EP3931442A1

EP3931442A1 - A longitudinal structure for an offshore wind turbine

Info

Publication number: EP3931442A1
Application number: EP20708429.4A
Authority: EP
Inventors: Torben Ladegaard Baun; Jesper Lykkegaard NEUBAUER
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2019-03-01
Filing date: 2020-02-25
Publication date: 2022-01-05
Also published as: WO2020177825A1

Abstract

A longitudinal structure (10), e.g. a monopile, for an offshore wind turbine (9) is disclosed. The longitudinal structure (10) comprises an outer wall enclosing a hollow interior (5), the outer wall comprising a first region (17) in which the outer wall is solid and a second region (1) in which the outer wall is provided with a plurality of passages (4) allowing fluid to pass to and from the hollow interior (5) of the longitudinal structure (10). The passages (4) have a non-circular elliptic shape with the major axis arranged substantially along a longitudinal direction (6) of the longitudinal structure (10). The passages (4) are arranged relative to each other in such a manner that a plurality of first passage-free areas (7) and a plurality of second passage-free areas (8) are formed on the outer wall of the second region (1), where each of the first passage-free areas (7) extends along a direction being parallel to the longitudinal direction (6) of the longitudinal structure (10), and each of the second passage-free areas (8) extends along a direction forming an angle with the longitudinal direction (6) of the longitudinal structure (10), said angle being within range of 30° to 60°.

Description

A LONGITUDINAL STRUCTURE FOR AN OFFSHORE WIND TURBINE

FIELD OF THE INVENTION

The present invention relates to a longitudinal structure for an offshore wind turbine. The longitudinal structure of the invention reduces fatigue loads on the longitudinal structure caused by waves and reduces the formation of scour holes in a region near the longitudinal structure.

BACKGROUND OF THE INVENTION

Offshore wind turbines, i.e. wind turbines located at sea, are sometimes mounted on an underwater foundation via a longitudinal structure, such as a monopile, a part of the tower, etc. In this case the longitudinal structure is arranged partly under water. Thereby the longitudinal structure is subjected to loads caused by waves colliding with the longitudinal structure, and this introduces fatigue in the longitudinal structure. Accordingly, it is necessary to design the longitudinal structure in a manner which prevents that the introduced fatigue decreases the lifetime of the longitudinal structure to an unacceptable level. This increases the manufacturing costs of the longitudinal structure.

Furthermore, the presence of a longitudinal structure, or another kind of structure connecting an offshore wind turbine to an underwater foundation, causes the underwater currents to remove material from the seabed in a region surrounding the foundation and the longitudinal structure. Such removal of seabed material is sometimes referred to as 'scour holes'. In order to prevent or reduce the formation of scour holes it is necessary to provide special measures or design the foundation in a special manner. This also adds to the

manufacturing costs of the wind turbine.

DE 10 2015 204 695 A1 discloses a monopile comprising at least two monopile segments being connected to each other in such a manner that their longitudinal axes extend substantially co-linearly. Each of the segment is provided with an open slit extending through the entire length of the segment.

WO 2017/204640 A1 discloses a wind turbine comprising a mast with a peripheral wall and an offshore support configured to support the mast. The offshore support comprises a first part comprising a monopile which is driven into a seabed and a second part which defines a throughflow part and is provided with at least one passage along a longitudinal length thereof.

WO 2010/003416 A1 discloses an offshore construction with a number of modules. One or more of the modules may be provided with a number of holes in the outer cladding such that a stream of water can pass through the module.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a longitudinal structure for an offshore wind turbine in which fatigue loads caused by impact of waves are reduced as compared to prior art longitudinal structures.

It is a further object of embodiments of the invention to provide a longitudinal structure for an offshore wind turbine in which formation of scour holes formed in the seabed around the longitudinal structure is reduced as compared to prior art longitudinal structures.

