EP3176329A1

EP3176329A1 - Gravity-based foundation for offshore wind turbines

Info

Publication number: EP3176329A1
Application number: EP14898896.7A
Authority: EP
Inventors: Miguel A. VAZQUEZ ROMERO; Noelia GONZALEZ PATIÑO; Elena Martin Diaz; Enrique DE FARAGÓ BOTELLA; Juan Manuel GONZALEZ ESTEBAN; Jonay CRUZ FERNANDEZ; Carlos Jesús POLIMÓN OLABARRIETA
Original assignee: Dragados SA; Drace Infraestructuras SA; Proes Consultores SA
Current assignee: Dragados SA; Proes Consultores SA; Drace Geocisa SA
Priority date: 2014-07-30
Filing date: 2014-07-30
Publication date: 2017-06-07
Anticipated expiration: 2034-07-30
Also published as: WO2016016481A1; ES2835551T3; DK3176329T3; EP3176329A4; EP3176329B1; LT3176329T; PT3176329T; PL3176329T3

Abstract

Gravity foundation for offshore wind turbines, fabricated using floating dock technology, comprising a caisson (1) with a circular cross-section, with hollow cells (11, 12, 13) for interior voiding, closed on the top by one or more covers (16), whose central cell (13) is extended into 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 caisson are filled with solid ballast (8) for the purpose of lowering its centre of gravity, such that the design of the foundation as a whole can be towed and sunk on the open sea without the need for special vessels or the use of additional means of flotation, because it has sufficient naval stability during all of the its phases.

Description

Object of the invention

This invention, as its title indicates, relates to a gravity foundation for offshore wind turbines, fabricated using floating dock technology.

Background of the invention

The foundations of marine wind turbines are usually either deposited directly on the sea floor (gravity) or are driven into it (monopile, tripod, or jacket). These types, as well as the variations based on them, account for approximately 95% of the foundations installed to date, with other types of foundations (artificial islands and floating foundations) observed only very infrequently. Generally, gravity solutions are proposed for shallower depths, with monopile and jacket solutions applied at depths of more than 35 m and up to 50 or 60 m. Floating solutions are applied at depths greater than 60 m.
As of the end of 2010, there were approximately 225 turbines with gravity foundations, very far from the more than 1,000 with monopile foundations. Also, a very high percentage of these were installed at shallow depths (less than 15 m), with the design of several of these early gravity foundations based on the conceptual criteria of bridge foundations, which demonstrates the limited experience even today in regard to this type of foundation, especially at depths of more than 20 m.
In regard to the higher cost of offshore wind farms in comparison with land-based wind farms, it is important to note that the turbines and blades themselves are very similar, and although marine turbines tend to be slightly larger, their cost is comparable. Marine equipment also requires systems to protect against the abrasive saline environment of the sea, which can result in an increase of 10 to 15 % in the cost of this equipment. Nevertheless, the principal differences are related to the costly processes of fabricating, transporting, and sinking the foundation structures, as well as the processes to install the turbine in offshore conditions.
Also, port caissons that are fabricated on floating docks are very well known. These are large reinforced concrete structures are able to float after they have been completed due to their voided (multi-cell) cross-section. This makes them highly versatile in terms of construction (using the slipform technique), floating transport, and placement (sinking) at the port works site, for docks, breakwaters, and other structures. Caisson breakwaters (protective works) and docks (mooring works) are used widely in Spanish ports, and the technique of fabrication using floating docks is well known in Spain, and the applicant companies are international leaders in the technology of slipform reinforced concrete construction on floating docks, because to date they have constructed more than 3,000 units.
In general, port caissons have a parallelepiped shape with a rectangular or square horizontal cross-section, although in some special cases, caissons with other shapes have been used in order to conform to the conditioning factors in each project.
Document ES 2,378,960 by INNEO describes a structure for a gravity foundation for marine wind turbines, with a conical base part, which cannot use slipforming on a floating dock and that lacks the auxiliary flotation structures required to maintain the stability of the assembly during the phases of the sinking process.
Document WO 2009/130343 by ACCIONA WINDPOWER describes a support for a marine wind turbine, comprising a reinforced concrete caisson that can be constructed on a floating dock as a gravity foundation. However, the upper part of the caisson remains exposed, so the range of application of this solution is limited in terms of depths, a condition imposed by the capacities of existing floating docks, and the drafts required on the fabrication docks. Therefore, for practical purposes, this solution cannot be applied for depths of more than 30 m, far from the 45-50 m depths covered by the solution proposed here. The caisson does not fully submerge during any of the sinking phases (it maintains the same sinking procedure of conventional port caissons), which avoids critical phases, at the cost of significantly increasing wave loads during the service phase, because the waves strike directly against the caisson. This increase in applied loads also generates a significant increase in materials (concrete, steel, and infill), in order to provide stability against these loads. Also, the caisson has a rectangular or square horizontal cross-section, rather than circular, increasing wave loads significantly.
The document ES 2,461,065 describes a floating foundation, with a prefabricated caisson, that attempts to resolve problems and conditioning factors that are entirely different from those that affect this invention, in which special consideration must be given to stability in response to the geotechnical failure modes, as opposed to the floating solution described in the document, which addresses the problems related to stability during the service phase as a floating object, requiring an auxiliary mooring system consisting of several lines anchored to the sea floor. The floating foundation described in the document is modular and therefore cannot be constructed on a floating dock, which increases complexity and its construction process is significantly more costly. Also, the structural behaviour of the floating foundation described in the document is completely different and is not subject to any type of conditioning factor, such as the draft depths on the fabrication docks and the range of depths in which the solution can be used as a gravity foundation for offshore wind turbines with a higher probability of success; quite the opposite, its dimensions are very large, on the order of 70 m in diameter, which also involves special installation and construction processes.

