SE2251139A1

SE2251139A1 - Hull structure for a semi-submersible wind power turbine platform

Info

Publication number: SE2251139A1
Application number: SE2251139A
Authority: SE
Inventors: Fredrik Arvidsson; Jimmy Bondesson; Joakim Nilsson; Robert Ludwigsson; Yungang Liu
Original assignee: Bassoe Tech Ab
Priority date: 2022-01-14
Filing date: 2022-09-30
Publication date: 2023-07-15
Also published as: SE545506C2

Abstract

The invention concerns a hull structure (10, 20, 30) for a semi-submersible wind power turbine platform (100), wherein the hull structure comprises:- first, second and third buoyant stabilizing columns (1, 2, 3) extending in a substantially vertical direction; and- first and second elongated submersible buoyant pontoon structures (11, 12) extending in a substantially horizontal direction;wherein the hull structure further is provided with first, second and third brace members (21, 22, 23) that extend between and connect upper parts of the columns (1, 2, 3) at a level above the first and second pontoon structures (11, 12),wherein a lower side of the third brace member (23) is located at a level that is higher than an upper side of at least a part of each of the first and second brace members (21, 22) so that a first and a second hull structure according to this claim can be positioned close to each other with the second column (2) of the first hull structure located between the first and second pontoon structures (11, 12) of the second hull structure and with the first and second brace members (21, 22) of the second hull structure extending below the third brace member (23) of the first hull structure.

Description

Hull structure for a semi-submersible wind power turbine platform TECHNICAL FIELD This invention relates to a hull structure for a semi-submersible wind power turbine platform.

BACKGROUND OF THE INVENTION There is a growing interest for offshore wind power, i.e. sea-based wind power stations/turbines that produce electricity. Such a wind turbine may have a fixed undenNater foundation or, in particular at water depths larger than around 50-60 m, may be arranged on a floating platform anchored to the bottom.

A floating wind power turbine platform may be of a semi-submersible type comprising a semi-submersible hull structure onto which a wind turbine tower is arranged. The hull structure is typically made up of a plurality of stabilizing buoyant columns connected by submersible buoyant pontoons or other connection members. The turbine tower is typically arranged onto one of the columns. An example of a semi-submersible wind power turbine platform is disclosed in WO2021/219787.

Platforms of this type are large constructions. For instance, each column of a 10 MW wind power turbine platform may have a height of 30 m and the distance between the columns may be 60-80 m. The total weight of the hull structure may be more than 3000 tonnes. The turbine tower may extend up to, say, 150 m above sea level and each turbine blade may be more than 100 m long.

A challenge in the field of offshore wind power is manufacturing, transportation and installation of the semi-submersible platforms. Towing of a platform with the wind turbine tower and blades etc. installed is complicated and challenging, and to reduce the towing distance for such a complete platform it is preferably arranged so that the turbine tower and the turbine blades etc. are installed onto the hull structure in a sheltered location relatively close to the final offshore location. A particular transportation challenge arises if the hull structure is manufactured at a construction yard located far away from the sheltered location, for instance because there is no construction yard suitable for such large and heavy hull structures available at or near the sheltered location. ln such a situation the hull structures need to be transported a relatively long distance.

A further challenge with regard to semi-submersible wind power turbine platforms is to design the hull structure so that the platform becomes robust and stable also under harsh offshore conditions and so that the platform withstands many years of operation under such conditions.

A still further challenge is of course that manufacturing, transportation, installation, operation, etc. of the platform or hull structure must be cost efficient for keeping and increasing the interest for offshore wind power.

SUMMARY OF THE INVENTION An object of this invention is to provide a hull structure for a semi- submersible wind power turbine platform, where the hull structure exhibits improved properties with regard to allow positioning close to each other, in particular for allowing close and efficient stowing of hull structures onto a deck of a marine transportation vessel but also for taking up less space when moored at a construction yard or harbour.

The invention concerns a hull structure for a semi-submersible wind power turbine platform, wherein the hull structure comprises: first, second and third buoyant stabilizing columns extending in a substantially vertical direction; and first and second elongated submersible buoyant pontoon structures extending in a substantially horizontal direction.

The first pontoon structure extends between and connects the first and the second column, wherein the first pontoon structure is connected to a lower part of each of the first and second columns. Similarly, the second pontoon structure extends between and connects the second and the third column, wherein the second pontoon structure is connected to a lower part of each of the second and third columns. The first and second pontoon structures are arranged in a V-shape in the horizontal plane with the first and second pontoon structures forming legs in the V-shape and with the second column located where the legs meet.

