Barge for assembly of a floating support structure

Abstract

The invention describes a barge for assembly of a floating support structure (1) comprising a lower buoyancy section (2) with a lower buoyancy volume VL and a working deck (3) comprising support elements (4) for receiving parts of the floating support structure to be assembled. The barge further comprises a ballasting system (7) for lowering and elevating the barge (1). The barge is characterized by a divisional section (6) holding the lower buoyancy section (2) and the working deck (3) in a vertical distance from each other and having a divisional buoyancy volume VD considerably smaller than lower buoyancy volume VL, such that the ballasting system, by pumping a quantity of water equal to VD in or out, is able to move the barge (1) between a lower position with a bottom side of the working deck (3) in the waterline, and an upper position with the top side of the lower buoyancy section (2) in the waterline.

Description

[0001] Title: BARGE FOR ASSEMBLY OF A FLOATING SUPPORT STRUCTURE

[0002] Technical field

[0003] The invention relates to off-shore wind turbines, more specifically to a barge used for assembly of a floating support structure for an offshore wind turbine.

[0004] Background

[0005] Floating support structures for offshore wind turbines are known in the art and several methods for assembly of wind turbines and the substructure have been described.

[0006] A common disadvantage for many of the known floating support structures are the issues arising from the assembly process. Issues that may arise are excessive space requirement on shore, low assembly rate, and limited depth by the quay side at assembly location. As the size, and thereof the weight, of the wind turbines have increased, the size of equipment for assembly have also increased and the depth of typical support structure for a 15 MW wind turbine is a lot more that 10 meters when the tower and nacelle is mounted. A harbor with a depth deeper than 10 meters are hard to find and / or expensive to build. Ideally, the assembly site should also be close to the final destination, i.e. the wind park, because towing of the wind turbine is expensive and time consuming. One partial solution is to assemble the floating support structure on shore and mount the tower and nacelle at sea, but this puts high demands on a crane big enough to rise a tower at sea.

[0007] In view of the above, it is an object of the invention to provide a process that mitigates several of the problems related to use of prior art solutions.

[0008] An object of the invention is to find a way to assemble a large floating support structure at a quay side with limited depth.

[0009] Another object of the invention is to reduce the assembly area needed near the quay side.

[0010] Another object of the invention is to reduce the time consumed when assembling a floating support structure and wind turbine.

[0011] Another object is to reduce the time and energy consumption during launching the completed floater / turbine assembly. Summary of the invention

[0012] In a first aspect of the invention it is described a barge for assembly of a floating support structure. The barge comprises a lower buoyancy section with a lower buoyancy volume VL, a working deck comprising support elements for receiving parts of the floating support structure to be assembled and a ballasting system for lowering and elevating the barge. The barge further comprises a divisional section holding the lower buoyancy section and the working deck in a vertical distance from each other and having a divisional buoyancy volume VD considerably smaller than lower buoyancy volume VL, such that the ballasting system, by pumping a quantity of water equal to VD in or out, is able to move the barge between a lower position with a bottom side of the working deck in the waterline, and an upper position with the top side of the lower buoyancy section in the waterline when the barge is unaffected by external buoyancy volumes from structures resting on the barge.

[0013] In an embodiment of the barge a major part of the divisional buoyancy volume VD resides in buoyancy structures extending in the entire height of the divisional section and being positioned in an area at least comprising each corner of the barge to provide stability when the lower buoyancy section is totally submerged.

[0014] In an embodiment of the barge the divisional section comprises stabilizing pillars at each corner of the barge extending between the lower buoyancy section and the working deck for stabilizing the barge when the lower buoyancy section is submerged.

[0015] In yet an embodiment of the barge the working deck comprises a safety buoyancy volume Vw.

[0016] In yet an embodiment of the barge the divisional section comprises support beams extending between the lower buoyancy section and the working deck.

[0017] In a second aspect of the invention it is described a floating support structure to be assembled on a barge as described above.

[0018] In an embodiment the floating support structure comprises a horizontal transverse part having a first and second vertical transverse buoyancy unit at each end respectively and a horizontal aft part connected to a midpoint of the transverse part and having a vertical aft buoyancy unit at the aft end thus providing an open space underneath the horizontal transverse and aft parts for accommodating the barge. The floating support structure further comprises a connecting flange for connecting a windmill tower. In an embodiment the floating support structure comprises a first, second and third main section each comprising a horizontal part and a vertical part, wherein the vertical part comprises a buoyancy unit. The floating support structure further comprises a central transition piece for connecting the respective three horizontal parts of the main sections by means of respective connector ends adapted to fit the respective horizontal parts, the central transition piece further comprising a connecting flange for connecting the windmill tower.

