Floating platform, in particular for an offshore wind turbine, having improved stability

Abstract

The invention relates to a floating platform (2), in particular a platform for a wind turbine (2), which affords improved stability by means of an arrangement of anchoring tendons (17, 18). The invention also relates to a method for assembling such a platform and the wind turbine thereof as well as to a method for anchoring said platform offshore.

Description

[0001] DESCRIPTION

[0002] Title: Floating platform, particularly for an offshore wind turbine, with improved stability.

[0003] The present invention relates to the field of offshore platforms, in particular platforms serving to support offshore wind turbines.

[0004] In particular, a platform supporting an offshore wind turbine must be as stable as possible to ensure electrical energy production, even in conditions of strong swell or strong wind within the operating limits set by the manufacturer. Indeed, beyond a certain wind speed, for example eighty kilometers per hour, the system is automatically shut down.

[0005] In addition, a more stable platform allows easier access to the wind turbine; the maintenance cost of the wind turbine is therefore reduced. Also, by reducing the stresses borne by the wind turbine, the need for maintenance and wear are reduced and its lifespan is increased, which increases productivity.

[0006] An aim of the invention is to provide a floating platform with improved stability and a method for installing an offshore wind turbine using such a platform.

[0007] According to a first subject of the invention, a floating platform, in particular an offshore platform, intended to support equipment and organized around a substantially vertical main axis, comprises a float, tendons, fixing means for holding an upper end of each of the tendons to the float and anchoring means for fixing a lower end of each of the tendons to a seabed, the tendons comprising at least three tendons of a first type, arranged vertically, and at least three tendons of a second type, arranged obliquely, between their attachment means and their anchoring means, the types of tendons being arranged alternately around the main axis and being designed so that, in an operational position, the float is kept submerged at a depth below the average surface of the water, and at least the vertical tendons are kept taut.

[0008] Preferably, all oblique tendons form with the main axis the same angle preferably greater than five degrees.

[0009] The oblique tendons may be rigid rods, preferably tubular. Advantageously, each end of each of the tendons is articulated to its fixing or anchoring means by ball and socket means. Equally advantageously, the attachment means are designed to transmit only tensile forces to the upper ends of the oblique tendons.

[0010] Oblique tendons can also be formed from strands of a continuous cable stretched between pulleys carried by the float and pulleys carried by respective anchors.

[0011] The float is preferably of revolution around the main axis; it can be annular.

[0012] The platform may comprise a riser and a seat for the equipment, this seat being arranged on the riser, the riser extending upwards from the float, and being designed so that in the use position of the platform there is an air draft under the seat. Advantageously, the riser comprises means of transparency to swell and wind, preferably a lattice structure, preferably a structure comprising posts and substantially horizontal bars connecting the posts together. Preferably the riser is substantially of revolution around the main axis. In addition, the platform may comprise inflatable buoy means capable of ensuring sufficient flotation of the platform if the float is fully ballasted or partially ballasted. Such buoy means are particularly useful during the tendon attachment phase or if an incident reduces the submerged volume of the platform.

[0013] A second object of the invention relates to an assembly comprising a platform according to the invention and equipment supported by this platform.

[0014] If the platform of this assembly comprises a seat, the equipment may comprise a base which forms swivel means with the seat of the platform. The equipment may then comprise counterweight means which extend downwardly from the base through the platform, these counterweight means preferably comprising a rod extending downwardly from the base and a mass fixed to a lower end of the rod, the seat comprising an axial passage for this rod. The equipment may be a wind turbine comprising a propeller having an axis of rotation and the swivel means having a center of rotation substantially on the axis of rotation.

[0015] A third object of the invention relates to a method for assembling a platform according to the invention comprising:

[0016] - provide a platform;

[0017] - arrange several assembly stations along the quay;

[0018] - provide a submersible barge;

[0019] - place the barge at one or more stations for the manufacture of the float; then,

[0020] - place the barge at one or more stations for the assembly of the riser and the base; then,

[0021] - place the barge at one or more positions for mounting the equipment on said platform; then,

[0022] - place the barge on another position; then - ballast the barge until the float alone ensures the flotation of the assembled unit.

