WO2025188350A1

WO2025188350A1 - Ring-based floating and fixed-floating wind turbine platforms

Info

Publication number: WO2025188350A1
Application number: PCT/US2024/044594
Authority: WO
Inventors: Eric Loth
Original assignee: University of Virginia Patent Foundation
Current assignee: UVA Licensing and Ventures Group
Priority date: 2024-03-08
Filing date: 2024-08-30
Publication date: 2025-09-12
Anticipated expiration: 2026-09-08
Also published as: WO2025188350A8

Abstract

A wind turbine platform, including a buoyancy tube disposed radially around a tower and/or stem for a floating wind turbine or around the tower and/or monopile for a fixed-bottom wind turbine, a plurality of buoyancy cables/spokes extending from the buoyancy tube to the stem and arranged to carry in tension an upward buoyancy force of the buoyancy tube, and a plurality of thrust cables/spokes extending from the buoyancy tube to the stem and/or tower and arranged along with the buoyancy cables to carry in tension a generally horizontal force imparted by at least one of rotor thrust, torque, wind, and wave forces.

Description

RING-BASED FLOATING AND FIXED-FLOATING WIND TURBINE PLATFORMS

TECHNICAL FIELD

The present application generally relates to wind power technologies and, more particularly, to a buoyant support platform for floating and fixed-bottom offshore wind turbines.

BACKGOUND

Offshore installations of wind turbines can be desirable to access wind power generation. Such installations generally utilize fixed-bottom or floating wind turbine arrangements. Fixed-bottom offshore turbines have foundations upon the lake underwater ground floor (lakebed) or upon the sea or ocean underwater ground floor (seabed) and include underwater substructures (such as monopiles) upon these foundations which support the tower, nacelle, and rotor above water level for power generation. The foundations and substructures for fixed-bottom turbines must provide support for gravitational forces and resist forces due to wind and waves. Floating wind turbine installations instead include a flotation structure upon which the turbine is mounted and supported for power generation. These floating installations are not fixed to the lakebed or seabed but are instead typically connected thereto with mooring lines and anchoring system.

Fixed-bottom offshore wind turbine installations are limited by the depth of the underwater ground floor and by the composition and characteristics of the soil. Such fixed-bottom installations are often too expensive to build in deep water, in which case a floating offshore turbine may be more appropriate. Floating installations must also provide adequate stability for the respective turbine against forces generated by its own rotor and by external forces generated by the wind and waves. However, current floating wind turbines are often too costly for deployment due, at least partially, to high platform costs. For more widespread deployment of wind turbines in deep water, a new platform system for supporting floating wind turbines and fixed-bottom offshore wind turbines is needed that is less expensive to build, install, and maintain, and which provides sufficient stability to the wind turbine in the face of gravitational forces and rotor, wind, and wave forces.

BRIEF SUMMARY

To provide high stability at low cost, a ring-based floatation system for a floating wind turbine is proposed. The system uses an outer buoyancy tube to provide buoyancy at a large maximum radial extent to enhance platform stability in terms of both inertia and metacentric height. This tube is coupled to the tower and stem with tension-based buoyancy cables/spokes (to support the buoyancy forces) and thrust cables/spokes (to support thrust forces). This design improvement can reduce the overall mass and cost of a floating platform for a wind turbine as compared to conventional floating wind turbine designs. The buoyancy tube can also be used to store compressed air in a structurally efficient manner. The compressed air can be used for energy storage and/or to reduce any compressive stresses in the structural shell of the buoyancy tube. The floatation tubes can also be used to support a surface platform to provide a convenient docking area near the wind turbine tower for water vessels used for installation, maintenance, or decommissioning of the turbine.

In one embodiment, a wind turbine platform is provided including a buoyancy tube disposed circumferentially around a stem or monopile of a wind turbine, a plurality of buoyancy cables and/or spokes extending radially inward and axially downward from the buoyancy tube to the stem and arranged to cany in tension to rimarily carry an upward buoyancy force of the buoyancy tube, and a plurality of thrust cables and/or spokes extending radially inward from the buoyancy tube to the tower and/or stem and arranged to primarily carry in tension generally horizontal forces imparled upon the turbine by a general combination of rotor thrust and rotor torque as well as wind and wave forces. The platform dynamics in pitch, roll and heave can also be damped by adding a horizontal porous layer or layers supported by the buoyancy tube and cable whereby the porous layer or layers are generally under water to increase drag and added mass. Furthermore, the platforms between turbines may be connected by a scries of connecting support lines to reduce the number of mooring cables and anchors needed for a floating wind farm. Notably, the ring-based floatation system can be employed for a variety of floating wind turbine platforms including (but not limited to) a semi-submersible configuration, a tension-leg platform configuration and a hybrid fixed-floating configuration. This ringbased support concept can also be applied to a fixed-bottom wind turbine, to allow such systems to be effective in deeper water than conventional fixed-bottom wind turbines. This ring-based support concept can also be applied to other offshore platforms, for example, platforms for storage or drilling of oil and/or natural gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-B is a side and partial cross-sectional view of a ring-based floating wind turbine platform in an exemplary embodiment with the thrust and buoyancy cables attached at the interior of the semi- submersed buoyancy tube and to the tower and stem, and a top plan view thereof, with the nacelle, rotor, and buoyancy cables removed;

FIG. 2 is a side and partial cross-sectional view as in FIG. 1 but with cable attachment points for both the thrust and buoyancy cables at a separate cross-sectional positions on the buoyancy tube;