[0019] In an embodiment the floating support structure is manufactured in a set of subparts comprising first second, third, etc subparts (20a, b, c...) constituting the floating support structure when assembled.

[0020] In an embodiment each subpart comprises at least a horizontal section for resting on a support structure on the barge 1 and wherein the vertical first and second transverse buoyancy parts and aft buoyancy part are attached to respective horizontal sections. In other words, the set of subparts may include subparts consisting of only one horizontal section, but no subpart will consist of only a vertical buoyancy section

[0021] In yet an embodiment of the floating support structure the set of subparts comprises a first subpart comprising the vertical aft buoyancy unit and the horizontal aft part and a second subpart comprising the first vertical transverse buoyancy unit, a major section of the horizontal transverse part including a connector for connecting the horizontal aft part to the midpoint of the horizontal transverse part. The set of subparts further comprises a third subpart comprising the second vertical transverse buoyancy unit and a minor section of the horizontal transverse part.

[0022] In yet an embodiment of the floating support structure the mentioned set of subparts comprises the first subpart comprising the central transition piece, and the second, third and fourth subpart comprising the first, second and third main section, respectively.

[0023] In yet an embodiment of the floating support structure the vertical first and second transverse buoyancy unit and aft buoyancy unit, the horizontal transverse part and the horizontal aft part have a tubular shape.

[0024] In yet an embodiment of the floating support structure each subpart of the set of subparts have suspension connectors positioned such that the horizontal sections of the subpart are horizontal when lifting lines are connected to the suspension connectors and suspended from the lifting device.

[0025] In a third aspect of the invention it is described a method for assembly of a floating support structure as described above by means of the described barge and a lifting device positioned at a quayside for lifting a set of subparts that constitutes the floating support structure when assembled. The method comprises the following steps: a. providing the set of subparts that constitutes the floating support structure, b. lifting a subpart onto the barge using the lifting device letting the subpart rest on the support elements and, if needed for stability, also letting the subpart remain suspended from the lifting device while the subpart is connected to a previously positioned subpart if the lifted subpart is not the first to be lifted onto the barge, in which case the subpart must be stabilized without the support of the lifting device c. when the assembly is completed, lowering the barge by means of the ballasting system until the floating support structure is free of all obstacles on the barge and d. removing the barge from underneath the floating support structure.

[0026] In an embodiment of the method the set of subparts comprises a first subpart comprising the vertical aft buoyancy unit and the horizontal aft part and a second subpart comprising the first vertical transverse buoyancy unit, a major section of the horizontal transverse part including a connector for connecting the horizontal aft part to the midpoint of the horizontal transverse part. The set of subparts further comprises a third subpart comprising the second vertical transverse buoyancy unit and a minor section of the horizontal transverse part. When provided with such a set of subparts step b comprises the following substeps:

[0027] 1. Lifting the second subpart onto the barge,

[0028] 2. Lifting the third subpart onto the barge and holding the second transverse subpart in position with the lifting device until it is connected to the first transverse subpart, and

[0029] 3. Lifting the first subpart onto the barge and connecting the aft horizontal part to the connector of the first transverse subpart.

[0030] In an embodiment of the method the set of subparts comprises the first subpart comprising the central transition piece, and the second, third and fourth subpart comprising the first, second and third main section, respectively. When provided with such a set of subparts step b of the method comprises the following substeps:

[0031] 1. lifting the first subpart (20b) onto the barge and positioning a connector end (35) in a direction parallel to the quayside,

[0032] 2. lifting the second subpart onto the barge and connecting it to the mentioned connector end of the first subpart (20a,

[0033] 3. turning the barge until the second subpart (20b) is perpendicular to and pointing outward from the quayside, 4. lifting the third and fourth subpart (20c and d) onto the barge and connecting them to the connector ends (35) of the first subpart (20a) in succession.

[0034] In an embodiment of the method step c comprises moving the barge to deeper water before lowering the barge.

[0035] In an embodiment of the method lifting device is positioned at a quay side.

[0036] Short description of the drawings

[0037] The following drawings are appended to facilitate the understanding of the invention. The drawings show embodiments of the invention, which will now be described by way of example only. Same numerals in different drawings refer to the same feature.

