The present invention relates to a support structure for a wind turbine, includ- ing a lattice tower having a top part and a bottom part and including at least three columns and a number of transverse braces interconnecting adjacent columns, the interconnections between columns and transverse braces forming nodes of the lattice tower. US 4, 704,051 discloses an offshore, bottom supported platform structure including a tower structure of low bending stiffness which is laterally supported at several elevations by inclined, pretensioned stay cables connected to anchorages on the seabed. The tower structure is composed of a number of vertical columns which at each stay elevation below the elevation of the uppermost one, are interconnected by horizontal bracing members. However, the attachment of the stay cables to the anchorages and the subsequent tensioning of the stay cables must be performed offshore and is thus labour intensive. In order to overcome this problem, the document suggests an alternative solution, according to which, at the coast, the tower is erected on top of a floating box-like foundation structure to which the stay cables are also connected. The cables are tensioned and subsequently the platform is towed to its final offshore location and installed. However, the floating box-like foundation structure further complicates the platform structure. In any way, the traffic of boats close to the platform may render the stay cables prone to damage.

US 6,115,004 discloses an antenna support system for a communications tower having three or more vertical steel legs which are joined along their lengths by non-metallic lattice members. The lattice members include hori- zontal braces and diagonal guy supports. Each separate guy support comprises two parallel, non-metallic composite fibre wires connected in tension. One separate guy support extends diagonally between an end of each horizontal brace to an opposite end of the horizontal brace located above. However, each separate guy support must be tensioned individually when mounting the guy supports. This is especially difficult in the case that a large ten- sional force should be required, as all separate guy supports should be tensioned uniformly in order to avoid devastating deformation of the support system.

EP 2 362 036 A1 discloses a lattice structure for an offshore structure. Col- umns and braces of the lattice structure are connected by means of node pieces having a spherical or polygonal form. The columns and braces are welded to said node pieces.

US 2007/0001464 A1 discloses a wind turbine system including a wind tur- bine generator having a rotor and a nacelle mounted atop a wind tower. The wind tower is mounted on a central caisson. The wind turbine system further includes a plurality of tensioned mooring lines flexibly secured to the central caisson. The mooring lines are further secured to a plurality of anchors fixed on to a sea bed. The anchors are adapted to support lateral loads on the wind turbine system. However, the attachment of the mooring lines to the seabed and the subsequent tensioning of the mooring lines must be performed offshore and is thus labour intensive. Furthermore, the traffic of boats close to the platform may render the mooring lines prone to damage. The object of the present invention is to provide a support structure for a wind turbine, whereby a high degree of stiffness of the support structure may be obtained at reduced fabrication and material costs.

In view of this object, the lattice tower is reinforced by means of at least one tensioned cable section, a first end of which is attached at the bottom part of the lattice tower and a second end of which is attached at the top part of the lattice tower, and the at least one tensioned cable section is extended along successive straight lines extending between nodes at opposed ends of adjacent transverse braces. Thereby, by a simple procedure, the lattice tower may be pretensioned by means of the tensioned cable section. This may reduce the impact of fatigue loading in the design of the lattice tower due to retention of loads in the pretension. Consequently, the columns and the transverse braces may be of a weaker design than what would be necessary in the case of a corresponding conventional lattice structure. The structure may be lighter, and in turn, transportation and handling costs may be reduced.

In a structurally advantageous embodiment, in the mounted position of the lattice tower, at each height position of a node of the lattice tower, a horizon- tally extending transverse brace is provided between each pair of neighbouring columns.

In an embodiment, by means of part of a tensioned cable section, the node at either end of each transverse brace is connected with the node at the oppo- site end of the adjacent transverse brace. Thereby, the braces of each pair of transverse braces will be mutually interconnected by means of crossing parts of tensioned cable sections, thereby increasing the structural stiffness of the lattice tower. The lattice tower may thereby be pretensioned by means of the tensioned cable section or sections to a level at which the columns and the transverse braces never experience tension and the tensioned cable section or sections never experience compression. This may reduce the impact of fatigue loading in the design of the lattice tower due to retention of loads in the pretension. In a structurally advantageous embodiment, two of said tensioned cable sections are attached to the lowermost node of each column and, correspond- ingly, two of said tensioned cable sections are attached to the uppermost node of each column.

In a further structurally advantageous embodiment, the lowermost node of each column is provided with a first tensioned cable section and a second tensioned cable section both attached to said node, and the first tensioned cable section is wound around the lattice tower in a clockwise direction, seen from above, stepwise from a node to a following node one step further in the direction of the top part of the lattice tower, and the second tensioned cable section is wound correspondingly around the lattice tower in a counter clockwise direction, seen from above. Thereby, by pretensioning each of the cable sections, a uniform pretensioning of the entire lattice tower may be achieved.

In an embodiment, the height of the lattice tower from its bottom part to its top part is at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres, and wherein each at least one tensioned cable section, before exposing the lattice tower to wind or waves, is pretensioned with a force corresponding to at least 150 tons, preferably at least 300 tons and most preferred at least 450 tons, divided by the number of tensioned cable sections. Thereby, it may be achieved that the lattice tower is pretensioned to a level at which the risk that the columns and the transverse braces will experience tension and the risk that the tensioned cable section will experience compression are very low. In an embodiment, the columns are maintained into engagement with the transverse braces by means of the tension force of the at least one tensioned cable section. Thereby, the assembly of the lattice tower may be facilitated, as welding or bolting of the interconnections between columns and transverse braces may be avoided. In an embodiment, the columns are composed by column sections engaging each other at nodes of the lattice tower, and the column sections are maintained into engagement with each other by means of the tension force of the at least one tensioned cable section. Thereby, the assembly of the lattice tower may be further facilitated, as welding or bolting of the interconnections between column sections may be avoided.

In an embodiment, node connection pieces are provided at nodes of the lattice tower, and each node connection piece is engaged with an upper and a lower column section and a left and a right transverse brace. Thereby, the assembly of the lattice tower may be further facilitated. The node connection pieces may be standardized and manufactured at a factory.

In a structurally advantageous embodiment, lowermost column sections are connected to a foundation structure.

In another embodiment, lowermost node connection pieces are provided at lowermost nodes of the lattice tower and are adapted to connect to a foundation structure, and each lowermost node connection piece is engaged with an upper column section and a left and a right transverse brace.

In a structurally advantageous embodiment, uppermost column sections are connected to a transition piece adapted to connect with a lower end of a wind turbine tower, such as a hollow cylindrical tower structure.

In another embodiment, uppermost node connection pieces are provided at uppermost nodes of the lattice tower and are connected to a transition piece adapted to connect with a lower end of a wind turbine tower, such as a hollow cylindrical tower structure, and each uppermost node connection piece is engaged with a lower column section and a left and a right transverse brace. In an embodiment, the node connection pieces are adapted to receive ends of the column sections and ends of the transverse braces, and the column sections and the transverse braces are maintained into engagement with the node connection pieces by means of the tension force of the at least one tensioned cable section. Thereby, the assembly of the lattice tower may be further facilitated. The ends of the column sections and ends of the transverse braces may have a simple configuration, thereby facilitating manufacture of these items.