FIGS. 3A-B is a side and partial cross-sectional view as well as a top view as in FIG. 1 but with thrust cables and buoyancy cables wrapping around and attaching on the outside of the buoyancy tube and with the thrust cables attaching at two different vertical locations on the tower/stem;

FIG. 4 is a side and partial cross-sectional view as in FIG. 3A but with the thrust cables attaching at only one vertical location on the tower; FIG. 5 is a side and partial cross-sectional view as in FIG. 3A but with cable attachment points for the thrust and buoyancy cables located at both the top and bottom of the buoyancy tube;

FIGS. 6A-B are top plan views thereof, with the nacelle, rotor, and buoyancy cables removed as in FIG. 3 but with bifurcated thrust cables to better distribute the cable loading on the buoyancy tube for thrust cables that meet the tower: a) tangentially and b) radially;

FIG. 7 is a side and partial cross-sectional view of a ring-based floating wind turbine platform as in Fig. 4 but additionally including a ballast ring disposed beneath the buoyancy tube and attached to the tube and stem with ballast cables;

FIGS. 8A-B is a side and partial cross-sectional view as well as a top view showing the addition of an inner buoyancy tube disposed radially inward of a second outer buoyancy tube (where other components such as thrust and buoyancy cables, attachments and turbine are not shown);

FIGS. 9A-B are top plan views of the buoyancy tube with the nacelle, rotor, and cables removed showing example segmentation planform embodiments for platform shapes which from above are: a) circular and b) square (where other components such as thrust and buoyancy cables, attachments and turbine are not shown);

FIGS. 10A-B is a side and partial cross-sectional view as well as a top view as in FIG. 3 but with the addition of an underwater porous mesh supported by the lower thrust cables for the underwater region circumscribed by the buoyancy tube (where mesh attachment points are not shown); FIGS. 11A-B is a side and partial cross-sectional view as in FIG. 4 as well as a top view with the addition of a surface platform fixed to the buoyancy tube and supported by the upper thrust cables which covers the region circumscribed by the buoyancy tube;

FIGS. 12A-B are top plan views thereof, with the nacelle, rotor, buoyancy, and thrust cables removed, with the addition of a surface platform which covers a portion of the region circumscribed by the buoyancy tube;

FIGS. 13A-B is a side and partial cross-sectional view as well as a top view as in FIG. 4 with the addition of multiple mooring lines attached to the buoyancy tube;

FIGS. 14A-B is a side and partial cross-sectional view as well as a top view as in FIG. 13 with both mooring lines and connecting support lines attached to the buoyancy tube;

FIG. 15 is a top view of several wind turbines comprising an offshore wind farm with both mooring lines and connecting support lines attached to the turbines such that surface vessels can have access between rows of turbines;

FIG. 16 is a top view of several wind turbines as in FIG. 15 but with shared anchor locations;

FIG. 17 is a side and partial cross-sectional view as in FIG. 4 with the addition of temporary lifting platform to facilitate assembly of the wind turbine and platform and to reduce the draft of the turbine prior to deployment;

FIG. 18 is a side and partial cross-sectional view as in FIG. 4 with the addition of nearly vertical mooring lines and with the buoyancy tube fully submerged underwater to provide a tension-leg platform configuration; and

FIG. 19 is a side and partial cross-sectional view as in FIG. 4 with the addition of a fixed monopile to provide a hybrid floating and fixed-bottom configuration. DETAILED DESCRIPTION

To provide buoyancy for a floating wind-turbine, a ring-based platform attached to the stem and/or tower with tension elements is proposed. The tower is defined herein as the central support structure for the nacelle and the rotor which is generally above water while the stem is defined herein as the central support structure for the tower which is generally under water. FIG. 1 shows an exemplary embodiment of a system comprising a floating wind turbine 10 having a tower 12 that supports a nacelle 16 and rotor 18 and a stem 14 which is supported below the tower 12. A ring-shaped buoyancy tube 20 encircles the stem 14 and tower 12 of the wind turbine 10 and is connected thereto with buoyancy cables 22 and thrust cables 24. The hollow buoyancy tube 20 provides the primary buoyancy (floatation) to balance the gravitational force (due to the mass of the wind turbine 10, stem 14, and floating platform) and as well as any downward forces due to a mooring system (discussed later). The buoyancy tube is designed to provide buoyancy at a large maximum radial extent to enhance platform stability in terms of increasing both inertia and metacentric height while minimizing structural mass. The stem 14 may also provide additional buoyancy forces but also may include ballast to provide additional gravitational forces. Along with the buoyancy cables, the thrust cables 24 support the wind turbine 10 with respect to thrust forces and torque generated by the spinning rotor 18 of the turbine 10 and with respect to other generally horizontal forces generated from wind and waves. The ring-based platform may have a semi-submersible configuration as shown in FIG. 1 in which at least a portion the buoyancy tube 20 is maintained above the water surface.

As shown in FIG. 1, the buoyancy tube 20 can have a toroidal shape with an outer ring circumference in the form of a circle (when viewed from above) and with a circular cross section of the tube (when viewed in a vertical cut plane) that is hollow to provide volumetric displacement of the water for buoyancy. Such a hollow toroidal geometry can carry a combination of compression, shear, and tensile loads in a structurally efficient manner. This concept helps eliminate costly and heavy pontoons that would otherwise connect the buoyancy platform to the tower and/or stem, which therefore allows the buoyancy volume to be much less than that for conventional semi-submersible floating wind turbine platforms. The cross-sectional shape of the tube 20 (when viewed in a vertical cut plane) can also be an ellipse or another shape that is structurally efficient. Wave loading can be mitigated because of the low vertical profile and rounded cross-sectional shape as well because larger waves can wash over the tube 20. The toroidal shape also acts as a moonpool and provides damping benefits to heave through vortex shedding and pitch/roll through sloshing of water in the toroidal structure as is common of annular barge hulls.