The invention provides a longitudinal structure for an offshore wind turbine, the longitudinal structure defining a longitudinal direction and the longitudinal structure comprising an outer wall enclosing a hollow interior, wherein the outer wall comprises a first region in which the outer wall is solid and a second region in which the outer wall is provided with a plurality of passages allowing fluid to pass to and from the hollow interior of the longitudinal structure, and wherein :

- the passages have a non-circular elliptic shape with the major axis

arranged substantially along the longitudinal direction of the longitudinal structure, and - the passages are arranged relative to each other in such a manner that a plurality of first passage-free areas and a plurality of second passage-free areas are formed on the outer wall of the second region, where each of the first passage-free areas extends along a direction being parallel to the longitudinal direction of the longitudinal structure, and each of the second passage-free areas extends along a direction forming an angle with the longitudinal direction of the longitudinal structure, said angle being within the range of 30° to 60°.

Thus, the present invention relates to a longitudinal structure for an offshore wind turbine. In the present context the term 'longitudinal structure' should be interpreted to mean a structure which defines a longitudinal direction. The longitudinal structure may, e.g., have a tubular, cylindrical or conical shape. The longitudinal structure may be a monopile, i.e. a structure which is driven directly into the seabed, and which is configured to have a wind turbine tower mounted thereon. Alternatively, the longitudinal structure could be a part which is attached to a separate underwater foundation, e.g. a foundation anchored in the sea bed or a floating foundation, or a part of an underwater foundation. As another alternative, the longitudinal structure may be a jacket, or it may be a part of a wind turbine tower.

In the present context the term 'offshore wind turbine' should be interpreted to mean a wind turbine which is positioned at an offshore location, i.e. at sea. The longitudinal structure is preferably at least partly submerged into the sea.

The longitudinal structure defines a longitudinal direction. The longitudinal direction preferably coincides with a longitudinal direction defined by a wind turbine tower being mounted on and supported by the longitudinal structure. Thus, when the longitudinal structure is arranged at an operating position for the wind turbine, the longitudinal direction is preferably substantially vertical.

The longitudinal structure comprises an outer wall enclosing a hollow interior.

The outer wall may have a substantially circular cross section along a direction being perpendicular to the longitudinal direction, in which case the longitudinal structure has a cylindrical or conical shape. Alternatively, the outer wall may have any other suitable cross section, such as a polygonal cross section.

The outer wall comprises a first region and a second region. In the first region the outer wall is solid, and it thereby defines a firm boundary between the hollow interior and the environment surrounding the longitudinal structure. In the second region the outer wall is provided with a plurality of passages allowing fluid to pass to and from the hollow interior of the longitudinal structure.

Accordingly, sea water surrounding the longitudinal structure is allowed to pass into the interior of the longitudinal structure, via the passages, and sea water which has entered the interior of the longitudinal structure in this manner is allowed to leave the interior of the longitudinal structure, via the passages. This has the effect that when waves hit the second part of the outer wall of the longitudinal structure, they are allowed to partly pass through the longitudinal structure. This reduces the force transferred from the waves to the outer wall of the longitudinal structure, thereby reducing the load impact on the longitudinal structure caused by the waves.

Furthermore, allowing the waves to partly pass through the longitudinal structure in the manner described above changes the turbulence pattern of the waves, and this results in reduced scour hole formation.

The passages have a non-circular elliptic shape with the major axis arranged substantially along the longitudinal direction of the longitudinal structure.

Accordingly, the passages have a shape which is not circular, but rather a shape which defines a major axis and a minor axis, where the minor axis is shorter than the major axis. The elliptic shape of the passages reduces stress introduced around the passages, and thereby weakens the longitudinal structure less than would be the case if circular passages were used.

Furthermore, the passages are arranged relative to each other in such a manner that a plurality of first passage-free areas and a plurality of second passage-free areas are formed on the outer wall of the second region. In the present context the term 'passage-free area' should be interpreted to mean a continuous area of the outer wall where there are no passages. Each of the first passage-free areas extends along a direction being parallel to the longitudinal direction of the longitudinal structure. Thus, the outer wall of the longitudinal structure comprises a plurality of continuous areas where no passages are arranged, which extend in the longitudinal direction of the longitudinal structure. Thereby a plurality of continuous paths can be defined on the outer wall, each path extending along the longitudinal direction of the longitudinal structure without intersecting a passage. Bending moments introduced in the longitudinal structure may advantageously travel along paths extending along the longitudinal direction of the longitudinal structure, and therefore the first passage-free areas are very suitable for handling such bending moments. Since there are no passages in these areas, they constitute strong parts of the longitudinal structure, and therefore the longitudinal structure is capable of efficiently handling bending moments, in spite of the presence of the passages, and in a cost efficient manner.