In an embodiment, each node connection piece is provided with a first sliding sleeve accommodating a first tensioned cable section slidably therein and a second sliding sleeve accommodating a second tensioned cable section slidably therein, and the first sliding sleeve is arranged at an angle to the second sliding sleeve and so that the first tensioned cable section does not touch the second tensioned cable section. Thereby, an entire cable section may be tensioned by tensioning an end thereof, because the cable section may slide at reduced friction in said sleeves and thereby even out the tension along the entire cable section. As the crossing cable sections do not touch each other, no friction will be present between them, and thereby wear of the cable sections may be minimized.

In an embodiment, the lattice tower is anchored to the seabed by means of anchor cables guided to follow the respective columns of the lattice tower from the bottom part to the top part of the lattice tower, a first end of each anchor cable being attached to an anchor driven into the seabed, such as a plate anchor, and a second end of each anchor cable being attached to the top part of the lattice tower. Thereby, anchoring the lattice tower to the seabed may be facilitated, as the lattice tower may be positioned on the seabed by dragging the anchor cables at the top part of the lattice tower, whereby the bottom part of the tower will be placed on the seabed and the cables will be tensioned, thereby firmly attaching the lattice tower to the seabed. As dragging the anchor cables may be performed at the top part of the lattice tower, no diving operations may be necessary in this respect, and thereby costs may be saved. The exact position of the anchors driven into the seabed need not be critical, as the cables may adapt to the position of the columns of the lattice tower by moving laterally through the material of the seabed. Therefore, a template for the positioning of the anchors may not be necessary. Furthermore, by anchoring the lattice tower to the seabed by means of anchor cables guided along the columns of the lattice tower, the columns of the lat- tice tower may be pretensioned to a certain level, whereby the at least one tensioned cable section extending along successive straight lines extending between nodes of the lattice tower may be relieved. Said tensioned cable section may therefore be of a weaker design. In a structurally advantageous embodiment, each anchor cable extends inside a hollow of the respective column of the lattice tower.

In an embodiment, the second end of each anchor cable is attached to the top part of the lattice tower by means of a tension adjusting device. Thereby, the tension of the anchoring cables may be easily readjusted at a later stage without diving operations being necessary.

In an embodiment, the lattice tower is further anchored to the seabed by means of feet having form of inverted buckets pressed at least partially down into the seabed. The feet having form of inverted buckets may take up horizontal forces, whereby it may be ensured that the lattice tower will not move laterally on the seabed.

The present invention further relates to an offshore wind turbine including a support structure as described above, wherein a hollow cylindrical tower structure carrying a nacelle with a wind turbine rotor is mounted on the top part of the lattice tower. Thereby, a standard hollow cylindrical tower structure may be employed offshore without large modifications, as the lattice tower may carry the hollow cylindrical tower structure above the water level. The lattice tower has an open structure, thereby minimizing the impact of waves.

The present invention further relates to a method of mounting a support structure for a wind turbine, the support structure including a lattice tower having a top part and a bottom part and including at least three columns and a number of transverse braces interconnecting adjacent columns, whereby columns and transverse braces are interconnected, thereby forming nodes of the lattice tower at the interconnections between columns and transverse braces. The method is characterised by, firstly, providing the lattice tower with at least one cable section, thereby attaching a first end of the at least one cable section at the bottom part of the lattice tower and attaching a second end of the at least one cable section at the top part of the lattice tower, and thereby extending the cable section along successive straight lines extending between nodes at opposed ends of adjacent transverse braces, and by, secondly, tensioning the least one cable section, thereby reinforcing the lattice tower. Thereby, the above explained features may be achieved.

In an embodiment, the node at either end of each transverse brace by means of part of a tensioned cable section is connected with the node at the opposite end of the adjacent transverse brace. Thereby, the above explained fea- tures may be achieved.

In an embodiment, two of said tensioned cable sections are attached to the lowermost node of each column and, correspondingly, two of said tensioned cable sections are attached to the uppermost node of each column. Thereby, the above explained features may be achieved. In an embodiment, the lowermost node of each column is provided with a first tensioned cable section and a second tensioned cable section that are both attached to said node, and the first tensioned cable section is wound around the lattice tower in a clockwise direction, seen from above, stepwise from a node to a following node one step further in the direction of the top part of the lattice tower, and the second tensioned cable section is wound correspondingly around the lattice tower in a counter clockwise direction, seen from above. Thereby, the above explained features may be achieved. In an embodiment, before raising the lattice tower to an upright position, the lattice tower is provided with the at least one cable section and said cable section is pretensioned partially, and, after raising the lattice tower to an upright position, said cable section is pretensioned to a final pretension. Thereby, partly it may be achieved that the integrity of the lattice tower may be ensured during raising of the tower, and partly it may be achieved that the tower may be finally pretensioned in upright position, whereby it may be ensured that the tower is not distorted during pretensioning.

In an embodiment, after having pretensioned said cable section to said final pretension, the lattice tower is transported to an offshore location when being in upright position, and the lattice tower is then subsequently mounted on the seabed at said offshore location. Thereby, pretensioning of the lattice tower may be entirely performed at the coast and not offshore, thereby avoiding costly diving operations.

In an embodiment, the height of the lattice tower from its bottom part to its top part is at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres, and each at least one tensioned cable section, before exposing the lattice tower to wind or waves, is pretensioned with a force corresponding to at least 150 tons, preferably at least 300 tons and most pre- ferred at least 450 tons, divided by the number of tensioned cable sections. Thereby, the above explained features may be achieved.

In an embodiment, the lattice tower is assembled by means of node connec- tion pieces that are engaged with upper and lower column sections and left and right transverse braces. Thereby, the above explained features may be achieved.

In an embodiment, before raising the lattice tower to an upright position, the lattice tower is assembled by means of the node connection pieces and is then provided with the at least one cable section, said cable section is then pretensioned at least partially, and, subsequently, the lattice tower is raised to an upright position. Thereby, partly it may be achieved that the components of the lattice tower are maintained in connected state during raising of the tower, and partly it may be achieved that the tower may be finally pretensioned in upright position, whereby it may be ensured that the tower is not distorted during pretensioning.

In an embodiment, the lattice tower being of the type having columns inclin- ing in the direction of each other, the lattice tower is assembled by means of the node connection pieces beginning from the top part of the lattice tower and ending at the bottom part of the lattice tower. Thereby, the braces that will extend transversally in the upright position of the tower, will be in a slightly inclined position leaning against upper columns during assembly of the tower, thereby maintaining columns and braces connected during assembly. Subsequently, the tower may be pretensioned partly by means of the at least one cable section, whereby columns and braces may be locked into connection with each other. In an embodiment, the at least one cable section is pretensioned by tensioning said cable section at one end thereof, whereby said cable section slides in sliding sleeves provided at the nodes of the lattice tower. Thereby, the above explained features may be achieved.