The buoyancy tube 20 must be designed to withstand the circumferential stresses introduced by the buoyancy cable 22 tension that imposes a radially inward and downward force. To support these stresses, the buoyancy tube 20 should have sufficient thickness to prevent structural failure, such as buckling. The buoyancy tube 20 can, for example, be fabricated out of steel, fiberglass or other materials that provide high strength and robustness per unit cost of the material. In addition, the shell of the tube 20 can be pretensioned by using compressed air on the inside to help reduce compression stresses. The terms buoyancy tube and buoyancy ring are used herein to broadly describe a buoyant volume constructed to extend around the stem or, as further discussed herein, the monopile of a wind turbine in a circumferential manner. The buoyancy tube 20 is preferably symmetric in shape when viewed from above and may be circular or formed of three or more straight sides, or may include a combination of curvilinear- and straight sides. The buoyancy tube 20 may include any desired cross-sectional shape sufficient to provide an adequate buoyant force to support the wind turbine. In one preferred embodiment, the buoyancy tube 20 is shaped as a torus as shown in FIGS. 1A-B.

The buoyancy and thrust cables 22, 24 are structural tension elements with a length that is much greater than their effective cross-sectional diameter and whose primary purpose is to cany tension loads and are pre-tensioned. They may be referred to as either cables or spokes in this regard. Although herein, the term ‘cable’ is used generally and shall be understood to broadly designate cables, spokes, and any other suitable structural tension element. The cables can be made of a material with high tensile strength per unit cost, such as steel, polyester, nylon, high-modulus polyethylene (HMPE) or the like.

The buoyancy cables 22 will generally cany all the upward buoyancy forces of the buoyancy tube. Combined with the buoyancy cables, the thrust cables 24 will support the rotor thrust, which acts nearly horizontally, and rotor torque as well as other substantially horizontally forces, which can result from wave and/or wind interactions. The buoyancy cables 22 will generally carry much higher loads than that of the thrust cables 24 because gravity forces of wind turbines are typically much larger than thrust forces. For example, the rated thrust can be eight or more times smaller than the gravity force for the mass of a typical floating wind turbine. In addition, the buoyancy cables 22 will also carry tension toward the stem 14 at an angle, so that the net tension force is greater than the gravity force for the mass of typical floating wind turbine.

The bottom of the wind turbine stem 14 must be designed to support the tension loads of the buoyancy cable 22 which will introduce a compressive stress on the stem 14. This support can be in the form of curved bottom of the stem 26 as shown in FIG. 1 in order to provide a stem-cable saddle. Such a saddle on the stem bottom 26 may be like that used to support cables on top of the pylons of a suspension bridge. The buoyancy cables 22 can clamped on the bottom surface of the stem 14 with a saddle-based system (as shown in FIG. 1) as used on suspension bridges or the buoyancy cables 22 can be attached to the stem bottom along its circumference or individually attached to the stem 14 with individual cable-stays.

In the illustrations of FIGS. 1A and 2, the buoyancy cables 22 extend radially outward and attach to the inside of the buoyancy tube 20. This attachment 28 may use a pin connection system or shackle-based system (as used to connect suspender cables to the main cable on a suspension bridge and as used to connect spoke cables on a Ferris wheel to the outer rim) or another point connection system. As shown in the illustrations of FIGS. 3 A and 4, the buoyancy cables 22 may also wrap around the buoyancy tube 20. This wrap attachment 30 may use a wrap-based cable band system as used to connect suspender cables to the main cable on a suspension bridge. As shown in the illustration of FIG. 5, the buoyancy cables l may also be attached to the top and bottom of the buoyancy tube 20. In other embodiments, the thrust and/or buoyancy cables 22, 24 can be connected to multiple cable rings or a truss-based ring structure that is attached to the buoyancy tube 20.

To ensure that the wind turbine always remains nearly vertical, the thrust cables 24 of FIGS. 1A-B are designed along with the buoyancy cables to resist rotor thrust and torque as well as horizontal wind and wave forces when the turbine 10 is deployed. As such, the thrust cables 24 effectively act like spokes on a bicycle or cables on a Ferris wheel. Similarly to the buoyancy cables 22, the thrust cables 24 can be attached to the interior of the buoyancy tube (as shown in FIGS. 1A-B and 2) and/or wrapped around a circumference of the buoyancy tube 20 (as shown in FIG. 3A-B and 4). The thrust cables 24 can be attached to one vertical location 12A on the tower 12 (as shown in FIGS. 1A-B, 2 and 4), or to two vertical locations 12B on the tower 12 (as shown in FIGS. 3A and 5) or three or more vertical locations on the tower 12. The buoyancy and thrust cables 22, 24 may be attached to the buoyancy tube 20 at the same cross-sectional location (as shown in FIG. 1A, 3A and 4) or in different cross-sectional locations (as shown in FIG. 2 and 5).

