ES2835551T3 - Gravity foundation for offshore wind turbines - Google Patents

Abstract

Gravity foundation for offshore wind turbines, manufactured using floating dock technology, consisting of a box (1) with a circular section, internally lightened by hollow cells (11, 12, 13), closed at the top by one or more covers (16), whose The central cell (13) is extended by a post-tensioned concrete mast (2), on which the metal tower (4) that supports the wind turbine is attached. The cells (11, 12, 13) of the box are filled with a solid ballast (8) in order to lower its center of gravity, so that the design of the foundation as a whole allows it to be towed and anchored in the open sea. without the need for special vessels, or the use of additional flotation means, as it has sufficient naval stability conditions in all its phases.

Description

DESCRIPTION

Gravity foundation for offshore wind turbines

Object of the invention

The present invention, as its title indicates, refers to a gravity foundation for offshore wind turbines, manufactured using floating dock technology.

Background of the invention

Usually the foundations of offshore wind turbines are either directly deposited on the seabed (gravity), or they are embedded in it (monopile, tripod or lattice). These typologies, as well as the variants based on them, cover around 95% of the foundations installed to date, with a very residual record of others (floating and artificial islands). Generally, gravity solutions are proposed for shallower depths, while above 35 m, monopile and lattice solutions (jackets) are usually conceived for depths up to 50 or 60 m. From 60 m, floating solutions are proposed.

At the end of 2010, approximately 225 turbines with gravity foundations could be counted, far from the more than 1,000 with monopiles. Furthermore, a very high percentage of them were at shallow depths (less than 15 m), and the design of several of these first gravity foundations was based on conceptual criteria for bridge foundations, which proves the limited experience with the one that is still counted today in this type of foundations, especially for depths of more than 20 m.

In relation to the higher cost of offshore wind installations compared to onshore ones, it should be noted that the turbines and blades themselves are very similar, although the marine ones tend to be slightly larger in size, although their cost is comparable. Likewise, marine equipment requires protection systems against the salty and abrasive environment of the sea, which can mean an increase of 10 to 15% in the value of this equipment. However, the main differences stem from the costly manufacturing, transportation and anchoring processes of the foundation structures, as well as the installation processes of the turbine in offshore conditions.

On the other hand, the port caissons manufactured in floating dock are well known. These are large-scale reinforced concrete structures that, due to their lightened cross-section (multicellular), can float once finished. This gives them great versatility in terms of construction (using the sliding formwork technique), floating transport and placement (anchoring) in the port work, whether for docks, docks or others. The docks (sheltering works) and docks (docking works) of caissons are a typology especially used in Spanish ports, whose manufacture by means of floating docks is well known in our country, being the leading international applicants for the technology for the construction of reinforced concrete caissons by sliding in a floating dock, since they have built more than 3,000 units to date.

In general, the port drawers have a parallelepipedic shape, with a rectangular or square plan, although in some special cases drawers with other shapes have been used in order to adapt to the conditions of each project.

Document ES 2378960, also published as WO2012038487, by INNEO TORRES SL ET AL, describes a structure for gravity foundations for marine wind turbines, with a frustoconical part of the base, which is not slippery in a floating dock and lacks the structures buoyancy aids, necessary to maintain the stability of the set during the anchoring phases

Document WO 2009/130343 from ACCIONA WINDPOWER describes a support support for a marine wind turbine, consisting of a reinforced concrete box that can be built on a floating dock as a gravity foundation. However, the box remains emerged in its upper part, so this solution limits its range of application in terms of drafts, a condition imposed by the capacities of the existing floating docks and by the drafts required in the manufacturing docks. Therefore, for practical purposes, this solution cannot be applied to drafts greater than 30 m, far from the 45-50 m depth covered by the solution proposed here. It does not lose cover at any stage during the anchoring (it maintains the same procedure as the anchoring of conventional port boxes), thus avoiding critical phases at the cost of substantially increasing wave loads in the service phase, since the waves directly impact on the drawer. This increase in stressing loads is also accompanied by a significant increase in materials (concrete, steel and filler), in order to give it stability against said loads. On the other hand, the box has a rectangular or square plan and is not circular, which translates into a significant increase in wave loads.