Description of the invention

Gravity foundations of marine wind turbines, also known as GBF (Gravity-Based Foundations) or GBS (Gravity-Based Structures), present a series of problems, or conditioning factors, which must be taken into account during the design of a new foundation, and which can basically be classified as follows:

Design-related conditioning factor: gravity foundations for offshore wind towers must allow the connection to the metal mast to be made at a level that is high enough to ensure that the point is protected from the direct action of the waves, even in the strongest storms. This normally translates into the placement of this connection point at an elevation at least 15 m above sea level. Also, the connection point is usually configured to serve as a platform to access the tower during the operation phase.
Conditioning factors during fabrication: the fabrication methods used up to this point require large expanses of land for the fabrication and stockpiling of the structures, in addition to large hoisting equipment to launch or transfer them to the vessel responsible for transporting them to their positions. Structures that are not fabricated on floating docks like those used for the fabrication of reinforced concrete caissons for port docks or breakwaters pose all of these problems. One of the conditioning factors that determines whether or not adequate docks are available for fabrication using floating docks is their draft.
Conditioning factors during transport: there are two general methods for transporting these structures from their point of fabrication to their final positions. The first consists of transporting them aboard a vessel. In the second case, they are transported by directly towing them, which requires the GBS, either itself or through auxiliary means, to have adequate floatability to provide naval stability during towing to the location where it will be sunk. In regard to the towing situation, in this phase, some certification companies impose the condition that the structure must be stable and not sink in case of accidental flooding of one of the exterior cells due to breakage or cracking of a part of the outer wall. This conditioning factor directly affects the design of the GBS.
Conditioning factors during positioning: The process of positioning the foundation on the sea floor or on a supporting foundation bed is also critical. There are two general ways to carry out this operation to position (sink) the structure:
- ∘ By progressive sinking with support from auxiliary hoisting equipment (floating crane, heavylift). This is the sinking procedure used if the GBS has been transported aboard a vessel;
- ∘ By progressive sinking without the need for any auxiliary hoisting equipment. There are two options in this latter case:
  - ▪ The GBS requires flotation elements in addition to its structure in order to provide sufficient stability during all of the phases of the sinking process.
  - ▪ The GBS, through its own design, has the necessary stability during all of the phases of the sinking process.
Conditioning factors during the infilling phase: one of the principal conditioning factors encountered with all gravity-based solutions for offshore wind turbine foundations is to make the procedure for filling the inside of the foundation compatible with the geometric configuration of the structure, with its structural type, and with the sea-based equipment required to carry out this operation. It is important to take into account that infilling is a fundamental factor in the behaviour of these structures during the service phase, because it provides a very high percentage of the weight to provide stabilization against loads.
Conditioning factors during the operation phase: the foundation must be designed to withstand the loads during the service phase. Basically, these loads may be: dead weight, environmental loads (including wind and waves), operating loads (those generated by the operation of the wind turbine), and accidental loads (for example, impact by a ship, an iceberg, etc.). Of all of these types of forces, the stabilizing forces correspond to dead weight, while the design must consider all of the other forces as destabilizing forces. The design must guarantee correct behaviour of the foundation in response to the geotechnical and equilibrium failure modes, and also guarantee its structural validity in compliance with the standards specified in the different regulations, in order to ensure that the wind turbine is operational and functional over the course of its entire life span.
Conditioning factors during dismantling: a normal requirement for this type of structure is that it must be able to be dismantled at the end of its life span. This factor may condition the design of the GBS.

