Field of the invention

The present invention relates to a floating wind turbine.

Background

A floating wind turbine is a wind turbine mounted on a floating structure that allows the wind turbine to generate electricity in water depths where bottom-mounted towers are not feasible. In the design of a floating wind turbine its dynamic behaviour, performance and costs of manufacture, operation and maintenance are important factors to consider.

Offshore wind turbines may be installed on floating structures when the water depth may be too large to accommodate a fixed structure. For example, when the water depth may be larger than 40 to 50 meters, a floating structure may be used to support the wind turbine. Such a floating structure may be a semi-submersible structure or a spar structure or a column structure or any other floating structure.

As regards the floating wind turbine's dynamic behaviour, it is important that the floating structure provides enough buoyancy to support the weight of the wind turbine, and sufficient stability to keep pitch, roll and heave motions within acceptable limits. Pitch and roll motions of the floating wind turbine may be effectively countered by increasing the dimensions of the floating structure. However, although greater dimensions results in a greater stability of the floating structure, they also result in higher manufacturing and/or installation and/or operating costs.

Summary of the invention

It is an object of the present invention to provide for an economically exploitable and stable floating wind turbine structure.

To this end, a first aspect of the present invention is directed to a wind turbine carrying semi-submersible comprising three buoyant stabilizing columns and a mooring system, said mooring system including at least one catenary mooring line that is connected to the semi-submersible at a point above an operational waterline thereof. The operational waterline may also be referred to as the sea level.

The presently disclosed floating wind turbine includes a floating structure in the form of a semi-submersible having at least three stabilizing columns. Such a semi-submersible combines a relatively shallow draft with overall good stability in both operational and transit conditions. This renders the semi-submersible more economical to commission and decommission than alternative structures, such as for instance deep-draft spars, or in themselves low-stability tension-leg platforms. The semi-submersible can be moored by means of one or more structurally simple and therefore cost-effective catenary mooring lines, which may be ballasted to provide additional tension and increase the floating structure's resistance to pitch and roll.

A particularly advantageous feature of the presently disclosed floating wind turbine is that the catenary mooring lines are connected to the semi-submersible at a point above an operational water line thereof, instead of below the operational waterline as in prior art semi-submersible floating wind turbine structures. Such a high connection of the mooring lines to the semi- submersible floating structure elevates the centre of gravity of the assembly, comprising the floating structure and the wind turbine positioned on it, and thus may reduce its stability. Contrary to what would have been expected, the overall effect on the stability is found to be surprisingly favourable. This may be because the high connection of the mooring lines to the semi-submersible reduces the mathematical arm between the point of application of the horizontal wind thrust force on the wind turbine and the points of application of the horizontal components of the restoring forces exerted by the mooring system, which may decrease the trimming moment on the assembly that tends to overturn it. Providing a connection of the mooring lines at or near deck level is contrary to the conventional way of connecting the mooring lines to a semi- submersible or a floating structure, i.e. at an underside of the submerged part of the structure.

In the following an example is given. The example is non -limitative and merely an illustration. As an example, for a 5 MW wind turbine, the horizontal wind load may be approximately 1 MegaNewton (MN) at 90 m above sea level. The horizontal restoring force may act at the level of the mooring line connection. If the mooring lines are connected at the bottom of the floater at e.g. 12 meter below the water line, the overturning moment may be 1 MN * (90m + 12m) = 102 MNm. If the mooring lines are connected at the main deck at 18 m above the water level, the overturning moment may be 1 MN * (90m - 18m) = 72 MNm, resulting in a more stable wind turbine carrying structure.

As a consequence of the increased stabilizing functionality of the mooring system, the semi-submersible wind turbine carrying structure may be dimensioned smaller, and may thus be manufactured and/or installed more cost effective and/or may be lighter and/or may be more easy and/or more cost effective in maintenance. Another aspect of the present invention is directed to a method of mooring a wind turbine carrying semi-submersible. The method includes providing a wind turbine carrying semi-submersible according to the first aspect of the present invention, and mooring the semi- submersible at an offshore location by means of the at least one catenary mooring line.