[0023] A fourth object of the invention relates to a method for mooring an assembly according to the invention on a site chosen for the operation of this assembly, comprising the following steps:

[0024] - arrange the anchoring means according to a template, in a desired position;

[0025] - fix the tendons to the anchoring means;

[0026] - bring the assembly by floating above the position;

[0027] - inflate the buoy means;

[0028] - ballast the float until it is sufficiently submerged;

[0029] - fix the upper ends of the tendons to the float;

[0030] - deballast the float;

[0031] - deflate the buoy means.

[0032] Embodiments and variants will be described below, by way of non-limiting examples, with reference to the appended drawings in which:

[0033] [Fig. 1] is a schematic elevation view of an assembly comprising a wind turbine mounted on a floating platform according to the invention;

[0034] [Fig. 2] is a schematic view of an underwater float for the platform of Figure 1 and a first mode of arrangement for these tendons;

[0035] [Fig. 3] is a schematic view of a float for the platform of Fig. 1 and a second mode of arrangement for these tendons;

[0036] [Fig. 4] is a schematic elevational view, in perspective and partly cut away, of the platform of Figure 1;

[0037] [Fig. 5] is a schematic elevation and perspective view of the platform float of Figure 1;

[0038] [Fig. 6] is a schematic elevation and perspective view of a swivel frame for the platform of Figure 1; [Fig. 7] is a schematic elevation and perspective view illustrating a dockside assembly process and stations for the assembly of Figure 1;

[0039] [Fig. 8] is a schematic elevation view of a sea positioning stage of the assembly of Figure 1; and,

[0040] [Fig. 9] is a schematic view of an underwater float for the platform of Figure 1 and a second embodiment for the tendons, these tendons being formed by a tackle system.

[0041] In particular, the terms "top", "bottom", "upper" and "lower", "horizontal" and "vertical" and other terms of the same type may be arbitrarily used in this description and generally refer to the positions illustrated in the figures.

[0042] Figure 1 illustrates an offshore wind power generation assembly 1. This assembly is substantially symmetrical around a main vertical axis XI and includes in particular a floating platform 2 and a wind turbine 3. The wind turbine rests on the platform 2. The platform is anchored to the seabed 4 by anchors 6.

[0043] The platform comprises an annular float 8 around the main axis XI; in the position of use illustrated in Figure 1, this float is kept submerged under an immersion height HS, measured from an upper surface of the float to a mean level NS of the surface S of the water. It also comprises a rise 9 mounted on the float and extending over a height H9 greater than the immersion height HS, so that it culminates at an air height HA=H9-HS, above the mean water level NS. The immersion height HS is chosen so that the forces of the swell are little or not felt by the float. Typically the immersion height is chosen between eight meters and fifteen meters, depending on the location of the platform.This float is watertight; it includes a buoyancy volume calculated to define an Archimedes thrust greater than the total weight of assembly 1, increased by the force values ​​to which it can be subjected in the strongest storm conditions foreseeable.

[0044] The platform further comprises, located at the top of the rise 9, a base 10 for installing equipment therein. In the example illustrated, the equipment carried by the platform is the wind turbine 3. The wind turbine comprises a hollow, tubular mast 11, which extends upwards from the base 10, a nacelle 12, arranged at the top of the mast 11 and a three-bladed propeller 13 fixed to the nacelle 12. The propeller 13 is movable around a substantially horizontal axis of rotation of the propeller X13.

[0045] The water depth H4 below the float 8 and above on ground 4 can be greater than seventy meters. The platform includes a mooring system 16 for attaching to the anchors 6. This system includes eight tendons 17, 18 including four vertical tendons 17 and four oblique tendons 18.

[0046] The tendons used in the example illustrated in Figures 1 and 4 are substantially rigid metal tubes whose length is adapted to the irregularities of the seabed 4; the resulting differences in length being small compared to the depth H4, they are negligible and do not significantly affect the operation of the mooring system. The diameter of the tubes is determined by their mechanical characteristics which allow them to remain within the elastic limit under maximum stress. In the example, these tubes have a diameter of eight hundred millimeters and a thickness of thirty millimeters.