[0038] Fig. 1 shows an embodiment of the barge in a perspective view.

[0039] Fig. 2 shows the embodiment from fig. 1 in a top view.

[0040] Fig. 3 shows the same embodiment from the side exposing the divisional section.

[0041] Fig. 4 a-c shows three stages of a method of assembly of an embodiment of the floating support structure using a barge according to the invention.

[0042] Fig 4d shows a typical configuration of barge and lifting device by a quay side.

[0043] Fig 5 a-d shows four stages of a method of assembly of an embodiment of the floating support structure by a quay side using a barge according to the invention.

[0044] Fig 6a-c shows three main stages of the lowering of the barge.

[0045] Fig. 7a and b shows transportation on a vessel of an embodiment of a set of subparts shown in fig. 4 a-c.

[0046] Fig. 8a and b shows transportation on a vessel of an embodiment of a set of subparts shown in fig. 5 a-d.

[0047] Fig. 9 shows a complete wind turbine mounted on a floating support structure. The arrow shows preferred towing direction.

[0048] Detailed description

[0049] A barge for assembly of a floating support structure as seen in fig. 1 - 3 is described below. The floating support structure will in most cases be a floating support structure for supporting an offshore wind turbine. The assembly process will be carried out with the parts to be assembled resting in support elements 4 or cribs 4 on a working deck 3 of the barge. For the assembly of the floating support structure 8 to function the floating support structure 8 is provided with an obstacle free volume underneath the floating support structure in order to allow the barge to be lowered and then moved from its position underneath the floating support structure 8. In most practical cases this will mean that the floating support structure 8 do not have any support structures, bracings or other obstacles between surfaces of the floating support structure resting on the barge and the maximum depth of the barge when fully ballasted in an area occupied by the barge below the floating support structure.

[0050] A floating support structures suitable for assembly using the barge according to the invention is described in N020200726 and in N020231009. Suitable floating support structures will be described in more detail later.

[0051] The barge 1 comprises a lower buoyancy section 2 having a lower buoyancy volume VL. In an embodiment the lower buoyancy section 2 has the shape of an ordinary barge typically with a flat bottom allowing passage in shallow waters.

[0052] The barge 1 further comprises a working deck 3, positioned above the lower buoyancy section 2, comprising support elements 4 for receiving parts of the floating support structure to be assembled. In a preferred embodiment the support element comprises a crib 4 matching the shape of the parts of the floating support structure to be assembled as seen in fig. 1 - 4. To further support and stabilize the parts to be assembled during assembly, the barge may be provided with side supports 5 preferably protruding from the side of the working deck 3 as seen in fig. 1 - 4. The side supports 5 are preferably retractable to ease removal of the barge after assembly of the floating support structure. Advantageously the working deck comprises working habitats 27 and a barge crane 26 as shown in fig. 2

[0053] Further, the barge 1 comprises a divisional section 6 for keeping the lower buoyancy section 2 and the working deck 3 at a distance from each other. A divisional buoyancy volume VD of the divisional section 6 is considerably smaller than the lower buoyancy volume VL of the lower buoyancy section 2. Preferably, the divisional section comprises buoyancy volumes at each corner of the barge extending from the lower buoyancy section to the working deck thus providing stability when the lower buoyancy section 2 is submerged. In an embodiment the divisional buoyancy volume VD comprises a stabilizing pillar 12 having a cylindrical buoyancy volume at each corner of the barge as seen in fig. 1 and 3. The mentioned distance between the working deck 3 and the lower buoyancy unit 2 is preferably of the same magnitude as the depth of the fully mounted wind turbine and floating support structure when floating freely.

[0054] In a preferred embodiment the working deck 3 also comprises a buoyancy volume Vw. The working deck buoyancy volume Vw is of a precautionary nature configured to keep the barge floating if a fault or damage occurs. The barge 1 further comprises a ballasting system 7 for ballasting and deballasting the barge as indicated in fig. 3. Several ballasting systems are on the market and will not be described in great detail. By pumping in or out a volume of water equal to the buoyancy volume VD of the divisional section the barge will be able to be lowered and elevated between a lower position with the lower side of the working deck at the waterline and an upper position with the upper side of the lower buoyancy section 2 at the waterline. Here we assume the barge is unaffected by external buoyancy volumes or forces. The size of the divisional buoyancy volume VD should be as small as possible to reduce the quantity of water pumped during ballasting, but still large enough to maintain stability during lowering and elevation of the barge in positions where the lower buoyancy section 2 is submerged and therefore do not contribute to stability.

