Patent Publication Number: US-2023160367-A1

Title: Foundation device for a wind turbine tower and assembly method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY 
     This patent application claims priority from PCT Application No. PCT/ES2021/070262 filed Apr. 20, 2021, which claims priority from Spanish Patent Application No. P202030320 filed Apr. 20, 2020. Each of these patent applications are herein incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention is comprised among wind platforms installed offshore, as well as among the means needed to perform said installation. 
     BACKGROUND OF THE INVENTION 
     When installing an offshore wind turbine, providing a system suitable for building the foundation thereof and stabilization is needed. 
     There are a number of methods and devices dedicated to such purpose. They can all be classified into two types of classifications: one corresponding to the type of foundation used and one corresponding to the type of structure being taken to the installation site. 
     According to the first classification, the devices can have a floating or weighted foundation. The first kind are bulkier but do not require very powerful means to take them to the installation site. The second kind are smaller but require large ships to transport them to the installation site. 
     According to the second classification, assembly can be performed on land (moving the entire assembly to the installation site) or at the installation site itself (moving the foundation and the tower separately). 
     The relevant technical problem for the present invention refers to weighted foundation devices, intended for being sunk entirely. The processes for assembling the tower, moving the foundation or assembling and sinking the foundation are critical and present their own technical requirements relating to buoyancy, inertias and equipment necessary for their use, being completely unrelated to those devices intended for providing a floating foundation. 
     Document WO 2018/150063 A1 discloses a maritime structure for laying the foundations of buildings, comprising a polygonal solid body with columns. 
     The present invention provides an alternative solution for problems related to devices dedicated to the process of installing an offshore wind turbine tower with a weighted foundation. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides an alternative solution to the problem mentioned above by means of a device according to claim  1 . The dependent claims define preferred embodiments of the invention. 
     Unless otherwise defined, all the (scientific and technical) terms used herein are to be interpreted as one skilled in the art would do. It will therefore be understood that commonly used terms are to be interpreted as one knowledgeable in the subject matter would do, and not in an idealized or strictly formal mode. 
     Throughout the text, the word “comprises” (and derivatives thereof, such as “comprising”) must not be understood in an exclusive manner, but rather must be understood in the sense that they allow for the possibility that what is defined may include additional elements or steps. 
     An object of the present invention relates to a foundation device for a wind turbine tower, the device being configured to be supported on the seabed, comprising
         a hollow main body comprising a lower face intended for being supported on the seabed and an upper face opposite the lower face; the device being characterized in that   both the lower face and the upper face contain a hollow space, each of the faces thus comprising an outer perimeter and an inner perimeter;   the main body further comprises an outer lateral wall arranged between the outer perimeter of the lower face and the outer perimeter of the upper face, and an inner lateral wall between the inner perimeter of the lower face and the inner perimeter of the upper face; and   the device further comprises a plurality of columns that project from the upper face of the main body.       

     Foundation devices of this type achieve better stability in relation to weight due to the inner space that is generated by the shape of the upper and lower faces. The displacement volume (understood as the volume of the device subject to Archimedes&#39; principle) is positioned far from its center of gravity, which increases hydrostatic stability. 
     The vertical columns provide the buoyancy needed during immersion of the structure. This arrangement increases the metacentric height of the main body and therefore the stability of the structure as it is being ballasted towards the seafloor. 
     In particular embodiments, the device further comprises a floor slab positioned on the lower face, the floor slab being a polygonal surface without inner spaces. 
     This floor slab is intended for covering the polygonal ring formed between the upper and lower faces. In this manner, the main structure is still a ring, but with a floor, such that, when the device is cast into the sea, water does not penetrate the inner space of the ring, reducing the draft in a port. 
     In particular embodiments, the distance between the upper face and the lower face is at least four times bigger than the height of the floor slab, where the height of the floor slab is measured in the direction parallel to the distance between the upper face and the lower face. 
     It is therefore a thin slab compared with the height of the assembly, understood in the direction from the lower face to the upper face (i.e., in the vertical direction when the device is in use). In this way, it does not significantly affect the advantages of stability and weight savings. 
     In particular embodiments, the hollow space inside the upper face and lower face is such that the ratio between the transverse metacentric radius of the device and the cube root of the displacement volume is greater than 0.003. 
     One of the main advantages of the invention is that a higher metacentric radius is achieved as compared with conventional solutions. One way to quantify this advantage is with a dimensionless parameter which consists of dividing the transverse metacentric radius between a characteristic length, calculated as the cube root of the displacement volume. This dimensionless parameter, regardless of the size of the foundation device, is greater in devices according to the present invention compared with known devices. The size of the inner space allows for this increase and this advantage. 
     In particular embodiments, at least one of the columns is hollow. In particular embodiments, the inside of at least one of the columns is in communication with the inside of the main body. 
     This allows the columns to also provide a hollow space for receiving the ballast and being able to better control the sinking of the device during the installation process. 
     In particular embodiments, one of the columns is an anchoring column and presents an anchoring surface with anchoring means configured for receiving the installation of a wind turbine tower. 
     Anchoring the wind turbine tower in one of the columns allows a better fixing and a better design of the foundation device, as it has with a fixed reference for the wind turbine tower. 
     In particular embodiments, the device further comprises
         a plurality of first plates anchored in a reinforcement ring of the anchoring column;   a plurality of second plates adapted to be fixed to a reinforcement ring of a wind turbine tower;   a plurality of lifting elements, each of which is configured to exert pressure between one of the first plates and one of the second plates.       

