Area of the invention

The present invention relates to a floating windmill construction comprising a foundation that floats in a body of water, where the floating foundation supports at least one tower equipped with a turbine, and said floating foundation comprises a submerged, elongated and stabilizing buoyancy part attached to the lower part of the tower. The floating foundation further comprises one or more outwardly extending stabilizer arm(s) that extend out in a direction transverse with respect to the elongated and stabilizing longitudinal direction of the buoyancy part, as said stabilizer arm penetrates the surface of the body of water.

The description relates to an invention in the technical field of offshore wind energy. A technical solution is described for achieving sufficient stability of a floating foundation for one or more windmills, also referred to as a platform or floating windmill construction.

Background of the invention

In order to reduce pollution and limit global warming, it is desirable to develop technologies that can make renewable energy more competitive. Offshore wind is a particularly interesting resource related to renewable energy recovery. There is great potential in utilizing ocean areas and the wind over such areas to extract electrical energy. One challenge related to the extraction of energy from offshore wind is the cost. In order for offshore wind to be a competitive energy source, it is crucial that costs related to production, installation and operation and maintenance of offshore wind turbines are reduced. To achieve this goal, it is important to develop new, cost- effective technology.

Many technical solutions have been developed for the production of offshore wind energy. Some of these are related to solid foundations, while others are related to floating foundations. The floating foundations, in many cases, are advantageous, especially for use at ocean depths, typically over 50 meters.

At sea, an offshore wind turbine and its foundation are exposed to strong forces from wind, waves and currents. Description of prior art

Many technical solutions have been developed for the production of offshore wind energy. Some of these are related to bottom-fixed foundations, where the lower structure is rigid, and rotary attached to the seabed. Typical but not limited names of such devices are Monopile, Tri-pod, Jacket, suction caisson and gravity base. While the upper part consists of a transition piece which is mounted on to the windmill tower with associated windmill.

Another main type is floating foundations which are anchored to the seabed. There are also several variants which, typically, but not limited terms are Barge, Semi- Submersible, multi-spar, spar and tension-leg platform. The majority of said devices are anchored in a manner which means that they cannot rotate but use a nacelle which can rotate relative to the tower.

Floating platforms are another type designed in a way that allow them to rotate about the anchorage point, further referred to as (rotating platforms). This type of device will be able to allow more of a wind turbine as it adjusts itself to the wind, which means that the turbines do not come in the shadow of each other, referred to as a waking effect.

Stability to be able to support a turbine is a challenge and cost driver for floating foundations, especially for floating foundations that are to be used in areas with potential for violent wind and wave conditions. In addition to the design of the floating foundation, how it is anchored is an important part of how it will behave under demanding conditions and thus be part of the overall cost picture for a wind farm. As a general rule, "tight" moorings will provide less movement on the platform, while slack moorings, often referred to as "catenary anchoring", will allow the platform significantly greater movement. The movements of a platform are mainly controlled by the influence of wind, as the wind will push the platform in the direction of the wind. Waves will also be able to contribute to significant movements on a floating platform. The waves will give a typical back and forth movement, in combination with up and down. Waves and wind will not always have the same direction, which makes the motion picture for a platform somewhat complicated.

The movement of the platform in the horizontal plane is often referred to as the "watch circle", which describes how much one can assume that the platform moves in relation to an imaginary center point. The advantage of keeping the watch circle as small as possible is to limit dynamic stresses on, for example, electrical cables that are connected to a platform. Furthermore, it is easier to plan optimization of a wind farm if the platform is as stationary as possible under changing weather conditions. Normally, one will see that the more rigid the anchors, the greater loads one can expect in connection with the anchoring systems, which can be cost-driving and thus negative.

Existing platform solutions are largely based on designs known from the oil industry. Of the main categories, one has the Tension Leg Platform (TLP), as an example, it can be referred to the TetraSpar floater. The main principle of a TLP platform is that it is stretched taut against a seabed, preferably with 3-6 chains or equivalent mooring lines or stays. The rigid clamping is achieved by using the buoyancy of the platform, where the clampings are a result of the platform pulling upwards in a body of water, while the anchorage holds it at a preset depth, set by the length of the anchor lines. This tight tension helps to limit the movement of the platform horizontally and vertically under the influence of, for example, waves and wind.

