Technologic Area:

The present invention relates to floating offshore wind turbines to be used in offshore wind energy and marine renewable energy sectors.

It relates to a submerged floating platform for offshore wind turbines designed to decrease the cost of the platform and to decrease the oscillations due to forces and rotations under various wind and wave conditions in all 6 degrees of freedom (DoF).

BACKGROUND ART:

Wind turbines installed on the sea can generate more power than their land-based counterparts because high wind speeds can be reached at lower altitudes. However, in order to feel this high wind speed, it is necessary to move at least a few kilometers away from the coastline. In regions of the world where water depths exceed 50 meters in a short distance, fixing the turbine towers to the seabed is not economically feasible. Therefore, installing offshore wind turbines by placing them on floating platforms is an important option.

Placing vertical or horizontal axis wind turbines on a floating platform and connecting this platform to the bottom with chains or ropes makes it possible to obtain a stable and economical wind power plant against factors such as wind, waves, and currents in the open sea. These turbines are called Floating Offshore Wind Turbines (FOWT). FOWTs consist of a turbine, a floating platform, and connections to the bottom. The most crucial part in terms of stability is the floating platforms. Floating platforms are connected to the seabed at several points with keel (catenary) or taut anchor ropes or chains. Although different platforms have been developed for floating wind turbines in the last 10-15 years, some typical platform geometries have come to the fore. These are the semi-submerged platform, the spar platform, and the tension-legged platform (TLP).

Semi-submerged platforms are stabilized by buoyancy and the large platform area on the water's surface. They usually consist of 3 semi-submerged cylinder columns. Liu et al. (2016) summarized the developments in the semi-submersible platform and reviewed different designs working on the principle of the semi-submersible platform.

The stability of the spar-type platform is provided by the deep water cut (draft) and by pulling the center of gravity down with the help of ballast (concrete or water) placed at the bottom of the structure. Improvements in the spar are made by connecting the mooring chains in the form of a delta, reducing the draft, or attaching swinging wings to the bottom.

The tension-legged platform (TLP) works on the principle of balancing the load with the mooring chains. The turbine tower is fixed to the platform under the water, while the platform is anchored to the bottom with taut steel cables. The platform can be crossshaped or rectangular or in the form of columns similar to a semi-submersible as in Gicon- TLP.

Gouppe et al. (2014) conducted an experimental study and compared semi-submerged, spar, and tension-legged platforms (TLP). They found that the spar has less longitudinal translation (surge) motion than the others, while the tension-legged type is advantageous in pitch motion. Bagbanci et al. (2015) compared spar and semi-submerged types with the help of numerical modeling. The study results found that longitudinal translation and heave movements are more common in semi-submerged platforms. The barge-type platform, which works on the principle of buoyancy like a semi-submersible, is the most straightforward design in terms of production. Still, since the platform is entirely on the sea surface where the wave is most effective, it is very much affected by wave forces, creating problems in stability and stable energy production (Jonkman and Matha, 2011 ). TLP appears stable in comparative studies (Oguz et al. 2018, Nihei et al. 2014). However, its attachment to the bottom with a large number of taut steel cables creates reservations regarding both economy and application in the deep sea (Jonkman and Matha, 2011 ). Therefore, each of these typical platforms has advantages and disadvantages, and efforts are ongoing to achieve a more stable and economical floating platform design for the commercialization and widespread deployment of floating wind turbines. The targeted platform should be resistant to rotations like a semi-submersible, up and down displacement like a spar, and not have a large water surface area like a barge-type platform. For this reason, three floats are used in the newly developed platform to compensate for rotational moments, but the floats were designed as fully submerged. Ballast weights and weight disks were used to reduce up and down displacement, but they were connected to the tower and floaters with chains. Using chains to connect the ballast weight to the platform aims to decrease the platform's cost.

The Chinese patent document CN214356565 (U) encountered in the literature research mentions a completely submerged platform. In the Chinese patent document, it is seen that three disks are used in the lower section. Since these disks are in a horizontal position, they allow only uniaxial balancing. In addition, rigid connecting rods are used between the weights instead of chains. This prevents the platform from remaining balanced due to wave-induced movements from different axes.

The PCT patent document numbered WO2021211121 (A1 ), which was encountered in the literature search, describes a floating submerged platform that is entirely underwater. No disk is used in the mentioned floating submerged platform. Only weights are used. These weights do not provide balancing performance in three different axes. For this reason, there is a high probability of encountering balance problems on the platform.

The Chinese patent document CN214401876 (U), which was encountered in the literature search, describes a semi-submersible platform for floating wind turbines. Unlike the platform subject to the description, the mentioned platform is semi-submerged. It also needs a design to balance the wave forces coming from three axes. For this reason, it is possible to encounter balance problems, especially excessive up and down movement. The rotations and displacements caused by wind and wave forces cannot be counterweighted sufficiently in the known floating submerged platforms. There is no design for balancing these forces in six degrees of freedom.

As a result, there is a need for a new floating platform where the known state of the technique is overcome, and its disadvantages are eliminated.

