Semi-submersible offshore support structure for wind turbine, method for assembly

Abstract

Semi-submersible offshore support structure for a wind turbine comprising: three semi-submersible columns and a receiving element for a wind turbine; a connection structure comprising a plurality of braces connecting the semi-submersible columns and / or the receiving element, wherein at least one of the braces is a truss-brace.

Description

[0001] P138258PC00

[0002] Title: Semi-submersible offshore support structure for wind turbine, method for assembly

[0003] The invention relates to a wind turbine offshore support structure, in particular to a semi-submersible offshore support structure for a wind turbine.

[0004] In view of the increasing demand for green energy, the supply of green energy by offshore wind turbines is increasing. Many offshore wind turbines or offshore wind turbine farms are positioned in near shore or relative shallow water areas. At these offshore locations, the wind turbines are fixedly mounted to the seabed, typically installed on fixed platforms or structures. However, the number of offshore sites for shallow wind farm areas becomes exhausted and there is a quest for harvesting wind power at deeper water. At these deeper water locations, a fixed support structure for the wind turbines is not feasible anymore against reasonable costs, so a floating support structure for the wind turbine is needed. Many configurations of floating offshore support structures for wind turbine installations have been designed and tested.

[0005] There seems to be a convergence to semi-submersible offshore support structures comprising at least three semi-submersible columns with a connection structure connecting the semi-submersible columns.

[0006] There is a continuous effort in improving the semi-submersible offshore structure. In particular, a lighter and / or less costly support structure is aimed for.

[0007] The invention thereto provides for a semi-submersible offshore support structure for a wind turbine according to claim 1.

[0008] The semi-submersible offshore support structure has a connection structure connecting the at least three semi-submersible columns. Further, there is provided a wind turbine receiving element to which the wind turbine can be mounted. The wind turbine receiving element can beprovided at any position to the connection structure. The connection structure typically comprises braces connecting the at least three semisubmersible columns. These braces can be quite long and / or can be subject to relatively heavy loads. In particular wave loads may have an impact on the braces. Wave loads may be slamming loads and may generate drag forces and inertia forces on the braces. Due to the length of a brace combined with these loads, braces of a connection structure may have quite some dimensions, thereby adding to an increasing weight of the offshore support structure. However, to reduce the weight, the inventors found that replacing at least one of the braces by a truss-brace may significantly reduce the loads on the said truss-brace, resulting in that the truss-brace can become lighter, while maintaining the structural integrity of the truss-brace as part of the support structure to distribute loads to the semi-submersible columns and / or to withstand any axial forces in the direction of the trussbrace between the semi-submersible columns.

[0009] Advantageously, at least one brace is being replaced by a trussbrace according to the invention. The at least one brace may be replaced by a truss-brace to optimally benefit from a smaller surface area, and thus wave impact force reduction. Optionally, other braces, above and / or below the waterline, can be replaced by truss-braces to benefit from weight reduction. Depending on the configuration of the support structure, more in particular, on the position of the wind turbine receiving element in the support structure, a second or third brace can be replaced by a truss brace as well.

[0010] Such a truss-brace may comprise at least three longitudinal trussbrace members joined by a plurality of connection members of which at least one truss-brace member is positioned as a lower truss-brace member, of which at least two truss-brace members are positioned as upper truss-brace members. Seen in cross-section such a truss-brace may have a substantially triangular shape, in case of three longitudinal truss-brace members, or asubstantially quadrangular shape, when four longitudinal truss-brace members are provided. In the following, reference may mostly be made to a triangular shaped truss-brace, however, all aspects of the invention are also applicable for truss-braces having a different cross-sectional shape.

[0011] By replacing at least one of the braces of the connection structure with a truss-brace, the area of the truss-brace, seen in side view, is smaller than the side view area of an integral brace, the area being proportional to the diameter of the brace. Indeed, with a truss brace typically there may be more members, longitudinal members joined by a plurality of connection members, but these members typically have a smaller diameter than the integral brace. A plurality of smaller diameters may still add up to less than the diameter of the integral brace. As the loads impacted onto the structure are for a large part proportional to the exposed area, the impact may be less when the exposed area is less. For example, slamming load and drag forces, induced by waves, may be proportional to the exposed area of the brace, respectively of the exposed areas of the members of the truss-brace, wherein the exposed area is understood to be the cross-sectional area of the respective brace or member transverse to the flow direction, such as the wave direction. Inertia forces may be proportional to the volume of the brace, respectively of the members of the truss-brace. It may be understood that providing a truss-brace may significantly reduce the exposed surface area and volume, thereby reducing forces and thus loads. The inventors found that a truss-brace could be made lighter than an integral brace, despite the many individual members and joints that are required to manufacture a truss brace. An integral brace is understood to be a brace having a cylindrical or prismatic shape, e.g. may be tubular or may have a polygonal cross-sectional shape.