Another main type is a Semi-Submersible which typically floats on a water surface, where stabilization solutions normally include at least three buoyancy towers that penetrate the water surface, where the buoyancy towers are mounted on a rigid platform where the main stability is a result of the area of the buoyancy towers in the water surface and the distance between the towers.

As an example of a Semi-Submersible, Wind Float can be highlighted. It consists of three towers that penetrate a surface where the tower is mounted on the top of one of the buoyancy towers. Furthermore, it is anchored with a set of mooring lines that are normally loosely attached between the floating foundation and anchoring on the seabed. A Star Wind Floater is another type of Semi-Submersible type platform, which consists of three buoyancy towers in a star formation. There is a tower in the center of the floating foundation where a windmill tower is mounted.

Spar-Buoy is another type of floating foundation, an example of such a solution is known as "Hywind".

The offshore wind solution «Hywind» includes an example of a foundation for an offshore wind turbine, where the foundation is a floating vertical spar buoy. To counteract tilting forces, the spar buoy reaches deep into the sea in an operational position, and it is often very heavy. The weight is largely due to the spar buoy solution's need for corrective ballast. Its size makes it expensive to manufacture and transport to a destination at sea, and it means that "Hywind" cannot be used in ocean areas of limited depth.

The different types of platform design and anchoring systems, have their clear advantages and disadvantages. Then in terms of costs, but also how the platform moves under varying weather conditions.

A Semi-Submersible platform with catenary anchoring will typically have, among other things, these advantages over a TLP platform:

• The platform maintains stability in the event of a break in one or more of the anchor lines, while a TLP here may suffocate.

• The platform will be able to be towed as it is to the final installation site, while a TLP will probably require special vessels.

• In some cases, the mooring systems will be cheaper.

• The tower and the nacelle will normally be able to be mounted while the platform is lying afloat, for a TLP a special vessel will normally be required or else the whole assembly must be put together on land.

A TLP platform will typically have, among other things, these advantages over a Semi-Submersible platform with catenary anchoring:

• Less structural mass

• Easier to industrialize

• Does not need expensive dynamic cables due to the rigid clamping

• No need for active ballasting to maintain stability.

US2009/091 136 A1 relates to a floating windmill construction, comprising of a foundation that floats in a body of water, the floating foundation supports a tower equipped with a turbine, and that said floating foundation is comprising a submerged, elongated and stabilizing buoyancy part arranged substantially vertically floating in the body of water and which is anchored to a bottom-fixed foundation.

KR 101956032 B1 shows a floating foundation which comprises stabilizing buoyancy parts arranged to float vertically in a body of water. The buoyancy part is anchored to the seabed by means of anchoring lines. Objects of the present invention

It is an object of the invention to provide a floating windmill construction which solves many of the challenges one can see in current constructions in relation to costs related to production, installation and operation and maintenance throughout the life of the construction.

This document describes a technical solution that has advantages over prior art in that one combines the known methods Tension Leg Platform design with the known Semi-Submersible platform designs. This with the intention of combining all the benefits of the two designs into one design. Furthermore, the invention has solved many of the disadvantages of the two previously described designs, in that one design solves the disadvantages of the other design and vice versa.

Summary of the Invention

The above object is achieved with a floating windmill construction comprising a foundation that floats in a body of water, where the floating foundation supports at least one tower equipped with a turbine, and said floating foundation comprises a submerged, elongated and stabilizing buoyancy part attached to the lower part of the tower. The floating foundation is further comprising one or more outwardly extending stabilizer arm(s) that extend in a direction transverse in relation to the stabilizing, longitudinal direction of the buoyancy part, as said stabilizer arm penetrates a water surface of the body of water. The submerged, elongated and stabilizing buoyancy part is arranged to float horizontally in the body of water and is rigidly anchored to one or more bottom-fixed foundations extending from a seabed.

Alternative embodiments are specified in the respective dependent claims.

A buoyancy body can be mounted on the free end of the extending stabilizer arm and extend up or down into the body of water.