The developed solution presented here for the foundation of marine wind turbines consists of a structure made up of a prefabricated reinforced concrete caisson that serves as a support and to transfer all of the load of the rest of the structure to the foundation bed, fabricated on a floating dock using the technique for the fabrication of port caissons. This caisson has a circular horizontal cross-section and solid concrete ballast at the bottom of the cells, with the thickness varying based on the conditions of the site, whose purpose is to guarantee stable conditions during the towing and sinking of the structure.
The floor of this caisson is thicker than the side walls and intermediate walls that separate the cells into which it is divided, which are distributed around a central cell, forming at least two concentric rings of cells distributed radially, which are equipped with means of communication between one another and with the exterior, equipped with drainage and fill devices to enable the self-regulation of the ballast level with seawater for sinking at the final location.
The ratio between the diameter of the base and the height of the caisson is between 3:2 and 8:5, and is preferably 11:7.
A mast extends from the central part of the caisson, with the upper end of the mast connected to the metal tower of the wind turbine by means of a metal transition piece. The geometry of this mast is almost cylindrical and slightly conical, and it is fabricated out of post-tensioned concrete, a lower portion fabricated inside the floating dock itself, and the upper area (approximately above 6 m) outside of the dock so that it can be slid outside of the caisson plant.
The height of the caisson is such that during the service phase, it will be completely submerged (but not the tower, which has a portion that extends above the water to facilitate connection to the remaining mast at an elevation that is high enough with respect to sea level). The interior of the caisson is divided into cells that are closed at the top by a reinforced concrete slab. In general, the height of the mast above the caisson is similar to the height of the caisson in question.
The outer wall of the caisson is voided by voids with a circular cross-section and/or voids in the top slab.
The radial partitions that separate the cells are also equipped with a series of gaps (windows) beginning at a certain height, so that adjacent cells are connected above that height.
The advantages of this proposal are described below:

During fabrication:
- ▪ Fabrication using the slipform technique on a floating dock is a standardized process that avoids large demands for resources, installations, and the occupation of land that are required by customary fabrication on land, increasing the number of ports with the capacity to house the entire fabrication process.
- ▪ The proposed design limits the capacity required of the fabrication dock in terms of draft, which is vital based on the availability of existing infrastructure capable of housing fabrication processes for gravity structures such as offshore wind turbine foundations.
- ▪ Safety and quality conditions are improved as a result of the standardized prefabrication.
- ▪ In turn, fabrication output is also increased substantially, because the use of floating docks makes the principal fabrication equipment available continuously, without the need for downtime to disassemble formwork, execute the launching process from land, and reassemble the formwork system.
- ▪ The caisson design makes it applicable for foundations of offshore wind turbines between depths of 35 m to 50 m, without the need to modify the caisson geometry, changing only the levels of solid ballast and the length of the top mast. This means that in all cases, the work inside the floating dock remains the same, despite the considerable increase in the number of potential sites in which this foundation could be used.
- ▪ This solution is less dependent on the price of steel than metal solutions.
- ▪ The use of conventional materials (concrete, steel for passive and active reinforcing) and local labour. There is not need to use uncommon materials (lightweight concrete, heavy materials for use as infill, etc.), whose availability could condition the fabrication and increase the cost of the solution.
During transport and positioning (sinking):
- ▪ Once the structure has been fabricated, it is transported to its final position by direct towing with a conventional tugboat and without the need for auxiliary equipment. This is possible because the GBS itself has adequate floatability, which gives it naval stability.
- ▪ The design has also been adjusted to comply with the strictest safety requirements in response to accidental situations during the towing process (flooding of an exterior cell), maintaining its stability and keeping it afloat.
- ▪ Likewise, the sinking process is carried out by the simple gravity ballasting of its cells with seawater, without the need for additional equipment, special large-capacity auxiliary vessels, or flotation elements apart from the structure itself, to provide it with naval stability, because by design, this structure complies with the requirements during all of the phases of the sinking process, keeping the value of the metacentric height above one metre at all times: GM ≥ 1.00 m.
- ▪ By avoiding the need for special vessels (which are difficult to obtain on the market) and auxiliary equipment for towing and sinking, the manoeuvre time is reduced and the construction calendar can be adjusted to the available windows of good weather, thus optimizing the execution process as a whole, because the time required to prepare the structure prior to these manoeuvres is minimal from the moment that there is a favourable weather forecast.
- ▪ As a result of this, the costs associated with these operations are reduced significantly.
- ▪ Also, the sinking process is reversible, so that once the caisson begins to sink, it is possible to refloat it by activating the system of valves and pumps until the liquid ballast level has been adjusted to the desired level.
Cell infilling:
- ▪ A cell infilling procedure that is compatible with the rest of the design of the structure has been developed. This procedure is based on the use of conventional suction dredges that infill the cells using hydraulic delivery.
- ▪ In addition to the aforementioned cell infill system, the design of the GBS design can also use an alternative method, consisting of the removal of the top covers and infilling using mechanical dredges. This is an important advantage that allows the process to be adapted to the conditioning factors in each specific site.

