Floatable renewable energy source platform

Abstract

A floatable renewable energy platform is provided that includes a superstructure including one or more substantially horizontal structural support members located above a wave. The superstructure is supported by a plurality of floats interconnected by structural members, and the platform is configured to provide an air gap in the range of 10 m to 30 m between the still water surface and the structural members.

Description

[0001] The present invention relates generally to renewable energy and, more particularly, although not exclusively, to floating platforms for carrying renewable energy collectors / converters.

[0002] Renewable energy is energy that comes from renewable resources that replenish naturally on human timescales.

[0003] Renewable resources include sunlight, wind, water movement, and geothermal heat.

[0004] This application claims priority from GB2300235.5 and GB2301458.2, the contents of which are incorporated herein by reference. Background Art

[0005] Floating energy collectors, such as floating wind turbines, require a stable floating platform to mount the collector (e.g., turbine).

[0006] like Figure 1 As shown in this report (Source: Report prepared by Frazer Nash for the UK Department for Energy Security and Net Zero), there are four common designs for floating wind platforms. These designs are based on structures used in the oil and gas industry. Each of the four platform designs follows different design principles to achieve the desired level of stability and low motion response in wind and waves.

[0007] Spar - achieves stability by having a deep hull draft and a low center of gravity.

[0008] Barge type - has the stability characteristics of an ordinary barge or ship.

[0009] Semisubmersible - designed to achieve low motion response in waves by arranging the natural periods of heave, roll and pitch to be longer than the typical wave period. Its underwater shape is also designed to minimize vertical wave forces and motions during critical wave periods.

[0010] Tension leg type - stability and low heave motion response in waves are achieved through taut vertical tethers.

[0011] The present invention is a new general design of floating platform based on different principles and intended to provide advantages for appropriate mid-deep water offshore locations and environments. Summary of the Invention

[0012] Aspects and embodiments of the present invention relate to a floating platform for supporting equipment, such as renewable energy devices, including wind turbines for generating electricity from wind currents.

[0013] The floating platform may include one or more design features that enable the platform to be constructed, installed, and operated more cost-effectively than existing designs.

[0014] One aspect of the present invention provides a floatable renewable energy platform comprising a superstructure comprising one or more substantially horizontal structural support members positioned above the waves, the superstructure being supported by a plurality of buoys interconnected by the structural members, and the platform being configured to provide an air gap in the range of 10 m to 30 m between the still water surface and the structural members.

[0015] Another aspect provides a permanently moored, self-floating renewable energy platform comprising a plurality of pontoons interconnected by horizontal structural members that are always above wave action and direct wave forces and have an air gap of 10m to 30m between them and the still water surface (which may be determined by the wave climate for which the platform is designed).

[0016] The superstructure may comprise a combination of horizontal, vertical and inclined structural members.

[0017] Some embodiments include spaced-apart bow and stern buoys to generate a ripple-like pitch response amplitude operator in a head wave. The platform may include spaced-apart buoys 100 to 300 meters apart to generate two or more frequencies within an operational wave period range of 4 to 12 seconds at which wave excitation forces or moments are in antiphase, thereby generating zero excitation force and response at those frequencies, as evidenced by a ripple-like pitch or heave response amplitude operator in a head wave within the operational wave period range.

[0018] Some embodiments relate to the shape of the floating body.

[0019] In some embodiments, the shape of the buoyancy body is configured to reduce bending loads in horizontal structural members.

[0020] Some embodiments provide that the floating body is a body of rotation.

[0021] Some embodiments contemplate placing the axis of the rotation body at the level of the horizontal structural member.

[0022] Some embodiments provide a floating body (which may or may not be a body of rotation) that places wave forces at the level of horizontal structural members in low steepness waves of a specific wavelength.

[0023] Some embodiments include a buoyancy body having a wet hull form, which is in the shape of a body of revolution, the axis of which is horizontal and at right angles to the longitudinal axis of the buoyancy body.

[0024] Some embodiments include a buoyancy shape that is intended to reduce bending loads in horizontal structural members by placing the axis of rotation at the neutral axis and point of application of the primary structural member to which the buoyancy is connected.