[0012] The connection structure may comprise a plurality of upper braces connecting the semi-submersible columns at a level above the waterline. It is noted that when the semi-submersible support structure is floating, apart of the semi-submersible column is above the waterline and a part is below the waterline. The mean waterline level however is usually known and may be indicated as a line on the semi-submersible column.

[0013] Advantageously, one of the upper braces is a truss-brace. The connection structure may also comprise a part that is below the waterline connecting the semi-submersible columns, but this may depend on the design of the floating support structure. In some embodiments, the connection structure may comprise lower braces connecting the semi-submersible columns, although other connections may be envisaged, such as pontoons, etc.

[0014] It appears that wave slamming loads mainly appear on one or more of the upper braces, which are typically above the waterline. One or more of the lower braces, which are typically below the waterline, may also be subject to varying loads for which it may be beneficial to replace an integral brace by a truss brace. So, providing one of the upper braces as a truss-brace may be beneficial, and may provide a significant weight reduction, while maintaining structural integrity of the support structure.

[0015] For example, it can be advantageous to provide the brace that may cross a longer distance between the columns-as a truss-brace. An upper brace may be substantially above the waterline and may be vulnerable to wave slamming loads and / or other loads. Waves then impacting the trussbrace come in contact with a smaller cross-sectional area and / or volume of the truss-brace, thus generating less forces, e.g. less slamming forces, drag force and / or less inertia force, that needs to be taken up by the support structure. Advantageously, two adjacent semi-submersible columns are connected by a truss-brace. For example, in the support structure being a substantially triangular shaped structure, two adjacent braces may be provided as truss-braces.

[0016] Advantageously, the semi-submersible column to be connected with the truss-brace is specifically configured to connect with an end of the trussbrace. Preferably, the semi-submersible column to be connected with thetruss-brace is provided with a receiving arrangement for receiving an end of the truss-brace, for example the receiving arrangement can be configured as a cut-out to which the end of the truss-brace can be engaged. The cut-out may have a shape corresponding to the shape of the end of the truss-brace. For example, if the end of the truss-brace has a substantially triangular cross-section, the cut-out may be substantially triangular shaped. The semisubmersible column may be provided as a hollow structure of an approximately cylindrical and / or an approximately prismatic shape having an outer wall. The cut-out may be provided in the outer wall of the semisubmersible column. The end of the truss-brace may then engage to the cutout of the semi-submersible column. By providing such a receiving arrangement, e.g. as a cut-out, connection of the end of the truss-brace to the associated semi-submersible column may be facilitated.

[0017] The end of the truss-brace to be connected to the associated semisubmersible column may be configured as an end connector. In particular in combination with the receiving arrangement of the associated semisubmersible column, connection of the truss-brace to the semi-submersible column may become relatively simple and / or straightforward. The end connector may be configured to fittingly engage with the cut-out of the associated semi-submersible column. Furthermore, the receiving arrangement and / or the end connector may be arranged to optimally transfer loads from the truss-brace to the semi-submersible column, e.g. by providing reinforcement elements, such as gusset plate connections, shear plates, girders and / or stiffeners at designated positions.

[0018] Advantageously, the receiving arrangement of the semisubmersible column may be provided at an upper side and / or a lower side of the column depending on the truss-brace to be connected being an upper brace and / or a lower brace respectively. Then, the truss-brace can be received to the receiving arrangement in a downward movement from above the receiving arrangement to below into the receiving arrangement. In casethe truss-brace is to be connected to a lower side of the column, it may be envisaged to provide a receiving arrangement at the lower end of the column, e.g. in a damper box structure at the lower end of the column. . Alternative connection steps may be possible as well. As such, assembly of the truss-brace to the semi-submersible columns can be done relatively easy as the truss-brace can be lowered vertically in a single handling until it engages to the receiving arrangement. Then, as a next step, the connection between the truss-brace, in particular the end connector of the truss-brace, may be done, e.g. by welding the end connector to the receiving arrangement. It may be that when the end connector is engaged to the receiving arrangement, that seams occur where parts of the end connector abut to and / or rest on parts of the receiving arrangement. Connecting the end connector to the receiving arrangement may then be done by for example by welding those seams. As such, the connection between the end connector and the semi-submersible column can be done in a relatively simple manner by using a technology - welding - that is well known in the field. By providing a dedicated receiving arrangement and / or an end connector, connection of the truss-brace to the semi-submersible column can be made relatively easy and thus rather cost effective.