     These adjustment elements allow an inclination adjustment once the tower is positioned in the anchoring column. The inclination adjustment is usually performed once the foundation device is already supported on the seabed, so as to be able to take a reliable reference of the vertical. The reinforcement rings allow stresses caused by the force of the lifting elements to be absorbed. 
     In particular embodiments, at least one portion of one of the lifting elements is embedded on a second plate. 
     The pistons of the lifting element can thereby serve as a guide during the levelling of the wind turbine tower, preventing its imbalance. 
     In particular embodiments, the device further comprises reversible pumping means to selectively fill or empty the inside of the main body. 
     The installation of pumping means which allow both the filling and the emptying of the inside of the main body is an advantage: the filling function allows to control the ballasting process, while the emptying function allows to refloat the foundation device so it can be dismantled or inspected, allowing the later reuse thereof. 
     In particular embodiments, both the upper face and the lower face have an annular polygon shape, wherein both the inner perimeter and the outer perimeter are polygons with the same number of sides. 
     The use of polygons in the upper and lower faces leads to great ease in manufacture, as it allows the lateral wall to be built by means of rectangular panels. 
     In particular embodiments, both the upper face and the lower face have a ring shape, such that both the inner perimeter and the outer perimeter are closed curves, such as circles or ellipses. 
     A single closed curve can offer other advantages, such as lower resistance to forward movement in the water. 
     In particular embodiments, the main body is made of concrete. 
     The manufacture of foundation elements using concrete is a widespread practice; the present invention is perfectly compatible with these methods of manufacture. 
     In a second inventive aspect, the invention provides a method of installing a wind turbine tower, comprising the steps of
         providing a foundation device according to any of the preceding claims;   providing a wind turbine tower;   positioning the assembly formed by the foundation device and the wind turbine tower at an offshore installation site; and   filling the hollow main body with a fluid, such that the sinking of the assembly formed by the foundation device and the wind turbine tower takes place.       

     This method allows for great stability while the foundation device is being taken to the installation site, due to its particular geometry that is hollow on the inside. 
     This method is compatible both for the option of installing the wind turbine tower on the foundation assembly on land and then move the entire assembly to the installation site, and for the option of assembling the wind turbine tower while it is afloat and then transporting and subsequently ballasting it. 
     In particular embodiments, the method comprises the steps of
         attaching the wind turbine tower to the installation column of the foundation device, and   moving the assembly formed by the foundation device and the wind turbine tower to an offshore installation site.       

     In this particular case, the tower is attached to the installation column before moving the entire assembly by sea to the installation site. The attachment of the tower on land facilitates the process and prevents moving complex machines to the installation site. Furthermore, on land, the attachment is not subjected to the movement of the waves. 
     In particular embodiments, the method further comprises the steps of
         applying pressure between a first plate and a second plate by means of a lifting device so as to vary the inclination of the wind turbine tower with respect to the anchoring column, creating a levelling gap between the anchoring surface and the wind turbine tower; and   introducing a securing element in the levelling gap.       

     This later step of levelling allows to correct the inclination of the tower once the foundation device rests on the seabed. Because it is not always possible to provide a completely flat floor, it is possible for the foundation device to have a slight inclination. This would inevitably be transmitted to the tower were it not for this additional step. 
    