Description of the drawings

To complement the description that is being provided, and in order to facilitate the comprehension of the characteristics of the invention, a set of drawings is attached to this description, which, for the purposes of illustration and not limitation, represent the following:

Figure 1 shows a general view of the installation of an offshore wind turbine (6); anchored to the foundation that is the object of the invention.
Figures 2 and 3 represent a cross-section in the horizontal and vertical planes, respectively, through the centre of said foundation.
Figure 4 is a detailed view of a series of voids (17) situated in the outer wall of the caisson (1).
Figure 5 represents a plan view below the slab (16) of the caisson (1), which shows the precast slabs (8) and the voids (81) in the precast slabs.
Figures 6 and 7 show the details of said precast slabs (8) and voids (81).
Figure 8 shows a diameter cross-sectional view, in the vertical plane, of the foundation when it is positioned for floated towing, prior to being sunk at sea (5).

Preferred embodiment of the invention

As shown in the figures, the caisson (1) that constitutes the base of this foundation, and ultimately the support for the offshore wind turbine structure as a whole, is a prefabricated reinforced concrete caisson with a circular horizontal cross-section, 33.00 m in diameter at the floor (14) and 32.00 m in diameter across the shaft (15). The floor (14) has a thickness of 1.20 m, while the cover (16) of the cells is 0.60 m thick. The total height of the shaft (15) is 19.20 m, and the height of the caisson (1) (including the floor, shaft, and the top cover slab) is 21.00 m.
A mast (2) extends from the central part of the caisson, with the connection with the metal tower (4) of the wind turbine (6) anchored at its upper end (24) by means of a metal transition piece (3). The geometry of the mast is almost cylindrical and slightly conical (the outer diameter at its base is 8.00 m and 6.00 m at the upper end). This mast is fabricated of post-tensioned concrete to withstand the stresses to which it will be subjected during the service phase. The first 6 metres (21) are fabricated in the caisson plant using slipforming after the caisson-base, while the upper section (22), which is slightly conical, is constructed outside of the floating dock due to its height. The post-tensioning cables are tensioned from the mast head (2) after it has been completed, with the passive anchors (25) of these cables installed in the floor of the caisson (14). The height of the mast (2) depends on the depth at which the foundation will be installed, such that the metal tower (4) has an elevation of connection with the post-tensioned concrete mast higher than 15 m with respect to sea level (51). This connection is made using the metal transition piece (3).
The circular cross-section has been feasibly demonstrated to reduce wave loads during the operation phase, as a gravity foundation for different depths, from 35 m to 50 m (always depending on the geotechnical conditions and the ocean climate of the area), and without the need to modify any of the dimensions of the caisson (only the height of the mast (2)). The design of this caisson (1) takes into account that it must be fabricated entirely on a floating dock, in order to take advantage of the benefits provided by this technique. For this reason, caisson shapes have been adopted that allow the walls to be slipformed, so that the construction process is the same as for a port caisson.
Another conditioning factor of the construction to be taken into account is due to the fact that the depth required in the fabrication docks in accordance with the described process must be limited, because in practice, the actual availability of large-draft docks may be very scarce, depending on the location of the offshore wind farm. The proposed GBS requires a depth at the fabrication dock of approximately 16.50 m. With this depth, all of the construction phases can be executed without the need for additional actions. In order to reduce this depth and limit the influence of this conditioning factor, a series of voids have been added to the structural elements of the designed GBS. There are three basic types of these voids:

The exterior wall has a series of voids (17) with a circular cross-section throughout the entire shaft. These voids can be made using the slipforming technique inside the caisson plant, meaning they only affect the design of the formwork.
The radial partitions of the interior cells have three windows (18), which, in addition to reducing the weight, connect the cells above a particular height. This is high enough to avoid affecting the water ballasting process (in all cases, the caisson is sunk to the required level with a smaller level of liquid ballast).
The precast top slabs (8), which are installed to form the top cover (16), have a series of structural voids (81) in the portion corresponding to the outer edge.