[0019] Optionally, the end connector of the truss-brace may extend at least partially through the cut-out into the semi-submersible column. The cut-out may be provided in the outer wall of the semi-submersible column, and the end connector of the truss-brace may extend through the cut-out into an interior of the semi-submersible column. As such, the load transfer between the end connector and the receiving arrangement may be more optimal. Optionally, the receiving arrangement may comprise the cut-out and for example reinforcement elements in the interior of the semisubmersible column. Such reinforcement elements may then cooperate with the end connector to provide for an optimal load transfer.In an interior of the semi-submersible column one or more deck plates can be provided. Additionally, at an inner side of the column, multiple horizontal girders may be provided at a certain distance from each other along the height of the column. Such a horizontal girder ring may be required for strength and may be provided approximately every 2 m to 2,5m over the height of the semi-submersible column. A wall of the semisubmersible column may be provided as a side shell. Typically, the first girder below the upper deck plate of the semi-submersible column is partly laid out as a deck plate, optionally with openings, as it may serve as a girder for the side shell of the semi-submersible column, and / or as flanges for any girders below the upper deck plate and / or for load introduction of the lower end of the braces, being an integral brace or a truss-brace. Such a girder may be considered as an interior deck plate. Other reasons for providing interior deck plates may be e.g. as a working platform, or to store equipment thereon etc. One such interior deck plate may be provided at a level where a lower side of the truss-brace engages with the semi-submersible column. For example, when a cut-out is provided in the outer wall of the semisubmersible column, the deck plate may be provided at a level of the bottom side of the cut-out. The deck plate may be part of the receiving arrangement, in that a lower end of the end connector may rest onto the interior deck plate, e.g. when the end connector extends partially in an interior of the semi-submersible column. Alternatively and / or additionally, the said interior deck plate, at the level of engagement with the truss-brace, may extend partially outside of the outer wall of the semi-submersible column. Then, the end connector may also rest onto the interior deck plate outside of the semi-submersible column, thereby improving load transfer from the truss-brace to the interior deck plate, further to the semi-submersible column.

[0020] Further, the semi-submersible column may be provided with an upper deck plate and / or a lower deck plate, for closing off an interior of thesemi-submersible column from environmental influences. An upper deck plate may also provide for available deck space. In case an upper truss-brace is to be connected to the semi-submersible column, preferably, the upper deck plate is provided at an upper side of the cut-out. The upper deck plate itself may also partially be provided with a cut-out as part of the receiving arrangement to receive the truss-brace, in particular, the end connector of the truss-brace. Thus, a sufficiently large receiving space can be provided in which the truss-brace, preferably the end connector thereof, can be received. After engagement of the end connector to the receiving arrangement, the end connector may be arranged flush with the upper deck plate such that a seam may be provided between the end connector and the upper deck plate. Such seam may be easily welded to connect the end connector to the upper deck plate.

[0021] In an embodiment, the truss-brace may comprise at least three longitudinal truss-brace members joined by a plurality of connection members of which at least one truss-brace member is positioned as a lower truss-brace member, and of which at least two truss-brace members are positioned as upper truss-brace members, preferably wherein at both ends of the truss-brace an end connector is provided. A truss-brace comprising three longitudinal truss-brace members may have, in cross-section, a substantially triangular shape. One of the truss-brace members may be positioned as a lower truss-brace member, while the other two truss-brace members may be positioned as upper truss-brace members. Then, the lower truss-brace member may be configured to engage with a lower side of a receiving arrangement of the semi-submersible column, preferably with a lower side of the cut-out of the receiving arrangement. Preferably the trussbrace is provided with an end connector at its ends to be configured to engage with the lower side of the receiving arrangement. Then a lower side of the end connector that may continue from the lower longitudinal trussbrace member may engage with the lower side of the receiving arrangement.Advantageously, the upper longitudinal truss-brace members may be approximately at the same level as an upper deck plate of the semisubmersible column. It may be understood that more than three longitudinal truss-brace members may be provided, resulting in e.g. a rectangular cross-section. Between the longitudinal truss-brace members, connection members are provided to connect the longitudinal truss-brace members and to provide for load transfer. Typically, such connection members may be connected to the longitudinal truss-brace members in an oblique manner.

