An introduction

WindSea AS is developing, testing and plans commercialize a new, innovative offshore wind turbine platform. WindSea is owned by FORCE Technology and NLI, both leading players in their respective fields.

WindSea is a new concept for offshore wind turbine platforms. The concept is beeing developed using the accumulated experience of the founders in the offshore industry. Issues particular to offshore wind turbine deployment such as fabrication, access, installation, and maintenance are effectively addressed by the WindSea design.

The WindSea concept consists of a floating platform supporting 3 wind mills.

The configuration of the platform is a semi-submersible vessel with 3 corner columns. Each column supports one wind turbine.

The semi-submersible vessel is moored to the sea bottom with the mooring lines connected to a turret at the vessel geometric center. This configuration allows the vessel to rotate. The vessel is therefore always able to orient the turbines optimally towards the incoming wind.

Due to the large diameters of the mills and the relative small dimension of the vessel, one of the mills will be partly in the shadow of the 2 others. To minimize this shadow effect the tower of the downwind mill is made higher.

The ability of the vessel to rotate, leads to the advantage that the turbines do not need a traditional yaw-system that allows them to rotate individually.This reduces cost.

Continued Development

• The development phase that started in Q1 2010, includes further studies and engineering work and continued optimization of the concept prior to beginning fabrication of a prototype.
• The prototype phase includes detail engineering, fabrication, procurement, commissioning and installation at a test location.
• After the installation of the prototype, there will be further on location testing which will lead up to full commercialisation of the WindSea system.

The advantages of WindSea

Today, offshore wind turbine farms are deployed in fixed (to the seabed), monopile ,jacket or gravity base configurations. The bottom fixed foundation concept for offshore windmills can only be installed in sea depths of 40-60 m. maximum, whereas some floating platforms require a minimum water depth of 110-130 m. As the WindSea concept requires only approximately 45m water, it can be installed in water depths ranging from 45m up to several hundred meters. In theory there are no limits to the depth at which WindSea could be installed. Thus, the main advantages of the WindSea floating platform are:

• Location flexibility; floating platform in 45-700+ m. water depths
• On-shore fabrication; can be constructed in a ship yard, including turbine installation
• "Plug and Play installation; easy connection to the mooring system
• On-shore (or on site)maintenance

The WindSea platform is designed to give the offshore developer flexibility and lower costs, helping to improve the financial return of wind farm - something developers want.

Windsea - development phases

The development of WindSea is divided into phases. The first phase was the Validation of Concept and the last will be construction, installation and operation of a full scale pilot with a generator capacity of 3 x 3.6 MW. Currently, the WindSea project has completed the Validation of Concept phase with excellent results. During this period extensive analyses was performed, both with internal and external competencies. Further, extensive model tests has been performed including tests in a water tank, wind tunnel, and combined wind exposed tests in water tank. The Validation of Concept phase is focused on some of the technical challenges connected with floating offshore wind turbines. The test results were positive, and the project moved on to the next phase, which is focused on optimizing the design concept prior to construction of the prototype.

Technical and financial challenges

Globally, there are today relatively few, mature offshore wind turbine installations. Reliability is a challenge, since not all technical problems connected with offshore wind turbines are fully realised - such as gear box problems. Floating offshore wind turbines are also much more movable, which require much more of the equipment such as generators, gearboxes, etc. Furthermore, the choice of correct materials also plays an important role, as the risk of corrosion is much higher with offshore installation compared to onshore installations. The WindSea team has more than 30 years experience in the design of floating platforms. By leveraging this experience, the WindSea design gives predictable and smooth movements in all sea conditions, which minimizes adverse effects on the turbines. As a consequence of the WindSea design, standard offshore turbines can be used with the same estimated reliability as standard turbines.
Based on the present design and the utilization of medium size turbines (3 x 3, 6 MW) the calculations show that the concept will produce electrical energy at competitive cost.

Conclusions

The testing to date confirms the expected behavior of the platform.
For the given wind parameters it has been proved that the effect of the wind on the vessel motions is small, and that the effect of the motion on the power output of the turbines is negligible.
The tests results are compliant with the results obtained by Risø DTU, one of the leading wind-power research institutions in Europe and the calculations performed by the project team.
A more extensive program for testing other wave-wind combinations together with a more optimal model of the turbines may also be undertaken on the basis of those results. Calculations for the production potential of WindSea is based on wind data aquired from the Ekofisk oil field on the Norwegian Continental Shelfe (NCS) The Ekofisk oilfield is the longest producing oilfield on the NCS, producing since early 1970s

Critical components (turret, electrical connection for power export, anchors and mooring lines) are designed using existing, proven technology.
The wake effect and turbulence action on the downwind turbine have been calculated. Those investigations conclude on the feasibility of the chosen wind turbine configuration.
Model tests in wind tunnel and wave basin have confirmed the validity of the design.
The calculated power production for the Ekofisk wind data is of 41,4 GWh/year which represents 93% of the production of three 3,6 MW stand-alone turbines.

The present design has proven to be a sound basis for further development.