In addition to the examples shown in FIGS. 1 A-B and 3, the buoyancy and thrust cables 22, 24 can be arranged in alternate patterns with respect to the stem 12 and can include alternate numbers of individual cables to maximize structural integrity with minimum cost and minimum maintenance. For example, these patterns can include spoke arrangements used on bicycle wheels and on Ferris wheels. These patterns can use bifurcated cables. For example, FIGS. 6A-B show thrust cables 24 which bifurcate at outer radial locations in order to distribute the load of the cables 24 more uniformly across the circumference of the buoyancy tube 20. The buoyancy cables can bifurcate in a similar manner. A configuration may include bifurcation in only the buoyancy cables 22, only in the thrust cables 24, or in both the buoyancy and thrust cables 22, 24. In another embodiment, the cables can be connected to a mesh wrap that extends over the outer circumference of the tube. This wrap can provide a nearly uniform distribution of the cables loads over the outer surface of the buoyancy tube to avoid stress concentrations. The wrap can be made of a material with high tensile strength per unit cost, such as steel, glass polyester, nylon, high-modulus polyethylene (HMPE) or the like.

The cables 22, 24 can also attach in a tangential or nearly tangential manner along the stem 14 or tower 12 as shown in FIG. 6a or in a radial or nearly radial manner to the stem or tower as shown in FIG. 6b. Nearly tangential connections provide additional support to withstand any torque loads about the tower and stem axis.

As with bicycle spokes and Ferris wheel cables, the thrust cables 24 can be pre-tensioned after being installed, so they are in tension when deployed, even when there is no thrust. The amount of pre-tension should be generally sufficient so that the cables stay in tension even when a counter-acting thrust forces or other forces act towards a cable to reduce the tension. The cables must also be capable of carrying a tensile load that is the sum of the pre-tension and the maximum tension caused by forces acting away from the cable, where such forces can include rotor thrust and torque as well as other horizontal forces due to wind, wave and mooring forces. The amount of pre-tension in the thrust cables can also be designed to optimize the stiffness of the combined tower and stem to minimize the cost of the tower and stem.

The ring-and-cable approach discussed above for the buoyancy tube can also be applied to integrate ballast into the system for providing increased platform stability. The ballast may be placed at the bottom of the stem as is conventionally done in many floating wind turbine platforms. However, the ballast may also be added in the form of a ballast ring 32 that is supported by ballast cables 34 attached to the buoyancy ring 20 and to the stem 14 as shown in FIG. 7. The ballast cables 34 may also be attached to the tower 12. Using multiple connections, allows for an effectively rigid ballast system. However, another option is to make the ballast ring free to move so it acts like a pendulum with a natural frequency based on the natural pitch and roll frequencies of the floating platform, so it acts as a pendulum-based tuned mass damper. To keep it inexpensive, the ballast ring 32 could, for example, be in the form of a steel pipe filled with concrete and may have a crosssection which is circular, square, rectangular, ellipsoidal, trapezoidal, tear-dropped, etc. Using a lower ring for the ballast allows the extra mass to be located far from the axis of symmetry to maximize inertia for pitch and roll motion, while also making the center of gravity as low as possible. The planform shape of the ballast ring 32 circumference may be similar to the shape of the buoyancy tube 20 circumference. The ballast ring 32 can also be designed to have an adjustable depth so it can be lowered to make the center of gravity even lower once the floating turbine reaches deep enough water.

As shown in FIGS. 8A-B, the floating wind turbine platform may include two buoyancy tubes 20 connected by buoyancy and thrust cables 22, 24. An inner buoyancy tube 20A is disposed proximate to the turbine tower 12 and stem 14, while a concentric outer buoyancy tube 20B is disposed radially outward of the inner tube 20A. This double tube arrangement provides safety and redundancy in case there is a flotation failure in one tube. The floating wind turbine platform may also include three or more buoyancy tubes 20 all connected by buoyancy and thrust cables 22, 24.

The buoyancy tube 20 disclosed herein may be fabricated in two or more circumferential sections 21 and then assembled together. For example, FIG. 9A shows a buoyancy tube based on four sections 21 of four curved segments to make a circular circumference profile in the horizontal plane (when viewed from above) while FIG. 9B shows four straight segments 21 joined at the corners to make a square circumference profile. The sections 21 can be connected to each other with weld connections, flanges and/or with overlapping joints. As such, the buoyancy tube may be much simpler to manufacture and assemble as compared to conventional semi-submersible floating wind turbine platforms.

It may be more cost-effective to manufacture the buoyancy tube 20 in sections of straight pipe instead of curved pipe. For example, eight straight tube sections 21 can be used to form an octagon shape in the horizontal plane. Many such straight sections 21 in a symmetric circumferential profile will have a structural efficiency approaching that of a circular shape in the horizontal plane. The tubes can also be compartmentalized to be water-tight for each section to provide stability redundancy. In this way, other sections can remain buoyant even if there is a flotation failure in one section. The shape of the tube cross-sections in the vertical plane can also vary, e.g., circular, square, rectangular, ellipsoidal, trapezoidal, tear-dropped, and combinations thereof, etc. An advantage of using buoyancy tubes with circular cross- sections is that these internal volumes can handle compressed air with good structural efficiency. The compressed air can be used to reduce compressive stresses in the shell of the buoyancy tube and/or for compressed air energy storage.