SUMMARY:

The present invention relates to a floating submerged platform that fulfills the abovementioned requirements, eliminates disadvantages, and brings additional advantages.

Based on the state of the art, the invention aims to provide a new floating platform that eliminates the stability problems encountered in offshore wind turbines caused by wind and wave-induced triaxial forces and rotational moments.

Another object of the invention is to provide a fully submerged floating platform to move the floating platform away from the water surface where wave forces are most effective.

Another aim of the invention is to provide a new floating submerged platform that, thanks to its weight disks on different axes, compensates for the various axial forces experienced by the wind turbine.

Another object of the invention is to provide a new floating submerged platform with improved balancing performance by connecting the weight disks with chains that allow freedom of movement as a counterweight instead of rigid rods.

The inventive design incorporates the best features of the three leading existing platforms. These are:

• Since the platform is submerged, the effects ofwave forces are kept to a minimum. • A ballast weight reduces pitch motion, which is especially important in generating stable energy. However, the stabilizer weight was not placed in a long draft of the platform as in the SPAR. The ballast weight was suspended from the turbine tower with chains. Therefore, a much more economical platform was obtained.

• As a result of the spatial spreading of the platform with three floaters, the pitch and roll oscillations are again reduced.

• To reduce the heave (in the direction of gravity), the weight disks used at the bottom of the semi-submersible columns in the existing technologies are suspended on chains in the invention.

• In the comparison studies using the numerical model, under the same conditions, the oscillations of the invented platform are less, and the cost is lower than the existing platforms like semi-submersible and spar.

BRIEF DESCRIPTION OF THE DRAWINGS:

The invention will be described with reference to the accompanying figures so that the features of the invention will be more clearly understood. However, this is not intended to limit the invention to these particular embodiments. On the contrary, it is also intended to cover all alternatives, modifications, and equivalents of the invention which may be included within the field defined by the appended claims. It is to be understood that the details shown are shown for the sole purpose of describing preferred embodiments of the present invention and are presented to provide the most useful and readily understood description of both the embodiment of the methods and the rules and conceptual features of the invention. In these drawings:

Figure 1 shows schematically in perspective view a detail of the present invention which is a submerged floating platform for offshore wind turbines according to an embodiment. The numbered parts in Fig.1 are explained below: Explanations:

• Turbine tower

• Legs to connect floaters to the tower

• Floater

• Ballast weight

• Chains

• weight disks

Detailed description of embodiments:

In this detailed description, the inventive floating submerged platform is explained only for a better understanding of the subject matter, with examples that will not have any limiting effect. The description describes a floating submerged platform with a design realized to reduce the up-and-down oscillation motion and rotational moments caused by sea waves and wind and increase its stability capability.

The floating platform, which is the subject of the present invention is fully submerged, including the part of the legs connecting the floaters and the tower. The wind turbine is placed on the turbine tower (1 ) in the platform's uppermost part. Three submerged floaters (3) are connected to the tower through three inclined legs (2). The number of floaters and the angle of the legs can vary. However, the best stability performance is achieved with the design seen in Figure 1. Floaters (3) are connected to a ballast weight (4) through a bundle of chains (5) in the platform.

While the weight discs (6) are generally positioned at the bottom of the floaters or the ballast weight in the existing platform technologies and called heave disks, in the present innovative platform, they are used both on the inclined chains (5) between the floaters (3) and the balancing weight (4) and the vertical chains (5) between the turbine tower (1) and the balancing weight (4). While the traditional heave disks can decrease the motion only on the vertical axis (heave motion), in this invention, they can balance the forces and rotations in 6 DoF since the weight discs (6) are positioned at different angles.

A ballast weight is used in the existing floating platform technologies. For example, in a spar platform, the ballast weight is placed at the bottom of the extended cylinder platform structure. It may also be suspended by rigid legs. In the present invented platform, the ballast weight (4) is connected to the tower(1 ) and floaters(3) through a bundle of chains(5).

In the present invention, at least one weight disc (6) is used on the chain (5) between the turbine tower (1) and the ballast weight (4). Weight disks (6) are positioned on the chain (5) to be perpendicular to the chain. However, it is also possible to place weight discs (6) on the chain at different angles changing between 0-90°. Different angles of weight disks may counterweight the forces in different directions. In addition, it is also possible to change the number of chains in a bundle and the diameter of the chains. Moreover, various types of material can be used in the floaters and the chains.

Similarly, in the prevent invention, at least one weight disc (6) is used on the chain (5) between the floaters (3) and the ballast weight (4). Weight disks (6) are positioned on the chain (5) to be perpendicular to the chain. However, it is also possible to place weight discs (6) on the chain at different angles changing between 0-90°. Different angles of weight disks may counterweight the forces in different directions. In addition, it is also possible to change the number of chains in a bundle and the diameter of the chains. Moreover, various types of material can be used in the chains.

Furthermore, in the present invention, there is also one weight disk (6) at the bottom of the ballast weight (4). The weight disk is similar to the heave disk and is used to decrease the motion in the Z direction.

Therefore, the platform has weight disks positioned at different angles that counterweight the forces in different directions.