    
     
       DESCRIPTION OF THE FIGURES 
       To complete the description and make it easier to understand the invention, a set of figures is attached to the description. These figures are part of the description and illustrate a particular example of the invention, which is not to be interpreted as being limiting of the scope thereof, but rather as a mere example of how the invention can be carried out. This set of figures comprises the following: 
         FIGS.  1   a  and  1   b    show two particular examples of foundation devices for a wind turbine tower according to the invention. 
         FIGS.  2   a  to  2   d    show different steps of a method of installation according to the invention. 
         FIG.  3    shows a detail of the connection between the column of a foundation device and a wind turbine tower, in which a system for correcting the inclination of a foundation device according to the invention can be seen. 
         FIG.  4    shows an alternative embodiment of the foundation device. 
     
    
    
     In order to help to better understand the technical features of the invention, the mentioned figures include a series of reference numbers, in which the following is depicted in an illustrative and non-limiting manner: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 1 
                 Main body 
               
               
                 2 
                 Lower face 
               
               
                 21 
                 Outer perimeter of the lower face 
               
               
                 22 
                 Inner perimeter of the lower face 
               
               
                 3 
                 Upper face 
               
               
                 31 
                 Outer perimeter of the upper face 
               
               
                 32 
                 Inner perimeter of the upper face 
               
               
                 4 
                 Outer lateral wall 
               
               
                 5 
                 Inner lateral wall 
               
               
                 6 
                 Columns 
               
               
                 60 
                 Anchoring column 
               
               
                 61 
                 Anchoring surface of the anchoring column 
               
               
                 62 
                 Anchoring means of the anchoring column 
               
               
                 63 
                 Reinforcement ring 
               
               
                 7 
                 First plate 
               
               
                 8 
                 Second plate 
               
               
                 9 
                 Lifting jacks 
               
               
                 10 
                 Foundation device 
               
               
                 11 
                 Pumping system 
               
               
                 12 
                 Floor slab 
               
               
                 100 
                 Wind turbine tower 
               
               
                   
               
            
           
         
       
     
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the present invention, which is provided for illustrating but not limiting purposes, is described below. 
       FIGS.  1   a  and  1   b    show two particular examples of foundation devices  10  for a wind turbine tower according to the invention. 
     Each device  10  comprises a hollow main body  1  with a lower face  2  intended for being supported on the seabed and an upper face  3  opposite the lower face  2 . 
     Both the lower face  2  and the upper face  3  contain a hollow space, each of the faces thus comprising an outer perimeter  21 ,  31  and an inner perimeter  22 ,  32 . 
     Likewise, both the upper face  3  and the lower face  2  have an annular polygonal shape. However, this polygonal shape can be changed in other embodiments. In  FIGS.  1   a  and  1   b   , a hexagon and a square are shown, but triangles, pentagons, heptagons, octagons, etc., can be used, or they can be made with closed curves, such as circles, ovals or ellipses. 
     Since both the upper face  3  and the lower face  2  have an annular polygon shape, hollow in the center thereof, the main body  1  further comprises an outer lateral wall  4  arranged between the outer perimeter  21  of the lower face  2  and the outer perimeter  31  of the upper face  3 , and an inner lateral wall  5  between the inner perimeter  22  of the lower face  2  and the inner perimeter  32  of the upper face  3 . 
     In this way, the transverse metacentric radius of the assembly, understood as the ratio between the transverse inertia of the device with respect to a horizontal axis and the displacement volume, is higher than in the known solutions, which allows for the same stability to be obtained with a much lower weight, facilitating moving the assembly and reducing the ballasting time. 
     In addition to this main body  1 , the foundation device  10  also comprises a plurality of columns  6  that project from the upper face of the main body. These columns  6  are hollow, and the inside thereof is in communication with the inside of the main body. 
     The vertical flotation columns allow the foundation device  10  to be immersed when needed without the aid of auxiliar, large tonnage vessels, by simply adding ballast water. These columns keep values of inertia in flotation at suitable stability values, with the main advantage being the reduction of installation costs, both in terms of materials and in terms of auxiliary means, and the immersion process can be performed in a safe, efficient and rapid manner. 
     The shape of these columns can be wide-ranging, provided that it maintains inertia in flotation at acceptable values. Therefore, circular or polygonal sections are allowable. 
     One of these columns is an anchoring column  60  and presents an anchoring surface  61  with anchoring means  62  configured for receiving the installation of a wind turbine tower. These anchoring means  62  are configured for providing a screwed attachment between the column and the tower. 
     Furthermore, as it can be observed in the figure, the foundation device  10  also includes reversible pumping means  11  to selectively fill or empty the inside of the main body  1  during the installation and ballasting operations and dismantling or flotation operations. 
     The pumping system is configured for reversible operation, allowing the deballasting of the structure and later transport thereof for dismantling once the service life of the platform has ended. To that end, it is designed with protection against corrosion, and both the pumps and a number of valves are replaceable throughout the life cycle of the structure. The pump chamber is located in a dry room in the foundation device  10 . 
     Both the main body  1  and the columns  6  are made of concrete. 
       FIGS.  2   a  to  2   d    show different steps of a method of installation according to the invention. 
       FIG.  2   a    shows the assembly formed by the foundation device  10  and a wind turbine tower  100 , which has been assembled on the installation column  60  on land. 
     Being able to install the equipment with land means and in a port setting reduces the logistics process and eliminates the risks inherent to any operation at sea and between two bodies with different relative movements due to their different buoyancies. 
     Next, as observed in  FIG.  2   b   , a number of tugboats may transport the structure to the installation site. 
     There, as shown in  FIG.  2   c   , the pumping system  11  will be activated for filling the main body  1  with water. This will give rise to the immersion of the platform, as illustrated in  FIG.  2   d   . During this process, only the assistance of the tugboats will be needed to keep the platform in position, due to the flotation columns which, as explained, ensure that during immersion no additional auxiliary means are needed. 
       FIG.  3    shows a detail of the connection between the anchoring column  60  and the wind turbine tower  100 , in which a system for correcting the inclination of the wind turbine tower  100  can be seen. 
     This system comprises elements both in the anchoring column  60  and in the wind turbine tower  100 :
         a plurality of first plates  7  anchored to a reinforcement ring  63  of the anchoring column  60 ;   a plurality of second plates  8  fixed to a reinforcement ring  63  of the wind turbine tower  100 ;   a plurality of hydraulic lifting cylinders  9 , each of which is configured to exert pressure between one of the first plates  7  and one of the second plates  8 .       

