A MECHANICAL FUSE FOR A TOWER CONSTRUCTION AND A TOWER

CONSTRUCTION COMPRISING A MECHANICAL FUSE

FIELD OF THE INVENTION

The present invention relates to the technical field of tower construction and design. More specifically the invention pertains to a mechanical fuse used in a tower construction, wherein the fuse protects the tower construction during extreme working conditions.

BACKGROUND OF THE INVENTION

Tower constructions, when exposed to extreme and/or improbable working conditions, can collapse and deform requiring significant repair or replacement of the tower construction. These tower constructions are used in numerous applications, such as wind turbines, chimneys and radio antennas.

Tower constructions have a range of measures in place to assist the structure under extreme and/or improbable conditions and thereby reduce the probability of structural collapse. In the case of wind turbines, brakes will stop the rotor and the blades feather to minimize the impact from the wind. The tower is also designed so air resistance is kept to a minimum. These design features attempt to prevent forces, induced from winds, which can affect the structural integrity of the tower construction. However, these measures may not be sufficient to deal with situations where the tower construction is exposed to improbable loads induced under extreme conditions such as e.g. seismic events, tsunami wave loading or typhoon wind conditions. Damage or failure of the tower construction would normally imply a complete replacement of the entire structure. Furthermore, in the case of wind turbine park re-powering, existing foundations would not have sufficient bending moment capacity to accommodate the base moments of larger towers.

It can therefore be advantageous if the improbable or extreme loads, induced under extreme conditions, could be absorbed locally in a controlled manner by a specially designed safety component as a part of the tower construction, and thereby avoid a complete replacement of the structure. The repair process will then only comprise a replacement of the damaged component. OBJECT OF THE INVENTION

It may be seen as an object of the present invention to provide a tower base, which in extreme and/or improbable conditions prevents further damage to the tower construction.

In particular, the objective of the invention is to provide an additional feature to the traditional substructure of the tower base, that is to deform in a controlled manner during extreme and/or improbable conditions, so to prevent any damage to the main parts of the tower construction.

An object of the present invention is to provide an alternative to the prior art.

SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a mechanical fuse at the bottom of the tower base, which is interlaid between the superstructure and substructure of the tower construction. In a first aspect, the invention relates to a tower construction comprising

an upright superstructure formed at least partly by a vertical segment extending above ground and having a base section;

a substructure which anchors the tower construction, and

a mechanical fuse having a vertical extension and which is attached to the base section of vertical segment at an upper end of the mechanical fuse and connected at a lower end to the substructure;

wherein

the mechanical fuse is configured to allow rotations in a controlled ductile manner at a smaller load exposure on the tower construction than the design load exposure which would lead to mechanical failure of the vertical segment and/or the substructure.

The present invention may allow for the protection of typical wind turbines and tower constructions in regions associated with extreme natural hazards such as earthquakes, tsunamis, and/or typhoons. The present invention may allow for the limitation of base moments, so as to allow for the placement of larger wind turbines on existing foundations, in the case of wind turbine park re-powering.

The present invention may be viewed as a complete alteration of the tower substructure interface, to accommodate for extreme events.

A fuse is normally associated with electrical circuits, acting as a safety component, activated only during critical events. The present invention may be viewed as utilizing a similar conceptual idea as for the electrical fuse. However, contrary to the electrical fuse, this invention may cover a structural component applied to or in a tower construction, acting as a mechanical fuse which could activate for critical extreme events such as e.g. seismic activity, tsunami wave loading, or typhoon wind conditions. This may lead to a sacrificial element whose primary task is to absorb the effects of an extreme event, rendering the entire tower construction intact and only damaging the fuse in a controlled manner. In case of such an event, the fuse would have to be replaced or repaired, but at low cost compared to the expense of strengthening the tower construction to

accommodate extreme natural hazards or compared to the cost of a complete failure of the tower construction. If designed appropriately, the addition of a mechanical fuse could make it possible to use conventional tower design, as for similar tower constructions exposed to normal environmental conditions, through the behaviour factor, q, and a reduction of forces.

The behaviour factor q, may be used in the design, to reduce the forces in a conventional linear elastic analysis. The behaviour factor is defined by the ratio of the forces that the structure would experience if its response was completely elastic, to the forces that the structure would actually experience in terms of the ability of the structure to plastically deform, without increasing internal forces, and still ensuring a satisfactory response of the structure.