Document ES 2461065, from UNIV STUTTGART PUBLIC LAW INSTITUTION ET AL, describes a foundation floating, with a prefabricated box, for which the problems and conditions that it tries to solve are totally different from those existing in the present invention, in which stability against geotechnical failure modes must be considered, while a floating solution described It faces the problems derived from stability in the service phase as a floating device, requiring an auxiliary mooring system made up of different lines anchored to the seabed. The floating foundation described in this document is modular and, therefore, cannot be built in a floating dock, which detracts from its simplicity and its construction process is significantly more expensive. On the other hand, the floating foundation described in this document has a totally different structural behavior and is not subject to any type of conditioning factors such as the drafts in the manufacturing piers and the range of drafts in which the solution can be used as gravity foundation of offshore wind turbines with a greater probability of success, on the contrary, it has very large dimensions, of the order of 70m in diameter, which also implies special construction processes and installation.

Document WO 2010/026098, from BOEGL MAX BAUUNTERNEHMUNG GMBH ET AL, describes an offshore foundation for offshore wind turbines comprising a floating base that can sink by flooding hollow chambers located in a base. The buoyancy of the structure is given by prefabricated floating devices formed by rings that surround the central axis that forms the base of the turbine support mast. This foundation is not manufactured in a floating dock, for the simple fact that it is a series of loose pieces, which do not form any box closed by the base so that it floats by itself; in fact, it is made up of three pieces: a substantially flat floor plate 3, a mast 4, made up of several segments 15, and a floating body 5, made up of several rings 19; In this foundation, the base of the mast 4, on which the metallic tower that supports the wind turbine is supported, is not an extension of the central cell of the box, firstly because in this development there is no box, and secondly because it is It is a different structure from the plate 3 and the rings 19 that form the enclosing body 5. The body 5 in charge of providing buoyancy to the foundation is made up of precast concrete rings 19 that are mounted around the base 4 of the mast and on the floor plate 3, it can be removed after submerging, or filled in to give the foundation more weight. Finally, this foundation does not include a solid ballast in the lower area so that the center of gravity of the assembly is low enough so that buoyancy conditions are maintained with a metacentric height greater than 1.00 m in all its phases, so that it can be towed and anchored in the open sea without the need for special vessels or the use of additional flotation means. All the components of this foundation, made of concrete or metal, are prefabricated on the ground and, on some occasions, are mounted on a pontoon that is used as a means of flotation for the contraption that has been formed by prestressing together with floor plate 3 rings that make up the base of the mast 4 and those that make up the casing 5; In fact, this structure by itself cannot be considered as a Gravity Base Foundation, which can withstand the actions under its own weight and good proof of this is that this document describes an advantageous alternative form of foundation using piles.

Description of the invention

The gravity foundations of marine wind turbines, also called GBF (Gravity Base Foundation) or GBS (Gravity Base Structures), present a series of problems, or conditioning factors, that must be taken into account in the design of a new foundation and that basically can be classified as follows:

- Design conditioner: the gravity foundation for offshore wind towers must allow its connection with the metal mast to be made at a sufficiently high elevation so that said point is safe from the direct actions of the waves even in the greatest storms. This usually means that said connection is made, at least, at an elevation more than 15 m above sea level. In addition, this connection point is usually configured as the access platform to the tower during the exploitation phase.

- Conditioning factors during manufacturing: through the manufacturing methods used up to now, large areas of land are required for the manufacture and storage of the structures, as well as large lifting means for their launching or their placement on board the vessel in charge of transport it to its place of positioning. Structures that are not manufactured in a floating dock of those used in the manufacture of reinforced concrete caissons for docks or docks pose all these problems. For the manufacture by means of floating dock, one of the determining factors that marks the availability or not of suitable docks is their draft.