In a first embodiment, the outwardly extending stabilizer arm can extend from the lower part of the tower in a substantially horizontal direction and in an area above the body of water.

In a second embodiment, the outwardly extending stabilizer arm can be immersed in the body of water and extend from the elongated and stabilizing buoyancy part in a substantially horizontal direction. The buoyancy body of the stabilizer arm can be equipped with a suspended weight.

Furthermore, the floating foundation can be rigidly anchored to the bottom-fixed foundation by a first rigid anchoring running from a first anchoring point on the elongated and stabilizing buoyancy part down to the bottom-fixed foundation, and by a second rigid anchoring running from a second anchoring point on the elongated and stabilizing buoyancy part and down to the bottom-fixed foundation.

Said first and second anchoring points are usually at respective distal ends of the elongated and stabilizing buoyancy part.

Furthermore, said first and second anchoring can be anchoring lines which can be tightened, or tension rods in the form of rigid pipes.

The first and second anchors can be tightened to form a rigid connection between the extended and stabilizing first anchoring point of the buoyancy part, the bottom- fixed foundation and the second anchorage point of the elongated and stabilizing buoyancy part.

The stabilizer arm can be anchored to the seabed. Alternatively, the buoyancy body of the stabilizer arm can be anchored to the seabed.

The suspended weight of the buoyancy body can also be anchored to the seabed.

The buoyancy part can comprise internal tanks that can be emptied and filled with ballast. The ballast can be transportable between the tanks.

In one embodiment, the bottom-fixed foundation can be provided with a rotary anchoring mechanism. The rotary anchoring mechanism can be a turntable anchorage with connections for the first and second anchors. Alternatively, the rotary anchoring mechanism can be an electric swivel with connections for the first and second anchors.

Using a rotary anchoring mechanism, the tower can be placed on a non-centered part of the buoyancy part. Likewise, the tower can be placed on the stabilizer arm. Furthermore, the stabilizer arm can be equipped with a heave compensating device which slows down movement in the water mass.

Description of figures

Preferred embodiments of the invention shall, in the following, be described in more detail below with reference to the accompanying figures, in which:

Figure 1 shows a floating windmill construction according to the invention.

Figure 2 shows a further variant of the windmill construction according to the invention.

Figure 3 shows a further variant of the windmill construction according to the invention.

Figure 4 shows a further variant of the windmill construction according to the invention.

Figures 5 and 6 are intended to illustrate the assembly and anchoring of a windmill construction according to the invention.

Description of preferred embodiments of the invention

A windmill construction according to the invention comprises a floating foundation 12 which floats partially immersed in a body of water 20, where the floating foundation 12 supports at least one tower 14 equipped with a wind turbine with turbine blades or a wind turbine 16. The lower part of the tower 14 will naturally form part of the floating foundation 12, and consequently the floating foundation 12 is perceived to comprise the lower part of the tower 14.

In alternative embodiments, the foundation 12 can comprise several towers 14, for example twin towers which are positioned side by side and tilted relative to each other.

The tower 14 extends up through the water surface of the body of water 20 and the lower part 4 of the tower is located down in the body of water 20 when the floating windmill construction is installed. The lower part 4 of the tower is further attached to an elongated and stabilizing buoyancy part 3 which is tautly tensioned and attached to a bottom-fixed foundation 25 on a seabed 30. The clamping provides a rigid anchorage which partially prevents vertical, horizontal and rotational movement of the foundation 12 in the body of water 20. The lower part 4 of the tower can, in some configurations, be a part of the tower 14, so that the tower 14 is mounted directly on the buoyancy part 3. The buoyancy part 3 can advantageously have an embodiment where it is elongated to achieve greater stability. It can preferably have a length of 50, 80, or over 100 m. By elongated is meant that the buoyancy part 3 is substantially longer in one length than in another direction. When using several stabilizer arms 6, the buoyancy part 3 can have a less elongated shape where the "diameter" becomes the length of the buoyancy part 3. The stability is then maintained by several stabilizer arms 6 that extend out from the buoyancy part 3 at different mutual angles relative to each other in the horizontal plane, and there is less need for an elongated buoyancy part.