Each of the second passage-free areas extends along a direction forming an angle with the longitudinal direction of the longitudinal structure, where the angle is within the range of 30° to 60°. Thus, the outer wall of the longitudinal structure further comprises a plurality of continuous areas where no passages are arranged, which extend along a direction which forms an angle between 30° and 60° with the longitudinal direction of the longitudinal structure, i.e. which is not parallel to the longitudinal direction of the longitudinal structure. Thereby a plurality of continuous paths can be defined on the outer wall, each path extending along the direction forming an angle with the longitudinal direction of the longitudinal structure without intersecting a passage. Torsional moments introduced in the longitudinal structure may advantageously travel along paths extending along such a direction, and therefore the second passage-free areas are very suitable for handling such torsional moments. Since there are no passages in these areas, they constitute strong parts of the longitudinal structure, and therefore the longitudinal structure is capable of efficiently handling torsional moments, in spite of the presence of the passages, and in a cost efficient manner.

Thus, due to this specific pattern formed by the passages formed in the second part of the outer wall, the longitudinal structure is capable of efficiently handling bending moments as well as torsional moments, while reducing the fatigue loads caused by waves as well as reducing scour hole formation, due to the presence of the passages.

The passages may be arranged in rows circumferentially on the outer wall, and the passages arranged in one row may be staggered with respect to the passages arranged in a neighbouring row.

According to this embodiment, the number of passages formed in the outer wall is optimized while ensuring that the passage-free areas described above are defined.

The outer wall may have an increased material thickness in the first passage- free areas and/or in the second passage-free areas. This should be interpreted to mean that the material thickness in these parts is larger than in other parts of the longitudinal structure than in other parts of the longitudinal structure.

According to this embodiment the parts of the longitudinal structure which handle bending moments and/or the parts of the longitudinal structure which handle torsional moments are strengthened by increasing the material thickness in these specific areas. However, this is obtained by a minimal increase in material consumption, since only the parts handling such moments are

strengthened.

The angle between the second passage-free areas and the longitudinal direction of the longitudinal structure may be substantially 45°, for instance within the interval 35°-55°, such as within the interval 40°-50°. Paths extending along a direction forming such an angle with the longitudinal direction of the longitudinal structure are very suitable for handling torsional moments.

The first region and the second region of the longitudinal structure may be formed as two separate parts being joined to each other. According to this embodiment, the part of the longitudinal structure having a solid outer wall is manufactured separately from the part of the longitudinal structure having the passages formed in the outer wall, and these two parts are subsequently joined to each other, e.g. by means of welding or by means of a bolt connection. For instance, the first region and the second region may be manufactured using two different manufacturing techniques.

The second region may be arranged above the first region. According to this embodiment, the part of the longitudinal structure having the passages formed in the outer wall is arranged closer to the sea level and further away from the seabed than the part having a solid outer wall. Thereby the passages are arranged in a region where the impact from waves is expected to be highest.

At least the second region may be manufactured by casting. According to this embodiment, the part of the longitudinal structure having the passages formed in the outer wall is manufactured by casting. Thus, the second region is, in this case, made from a material which is suitable for casting. Thereby the passages may be provided directly in the outer wall during the casting process, thereby reducing material waste. Furthermore, this allows for easy customisation of the design of the passages, e.g. with respect to the rim of the passages.

As an alternative, the second region may be manufactured using any other suitable manufacturing technique, including rolling and subsequent cutting of the passages.

The first region may also be manufactured by casting, or it may be

manufactured using any other suitable manufacturing technique.

At least the second region may be formed from two or more segments being joined to each other along assembly portions extending along the longitudinal direction of the longitudinal structure. According to this embodiment, the second region is formed from at least two angularly extending segments. Each segment is manufactured separately, and possibly transported separately to the operating site of the wind turbine, where they are assembled to form the second region of the longitudinal structure. This allows for easy handling, including manufacture and transport, of even very large longitudinal structures.