[0022] Advantageously, the end connector comprises a downwardly extending flange mounted to the lower truss-brace member that is configured to engage with an interior deck plate of the semi-submersible column. Such downwardly extending flange may be provided as a shear plate passing loads from the lower truss-brace member via shear to the receiving arrangement and further to the semi-submersible column. The downwardly extending flange may be provided over a length along the lower truss-brace member of about 3 - 8 times a diameter or a height of the said member. By providing such a downwardly extending flange, that may rest onto the interior deck plate and / or onto an outwardly protruding part of the interior deck plate, a connection between the flange and the interior deck plate, substantially along a line, can be provided. Such a line connection may be beneficial for welding. Alternatively, the lower truss-brace member may at an end thereof be provided as a member with a square cross-section that may rest onto the interior deck plate of the receiving arrangement, which may allow for an easy connection, preferably by welding.

[0023] Alternatively, the lower truss-brace member may extend into a plate at a center line of the lower truss-brace member, which plate may then engage, preferably flush, with the interior deck plate or with a bulkhead plate inside of the column. Such plate end at the lower truss-brace member may be oriented horizontally or vertically to engage respectively with a deck plateor a bulkhead plate. The lower truss-brace member or any extension to the lower truss-brace member, such as the aforementioned plates, may extend further than the upper truss-brace member.

[0024] Typically a longitudinal truss-brace member may be a pipe member with a round cross-section, that may make connection with the receiving arrangement difficult. By providing a downwardly extending flange, or a different shape of the cross-section at its end, connection with the receiving arrangement, e.g. with the interior deck plate of the receiving arrangement and / or the cut-out may be facilitated. Advantageously, the downwardly extending flange and / or the different cross-section of the longitudinal trussbrace member may extend about 3 - 8 times a diameter of the longitudinal truss-brace member, such that there may be sufficient length to engage with the receiving arrangement of the semi-submersible column. The main load transfer from the lower truss-brace member to the receiving arrangement may occur via shear, so the downwardly extending flange is preferably sufficiently larger to transfer the shear force.

[0025] The end connector may further comprise an upper connection plate, wherein the upper connection plate may extend from the upper longitudinal truss-brace members, preferably from a center line of the upper longitudinal truss-brace members. Advantageously, when the end connector is engaged to the semi-submersible column, the upper connection plate may preferably be flush with an upper deck plate of the semi-submersible column, so as to provide an uninterrupted deck area.

[0026] Optionally, the upper connection plate of the end connector may be provided with reinforcement elements, which may be in line with the longitudinal truss-brace members from which the upper connection plate is extending. The reinforcement elements preferably are provided at an underside of the upper connection plate, such that, when connected to the semi-submersible column, the reinforcement elements may be in an interior of the semi-submersible column. The upper deck plate of the semi-submersible column may be provided with corresponding reinforcement elements at an underside thereof. When assembled, the reinforcement elements of the upper deck plate and the reinforcement elements of the upper connection plate of the end connector are in line with each other and can be connected to each other, e.g. by welding.

[0027] Optionally, the lower longitudinal truss-brace member may extend further outwardly than the upper longitudinal truss-brace members. The upper connection plate of the end connector may then extend over the length, seen in axial direction of the truss-brace, from the upper longitudinal truss-brace members to about or further than the lower longitudinal truss-brace member.

[0028] In an embodiment, the wind turbine receiving element may be positioned in line between two adjacent semi-submersible columns, preferably wherein the wind turbine receiving element is connected to its adjacent semi-submersible columns by non-truss braces. The other braces of the connection structure, connecting the third semi-submersible column to the aforementioned semi-submersible columns are preferably truss-braces.

[0029] The end connector may further comprise a number of end elements configured to fit to the semi-submersible column, preferably to a receiving arrangement of the semi-submersible column. Such end elements can be configured to engage to corresponding parts of the receiving arrangement.

[0030] In another aspect of the invention, there is provided a method for assembly of a truss-brace to a semi-submersible column of a semisubmersible offshore support structure for a wind turbine, the method comprising providing an end connector to respective ends of the truss-brace; providing a receiving arrangement to the semi-submersible column, the receiving arrangement at least having a shape corresponding to a shape of the end connector of the truss-brace; lowering the truss-brace into the cutout until a lower side of the end-connector and / or truss-brace abuts a lowerside of the receiving arrangement; forming a connection by joining the endconnector to the receiving arrangement of the semi-submersible column.

[0031] By providing the receiving arrangement to the semi-submersible column, for example the receiving arrangement comprising a cut-out having a shape corresponding to a cross-sectional shape of the truss-brace, the truss-brace can be lowered from above the receiving arrangement downwardly to the receiving arrangement until engagement may be obtained. Then, when engaged, joining of the truss-brace to the semisubmersible column can be done e.g. by welding or any other known technique.