[0025] In some embodiments, a mooring device is provided on the bow buoy. The mooring device may be connected to the buoy by means of a yoke, for example in the form of a rigid frame or equivalent chain, suspended and hinged at the level of the neutral axis of a horizontal structural member connected to the buoy.

[0026] The length of the buoy may be greater than its width. Some embodiments are configured as floating wind turbines. A floating wind energy platform may include spaced stern buoys located below one or more wind turbines, and a bow buoy.

[0027] The platform may be configured to have a natural pitch, heave, and optionally roll period of less than 10 seconds, and a longitudinal metacentric altitude of more than one kilometer.

[0028] Some embodiments have a power rating greater than 5 MW and are configured to operate at drafts less than 7 m.

[0029] The buoyancy body and / or the structural members may be made of composite materials, for example.

[0030] The buoyancy body and / or the structural members may be made of fiberglass reinforced plastic (FGRP).

[0031] The buoyancy body and / or structural members may be made, for example, from conventional shipbuilding materials including steel, aluminum, and concrete.

[0032] The structural members may be formed as lattice structures. The chords and cross braces in the structural members may be of airfoil cross-section to reduce direct aerodynamic loads.

[0033] In some aspects and embodiments, the platform has a generally flat, folded configuration and an erected configuration.

[0034] The platform may include a plurality of structural members including hinges.

[0035] Some embodiments may be configured to fold flat in the manner of a recliner, down to the level of a horizontal structural member between the floats, with multiple hinges depending on the arrangement of the structural members.

[0036] Some aspects and embodiments provide a platform configured as a floating wind turbine and including one or more wind turbines.

[0037] The platform can be equipped with one or more thrusters for heading control. Adding a thruster to the stern buoy allows for controlled windvaning. This is desirable because vessels with windvaning moorings are prone to "fishtailing" instabilities in deep water, thus stabilizing the platform's heading. It can also be used to precisely set the heading into the wind, if this is beneficial to reduce loads on the turbine blades, particularly in survival conditions where the turbine is stationary. Alternatively, it can be used to precisely set the heading into the sea, if this is beneficial to reduce transverse wave loads on the buoy. Finally, it can be used in calm conditions to fully rotate the wind turbine about its vertical axis, thereby unwinding the power cable to the seabed. This eliminates the need for expensive electrical swivels.

[0038] This stable platform also has uses in other applications that require a stable platform in waves, such as floating solar panels.

[0039] In some embodiments, the platform is configured as a floating wind turbine and includes one or more wind turbines.

[0040] Another aspect provides a floatable renewable energy platform having a generally flat, folded configuration and an erected configuration.

[0041] The platform is movable between a generally flat, folded configuration and an erected configuration.

[0042] The platform may be provided with floating bodies which may be interconnected by structural members which lie above the action of the waves.

[0043] Additional aspects and embodiments are listed below.

[0044] 1) A floating wind turbine designed according to principles (a)-(c): a) increase the hydrostatic roll and pitch stiffness so that the natural roll and pitch periods fall well within the range of typical wave periods; b) reduce the platform draft so that the resonant roll and pitch motions are well damped by wave radiation; and c) select the float length so that the vertical wave forces acting on the float cancel each other along its length during the wave period that excites the platform pitch resonance.

[0045] 2) A floating wind turbine that incorporates hinges that allow it to fold flat like a deckchair, enabling assembly at a low level (e.g., on a construction barge), thereby minimizing the need for a high hook height crane and allowing the entire floating wind turbine to be assembled on the construction barge in a sheltered anchorage.

[0046] 3) A floating wind turbine with a float in the form of a body of revolution about a horizontal axis of rotation at right angles to the longitudinal axis of the body. The axis of rotation is located at the neutral axis and action point of the main structural member connected to the float. This reduces the bending moment transmitted to the structural member.

[0047] 4) A floating wind turbine with thrusters to control its heading.

[0048] 5) A floating platform with a natural pitch and heave period of less than 10 seconds and a metacentric height exceeding one thousand meters.

