket with sleeves for piles).
Highly populated coastal areas like northern Spain, western France, western Norway, countries facing the Mediterranean Sea, Japan, west and east coasts of USA and the east coast of China, where, due to high electricity demand and good wind resources, implementation of offshore wind farms would be ideal. However due to the fact their coastlines have high water depths (higher then 30-40 m) even after a few kilometres from the shore, the actual available technologies (fixed-bottom offshore wind farms) would be too expensive and not feasible.
In reality, floating structures, as shown in Fig. 1, have already been successfully developed by the offshore oil & gas industry over many years. As the concept has been successful in this industry it is credible to believe that also for the offshore wind industry a similar development (deep water installations) will be soon seen in the future.
Fig. 1 – Existing offshore oil & gas structures

The main challenges for floating offshore wind turbines are to combine stability and acceptable motions at low costs.
The main benefits for deepwater offshore floating wind farms will be the following:
- Greater choice of sites & countries
- Greater choice of concepts
- Greater flexibility of construction & installation procedures
- Easier installation, decommissioning and removal

1. Floating stabiliser classification
The floating structure has to provide enough buoyancy to support the weight of the turbine and to limit the motions within acceptable limits.
Limitation of the various motions can be obtained with different principles. Floating platforms can be divided into three main categories (see Fig. 2) based on their strategy used to achieve static stability:
1) Ballast stabiliser - SPAR: stability achieved by using ballast weights positioned in the lower part of a buoyancy tank, which creates a righting moment and inertial resistance to pitch and roll motions.
2) Mooring lines stabiliser – Tension Leg Platform (TLP): stability achieved through the use of mooring line tension.
3) Buoyancy stabiliser - FLOAT: stability achieved through the use of distributed buoyancy, taking advantage of weighted water plane area for righting moment.
Fig. 2 – Typical floating platform systems

2. Design challenges
The main dynamic challenges of a floating wind turbine are related to the combined wave and wind loads and the choice of blade-pitch control strategy. Typically, the overall architecture of a floating platform will be determined by its static stability driving the design parameter for the stabiliser due to the large wind overturning moment. However, as well as the rotor thrust force, the motions due to waves and current interaction are also important parameters to be taken into consideration at design stage.
The two main important design parameters are the flexibility of the wind turbine to operate within certain limits (max motions’ amplitudes and accelerations) and the breaking strength of the mooring lines. Winds, waves and currents are stochastic problems and due to their nature the prediction of the whole system’s motions is difficult to analyse with mathematical models.

3. Economic aspects
In order to keep the cost of the wind turbine itself within acceptable value, the movements and the accelerations of the whole system shall be kept within proper limits to be addressed by the manufacturers.
The economics of floating offshore wind turbines will be mainly dictated by the additional cost of the floating stabiliser, mooring system and power distribution system, which could be offset by higher and steadier wind speeds, proximity to highly populated areas and greater public acceptance due to lower visual and noise impacts.
Although the characteristics of proven offshore floating platforms used by the oil & gas industry are similar to the concepts being considered for floating wind turbines platforms, it is their differences that will allow the necessary cost reductions according to the following points:
- Oil platforms must provide additional safety margin to provide permanent residences for personnel. Wind platforms do not.
- Oil platforms must provide additional safety margin and stability for oil spill prevention. This is not a concern with wind platforms.
- Wind platforms will be deployed in water depths up to around 200 m. Floating oil tension leg platforms range in depths from 450 m up to 1,000 m.
- Submerging wind platforms minimizes the structure exposed to wave loading. Oil platforms maximize them above water deck/payload area.
- Wind platforms will be mass-produced and will benefit from a steep learning curve.

4. Environmental considerations
Implementation of offshore wind farms is less influenced by noise and visual impact constraints than offshore farms however other important factors have to be taken into account. Commercial shipping lines, fishing and fish breeding, birds migratory lines, military restrictions, oil & gas industry, dredging and conservation areas are some of the most important ones. As floating wind turbines are not restricted by their physical position (no limits on sea depth), it would be easier to find a suitable site for a wind farm (greater choice of sites) satisfying all the constraints.
Impact on the seabed would be less intrusive then fixed bottom structures. Furthermore the noise produced during installation process would be highly reduced.
At the end of the life of a project, turbines can be easily removed without leaving any permanent trace in the environment.

5. Current and future projects
For better understanding of what is going on around the world a brief description of undergoing projects is described here below.
5.1 Norsk Hydro – Norway
Norsk Hydro – Norwegian waters off Karmøy.
This demonstration project “Hywind” uses a Spar-buoy concept. Hywind (shown in Fig. 3) is designed such that all modes of motions that are exited by the wave forces have natural periods outside the wave frequency range. At the same time the pitch restoring force is such that the static tilt is sufficiently small.
This kind of solution would be acceptable only for water depth above 150 m, where a slender deep draft hull makes static requirements easy to fulfil and keeps moderate the wave loads.
Fig. 3 – Hywind concept

5.2 SWAY – Norway
The SWAY concept (shown in Fig. 4), similar to the Hywind proposal, is based on a floating elongated pole extending far below the water surface with ballast at the bottom part [10].
This system consists of a floating foundation capable of supporting a 5 MW wind turbine in water depths from 80m to more than 300m. The motions at the top of the tower are sufficiently small to allow the wind turbine to function efficiently.
Fig. 4 – Sway concept

5.3 BlueH - Holland
Blue H Technologies launched last December the first ever large scale prototype Submerged Deepwater Platform (SDP) which has been anchored in 108 meters waters at a distance of approximately 11 nautical miles from the coast in southern Italy (see Fig. 5).
This prototype is now used, not only to test the assembly, launch, float-over and installation of the tension legged wind energy converter, but also to serve as a metering platform.
The construction of the first real scale unit is now underway and will be soon moored nearby the prototype, to be followed shortly thereafter by other more units reaching the final rated capacity of 92 MW.
Fig. 5 – BlueH concept

On the 10th of March 2008 BlueH submitted a nomination for lease to the US Minerals Management service to install a 420 MW commercial wind energy project located at approximately 25 nautical miles from shore off New Bedford (Massachusetts) in a water depth of about 55 meters.

5.4 WindSea - Norway
WindSea is working to a new idea. A triangular floating platform having three wind turbines (see Fig. 6). One at each corner of the platform. Each platform will have a total rated output of 10 MW.
Statkraft, NLI and FORCE are the three companies collaborating together for this project.
Fig. 6 – WindSea concept

5.5 First floating desalination platform powered by wind
The Aegean Sea has the first floating desalination platform in the world. Fig. 10 shows the system that consists in a floating wind turbine plus photovoltaic panels. It produces the necessary energy used to turn seawater into drinking water and it is built in such way that can operate in the most adverse weather conditions, while the platform can be moved to different islands to supply them with drinking water.
Fig. 7 – Desalination platform

6. Conclusion
Deepwater offshore wind development could become practical with a proactive R&D agenda involving close collaborations between the oil & gas industry and the offshore wind community. Technical viability and cost effectiveness of this new technology for large scale offshore applications will be critical to securing financing and insurance in the earlier stages.
Cost is still the key issue for offshore wind, but beyond 30-40 m of depth it is credible to predict that cost increases will diminish with development of new concepts and lessons learned.
However interest in deep-water offshore wind is growing and future implementation of it will be surely seen soon in countries like USA, Japan, Norway and China. The main driver for this new industry will be the greater choice of sites and countries, greater choice of concepts, greater flexibility of construction & installation procedures, easier installation, removal and decommissioning.


Posted by Filippo