[0055] The effect of the divisional section 6 having a much smaller buoyancy volume VD than the lower buoyancy volume VL is that the barge can be lowered and elevated between the lower and upper position by pumping a lot less water thus saving time and energy.

[0056] The process of lowering the barge can be seen in fig. 6a - 6c. In fig. 6a the barge is holding the load of the floating support structure and tower 9 included nacelle 10 and turbine blades 11. The tower, nacelle and blades are shown in fig. 9. The buoyancy volumes of the barge must be adapted to match the combined weight of the floating support structure, tower 9, nacellelO and turbine blades 11. This combined weight, minus an adjustment for any parts of the floating support structure 8 that is submerged when the floating support structure is resting on the barge, must be supported by the lower buoyancy section 2 of the barge 1. The effect of this is that the barge 1 and wind turbine with floating support structure 8 can float by a quay side with ordinary depth.

[0057] The total weight of water that must be taken in by the ballasting system 7 is equal to or larger than the mentioned total weight of the wind turbine and support structure (minus an adjustment if parts of the floating support structure is immersed in water when resting on the barge). In addition a further volume must be allowed into the ballasting tanks to be able to lower the barge 1 further down after the floating support structure 8 is floating by itself in order to allow the barge being extracted from below the floating support structure without any obstacles on the barge 1 interfering with the floating support structure 8. Roughly speaking, the ballasting system of the barge should be able to fill the ballasting system 7 with water equal to the weight of the combined floating support structure and wind turbine plus at least 10 %.

[0058] Assembly of two different embodiments of the floating support structure 8 is shown in fig 4a-d and fig. 5a-d. During assembly the barge will remain in the upper position and the lower buoyancy unit 2 will take the load of the combined weight of the floating support structure and the wind turbine, minus a submerged part of the support structure if this is the case. A typical configuration is seen in fig. 6a. The floating support structure 8 will remain resting in the support elements 4 on the work deck 3 until the assembly is complete including mounting of tower 9, nacelle 10 and turbine blades 11. When the assembly is completed and the wind turbine is ready to be towed to its final destination, the barge is moved to deeper water, where the barge 1 may be lowered to a level shown in fig. 5c where the support elements 4 and all structures on the work deck 3 are below the horizontal parts of the floating support structure.

[0059] With the current size of wind turbines of 15 MW the barge and wind turbine with floating support structure will be able to float on 6 meters of depth by the quay side and when removing the barge from underneath the floating support structure a depth of about 20 meters is needed. Thus, most ordinary quays along the coastline will be able to receive the barge and perform an assembly of a floating support structure according to the invention. However, the barge can be built to match all sizes of floating support structures. An advantage of the invention is that a crane mounted on shore can be used for all the steps in the assembly process of the floating support structure and the wind turbine even if the quay side have limited depth.

[0060] The floating support structure 8 to be assembled must be able to rest on the barge as described above. An example of such a floating support structure is seen in fig. 6a, but a range of other embodiments like two pontoons or four legs positioned in each corner of a square are possible. A natural choice would be a floating support structure with a large horizontal section, where the horizontal section is positioned above the sea surface and different parts of the horizontal section are configured to rest on the barge before assembly. One class of embodiments of the floating support structure comprises three buoyancy units 31. In one embodiment of the floating support structure 8 seen in fig. 4a-d the three buoyancy units are constituted by a vertical first and second buoyancy units 17a, 17b connected to a horizontal transverse part 16 and aft buoyancy unit 19 connected to a horizontal aft part 18, thus providing an open space underneath the horizontal parts 16, 19 for accommodating the barge 1. The floating support structure also comprises a connecting flange 15 for connecting a windmill tower 9 to the floating support structure as shown in fig. 6a.

[0061] In a preferred embodiment, a damping device 22 is attached at the lower end of the vertical buoyancy units. The damping device is a plate extending horizontally outside the perimeters of the buoyancy units as seen in 4a-c and fig. 6a - c. In order to enable transport of the floating support structure 8 the floating support structure may be manufactured in a set of smaller subparts 20 where each subpart 20 comprises at least a horizontal section for resting on the barge and where at least some of the subparts 20 have a buoyancy unit 31, for example a vertical first, second or aft buoyancy unit 17a, 17b, 19 attached to at least one end as seen in fig. 4a-d. The subparts 20 having a vertical buoyancy unit attached may also, in addition to resting on the barge, be supported by side supports 5 and / or the lifting device 14 until it is firmly connected to the rest of the floating support structure. When assembled on the barge the subparts 20 constitutes the entire floating support structure.