[0049] 6) A floatable renewable energy platform whose design is based on one or more of the following: a) Increased hydrostatic heave and pitch stiffness, minimizing pitch angles under wind loads by having a metacentric height exceeding one kilometer (high heave, pitch, and roll stiffness can also be achieved with tension-leg mooring, as in some oil rigs and floating wind turbines—these are not included in this invention). Large ballast systems are generally not required to counteract wind-induced pitch, a significant advantage. High heave and pitch stiffness means that the natural heave and pitch periods fall well within the range of typical wave periods. At high wind speeds, this design feature is highly advantageous for wind turbine control systems, which attempt to limit power during gusty wind speed increases by reducing wind loads on the turbine. The platform's pitch motion appears to the turbine control system as a gust: reduced wind loads in this case correspond to negative pitch damping. This can lead to dangerous pitch dynamic instabilities. Consequently, the platform's natural pitch period is short, similar to the natural period of a comparable land-based wind turbine tower, which is highly advantageous. b) Reduce the platform's draft so that the resonant heave and pitch motions are well damped by wave radiation. Use a shallow-draft buoy. c) Use a "weathervane effect" mooring similar to a single-point moored FPSO (Floating Production Storage and Offloading) (and some other floating wind turbines), so that the platform faces primarily the wind and waves, and the wind turbines on the platform operate downwind. d) Make the buoy longer than it is wide, like a ship, and point it toward the waves. The mooring is located on the bow buoy (rather than on the stern buoy below the turbines). e) Choose the buoy length so that during the wave period that excites the platform's pitch resonance (platform pitch direction = buoy pitch direction), the vertical wave forces acting on the buoy cancel each other out along its length (for example, choose the buoy length to be approximately equal to the wavelength). This will further reduce the resonant roll and pitch motions. The buoy length, L, is given by the following formula: L = 2πx(1+Cm)xD. Here, Cm is the heave added mass coefficient of the float, and D is the draft of the float. f) The floats are spaced very widely apart, typically several float lengths. This produces a "ripple-like" roll, heave, and surge RAO in head waves because, at some wavelengths, the wave forces acting on the bow and stern floats act in opposite directions, while at other wavelengths they act in the same direction. Overall, motion in the waves is reduced, a significant advantage. g) The floats are interconnected using structural members that are always above the water surface. This eliminates direct wave loads on the members, which could otherwise be a significant design issue. As a result, the structural members can be lightweight lattice structures, like crane booms. They and the floats can be made of composite materials, such as fiberglass reinforced plastic (FGRP), instead of steel, reducing the cost and weight of the platform. To reduce direct aerodynamic loads, the chords and cross braces in the structural members can have airfoil-shaped cross-sections.h) Hinges are integrated into the ends of some structural members, allowing the platform to fold flat like a deckchair. This allows the entire floating wind turbine to be assembled on a construction barge in a sheltered anchorage. i) Because the horizontal structural members are located some distance from the still water surface, as mentioned above, horizontal wave loads acting on the floating body can act at a considerable distance below them, causing bending loads in the structural members. This can be a significant design problem. Bending loads can be greatly reduced by shaping the floating body so that the horizontal wave loads acting on it are at the same level as the neutral axis (usually the centerline) of the structural members. The simplest such floating body shape is a sphere, with its center at this level (of course, only the wetted surface of the floating body needs to have this shape). All pressure loads acting on the sphere pass through its center, so their horizontal components are at the level of the center. Another floating body shape with this property is any bilaterally symmetrical solid of revolution with its axis parallel to the wave crest and at the same level as the center of the sphere. To reduce bending loads as described, they do require waves to propagate from the bow buoy to the stern buoy (known in nautical terms as "heading waves"), but this is often the case due to the "weathervaning" mooring described above. Other buoy shapes can be designed using diffraction programs (see Reference 1, Chapter 4) or other methods to produce zero bending loads in heading waves of low steepness and specific wavelengths; this may be sufficient. j) Another design feature is the addition of thrusters to the stern buoy to control the windvaning effect. This is desirable because vessels with windvaning moorings are prone to "fishtailing" instabilities in deep water, and can be used to stabilize the platform's heading. It can also be used to precisely set the heading into the wind if this is beneficial to reducing loads on the turbine blades, especially in survival conditions where the turbine is stopped. Alternatively, it can be used to precisely set the heading into the sea if this is beneficial to reducing transverse wave loads on the buoy. Finally, it can be used in calm conditions to fully rotate the wind turbine about its vertical axis, thereby unwinding the power cables to the seabed. This eliminates the need for expensive electrical rotary joints. 7) A floating wind turbine designed according to principles (a)-(c). 8) A floating wind turbine incorporating hinges that allow it to fold flat like a deckchair, enabling assembly at a low level (e.g., on a construction barge). 9) A floating wind turbine having a float shaped as a body of rotation with bilateral symmetry as described herein.

