A floatable renewable energy platform

Abstract

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

Description

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

[0002] Renewable energy is energy from renewable resources that are naturally replenished on a human timescale.

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

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

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

[0006] There are four generic designs of floating wind platform as shown in FIG. 1 (source: Frazer Nash report prepared for the UK Department for Energy Security and Net Zero). 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 type—achieves stability by having a deep vessel draught and low centre of gravity.

[0008] Barge type—has the stability characteristics of a normal barge or ship.

[0009] Semisubmersible type—designed to have low motion response in waves by arranging that the natural periods of heave, roll and pitch to be longer than typical ocean wave periods. Also the underwater form is designed to minimise vertical wave forces and motions at critical wave periods.

[0010] Tension leg type—stability and low heave motion response in waves is achieved by taut vertical tethers.

[0011] The present invention is a new floating platform generic design based on different principles which seeks to provide advantages for appropriate mid and deep water offshore locations and environments.SUMMARY OF THE INVENTION

[0012] Aspects and embodiments of the invention relate to a floating platform for supporting equipment such as renewable energy devices (including wind turbines which generate electricity from wind flow).

[0013] The floating platform may include one or more of several design features which allow the platform to be constructed, installed and operated more cost effectively than existing designs.

[0014] An aspect of the present invention provides a floatable renewable energy platform comprising a superstructure including one or more generally horizontal, above-wave structural support members, the superstructure is supported by of plurality of floats interconnected by the structural members, and the platform is configured to provide an air gap between still water surface and the structural members in the range 10 m to 30 m.

[0015] A further aspect provides a permanently moored, self-floating renewable energy platform, comprising a plurality of floats interconnected by horizontal structural members above wave action and direct wave forces at all times with an air gap of 10 m to 30 m between them and the still water surface (possibly determined by the wave climate for which the platform is designed).

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

[0017] Some embodiments comprise spaced bow and stern floats giving a corrugated pitch response amplitude operator in head seas.

[0018] Platforms may comprise spaced floats, in the range 100 m to 300 m apart, to produce two or more frequencies where wave excitation forces or moments are anti-phase in operating wave periods in the range 4 to 12 seconds, therefore producing zero excitation force and response at that frequency as evidenced by a corrugated pitch or heave response amplitude operator for head seas in that range of operating wave periods.

[0019] Some embodiments relate to shaping of the floats.

[0020] In some embodiments floats are shaped / configured to reduce bending loads in horizontal structural members.

[0021] Some embodiments provide floats which are a solid of revolution.

[0022] Some embodiments cover putting the axis of that solid of revolution at the level of the horizontal structural members.

[0023] Some embodiments provide floats (which may or may not be solids of revolution) which put the wave force at the level of the horizontal structural members, in low-steepness waves of a particular wavelength.

[0024] Some embodiments comprise floats which have a wetted hull form in the shape of a solid of revolution with axis that is horizontal and at right angles to the longitudinal axis of the float.

[0025] Some embodiments comprise floats shaped to reduce bending loads in horizontal structural members by locating the axis of revolution at the neutral axis and work point of the main structural members attached to the float.

[0026] In some embodiments a mooring is provided on a bow float. The mooring may be attached to the float by means of a yoke, for example in the form of a rigid frame or chain equivalent, suspended and hinged at the level of the neutral axis of the horizontal structural members connected to the float.

[0027] Floats may be longer than they are wide.

[0028] Some embodiments are configured as a floating wind turbine. Floating wind energy platforms may comprise spaced stern floats positioned under one or multiple wind turbines, and a bow float.

[0029] Platforms may be configured for natural pitch, heave and optionally roll, periods below 10 seconds and longitudinal metacentric height more than one thousand metres.

[0030] Some embodiments have a rated power above 5 MW and configured for operational draught below 7 m.

[0031] Floats and / or structural members may, for example, be formed from composite material.