- Conditioning factors during transport: there are two general methods of transporting these structures from their place of manufacture to the place of final positioning. The first of them consists of transporting them on board a boat. In the second case, its direct towing is carried out, for which it is necessary that the GBS has by itself, or by an auxiliary means, the adequate buoyancy that gives it naval stability during its towing to the anchoring place. In relation to the towing situation, some of the certifying companies impose as a condition in this phase that the structure is stable and does not sink in the accidental case of flooding of one of the outer cells due to breakage or fissure of a part of the wall Exterior. This condition directly affects the design of the GBS.

- Conditioning factors during positioning: The positioning process on the seabed or on a support bench is also critical. There are two generic ways to proceed in this positioning maneuver (anchoring) of the structure:

^or by progressive sinking with support by an auxiliary lifting device (floating crane, heavylift). This is the proper anchoring procedure for the case in which the GBS has been transported aboard a vessel;

^or by progressive sinking without the need for any auxiliary lifting means.

In this second case there are also two options:

■ that the GBS requires an additional flotation element to its structure to have sufficient stability during all phases of its sinking

■ that the GBS has, by its own design, the necessary stability during all phases of anchoring.

- Conditioning factors during the internal filling phase: one of the main conditioning factors found in all gravity solutions for foundations of offshore wind turbines is to be able to make the internal filling procedure compatible with the geometric configuration of the structure, with its structural typology and with the maritime means necessary to carry out said operation. It must be taken into account that the interior filling is a fundamental part in the behavior of these structures in the service phase, since it contributes a very high percentage of the stabilizing weight against loads.

- Determinants during the exploitation phase: the foundation must be designed to withstand the loads during the service phase. Basically the loads can be: own weight, environmental loads (including wind and waves), operational loads (those due to the operation of the wind turbine) and accidental loads (for example, the impact of a ship, an iceberg, etc.). Of all these types of forces, the stabilizers correspond to their own weight, while the design must be carried out considering all the others destabilizing. The design must guarantee the correct behavior of the foundation in the face of equilibrium and geotechnical failure modes, as well as guarantee its structural validity, complying with the standards set out in the various regulations, so as to ensure its functionality and operability throughout entire shelf life.

- Determinants during the dismantling phase: a common requirement in this type of structure is that it must be dismantled at the end of its useful life. This factor can condition the design of the GBS.

The solution developed that is presented here for the foundations of marine wind turbines consists of a structure consisting of a precast reinforced concrete box, which serves as support and to transmit all the load of the rest of the structure to the foundation bench, manufactured in dock floating with the technique of manufacturing port caissons. This box has a circular plan and in the lower part of the cells it has a solid concrete ballast of variable thickness depending on the site conditions, whose mission is to guarantee the stability conditions during the towing and anchoring of the structure.

The hearth of this box is thicker than the lateral and intermediate walls that separate the cells into which it is divided, which are distributed from a central cell, forming at least two concentric rings of cells distributed radially, which are provided of means of communication with each other and with the outside, equipped with emptying and filling devices that allow self-regulation of the ballast level for anchoring in its final location.

The ratio between the diameter of the base and the height of the drawer is between 3: 2 and 8: 5, and is preferably 11: 7.

A mast rises from the central part of the box, at the upper end of which the connection with the metal tower of the wind turbine will be realized by means of a metal transition piece. This mast has an almost cylindrical geometry, with a slight taper, and is made of post-tensioned concrete, a lower part inside the floating dock itself and the upper part (approximately from 6 m) outside it so that it can slide out of the box.

The height of the box is such that in the service phase it is totally submerged (not the tower, which has an emerged part to facilitate connection with the remaining mast at a sufficiently high elevation with respect to sea level). Internally, the box is divided into cells that are closed at the top by means of a reinforced concrete slab. In general, the height of the neck above the box is similar to the height of the box.