With regard to the terminology used in this text, the following may be noted. The term 'semi-submersible' as used in this text may be construed to refer to a type of specialized marine vessel - often used in the offshore industry as a basis for, inter aha, drilling, production, hotel services, crane operations, and wind turbines - having a waterplane area, i.e. the horizontal cross- sectional area of the vessel at the operational water line/water surface, that is smaller than the cross-sectional area of the vessel below the operational water line. The term 'catenary mooring line' may be construed to refer to a mooring line, typically in the form of a cable, chain or the like, that is configured to be supported only at its ends, such that the line, in between those ends, is freely suspended and describes a catenary. A 'catenary' is the curve that a hanging cable assumes under the influence of its own weight, when supported only at its ends. Mathematically, the catenary is defined by a hyperbolic cosine. The term 'catenary mooring line' may be construed to include a (ballasted) catenary mooring line that, at one or more positions along its length, has been provided with for instance steel or concrete clump weights to obtain specific

characteristics. The clump weights may in particular serve to reduce tension spikes in the mooring line, and to ensure that it pulls in a generally horizontal direction on an anchor attached thereto. A 'catenary mooring line' should be distinguished from a taut mooring line that is configured to work under large pre-tension.

These and other features and advantages of the invention will be more fully understood from the following detailed description of certain embodiments of the invention, taken together with the accompanying drawings, which are meant to illustrate and not to limit the invention.

Brief description of the drawings

Fig. 1 schematically illustrates, in a perspective view, an exemplary embodiment of a wind turbine carrying semi-submersible according to the present disclosure;

Fig. 2 schematically illustrates a side view of the wind turbine carrying semi-submersible of Fig. 1; and

Fig. 3 schematically illustrates a top view of the wind turbine carrying semi-submersible of Figs. 1 and 2. Detailed description

It is noted that the figures are only schematic representations of embodiments of the invention that are given by way of non-hmiting example. In the figures, the same or corresponding parts are designated with the same reference numerals.

For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.

The values given in this description are only given by way of example and illustration and for ease of understanding. They may not be considered limitative. Depending on various design parameters, the

dimensions and/or size of the semi-submersible wind turbine carrying structure may vary.

Figs. 1-3 schematically illustrate an exemplary embodiment of a wind turbine carrying semi-submersible 10 according to the present invention. Below its construction is described in general terms, where appropriate with reference to the Figures.

The semi-submersible 10 may support a wind turbine 30, which in itself may be of a conventional design. As in the depicted embodiment, the wind turbine 30 may, for instance, be a horizontal-axis towered wind turbine, including a tower 36 that extends upwards from a deck structure 16 of the semi-submersible 10. At its top the tower 36 may support a nacelle that accommodates a generator assembly, and that is rotatably connected to a rotor assembly. The tower 36 may have for example a height of approximately 50 - 100 m. The shaft of the nacelle may for example be positioned at a height of approximately 90 m above sea level. The rotor assembly may include a plurality of elongate rotor blades, typically having a length in the range of 30- 80 meters, which may be connected to a central hub by means of a pitch adjustment mechanism. The generator assembly may typically include a drive shaft that couples the rotor hub to a gear box, which in turn may be operably connected to an electrical generator. Accordingly, the rotor assembly may be operably connected to the generator assembly to enable the conversion of wind- induced rotational motion of the rotor blades into electrical energy. In other embodiments, the wind turbine may be of a different design, and for instance include a vertical-axis wind turbine. It will be clear that the physical

dimensions and mass of a wind turbine, which may be related directly to its capacity, may influence the dynamic behaviour of the semi-submersible 10 carrying it. Accordingly, the dimensions of the wind turbine 30 and the semi- submersible 10 may preferably be geared towards each other for optimal overall stabihty. In the proposed configuration, a ratio between an operational displacement of the semi-submersible (in tons) and an installed turbine power (in kW) may preferably be less than 1. Many variants of wind turbines may be possible.