     The plates  7 ,  8 , arranged perpendicular to the respective reinforcement ring  63  are in charge of housing the body or base of the hydraulic cylinders  9 . The plates  7 ,  8  perpendicular to the reinforcement rings  63  are in charge of receiving the force of the piston of the hydraulic cylinder  9 . 
     To prevent the tower from becoming unbalanced when it is raised up, the pistons themselves act like a guide, the hydraulic cylinder  9  being embedded on the plate  8  of the tower  100 . 
     Although the hydraulic cylinder  9  acts only on the plate  8  of the tower  100 , auxiliary guides can be used to prevent the tower  100  from becoming destabilized during the levelling process. 
     These elements shown in  FIG.  3    are used in a step of correction, performed after the step illustrated in  FIG.  2   d   . Once the foundation device  10  has been moored, this step of correction involves
         using the hydraulic lifting cylinders  9  for applying pressure between a first plate  7  anchored to a reinforcement ring  63  of the anchoring column  60  and a second plate  8  anchored to a reinforcement ring  63  of the wind turbine tower  100 , so as to vary the inclination of the wind turbine tower  100  with respect to the anchoring column  60 , creating a levelling gap between the anchoring surface and the wind turbine tower; and   introducing a securing element in the levelling gap.       

     To close the space generated by the levelling gap, different solutions, such as, for example, metal sheets, an epoxy paste and metallic material or an epoxy liquid and metallic material, can be used. 
     In this way, the fixing of the wind turbine tower  100  in its new inclination thereby takes place, endeavoring for the final result to be as vertical as possible. 
       FIG.  4    shows an alternative embodiment of the foundation device. In this case, starting from the device of  FIG.  1   b   , a floor slab  12  has been added on the lower face of the device. 
     The height of this floor slab  12 , measured in the vertical direction, from the lower face  2  to the upper face  3  is considerably less than the distance between said lower face  2  and upper face  3 . In this case, the ratio between the height of the floor slab  12  and the distance between said faces is 15%. 
     By means of this floor addition, the weight of the device increases slightly, but an important reduction of the draft in a port is obtained, therefore facilitating the launching of the device and the initial transport steps.