[0032] Floats and / or structural members may be formed from fibre glass reinforced plastic (FGRP).

[0033] Floats and / or structural members may, for example be formed from conventional ship building materials including steel, aluminium and concrete.

[0034] Structural members may be formed as lattice structures. Chords and cross-bracing in structural members may be of aerofoil cross-section to reduce direct aerodynamic loads.

[0035] In some aspects and embodiments the platform has a generally flat, folded configuration and an erected configuration.

[0036] The platform may comprise a plurality of structural members incorporating hinges.

[0037] Some embodiments may be configured to fold flat down to the level of the horizontal structural member between floats, in the way that a deck chair folds flat, with multiple hinges depending on the arrangement of structural members.

[0038] Some aspects and embodiments provide a platform configured as a floating wind turbine and comprising one or more wind turbines.

[0039] Platforms may be provided with one or more thrusters to control heading. The addition of thrusters to the stern floats, enables weathervaning to be controlled. This is desirable to stabilise the platform heading, since vessels with weathervaning moorings are prone to a “fishtailing” instability in deep water. It can also be used to set the heading exactly into the wind, if this is desirable to ease the blade loads on the turbine, especially in survival conditions when the turbine is stopped. Alternatively, it can be used to set the heading exactly into the waves, if this is desirable to reduce the transverse wave loads on the floats. Finally it can be used to rotate the wind turbine completely about a vertical axis, in calm conditions, and thus unwind the power cable to the seabed. This eliminates the need for an electrical swivel, an expensive item.

[0040] Such a stable platform has other applications requiring a stable platform in waves, e.g. floating solar panels.

[0041] In some embodiments the platform is configured as a floating wind turbine and comprising one or more wind turbines.

[0042] A further aspect provides a floatable renewable energy platform, the platform having a generally flat, folded configuration and an erected configuration.

[0043] The platform may be movable between the generally flat, folded configuration and the erected configuration.

[0044] Floats may be provided for the platform. Floats may be interconnected by structural members above wave action.

[0045] Further aspects and embodiments are listed below.

[0046] 1) A floating wind turbine designed according to the principles (a)-(c):

[0047] a) increase the hydrostatic roll and pitch stiffness so that the natural roll and pitch periods are well within the range of typical wave periods;

[0048] b) reduce the platform draught, so that the resonant roll and pitch motions are well damped by wave radiation; and

[0049] c) choose the float length so in the wave periods which excite pitch resonance in the platform the vertical wave force on a float cancels out down its length.

[0050] 2) A floating wing turbine incorporating hinges, so that it can fold flat like a deck chair, enabling the assembly to be performed at low level, e.g. on construction barges, thus minimising the requirement for cranes with high hook height and allows the whole floating wind turbine to be assembled on construction barges, in a sheltered anchorage.

[0051] 3) A floating wind turbine with floats in the shape of a solid of revolution about an axis of rotation that is horizontal and at right angles to the longitudinal axis of the float. The axis of rotation is located at the neutral axis and work point of the main structural members attached to the float This reduces the bending moments transferred to the structural members.

[0052] 4) A floating wind turbine with thrusters to control its heading.

[0053] 5) A floating platform with natural pitch and heave periods below 10 seconds and metacentric height more than one thousand metres.

[0054] 6) A floatable renewable energy platform designed based on one or more of the following:

[0055] a) Increase the hydrostatic heave and pitch stiffness, by having a metacentric height of more than a thousand metres to minimise pitch angles under wind loads (high heave, pitch and roll stiffness can also be achieved by a tension-leg mooring, as in some oil rigs and floating wind turbines—these are excluded from this invention). Typically a large ballasting system is not required to counter wind pitch, a significant advantage. The high heave and pitch stiffness means that the natural heave and pitch periods are well within the range of typical wave periods. In high wind speeds, this design feature is highly beneficial to the wind turbine control system, which seeks to limit power when the wind speed increases in a gust, by reducing the wind load on the turbine. Pitch motions of the platform appear to the turbine control system as gusts: reducing the wind load in such cases corresponds to negative pitch damping. This can give a dangerous dynamic instability in pitch. It is thus very advantageous that the natural pitch period of the platform is short, like the natural period of the tower of a comparable land-based wind turbine.