[0062] In an embodiment, seen in fig. 7a and b, the floating support structure described above is manufactured and transported in a carrier vessel 32 in a set of three subparts 20a, b and c. The set of subparts 20 may comprise a first subpart 20a comprising the vertical aft buoyancy unit 19 and the horizontal aft part 18. The set of subparts may further comprise a second subpart 20b comprising the first vertical transverse buoyancy unit 17a and a major section of the horizontal transverse part 16 including a connector 23 for connecting the horizontal aft part to the horizontal transverse part 16, and a third subpart 20c comprising the second vertical transverse buoyancy unit 17b and a minor section of the horizontal transverse part 16.

[0063] In a preferred embodiment shown in fig. 5a-d the floating support structure 8 may comprise a first, second and third main section 33a, 33b, 33c each comprising a horizontal part 29 and a vertical part 30, wherein the vertical part comprises a buoyancy unit 31. The floating support structure further comprise a central transition piece 34 for connecting the three main sections. The main sections 33a, b and c are preferably symmetrically distributed with a 120 degree angle between them, thus providing an open space underneath the horizontal parts 29 for accommodating the barge 1 in three possible directions. The floating support structure also comprises a connecting flange 15 for connecting a windmill tower 9 to the floating support structure as shown in fig. 5a.

[0064] In a preferred embodiment, seen in fig. 8a and b, the floating support structure described above is manufactured and transported in a vessel in a set of four subparts 20a, b, c and d. The set of subparts 20 may comprise a first subpart 20a comprising three connector ends 35, for connection to a second, third and fourth subparts 20 b, c and d comprising the first second and third main sections (33a, 33b, 33c) respectively. Each of the mains sections comprises a horizontal part 29 and a vertical part 30 wherein the vertical part comprises a buoyancy unit 31.

[0065] Each subpart in the set of subparts 20 may have suspension connectors 21 for attaching a suspension line to be hooked onto the lifting device 14 as indicated in fig. 6a. The suspension connectors 21 are positioned to allow the horizontal part of the subparts 20 to be suspended in a horizontal position when suspended from the lifting device 14. This will ease the task of placing the subparts 20 on the barge 1.

[0066] Preferably, the horizontal transverse part 16 and the horizontal aft part 18 or any of the subparts and also the buoyancy units 31 have a tubular shape. A circular crosssection provides a good strength to weight ratio. A method for assembly and transportation of a floating support structure 8 is described and illustrated in fig. 4a-d and fig. 5a-d representing two different embodiments of the floating support structure. The assembly is done by means of a barge 1 as described and a lifting device 14 for lifting the subparts in a set of subparts 20 that constitutes the floating support structure 8 when assembled, the method comprising the steps: a. providing a set of subparts 20 that constitutes the floating support structure, b. lifting a subpart 20 onto the barge using the lifting device 14 for resting on at least one support element 4 and, if needed to obtain stability, being suspended from the lifting device while the subpart is connected to a previously positioned subpart if the lifted subpart is not the first to be lifted onto the barge 1, c. when the assembly of the floating support structure is completed, mounting tower 9, nacelle 10 and turbine blades 11, d. lowering the barge by means of the ballasting system 7 until the floating support structure is free of all obstacles on the barge (1) and e. removing the barge from underneath the floating support structure f. towing the floating support structure and wind turbine to a final destination.

[0067] If the depth by the quayside 25 do not allow the barge to be lowered point d must include first moving the barge and floating support structure to deeper water.

[0068] In an embodiment of the method where the set of subparts is constituted by second and third subpart 2a, 20 comprising the first and second transverse subpart 20b, 20c respectively and a first subpart 20a comprising the aft subpart 20a as described previously and shown in fig. 4a-d, step b comprises the following substeps, which is shown in fig. 4a, 4b and 4c:

[0069] 1. lifting the second subpart 20b onto the barge

[0070] 2. lifting the third subpart 20c onto the barge and holding the third subpart 20c in position with the lifting device 14 until it is connected to the first transverse subpart 21

[0071] 3. lifting the first subpart 20a onto the barge and connecting the aft horizontal part of the first subpart 20a to the connector 23.

