NL2010709C2

NL2010709C2 - Floating wind turbine.

Info

Publication number: NL2010709C2
Application number: NL2010709A
Authority: NL
Inventors: Johannes Wilhelmus Jacobus Mikx; Fons Adriaan Huijs
Original assignee: Gustomsc Resources B V
Priority date: 2013-04-25
Filing date: 2013-04-25
Publication date: 2014-10-29

Description

Title: Floating wind turbine

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 behavior, performance and costs of manufacture and operation are important factors to consider.

As regards the floating wind turbine’s dynamic behavior, 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 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.

To this end, a first aspect of the present invention is directed to a wind turbine carrying semi-submersible comprising three buoyant stabihzing 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 presently disclosed floating wind turbine includes a floating structure in the form of a semi-submersible having at least three stabihzing 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 at or below the operational waterline as in most conventional floating structures. Although such a high connection of the mooring lines to the semi-submersible elevates the center of gravity of the assembly and thus disadvantageously reduces its stability, the overall effect on its stability is surprisingly favorable. This is mainly 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 decreases the trimming moment on the assembly that tends to overturn it. As a consequence of the increased stabilizing functionality of the mooring system, the semi-submersible may be dimensioned smaller, and thus be manufactured cheaper.

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 alia, 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 ihustrates 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

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 rotor assembly may include a plurabty 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 stability. 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.

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 precisely 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 (rounded) square transverse cross-section, as in the embodiment of Fig. 1; alternatively, their transverse cross-sections may be otherwise polygonal, or circular.

At their top ends, the stabilizing columns 12 may be interconnected by and 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 main deck 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 center 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.

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 additional stability.

The semi-submersible 10 may also include a mooring system 50 for securing the semi-submersible at a desired offshore location. The mooring system must 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. At one end, each catenary mooring line 54 may be connected to an anchor, e.g. a drag embedment anchor, that is to be placed on the seabed. At the other end, the respective catenary mooring line 54 may be connected to the semi-submersible 10 at a point above an operational waterline thereof. In one embodiment, the at least one catenary mooring line 54 may be connected to the deck structure 16, preferably at main deck level 18. Such an unconventionally high connection of the catenary mooring line 54 to the semi-submersible 10 affects 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 the (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 catenary 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 stabilizing 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 stabilizing 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 catenary 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 catenary mooring line 54, which may then be connected to the semi-submersible 10 at that respective, associated stabibzing 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 devices may, for instance, include a chain jack, windlass or a winch.

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 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

Claims

1. Een windturbine (30) dragend half-afzinkbaar vaartuig (10) omvattende drie stabiliserende kolommen (12) en een verankeringssysteem (50), waarbij genoemd verankeringssysteem ten minste één catenaire meerlijn (54) omvat die met het half-afzinkbare vaartuig is verbonden op een punt boven een operationele waterlijn (70) daarvan.A wind turbine (30) bearing semi-submersible vessel (10) comprising three stabilizing columns (12) and an anchoring system (50), said anchoring system comprising at least one catenary mooring line (54) connected to the semi-submersible vessel at a point above an operational water line (70) thereof.

2. Het windturbine dragende half-afzinkb are vaartuig volgens conclusie 1, waarbij elk van de stabiliserende kolommen (12) van het half-afzinkb are vaartuig (10) is geassocieerd met een catenaire meerlijn (54) die met het half-afzinkb are vaartuig is verbonden aan een respectieve stabiliserende kolom.The wind turbine-carrying half-sink vessel according to claim 1, wherein each of the stabilizing columns (12) of the half-sink vessel (10) is associated with a catenary mooring line (54) associated with the half-sink vessel is attached to a respective stabilizing column.

3. Het windturbine dragende half-afzinkb are vaartuig volgens conclusie 2, waarbij de drie stabiliserende kolommen (12) zijn geplaatst op de hoeken van een gelijkbenige, en bij voorkeur gelijkzijdige driehoek.The wind turbine-bearing semi-submersible vessel according to claim 2, wherein the three stabilizing columns (12) are positioned at the corners of an isosceles, and preferably equilateral, triangle.

4. Het windturbine dragende half-afzinkb are vaartuig volgens een van de conclusies 1-3, waarbij - bezien in een bovenaanzicht - de windturbine (10) op een geometrisch middelpunt van de stabiliserende kolommen (12) is geplaatst.The wind-sinking semi-submersible vessel according to any one of claims 1-3, wherein - viewed in a plan view - the wind turbine (10) is positioned at a geometric center of the stabilizing columns (12).