[0047] In the example illustrated in Figure 9, the oblique tendons 18 are formed by a continuous cable connected to the anchors 6 and to the float 8 by a pulley system; this system of oblique tendons formed by a cable and pulley system makes it possible in particular to take up very high forces. As particularly illustrated in Figure 5, the float 8 has an annular shape of rectangular radial section. It comprises radial partitions 21 regularly distributed around the main axis XI; in the example illustrated, the partitions 21 are eight in number. The walls 21 divide the float into eight independent watertight boxes. They also constitute reinforcements so that the float can withstand the water pressure. The float further comprises eight fasteners 22. In the example of Figure 5, each is arranged on a lower edge 23 of the cylindrical external wall 24 of the float.In the example illustrated, each fastener has the shape of a tab 22; it is formed in the radial extension of one of the eight respective radial walls. Each tab is provided to hold an upper end of a respective one of the eight tendons. A spherical guide allows the fasteners to be left free to rotate.

[0048] Each upper end of a vertical tendon 17 is fixed to its respective leg 22, so that the leg cannot slide vertically, along the tendon.

[0049] Each upper end of an oblique tendon 18 is held at its respective attachment by a stop 26 (see Figure 4), fixed to the tendon and arranged above the tab. Thus arranged, the tab can transmit a tensile force to the tendon, but does not transmit substantially any buckling force. Furthermore, each upper or lower end of each tendon 17, 18, forms a ball joint with the respective attachment. Thus, substantially no compressive force is transmitted to the oblique tendons.

[0050] Each oblique tendon 18 is also connected, by a lower end, to its respective anchor by respective ball joint means. Each oblique tendon is arranged substantially in a vertical radial plane comprising the main axis XL. Each axial plane comprising an oblique tendon is angularly equidistant from two axial planes comprising a vertical tendon. Thus, the tendons are arranged alternately around the main axis XL. That is to say, when one goes around this axis XI, one alternately encounters a vertical tendon then an oblique tendon.

[0051] In the embodiment of Figures 1, 3, 4 and 8, the tendons are external, that is, they move away from the main axis XI when moving from top to bottom.

[0052] In the embodiment of Figure 2, the tendons are internal, that is to say they approach the main axis XI when moving from top to bottom. In the example of Figure 2, the lower ends of the tendons 18 are substantially joined on the axis XI.

[0053] Preferably, the oblique tendons 18 form with the vertical an angle Al 8 greater than five degrees of angle.

[0054] In the example illustrated in the figures, the rise 9 is of substantially truncated cone shape. It comprises inclined posts 31 regularly arranged around the main axis XI and extending from bottom to top along generatrices of the conical shape. It also comprises hoops 32 arranged horizontally, connecting the posts together in order to prevent the posts from buckling under the weight of the base and the equipment, here the wind turbine 3. The rise can, for example, be made of steel or concrete, or a combination of the two materials.

[0055] The shape of the 9 rise, very open, allows it to be substantially transparent to the effects of wind and swell, which can pass through it with reduced effort.

[0056] The seat 10 has the shape of a spherical cradle, concave upwards, one axis of revolution of which is the main axis XI and the center of revolution of which is as close as possible to the axis of rotation X13 of the propeller 13. In the example illustrated, the seat comprises a structure 34 formed of radial beams 35 and concentric rings 36. The beams extend substantially horizontally radially to the main axis XI, they are preferably metallic and hollowed out near their neutral fiber. This structure, composed of hollowed beams and rings, is thus lightened, which makes it possible to maintain a low center of gravity for the assembly 1.

[0057] Among the concentric rings, the ring 36 C, the most central, forms a free cylindrical passage 37 around the main axis XL

[0058] In the example illustrated, the wind turbine mast is a hollow mast, substantially tubular around the main axis XI; it rests on the base. It has, on the one hand, a base 41 which is substantially spherical, convex downwards, designed to rest, at least indirectly, on the base and to form a quasi-ball joint with the base, and, on the other hand, a concave homothetic surface whose focus is that of the center of the base. The spherical base 41 has a center approximately identical to the center of revolution of the base. For safety, this concave surface is guided with play in a cover whose lower part is convex spherical with a center identical to that of the base.

[0059] Several connecting means can be provided between the base 41 and the seat 10. For example, ball bearings can be used. Elastic contacts, for example neoprene bearings, can also be used. In fact, the maximum angular displacements are very small, of the order of plus or minus five hundredths of an angle degree, i.e., for a mast one hundred and twenty meters high, or more, a displacement of less than nine centimeters.