[0072] It is possible to do the three substeps in any order as long as the first transverse subpart is lifted on to the barge before the second transverse part.

[0073] Preferably, the side supports 5 are retractable and are retracted during step d of the method. In a preferred embodiment of the method the set of subparts is comprised by a central transition piece 34 constituting a central subpart 20a and three main sections 33b-d constituting first second and third radial subpart 20b, 20c and 20d as described previously and shown in fig. 5a-d. In this embodiment step b comprises the following substeps, which is shown in fig. 5a-d:

[0074] 1. lifting the first subpart 20a onto the barge with a connector pointing in a direction substantially parallel with the quayside 25

[0075] 2. lifting the second subpart 20b onto the barge for connection to the connector pointing in a direction substantially parallel with the quayside 25 and holding the first radial subpart 20c in position with the lifting device 14 until it is connected to the first transverse subpart 21,

[0076] 3. turning the barge until the second subpart 20b is perpendicular to and pointing outward from the quayside 25,

[0077] 4. lifting the third and fourth subpart 20c and d) onto the barge and connecting them to the connector ends 35 of the first subpart (20a) in succession.

[0078] An advantage using this method is that the maximum weight of the subparts are low because they are similar and that the reach of the crane is kept to a minimum due to the turning of the barge, which allows a smaller crane to do the job.

[0079] In a preferred embodiment the barge 1 comprises thrusters (not shown) to be able to retract the barge 1 from underneath the floating support structure 8 without using a towing boat. Furthermore, the thrusters on the barge can advantageously be connected to a positioning system, such as a GPS-system, that also is linked to the positioning of the floating support structure in a suitable manner to avoid impact between barge and floating support structure during step e of the method.

[0080] If assembly of the floating support structure is done in shallow water, step c comprises moving the barge to deeper water before lowering the barge. This is mostly the case if the lifting device 14 and barge 1 is positioned at a quay side.

[0081] References

[0082] 1 Barge

[0083] 2 Lower buoyancy section

[0084] 3 Working deck

[0085] 4 Support element / Crib

[0086] 5 Side supports

[0087] 6 Divisional section

[0088] 7 Ballasting system

[0089] 8 Floating Support structure

[0090] 9 Tower

[0091] 10 Nacelle

[0092] 11 Turbine blades

[0093] 12 Stabilizing pillars

[0094] 13 Support beams

[0095] 14 Lifting device

[0096] 15 Connecting flange

[0097] 16 Horizontal transverse part

[0098] 17 a, b First and second vertical transverse buoyancy part

[0099] 18 Horizontal aft part

[0100] 19 Vertical aft buoyancy unit

[0101] 20 Set of subparts of the floating support structure / subparts

[0102] 20a Aft subpart

[0103] 20b First transverse subpart

[0104] 20c Second transverse part

[0105] 21 Suspension connectors

[0106] 22 Damping structures

[0107] 23 Connector

[0108] 25 Quayside

[0109] 26 Barge crane

[0110] 27 Working habitat

[0111] 28 Section connector

[0112] 29 Horizontal part

[0113] 30 Vertical part

[0114] 31 Buoyancy unit

[0115] 32 Carrier vessel

[0116] 33a, b, c First, Second and Third main section

[0117] 34 Central transition piece

[0118] 35 Connector ends

Claims

CLAIMS1. Barge (1) for assembly of a floating support structure (8) comprising: a lower buoyancy section (2) with a lower buoyancy volume VL, a working deck (3) comprising support elements (4) for receiving parts of the floating support structure to be assembled, a ballasting system (7) for lowering and elevating the barge (1), characterized by a divisional section (6) holding the lower buoyancy section (2) and the working deck (3) in a vertical distance from each other and having a divisional buoyancy volume VD considerably smaller than the lower buoyancy volume VL, such that the ballasting system, by pumping a quantity of water equal to VD in or out, is able to move the barge between a lower position with a bottom side of the working deck (3) in the waterline, and an upper position with the top side of the lower buoyancy section (2) in the waterline when the barge is unaffected by external buoyancy volumes.

2. Barge according to claim 1, wherein a major part of the divisional buoyancy volume VD resides in buoyancy structures (12) extending in the entire height of the divisional section (6) and being positioned in an area at least comprising each corner of the barge to provide stability when the lower buoyancy section is totally submerged.