In another embodiment, the floating wind turbine platform may include a porous mesh 36 inside of the buoyancy ring 20 which is suspended along some of the thrust cables 24 as shown in FIGS. 10A-B so at least a portion the porous mesh 36 is generally below the water surface. The porous mesh 36 can be horizontal or at an angle to the horizon so that the mesh is consistent with the thrust cables 24 on which it is suspended. Multiple porous meshes may be used. A porous mesh or porous meshes may also or instead be suspended by the buoyancy cables. This mesh 36 can increase drag and the added fluid mass for motion that is perpendicular to the mesh 36. As such, the mesh 36 can damp platform dynamics in pitch, roll, and heave motion. The damping will generally be proportional to the coverage area and the solidity of the mesh 36. To be cost-effective, the mesh 36 will generally have a relatively small thickness but must be composed of elements that are thick enough and have good strength to withstand the associated drag and added mass forces. For structural efficiency, the porous mesh 36 can be composed of wires that are woven together where the wires are composed of a material or materials that have high tensile strength per unit cost, such as steel, polyester, nylon, high-modulus polyethylene (HMPE) or the like.

In a further embodiment, a surface platform 38 may be placed on top of the buoyancy tube 20 and/or the thrust cables 24 to provide a platform for access to the turbine 10 as shown in FIGS. 11A-B. The surface platform 38 may extend over all or most of the area occupied by the buoyancy tube 20, as shown in FIG. 11B. Alternatively, the surface platform 38 may only cover part of the area above the buoyancy tube 20 in order to reduce the platform size and cost. Examples are shown in FIG. 12a for a buoyancy ring 20 with a circular profile and shown in FIG. 12b for a buoyancy ring 20 with a square profile. The surface platform 38 can be designed to support personnel, supplies, payload, and equipment. The surface platform 38 can be extended beyond the outer edge of the buoyancy tube 20 (as shown in FIGS. 12A-B) to provide docking for boats or ships. The above concepts are illustrated for a semi-submersible configuration which can include taut, semi-taut, or catenary mooring lines 40 which are anchored underwater at the ground (c.g. the seabed). The mooring lines arc extended below the turbine 10 to the anchors. The mooring lines can extend radially outward, i.e. the mooring lines 40 can be anchored at radial locations, relative to the tower 12 and stem 14, that are greater than that of the buoyancy tube. The mooring lines 40 may be fixed and connected to the buoyancy tube 20 as shown in FIGS. 13A-B. For a single turbine 10 with its own mooring lines 40, generally three or more mooring lines 40 are needed for stability and these lines 40 may be attached at approximately equal distances around the circumference of the buoyancy ring 20. For example, the top view of FIG. 13B shows four mooring lines 40 attached to the buoyancy ring 20 using a cable wrap attachment 30. The mooring lines 40 can also be attached with a pin connection system or shackle-based system 28 (as used to connect suspender cables to the main cable on a suspension bridge and as used to connect spoke cables on a Ferris wheel to the outer rim) or another point connection system 28.

In another embodiment, a semi-submersible floating wind turbine can be connected to other semi-submersible floating wind turbines using connecting support lines 42 to help maintain position and stability of the turbine platforms. This may be especially helpful in very deep waters (e.g. depths of hundreds of meters) where the mooring line and anchor systems become a substantial part of the system costs. The connecting support lines 42 can help reduce the number of mooring lines 40 and anchors 46 needed for a floating wind farm, which is a collection of floating wind turbines that are distributed in a local offshore region. For example, FIGS. 14A-B show a turbine 10 with two mooring lines 40 (which attach to anchors 46 at the ground level underwater, see FIGS. 15-16) and two connecting support lines 42 (which attach to other wind turbines). The connecting support lines 42 can be used to form a row 44 of wind turbines 10 using structural tension connections as shown in FIG. 15. The ideal space between turbines relative to a primary wind 48 direction is generally smaller in the lateral direction than it is in the streamwise direction. For example, the streamwise spacing between turbines may be about seven or eight rotor diameters apart, while the lateral spacing between turbines may be about three rotor diameters apart, when considered relative to the primary wind direction 48. The connecting support lines 42 can extend along the direction which has the closest wind turbine spacing, which is generally lateral (perpendicular) to the primary wind direction 48. The connecting support lines 42 are pre-tensioned by mooring lines 40 and anchors 46 at either end of a row 44 to ensure the turbines 10 stay separated from each other along the row 44. To provide good station-keeping performance, the connecting support lines 42 may be designed to be close to the surface and composed of lower density materials (e.g. non-metal materials) to minimize effects of gravitational sinking. Lower density materials for the connecting support lines 42 may include synthetic materials such as polyester, nylon, high-modulus polyethylene (HMPE) or the like. The electrical lines used for electrical power and/or communication connecting turbines in a row can be attached or be part of these connecting support lines.

As shown in FIG. 15, additional mooring lines 40 with anchors 46 can be placed in the center of the row 44 to help prevent the movement of the turbine locations. Additional mooring lines 40 with anchors 46 can be placed along the row 44 to help prevent the movement of the turbine locations. As shown in FIG. 15, multiple rows 44 can be used to provide extended coverage of an area. Relative to the primary wind direction 48, the spacing between rows 44 will generally be larger than the lateral spacing between turbines 10 along a single row 44 since their rotor wakes tend to be oriented in the direction of the primary wind direction. The available spacing between rows 44 can be used to provide surface vessel access (for boats and ships) since the mooring lines 40 will be generally far below the surface. To provide additional clearance for vessel access, the mooring lines 40 can be attached below the buoyancy tube 20, e.g., on the stem 14 and/or on a ballast ring 32. As shown in FIG. 16, mooring lines 40 from adjacent rows 44 can employ shared anchors 46. Also, as shown in FIG. 16, more than two mooring lines 40 can be employed at the end of each row 44 to provide additional safeguards in case of a mooring line or anchor failure.