The hull structure is further provided with first, second and third brace members that extend between and connect upper parts of the columns at a level above the first and second pontoon structures, wherein the first brace member is arranged between the first and second columns, wherein the second brace member is arranged between the second and third columns, wherein the third brace member is arranged between the first and third columns.

A lower side of the third brace member is located at a level that is higher than an upper side of at least a part of each of the first and second brace members so that a first and a second hull structure of this type can be positioned close to each other with the second column of the first hull structure located between the first and second pontoon structures of the second hull structure and with the first and second brace members of the second hull structure extending below the third brace member of the first hull structure.

Brace members are useful for improving strength and stability of a hull structure of the above type. Although the brace members typically are relatively small and light compared to the columns and the pontoons, the brace members are still large and heavy components in absolute terms, and they are difficult to handle also with large cranes. lt is therefore an advantage if also the brace members can be mounted as completely as possible already at the construction yard. Typically, such brace members are indeed mounted at the construction yard (or are proposed to be mounted at the construction yard for not yet built hull structures). For a hull structure with three columns, three bracings are typically arranged at a similar vertical level between all columns of the hull structure. Such hull structures cannot be positioned close to each other even if the general shape of the hull structure would allow this, such as where the first and second pontoon sections define a V-shape so that one hull structure in principle could be partly inserted with its V-tip between the legs of the V of another hull structure. The brace members prevent this. ln the inventive hull structure, the brace member at the open end of the V- shape (or, if the hull structure is A-shaped, at the side of the hull structure where a third pontoon is located as will be further described below) is located at a higher level than the two other brace members so that two hull structures of the inventive type can be positioned close to and partly inserted into each other even when the brace members are mounted. The first and second brace members fit below the third brace member.

Such close positioning of hull structures is useful for efficient stowing of a set of hull structures onto a deck of a marine transportation vessel, typically for transporting hull structures from a construction yard located far away from a sheltered location relatively close to the final offshore location as mentioned above. Such transportation is very costly so the costs can be significantly reduced if a higher number of hull structures can be stowed onto the marine vessel. Close positioning of a set of hull structures may also be advantageous for taking up less space when moored at a construction yard or harbour.

Each of the first, second and third columns has an intended operational waterline at least approximately corresponding to a water surface when the hull structure is set in an operational state with the first and second pontoon structures submerged beneath the water surface and with the first, second and third columns extending through the water surface.

That the platform, and the hull structure, is semi-submersible means that the platform/hull structure can be partly located beneath the water surface when in operation. The entire pontoon structures and parts of the columns are typically to be located beneath the surface. Anchoring/securing of the platform/hull structure to the bottom can be arranged in different ways. lt may not be necessary that the third brace member is located above the first and second brace members along the entire length of the first and second brace members. ln an embodiment, the lower side of the third brace member is located at a level that is higher than the upper side of each of the first and second brace members along at least a half of each of the first and second brace members located closest to the second column. ln an embodiment, the lower side of the third brace member is located at a level that is higher than substantially the entire upper side of each of the first and second brace members. ln an embodiment, at least the third brace member extends in a substantially horizontal direction. Also the first and second brace members may extend in a substantially horizontal direction. ln an embodiment, the hull structure comprises a third elongated submersible connection structure extending in a substantially horizontal direction between the lower part of the first column and the lower part of the third column and/or between the first and second pontoon structures so as to form a A- or A- shape in the horizontal plane together with the first and second pontoon Structures. The third connection structure may or may not be a buoyant pontoon structure. Also A- and A-shaped hull structures may in principle be partly inserted into each other and provide for close positioning and efficient stowing when the brace members are arranged so as to allow this. ln an embodiment, the second column has a height that is less than a vertical distance between an upper side of the third connection structure and the lower side of the third brace member. This way the second column of a horizontally oriented first hull structure fits vertically between the third connection structure and the third brace member of a horizontally oriented second hull structure. ln an embodiment, the third connection structure has a height that is less than that of each of the first and second pontoon structures, and wherein the third connection structure is arranged so that the upper side of the third connection structure is located at a lower level in the horizontal direction than an upper side of each of the first and second pontoon structures. This makes it easier to stow two hull structures close together onto a flat surface, such as a cargo deck of a marine vessel, since the first and second pontoon structures of a first hull structure inserted into another hull structure will be located onto the third connection structure of that second hull structure, and the lower the third connection structure, the less inclined position will the first hull structure need to occupy. ln an embodiment, each of the first, second and third pontoon or connection structures has a lower side facing downwards and wherein the lower sides of the first, second and third pontoon or connection structures are substantially aligned with each other in the horizontal plane. Preferably, the lower sides of the first, second and third pontoon or connection structures are substantially aligned with downwardly facing lower sides of each of the first, second and third buoyant stabilizing columns. This way the entire lower side of the hull structure will be substantially aligned and flat (except, for instance, an inclined surface that is described further below). ln an embodiment, the height of the third connection structure is less than 50% of the height of at least one of the first and second pontoon structures.