In an embodiment, the support structure is anchored to the seabed by mount- ing anchor cables guided to follow the columns of the lattice tower from the bottom part to the top part of the lattice tower, attaching a first end of each anchor cable to an anchor, such as a plate anchor, and attaching a second end of each anchor cable to the top part of the lattice tower, preferably by means of a tension adjusting device, whereby each anchor is driven into the seabed and, subsequently, the second end of each anchor cable is tensioned, thereby firstly positioning the support structure on the seabed and secondly firmly attaching the support structure to the seabed. Thereby, the above explained features may be achieved. In a structurally advantageous embodiment, each anchor cable is extended inside a hollow of the respective column of the lattice tower.

In an embodiment, the lattice tower is further anchored to the seabed by means of feet having form of inverted buckets that are pressed at least par- tially down into the seabed as the anchor cables are tensioned. Thereby, the above explained features may be achieved.

In an embodiment, subsequently to anchoring the lattice tower on the seabed, a hollow cylindrical tower structure carrying a nacelle with a wind turbine rotor is mounted on the top part of said support structure. Thereby, the above explained features may be achieved.

The invention will now be explained in more detail below by means of examples of embodiments with reference to the very schematic drawing, in which

Fig. 1 is a front view of a support structure for a wind turbine; Fig. 2 is a side view seen from the right of the support structure shown in Fig 1 ;

Fig. 3 is a perspective view of the support structure shown in Fig. 1 ;

Fig. 4 shows a detail on a larger scale of the support structure shown in Fig.

3;

Fig. 5 shows a transition piece at a top part of the support structure shown in Fig. 3;

Fig. 6 shows a node connection piece of the support structure shown in Fig.

3; Figs. 7 to 9 illustrate different embodiments of the arrangement of wire on the support structure shown in Fig. 3;

Figs. 10A to 10E illustrate a method of mounting a support structure as shown in Figs. 1 to 4 on the seabed;

Fig. 11 illustrates yet another embodiment of the arrangement of wire on the support structure shown in Fig. 3;

Fig. 12 is a side view of the support structure shown in Fig 11 ;

Fig. 13 is a cross-section illustrating a saddle to be mounted on a column of the support structure according to the invention;

Fig. 14 is a side view of the saddle in Fig. 13 mounted on a column of the support structure according to the invention; Fig. 5 is a top view of the mounted saddle in Fig. 14;

Fig. 16 illustrates an embodiment of a node connection piece seem from above;

Fig. 17 illustrates a longitudinal cross section through the node connection piece of Fig. 16; and

Fig. 18 illustrates a longitudinal cross section through another embodiment of a node connection piece.

Figs. 1 to 4 illustrate a support structure 1 for a not shown wind turbine. The support structure 1 is preferably mounted on a seabed and may be adapted to carry a traditional hollow cylindrical tower structure of a wind turbine.

The support structure 1 includes a lattice tower 2 forming a so-called jacket and having a top part 3 and a bottom part 4. The lattice tower 2 is illustrated in its mounted orientation and includes three columns 5, 6, 7, also called jacket legs, and a number of transverse braces 8 interconnecting adjacent columns 5, 6, 7. The lattice tower 2 may, however, include four or more columns, and each pair of columns may be interconnected by any suitable number of braces. As illustrated, it is preferred that the columns 5, 6, 7 of the lattice tower 2 converge in the direction from the bottom part 4 to the top part 3 of the lattice tower 2.

The interconnections between columns 5, 6, 7 and transverse braces 8 form nodes 9 of the lattice tower 2. In the illustrated embodiment, at each height position of a node 9 of the lattice tower 2, a horizontally extending transverse brace 8 is provided between each pair of neighbouring columns 5, 6, 7. How- ever, many other configurations are conceivable, for instance, some or all of the braces 8 may be inclined to the horizontal, and braces interconnecting one pair of columns may be staggered in relation to some or all of the braces interconnecting another pair of columns.

The lattice tower 2 shown in Figs. 1 to 4 is reinforced by means of six ten- sioned cable sections 10, 11 , 12, 13, 14, 15. A first end 16 of each tensioned cable section 10, 11, 12, 13, 14, 15 is attached at the bottom part 4 of the lattice tower 2, and a second end 17 of each tensioned cable section 10, 11 , 12, 13, 14, 15 is attached at the top part 3 of the lattice tower 2. Thereby it is noted that two of said tensioned cable sections 10, 11 , 12, 13, 14, 15 are at- tached to the lowermost node 9 of each column 5, 6, 7, respectively, and, correspondingly, two of said tensioned cable sections 10, 11 , 12, 13, 14, 15 are attached to the uppermost node 9 of each column 5, 6, 7. It is noted that two or more of the cable sections 10, 11 , 12, 13, 14, 15 may be formed as one continuous cable, so that one or more mid sections of such a cable may be attached to a lowermost and/or an uppermost node 9 of columns 5, 6, 7.

In the embodiment shown in Figs. 1 to 4, the lowermost node 9 of the column 5 is provided with a first tensioned cable section 11 and a second tensioned cable section 10 both attached to said lowermost node 9, and said first ten- sioned cable section 11 is wound around the lattice tower 2 in a clockwise direction, seen from above, stepwise from a node 9 to a following node 9 one step further in the direction of the top part 3 of the lattice tower 2, and said second tensioned cable section 10 is wound correspondingly around the lattice tower 2 in a counter clockwise direction, seen from above. Said first ten- sioned cable section 11 is attached to the uppermost node 9 of the column 7, and said second tensioned cable section 10 is attached to the uppermost node 9 of the column 6. Correspondingly, the lowermost node 9 of the column 6 is provided with a first tensioned cable section 13 and a second tensioned cable section 12 both attached to said lowermost node 9, and said first tensioned cable section 13 is wound around the lattice tower 2 in a clockwise direction, seen from above, stepwise from a node 9 to a following node 9 one step further in the direction of the top part 3 of the lattice tower 2, and said second tensioned cable section 12 is wound correspondingly around the lattice tower 2 in a counter clockwise direction, seen from above. Said first tensioned cable section 13 is attached to the uppermost node 9 of the column 5, and said second tensioned cable section 12 is attached to the uppermost node 9 of the column 7. Correspondingly, the lowermost node 9 of the column 7 is provided with a first tensioned cable section 15 and a second tensioned cable section 14 both attached to said lowermost node 9, and said first tensioned cable section 15 is wound around the lattice tower 2 in a clockwise direction, seen from above, stepwise from a node 9 to a following node 9 one step further in the direction of the top part 3 of the lattice tower 2, and said second tensioned cable section 14 is wound correspondingly around the lattice tower 2 in a counter clockwise direction, seen from above. Both said first tensioned cable section 15 and said second tensioned cable section 14 are attached to the uppermost node 9 of the column 5. Said first tensioned cable section 15 is attached to the uppermost node 9 of the column 6, and said second tensioned cable section 14 is attached to the uppermost node 9 of the column 5. Each tensioned cable section 10, 11 , 12, 13, 14, 15 is thus extended along successive straight lines extending between nodes 9 at opposed ends of adjacent transverse braces 8. Referring to the particular nodes 9' and transverse braces 8' shown in Fig. 4, this is exemplified by the tensioned cable section 10 extended along a straight line extending between the nodes 9' at opposed ends of the adjacent transverse braces 8', namely between the node 9' at the left end of the lower transverse brace 8' and the node 9' at the right end of the upper transverse brace 8'; said right end of the upper transverse brace 8' being opposed to said left end of the lower transverse brace 8'. Furthermore, referring to elements shown in Fig. 4, it is noted that the nodes 9', 9" at either end of each transverse brace 8' by means of part of a tensioned cable section 10, 13 is connected with the nodes 9', 9" at the opposite ends, respectively, of the adjacent transverse brace 8'.