[0050] 10) A floating wind turbine having thrusters for controlling its heading, as herein described.

[0051] 11) A floating wind turbine or the like having a natural pitch and heave period of less than 10 seconds and a metacentric height exceeding one kilometer.

[0052] 12) A floating wind turbine or the like having a rated power greater than 5 MW and an operating draft less than 7 m.

[0053] 13) A floating wind turbine or the like, wherein the length of the float is given by the equation in (e).

[0054] 14) A floating wind turbine or the like having widely spaced floats to produce a "rippled" pitch RAO, such as described in (f).

[0055] 15) A floating wind turbine or the like, the buoyancy of which is interconnected by structural members which lie above the action of waves, and both advantageously made of FGRP, for example as described in (g).

[0056] 16) A floating wind turbine or the like that can be folded flat like a deck chair, for example as described in (h).

[0057] 17) A floating wind turbine or the like having a float shape designed to reduce bending loads in horizontal structural members, e.g. as described in (i).

[0058] 18) A floating wind turbine or the like having thrusters to control its heading, e.g. as described in (j).

[0059] The different aspects and embodiments of the present invention can be used alone or together.

[0060] Other particular and preferred aspects of the present invention are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and may be combined in combinations not explicitly set out in the claims. Each aspect may be implemented independently of the other aspects, or in combination with one or more of the other aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] The present invention will now be described in more detail with reference to and as shown in the accompanying drawings, in which:

[0062] Figure 1 Universal design for existing floating wind platforms;

[0063] Figure 2 A floating wind turbine platform formed in accordance with one embodiment;

[0064] Figure 3 for Figure 2 a side view of the platform showing the air gap;

[0065] Figure 4 It is a typical ripple-shaped pitching RAO in the top wave;

[0066] Figure 5floatable platforms that fold flat for construction or maintenance;

[0067] Figure 6 for Figure 5 The platform is in an upright position;

[0068] Figure 7 To fold / erect the floating turbine with the help of a crane;

[0069] Figure 8 To fold / erect the floating turbine using built-in winches;

[0070] Figure 9 A platform with shaped floats to minimize bending loads in structural members; and

[0071] Figure 10 For mooring, the yoke pivots around the bearing axis together with the mooring rope.

[0072] The example embodiments are described in sufficient detail to enable one of ordinary skill in the art to embody and implement the systems and processes described herein.It is important to understand that the embodiments may be provided in many alternative forms and should not be construed as limited to only the examples set forth herein.

[0073] Therefore, while the embodiments may be modified in various ways and presented in various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples. It is not intended to limit the invention to the particular forms disclosed. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims are intended to be included therein.

[0074] In the drawings and detailed description, elements of the example embodiments are represented by like reference numerals throughout, where appropriate.

[0075] Unless otherwise defined, all terms used herein (including technical and scientific terms) should be interpreted according to common practice in the art. It should also be understood that commonly used terms should also be interpreted according to common practice in the relevant art and should not be interpreted in an idealized or overly formal sense unless explicitly defined as such herein.

[0076] In the following description, all directional terms, such as upper, lower, radial, and axial, are used relative to the drawings and should not be construed as limiting the present invention.

[0077] For definitions of technical terms (e.g., roll and pitch), see Reference 1. DETAILED DESCRIPTION

[0078] This application relates to a new general design for a floating wind platform. Typical examples of this general design are Figure 2 shown.

[0079] It has a floatable renewable energy platform 10 comprising a superstructure containing one or more generally horizontal structural support members 20 positioned to avoid wave impact. The superstructure is supported by a plurality of buoys 30 interconnected by structural members, and the platform is configured to provide an air gap in the range of 10m to 30m between the still water surface and the structural members.

[0080] In this embodiment, one bow buoy 30a and two stern buoys 30b, 30c are provided.