The outer wall of the box is lightened by means of lightening of circular cross section and / or in the upper slab.

Likewise, the radial cell separation partitions have gaps (windows) from a certain height, so that from that height the adjacent cells are communicated.

Below are the advantages of this proposal:

- In manufacturing:

■ Manufacturing using the floating dock sliding technique is a standardized process that avoids the great requirement of resources, facilities, and land area occupation that the usual onshore manufacturing supposes, increasing the number of ports with the capacity to host the entire process of manufacturing.

■ The proposed design limits the need for the capacity of the manufacturing dock in terms of draft, which is vital given the availability of existing infrastructures capable of accommodating manufacturing processes for gravity structures such as foundations for offshore wind turbines.

■ Safety and quality conditions are improved due to standardized prefabrication.

■ At the same time, it substantially increases manufacturing performance, since the use of the floating dock allows the main manufacturing means to be continuously available, without the need for downtime due to the need to dismantle the formwork, carry out the launching process from the ground and reassemble. of the formwork system.

■ The design of the box makes it applicable for offshore wind tower foundations from 35 m to 50 m depth, without the need to change the geometry of the box but only the solid ballast levels and the length of the upper mast. Therefore, in any case, the works within the floating dock remain unchanged even though the potential sites that could use this foundation have been considerably increased.

■ This solution is less dependent on the price of steel than metallic solutions

■ Use of conventional materials (concrete, steel for passive and active reinforcements) and local labor. It is not necessary to use unusual materials (light concrete, heavy materials for use as fillers, etc.) whose availability would condition manufacturing and make the solution more expensive.

- In transport and positioning (anchoring):

■ Once this structure is manufactured, it is transported to its final location by direct towing with a conventional tugboat and without the need for auxiliary means. This is due to the fact that the GBS has by itself an adequate buoyancy that gives it naval stability.

■ The design has also been adapted to meet the strictest requirements in terms of safety against accidental situations during towing (flooding of an external cell) while maintaining stability and maintenance afloat conditions.

■ Likewise, the anchoring process is carried out simply by gravity ballasting its cells with seawater, without the need for any additional means, or special auxiliary vessels of large capacities, or flotation elements unrelated to the structure itself, to confer naval stability since, due to its design, this structure meets the requirements demanded during all phases of the anchoring process, maintaining at all times the value of the metacentric height greater than one meter: GM> 1.00 m.

■ By avoiding the need to use special boats (difficult to find on the market) and auxiliary means for towing and anchoring, maneuvering times are reduced and the execution schedule can be adjusted to the available good weather windows, thus optimizing the execution process as a whole, since the time required to prepare the structure prior to said maneuvers is minimal from the moment a favorable weather forecast is available.

■ As a result of the foregoing, the costs associated with these operations are substantially reduced. ■ In addition, the anchoring process is reversible, so that once the box begins to sink, it can be refloated by actuating the valve and pump system until the liquid ballast level is adjusted to the desired level.

- Cell filling:

■ A cell filling procedure has been developed that is compatible with the rest of the structure design. This procedure is based on the use of conventional suction dredgers that fill the cells by means of hydraulic drive.

■ In addition to the previous cell filling system, the GBS design is capable of employing an alternative method, consisting of removing the top covers and filling by means of mechanical dredgers. This is an important advantage that allows adapting to the conditions of each specific location.

Description of the figures

To complement the description that is being made and in order to facilitate the understanding of the characteristics of the invention, the present specification is accompanied by a set of drawings in which, by way of illustration and not limitation, the following has been represented:

Figure 1 shows a general view of the installation of an offshore wind turbine (6), fixed on the foundation object of the invention.

Figures 2 and 3 respectively represent a section according to a horizontal plane and a vertical plane through the center of said foundation.

Figure 4 is a detail of some lightening (17) located on the outer wall of the drawer (1).