[0056] b) Reduce the platform draught, so that the resonant heave and pitch motions are well damped by wave radiation. Use shallow-draught floats.

[0057] c) Adopt a “weathervaning” mooring like a single-point-moored FPSO (and some other floating wind turbines), so that the platform predominantly heads into the wind and waves and wind turbines on the platform operate in a downwind direction.

[0058] d) Make the floats longer than they are wide, like a ship, and point them into the waves. The mooring is on the bow float (as opposed to the stern floats, under the turbine).

[0059] e) Choose the float length so in the wave periods which excite pitch resonance in the platform (platform pitch direction=float pitch direction), the vertical wave force on a float cancels out down its length (e.g. choose float length=wavelength, approximately). This will further reduce resonant roll and pitch motions. The following formula L=2π×(1+Cm)×D gives the float length L. Here Cm is the heave added mass coefficient of the float, and D is the draught of the float.

[0060] f) Choose a very wide float spacing, typically many float lengths. This will produce a “corrugated” pitch, heave and surge RAO in head seas, because at certain wavelengths the wave forces on bow and stern floats act in opposite direction, and at other wavelengths they act in the same direction. Overall, motions in waves are reduced, a significant advantage.

[0061] g) Interconnect the floats with structural members that are above the water surface at all times. This eliminates direct wave loads on the members, which could otherwise be a significant design issue. The structural members can thus be light lattice structures, like a crane boom. They, and the floats, can be made from composite material such as fibre glass reinforced plastic (FGRP) instead of steel, reducing the cost and weight for the platform. To reduce direct aerodynamic loads, the chords and cross-bracing in the structural members can be of aerofoil cross-section.

[0062] h) Incorporate hinges at the ends of some of the structural members, so that the platform can be folded flat like a deck chair. This allows the whole floating wind turbine to be assembled on construction barges, in a sheltered anchorage.

[0063] i) Because the horizontal structural members are some distance above the still water surface, as described above, the horizontal wave load on the floats may act considerably below them, thus putting bending loads into the structural members. This may be a significant design issue. The bending loads can be greatly reduced by shaping the floats so that the horizontal wave loads on them act at the same level as the neutral axis (typically the centreline) of the structural members. The simplest such float shape is sphere, with its centre at this level (only the wetted surface of the float need have such a shape, of course). All pressure loads on a sphere act though its centre, so their horizontal component is at the level of the centre. Another float shape with this property is any port-starboard symmetric body of revolution with its axis parallel to the wave crests, and at the same level as the centre of the sphere. To reduce bending loads as described, they do require the waves to be travelling from the bow float towards the stern ones (“head seas” in the nautical jargon), but this will generally be the case because of the “weathervaning” mooring described above. Other float shapes can be designed with a diffraction program (see ref. 1, chapter 4), or otherwise, to produce zero bending loads in head seas of low steepness and a particular wavelength; this may be sufficient.

[0064] j) A further design feature is the addition of thrusters to the stern floats, so that the weathervaning can be controlled. This is desirable to stabilise the platform heading, since vessels with weathervaning moorings are prone to a “fishtailing” instability in deep water. It can also be used to set the heading exactly into the wind, if this is desirable to ease the blade loads on the turbine, especially in survival conditions when the turbine is stopped. Alternatively, it can be used to set the heading exactly into the waves, if this is desirable to reduce the transverse wave loads on the floats. Finally it can be used to rotate the wind turbine completely about a vertical axis, in calm conditions, and thus unwind the power cable to the seabed. This eliminates the need for an electrical swivel, an expensive item.