[0060] As particularly illustrated in Figure 1, the wind turbine 3 comprises a rod 42 which extends substantially vertically downward from the base 41 of the mast. The rod is substantially rigid and rigidly fixed to the base. The rod passes through the passage 37 in the base 10. It carries, at a lower end, a mass 43. The mass is arranged inside the cylindrical interior space formed by the annular float 8. The rod 42 is preferably a steel tube. The mass 43 can be of any shape, it can in particular comprise a propeller of an electric generator.

[0061] As particularly illustrated in Figure 4, the platform 2 comprises an inflatable buoy 46 arranged under the seat 10, inside the rise 9. The buoy is preferably in the form of a bellows and made of a neoprene-type material. As illustrated in Figure 8, the buoy is hollow and forms a cylinder which allows the rod 42 to pass through its interior space.

[0062] In Figure 4, the buoy is shown deflated, so that it does not come into contact with the water. In Figure 8, the buoy 46 is shown inflated so that it extends downwards, from the seat 10 into the interior space 44 of the float 8, below the surface S of the water.

[0063] During inflation, the buoy is guided vertically by vertical guides 47. For each guide 47, the buoy comprises, on a lower peripheral edge 45, a respective slide 50 ensuring sliding guidance during inflation of the buoy. In the example illustrated in Figure 4, there are three guides; there could be more.

[0064] Each of the guides is fixed on the one hand under the seat 10 and on the other hand to an internal wall 48 of the float, to a free end of a console 49 extending radially from the wall 48 in the direction of the main axis XL. The consoles advantageously serve as a stop for the downward deployment of the buoy during its inflation.

[0065] Preferably, in the inflated position, the lower edge of the buoy 46 is above a free flotation plane of the platform. Free flotation means the flotation of the platform when it is not pulled towards the bottom by the tendons 17, 18 but is floating freely.

[0066] In its inflated position, the buoy forms with the float 8, the riser 9 and the base 10 a substantially rigid assembly.

[0067] The buoy constitutes a safety device preventing the destruction of the assembly 1. Thus, for example, in the event of a rupture of the watertightness of a float box or a rupture of a tendon, the platform includes means for immediately inflating the buoy so as to ensure the flotation of the assembly until the platform is repaired.

[0068] The anchors 6 are advantageously suction anchors. They can also be fixed to piles. In order to install the anchors as precisely as possible, and to allow for good arrangement of the tendons, a template is preferably used.

[0069] We will now describe a method of assembling an electrical production unit according to the invention, with reference to figures 7 and 8.

[0070] According to the invention, the assembly is carried out at the port on an assembly line 100 comprising several stations. The line 100 illustrated in Figure 7 comprises five stations P1-P5 distributed along a quay (not shown), each station corresponding to a particular assembly phase.

[0071] A submersible barge, not shown, allows the assembly to be moved during assembly, parallel to the quay, from one position to the next.

[0072] At the first PI station, the float described here being made of reinforced concrete, the formwork 99 serving as a mold for the concrete of the float is assembled on the barge. The reinforcement for the concrete is placed in the formwork. Inserts are included, in particular for the legs 22 for holding the tendons and pipes in order to be able to ballast the caisson.

[0073] At the second station P2, the concrete is poured, preferably continuously to avoid rework, which could cause cracking that would be detrimental to the float's watertightness. The float 8 is then removed from the formwork once sufficient strength has been achieved. It is also possible to use a formwork type described in the patent published under number FR 3 111 651, in the name of the company Coffratherm. At the third station P3, the riser 9 and the base 10 are mounted on the float 8 thus produced.

[0074] At the fourth station P4, wind turbine 3 is mounted on platform 2 thus created.

[0075] At the fifth station P5, with assembly 1 thus completed, the barge is ballasted and evacuated between the bottom of the port and the float. It is then the float which ensures the flotation of assembly 1.

[0076] We then begin a sixth phase P6, corresponding to the installation of set 1.

[0077] As illustrated in Figure 8, the assembly 1 thus assembled is brought to sea by tugs 102 to be anchored at its place of operation. In order to increase the stability of the assembly during towing, once out of the construction zone and as soon as the water depth is sufficient, the center of gravity of the assembly will be lowered. To do this, the caisson is ballasted while taking the precaution of maintaining positive buoyancy.