[0032] The receiving arrangement of the semi-submersible column may have an open end, or an open upper side, such that the truss-brace can be lowered therein from above. This allows easy assembly, as then the whole truss-brace can be lowered from above into the receiving arrangements of the associated semi-submersible columns. It may also allow parallel and / or independent manufacturing. For example, the truss-braces can be manufactured and / or produced, while in parallel, work on the semisubmersible columns and / or the support structure may be done. As such, when most of the work on the support structure may be done, or at least when the position of the semi-submersible columns with respect to each other may be fixated, the completed truss-braces can be picked up and taken towards the partly assembled support structure. Then the truss-brace can be positioned, e.g. when held in a crane, above the semi-submersible columns, more specifically above the receiving arrangements of the respective semi-submersible columns. Then, when lowering the truss-brace downwardly the truss-brace is being received to the semi-submersible column. Then, the truss-brace can be joined to the semi-submersible column to provide for a fixed connection.It is understood that where upper brace is mentioned, all aspects are equally valid for any lower braces to be connected to a lower side of the semi-submersible column.

[0033] Further advantageous embodiments are provided in the subclaims.

[0034] In the following, the invention will be explained further using examples of embodiments and drawings. In the drawings show:

[0035] Figure 1 a perspective view of an embodiment of a support structure according to the invention;

[0036] Figure 2 a top view of another embodiment of a support structure according to the invention;

[0037] Figure 3 a perspective exploded view of the support structure of Figure 1 with the truss-braces above the semi-submersible columns;

[0038] Figure 4 a perspective top view of an end connector of a truss brace and a receiving arrangement of a semi-submersible column with the trussbrace above the semi-submersible column;

[0039] Figure 5 a the view of Figure 4 with an upper deck plate of the semi-submersible column removed;

[0040] Figure 6 a perspective view of the end connector and the receiving arrangement engaged to each other with the upper deck plate removed;

[0041] Figure 7 a perspective bottom view of the end connector and the receiving arrangement with the end connector above the receiving arrangement;

[0042] Figures 8a, 8b, 8c welding lines between the end connector and the receiving arrangement.

[0043] The drawings are schematic and merely show an example. In the drawings, corresponding elements are provided with corresponding reference signs. For clarity of the drawings, some elements and / or references signs may be omitted from some of the figures, wherein thepresence of such elements may nevertheless be understood in view of one of more other figures and / or the description.

[0044] Figure shows a perspective view of an embodiment of a support structure 100 for a wind turbine. The support structure 100 comprises three semi-submersible columns 110, and a wind turbine receiving element 120. The support structure further comprises a connection structure 130 connecting the three semi-submersible columns 110 and the wind turbine receiving element 120. The connection structure 130 comprises a plurality of braces 140 and two truss-braces 200. The two truss-braces 200 here connect adjacent semi-submersible columns 110 at an upper side 150 of the semisubmersible columns 110. Lower sides 160 of the semi-submersible columns 110 are connected to each other by means of the braces 140. The braces 140 are so-called integral braces 140, in that they are provided as a cylindrical and / or prismatic pipe. In this embodiment, the wind turbine receiving element 120 is in line between two adjacent semi-submersible columns 110, so forming one side of the triangular support structure 100. The trussbraces 200 are provided at the other two sides of the triangular support structure 100. Further in this figure 1 can be seen that the semisubmersible columns 110 at their lower sides 160 may be provided with damper boxes 170. Optionally, such damper boxes may be absent or may be embodied differently. More specifically, the lower braces 140 of the connection structure 130 connect to the damper boxes 170. However, it is understood that the connection structure can connect the semi-submersible columns 110 directly. Also, it is understood that the connection structure 110 can be embodied differently to connect a lower side 160 of the semisubmersible columns, e.g. as pontoons. Here, at the damper boxes 170 mooring attachments 180 are shown to which mooring lines can be connected. It is however understood that the mooring attachments 180 can be provided at other parts of the support structure 100, e.g. at an upper side 150 of the semi-submersible columns 110.Figure 2 shows an alternative embodiment of a support structure 100 in top view. Here, the wind turbine receiving element 120 is positioned centrally in the triangular configuration of the three semi-submersible columns 110. The braces 140 of the connection structure 130 connect here the semi-submersible columns 110 with the wind turbine receiving element 120. The connection structure 130 comprises in this embodiment three truss-braces 200 each connecting two semi-submersible columns 110 at an upper side thereof.