In the above embodiments, it can be noted that increasing the depth of the stem 14 relative to the buoyancy tube 20 can reduce the tension forces acting on the buoyancy cables 22 and reduce the connection forces at the stem 14 and at the buoyancy tube 20. In addition, increasing the stem depth can be helpful to locate any ballast at the lowest possible depth to enhance stability. However, it is also desirable to limit the stem draft (water depth) to allow the system to be assembled and placed in the water at port-side where water depths are shallower. When taking the floating turbine 10 from the port to the site, the waters along the way may also be shallow which limits the stem draft. To temporarily decrease the draft (water depth) of the stem 14 for such temporary shallow water conditions, a temporary (and generally detachable) lifting platform 50 can be placed below the buoyancy tube 20 as shown FIG. 17. The lifting platform 50 is filled with air (or another gas) to provide additional buoyancy to raise the entire substructure and temporarily reduce the stem draft. The lifting platform 50 would be designed to maintain turbine stability in shallow water with no turbine operation (no rotor thrust and no rotor torque). To simplify the system and reduce cost, the lifting platform 50 can also be designed to handle reduced wave and wind conditions compared to those expected in deep water. The lifting platform 50 can be attached to the buoyancy tube 20 and the stem 14 with temporary lifting cables 52 and lifting cable attachments 28 as shown in FIG. 17. The lifting platform 50 may be located at a somewhat larger radial extent (as shown in FIG. 1) to minimize interactions with the buoyancy cables 22.

The lifting platform 50 may have a profile shape (viewed from above) that is similar to that of the buoyancy tube 20. For example, if the buoyancy tube 20 has a circular- horizontal profile as in FIG. 9A, the lifting platform 50 may also have a circular horizontal profile.

As another example, if the buoyancy tube 20 has a square shape for its horizontal profile as in FIG. 9B, the lifting platform 50 may also have a square horizontal profile. In addition, the lifting platform 50 need not support the entire circumference of the buoyancy tube 20, i.c. the lifting platform 50 can be in segments that only support a portion of the buoyancy tube 20. This segmentation may allow the lifting platform 50 to be more readily assembled and dissembled.

Once the turbine 10 has been towed out to deep enough water, the lifting platform 50 can be removed from the rest of the platform structure. This can be accomplished by removing the lifting cables 52 and filling the lifting platform structure 50 with sufficient water so it detaches to allow disengagement and sinks below the permanent platform and can then be removed laterally while beneath the floating platform. If the lifting platform 50 is composed of segments, then some or all of these segments can be separated to help remove it the rest of the platform structure. Once removed, the lifting platform 50 can then refilled with air in order to displace the water from its interior and restore buoyancy so it can be returned back to the shallow water areas and port to be used for the assembly and transport of another floating wind turbine.

As such, the lifting platform 50 acts as a reusable buoyant jack system to assist in floating turbine deployment. This temporary lifting platform may allow longer stem lengths in deep water which can reduce the tension on the buoyancy cables and increase stability while still allowing deployment in ports with shallow water.

In the above embodiment, the lifting platform 50 is envisioned as a rigid structure. In another embodiment, the lifting platform 50 can employ a flexible material (such as Polyvinyl Chloride, Polyurethane, Neoprene Hyapalon, or the like) so it can be inflated for initial use to support the turbine 10 for assembly and transport. The flexible lifting platform structure 50 can then be deflated to allow disengagement for turbine deployment so it sinks below the permanent platform and can then be removed laterally while beneath the floating platform.

The above concepts are illustrated for a semi-submersible configuration but the ring-based concept can also be applied to floating turbines with a tension-leg platform configuration. For example, FIG. 18 shows the buoyancy tube 20 and cable configuration 22, 24 of FIG. 1 but in a tension-leg platform where the buoyancy tube 20 is now fully submersed underwater and the mooring lines 40 are nearly vertical and under higher tension to provide an additional downward force for stabilization. This embodiment again avoids the radially extending pontoons as are typically used in tension-leg platform configurations. As such, the buoyancy tube design may be much simpler to manufacture and assemble as well as being less costly as compared to conventional tension-leg platform designs. While the above configurations are based on a floating wind turbine platform, the buoyancy ring concept can also be applied to enable fixed-bottom turbines in water locations that would be ordinally too deep for a conventional fixed-bottom approach or at sites where the seabed soil is not practical for a conventional fixed-bottom foundation. In particular, the buoyancy tube 20, buoyancy cables 22, and thrust cables 24 can be attached to the tower 12 and underwater structure of a fixed-bottom turbine 100 to create a hybrid “fixed- floating” wind turbine. For example, FIG. 19 shows the ring-based floating platform design of FIG. 4 applied to a wind turbine 100 with a soil-embedded monopile foundation 102. The soil-embedded monopile foundation 102 can be used to ensure station keeping and support export line connections while the buoyancy support 20 can help keep the turbine 100 stable and nearly vertical, even when there are large thrust and/or wave loads, thereby reducing/relaxing the local foundation requirements compared to a conventional fixed-bottom turbine at this same depth. This can allow the increased flexibility associated with monopile systems to reduce costs and/or allow greater water depths. While FIG. 19 is based on a soil-embedded monopile, the hybrid fixed-floating concept can be applied to other fixed-bottom foundations such as gravity base, suction caisson, jacket/truss, and tripod foundations. Notably, the foundation and parts of the underwater structure can be designed to allow some flexibility or articulation at the ground level since the buoyancy ring can serve to provide sufficient forces to keep the above-water turbine nearly vertical. The size and vertical location of the buoyancy ring should ideally be set so that all portions of the circumference are at least partially submerged at all times. In addition, use of a hybrid fixed-floating approach can reduce concerns of foundation scouring and settling.