These three types of voids reduce the weight by approximately 950 T, reducing the draft depth in the fabrication phase by approximately 1.20 m.
In order to adapt the design to the conditioning factors during the towing and sinking phases, in an attempt to avoid the need for additional equipment for the towing and sinking of the GBS, which requires a metacentric height greater than or equal to one metre: GM ≥ 1.00 m, the length of the mast will be adjusted according to the depth at which the offshore wind tower will be located, because the top elevation must be at least at the level of + 15.00 m. This generates different stability conditions during the naval phase (towing and sinking), because the distribution of weight differs depending on the length of the mast in each case. This variability is resolved by applying different amounts of solid ballast (7) (plain concrete) inside the cells (11 and 12) of the caisson (1). This means that for a caisson for installation at a depth of 35 m, a thickness of 0.415 m of solid ballast will suffice, while for a caisson for installation at the level of - 50 m, a thickness of approximately 2.30 m is required.
It is also necessary to maintain stability and flotation capacity without submerging in case of accidental flooding of one of the exterior cells (11) during towing of the GBS, which significantly conditions the design. The solution proposed here is compatible with this conditioning factor, simply by adding more or less solid ballast in the bottom of the cells.
As explained earlier, plain concrete is used as solid ballast (7), with no structural function and for the sole purpose of providing sufficient weight at a low elevation in order to lower the centre of gravity of the structure and improve its conditions of naval stability. The application of this solid ballast is entirely compatible with the proposed construction process, because it is carried out by simply pouring plain concrete once the caisson has exited the floating dock.
Figure 8 shows how this accidental conditioning factor could be addressed, using a caisson corresponding to a foundation at a depth of 35 m as an example. In this case, the caisson has 0.85 m (level 52) of solid ballast and does not have liquid ballast (water), so its draft during towing is 13.55 m, and its freeboard is therefore 7.45 m, with a GM >1.00 m. In this case, should there be accidental flooding of one of the exterior cells of the caisson, the caisson would heel approximately 15°, but would remain afloat without submerging, thus allowing the towing process to be completed. In this situation, the liquid ballast aperture valves would be activated to allow seawater to be enter by gravity into the cells of the opposite side, so that the caisson would be sunk progressively, but with even higher GM values in all of its phases.
Understandably, there are an infinite number of intermediate situations between a foundation at a depth of 35 m and one at a depth of 50 m, which require a different combination of solid and liquid ballast levels. However, since the implementation of this ballast is a simple process (pouring concrete and opening valves to allow seawater to enter the cells by gravity, respectively) that is completely integrated into the general construction process, this variability does not affect the general design of the caisson, because the only aspect that needs to be adapted is the quantity of solid ballast (plain concrete) that must be poured into the cells in each case. And this process of pouring concrete is simple and does not affect the process of fabricating the caisson on the floating dock, because it is carried out after the caisson has left the dock.
The gravity structure designed in this way can be towed, using the tugboats common found in ports, to the locations where they are to be installed, where they will then be sunk by adding ballast to the interior cells of the caisson with seawater, until the caisson is permanently supported on the rockfill foundation bed. The ballasting process is done by adding seawater into the caisson by gravity, using a system of valves installed on the exterior wall of the caisson, and the corresponding system of interior communication between the cells.
During the sinking process, the caisson is connected by mooring lines to conventional tugboats, which use winches to act on the lines to apply different amounts of tension, allowing the structure to be positioned on the horizontal plane at the specified location and within the permitted tolerances.
The sinking process avoids the use of special vessels or flotation elements not integrated into the structure itself, with the design of the GBS itself providing characteristics of stability during all of the intermediate phases.
Once the caisson has been sunk in the location where the wind turbine will be installed, the next phase consists of infilling the caisson cells with granular material, which is a relatively complex operation because they are underwater and covered by slabs. Also, since these are offshore structures, they can only be accessed using sea-based equipment.
One of the alternatives for the process of infilling the cells consists of using hydraulic equipment (such as suction dredges) by delivering the material from the dredge through a system of pipes that are connected to the caisson by flanged connection openings in the top cover slabs of the caisson. This infilling provides the GBS with the necessary weight to guarantee the stability of the foundation for the entire life span of the structure. A system of valves is installed in the walls of the caisson to allow air and water to enter and exit during the flooding and cell infill phases. Using this system, the excess pressure inside the cells due to the progressive entry of water delivered from the dredge is limited and dissipates gradually.
As an alternative to this process for the infilling of cells, the GBS design can be adapted to allow the cover slabs to be removed after the caisson has been sunk and the flooding of all of its interior cells has been completed. At this time, when the water pressure inside and outside the cells has equalized, the covers that form the top cover of the cells can be removed using a floating crane. The connection of the covers to the caisson walls is configured in such a way as to allow them to be disconnected easily, by manipulating a series of latch-type closures. Once the covers have been removed and the cells are submerged but exposed, the cell infill process is facilitated, allowing it to be done by direct delivery to the inside of the cells, or mechanically using a clamshell dredge. In this case, in which the caisson's covers are removed, the infill material is protected at the top by applying two layers of rockfill that are heavy enough to withstand the action of the currents and guarantee the stability of the infill in the cells for the entire life span of the structure.
The foundation has a circular shape that has been feasibly demonstrated to reduce wave loads during the operation phase, as a gravity foundation for different depths, from 35 m to 50 m (always depending on the geotechnical conditions and the ocean climate of the area), and without the need to modify any of the dimensions of the caisson (only the height of the mast).
The design of this caisson also allows the dismantling operation to be carried out without additional hoisting or flotation equipment, because the GBS has the necessary stability during all phases of the flotation phases.