Typically, the first and second pontoon structures have the same height (and similar dimensions also in other directions). ln an example, the height of each of the first and second pontoon structures is around 7 m while the height of the third connection structure is around 3 m, thus less than 50% of the height of the first and second pontoon structures. ln a further example, the height of the third connection structure is 2-4 m. ln an embodiment, the hull structure as a whole is arranged to exhibit a V- shape or a A-shape or an A-shape in the horizontal plane where the first and second pontoon structures form the V-shape or two sides in the A- or A- shape. The terms V-, A- and A-shape refers to the general shape set by the two or three pontoon/connection structures mentioned above. ln an embodiment, the hull structure has no additional column besides the first, second and third columns. Preferably, the hull structure is not provided with any further pontoon structure connected to the second column. ln an embodiment, the third brace member is a stiff structure. A stiff structure, such as a pipe or beam made of steel, is useful for strengthening the hull structure both during transport of the hull structure as well as during operation of the wind power turbine platform. ln an embodiment, the third brace member is a non-stiff structure. A non-stiff structure, such as a wire or rope, is in particular useful for strengthening the hull structure during transport of the hull structure. Such a non-stiff brace member may be pre-tensioned when installed onto the hull structure.

The invention also concerns a method for loading a set of hull structures of the above type onto a semi-submersible cargo carrying marine vessel in the form of a semi-submersible cargo carrying marine vessel configured to be lowered partly below the water surface into a lower position and be raised to an upper position so as to load onto the vessel cargo that is located at the water surface above the vessel, the method comprising: - providing the set of hull structures floating in water; - arranging the set of hull structures in a row above the marine vessel when the marine vessel is in its lower position; and ~ raising the marine vessel to its upper position so as to load the row of hull structures onto the marine vessel.

Close positioning of a set of hull structures can be useful not only for vessel transport but also for reducing the space required for storage or mooring. The invention also concerns a method for positioning a set of hull structures close together, wherein the set of hull structures comprises at least a first and a second hull structure of the above type. The method comprises: providing the set of hull structures floating in water; and arranging the set of hull structures in a row, where the latter step comprises arranging the first and second hull structures adjacent each other so that the second column of the second hull structure is positioned closer to the second column of the first hull structure than to the first and third columns of the first hull structure.

The method for positioning a set of hull structures close together may further comprise arranging the first and second hull structures adjacent each other so that the second hull structure is located above any third connection structure of the first hull structure with the second column of the second hull structure positioned between the first and second connection structures of the first hull structure.

The method may further comprise: setting at least one of the first and second hull structures in an inclined position so as to allow the second hull structure to float into position adjacent the first hull structure.

BRIEF DESCRIPTION OF DRAWINGS ln the description of the invention given below reference is made to the following figure, in which: Figure 1 shows a perspective view of a first embodiment of a hull structure according to this disclosure.

Figure 2 shows different views (figures 2A and 2B) of the embodiment of figure 1.

Figure 3 shows a top view of the embodiment of figure 1.

Figure 4 shows a perspective view of a second embodiment of a hull structure according to this disclosure.

Figure 5 shows a side view of the embodiment of figure 4.

Figure 6 shows how a set of floating hull structures according to figure 4 are arranged in a row in close position to one another.

Figure 7 shows a perspective view of a third embodiment of a hull structure according to this disclosure.

Figure 8 shows how a set of floating hull structures according to figure 7 are arranged in a row in close position to one another.

Figure 9 shows a set of floating hull structures according to figure 7 stowed onto a marine transportation vessel.