[0045] Figure 3 shows the configuration of Figure 1 with the two trussbraces 200 above the semi-submersible columns 110. The truss-braces 200 have an end connector 300 at both ends thereof, and the semi-submersible columns 110 have a corresponding receiving arrangement 400 to receive the associated end connector 300. As such, the support structure 100 may be manufactured and / or assembled, while, in parallel and / or separately, the truss-brace 200 can be manufactured. Then, when the truss-brace 200 is completed it can be picked up and be positioned above the receiving arrangements 400. The truss-brace 200 may be lifted and handled by a crane, not shown. Then, the truss-brace 200 can be lowered towards the receiving arrangement 400 until it engages with the semi-submersible column 110. Thus, assembly of the truss-brace 200 to the semi-submersible columns 110 can be done at a relatively late stage of the assembly of the support structure 100, for example when the three semi-submersible columns are fixed in position with respect to each other, e.g. due to other part of the connection structure 130 already being in place.

[0046] Figure 4 shows a detail of figure 3 with a focus on the end connector 300 and the receiving arrangement 400. The truss-brace 200 has here three longitudinal truss-brace members 202 that are interconnected to each other by a plurality of connection members 204. The connection members 204 may have an oblique orientation with respect to the longitudinal truss-brace members 202. The truss-brace 200 here has twoupper longitudinal truss-brace members 202u and one lower longitudinal truss-brace member 2021, giving the truss-brace in cross-section a triangular shape. It can be seen that the lower longitudinal truss-brace member 2021 extends somewhat further in a longitudinal direction of the truss-brace 200 than the upper longitudinal truss-brace members 202u. This may facilitate engagement of the truss-brace 200 to the semi-submersible column 110.

[0047] At an end of the truss-brace 200, the end connector 300 is provided. More specifically, as can be seen in figure 3, at both ends of the truss-brace 200, an end connector 300 is provided. The end connector 300 comprises an upper connection plate 302, a downwardly extending flange 304, reinforcement elements 306 (visible in figure 7), end elements 308. The upper connection plate 302 extends from the upper longitudinal truss-brace members 202u, preferably from a center line thereof. After assembly, the upper connection plate 302 preferably is approximately flush with an upper deck plate of the semi-submersible column 110. Over a certain length of the lower longitudinal truss-brace member 2021, the downwardly extending flange 304 is provided. Advantageously, the flange 304 extends over a length between about 3 - 8 times the diameter of the lower longitudinal truss-brace member 2021. The downwardly extending flange 304 can be seen as extending vertically downward, when the truss-brace 200 is held with the longitudinally extending members 202u, 2021 in horizontal position.

[0048] Further, the end connector 300 comprises a number of end elements 308. For example, an end of the lower longitudinal truss-brace member 2021 is provided with an end plate 308-1 transverse to the longitudinal direction of the member 2021. Further, end connection elements 308-2 are provided to connect the lower longitudinal truss-brace member 2021 with respective upper longitudinal truss-brace member 202u. The end connection elements 308-2 can be provided as plate elements. Between the end connection elements 308-2 a connection plate 308-3 can be provided. The plate 308-3 can close a cut-out in an outer wall of the semi-submersiblecolumn 110 after assembly. At an inner side of the plate 308-3, stiffeners 309 are provided, which can be seen in figures 4, 5, 6. Further downward of the end connection elements 308-2, an end plate 308-4 can be provided at both sides of the longitudinal truss-brace member 2021. Such end plates 308-4 too, may fill a cut-out in an outer wall of the semi-submersible column 110.

[0049] The various elements of the end connector 300 may aid in fitting engagement of the end connector 300 to the receiving arrangement 400. The receiving arrangement 400 is provided in the semi-submersible column 110, preferably at an upper side 150 thereof. The receiving arrangement 400 may comprise a cut-out 402 in an outer wall 112 of the semi-submersible column 110. The outer wall 112 is at an inner side thereof provided with a plurality of stiffeners 113 to provide for strength and / or stiffness to the outer wall 112, which may increase the load bearing capacity of the outer wall 112. At the upper side of the semi-submersible column 110, an upper deck plate 114 can be provided. The cut-out 402 may further extend to the upper deck plate 114, as such the cut-out 402 may have an open upper side in which the end connector 300 of the truss-brace 200 can be received. In figure 5 and figure 6, the upper deck plate 114 is being removed so a view on an interior 116 of the semi-submersible column 110 can be given. So, it can be seen that an inner side of the upper deck plate facing the interior 116 is provided with reinforcement elements 404 of which at least two reinforcement elements 404 are oriented such that, when the end connector 300 is received in the receiving arrangement 400, the reinforcement elements 404 are in line with the reinforcement elements 306 of the end connector 300, and with the upper longitudinal truss-brace members 202u of the truss-brace 200. The reinforcement elements 404 end at a vertical girder 118 which is present in the interior of the semi-submersible column 110 to provide for stiffness. At a bottom side 406 of the cut-out 402 an interior deck plate 408 can be provided. Over a height of the semi-submersible column 110 multipleinterior deck plates can be provided. The interior deck plate 408 can be provided at a level of the bottom side of the cut-out 402 or the cut-out 402 can be extended until it reaches the interior deck plate 408. The end connector 300 may rest onto the deck plate 408, as can be seen in figure 6. In particular, the flange 304 may engage the deck plate 408. The deck plate 408 may also extend outwardly of the outer wall 112 forming a deck plate extension 408e, as can be seen in figure 7. The flange 304 may then rest onto the outwardly extending part of the deck plate 408 thus improving load transfer. The deck plate extension 408e is on itself supported or reinforced by a flange or bracket 409.