3. Barge according to claim 2, wherein the divisional section (6) comprises stabilizing pillars (12) at each corner of the barge (1) extending between the lower buoyancy section (2) and the working deck (3) for stabilizing the barge when the lower buoyancy section is submerged (6).

4. Barge according to any of the preceding claims, wherein the working deck (3) comprises a safety buoyancy volume Vw.

5. Barge according to any of the preceding claims, wherein the divisional section (6) comprises support beams (13) extending between the lower buoyancy section (2) and the working deck (3).

6. A floating support structure (1) to be assembled on a barge according to claim 1- 5.

7. The floating support structure according to claim 6 comprising: a horizontal transverse part (16) having a first and second vertical transverse buoyancy unit (17a, 17b) at each end respectively and a horizontal aft part (18) connected to a midpoint of the transverse part (16) and having a vertical aft buoyancy unit (19) at the aft end thus providing an open space underneath the horizontal transverse and aft parts (16, 19) for accommodating the barge (1) anda connecting flange (15) for connecting a windmill tower (9).

8. The floating support structure (1) according to claim 6 comprising: a first, second and third main section (33a, 33b, 33c) each comprising a horizontal part (29) and a vertical part (30), wherein the vertical part comprises a buoyancy unit (31), a central transition piece (34) for connecting the respective three horizontal parts of the main sections by means of respective connector ends (35) adapted to fit the respective horizontal parts, the central transition piece further comprising a connecting flange (15) for connecting the windmill tower (9).

9. The floating support structure according to claim 6 wherein the floating support structure is manufactured in a set of subparts (20) comprising first, second, third, etc. subparts (20a, b, c...) constituting the floating support structure when assembled.

10. The floating support structure according to claim 7 and 9, wherein the set of subparts (20), comprises: the first subpart (20a) comprising the vertical aft buoyancy unit (19) and the horizontal aft part (18), the second subpart (20b) comprising the first vertical transverse buoyancy unit (17a), a major section of the horizontal transverse part including a connector (23) for connecting the horizontal aft part to the midpoint of the horizontal transverse part and, and the third subpart (20c) comprising the second vertical transverse buoyancy unit (17b) and a minor section of the horizontal transverse part.

11. The floating support structure according to claim 8 and 9, wherein the set of subparts (20), comprises: the first subpart (20a) comprising the central transition piece (34), and the second, third and fourth subpart (20b-d) comprising the first, second and third main section (33a, 33b, 33c) respectively.

12. The floating support structure according to any of claims 9 - 11 wherein each subpart of the set of subparts (20) have suspension connectors (21) positioned such that the horizontal sections of the subpart are horizontal when lifting lines are connected to the suspension connectors (21) and suspended from the lifting device (14).

13. Method for assembly of a floating support structure (8) according to claim 9 by means of a barge according to 1-5 and a lifting device (14) positioned at aquayside (25) for lifting a set of subparts (20) that constitutes the floating support structure (8) when assembled, the method comprising the steps: a. proving the subparts (20) that constitutes the floating support structure (8), b. lifting a subpart onto the barge (1) using the lifting device (14) letting the subpart (20) rest on the support elements (4) and, if needed for stability, also letting the subpart remain suspended from the lifting device while the subpart is connected to a previously positioned subpart if the lifted subpart is not the first to be lifted onto the barge, c. when the assembly is completed, lowering the barge by means of the ballasting system (7) until the floating support structure is free of all obstacles on the barge (1) and d. removing the barge from underneath the floating support structure.

14. Method according to claim 13, when dependent on claim 10 wherein step b comprises the following substeps:

1. Lifting the second subpart (20b) onto the barge,2. Lifting the third subpart (20c) onto the barge and holding the second transverse subpart (20c) in position with the lifting device until it is connected to the second subpart (20b), and3. Lifting the first subpart (20a) onto the barge and connecting the aft horizontal part to the connector (23) of the first transverse subpart (20b).

15. Method according to claim 13, when dependent on claim 11 wherein step b comprises the following substeps:

1. lifting the first subpart (20b) onto the barge and positioning a connector end (35) in a direction parallel to the quayside (25),2. lifting the second subpart onto the barge and connecting it to the first subpart (20a,3. turning the barge until the second subpart (20b) is perpendicular to and pointing outward from the quayside (25),4. lifting the third and fourth subpart (20c and d) onto the barge and connecting them to the connector ends (35) of the first subpart (20a) in succession.

16. Method according to any of the claims 13 - 15 wherein step c comprises moving the barge to deeper water before lowering the barge.

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