Figure 10 shows a semi-submersible wind power turbine platform comprising a hull structure according to figure 4.

DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION Figures 1-3 show a hull structure 10 comprising first, second and third buoyant stabilizing columns 1, 2, 3 extending in a substantially vertical direction. First and second elongated submersible buoyant pontoon structures 11, 12 extend in a substantially horizontal direction and connect the columns. An intended operational waterline 33 is indicated on the columns. The waterline 33 indicates approximately a water surface when the hull structure 10 is set in an operational mode with the pontoon structures 11, 12 submersed beneath the water surface.

The first and second pontoon structures 11, 12 are arranged in a V-shape in the horizontal plane with the first and second pontoon structures 11, 12 forming legs in the V-shape and with the second column 2 located where the legs meet.

The hull structure 10 is provided with first, second and third brace members 21, 22, 23 that extend between and connect upper parts of the columns 1, 2, 3 at a level above the first and second pontoon structures 11, 12. The first brace member 21 is arranged between the first and second columns 1, 2, the second brace member 22 is arranged between the second and third columns 2, 3, and the third brace member 23 is arranged between the first and third columns 1, 3.

All brace members 21, 22, 23 extend in this case substantially horizontally. A lower side of the third brace member 23 is located at a level that is higher than an upper side of each of the first and second brace members 21, 22. As will be shown further down, this allows a first and a second hull structure of the type shown in figures 1-3 to be positioned close to each other with the 11 second column 2 of the first hull structure located between the first and second pontoon structures 11, 12 of the second hull structure and with the first and second brace members 21, 22 of the second hull structure extending below the third brace member 23 of the first hull structure.

As an example of size, the columns 1, 2, 3 may have a height of around 30- 35 m and a diameter of around 13 m. Each of the pontoon structures 11, 12 may have a length of around 50-70 m and a width of 6-10 m. The height of the first and second pontoon structures 11, 12 may be 6-9 m.

As shown in figures 1-3, the second column 2 has lower height than the first and third columns 1,3. Further, the first and second brace members 21, 22 are arranged at an upper side of the second column 2 and extend horizontally to a connection point at the upper part of the first and third columns 1, 3, respectively, located somewhat below an upper side of these columns.

Figures 2A and 2B indicate a vertical distance D1 between the center of the third brace member 23 and the common center of the first and second brace members 21, 22. The distance D1 should be greater than half the height (vertical thickness) of the third brace member 23 plus half the height (vertical thickness) of the first/second brace member 21/22 to locate the lower side of the third brace member 23 at a level that is higher than the upper side of each of the first and second brace members 21, 22. Some additional margin is suitable, as shown in figures 1-3.

Figure 2 further indicates a water surface 34. Figures 4-5 show a second embodiment of a hull structure 20 that principally is arranged in a similar way as the embodiment shown in figures 1-3, and the same reference numbers are used for similar components. 12 The hull structure 20 of figures 4-5 comprises a third elongated submersible buoyant pontoon structure 13 that extends in a substantially horizontal direction between the lower part of the first column 1 and the lower part of the third column 3. The hull structure 20 as a whole thus exhibits a A-shape in the horizontal plane.

As indicated in figure 5, the second column 2 of the hull structure 20 has a height D2 that is less than a vertical distance D3 between an upper side of the third pontoon structure 13 and the lower side of the third brace member 21. The second column 2 thus fits vertically between the third pontoon structure 13 and the third brace member 23 when two hull structures of the type shown in figures 4-5 are to be located close together.

As shown in figure 4, additional brace supports 24 are arranged on top of the first and third columns 1, 3 that set the third brace member 23 at a higher level than if the third brace member 23 would have been mounted directly onto the upper side of the columns.

The second column 2 is in this example provided with, or comprises, a wind turbine support 101 that forms an upper part of the second column 2.

The third pontoon structure 13 has a height that is less than that of each of the first and second pontoon structures 11, 12, and the third pontoon structure 13 is arranged so that the upper side of the third connection structure 13 is located at a lower level in the horizontal direction than an upper side of each of the first and second pontoon structures 11, 12. Each of the first, second and third pontoon structures 11, 12, 13 has a lower side facing downwards and the lower sides of all pontoon structures are substantially aligned with each other in the horizontal plane. Further, the lower sides of the pontoon structures 11, 12, 13 are aligned with downwardly facing lower sides of each of the columns 1, 2, 3. The height of the third 13 pontoon structure 13 is around 30% of the height of first and second pontoon structures 11, 12.