[0050] Advantageously, a shape of the cut-out 402 of the receiving arrangement 400 approximately corresponds to a shape of the end connector 300, as can be seen in figure 7. The truss-brace 200, and thus the end connector 300 is approximately triangular shaped, when seen in crosssection. The cut-out 402 then may also have a substantially triangular shape such that the end connector 300 may fittingly engage to the receiving arrangement 400.

[0051] Figures 8a, 8b and 8c show the end connector 300 engaged to the receiving arrangement 400, after the truss-brace 200 has been lowered into the receiving arrangement 400 until the flange 304 abuts the deck plate 408. Then, to fixedly connect the truss-brace 200 to the semi-submersible column 110, the end connector 300 may be joined to the receiving arrangement 400 e.g. by means of welding. Between the end connector 300 and the receiving arrangement 400 there occur seams 500. By welding along these seams 500, indicated with a thick line in the figures 8a, 8b, 8c, a fixed connection between the connector end 300 and the semi-submersible column 110 is obtained. Due to the configuration of the end connector 300, most of the seams are straight lines, thus making welding relatively easy. As such as relative cost effective way of assembling the truss-brace 200 to the semisubmersible column 100 can be provided. Not only can the truss-brace 200including the end connector 300 being manufactured separately and / or in parallel to the manufacturing of the semi-submersible columns 110, but also can the truss-brace 200 relatively simple be lowered into a receiving arrangement 400 of the semi-submersible column 110. Finally, the joining of the end connector 300 to the semi-submersible column 110 can be done by welding along mostly straight seams.

[0052] Figure 8c shows a cross-section trough a center line of the lower longitudinal truss-brace member 2021 showing the reinforcement element 308-2 and the reinforcement element 306 which may be embodied as web elements. The arrows P in the figure 8c indicate that the plate forming the web element 306 is continuous and that any strips, indicated as horizontal lines in the drawing of figure 8c, are welded on top. In scantling plans this is a common way to indicate with these type of arrows which structure is continuous. A load may be transferred from the oblique connection member 204 to the web element 308-2 which then further is being transferred to the structure of the semi-submersible column 110.

[0053] Although the invention has been explained further herein using examples of embodiments and drawings, these do not limit the scope of the invention as defined by the claims. In particular, the number of truss-braces can vary, as well as the position. Here, the invention is explained in the configuration of an upper truss-brace, but all is equally well applicable for a truss-brace at a lower side of the semi-submersible column. Within said scope, many variations, combinations and extensions are possible, as shall be appreciated by the skilled person having the benefit of the present disclosure. All such variants are included in the scope of the invention as defined by the following claims.

Claims

Claims1. Semi-submersible offshore support structure for a wind turbine comprising:- three semi-submersible columns and a receiving element for a wind turbine;- a connection structure comprising a plurality of braces connecting the semi-submersible columns and / or the receiving element, wherein at least one of the braces is a truss-brace.

2. Semi-submersible offshore support structure according to claim 1, wherein the connection structure comprises a plurality of upper braces connecting the semi-submersible columns and / or the receiving element to each other at a level above the waterline, wherein at least one of the upper braces is a truss-brace.

3. Semi-submersible offshore support structure according to claim 1 or 2, wherein the truss-brace connects two adjacent semi-submersible columns.

4. Semi-submersible offshore support structure according to any of the preceding claims, wherein the semi-submersible column to be connected with the truss brace is provided with a receiving arrangement.

5. Semi-submersible offshore support structure according to claim 4, wherein the receiving arrangement is provided as a cut-out, for receiving an end of the truss-brace, preferably wherein the cut-out has a shape corresponding to the shape of the truss-brace, for example wherein an outer wall of the semi-submersible column is provided with the cut-out.