Figure 5 represents a plan view below the slab (16) of the drawer (1), in which the pre-slabs (8) and the lightening (81) present in them can be seen.

Figures 6 and 7 show details of said pre-slabs (8) and lightening (81).

Figure 8 represents a sectional view, according to a vertical, diametrical plane of the foundation when it is ready to be towed floating, before being anchored in the sea (5).

Preferred embodiment of the invention

As can be seen in the figures, the box (1) that constitutes the base of this foundation and ultimately the support of the whole structure of the offshore wind turbine, is a precast reinforced concrete box, which has a circular ground plan, 33.00 m in diameter. in the base (14) and 32.00 m in diameter in the shaft (15). The hearth (14) has a thickness of 1.20 m, while the top (16) of the cells is 0.60 m. The total height of the shaft (15) is 19.20 m, while that of the box (1) (including the base, shaft and the upper closing slab) is 21.00 m.

A mast (2) arises from the central part of the box (2) at whose upper end (24) the connection with the metallic tower (4) of the wind turbine (6) is fixed by means of a metallic transition piece (3). The mast has an almost-cylindrical geometry, with a slight conicity (it has an outer diameter of 8.00 m at its start and 6.00 m at its upper end). This mast is made of post-tensioned concrete to withstand the stresses to which it will be subjected in the service phase. The first 6 meters (21) are manufactured by sliding in the box itself after the box-base, while the upper portion (22) has a slight conicity and is built outside the floating dock due to its height. The post-tensioning cables are tensioned from the head of the mast (2) once it is completed, while said cables have their passive anchors (25) installed in the bottom of the box (14). The mast (2) has a height dependent on the depth at which the foundation will be located, so that the metal tower (4) has a connection height with the post-tensioned concrete mast above 15 m with respect to the level. (51) from the sea. Said connection is materialized through the metallic transition piece (3).

The circular section of the foundation makes it possible to reduce wave loads, having verified its viability, during the exploitation phase, as a gravity foundation for different drafts, from 35 m to 50 m (always depending on the geotechnical conditions and the maritime climate of the area) and without the need to modify any of the dimensions of the box (only the height of the mast (2)). In the design of this caisson (1) it has been taken into account that it must be manufactured entirely in a floating dock, in order to take advantage of the advantages that this technique provides. For this, some forms of the drawer have been adopted that allow the walls to slide, so that the construction process is the same as for a port drawer.

Another constructive conditioning factor to take into account lies in the fact that the required draft in the manufacturing dock (s) according to the described process must be limited, since in practice the actual availability of large draft docks may be scarce in depending on the location of the offshore wind farm. The proposed GBS requires a draft at the fabrication quay of about 16.50 m. With this draft, all construction phases can be executed without the need for additional actions. In order to reduce this draft and limit the influence of this determining factor, the designed GBS has lightening in its structural elements. Basically these lightening are of 3 types:

- The outer wall has some lightening (17) of circular section throughout the shaft. These lightening can be carried out using the sliding technique inside the drawer, so they only influence the design of the formwork.

- The radial partitions of the interior cells have three windows (18) that, in addition to reducing weight, allow communication between cells from a certain height. This is high enough not to influence the water ballasting process (in all cases the box is anchored at the required level with a lower liquid ballast level).

- The upper pre-slabs (8), which are placed to form the upper cover (16), have structural lightening (81) in the part corresponding to the outer crown.

By means of these three lightening, it is possible to reduce some 950 T of weight, reducing the draft in the manufacturing phase by around 1.20 m.