As shown in figure 5, the lower side of the third pontoon structure 13 and a part of the lower sides of the first and third columns 1, 3 are inc|ined as indicated by reference number 8 in figure 5. The inc|ination may be around 10°. The purpose of the inc|ined surface 8 is to better support the hull structure 20 when forming part of a set of similar hull structures closely stowed in a row onto a flat surface, such as the deck of a marine transportation vessel. When stowed closely, the hull structures will occupy an inc|ined position corresponding to the angle of the inc|ined surface 8, which thus will be parallel with the flat surface/vessel deck.

As further shown in figure 5, a supporting surface 6 is arranged close to the second column 2, which supporting surface 6 is arranged on the same horizontal level as the upper side of the third pontoon structure 13. When stowing several hull structures close to each other onto e.g. the vessel deck, a first hull structure is supported by the supporting surface 6 and the upper side of the third pontoon structure 13 of an adjacent second hull structure.

Figure 6 shows how a set of floating hull structures 20a. 20b, 20c according to figures 4-5 are arranged in a row in close position to one another. Figure 6A shows that a first hull structure 20a may be tilted/inclined (using a controllable ballast system arranged on the hull structure) so as to allow a second hull structure 20b to be located "inside" the first hull structure and occupy the same inc|ination angle (figure 6C). The same procedure is used for a third hull structure 20c. When the set of hull structures 20a-20c are closely arranged in a row according to figure 6E, the set of hull structures may be loaded onto a marine transportation vessel in the form of a semi- submersible cargo carrying marine vessel 60 (see figure 9) configured to be lowered partly below the water surface into a lower position and be raised to 14 an upper position so as to load onto the vessel cargo that is located at the water surface above the vessel.

Figure 6E shows that the inclined surface 8 of all hull structures 20a-20c now is positioned horizontally. lt is further shown that the second and third hull structures 20b, 20c is supported (or will be supported if the set of hull structures is raised by the deck of the marine vessel) by the supporting surface 6 and the upper side of the third pontoon structure 13 of the adjacent hull structure to the right. The first hull structure 20a to the far right may be supported by an additional support arranged onto the vessel deck.

Figure 7 shows a third embodiment of a hull structure 30, which in most ways is similar to the embodiment of figures 1-3. A difference is that the hull structure 30 is provided with a fourth brace member 24 extending between the first and third column 1, 3 below the third brace member 23. To still allow for close stowing/positioning of a set of similar hull structures, the fourth brace member 24 is divided into three parts; a first short end part or mounting support 24a fixed to the third column 3, a second short end part or mounting support 24c fixed to the first column 1, and a central part 24b that constitutes most or substantially all of the total length of the fourth brace member 24.

The central part 24b of the fourth brace member 24 is releasably arranged at and along the third brace member 23. The arrangement of the central part 24b is suitably made at the construction yard in connection with manufacturing of the hull structure 30. Also the end parts/mounting supports 24a, 24c are preferably completed at the construction yard. When the transport of the hull structure 30 is completed (or when there is no need for close stowing/positioning), the central part 24b can be fixed to the mounting supports 24a, 24c relatively easily by first lowering the central part 24b using winches or similar and then fixing it. This way it is not necessary to use large cranes to position the central part 24b properly, instead the central part 24b is already properly positioned Iongitudinally and needs only to be lowered to correct vertical position. lf the central part 24b is located below the third brace member 23 as shown in figure 7, there should be a good vertical margin between the third brace member 23 and the first and second brace members 21, 22 so that close stowing/positioning of similar hull structures still is possible.

The second column 2 is in the example of figure 7, as in the example of figures 4-6, provided with, or comprises, a wind turbine support 101. ln the example of figure 7, an upper side of the wind turbine support 101 is located higher than the third brace member 23.

Figure 8 shows how a set of floating hull structures 30a-30c according to figure 7 are arranged in a row in close position to one another. As shown in figures 8B and 8C, the hull structures are tilted/inclined (using a controllable ballast system arranged on the hull structure) so as to achieve the close positioning. Compared to figure 6, the hull structures 30a-30c are tilted in the opposite direction. As the hull structures 30a-30b are not provided with any pontoon structure between the first and third columns 1, 3, the hull structures can be oriented horizontally when arranged in the row, see figure 8D.