[0065] 7) A floating wind turbine designed according to the principles (a)-(c).

[0066] 8) A floating wing turbine incorporating hinges, so that it can fold flat like a deck chair, enabling the assembly to be performed at low level, e.g. on construction barges.

[0067] 9) A floating wind turbine with floats in the shape of a solid of revolution with port-starboard symmetry, as described herein.

[0068] 10) A floating wind turbine with thrusters to control its heading, as described herein.

[0069] 11) A floating wind turbine etc. with natural pitch and heave periods below 10 seconds and metacentric height more than one thousand metres.

[0070] 12) A floating wind turbine etc. of rated power above 5 MW with operational draught below 7 m.

[0071] 13) A floating wind turbine etc. with floats of length given e.g. by the equation in (e).

[0072] 14) A floating wind turbine etc. with widely spaced floats giving a “corrugated” pitch RAO e.g. as described in (f).

[0073] 15) A floating wind turbine etc. with its floats interconnected by structural members above wave action, and both advantageously made of FGRP, e.g. as described in (g).

[0074] 16) A floating wind turbine etc. which folds flat like a deck chair, e.g. as described in (h).

[0075] 17) A floating wind turbine etc. with floats in the shaped to reduce bending loads in the horizontal structural members, e.g. as described in (i).

[0076] 18) A floating wind turbine etc. with thrusters to control its heading, e.g. as described in (j).

[0077] Different aspects and embodiments of the invention may be used separately or together.

[0078] Further 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 the features of the independent claims as appropriate, and in combination other than those explicitly set out in the claims. Each aspect can be carried out independently of the other aspects or in combination with one or more of the other aspects.BRIEF DESCRIPTION OF THE DRAWINGS

[0079] The present invention will now be more particularly described with reference to, and as shown in, the accompanying drawings, in which:

[0080] FIG. 1. Existing generic designs of floating wind platform;

[0081] FIG. 2. A floating wind turbine platform formed according to an embodiment;

[0082] FIG. 3. A side view of the platform of FIG. 2 illustrating an airgap;

[0083] FIG. 4. Typical corrugated pitch RAO in head seas;

[0084] FIG. 5. A floatable platform folded flat for construction or maintenance;

[0085] FIG. 6. The platform of FIG. 5 shown erected;

[0086] FIG. 7. Folding / erecting the floating turbine with the aid of cranes;

[0087] FIG. 8. Folding / erecting the floating turbine using a built-in winch;

[0088] FIG. 9. Platform with shaped floats to minimise bending loads in structural members; and

[0089] FIG. 10. Mooring yoke, pivoting with mooring line about bearing shaft.

[0090] The example embodiments are described in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. It is important to understand that embodiments can be provided in many alternative forms and should not be construed as limited to the examples set forth herein.

[0091] Accordingly, while embodiments can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples. There is no intent to limit to the particular forms disclosed. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included. Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate.

[0092] Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealised or overly formal sense unless expressly so defined herein.

[0093] In the following description, all orientational terms, such as upper, lower, radially and axially, are used in relation to the drawings and should not be interpreted as limiting on the invention.

[0094] See ref. 1 for definitions of technical terms, e.g. roll and pitch.DETAILED DESCRIPTION OF THE DRAWINGS

[0095] This application concerns a new generic design for floating wind platforms. A typical example of this generic design is shown in FIG. 2.

[0096] It features a floatable renewable energy platform 10 comprising a superstructure including one or more generally horizontal, structural support members 20 positioned to avoid wave impact. The superstructure is supported by of plurality of floats 30 interconnected by the structural members, and the platform is configured to provide an air gap between still water surface and the structural members in the range 10 m to 30 m.

[0097] In this embodiment one bow float 30a and two stern floats 30b, 30c are provided.

[0098] In this embodiment there are three horizontal support members 20a, 20b, 20c arranged in a generally triangular configuration and three uprights (uprights could be generally vertical or generally oblique, for example) 40a, 40b, 40c, together providing a generally triangular pyramidal superstructure. The superstructure supports a wind turbine 50.