In order to adapt the design to the conditions during the towing and anchoring phases, trying to avoid additional means for towing and anchoring the GBS, which require a metacentric height of at least greater than one meter: GM> 1.00 m has been planned adapt the length of the mast depending on the depth at which the offshore wind tower is to be located, since its crest elevation must always be at least 15.00 m. This fact implies different stability conditions in the naval phase (towing and anchoring), since the weight distribution is different depending on the length of the mast in each case. This variability is solved by applying different amounts of solid ballast (7) (mass concrete) inside the cells (11 and 12) of the box (1). Thus, for a box that is going to be installed at a depth of 35 m, a solid ballast thickness of 0.415 m is sufficient, while for a box that is going to be anchored at -50 m it requires about 2.30 m of thickness. . On the other hand, it is necessary to maintain stability and buoyancy without loss of cover in the event of accidental flooding of one of the outer cells (11) during GBS towing, which strongly conditions the design. The solution proposed here is compatible with this conditioner, simply adding more or less solid ballast at the bottom of the cells.

As previously explained, mass concrete is used as solid ballast (7), without structural function, whose sole objective is to give enough weight to a low elevation, so that the center of gravity of the structure is lowered and its naval stability conditions. The application of said solid ballast is fully compatible with the proposed construction process, as it is carried out by simply placing mass concrete once the box has left the floating dock.

Figure 8 shows how this accidental conditioner can be met, taking as an example a box corresponding to a 35 m deep foundation. In this case, the box has a solid ballast of 0.85 m (elevation 52) and has no liquid ballast (water), so it has a draft of 13.55 m during towing, and therefore a freeboard of 7.45 m, with a GM> 1.00 m. In this situation, in the event of accidental flooding of one of the outer cells of the box, the box would list about 15 ° but would remain afloat without losing cover, thus completing the trailer. From this situation, the liquid ballast opening valves would be actuated to allow the entry by gravity of seawater into the cells on the opposite side, so that the box would be progressively anchored, but with GM values still older in all its phases.

As can be understood, between the 35 m and 50 m deep foundations there are an infinity of possible intermediate situations that require a different combination of solid and liquid ballast levels. However, since the implementation of these ballasts is a simple process (pouring concrete and opening valves for the introduction by gravity of seawater into the cells, respectively) fully inserted into the general construction process, this variability does not affect to the general design of the box, since the only thing that has to be adapted is the amount of solid ballast (mass concrete) that has to be poured into the cells in each case. And this process of pouring concrete is simple and does not affect the manufacturing process of the box in floating dock, as it is done once the box has left it.

The gravity structure thus conceived can be towed with the usual tugboats in the ports to the place where it must be installed, then proceeding to anchor it by ballasting the interior cells of the box with seawater, until the moment the The drawer is permanently supported on the breakwater bench. The ballasting process is carried out by means of the introduction by gravity of sea water inside the box by means of a system of valves arranged on the outer wall of the box, and by means of the corresponding internal communication system between cells.

During the anchoring process, the box is connected by means of mooring lines to conventional tugboats that, by means of winches, act on said lines, giving them different tensions and allowing the positioning of the structure on the ground in the established location and within the allowable tolerances.

The anchoring process avoids the use of special vessels or flotation elements unrelated to the structure itself, the GBS design itself being the one that gives it stability characteristics in all intermediate phases.

Once anchored in the place of installation of the wind turbine, the next stage consists of filling the cells of the box with granular material, an activity that involves a certain complexity as they are submerged and closed by means of the slab. Likewise, as they are offshore structures, access to these structures will only be available by maritime means.

One of the alternatives for the cell filling process consists in the use of hydraulic means (type dredgers suction) by driving the material by the dredge through a system of pipes that are connected to the box by means of flanged connection ports located on the upper closing slabs of the box. By means of this internal filling, the GBS already has the necessary weight to guarantee the stability of the foundation throughout the useful life of the structure. A system of valves is located on the walls of the drawer that allow the entry and exit of air and water both during the flooding and cell filling phases. By means of this system, the overpressures inside the cells due to the progressive entry of water by impulsion from the dredge, is limited and dissipates.