It is noted that the lattice tower 2 shown in Figs. 1 to 4 may be pretensioned by means of the six tensioned cable sections 10, 1 , 12, 13, 14, 15 to a level at which the columns 5, 6, 7 and the transverse braces 8 never experience tension and the tensioned cable sections 10, 11 , 12, 13, 14, 15 never experience compression, even when the support structure is subjected to the influence of wind and waves. This may reduce the impact of fatigue loading in the design of the lattice tower due to retention of loads in the pretension. Consequently, the columns and the transverse braces may be of a weaker design than what would be necessary in the case of a corresponding conventional lattice structure. The structure may be lighter, and in turn, transportation and handling costs may be reduced. However, one or more of the six tensioned cable sections 10, 11 , 12, 13, 14, 15 of the embodiment of Figs. 1 to 4 may be omitted, still obtaining advantageous pretension of the lattice tower 2, although six tensioned cable sections 10, 11 , 12, 13, 14, 15 are preferred.

Figs. 7 to 9 illustrate by means of a bold, broken line different arrangements of a single cable section on the lattice tower 2. Fig. 7 illustrates how one 11 of the six tensioned cable sections 10, 11 , 12, 13, 14, 15 of the embodiment shown in Figs. 1 to 4 is arranged. The remaining tensioned cable sections 10, 12, 13, 14, 15 are arranged in a similar manner as described above. Fig. 8 illustrates another arrangement of a single cable section 18 on the lattice tower 2. According to this embodiment, the cable section 18 is zigzagged between nodes 9 of the adjacent columns 5, 6, beginning at the lowermost node 9 of the column 5 and continuing stepwise from a node 9 to a following node 9 one step further in the direction of the top part 3 of the lattice tower 2 and finally ending at the uppermost node 9 of the column 6. A further cable section 18 may then be zigzagged correspondingly between nodes 9 of the adjacent columns 5, 6 beginning at the lowermost node 9 of the column 6 and ending at the uppermost node 9 of the column 5. In the same way, two cable sections may be zigzagged between nodes 9 of the adjacent columns 6, 7 and further two cable sections may be zigzagged between nodes 9 of the adjacent columns 7, 5.

Fig. 11 illustrates an arrangement of cable sections 18, 39, 40, 41 on the lattice tower 2 corresponding to the arrangement described just above and illustrated in Fig. 8. It is noted that although the embodiment illustrated in Fig. 11 has less transverse braces 8 from bottom to top than the embodiment illustrated in Fig. 8, this is only a matter of illustration. Any suitable number of transverse braces 8 from bottom to top is possible. The zigzagged cable sections illustrated in Fig. 8 may be attached at the nodes 9 of the lattice tower 2 in many different ways, for instance in a way corresponding to the one illus- trated in Fig. 4 and 6, whereby the cable sections are attached on the side of the columns 5, 6, 7 directed away from a vertical centre axis of the lattice tower 2 by being inserted into a sliding sleeve 26, 27 of a node connection piece 21. According to the arrangement illustrated in Fig. 11 , the cable sections 18, 39, 40, 41 are attached by being coiled approximately 180 degrees around the respective columns 5, 6, 7. It is noted that Fig. 11 illustrates cable sections 18, 39 arranged between respective columns 5, 6 and furthermore cable sections 40, 41 arranged between respective columns 5, 7. However, in a similar way, two not shown cable sections should be arranged between the respective columns 6, 7.

Referring now to Fig. 12 illustrating a side view of the lattice tower 2 in Fig. 11 , it is preliminary noted that when a cable section 18, 39 extends behind a column 5, 6, the cable section 18, 39 is illustrated by a broken line, and when a cable section 18, 39 extends in front of a column 5, 6, the cable section 18, 39 is illustrated by a continuous line. In Fig. 12, it is seen that the cable section 18 is fixed at the bottom of the right column 5 at a node 9a and is in fact extending behind the right column 5 at this position, and there from the cable section 18 is extended along a straight line to a following node 9b at the left column 6 one step further in the direction of the top part 3. At the node 9b at the left column 6, the cable section 18 extends firstly behind the left column 6 and then in front of the left column 6, whereby the cable section 18 is coiled approximately 180 degrees around the left column 6 in a spiral-like fashion, and from this position, the cable section 18 is extended along a straight line to a following node 9c at the right column 5 one step further in the direction of the top part 3. At the node 9c at the right column 5, the cable section 18 ex- tends firstly in front of the right column 5 and then behind the right column 5, whereby the cable section 18 is coiled approximately 180 degrees around the right column 5 in a spiral-like fashion, and from this position, the cable section 18 is extended along a straight line to the back side of a following node 9d at the left column 6 one step further in the direction of the top part 3 (in the illustrated case, one of the uppermost nodes).

Still referring to Fig. 12, it is seen that a second cable section 39 is fixed at the bottom of the left column 6 at a node 9e and is in fact extending in front the left column 6 at this position, and there from the cable section 39 is ex- tended along a straight line to a following node 9f at the right column 5 one step further in the direction of the top part 3. At the node 9f at the right column 5, the cable section 39 extends firstly in front of the right column 5 and then behind the right column 5, whereby the cable section 39 is coiled approximately 180 degrees around the right column 5 in a spiral-like fashion, and from this position, the cable section 39 is extended along a straight line to a following node 9g at the left column 6 one step further in the direction of the top part 3. At the node 9g at the left column 6, the cable section 39 extends firstly behind the left column 6 and then in front of the left column 6, whereby the cable section 39 is coiled approximately 180 degrees around the left column 6 in a spiral-like fashion, and from this position, the cable section 39 is extended along a straight line to the front side of a following node 9h at the right column 5 one step further in the direction of the top part 3 (in the illustrated case, one of the uppermost nodes).