A mechanical fuse could also be implemented in tower constructions located in regions with low or no environmental hazards, where the addition of a mechanical fuse could make it possible to upgrade the superstructure without replacing the existing substructure. Here the mechanical fuse may be seen as a safety precaution, protecting the substructure for ultimate/strength limit state loads, which exceeds the design loads that the substructure originally was designed for. In another embodiment the mechanical fuse may comprise one or more

mechanical stops, that will activate at a predefined maximum rotation, designed to prevent the fuse from reaching uncontrolled deformations, leading to the complete failure of the fuse and a resulting loss of the tower construction. This could be achieved by the addition of internal contact faces, that are only in contact at larger deformation of the mechanical fuse. It could also be achieved by use of slack cables, which at larger deformation will undergo tensioning. In one embodiment the product of the elastic section modulus times the material strength of the mechanical fuse is smaller than, such as 10% smaller, or equal to the product of the elastic section modulus times the material strength of the vertical segment (2) evaluated at the base section (3) of the vertical segment (2). As presented herein, the second moment of area (I) may be one of three parameters affecting the moment capacity, the other two may be the cross section height (z) and the material strength (fy). The elastic section modulus (W) may be equal to second moment of area (I) divided by cross section height (z); thus S=I/z. The moment capacity may be calculated as My = W * fy

The mechanical fuse could be constructed in various ways, with various structural configurations and materials. For example a ductile hollow circular reinforced concrete element could, through yielding of reinforcement, be able to satisfy the intended behaviour of a mechanical fuse.

The position of the fuse preferably typically defines the segments "superstructure" and "substructure" of the wind turbine or tower construction in general, since elements of the tower construction belonging to the substructure are positioned below the lower end of the fuse, and elements of the tower construction belonging to the superstructure are positioned above the upper end of the fuse. The substructure may in certain preferred embodiments be positioned at least partly below ground or waterline.

In one embodiment of the invention the cross sectional shape of the vertical element may be comparable, when evaluated in close proximity, to the base section. The shape of the base section and the mechanical fuse could in an advantageous embodiment be circular. The mechanical fuse could have a ratio between inner and outer diameter within 0.7-0.85.

In one embodiment the mechanical fuse could comprise of a number of vertical elements releasable assembled to form the mechanical fuse. The vertical elements could be manufactured as reinforced concrete with mechanicals stops incorporated in them.

In one embodiment of the invention, the mechanical fuse comprises a lower and upper connecting flange, which connects the lower end of the fuse with the substructure and the upper end with the vertical segments of the tower construction. This could allow for an easy repair and installation stage.

In another embodiment the fuse could be used in offshore application where the mechanical fuse is adapted to be assembled on a monopile foundation. A transitional piece is connected at the lower end of mechanical fuse. In one embodiment of the invention the tower construction could be an offshore wind turbine.

The mechanical fuse can in general be used in any application, where the tower construction comprises a sub and superstructure.

Terms are used in manner being ordinary to a skilled person. However some of these terms are elaborated below:

"Design load" is preferably used in the meaning of the forces and deformations a tower structure is designed to withstand.

" Ultimate/strength limit state" is preferably used in the meaning of a structure designed to withstand improbable forces and deformations without a total collapse.

"Extreme conditions" is preferably used in the meaning of conditions, which is associated with the ultimate/strength limit state in regions with extreme environmental hazards. Such extreme conditions could be e.g. seismic activity, tsunami wave loading, or typhoon wind conditions.

"Equivalent diameter” is preferable used in the meaning of the circumference divided by p (pi).

"Elastic section modulus” is preferably used in the meaning of the modulus of the second moment of area divided by the cross section height. The elastic section modulus (W) is preferably equal to second moment of area (I) divided by cross section height (z); thus S=I/z.

"Behaviour factor" is preferable used in the meaning of a factor, which accounts for the plastic capacity of a structure, and may be used in the design to reduce the forces in a conventional linear elastic analysis.

According further embodiment of a mechanical fuse for use in a tower

construction, the mechanical fuse may comprise a ductile section, a mechanical stop and one or more connecting pieces. According to such embodiments, the ductile section may allow controlled rotations at a predefined load exposure threshold and the mechanical stop may prevent rotations over a certain threshold. In some preferred embodiments, the majority of the mechanical element of a tower, such as 50% or more, may be designed to act as a ductile plastic hinge.

Further embodiments and aspects are presented in the following and in the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

The present invention and in particular preferred embodiments thereof will now be described in more details with regard to the accompanying figures. The figures show ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set: Figure 1 schematically illustrates a first embodiment of a tower construction comprising a mechanical fuse being connected to a substructure at its lower end and connected to the vertical segment at its upper end,

Figure 2 schematically illustrates a second embodiment of a tower construction, illustrated as a wind turbine, comprising a mechanical fuse being connected to a transition piece of a monopile foundation at its lower end and to the vertical segment at its upper end,

Figure 3 is a graph illustrating a working principle of a mechanical fuse according to the invention,

Figure 4 schematically illustrates a tower construction, illustrated as a wind turbine, without a mechanical fuse (left hand side of figure 4), which has experienced buckling, and a tower construction with a mechanical fuse (right hand side of figure 4), which has experienced a load activating the fuse; figure 4b illustrates close-ups of base sections of the towers of figure 4a,

Figure 5 schematically illustrates a mechanical fuse according to one embodiment in which the mechanical fuse comprises a number of vertical elements,