Figure 9 shows a set of floating hull structures 30a-30c according to figure 7 stowed onto a deck 65 of a marine transportation vessel 60 in the form of a semi-submersible cargo carrying marine vessel configured to be lowered partly below the water surface into a lower position and be raised to an upper position so as to load onto the vessel cargo that is located at the water surface above the vessel.

A method for loading the set of hull structures 30a-30c (or, for instance, the set of hull structures 20a-20c) onto the semi-submersible cargo carrying marine vessel 60 comprises in general the following steps: 16 - providing the set of hull structures 30a-30c ﬂoating in water; - arranging the set of hull structures 30a-30c in a row above the marine vessel 60 when the marine vessel is in its lower position; and ~ raising the marine vessel 60 to its upper position so as to load the row of hull structures 30a-30c onto the marine vessel 60.

The step of arranging the set of hull structures in the row, wherein the set of hull structures comprises at least a first 20a and a second 20b hull structure, may comprise: arranging the first and second hull structures 20a, 20b adjacent each other so that the second hull structure 20b is located above the third connection structure 13 of the first hull structure 20a with the second column 2 of the second hull structure 20b positioned between the first and second connection structures 11, 12 of the first hull structure 20a.

The step of arranging the set of hull structures in the row, wherein the set of hull structures comprises at least a first 20a, 30a and a second 20b, 30b hull structure, may also comprise: arranging the first and second hull structures 20a, 20b, 30a, 30b adjacent each other so that the second column 2 of the second hull structure 20b, 30b is positioned closer to the second column 2 of the first hull structure 20a, 30a than to the first and third columns 1, 3 of the first hull structure 20a, 30a.

Figure 10 shows a semi-submersible wind power turbine platform 100 comprising a hull structure 30 according to figure 4. The platform 100 is provided with a wind turbine tower 102 in turn provided with three blades 103 (as well as a generator etc., which is not shown in the figures). 17 The invention is not limited by the embodiments described above but can be modified in various ways within the scope of the claims. For instance, the cross section of the columns, pontoon structures and brace members may be different than exemplified, such as polygonal columns or brace members and circu|ar or polygonal pontoon structures.

The third brace member 23 may be a stiff structure typically capable of carrying a load in both longitudinal directions, i.e. it can withstand both tensile and compression forces directed along its longitudinal axis. Alternatively, the third brace member 23 may be a wire, rope or other non-stiff structure, which may be pre-tensioned when installed, typically capable of carrying a load mainly, but not exclusively, when subject to longitudinally directed tensile forces. As shown in figure 9, the first and third columns 1, 3 are located outside of the deck 65 of the vessel. This means that the first and third columns 1, 3, or at least the upper parts thereof, tend to be forced somewhat apart, i.e. the columns tend to bend or flex outwards at the top, during transport, which in turn means that the third brace member 23 mainly will be subject to tensile forces during transport. Such forces or loads can be well carried by either a stiff or a non-stiff brace member. lrrespective of whether the third brace member 23 is stiff or non-stiff, it may be removed after transport. That is, when the wind power turbine platform is in operation it may not be provided with the third brace member 23.

A stiff brace member may be made of a metallic material, such as steel, and may form a pipe or beam. A non-stiff brace member may be in the form of a wire or a rope and may be pre-tensioned so as to reduce the forces acting onto different parts of the hull structure during transport.

Claims

1. A hull structure (10, 20, 30) for a semi-submersible wind power turbine platform (100), wherein the hull structure comprises: - first, second and third buoyant stabilizing columns (1, 2, 3) extending in a substantially vertical direction; and - first and second elongated submersible buoyant pontoon structures (11, 12) extending in a substantially horizontal direction; wherein the first pontoon structure (11) extends between and connects the first and the second column (1, 2), wherein the first pontoon structure (11) is connected to a lower part of each of the first and second columns (1, 2); wherein the second pontoon structure (12) extends between and connects the second and the third column (2, 3), wherein the second pontoon structure (12) is connected to a lower part of each of the second and third columns (2, 3); wherein the first and second pontoon structures (11, 12) are arranged in a V- shape in the horizontal plane with the first and second pontoon structures (11, 12) forming legs in the V-shape and with the second column (2) located where the legs meet; wherein the hull structure further is provided with first, second and third brace members (21, 22, 23) that extend between and connect upper parts of the columns (1, 2, 3) at a level above the first and second pontoon structures (11, 12), wherein the first brace member (21) is arranged between the first and second columns (1, 2), wherein the second brace member (22) is arranged between the second and third columns (2, 3), wherein the third brace member (23) is arranged between the first and third columns (1, 3), wherein a lower side of the third brace member (23) is located at a level that is higher than an upper side of at least a part of each of the first and second brace members (21, 22) so that a first and a second hull structure according to this claim can be positioned close to each other with the second column (2) of the first hull structure located between the first and second pontoonstructures (11, 12) of the second hull structure and with the first and second brace members (21, 22) of the second hull structure extending below the third brace member (23) of the first hull structure.