The length or diameter of the buoyancy part will be able to increase the stability of the foundation 12, both in a floating position and in a submerged, tensioned position. The buoyancy part 3 can have varying volumes which displace the body of water 20 in its longitudinal direction, so that one can optimize the stability of the foundation 12 from a submerged position, through an immersion sequence, until it is rigidly anchored submerged in the body of water 20. The buoyancy part 3 can advantageously have a smaller area at its ends 3a, this to reduce the hydrostatic load of the structure in a submerged position.

The rigidly tensioned buoyancy part 3 and the lower part 4 of the tower can be said to constitute or provide the rear buoyancy of the foundation 12. The tower 14 can, to that extent, be placed arbitrarily in the longitudinal direction of the elongated buoyancy part 3 but is preferably advantageously placed in an area which provides equilibrium, freely floating in the body of water 20 or fixedly clamped against the anchors.

By using a rotary anchoring mechanism 26, the tower 14 can be placed at or near one end of the elongated buoyancy part 3, i.e., the tower 14 is placed on a noncentered part of the buoyancy part 3. The tower with turbine, under the influence of wind forces, will rotate the elongated buoyancy part 3 in its longitudinal direction, in a direction approximately equal to the wind direction. In such a configuration it will then naturally be possible to fit several wind turbines on the floating foundation 12.

Furthermore, the floating foundation 12 as mentioned comprises at least one outwardly extending stabilizer arm 6 which extends in a direction substantially transverse to the elongated and stabilizing longitudinal direction of the buoyancy part 3. The stabilizer arm 6 thus forms part of the floating foundation 12. The length of the stabilizer arm 6 will have an effect on the floating foundation 12. In essence, a longer stabilizer arm 6 will add more stability, and one will further be able to reduce the buoyancy of the buoyancy body 8, where typically the length of the stabilizer arm 6 can be 50, 80, or over 100 m. The stabilizer arm 6 comprises a buoyancy body 8 which penetrates the water surface of the body of water 20. The buoyancy body 8, in one embodiment, is mounted on the free end of the extending stabilizer arm 6 and extends up or down into the body of water 20.

The buoyancy body 8 and/or the stabilizer arm 6 is further described here as constituting or producing the front buoyancy of the foundation 12. Depending on the installation location and configuration, the stabilizer arm(s) 6 can be placed on either side of the buoyancy part 3, or in front and behind.

The tower 14, in some project-specific configurations, can be placed completely on the stabilizer arm 6, instead of on the lower part 4 of the tower. The tower 14 can also be placed partially over the stabilizer arm 6 and the lower part 4 of the tower. Alternatively, the tower 14 can also be an integrated part of the buoyancy body 8. The tower 14 can thus be located at both ends of the stabilizer arm 6.

The figures show a stabilizer arm 6 which extends substantially perpendicularly and transversely in respect to the buoyancy part 3. However, the stabilizer arm 6 can also extend at an angle which is not perpendicular. In a variant not shown, two stabilizer arms 6 can extend transversely in relation to the buoyancy part 3, where the angle of each stabilizer arm 6, for instance, can be approximately 45 ^s in relation to the buoyancy part 3. The invention will also cover two stabilizer arms 6 which extend out in opposite directions in relation to the buoyancy part 3.

The buoyancy body 8 can be a pipe part which is mounted approximately perpendicular to the stabilizer arm 6, i.e., perpendicular in relation to the longitudinal axis of the stabilizer arm 6. By perpendicular is not meant an angle that is necessarily 90 degrees, but an angle that causes the buoyancy body 8 to extend up and down in the body of water in an approximately vertical direction.

The outwardly extending stabilizer arm 6 preferably extends from the upper part of the lower part 4 of the tower 14, in a substantially horizontal direction. Depending on the operating conditions, it is conceivable that the stabilizer arm 6 is angled in a way that allows it to penetrate the body of water in its front part without the need for the buoyancy body 8, or the stabilizer arm 6 and the buoyancy body 8 are regarded as an integrated part.