The assembly portions may include flanges formed on the segments, in which case the flanges may be connected to each other by means of bolts. The assembly portions may extend through some of the passages. According to this embodiment, the assembly portions extend through parts of the longitudinal structure which are already weakened due to the presence of the passages, and the passage-free areas, which handle the bending moments and the torsional moments, are only minimally affected by the weakness introduced by the assembly portions. Furthermore, the assembly interfaces between the segments are minimised, since they are limited to the parts where there are no passages.

An upper part of the second region may be provided with one or more

attachment portions for attaching a transition platform to the longitudinal structure. According to this embodiment, a transition platform can be readily mounted on the longitudinal structure. In the case that the second region is formed by casting the attachment portions may be formed directly on the second region as part of the casting process.

Alternatively or additionally, an upper part of the second region may be provided with an access opening providing access to an interior part of a wind turbine tower being mounted on the longitudinal structure, i.e. the access opening formed in the upper part of the second region is configured to provide access to the interior of the wind turbine tower once the wind turbine tower has been mounted on the longitudinal structure. According to this embodiment, a door opening providing access to the interior of the wind turbine tower is formed directly in the longitudinal structure. Thereby such a door opening need not be provided in the wind turbine tower, and this allows for easy and cost effective manufacturing of the wind turbine tower. In the case that the second region is formed by casting the access opening may be formed directly on the second region as part of the casting process.

The passages may be provided with a reinforcement rim. According to this embodiment the passages are capable of efficiently handling the forces originating from the waves passing the longitudinal structure via the passages.

The reinforcement rim may have a hydrodynamic shape. According to this embodiment the water passing into and out of the interior of the longitudinal structure via the passages is guided in a suitable manner, thereby minimising the impact of the passing waves on the longitudinal structure, in particular in the areas surrounding the passages.

The second region may extend across a normal sea level. According to this embodiment the surface of the water meets the longitudinal structure in the second region, i.e. the region having the passages formed therein. This is advantageous since this is where the highest impact from the waves on the longitudinal structure is expected. For instance, the second region may extend from a position approximately 15 m below normal sea level to a position 20 m above normal sea level, such as from a position approximately 10 m below normal sea level to a position 15 m above normal sea level. In the present context the term 'normal sea level' should be interpreted to mean a

representative and/or average sea level, taking tidal effects etc. into account.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1 is a perspective view of a second region for a longitudinal structure, in the form of a monopile, according to an embodiment of the invention,

Fig. 2 is a side view of a segment of the second region of Fig. 1,

Fig. 3 is a perspective view of a wind turbine comprising a longitudinal structure, in the form of a monopile, according to an embodiment of the invention,

Fig. 4 illustrates a prior art longitudinal structure and a longitudinal structure according to an embodiment of the invention, respectively, during use, and

Fig. 5 shows details of passages formed in a longitudinal structure according to an embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a perspective view of a second region 1 of a longitudinal structure, in the form of a monopile, according to an embodiment of the invention. The second region 1 is made of three segments 2 joined to each other by means of flange connections 3.

The second region 1 is provided with a plurality of passages 4 allowing fluid to pass to and from a hollow interior 5 defined inside the second region 1. This has the effect that when waves hit the second region 1 of the monopile, they are allowed to partly pass through the monopile. This reduces the force transferred from the waves to the outer wall of the monopile, thereby reducing the load impact on the monopile caused by the waves. Furthermore, the scour hole formation in a region surrounding the monopile is reduced. This will be described in further detail below.

The passages 4 have a non-circular elliptic shape with the major axis arranged substantially along a longitudinal direction of the monopile, illustrated by arrow 6. This reduces stress introduced around the passages 4, and thereby ensures that the design of the second section 1 is very strong, in spite of the fact that it is provided with the plurality of passages 4.

The passages 4 are furthermore arranged relative to each other in accordance with a specific pattern. This pattern defines a number of passage-free areas, i .e. continuous areas of the second section 1 where no passages 4 are arranged. The passage-free areas can be divided into two kinds of passage-free areas, i .e. first passage-free areas and second passage-free areas, respectively. The first passage-free areas extend along a direction being parallel to the longitudinal direction 6 defined by the monopile. One of the first passage-free areas is illustrated by dashed line 7. The second passage-free areas extend along a direction forming an angle with the longitudinal direction 6 defined by the monopile, the angle being within the range of 30° to 60°. Two of the second passage-free areas are illustrated by dashed lines 8. Each of the first passage-free areas 7 defines a continuous path which extends along the direction indicated by dashed line 7, and which does not intersect any of the passages 4. Accordingly, the first passage-free areas 7 are very suitable for handling bending moments introduced in the monopile, since they constitute strong parts of the monopile via which such bending moments may

advantageously travel.