2. The hull structure (10, 20, 30) according to claim 1, wherein the lower side of the third brace member (23) is located at a level that is higher than the upper side of each of the first and second brace members (21, 22) along at least a half of each of the first and second brace members (21, 22) located closest to the second column (2).

3. The hull structure (10, 20, 30) according to claim 1 or 2, wherein the lower side of the third brace member (23) is located at a level that is higher than substantially the entire upper side of each of the first and second brace members (21, 22).

4. The hull structure (10, 20, 30) according to any of the above claims, wherein at least the third brace member (23) extends in a substantially horizontal direction.

5. The hull structure (10, 20, 30) according to any of the above claims, wherein the hull structure comprises a third elongated submersible connection structure (13) extending in a substantially horizontal direction between the lower part of the first column (1) and the lower part of the third column (3) and/or between the first and second pontoon structures (11, 12) so as to form a A- or A-shape in the horizontal plane together with the first and second pontoon structures (11, 12).

6. The hull structure (10, 20, 30) according to claim 5, wherein the second column (2) has a height that is less than a vertical distance between an upper side of the third connection structure (13) and the lower side of the third brace member (23).

7. The hull structure (10, 20, 30) according to claim 6, wherein the third connection structure (13) has a height that is less than that of each of the first and second pontoon structures (11, 12), and wherein the third connection structure (13) is arranged so that the upper side of the third connection structure (13) is located at a lower level in the horizontal direction than an upper side of each of the first and second pontoon structures (11, 12).

8. The hull structure (10, 20, 30) according to any of claims 5-7, wherein each of the first, second and third pontoon or connection structures (11, 12, 13) has a lower side facing downwards and wherein the lower sides of the first, second and third pontoon or connection structures are substantially aligned with each other in the horizontal plane, and, preferably, the lower sides of the first, second and third pontoon or connection structures (11, 12, 13) are substantially aligned with downwardly facing lower sides of each of the first, second and third buoyant stabilizing columns (1, 2, 3).

9. The hull structure (10, 20, 30) according to any of claims 5-8, wherein the height of the third connection structure (13) is less than 50% of the height of at least one of the first and second pontoon structures (11, 12).

10. The hull structure (10, 20, 30) according to any of the above claims, wherein the hull structure as a whole is arranged to exhibit a V-shape or a A- shape or an A-shape in the horizontal plane where the first and second pontoon structures (11, 12) forms the V-shape or two sides in the A-shape or A-shape.

11. The hull structure (10, 20, 30) according to any of the above claims, wherein the hull structure has no additional column besides the first, second and third columns (1, 2, 3) and, preferably, that the hull structure is not provided with any further pontoon structure connected to the second column

12.(2)-12. The hull structure (10, 20, 30) according to any of the above claims, wherein the third brace member (23) is a stiff structure.

13. The hull structure (10, 20, 30) according to any of claims 1-11, wherein the third brace member (23) is a non-stiff structure.

14. A method for loading a set of hull structures (20a-20c, 30a-30c) according to any of the above claims onto a semi-submersible cargo carrying marine vessel (60) in the form of a semi-submersible cargo carrying marine vessel configured to be lowered partly below the water surface into a lower position and be raised to an upper position so as to load onto the vessel (60) cargo that is located at the water surface above the vessel (60), the method comprising: - providing the set of hull structures (20a-20c, 30a-30c) floating in water; - arranging the set of hull structures (20a-20c, 30a-30c) in a row above the marine vessel (60) when the marine vessel is in its lower position; and ~ raising the marine vessel (60) to its upper position so as to load the row of hull structures (20a-20c, 30a-30c) onto the marine vessel (60).