In addition, all combinations of the above described embodiments (including those of Figs 1-19) arc envisioned.

Various embodiments of the present invention are described herein with reference to the related drawings. Alternative embodiments can be devised without departing from the scope of this invention. It is noted that various connections and positional relationships (e.g., over, below, adjacent, etc.) are set forth between elements in the following description and in the drawings. These connections and/or positional relationships, unless specified otherwise, can be direct or indirect, and the present invention is not intended to be limiting in this respect. Accordingly, a coupling of entities can refer to cither a direct or an indirect coupling, and a positional relationship between entities can be a direct or indirect positional relationship.

The term "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments or designs. The terms "at least one" and "one or more" are understood to include any integer number greater than or equal to one, i.e. one, two, three, four, etc. The terms "a plurality" are understood to include any integer number greater than or equal to two, i.e. two, three, four, five, etc. Terms such as "connected to", “affixed to”, etc., can include both an indirect "connection" and a direct "connection."

The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the ail to understand the embodiments disclosed herein.

LISTING OF REFERENCE NUMERALS:

10 floating wind turbine

12 tower

12A single thrust cable tower attachment

12B double thrust cable tower attachment

14 stem

16 nacelle

18 rotor

20 buoyancy tube

20A inner buoyancy tube

20B outer buoyancy tube

21 tube sections

22 buoyancy cable

24 thrust cable

26 stem bottom

28 point attachment

30 wrap attachment

32 ballast ring

34 ballast cables

36 porous mesh

38 surface platform

40 mooring lines

42 connecting support lines

44 row of turbines

46 anchor

48 primary wind direction

50 lifting platform

52 lifting cables

100 fixed-bottom wind turbine

102 monopile

Claims

CLAIMS:

1. A wind turbine platform for flotation, comprising: a buoyancy tube disposed circumferentially around a tower and an underwater support structure of a wind turbine, the underwater support structure comprising a stem for a floating wind turbine or monopile or other support structure for a fixed-bottom wind turbine; a plurality of buoyancy cables extending generally radially inward and axially downward from the buoyancy tube to the underwater structure and arranged primarily to cany in tension an upward buoyancy force of the buoyancy tube; and a plurality of thrust cables extending generally radially inward from the buoyancy tube to the underwater support structure and/or tower that are arranged, along with the buoyancy cables, to carry in tension a generally horizontal force imparted upon the stem by a combination of rotor thrust, torque, wind, and wave forces.

2. The wind turbine platform of claim 1, wherein each of the buoyancy and thrust cables is affixed at a one end to the buoyancy tube by a tube attachment or by wrapping around the tube, and wherein each buoyancy cable attaches at or near a bottom of the stem and either terminates or continues to another point at the buoyancy tube, and wherein an opposite end of each thrust cable is affixed to the tower or stem and either terminates or continues to another point at the buoyancy tube.

3. The wind turbine platform of claim 2, wherein the tube attachment includes connections with a tube wrap, cable band, pin connection, cable rings, a truss-based ring structure, or the like.

4. The wind turbine platform of claim 1, wherein the buoyancy tube is a volume having a planform circular shape or a shape with two or more curved edges or three or more curved edges and wherein the buoyancy tube is circumferentially and symmetrically disposed around the wind turbine tower and the underwater support structure.

5. The wind turbine platform of claim 4, wherein the buoyancy tube is formed of multiple distinct straight or curved segments that are fastened together and which are individually watertight.

6. The wind turbine platform of claim 1 for a floating wind turbine configuration, further comprising a ballast placed at a bottom of the stem and/or a ballast ring circumferentially disposed around the wind turbine stem and tower wherein this ballast ring is beneath the buoyancy tube and connected to the buoyancy tube and/or stem with ballast cables to form a rigid ballast system or to allow free movement of the ballast ring to form a pendulumbased tuned mass damper to dampen pitch and/or roll of the floating platform.

7. The wind turbine platform of claim 1, further comprising a second buoyancy tube disposed radially outward of the first buoyancy tube and secured to the first buoyancy tube and by the buoyancy and thrust cables.

8. The wind turbine platform of claim 1, further comprising a fixed surface above-water platform disposed supported by thrust cables and configured to provide access for personnel, payload, and/or equipment, wherein the surface platform may cover all, more, or less than an area defined by an outer extent circumscribed by the buoyancy tube.

9. The wind turbine platform of claim 1 for a floating wind turbine configuration, further comprising mooring lines connected and fixed to the buoyancy tube at one end and secured to an underwater ground floor by an anchoring system.

10. The wind turbine platform of claim 1 for a floating wind turbine configuration, further comprising a temporary lifting platform disposed around the wind turbine stem and beneath the buoyancy tube in supporting engagement therewith to increase the buoyancy force and temporarily reduce platform draft where the lifting platform is composed of rigid structural members that are filled with water to allow the lifting platform to be removed for disengagement or is composed of a flexible material to allow the lifting platform to be deflated for disengagement.