5. Het windturbine dragende half-afzinkb are vaartuig volgens een van de conclusies 1-4, waarbij de drie stabihserende kolommen (12) een dekconstructie (16) ondersteunen die de drie stabiliserende kolommen onderling boven de operationele waterlijn verbindt, en waarbij de ten minste ene catenaire meerhjn (54) is bevestigd aan de dekconstructie (16).The wind turbine-bearing semi-submersible vessel according to any of claims 1-4, wherein the three stabilizing columns (12) support a deck structure (16) connecting the three stabilizing columns to each other above the operational water line, and wherein the at least one catenary joint (54) is attached to the deck structure (16).

6. Het windturbine dragende half-afzinkb are vaartuig volgens conclusie 5, waarbij de dekconstructie (16) een bovenste hoofddek (18) verschaft, en waarbij de ten minste ene catenaire meerlijn (54) aan het half-afzinkbare vaartuig is bevestigd (10) op hoofddek-niveau.The wind turbine-bearing semi-submersible vessel according to claim 5, wherein the deck structure (16) provides an upper main deck (18), and wherein the at least one catenary mooring line (54) is attached to the semi-submersible vessel (10) at main deck level.

7. Het windturbine dragende half-afzinkbare vaartuig volgens conclusie 5 of 6, voorts omvattende een meerlijngeleidingsgoot met een vloeiend gekromd, naar beneden hellend bodemoppervlak.The wind turbine-bearing semi-submersible vessel according to claim 5 or 6, further comprising a multi-line guide trough with a smoothly curved, sloping bottom surface.

8. Het windturbine dragende half-afzinkbare vaartuig volgens conclusie 7, waarbij een longitudinaal dwarsdoorsnedeprofiel van genoemd bodemoppervlak een elliptische, en bij voorkeur cirkelvormige, boog beschrijft die een hoek in het bereik van 70-90 graden onderspant.The wind turbine-bearing semi-submersible vessel according to claim 7, wherein a longitudinal cross-sectional profile of said bottom surface describes an elliptical, and preferably circular, arc that underlies an angle in the range of 70-90 degrees.

9. Het windturbine dragende half-afzinkbare vaartuig volgens een van de conclusies 1-8, waarbij de windturbine (30) een op een mast geplaatste windturbine met een horizontale as is.The wind turbine-bearing semi-submersible vessel according to any of claims 1-8, wherein the wind turbine (30) is a wind turbine with a horizontal axis placed on a mast.

10. Het windturbine dragende half-afzinkbare vaartuig volgens een van de conclusies 1-9, waarbij een verhouding tussen een operationele verplaatsing van het half-afzinkbare vaartuig (10) (in metrische tonnen) en een geïnstalleerd vermogen van de windturbine (30) (in kW) kleiner is dan 1.The wind-turbine-bearing semi-submersible vessel according to any of claims 1-9, wherein a ratio between an operational displacement of the semi-submersible vessel (10) (in metric tons) and an installed power of the wind turbine (30) ( in kW) is less than 1.

11. Het windturbine dragende half-afzinkbare vaartuig volgens een van de conclusies 1-10, waarbij de stabiliserende kolommen (12) een in hoofdzaak identieke vorm hebben, en waarbij, voor elk paar van twee stabiliserende kolommen, een verhouding tussen, enerzijds, een afstand tussen de respectieve twee stabiliserende kolommen in het kwadraat, en anderzijds, een buitendiameter van de stabihserende kolommen, ligt tussen 400 en 600.The wind turbine-bearing semi-submersible vessel according to any of claims 1-10, wherein the stabilizing columns (12) have a substantially identical shape, and wherein, for each pair of two stabilizing columns, a ratio between, on the one hand, a distance between the respective two stabilizing columns squared, and on the other hand, an outer diameter of the stabilizing columns, is between 400 and 600.

12. Werkwijze, omvattende: • het verschaffen van een windturbine dragend half-afzinkbaar vaartuig (10) volgens een van de conclusies 1-11; en • het afmeren van het half-afzinkbare vaartuig (10) op een offshore locatie door middel van de ten minste ene catenaire meerhjn (54).A method, comprising: • providing a wind turbine-bearing semi-submersible vessel (10) according to any of claims 1-11; and • mooring the semi-submersible vessel (10) at an offshore location by means of the at least one catenary lake (54).