By the arrangement of the cable sections 18, 39 as illustrated in Figs. 1 1 and 12 and described above, it may be obtained that two cable sections 18, 39 do not touch each other where they cross each other, that is at the nodes 9 and where the cable sections 18, 39 are extended along a straight line and cross each other, for instance at the area 42 illustrated in the figures. Thereby, it may be avoided that cable sections slide against each other and thereby wear down. Referring for instance to Fig. 1 1 , it is seen that the two cable sections 39, 40 are in fact crossing each other at the node 9f; however, they are not touching each other as they are led around the right column 5 so that they are mutually spaced. Furthermore, where the two cable sections 18, 39 cross each other at the area 42 illustrated in the figures, the two cable sec- tions 18, 39 are mutually spaced by a distance corresponding to the diameter of the columns 5, 6, because they are in fact extended on either side of the columns 5, 6. With reference to the vertical centre axis of the lattice tower 2, the cable section 18 is extended at the inside of the columns 5, 6, and the cable section 39 is extended at the outside of the columns 5, 6.

Fig. 9 illustrates yet another arrangement of a single cable section 19 on the lattice tower 2. According to this embodiment, the cable section 19 is wound around the lattice tower 2 in a clockwise direction, seen from above, stepwise up and down, from a node 9 of a column 5 to a following node 9 one step further in the direction of the top part 3 of the lattice tower 2, then one step backwards in the direction of the bottom part 4 of the lattice tower 2 and so forth until said column 5 is reached again, thereby forming one turn about the lattice tower 2. The winding starts from a lowermost node 9 of the column 5 and, subsequently, further turns are formed by the cable section 19 until one of the uppermost nodes 9 is reached. Finally, another cable section is, correspondingly, wound around the lattice tower 2 in a counter clockwise direction, seen from above, stepwise up and down, starting from the lowermost node 9 of the column 5 and finishing at one of the uppermost nodes 9. In the shown embodiment, the lattice tower 2 has three columns 5, 6, 7; however, in another embodiment, the lattice tower 2 could be provided with an equal num- ber of columns, such as for instance four. Thereby, by forming one turn about the lattice tower 2, the cable section 19 reaches the same node 9 again, from where it started. Therefore, in order to proceed further in the direction from the bottom part 4 to the top part 3 of the lattice tower 2, according to this embodiment, after every turn of the cable section 19, the cable section 19 is led along the column of this node 9 up to the next node 9 on the same column. Therefrom, the next turn of the cable section 19 may start out.

According to each of the embodiments illustrated in Figs. 7 to 9, it is preferred that the nodes 9, at either end of each transverse brace 8 by means of part of a tensioned cable section 10, 11 , 12, 13, 14, 15, 18, 19 is connected with the nodes 9 at the opposite ends, respectively, of the adjacent transverse brace 8. In order to obtain this, the embodiment illustrated in Fig. 7 is provided with six tensioned cable sections 10, 11, 12, 13, 14, 15, wherein a first end of each is attached at the bottom part 4 of the lattice tower 2, and a second end of each is attached at the top part 3 of the lattice tower 2. Similarly, in order to obtain this, the embodiment illustrated in Fig. 8 is provided with six tensioned cable sections 18, wherein a first end of each is attached at the bottom part 4 of the lattice tower 2, and a second end of each is attached at the top part 3 of the lattice tower 2. Similarly, in order to obtain this, the embodiment illustrated in Fig. 9 is provided with two tensioned cable sections 19, wherein a first end of each is attached at the bottom part 4 of the lattice tower 2, and a second end of each is attached at the top part 3 of the lattice tower 2. However, some of said cable sections or part of some of said cable sections may be omitted. Other arrangements of tensioned cable sections on the lattice tower 2 than those arrangements illustrated in Figs. 1 to 4 and 7 to 8 and described above are conceivable. The height of the lattice tower from its bottom part 4 to its top part 3 may be at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres. Each at least one tensioned cable section 10, 1 1 , 12, 13, 14, 15, 18, 19, may, before exposing the lattice tower 2 to wind or waves, be preten- sioned with a force corresponding to at least 150 tons, preferably at least 300 tons and most preferred at least 450 tons, divided by the number of tensioned cable sections.

In the embodiments of the support structure 1 shown, the columns 5, 6, 7 may be maintained into engagement with the transverse braces 8 by means of the tension force of the at least one tensioned cable section 10, 11 , 12, 13, 14, 15, 18, 19. Thereby, assembly of the lattice tower 2 may be facilitated, as other types of attachment, such as welding or bolting, may be avoided. The columns 5, 6, 7 may be composed by column sections 20 engaging each other at nodes 9 of the lattice tower 2, and the column sections 20 may be maintained into engagement with each other by means of the tension force of the at least one tensioned cable section, thereby further facilitating assembly.

In an embodiment illustrated in Fig. 6, node connection pieces 21 are provided at nodes 9 of the lattice tower 2. Each node connection piece 21 is en- gaged with an upper and a lower column section 20 and a left and a right transverse brace 8. The node connection pieces 21 may simply be formed with holes in which ends of column sections 20 and ends of transverse braces 8, respectively, may be received. Alternatively, the node connection pieces 21 may be provided with studs fitting into holes provided at ends of column sections 20 and ends of transverse braces 8, respectively. Other configurations for the connection between node connection pieces 21 and col- umn sections 20 or transverse braces 8 are conceivable. The column sections 20 and the transverse braces 8 are maintained into engagement with the node connection pieces 21 by means of the tension force of the at least one tensioned cable section. However, additionally, the column sections 20 and the transverse braces 8 may be attached permanently or preliminary to the node connection pieces 21 by other connection means, such as by means of welding or bolts. Preliminary attachment by means of welding may be achieved by means of so-called strongbacks that are subsequently removed.

Figs. 16 and 17 illustrate another embodiment of a node connection piece 21 made in steel. The concept is based on the transverse brace 8 be sniped off near the column 5 ending in a flat plate 48 that just interact with the column 5 by compression/contact. The column wall is backed-up by an internal dia- phragm in the same plane. One installation method for the transverse brace 8 is to just push it into place, sliding against the inner face of the columns 5, 6, 7. Therefore some stiffening of the column skin may be required. The same stiffening is applied behind the outer side of the column, so that the loads from the saddle element and cable section can be transferred to the diaphragm.

Fig. 18 illustrates yet another embodiment of a node connection piece 21 made in concrete. The concept comprises:

• local thickening of column structure,

· spherical concrete cap 49 cast with the transverse brace 8,

• intermediate compression pad 50 (possibly fibre reinforced concrete), whereby all elements may in principle be held in place only by contact and friction. The contact joints are preferably "close fit" to secure a uniform contact stress. On the other hand, the joint may preferably allow some angular rotation when the elements shall be preinstalled and during stressing of the cable sections. The thickening of the column 5 at the node 9 may be made in spun cast concrete, for instance. It may be assumed that spinning of the form with fresh concrete will result in a substantially constant inner wall surface throughout the length of the casting, and that it will be possible to vary the external face locally. The fibre reinforced concrete pad 50 may be a delicate element, subjected to various pulsating load effects. The pad 50 may therefore need to be as ductile as can be, so that any unintended initial cracking will not propagate.