Each of the second passage-free areas 8 defines a continuous path which extends along one of the directions indicated by dashed lines 8, and which does not intersect any of the passages 4. Accordingly, the second passage-free areas 8 are very suitable for handling torsional moments introduced in the monopile, since they constitute strong parts of the monopile via which such torsional moments may advantageously travel.

Thus, the second region 1 illustrated in Fig. 1 reduces fatigue loads on the monopile caused by waves as well as reduces scour hole formation, due to the presence of the passages 4. At the same time, the second region 1 is capable of handling bending moments as well as torsional moments, due to the specific pattern formed by the passages 4, defining the passage-free areas 7, 8.

The flange connections 3 joining the segments 2 to each other pass through some of the passages 4. Thereby the flange connections 3 are formed in a part of the second region 1 which is already weakened, due to the presence of the passages 4, rather than in one of the passage-free areas 7, 8, which are supposed to handle bending moments or torsional moments. Furthermore, the total length of the flanges which have to be joined to each other to form the flange connections 3 is minimised.

Fig. 2 is a side view of one of the segments 2 of the second region 1 of Fig. 1. The first passage-free areas 7 and the second passage-free areas 8 can be clearly seen.

Fig. 3 is a perspective view of an offshore wind turbine 9 comprising a

longitudinal structure, in the form of a monopile 10, according to an

embodiment of the invention. The wind turbine 9 further comprises a wind turbine tower 11 mounted on the monopile 10 and having a nacelle 13 mounted thereon. The nacelle 13 carries a rotor 14 carrying three wind turbine blades 15 for extracting energy from the wind.

The nacelle 13 is mounted rotationally on the wind turbine tower 11, thereby allowing it to perform yawing movements in accordance with the direction of the incoming wind.

The wind turbine 9 further comprises a transition platform 16 mounted on the wind turbine tower 11. The transition platform 16 allows personnel and equipment to be transferred from a seagoing vessel and enter the interior of the wind turbine tower 11 via an access door (not shown) formed in the wind turbine tower 11 or the upper part of the monopile 10.

The monopile 10 comprises a first region 17 and a second region 1. In the first region 17 an outer wall of the monopile 10 is solid, i.e. it defines a firm

boundary between a hollow interior of the monopile 10 and the environment surrounding the monopile 10. In the second region 1 the outer wall is provided with a plurality of passages 4 allowing fluid to pass to and from the hollow interior of the monopile 10. The second region 1 of the monopile 10 could, e.g., be similar to the second region 1 illustrated in Figs. 1 and 2. In particular, the passages 4 have a non-circular elliptic shape and are arranged in a pattern defining passage-free areas, as described above with reference to Fig. 1.

As described above the passages 4 formed in the second region 1 of the monopile 10 allow fluid to pass to and from the hollow interior of the monopile 10. Thereby sea water surrounding the monopile 10 is allowed to pass into the hollow interior of the monopile 10, via the passages 4, and sea water which has entered the hollow interior of the monopile 10 in this manner is allowed to leave the hollow interior of the monopile 10, via the passages 4. This has the effect that when waves hit the second part 1 of the monopile 10, they are allowed to partly pass through the monopile 10. This reduces the force transferred from the waves to the outer wall of the monopile 10, thereby reducing the load impact on the monopile 10 caused by the waves. Furthermore, allowing the waves to partly pass through the monopile 10 in the manner described above changes the turbulence pattern of the waves, and this results in reduced scour hole formation. This will be described in further detail below with reference to Fig. 4.

The sea level at the position of the monopile 10 is indicated by arrow 18. Thus, the sea level 18 meets the monopile 10 in the second region 1, i.e. the region where the passages 4 are formed. Accordingly, the second region 1 extends across the sea level 18.

Fig. 4 illustrates a prior art longitudinal structure 10' and a longitudinal structure 10 according to an embodiment of the invention, respectively, during use. The prior art longitudinal structure 10' has an outer wall which is solid. Waves meeting the longitudinal structure 10' thereby create turbulence which in turn forms a large scour hole 19' around the longitudinal structure 10'.