As an alternative to this process for cell filling, it is possible to adapt the GBS design to allow the removal of the upper slabs once the box has been anchored and the flooding of all its interior cells has been completed. At that moment, when the water pressures inside and outside the cells have been equalized, it is possible to remove the caps that make up the upper cell closing slab, using a floating crane. On the other hand, the union of the lids to the walls of the drawer is configured in such a way that they can be easily unlinked, acting on bolt-type closures. Once the covers have been removed and the cells submerged but uncovered, the process of filling the cells is facilitated, being able to be carried out either by driving directly into the cells, or mechanically by means of a scoop dredger. In this case in which the drawer is left without lids, the inner filling material is protected in its upper part by having two layers of breakwaters of sufficient weight to withstand the actions of the currents and guarantee the stability of the filling inside the cells during the entire useful life of the structure.

The foundation is circular in shape to reduce wave loads, its viability having been verified, during the exploitation phase, as a gravity foundation for different drafts, from 35 m to 50 m (always depending on the geotechnical conditions and the climate. area) and without the need to modify any of the dimensions of the box (only the height of the mast).

Likewise, the design of this box allows the dismantling operation to be carried out without additional lifting or flotation means, as the GBS has the necessary stability in all the flotation phases.

Claims (8)

1. - Gravity foundation for offshore wind turbines, manufactured using floating dock technology, comprising a base (1) prefabricated of reinforced concrete, circular section, internally lightened by hollow cells (11, 12, 13), closed at the top by means of a or more covers (16), and provided with structural lightening that reduces its weight so that the whole of the structure remains afloat and can be manufactured in a pier with a draft less than the height of said base; from which a post-tensioned concrete mast (2) emerges, on which the metal tower (4) that supports the wind turbine is attached, characterized in that said base (1) is a box, closed at the bottom by a base (14) of greater thickness than the lateral and intermediate walls that separate the cells (11, 12, 13) into which it is divided, and its distribution in plan presents a central cell (13), from which the mast (2) starts, which presents a cylindrical configuration in the lower area (21), which is also manufactured by sliding in the box of the floating dock, while the upper area (22), preferably with a slight taper, is manufactured a posteriori outside the factory of the floating dock ; where the lower area of the cells (11, 12) into which the box (1) is divided once built, is filled with a solid ballast (7) that aims to lower the center of gravity of the set to, while maintaining its buoyancy conditions with a metacentric height greater than 1.00 m in all its phases, to be able to be towed and anchored in the open sea without the need for special vessels or the use of additional flotation means. 2. - Foundation, according to the preceding claim, characterized in that the drawer (1) has a plan distribution with a central cell (13) around which at least two concentric rings of cells (12) and (11) are formed ), which have the same radial distribution and have means of communication with each other and with the outside, equipped with emptying and filling devices that allow self-regulation of the level of ballast with seawater for anchoring in its final location. 3. - Foundation according to the preceding claims, characterized in that the ratio between the diameter of the base and the height of the drawer (1) is between 3: 2 and 8: 5, and preferably 11: 7. 4. - Foundation, according to the preceding claims, characterized in that the mast (2) has a height dependent on the depth at which the foundation will be located, so that its connection with the offshore wind tower (4) by means of the corresponding metallic transition piece (3) is at an elevation of at least 15 m with respect to sea level (51). 5. - Foundation, according to the preceding claims, characterized in that the cover or covers of the prefabricated drawer (1) have means that facilitate the opening of the drawer that allow filling the interior cells with a granular material, once the foundation has been ballasted in the place of installation, in order to guarantee its stability in the service phase. 6. - Foundation according to the preceding claims, characterized in that the outer wall of the box (1) has lightening (17) of circular section throughout the shaft. 7. - Foundation, according to the preceding claims, characterized in that the radial partitions of the interior cells of the drawer (1) have windows (18) that, in addition to reducing weight, allow communication between cells from a certain height . 8. - Foundation according to the preceding claims, characterized in that the upper slab that forms the lid (16) has pre-slabs with structural lightening (81).