[0078] Preferably, three tugs are used in a star arrangement, two of which are visible in the figure. The anchors 6 and the tendons 17, 18 are previously arranged on the seabed 4. The tendons are held substantially vertically. The assembly is directly above the anchors. If this has not been done in the towing phase, the buoy 46 is fully inflated and the float is ballasted, so that residual flotation is provided by the buoy 46; the use of the buoy makes it possible to control the immersion depth of the float.

[0079] Once the float is at the correct depth, the vertical tendons 17 are first attached, then the oblique tendons 18.

[0080] The float 8 is then deballasted, which puts tension on the tendons 17, 18. The buoy 46 is then deflated. It is the internal volume of the float, filled with air, which then ensures the buoyancy of the assembly 1. Such an assembly can typically weigh 12,000 tonnes for a volume of 21,000 cubic metres.

[0081] The whole system is then operational and can enter the operating phase as soon as it is electrically connected to an electrical network or to an energy storage unit.

[0082] This assembly process minimizes the operations to be carried out at sea. It also allows for the speedy production of a large number of assemblies. This is particularly advantageous for wind farms that may include several hundred wind turbines.

[0083] The internal volume of the underwater float is greater than that required to balance its own weight, the weight of all the equipment, in particular that of the wind turbine, and the vertical thrust of hundred-year swells exerted on this float while maintaining a resulting upward thrust; this upward thrust makes it possible to maintain traction at least on the vertical tendons. In addition, this internal volume is calculated so that the draft of assembly 1 is sufficiently low to be able to evacuate the barge, at berth 5, then to tow the assembly into the port where it was assembled, out to sea.

[0084] Figure 9 illustrates oblique tendons formed from a cable 101 stretched over pulleys 102 and forming a hoist-type system. In the example illustrated, a single cable is used.

[0085] Upper yokes 122 and respective pulleys 102 form attachments for the cable 101 to the float 8. Lower yokes 120 and respective pulleys 102 form attachments for the cable to respective anchors 6. Preferably, the yokes 120, 122 are articulated, so that the pulleys can freely orient themselves under forces exerted on each pulley by the tension of the cable 101.

[0086] As illustrated, when an anchor 6 comprises a single pulley 102, the facing attachment 22 comprises two, and conversely, when an anchor 6 comprises two pulleys 102, the facing attachment comprises two. Thus, each oblique tendon 18 is formed of two strands 118 of the cable 101. These two strands 118 of the same tendon 18 are parallel to each other.

[0087] Each pulley 102 of an attachment comprising two is connected to another pulley of a neighboring anchor which comprises two, by a bias strand 119 of the cable 101.

[0088] Of course, the invention is not limited to the examples just described. On the contrary, the invention is defined by the claims which follow.

[0089] It will indeed appear to those skilled in the art that various modifications can be made to the embodiments described above, in light of the teaching which has just been disclosed to them. Thus, instead of being of rectangular section, the annular float can have a trapezoidal or any other section; the float can also be of toroidal shape. In an embodiment not comprising a balance, the float may not be annular and, thus, not comprise a central recess.

[0090] The float is preferably made of concrete. It can also be made of steel or any other material compatible with the forces applied to it and with the marine environment.

[0091] The hoops of the rise are represented in the shape of an arc of a circle connecting two neighboring posts. They can, for example, be replaced or supplemented by straight, horizontal or oblique rods, connecting two neighboring posts or not.

[0092] The number of tendons may be different from that previously described, i.e. four vertical tendons and four oblique tendons. The tendons may be three of each type, or more.

[0093] Also, the number of tendons of one type can be a multiple of the number of those of another type. For example, we can have three oblique tendons and six oblique tendons; in this case, the alternation of tendons can be: two vertical tendons followed by one oblique tendon, then two vertical tendons then one oblique tendon, etc.

[0094] Instead of being distributed on the external periphery of the external lower edge 23 of the float, all or part of the legs 22 can be arranged on a lower edge of the internal wall 48.

[0095] The platform can accommodate any type of wind turbine, including high-power ones, in excess of fifteen megawatts. A platform of the type described can support any type of equipment other than a wind turbine. The equipment could, for example, be drilling equipment or have a surface area intended to accommodate technical or residential buildings.