The longitudinal structure 10 according to the invention comprises a first region 17 having a solid outer wall and a second region 1 provided with passages 4 as described above. Since the passages 4 allow the waves to partly pass through the longitudinal structure 10, the turbulence conditions are changed as compared to the prior art longitudinal structure 10'. This has the consequence that the scour hole 19 formed around the longitudinal structure 10 is

significantly smaller than is the case for the prior art longitudinal structure 10'.

In the longitudinal structure 10 of Fig. 4, the first region 17 and the second region 1 are formed as two separate parts which have subsequently been joined to each other.

Fig. 5 is a cross sectional view of a second region 1 of a longitudinal structure according to an embodiment of the invention, illustrating two of the passages 4 formed therein. The passages 4 are provided with a reinforcement rim 20 having a hydrodynamic shape. Thereby water passing into and out of the interior 5 of the longitudinal structure via the passages 4 is guided in a suitable manner, thereby minimising the impact of the passing waves on the longitudinal structure, in particular in the areas surrounding the passages 4.

Claims

1. A longitudinal structure (10) for an offshore wind turbine (9), the longitudinal structure (10) defining a longitudinal direction (6) and the longitudinal structure (10) comprising an outer wall enclosing a hollow interior (5), wherein the outer wall comprises a first region (17) in which the outer wall is solid and a second region (1) in which the outer wall is provided with a plurality of passages (4) allowing fluid to pass to and from the hollow interior (5) of the longitudinal structure (10), and wherein :

- the passages (4) have a non-circular elliptic shape with the major axis arranged substantially along the longitudinal direction (6) of the

longitudinal structure (10), and

- the passages (4) are arranged relative to each other in such a manner that a plurality of first passage-free areas (7) and a plurality of second passage-free areas (8) are formed on the outer wall of the second region (1), where each of the first passage-free areas (7) extends along a direction being parallel to the longitudinal direction (6) of the longitudinal structure (10), and each of the second passage-free areas (8) extends along a direction forming an angle with the longitudinal direction (6) of the longitudinal structure (10), said angle being within the range of 30° to 60°.

2. A longitudinal structure (10) according to claim 1, wherein the passages (4) are arranged in rows circumferentially on the outer wall, and wherein the passages (4) arranged in one row are staggered with respect to the passages (4) arranged in a neighbouring row.

3. A longitudinal structure (10) according to claim 1 or 2, wherein the outer wall has an increased material thickness in the first passage-free areas (7) and/or in the second passage-free areas (8).

4. A longitudinal structure (10) according to any of the preceding claims, wherein the angle between the second passage-free areas (8) and the

longitudinal direction (6) of the longitudinal structure (10) is substantially 45°.

5. A longitudinal structure (10) according to any of the preceding claims, wherein the first region (17) and the second region (1) are formed as two separate parts being joined to each other.

6. A longitudinal structure (10) according to any of the preceding claims, wherein the second region (1) is arranged above the first region (17).

7. A longitudinal structure (10) according to any of the preceding claims, wherein at least the second region (1) is manufactured by casting.

8. A longitudinal structure (10) according to any of the preceding claims, wherein at least the second region (1) is formed from two or more segments (2) being joined to each other along assembly portions (3) extending along the longitudinal direction (6) of the longitudinal structure (10).

9. A longitudinal structure (10) according to claim 8, wherein the assembly portions (3) extend through some of the passages (4).

10. A longitudinal structure (10) according to any of the preceding claims, wherein an upper part of the second region (1) is provided with one or more attachment portions for attaching a transition platform (16) to the longitudinal structure (10).

11. A longitudinal structure (10) according to any of the preceding claims, wherein an upper part of the second region (1) is provided with an access opening providing access to an interior part of a wind turbine tower (11) being mounted on the longitudinal structure (10).

12. A longitudinal structure (10) according to any of the preceding claims, wherein the passages (4) are provided with a reinforcement rim (20).

13. A longitudinal structure (10) according to claim 12, wherein the reinforcement rim (20) has a hydrodynamic shape.

14. A longitudinal structure (10) according to any of the preceding claims, wherein the second region (1) extends across a normal sea level (18).