The rear part of the stabilizer arm 6 is attached to the lower part 4 of the tower in an area above the body of water 20, and then preferably at a height which places it above the wave zone. In an alternative embodiment, at least the outwardly extending stabilizer arm 6 can be placed submerged in the body of water 20 and extend from the elongated and stabilizing buoyancy part 3 in a substantially horizontal direction, or in the lower part 4 of the lower part of the tower or a combination of the buoyancy part 3 and the lower part of the tower 4. In the embodiment where the stabilizer arm 6 is submerged, it will be able to contribute to the buoyancy of the foundation 12.

The variant with submerged stabilizer arm 6 is shown in figure 2, while the remaining figures show the first variant.

The stabilizer arm 6 can be equipped with a heave compensating device which slows down movement in the body of water. As an example, reference is made to figures 3 and 4 where it appears that the buoyancy body 8 or the front part of the stabilizer arm 6 can comprise one or more suspended weights 10, suspended via a stay, chain or the like, and which further contribute to the stabilizing effect on the floating foundation 12, as the weight 10 hangs further down in the body of water 20. The weight 10 can have a design such as, for example, a cone, plate, or other shape which causes it to slow down movement in the body of water 20. The weight 10, in some embodiments, can be advantageously attached to the seabed 30 either as a rigid anchorage, or as a loose anchorage.

Connection of the floating windmill construction, and then in particular the tower 14 and associated lower part 4, as well as stabilizer arm(s) 6 can be done by means of a flange connection, where the screwed flange connections have an inner and an outer ring for bolts in relation to the supporting structure.

As mentioned, the floating foundation 12 is rigidly anchored to the bottom-fixed foundation 25. This can be carried out in an embodiment of the invention in that a first rigid anchoring 22 runs from a first anchoring point 28a on the elongated and stabilizing buoyancy part 3 and down to the bottom-fixed foundation 25, and in that a second rigid anchorage 24 runs from a second anchorage point 28b on the elongated and stabilizing buoyancy part 3 and down to the bottom-fixed foundation 25. The first and second anchorage points 28a, 28b can be produced at respective distal ends of the elongated and stabilizing buoyancy part 3, or at other suitable places on the buoyancy part 3.

The anchors 22, 24, for instance, can be chains, tension rods or the like which extend tautly down towards and to the bottom-fixed foundation 25 on the seabed 30 in order to keep the buoyancy part 3 below the sea surface. The buoyancy part 3 is arranged to have enough buoyancy to keep the anchors 22,24 against the bottom- fixed foundation 25 rigid. In order to increase the stability of the floating windmill construction in one direction, it will be advantageous to have one or more clamps placed apart in the direction in which increased stability is desired.

The first and second anchors 22,24 can thus be tightened to form a rigid connection between the first anchoring point 28a of the elongated and stabilizing buoyancy part 3, the bottom-fixed foundation 25 and the second anchoring point 28b of the elongated and stabilizing buoyancy part 3.

Furthermore, in one embodiment the bottom-fixed foundation 25 can be provided with a rotary anchoring mechanism 26, for example the rotary anchoring mechanism 26 can be a turntable anchorage with connections for the first and the second anchorage 22,24. Another example is an electric swivel.

The bottom-fixed foundation 25, for example, can be one or more suction anchors, anchors, or a combination of different types of anchoring, which are suitable for Tension Leg solutions. The bottom-fixed foundation 25 extends from the seabed 30 and a distance up into the body of water 20, however so that the foundation does not extend from the body of water 20. In an advantageous embodiment, said anchors 22, 24 are inclined inwards from each side of the buoyancy part 3 towards the common attachment in the bottom-fixed foundation 25. This type of inclined attachment will reduce the movement of the windmill construction in some directions, as well as probably reduce the cost, but it can also use several attachments that can either protrude vertically down or in an angular configuration as shown in figure 4.

Several examples of bottom-fixed foundations 25 for use with the invention can be gravity-based foundations, monopile foundations, lowered box foundations, multipile foundations, and foundations with multiple lowered boxes. Common to these is a first part that is introduced into the seabed and a second part that extends up from the seabed 30.

The monopile construction constitutes the most used variant today and comprises a pipe that is piled (driven) into the seabed until it is fixed. In the upper part, the pipe comprises a transition piece for receiving a tower of a windmill. The windmill construction according to the invention can be attached to said adapter.