As illustrated in Figs. 1 to 4 and 7 to 9, lowermost node connection pieces 21' are provided at lowermost nodes 9 of the lattice tower 2 and are connected to a foundation structure, such as bucket foundations 22. A bucket foundation 22 is well-known in the art and may be formed by an inverted bucket-like steel component that may be pressed down into the seabed by means of for instance a so-called driven plate anchor inserted into the seabed or by means of suction pressure provided in the bucket-like component by means of a vacuum pump. Each lowermost node connection piece 21' is engaged with an upper column section 20 and a left and a right transverse brace 8. The lowermost node connection pieces 21' may be connected directly to the bucket foundations 22 or the like, or the bucket foundations 22 or the like may be attached to the lower ends of the columns 5, 6, 7 or lowermost column sections 23 by means of welding or other suitable means. As seen in the figures, the lowermost column sections 23 may be shorter than other col- umn sections 20. By the use of lowermost column sections 23 for the connection to the bucket foundations 22 or the like, the lowermost node connection pieces 21' may be identical to the remaining node connection pieces 20 of the lattice tower 2. As further illustrated in Figs. 1 to 4 and 7 to 9, uppermost node connection pieces 21" are provided at uppermost nodes 9 of the lattice tower 2 and are connected to a transition piece 24 adapted to connect with a lower end of a wind turbine tower, such as a hollow cylindrical tower structure. The uppermost node connection pieces 21" may be connected directly to the transition piece 24, or the transition piece 24 may be attached to the upper ends of the columns 5, 6, 7 or uppermost column sections 25 by means of welding or other suitable means. As seen in the figures, the uppermost column sections 25 may be shorter than other column sections 20. By the use of uppermost column sections 25 for the connection to the transition piece 24, the uppermost node connection pieces 21" may be identical to the remaining node connection pieces 20 of the lattice tower 2.

The transition piece 24 may be fabricated as a conical cylinder with sliced steel beams welded on both the outside and inside of the cylinder. Inside the transition piece it will be possible to mount the wind turbine generator cable plug and other equipment which may be possible to mount on the quayside in very limited height, hence the use of crane activity may be limited. A platform with space for spare parts and other equipment may also be mounted on the transition piece 24 from fabrication site - even in limited height. Each of Figs. 7 to 8 thus illustrates a method of mounting a support structure 1 for a wind turbine as described above, including the following steps: firstly, providing the lattice tower 2 with at least one cable section 10, 11 , 12, 13, 14, 15, 18, 19, thereby attaching a first end of the at least one cable section at the bottom part 4 of the lattice tower 2 and attaching a second end of the at least one cable section at the top part 3 of the lattice tower, and thereby extending the cable section along successive straight lines extending between nodes 9 at opposed ends of adjacent transverse braces 8, and by, secondly, tensioning the least one cable section, thereby reinforcing the lattice tower. A not shown offshore wind turbine may include the support structure 1 , so that a not shown hollow cylindrical tower structure carrying a nacelle with a wind turbine rotor is mounted on the top part 3 of the lattice tower 2. The lower end of the hollow cylindrical tower structure may be adapted to receive a top part of the transition piece. Referring to Fig. 6, each node connection piece 21 is provided with a first sliding sleeve 26 accommodating a first tensioned cable section 11 slidably therein and a second sliding sleeve 27 accommodating a second tensioned cable section 10 slidably therein. The sliding sleeves 26, 27 may be made of graphite whereby they may be self-lubricating. The first sliding sleeve 26 is arranged at an angle A to the second sliding sleeve 27 and so that the first tensioned cable section 11 does substantially not touch or does not touch the second tensioned cable section 10. In the embodiment shown, this arrangement is achieved by forming the first sliding sleeve 26 as a groove in a surface 28 of the node connection piece 21 and forming the second sliding sleeve 27 as a groove in the surface 28 of the node connection piece 21 ; the groove forming the first sliding sleeve 26 having a depth d being smaller than a depth D of the groove forming the second sliding sleeve 27. Preferably, the depth d is smaller than depth D plus a thickness of a cable forming the second cable section 10. In this way, it may be achieved that the first tensioned cable section 11 does not touch the second tensioned cable section 10. Thereby, wear caused by the cable sections 10, 11 sliding against each other may be avoided. Alternatively, the first and second sliding sleeves 26, 27 may be formed by tubes through which the cable sections 10, 11 are led. In other possible arrangements, the cable sections 10, 11 may be carried by rollers, for instance running in a groove of the roller; the rollers being mounted on the node connection piece 21 , preferably also arranged so that the cable sections 10, 11 do substantially not touch or do not touch each other. Other configurations are possible, such as combinations of grooves forming sleeves, tubes forming sleeves and rollers carrying cable sections. Referring to Fig. 5, it is seen that upper, second ends 17 of cable sections

11 , 14 are connected to a cable strainer 29 in the form of an eye bolt 30 mounted through a hole 31 in a flap 32 attached by welding or other suitable method to the transition piece 24. Said cable ends are attached to the eye of the eye bolt 30 and the bolt of the eye bolt us maintained in the hole 31 by means of a bolt 33 screwed onto the eye bolt. Thereby, it is possible to tighten the cable sections 11 , 14 by tightening the bolt further. In correspondence with the description above, the cable sections 11 , 14 may be formed by one cable just passing through the eye of the eye bolt 30, as illustrated in the figures. Alternatively, the cable sections 11 , 14 may be connected to each other at the eye bolt or they may each be attached separately to the eye bolt. Many other configurations of cable strainers are possible. Cable strainers may also be positioned further downwards the lattice tower 2 than illustrated in the figures, it is, however, preferred that cable strainers are situated above the sea level as this facilitates the operation of cable straining. Furthermore, the cable sections may be strained by means of a separate device not forming part of the support structure 1 , which device may be removed after attachment of the cable sections on the lattice structure 2.

It is noted that the lower, first ends 17 of the tensioned cable sections 10, 11 ,

12, 13, 14, 15, 18, 19 may just as the upper, second ends 17 thereof be connected to a cable strainer or simply to an eye bolt or the like situated below the lower node connection pieces 21'. In the figures, however, the cable sections are just shown ending at the lower node connection pieces 21 ', which may, of course, also be the case, provided that the lower node connection pieces 21' are adapted for fixation of the cable sections thereto.