The semi-submersible 10, i.e. the floating structure that supports or carries the wind turbine 30, may include at least three, optionally structurally identical, buoyant stabilizing columns 12. The stabilizing columns 12 may be mutually spaced apart, preferably such that - seen in a top view - they are arranged on the vertices of a regular polygon. In case the semi-submersible includes three stabilizing columns, they may be arranged on the vertices of an isosceles, and preferably equilateral triangle. The stabilizing columns 12 may have a generally square or square with rounded corners horizontal cross- section, as in the embodiment of Fig. 1. Alternatively, their transverse cross- sections may be otherwise polygonal, or circular or may have any other different cross-section. A diameter of a column 12 with a circular or a square cross-section may be approximately 8 m. At their top ends, the stabilizing columns 12 may be interconnected by and/or support a deck structure 16 that provides for a main deck 18. The semi-submersible 10 may be configured such that an operational waterline extends well below the deck structure 16, and the highest expected waves will not reach the bottom of the deck box. For example, a main deck of the deck structure 16 may be approximately 18 m above the sea level. The wind turbine 30 may be connected to the deck structure 16 and extend upwards from the main deck 18. Seen in a top view, the wind turbine 30 may be preferably be symmetrically disposed at a geometric centre of the three stabilizing columns, as in the embodiment of Fig. 1. In an alternative embodiment, the wind turbine 30 may be asymmetrically disposed on top of one of the stabilizing columns 12. For example, when the columns 16 are in a polygonal configuration with the deck structure 16 interconnecting inbetween, the radius of the centre of the column 12 with respect to the middle of the semi-submersible structure 10 may be e.g. 36 m. Of course, the wind turbine carrying structure may be smaller or larger. The draft of a column 12, i.e. the length of the column 12 extending below the waterline, may be approximately 12 m, or may be smaller or larger in other embodiments. The height of the deck structure 16 may be approximately 6 m, or smaller or larger depending on the embodiment. Depending on various design conditions, such as the weight and/or height of the wind turbine and/or environmental conditions, such as wind, waves, water level etc. the dimensions and size of the semi-submersible wind turbine carrier may vary.

The semi-submersible wind turbine carrying structure 10 may be installed in an offshore wind farm. The various wind turbine supporting structures 10 may then be interconnected to each to transport the produced electricity to a hub and/or a central platform and/or to land. Thereto, two electricity transport cables 20 are provided. The transport cables 20 may be on the seabed and may rise at each wind turbine supporting structure 10. One transport cable may be an input line and one transport cable may be an output line going to the next wind turbine supporting structure.

At least one of the stabilizing columns 12 may be provided with a boat landing structure, and with a staircase extending therefrom to reach the main deck 18. In addition, the lower end of one or more of the stabilizing columns 12 may be provided with a transversely extending damper box 14 that provides for hydrodynamic added mass and damping, and so for improved heave, pitch and roll motions.

The semi-submersible 10 may also include a mooring system 50 for securing the semi-submersible at a desired offshore location to the sea bottom. The mooring system 50 may be able to resist environmental loads while allowing for first order wave motions. The mooring system 50 may include at least one catenary mooring line 54, which may be made of any suitable material, such as, for instance, metal chain, wire rope, artificial fibers (e.g. polyester) or combinations thereof. Depending on the water depth, the mooring line may have an other configuration than catenary, e.g. straight.

At one end, each mooring line 54 may be connected to an anchor, e.g. a drag embedment anchor, that is to be placed on the seabed. Other anchoring structures may be possible, There may be anchors that embed into the seabed. At the other end, the respective mooring line 54 may be connected to the semi- submersible 10 at a point above an operational waterline70 thereof. In one embodiment, the at least one mooring line 54 may be connected to the deck structure 16, preferably at main deck level 18. The connection of the mooring line 54 may be for example some 18 m above waterline. Such an

unconventionally high connection of the mooring line 54 to the semi- submersible 10 may affect the latter's stability in various, complex ways. The overall effect for the present design, however, appears advantageous. Without wishing to be bound to theory, a key factor in the interplay between the different effects and forces may, in simplified terms, be understood as follows.