11. The wind turbine platform of claim 1, wherein the wind turbine comprises a underwater support structure fixed to the seabed or lakebed and wherein the buoyancy tube is arranged around a vertical axis that extends through the wind turbine tower and the underwater support structure, and wherein the buoyancy cables are affixed to the underwater support structure and/or to a seabed or lakebed foundation and wherein a size and vertical location of the buoyancy tube are configured such that the buoyancy tube provides a substantial amount or a majority of the turbine’s gravitational support relative to that provided by the underwater support structure.

12. The wind turbine platform of claim 4, wherein the buoyancy tube comprises a toms shape and where the toms shape is circumferentially and symmetrically disposed around the wind turbine tower and the underwater structure.

13. The wind turbine platform of claim 2, wherein the buoyancy cables extend around the bottom of the stem through a stem-cable saddle for a floating wind turbine and wherein the thmst cables are connected to the tower and/or the remaining underwater structure at one or multiple locations.

14. The wind turbine platform of claim 1, wherein the buoyancy and thrust cables are affixed together at respective common connection points at one of an interior, an exterior, at top, or a bottom of the buoyancy tube.

14. The wind turbine platform of claim 1, wherein the buoyancy and thrust cables are affixed separately at distinct connection points at one of an interior, an exterior, at top, or a bottom of the buoyancy tube.

15. The wind turbine platform of claim 1, wherein at least one of the buoyancy and thrust cables is attached generally tangential and/or is attached generally radial to the underwater support structure.

16. The wind turbine platform of claim 1, wherein at least one of the buoyancy and thrust cables is bifurcated at at least one of first and second opposing ends to delimit additional points of connection with the buoyancy tube.

17. The wind turbine platform of claim 1, further comprising one or more porous meshes disposed inside of the buoyancy tube and suspended along at least some of the thrust cables and/or the buoyancy cables such that at least a portion of the one or more porous meshes is located below the water surface to provide increased added mass and/or damping of pitch, roll, and/or heave motion of a floating wind turbine.

18. The wind turbine platform of claim 17, wherein the porous mesh is composed of wires woven together where the wires comprise a material having a high tensile strength per unit cost, such as steel, polyester, nylon, high-modulus polyethylene (HMPE) or the like.

19. The wind turbine platform of claim 1, wherein a plurality of said platforms are connectable together to form one or more rows of wind turbines, wherein a connecting support line or lines in tension extends between adjacent floating platforms in a row to help maintain position and stability of the turbine platforms.

20. The wind turbine platform of claim 19, wherein mooring lines extend from the underwater ground floor to buoyancy tubes at ends and/or interior locations of the row.

21. The wind turbine platform of claim 20, wherein mooring lines of adjacent rows are secured to distinct and/or shared anchors at the underwater ground floor.

22. A floating wind turbine farm, comprising: a row or a plurality of rows of floating wind turbines; a connecting support line or lines extending between adjacent floating wind turbines in a row to provide tensioned connections between turbines to help maintain position and stability of the turbine platforms; electrical lines connecting turbines in the row, wherein the electrical lines arc for electrical power and/or communication and are independent of connecting support lines or are combined to form a part of the connecting support lines; mooring lines extending from the underwater ground floor to at least two or more of the floating wind turbines that are located at ends and/or interior locations of a row; and distinct and/or shared anchors disposed on the underwater ground floor between adjacent rows wherein at least some of the mooring lines from the adjacent rows are secured to these distinct and/or shared anchors.

23. The wind turbine farm of claim 22, wherein each of the floating wind turbines comprise a wind turbine platform according to claim 1.

24. The wind turbine platform of claim 1 for a floating wind turbine, wherein the buoyancy tube is partially submerged to allow a semi-submersible platform configuration or wherein the buoyancy tube is fully submerged to allow a tension-leg platform configuration.

25. The wind turbine platform of claim 1, wherein the buoyancy tube is fabricated out of steel, fiberglass and/or other materials that provide high strength and robustness per unit cost of the material.

26. The wind turbine platform of claim 1 for a floating wind turbine, wherein an amount of pre-tension in the thrust cables is set to optimize a stiffness of the combined tower and stem to minimize a cost of the combined tower and stem.

27. The wind turbine platform of claim 1, wherein the buoyancy tube is configured to store compressed air, wherein the compressed air provides energy storage and/or reduces compressive stresses in a structural shell of the buoyancy tube.

28. A floating wind turbine platform, comprising one or more porous meshes supported by pontoons, buoyancy components, and/or cables such that at least a portion of the one or more porous meshes is located below a water surface to provide increased added mass and/or damping of pitch, roll, and/or heave motion of the wind turbine.

29. The wind turbine platform of claim 1, wherein the buoyancy and thrust cables comprise at least one of a cable, a spoke, and/or a structural tension element.

30. An offshore flotation platform for storage or drilling of oil and/or natural gas, comprising: a buoyancy tube; a plurality of buoyancy cables, spokes, and/or structural tension elements extending generally radially inward and axially downward from the buoyancy tube to an underwater structure and arranged primarily to carry in tension an upward buoyancy force of the buoyancy tube; and a plurality of thrust cables, spokes and/or structural tension elements extending generally radially inward from the buoyancy tube to the underwater support structure and/or tower that are arranged, along with the buoyancy cables, to carry in tension a generally horizontal force imparted upon the stem by a combination of wind and wave forces.