[0081] In this embodiment, there are three horizontal support members 20a, 20b, 20c arranged in a generally triangular configuration, and three columns (which may be generally vertical or, for example, generally inclined) 40a, 40b, 40c, which together provide a generally triangular pyramidal superstructure supporting a wind turbine 50.

[0082] The air gap below the superstructure is Figure 3 It is chosen so that even under the most extreme conditions, wave crests will not impact the structural components. The size of the air gap can be calculated using standard procedures based on the wave climate statistics for the platform's location.

[0083] The spacing between the bow buoy 30a and the stern buoys 30b, 30c is in the range of 100m to 300m, depending on the wave environment. By spacing the buoys in this way, at certain frequencies, the wave forces and moments acting on the buoys are out of phase, resulting in zero excitation forces at many wave cycles within the typical operating wave period of 4 to 12 seconds. This is particularly useful in situations such as Figure 4 The corrugated nature of the motion response magnitude operator is evident in the diagram. This reduces hull motions that could be detrimental to structural integrity and energy production.

[0084] Figures 5 to 8 A floatable renewable energy platform 110 is shown formed according to another aspect. The platform 110 has a generally flat, folded configuration ( Figure 5 ) and vertical structures ( Figure 6 ).

[0085] The fold-flat configuration may be useful for rapid installation of wind turbines and blades with minimal requirements for high hook height cranes.

[0086] In this embodiment, the primary structural members have a lattice structure. The hinges in the platform 110 are Figure 7 and Figure 8 They could equally well be located in any other position that would allow the structure to fold flat like a recliner. For example, the hinges could be located at the other end of the longest structural member.

[0087] like Figure 7 As shown, the platform 110 requires two cranes to lower it into the folded position and to raise it into the erected position. For large wind farms, the cost of such cranes may be negligible, as the folding / erecting operations are fast, so a pair of cranes may be sufficient for the construction and maintenance of the entire wind farm. However, for small wind farms, it may be more cost-effective to provide a built-in winch function on the platform, such as Figure 8 The winch can then raise and lower the turbine on its own. The additional structure required is very light compared to the turbine itself, so it can be erected using a small crane, lifting from a point on the structural member close to the hinge.

[0088] According to another embodiment, the platform 210 is formed as Figure 9 shown.

[0089] Platform 210 has a metacentric height exceeding one kilometer to minimize pitch angles under wind loads. It is equipped with two wind turbines and comprises spaced stern pontoons located below the wind turbines, and a bow pontoon. The pontoons are interconnected by structural members that remain above water.

[0090] The platform is moored using a "weathervaning effect" similar to single-point mooring for FPSOs (and some other floating wind turbines), so that the platform faces primarily the wind and waves, and the wind turbines on the platform operate downwind. The mooring is located on the bow buoy (rather than the stern buoy below the turbines).

[0091] In this embodiment, a mooring device is provided on the bow buoy. The bow mooring device may be in the form of a rigid frame yoke or an equivalent chain structure, suspended and hinged at the level of the neutral axis of a horizontal structural member connected to the buoy, such as Figure 10 shown.

[0092] The structural members are lattice structures, like a crane boom. The chords and cross braces in the structural members can have airfoil-shaped cross-sections. The structural members and the buoy are made of fiberglass reinforced plastic (FGRP).

[0093] Because, as mentioned above, horizontal structural members are some distance from the still water surface, horizontal wave loads acting on the float may act at a considerable distance below it, causing bending loads in the structural members. This can be a significant design problem. Bending loads can be greatly reduced by shaping the float so that the horizontal wave loads acting on it are at the same level as the neutral axis (usually the centerline) of the structural member. The simplest such float shape is a sphere, with its center at this level (of course, only the wetted surface of the float needs to have this shape). All pressure loads acting on the sphere pass through its center, so their horizontal components are at the level of the center. Another float shape with this property is any bilaterally symmetrical body of revolution with its axis parallel to the wave crests and at the same level as the center of the sphere.

[0094] This type of float (e.g. biconical float) is Figure 9 To reduce the bending loads as described, they do require waves to propagate from the bow buoy to the stern buoy (called a "head wave" in nautical terms), but this is often the case due to the "weathervaning effect" of mooring. Other buoy shapes can be designed using diffraction programs (e.g., see Reference 1, Chapter 4) or other methods to produce zero bending loads in head waves of low steepness and specific wavelengths.