Claims

A gravity foundation for offshore wind turbines, fabricated using floating dock technology, comprising:
- a prefabricated reinforced concrete caisson (1), with a circular cross-section, with hollow cells (11, 12, 13) for interior voiding, closed on top by one or more covers (16), and equipped with structural voiding that reduces its weight, in order to allow the structure as a whole to remain afloat and allow fabrication on a dock with a draft that is less than the height of the caisson in question;

- a post-tensioned concrete mast (2), on which the metal tower (4) that supports the wind turbine is attached, which is the extension of the central cell (13) of the caisson and has a cylindrical configuration in its lower portion (21), which is also fabricated by slipforming in the caisson plant on the floating dock, while the top portion (22), preferably with a slightly conical shape, is fabricated later outside of the factory of the floating dock;

- solid ballast (8) that fills the lower portion of the cells (11, 12) into which the caisson is divided (1) after it has been constructed, whose purpose is to lower the centre of gravity of the assembly in order to, by maintaining its floatability conditions with a metacentric height greater than 1.00 m in all phases, allow it to be towed and sunk in the open ocean without the need for special vessels or the use of additional means of flotation.
The foundation according to the previous claim, characterised in that the floor of this caisson (1) is thicker than the side walls and intermediate walls that separate the cells into which it is divided, and its distribution on the horizontal plane presents 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 which are equipped with means of communication between one another and with the exterior, equipped with drainage and fill devices to enable the self-regulation of the ballast level with seawater for sinking at the final location.
The foundation according to the previous claims, characterised in that the ratio between the diameter of the base and the height of the caisson (1) is between 3:2 and 8:5, and preferably 11:7.
The foundation according to the previous claims, characterised in that the height of the mast (2) depends on the depth at which the foundation will be installed, such that its connection to the offshore wind tower (4) by means of the corresponding metal transition piece (3) is at an elevation of at least 15 m with respect to sea level (51).
The foundation according to the previous claims, characterised in that the cover or covers of the prefabricated caisson (1) are equipped with means to facilitate the opening of the caisson to enable infilling of the interior cells with a granular material, once the foundation has been ballasted in its installation location, in order to guarantee its stability during the service phase.
The foundation according to the previous claims, characterised in that the exterior wall of the caisson (1) has a series of voids (17) with a circular cross-section through the entire shaft.
The foundation according to the previous claims, characterised in that the radial partitions of the interior cells of the caisson (1) have a series of windows (18), which, in addition to reducing the weight, connect the cells above a particular height.
The foundation according to the previous claims, characterised in that the top slab that forms the cover (16) includes a series of precast slabs with structural voids (81).