[0099] An air gap below the superstructure is illustrated in FIG. 3. It is chosen so that even in the most extreme conditions, the wave crests do not impact the structural members. The size of the air gap may be calculated from statistical data on the wave climate at the platform site, by a standard process.

[0100] The separation between the bow float 30a and the stern floats 30b, 30c is in the range 100 m to 300 m, depending on wave environment. By spacing the floats in this way, wave forces and moments on the floats are out of phase at some frequencies resulting in zero excitation force at multiple wave periods in typical operating waves of periods from 4 to 12 seconds. This is evident in the corrugated nature of motion response amplitude operators as illustrated in FIG. 4. This reduces vessel motions which can be detrimental to structural integrity and energy production.

[0101] FIGS. 5 to 8 show a floatable renewal energy platform 110 formed according to a further aspect. The platform 110 has a generally flat, folded configuration (FIG. 5) and an erected configuration (FIG. 6).

[0102] The folded flat configuration may be useful for rapid installation of wind turbine and blades with minimal requirement for a crane with a high hook height.

[0103] The main structural members have a lattice construction in this embodiment. The hinges in the platform 110 are illustrated in FIGS. 7 and 8. They could equally be at any other positions which allow the structure to fold flat like a deck chair. The hinge could be at the other end of the longest structural member, for example.

[0104] As shown in FIG. 7, the platform 110 requires two cranes to lower it into the folded position, and to lift it into the erected position. For a large wind farm, the cost of such cranes may be insignificant, because the folding / erecting operation is quick, so a single pair of cranes would be sufficient for the construction and maintenance of the whole farm. For a small wind farm, however, it may be cost-effective to provide a built-in winching functionality on the platform, as shown in FIG. 8. A winch can then raise and lower the turbine on its own. The additional structure required is very light compared with the turbine, so it can be erected with a small crane, lifting from points on the structural members that are nearer the hinges.

[0105] A platform 210 formed in accordance with a further embodiment is shown in FIG. 9.

[0106] The platform 210 has a longitudinal metacentric height of more than a thousand metres to minimise pitch angles under wind loads.

[0107] The platform 210 is configured with two wind turbines and comprises spaced stern floats positioned the wind turbines, and a bow float. The floats are interconnected with structural members that are above the water surface at all times.

[0108] A “weathervaning” mooring like a single-point-moored FPSO (and some other floating wind turbines), so that the platform predominantly heads into the wind and waves and wind turbines on the platform operate in a downwind direction. The mooring is on the bow float (as opposed to the stern floats, under the turbine).

[0109] In this embodiment a mooring is provided on the bow float. The bow mooring may take the form of a rigid frame yoke or an equivalent chain structure suspended and hinged at the level of the neutral axis of the horizontal structural members connected to the float as shown in FIG. 10.

[0110] The structural members are lattice structures, like a crane boom. The chords and cross-bracing in the structural members can be of aerofoil cross-section. The structural members and the floats are made from fibre glass reinforced plastic (FGRP).

[0111] Because the horizontal structural members are some distance above the still water surface, as described above, the horizontal wave load on the floats may act considerably below them, thus putting bending loads into the structural members. This may be a significant design issue. The bending loads can be greatly reduced by shaping the floats so that the horizontal wave loads on them act at the same level as the neutral axis (typically the centreline) of the structural members. The simplest such float shape is sphere, with its centre at this level (only the wetted surface of the float need have such a shape, of course). All pressure loads on a sphere act though its centre, so their horizontal component is at the level of the centre. Another float shape with this property is any port-starboard symmetric body of revolution with its axis parallel to the wave crests, and at the same level as the centre of the sphere.