During operation, wind may exert a thrust on the wind turbine at a vertical level high above the operational waterline 70. This thrust may cause horizontal downwind displacement of the semi-submersible 10, and

additionally generate a moment that tends to tilt or trim it. As the semi- submersible 10 displaces and tilts, both its stabilizing columns 12 and its catenary mooring system 50 develop restoring forces. The horizontal restoring forces generated by the mooring system 50, however, disadvantageously cooperate with the wind thrust in tilting the semi-submersible 10. By

attaching the at least one catenary mooring line 54 to the semi-submersible 10 at a relatively high location, the distance between the points of application of the horizontal wind thrust force on the wind turbine 30 and the points of application of the horizontal components of the restoring forces exerted by the mooring system 50 is reduced. This, in turn, reduces the effective moment on the semi-submersible 10 that tends to overturn it. As a result of the thus increased stabihzing functionality of the mooring system 50, the semi- submersible 10 may be dimensioned smaller, and thus be manufactured cheaper. In a preferred embodiment, for instance, a ratio between a squared distance between two adjacent stabilizing columns 12 and an effective outer diameter of the respective stabihzing columns 12 may be between 400 and 600, e.g. about 500. Where a stabilizing column 12 has a polygonal cross-sectional shape, the diameter of its circumscribed circle may be taken as its effective diameter.

The number of mooring lines 54 of the mooring system 50 may vary for different embodiments, and typically be between three and twelve. In a preferred embodiment, the mooring system 50 may include at least as many catenary mooring lines 54 as there are stabilizing columns 12. This allows each stabilizing column 12 of the semi-submersible 10 to be associated with at least one mooring line 54, which may then be connected to the semi-submersible 10 at that respective, associated stabilizing column 12 to obtain a symmetric mooring configuration.

In particular when the catenary mooring lines 54 are connected to the deck structure 16 of the semi-submersible 10 at main deck level 18, the semi-submersible 10 may include one or more mooring line guidance chutes or bending shoes 52 via which a respective catenary mooring line 54 may be angled down and outward from the deck structure 16 to the sea bed. A mooring line guidance chute 52 may preferably include a smoothly curved, downward sloping bottom surface. A longitudinal cross-sectional profile of the bottom surface may describe an elliptical, preferably circular, arc subtending an angle in the range of 70-90 degrees to prevent excessive bending of a guided mooring line 54, so as to limit wear thereof.

The semi-submersible 10 may further include one or more tensioning device that enables adjustment of the length of, tension in, and/or initial departing angle from the semi-submersible, relative to the main deck 18 of individual mooring lines 54. A tensioning device may, for instance, include a chain jack, windlass or a winch. In case of a three line mooring system, a tensioning device for only one of the lines may be sufficient to adjust the tension in all three lines up to the required level. A chain cable can also be tensioned away from the semi-submersible by an anchor handling tug, using e.g. a Vrijhof Stevtensioner. The tensioning device, such as a winch, may be present on the semi-submersible carrying structure, and/or may be only brought aboard when needed. In fact, tensioning of the mooring lines may be done once at installation of the semi-submersible carrying structure 10, for example by adjustment of the length of at least one of the mooring lines.

During use, e.g. due to wear, corrosion, stretch etc, the tension in the mooring lines may become less. When required, the mooring system may be re- tensioned e.g. after some years of use. To that a tensioning device may be required, Such a tensioning device may be positioned inside the deck structure 16. However, such a tensioning device may also be brought on board and/or be provided by a support vessel when it may be needed.

Contrary to some prior art mooring systems, there is no active mooring system provided. Such an active mooring system controls the tensioning of the mooring lines during operation, i.e. there is a more or less continuous feedback of the mooring line tension and a more or less continuous adjustment thereof. Such an active mooring system, or an active tensioning device, is not provided in the semi-submersible wind turbine carrying structure 10 according to the invention. To the contrary, the mooring system of the semi-submersible structure 10 is a passive mooring system. During normal operations, the mooring system is passive and the connection of the mooring lines to the semi- submersible floater 10 is fixed.

Although illustrative embodiments of the present invention have been described above, in part with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the

embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an

embodiment" in various places throughout this specification are not

necessarily all referring to the same embodiment. Furthermore, it is noted that particular features, structures, or characteristics of one or more embodiments may be combined in any suitable manner to form new, not explicitly described embodiments.

List of elements

10 semi-submersible

12 stabilizing column

14 damper box / footer plate 16 deck structure

18 main deck

20 electricity cables 30 wind turbine

32 rotor assembly

34 generator assembly

36 tower 50 mooring system

52 mooring line guidance chute

54 catenary mooring line

70 operational waterline