The buoyancy body 8 mounted on the front part of the stabilizer arm 6 is arranged to carry some of the weight of the windmill construction. The buoyancy body can further in an embodiment, as shown in figure 4, be arranged to be attached tautly to a bottom-fixed foundation 25 on the seabed 30 by using a chain 32 or the like, so that the buoyancy body 8 is not allowed to move in the vertical direction. The buoyancy body 8 can be designed as a cylinder, cone, cube, or have any other shape which is suitable for the purpose of the buoyancy body 8. Similarly, the weight 10 suspended in the buoyancy body 8 in an embodiment, as shown in figure 4, can be arranged to be attached tautly to the buoyancy part 3, or the bottom-fixed foundation 25 on the seabed 30, by using a chain 34 or the like, so that the movement of the weight 10 is limited.

The buoyancy body 8 with the associated stabilizer arm 6 has the task of supplying stability to the floating windmill construction. The second or inner end part of the stabilizer arm 6 is attached to the rigidly tensioned buoyancy part 3 so that enough stability is obtained in all directions.

The buoyancy body 8 can further share the anchorage with the rigidly clamped buoyancy part 3, in the sense that an anchorage 32 can run down into the body of water to one or more bottom-fixed foundations 25. Alternatively, the buoyancy body 8 can be loosely fastened using at least one traditional anchorage where one uses a longer anchoring line attached to an anchorage on the seabed 30.

The connection between the rear part of the stabilizer arm 6 and the rear buoyancy of the structure in the form of the elongated and stabilizing buoyancy part 3 is arranged to be bolted and/or welded together, as shown in figure 6. The described interconnection can be carried out while the parts are floating in a body of water. Between the lower part 4 of the tower and the elongated buoyancy part 3, inclined braces 11 , designed as struts, pipes or chains, wire configurations can also be mounted.

The invention further facilitates the connection of the windmill construction to the anchoring device by means of anchors 22,24 (or anchoring lines as shown in figure 4) before it reaches the final installation site. This is in that the stabilizer arm 6 is arranged in such a way that one can fasten and/or hoist the anchoring device, i.e., the anchoring mechanism 26 which is attached to the bottom-fixed foundation 25, up below the stabilizer arm 6, as shown in figure 5. This can be done before transport to the installation site. The windmill construction is arranged for fastening of fixed or temporary hoisting devices for raising and lowering said anchoring devices.

The windmill construction with fitted tower 14 and turbine 16 with turbine blades will float in the normal position in a way where the elongated buoyancy part 3 will partially penetrate the surface and thus retain stability at a quay and during towing. During installation, it will be possible to lower anchoring devices down to the seabed 30 in the desired position. By using, for example suction anchors as an anchoring device, it will be possible to use the anchorage to pull the windmill construction down into the body of water 20 to a depth where the upper part of the elongated buoyancy part 3 is below and in the body of water 20, so that its buoyancy will lead to a rigid anchoring of the windmill construction.

Another way to achieve a rigid anchorage is by filling sufficient ballast water into the windmill construction so that it will settle to a depth and so that the windmill construction can be attached to the anchorage in the form of the bottom-fixed foundation 25, where then ballast water can be evacuated so that the anchors 22,24 (or the lines shown in figure 4) are tightened up until they have achieved the desired tension.

In order to increase the stability when anchoring the windmill construction during installation and/or operation, permanent or removable stabilizer legs 7 can be mounted on the elongated buoyancy part 3.

To increase the stability and better control the angle of the foundation 12 during operation and installation of, for example tower and nacelle, as also installation in wind farm, tanks can be used inside the elongated buoyancy part 3 and the stabilizer arm 6 with its mounted buoyancy body 8. The tanks (not shown) are connected to a pipe system, so that the water can be transported actively and controlled from one tank to another tank in the pipe system. By active and controlled is meant a closed loop that pumps water automatically, based on parameters and or sensors. If necessary, for example if one has large waves, the stabilizer arm 6 or its buoyancy body 8 can be anchored in the form of rigid anchoring or traditional anchoring with slack lines against an anchor attachment, which, for example can be the same bottom-fixed foundation 25 as mentioned.