[0112] Floats of this type (e.g. floats of double-conical shape) are shown in FIG. 9. To reduce bending loads as described, they do require the waves to be travelling from the bow float towards the stern ones (“head seas” in the nautical jargon), but this will generally be the case because of the “weathervaning” mooring described. Other float shapes could be designed with a diffraction program (e.g. see ref. 1, chapter 4), or otherwise, to produce zero bending loads in head seas of low steepness and a particular wavelength.

[0113] In FIG. 10 the bow float is also subject to loads from the mooring. These can also be made to act at the level of the structural members, by means of the mooring yoke shown. Such a yoke has been used before, on the Pelamis wave power machine, but there its function is quite different, to help restrain roll motions rather than prevent structural bending. On a floating wind turbine etc. it has another advantage apart from reducing bending loads in the structural members: it increases the effective mooring length, allowing operation in shallower waters.

[0114] Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiments shown and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.REFERENCE

[0115] 1. Faltinsen, O. M. 1990 Sea Loads on Ships and Offshore Structures. Cambridge University Press

Claims

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

2. A permanently moored floating renewable energy platform, comprising a plurality of floats interconnected by horizontal structural members that are above wave action and direct wave forces at all times, with an air gap of 10 to 30 m between the members and the still water surface level.

3. A platform as claimed in claim 1, comprising spaced floats, in the range 100 m to 300 m apart, so producing two or more frequencies where wave excitation forces or moments are anti-phase in operating wave periods in the range 4 to 12 seconds, therefore producing zero excitation force and response at that frequency as evidenced by a corrugated pitch or heave response amplitude operator for head seas in that range of operating wave periods.

4. A platform as claimed in claim 1, in which floats are shaped to reduce bending loads in the horizontal structural members.

5. A platform as claimed in claim 1, with floats which have a wetted hull form in the shape of a solid of revolution with axis that is horizontal and at right angles to the longitudinal axis of the float.

6. A platform as claimed in claim 5, with floats shaped to reduce bending loads in horizontal structural members by locating the axis of revolution at the neutral axis and work point of the main structural members attached to the float.

7. A platform as claimed in claim 1, in which a mooring is provided on the bow float.

8. A platform as in claim 1, with a mooring on a bow float, attached to the float by means of a yoke, in the form of a rigid frame or chain equivalent, suspended and hinged at the level of the neutral axis of the horizontal structural members connected to the float.

9. A platform as claimed in claim 1, in which floats are longer than they are wide.

10. A platform as claimed in claim 1, configured as a floating wind turbine and comprising spaced stern floats positioned under one or multiple wind turbines, and a bow float.

11. A platform as claimed in claim 1, configured for natural pitch, heave and optionally roll, periods below 10 seconds.

12. A platform as claimed in claim 1, having longitudinal metacentric height more than one thousand metres.

13. A platform as claimed in claim 1, with a rated power above 5 MW and configured for operational draught below 7 m.

14. A platform as claimed claim 1, in which floats and / or structural members are formed from composite material.

15. A platform as claimed in claim 14, in which floats and / or structural members are formed from fibre glass reinforced plastic (FGRP).

16. A platform as claimed in claim 1, in which floats and / or structural members are formed from one or more of: steel, aluminium and concrete.

17. A platform as claimed claim 1 comprising structural members formed as lattice structures.

18. A energy platform as claimed in claim 17, in which chords and cross-bracing in the structural members is of aerofoil cross-section to reduce direct aerodynamic loads.

19. A platform as claimed in claim 1, the platform having a generally flat, folded configuration and an erected configuration.

20. A platform as claimed in claim 19, in which the platform comprises a plurality of structural members incorporating hinges.

21. A platform as in claim 19, configured to fold flat down to the level of the horizontal structural member between floats, in the way that a deck chair folds flat, with multiple hinges depending on the arrangement of structural members.

22. A platform as claimed in claim 1, with thrusters to control its heading.

23. A platform as claimed in claim 1 and configured as a floating wind turbine and comprising one or more wind turbines.

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