[0095] exist Figure 10 In the case of the Floating Hull, the bow hull also bears the loads from the mooring system. These loads can also be directed to the level of the structural components by means of a mooring yoke, as shown in the figure. This type of yoke has previously been used on the Pelamis wave energy machine, but there its function is completely different: to help dampen rolling motions rather than prevent bending of the structure. In addition to reducing bending loads in structural components, it has another advantage in devices such as floating wind turbines: it increases the effective mooring length, allowing operation in shallower waters.

[0096] Although illustrative embodiments of the present invention have been disclosed herein in detail with reference to the accompanying drawings, it should be understood that the invention is not limited to the precise embodiments shown and that various changes and modifications may be made therein by those skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

[0097] References

[0098] 1. Faltinsen, OM 1990. Sea loads on ships and offshore structures. Cambridge University Press.

Claims

1. A floating renewable energy platform comprising: a superstructure comprising one or more generally horizontal structural support members positioned above the waves, the superstructure being supported by a plurality of buoys interconnected by the structural members, and the platform being configured to provide an air gap in the range of 10 m to 30 m between the still water surface and the structural members.

2. A permanently moored floating renewable energy platform comprising: A plurality of floating bodies interconnected by horizontal structural members which are always above the wave action and direct wave forces with an air gap of 10 to 30 m between the members and the still water surface level.

3. A platform according to claim 1 or 2, comprising buoys spaced 100m to 300m apart so as to produce two or more frequencies within an operational wave period in the range of 4 to 12 seconds at which wave excitation forces or moments are in anti-phase, thereby producing zero excitation force and response at that frequency, as evidenced by a wave-like pitch or heave response amplitude operator to a head sea within said operational wave period range.

4. A platform according to any preceding claim, wherein the shape of the buoyancy body is intended to reduce bending loads in the horizontal structural members.

5. A platform according to any preceding claim, the buoyant body having a wet hull form in the shape of a body of revolution, the axis of which is horizontal and at right angles to the longitudinal axis of the buoyant body.

6. The platform of claim 5, wherein the shape of the floating body reduces bending loads in horizontal structural members by placing the axis of rotation at the neutral axis and action point of the primary structural member connected to the floating body.

7. A platform according to any preceding claim, wherein a mooring arrangement is provided on the bow buoy.

8. Platform according to any of the preceding claims, having on the bow buoy a mooring device connected to the buoy by means of a yoke in the form of a rigid frame or equivalent chain suspended and articulated at the level of the neutral axis of a horizontal structural member connected to the buoy.

9. A platform according to any preceding claim, wherein the buoyancy body is longer than it is wide.

10. A platform according to any preceding claim, configured as a floating wind turbine and comprising spaced stern buoys located below one or more wind turbines, and a bow buoy.

11. A platform according to any preceding claim, configured for a period of natural pitch, heave and optionally roll of less than 10 seconds.

12. A platform according to any preceding claim, having a longitudinal metacentric height exceeding one kilometre.

13. A platform according to any preceding claim, having a power rating greater than 5 MW and configured for an operating draft less than 7 m.

14. A platform according to any preceding claim, wherein the buoyancy body and / or the structural members are made of composite material.

15. The platform according to claim 14, wherein the buoyancy body and / or the structural member are made of fiberglass reinforced plastic (FGRP).

16. The platform according to any one of claims 1 to 14, wherein the buoyancy body and / or the structural member are made of one or more of the following materials: steel, aluminum and concrete.

17. A platform according to any preceding claim, comprising said structural members formed as a lattice structure.

18. The energy platform according to claim 17, wherein the chord members and cross braces in the structural members are of airfoil cross-section to reduce direct aerodynamic loads.

19. A platform according to any preceding claim, having a generally flat folded configuration and an erected configuration.

20. The platform of claim 19, comprising a plurality of said structural members including hinges.

21. A platform according to claim 19 or 20, configured to fold flat in the manner of a deckchair, down to the level of a horizontal structural member between the pontoons, with a plurality of said hinges being arranged according to the arrangement of said structural members.

22. A platform according to any preceding claim, provided with thrusters to control its heading.

23. A platform according to any preceding claim, configured as a floating wind turbine and comprising one or more wind turbines.

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