[0096] It appears that the arrangement of the tendons ensures almost perfect stability of the assembly. As the platform has thus become substantially insensitive to the effects of swell and wind, electricity production is now limited only by the maximum rotation speed permitted by the wind turbine blades. In addition, the stability of the assembly reduces maintenance costs and increases its lifespan.

Claims

Claims 1. Floating platform (2), in particular an offshore platform, intended to support equipment (3) and organized around a substantially vertical main axis (XI), characterized in that it comprises a float (8), tendons (17, 18), fixing means (22) for holding an upper end of each of said tendons to said float and anchoring means (6) for fixing a lower end of each of said tendons to a seabed (4), said tendons comprising at least three tendons of a first type (17), arranged vertically and at least three tendons of a second type (18), arranged obliquely, between their attachment means and their anchoring means, the types of tendons being arranged alternately around said main axis (XI), being designed so that, in an operational position, said float (8) is kept submerged at a depth (HS) below the mean surface (S) of the water, and, at least the vertical tendons (17) are kept taut.

2. Platform according to claim 1, characterized in that all the oblique tendons (18) form with the main axis (XI) the same angle (A18), preferably greater than five degrees.

3. Platform according to one of claims 1 and 2, characterized in that the oblique tendons are rigid rods, preferably tubular.

4. Platform according to claim 3, characterized in that each end of each of the tendons (17, 18) is articulated to its fixing or anchoring means by ball joint means.

5. Platform according to one of claims 3 and 4, characterized in that the attachment means (22) are designed to transmit only tensile forces to the upper ends of the oblique tendons (18).

6. Platform according to one of claims 1 and 2, characterized in that the oblique tendons are formed from strands (118) of a continuous cable (101) stretched between pulleys (102) carried by the float (8) and pulleys (102) carried by respective anchors (6).

7. Platform according to one of claims 1 to 6, characterized in that the float (8) is of revolution around the main axis, preferably the float is annular.

8. Platform according to one of the preceding claims, characterized in that it further comprises a riser (9) and a seat (10) for said equipment, said seat being arranged on said riser, said riser extending upwards from the float (8), and being designed so that in the position of use there is an air draft (HA) under said seat.

9. Platform according to claim 8, characterized in that the rise comprises means of transparency (31, 32) to swell and wind, preferably a lattice structure, even more preferably a structure comprising posts (31) and substantially horizontal bars (32) connecting the posts together.

10. Platform according to one of claims 1 to 9, characterized in that it further comprises inflatable buoy means (46) capable of ensuring flotation of the platform if the float (8) is fully ballasted.

11. Assembly (1) comprising a platform (2) according to one of the claims 1 to 10 and equipment (3) supported by said platform.

12. Assembly according to claim 11, comprising a platform of claim 8, characterized in that the equipment (3) comprises a base (41) which forms swivel means with the seat (10) of said platform.

13. An assembly according to claim 12, characterized in that the equipment comprises counterweight means (42, 43) extending downwards from the base (41) through the platform (2), said counterweight means preferably comprising a rod (42) extending downwards from said base and a mass (43) fixed to a lower end of said rod, said seat comprising an axial passage (37) for said rod.

14. Assembly according to one of claims 12 and 13, characterized in that the equipment is a wind turbine (3) comprising a propeller (13) having an axis of rotation (X13) and in that the swiveling means have a center of rotation substantially on said axis of rotation (X13).

15. Method for assembling a platform according to claim 9, characterized in that it comprises: - provide a platform; - arrange several assembly stations (P1-P5) along said platform; - provide a submersible barge; - place the barge at one or more stations (P1-P2) for the manufacture of the float (8); then, - place the barge at one or more stations (P3) for the assembly of the rise (9) and the base (10); then, - place the barge at one or more stations (P4) for mounting the equipment on said platform; then, - place the barge on another position (P5); then - ballast the barge until the float alone ensures the flotation of the assembled unit.

16. Method for mooring an assembly (1) according to one of claims 12 to 14 comprising a platform (2) according to claim 10 characterized in that it comprises the following steps: - arrange the anchoring means (6) according to a template, in a desired position; - fixing the tendons (17, 18) to said anchoring means; - bring the assembly by floating above said position; - inflate the buoy means (46); - ballast the float until it is sufficiently submerged; - fix the upper ends of said tendons to the float; - deballast the float; - deflate the buoy means.

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