Furthermore, the cable sections may be strained by means of automatic cable strainer 29, such as for instance spring biased cable strainers adapted to maintain a certain pretension of each cable section. This may be advantageous, especially if relatively large variations of cable length is expectable, such as for instance in relation with land based wind turbines, where the temperature variations of the construction may be larger than it is the case for offshore wind turbines. Referring to Fig. 6, the first and second sliding sleeves 26, 27 may be provided with fixation devices for fixation of the first and second tensioned cable sections 10, 11 , respectively, in their respective sliding sleeves 26, 27 after that the cable sections have been tensioned by means of the cable strainer 29. Of course, each cable section 10, 11 , 12, 13, 14, 15, 18, 19 may be fixed in one or more of its sliding sleeves in this way. Thereby, the lattice tower 2 may be secured against failure in the case that a single cable section should brake.

As discussed above referring to Fig. 6, each node connection piece 21 may be provided with sliding sleeves 26, 27 accommodating tensioned cable sections slidably therein. However, it may be preferred that the tensioned cable sections are arranged non-slidably, i.e. fixed, in saddle elements arranged at the nodes 9. Figs. 13 to 15 illustrate a saddle element 43 having the form of an elongated trough with a U-formed cross-section and being arranged in a spiral-like way around the column 5 of the lattice tower 2. The aim of the saddle element 43 is to guide a cable section 18 around the column 5, whereby the cable section 18 is allowed to be rotated about its longitudinal axis in a way that is natural for such cable section. Furthermore, the aim of the saddle element 43 is to fix the cable section 18 non-slidably to the col- umn 5. The saddle element 43 illustrated in Figs. 13 to 15 is generally applicable two the different arrangements of the cable sections illustrated in Figs. 7 to 9, 11 and 12; however, the specific design illustrated in Figs. 13 to 15 is specifically directed at the arrangement of cable sections illustrated in Figs. 11 and 12, as it will be apparent from the following description. As discussed above, according to the arrangement illustrated in Figs. 11 and 12, the cable sections 18, 39, 40, 41 are attached by being coiled approximately 180 degrees around the respective columns 5, 6, 7. Because of the curvature of the cable section around the column, as it is well known in the art, in this situation, any type of cable will naturally seek to rotate approximately 360 degrees about its longitudinal axis along the coiled part of the cable section. If the cross section of the type of cable employed is approximately circular, the rotation of the cable about its longitudinal axis may in reality not be easily guided by the form of the saddle element 43. However, if the cross section of the type of cable employed is not circular, the rotation of the cable about its longitudinal axis may in fact be guided by the inner form of the saddle element 43. As it will be understood by those skilled in the art, by guiding a cable section that is coiled spirally approximately 180 degrees around a respective column 5, 6, 7 so that it is also rotated approximately 360 degrees about its longitudinal axis along the coiled part of the cable section, it may be achieved that all possible individual strands of the cable section are stressed substantially equally, thereby avoiding excessive stress in certain cable strands. In the embodiment illustrated in Fig. 13, the cable sections 18, 39, 40, 41 are formed by cables 44 composed by so-called monostrands 45 normally arranged in parallel in the cable. Each monostrand 45 may be composed of a number of not illustrated wires, such as metal wires, and each monostrand may be coated by plastic, for instance they may be PE-sheathed.

The specific saddle element 43 illustrated in Figs. 13 to 15 supports the entire cable 44 composed by 55 monostrands 45 (so-called cohestrands) in a common trough 46. The saddle element 43 may be made from cast steel fabricated to suit the complex 3D-geometry. The saddle may be supported by direct contact pressure to the node 9 or column 5, only minor fixations may possibly be required for temporary purpose during mounting. The curvature of the trough 46 is adequate for the cross sectional dimensions of the monostrands 45. Enough friction may be ensured by a full 0-360 degree twist of the cable 44 while being supported by the saddle element 43. The twist is ensured by grooving the hexagonal shape as illustrated of the trough 46 formed by the inner faces of the saddle element 43. The twist may ensure that the cable 44 may be bent around the column 5, and that all individual monostrands 45 may achieve the same length, i.e. that the pre-stressing force will be distributed evenly. The rotation of the cable 44 may have a very important function, as all monostrands 45 at some point during the 0-360 degree twist will be at the bottom of the trough 46; i.e. these monostrands will be efficiently compressed by the other monostrands on top, and thereby a significant shear force can be transferred. The monostrands 45 will therefore not slip relative to each other as would have been the case if there had been no twist.

In Fig. 13, the cross section of the saddle element 43 is illustrated. The outside form of the saddle element 43 may be constant during the above described 0-360 degree twist, so that a bottom side 47 of the saddle element 43 abuts the column 5 during the entire twist. However, the inner hexagonal shape of the trough 46 formed by the inner faces of the saddle element 43 is thereby rotated during the twist. For illustrative purposes, said inner hexagonal shape of the trough 46 is illustrated in the figure at two different positions during the twist: A first, initial position of the inner hexagonal shape is illus- trated by a continuous line and a second position of the inner hexagonal shape is illustrated by a broken line. Of course, in order to guide the cable as described above, the cross section of the inner of the trough 46 need not be hexagonal, any suitable non-circular form will be possible. A form that may be composed by a number of monostrands may be preferable. As discussed above, in all embodiments of the lattice tower 2 described, the cable sections may be tensioned by means of cable strainers 29 or the like. However, in the following, an alternative procedure of tensioning, likewise applicable to all embodiments of the lattice tower 2 described, will be ex- plained. According to this procedure, the cable sections 10, 1 1 , 12, 13, 14, 15, 18, 19, 39, 40, 41 of the lattice tower 2 are arranged at the lattice tower 2 in one of the described way, whereby, however, each cable section is attached at either of its ends to the lattice tower 2, whereby no cable strainers are applied. Subsequently, the cable sections 10, 11 , 12, 13, 14, 15, 18, 19, 39, 40, 41 of the lattice tower 2 are tensioned by spreading the columns 5, 6, 7 of the lattice tower 2 further apart from each other. This may be done in that each transverse brace 8 is inserted after that the corresponding two columns 5, 6, 7 have been forced so much apart by means of a hydraulic jack or like that said transverse brace 8 may be inserted between the two columns 5, 6, 7. Alternatively, the transverse braces may 8 may expandable in their longitudinal direction, for instance in a telescopic way. According to this procedure of tensioning the cable sections, it may be preferred that saddle elements for the guidance of the cable section around the columns are p remounted on the cable sections at correct positions. This may facilitate the mounting of the lattice tower in that no adjustment may be necessary during mounting. The pretensioning of the cable sections may be defined simply by the dimensions of the columns 5, 6, 7 and transverse braces 8, the length of the cable sections, and the position of the saddle elements on the cable sections. For instance, a saddle element 43 as illustrated in Figs. 13 to 15 above may be employed premounted on so-called cohestrands.