6. Semi-submersible offshore support structure according to any of the preceding claims, wherein an end of the truss brace is configured as an end connector to engage with the associated semi-submersible column.

7. Semi-submersible offshore support structure according to claim 6 and claim 4, wherein the end connector is configured to engage with a corresponding receiving arrangement, preferably wherein the end connector is configured to fittingly engage with the cut-out of the corresponding receiving arrangement of the associated semi-submersible column.

8. Semi-submersible offshore support structure according to claim 6, wherein the end connector of the truss-brace extends at least partially into the associated semi-submersible column.

9. Semi-submersible offshore support structure according to any of the preceding claims 4- 8, wherein at a bottom side of the receiving arrangement an interior deck plate is provided in an interior of the semisubmersible column, optionally wherein the deck plate extends outwardly of an outer wall of the semi-submersible column.

10. Semi-submersible offshore support structure according to any of the preceding claims 4- 9, wherein at an upper side of the receiving arrangement an upper deck plate is provided.

11. Semi-submersible offshore support structure according to any of the preceding claims, wherein the truss-brace comprises at least three longitudinal truss-brace members joined by a plurality of connection members of which at least one truss-brace member is positioned as a lower truss-brace member, of which at least two truss-brace members arepositioned as upper truss-brace members, preferably wherein at both ends of the truss-brace an end connector is provided.

12. Semi-submersible offshore support structure according to any of the claims 4 - 11, wherein a lower truss-brace member is configured to engage with a lower side of the receiving arrangement, preferably with the cut-out and / or to engage with an interior deck plate of the semi-submersible column.

13. Semi-submersible offshore support structure, according to any of the claims 4 - 12, wherein an end of the truss-brace, preferably the end connector, comprises a downwardly extending flange mounted to a lower truss-brace member that is configured to engage with the receiving arrangement, optionally with an interior deck plate of the semi-submersible column.

14. Semi-submersible offshore support structure according to any of the claims 4 - 13, wherein an end of the truss-brace, preferably the end connector, comprises an upper connection plate to engage with the receiving arrangement.

15. Semi-submersible offshore support structure according to claim 14 and according to claim 11, wherein the upper connection plate joins the at least two upper truss-brace members; optionally wherein the upper connection plate is provided with reinforcement elements in line with the upper truss-brace members, optionally wherein the said reinforcement elements are in line with corresponding reinforcement elements at an inner side of the upper deck plate of the semi-submersible column.

16. Semi-submersible offshore support structure according to any of the claims 11 - 15, wherein at the ends of the truss-brace the lower trussbrace member extends further outwardly than the upper truss-brace members.

17. Semi-submersible offshore support structure according to any of the claims 14 - 16, wherein the upper connection plate is flush with an upper deck plate of the semi-submersible column and / or fittingly engages to the upper deck plate of the semi-submersible column.

18. Semi-submersible offshore support structure according to any of the preceding claims, wherein the offshore support structure comprises two truss braces, both connecting upper sides of respective semi-submersible columns.

19. Semi-submersible offshore support structure according to any of the preceding claims, wherein the wind turbine receiving element is positioned in line between two adjacent semi-submersible columns, preferably wherein the wind turbine receiving element is connected to its adjacent semi-submersible columns by non-truss braces.

20. Semi-submersible offshore support structure according to any of the claims 6 - 19, wherein the end connector comprises a number of end elements configured to fit to the associated semi-submersible column, in particular to a receiving arrangement of the associated semi-submersible column, preferably configured to be joined, e.g. by welding, to the associated semi-submersible column, in particular to the receiving arrangement thereof.

21. Method for assembly of a truss-brace to a semi-submersible column of a semi-submersible offshore support structure for a wind turbine, the method comprising- providing an end connector to respective ends of the truss-brace;- providing a receiving arrangement to the semi-submersible column, the receiving arrangement at least having a shape corresponding to a shape of the end connector of the truss-brace;- lowering the truss-brace into the receiving arrangement until a lower side of the end-connector and / or truss-brace abuts a lower side of the receiving arrangement;- forming a connection by joining the end-connector to the receiving arrangement of the semi-submersible column.

22. Method according to claim 21, wherein joining of the truss brace to the semi-submersible column comprises welding seams between the end connector of the truss brace and the receiving arrangement of the semisubmersible column.

23. Method according to any of the claims 21 - 22, wherein the receiving arrangement of the semi-submersible column has an open end allowing the truss-brace to be lowered therein from above.

24. Truss-brace for connection to semi-submersible columns comprising an end connector at both opposite ends configured to fittingly engage with the semi-submersible columns.

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