Preferably, before raising the lattice tower 2 to an upright position, the lattice tower 2 is provided with the at least one cable section 10, 1 1 , 12, 13, 14, 15, 18, 19 and said cable section is pretensioned partially, and, after raising the lattice tower to an upright position, said cable section is pretensioned to a final pretension. Preferably, after having pretensioned said cable section 10, 11 , 12, 13, 14, 15, 18, 19 to said final pretension, the lattice tower is transported to an offshore location when being in upright position, and the lattice tower is then subsequently mounted on the seabed at said offshore location.

Preferably, before raising the lattice tower 2 to an upright position, the lattice tower is assembled by means of the node connection pieces 21 and is then provided with the at least one cable section 10, 11 , 12, 13, 14, 15, 18, 19, said cable section is then pretensioned at least partially, and, subsequently, the lattice tower is raised to an upright position.

Preferably, as mentioned above, the lattice tower is of the type having columns 5, 6, 7 inclining in the direction of each other, and in this case, the lattice tower 2 is preferably assembled by means of the node connection pieces 21 , beginning from the top part 3 of the lattice tower 2 and ending at the bottom part 4 of the lattice tower. Thereby, the braces 8 that will extend trans- versally in the upright position of the tower 2, will be in a slightly inclined position leaning against upper columns during assembly of the tower, thereby maintaining columns and braces connected during assembly. Subsequently, the tower may be pretensioned partly by means of the at least one cable section, whereby columns and braces may be locked into connection with each other.

Figs. 10A to 10E illustrate a method of mounting a support structure 1 as de- scribed above, whereby the lattice tower 2 is anchored to the seabed by mounting anchor cables 34 guided to follow the columns of the lattice tower 2 from the bottom part 4 to the top part 3 of the lattice tower 2. In the embodiment illustrated, each anchor cable 34 is extended inside a hollow of the respective column 5, 6, 7 of the lattice tower 2. However, the anchor cables 34 may also be extended and guided at the outside of the respective columns 5, 6, 7. For instance, the anchor cables 34 may be guided by a number of eyes attached to the columns 5, 6, 7 along these.

According to said method, a first end 36 of each anchor cable 34 is attached to an anchor, such as a plate anchor 35, and a second end 37 of each anchor cable 34 is attached to the top part 3 of the lattice tower 2, preferably by means of a not shown tension adjusting device that may be similar to the above described possibly automatic cable strainer 29. Each anchor 35 is driven into the seabed, for instance 10 to 15 metres, see Fig. 0A, and, sub- sequently, as the support structure 1 is carried by a vessel 38 floating on the sea surface, see Fig. 10B, the second end 37 of each anchor cable 34 is ten- sioned. Thereby, firstly, the support structure 1 is positioned on the seabed, see Figs. 10C and 10D, and, secondly, the support structure 1 is firmly attached to the seabed, see Fig. 10E. Furthermore, according to this method, the lattice tower 2 may further be anchored to the seabed by means of feet having form of inverted buckets, such as such as the bucket foundations 22 described above that are pressed at least partially down into the seabed as the anchor cables 34 are tensioned. The bucket foundations 22 may take the horizontal forces, relatively minimized moments and vertical loads from grav- ity.

According to said method, dragging the anchor cables 34 may be performed at the top part of the lattice tower 2, so diving operations may not be necessary in this respect. The anchor cables 34 may adapt to the position of the columns of the lattice tower by moving laterally through the material of the seabed, so that a template for the positioning of the anchors 35 may not be necessary.

Apart from facilitating the mounting of a support structure 1 on the seabed, the method of mounting the support structure 1 as described above by means of anchor cables 34 guided to follow the columns of the lattice tower 2 from the bottom part 4 to the top part 3 of the lattice tower 2 may also be used in order to pretension the lattice tower 2 in a way reducing the risk that the columns will experience tension. In the case that the lattice structure is already pretensioned by means of tensioned cable sections 10, 11 , 12, 13, 14, 15, 18, 19 as described above, the load on these cable sections may be reduced by for instance up to 35 per cent, and consequently these cable sections may be dimensioned correspondingly weaker than otherwise necessary. It is noted that the method of mounting a support structure 1 as described above by means of anchor cables 34 is also applicable for other support structures not as described above. Such other support structures may for instance include conventionally welded jacket structures without pretension- ing cable sections attached to the structure other than the anchor cables 34 guided to follow columns of such jacket structure.

It is noted that by the term cable is meant any suitable type of wire, rope, etc. made of any kind of suitable material such as metal, polyethylene, nylon among others. Suitable examples of cable types may be any of the following: bare post-tensioning strand (mono strand), post-tensioning strand in pe- sheathing (mono strand), post-tensioning strand bonded with pe-sheathing (Cohestrand), wire rope cables, spiral strand cables, locked coil cables, parallel strand cables, glass fibre FRP, aramid FRP and carbon FRP. The height of the lattice tower 2 from its bottom part 4 to its top part 3 depends on the depth of water and may be for instance at least 40 metres, at least 50 metres or at least 60 to 70 metres. Preferably about 10 to 20 metres of the lattice tower may be above the water. The pretentioning force of each tensioned cable section 10, 11 , 12, 13, 14, 15, 18, 19 and anchor cable 34 may be determined on the basis of the wind turbine tower placed on the top part of the lattice tower and on the basis of the wind and wave conditions on the site of construction.

In an embodiment, the height of the not shown wind turbine tower mounted on the support structure 1 may be about 60 to 70 metres, and each at least one tensioned cable section may, before exposing the lattice tower and wind turbine to wind or waves, be pretensioned with a force corresponding to at least 150 tons, preferably at least 300 tons and most preferred at least 450 tons, divided by the number of cable sections of the lattice tower. If this lattice tower is provided with six cable sections, the pretensioning force of each cable section may then be 25 tons, preferably at least 50 tons and most preferred at least 75 tons.

In an embodiment, the height of the not shown wind turbine tower mounted on the support structure 1 may be about 90 metres, and each at least one tensioned cable section may, before exposing the lattice tower and wind turbine to wind or waves, be pretensioned with a force corresponding to at least 300 tons, preferably at least 600 tons and most preferred at least 900 tons, divided by the number of cable sections of the lattice tower. If this lattice tower is provided with six cable sections, the pretensioning force of each cable section may then be 50 tons, preferably at least 100 tons and most preferred at least 150 tons.

The columns 5, 6, 7 of the lattice tower 2 may have the form of hollow con- crete beams, preferably CRC reinforced, the transverse braces 8 may have the form of preferably solid steel beams, and the node connection pieces 21 may be made of steel, preferably cast steel.

The concrete beams may be produced everywhere in the world due to the limited complexity. This means that the lead time of the fabrication may be reduced dramatically. One could even imagine fabrication at site location or simultaneous fabrication in parallel at several locations. The concrete beams may be very easy to transport and handle due to limited size and weight. The utilization of sub part transport vessels can be extremely high, due to the limited space required. A potential large saving may therefore be within reach.

Although the present invention has been described with different embodiments, a multitude of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformations, and modifications as fall within the scope of the appended claims.