Figure 6 schematically illustrates a mechanical fuse according one embodiment in which the mechanical fuse comprises a number of mechanical stops,

Figure 7 schematically illustrates a tower construction, illustrated as an off-shore wind turbine, comprising a monopile as base foundation, a mechanical fuse and a transition piece connecting the monopile and the mechanical fuse; figure 7b is a close-up, cross sectional view of the mechanical fuse shown in figure 7a, and

Fig. 8 schematically illustrates another embodiment of a mechanical fuse according the present invention; figure 8a illustrates the mechanical fuse in 3D- view whereas figure 8b illustrates a 3D-cross sectional view of the mechanical fuse. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 shows schematically an embodiment of the mechanical fuse (10) used in connection with a tower construction (1). In the embodiment shown, the tower construction consist of a vertical segment (2), connected to the mechanical fuse (10), connected to the substructure (4), which anchors the tower construction (1). The tower structure could in an embodiment be a mast, a chimney or a cantilever structure.

The mechanical fuse (10) comprises such as consists of a lower end (5) connected to the substructure by means of a connecting flange (6) and to the base section (3) of the vertical element (2) by means of a connecting flange (6a).

The mechanical fuse (10) will, when operating in extreme and/or improbable conditions, deform plastically and protect the tower construction (1). The plastic deformation will occur from a rapid decrease of the stiffness of the fuse that will not allow further increases of sectional forces across the fuse. This feature will prevent damage to the tower and/or substructure and avoid complete

replacement of the structure.

In another embodiment of the invention the fuse is configured to allow large rotations in a controlled ductile manner at a smaller load exposure on the tower construction (1), than the design load exposure which would lead to mechanical failure of the vertical segment (2) and/or the substructure (4). This would have the advantage that the mechanical fuse (10) could be used in connection with existing foundation/substructure (4), when an upgrade to a larger superstructure is required. The mechanical fuse (10) would prevent damage to the wind turbine and/or substructure by activating before a critical load would occur. It would also activate under extreme and/or improbable conditions where the tower

construction could potentially experience forces that would lead to damaging or mechanical failure of one of the main parts of the tower construction.

In one embodiment, the design load exposure would correspond to loads according to the extreme events of the ultimate/strength limit state design. This would have the effect that the fuse is only activated in situations where an activation is warranted and a structural collapse of the tower construction (1) is imminent.

In another embodiment, the ratio between the bending moment and the moment capacity, being the minimum of either the maximum elastic bending moment or the moment at which buckling occurs, is higher for the mechanical fuse (10) compared to any section of the vertical segment (2).

Figure 2 shows schematically one embodiment of the mechanical fuse (10) used in connection with a tower construction (1), illustrated as a wind turbine. In the embodiment shown in Figure 2, the substructure consists of a monopole (16) and a transition piece (14).

The mechanical fuse (10) consists of a lower end (5) connected to the transition piece (14) by means of a connecting flange (6b) and to the base section (3) of the vertical element (2) by means of a connecting flange (6a).

Figure 3 shows the working principle of the fuse, by a moment-rotation

relationship, connected to the tower base. It shows the relation between the base rotation and the base moment, M. It is shown in the graph that rotations in the fuse will, under normal working conditions, be within the elastic range with incremental increases in the rotation as a function of the base moment. When the base moment exceeds the value governed by normal design loads, the rotation of the fuse will increase rapidly, allowing for large displacements of the tower, without reaching bending moments which would damage the tower.

The equivalent relationship of the tower base without the mechanical fuse is also shown in Figure 3. If the tower construction is exposed to loads beyond the normal design loads, the tower may either undergo plastic deformations or buckle. The invention should not be considered limited to the case seen in figure 3, where the stiffness of the mechanical fuse during normal conditions is lower than the stiffness of the tower base.

It is shown that the maximum bending moment of the mechanical fuse is aligned with the maximum elastic bending moment of the tower (2), hence controlling that damage will be located within the mechanical fuse and protecting the tower from any permanent damage in terms of plastic deformations. The invention should not be considered limited to the case where the critical moment is the elastic moment capacity of the tower base. The critical moment can be governed by the minimum moment at which any section of the tower is damaged, e.g. undergoes yielding or buckling. The bending moment capacity of the mechanical fuse does not necessarily have to be lower than or aligned with the minimum moment at which the tower is damaged. At large plastic rotations of the mechanical fuse, the moment capacity of the fuse might be higher than the minimum damaging moment of the tower. However, this would define the limit at which the fuse no longer fully relieves the tower.

During extreme working conditions or improbable load scenarios, with loads exceeding the design loads, the fuse becomes active and all plastic deformations are concentrated in the fuse. No plastic deformations occur in the tower or substructure. During the most extreme conditions, the fuse could possibly reach severe deformations. Before the fuse reaches its ultimate plastic limit and undergoes uncontrolled deformations, the inclusion of a mechanical stop will deactivate the fuse, and all further deformations will result from plastic

deformations in the tower and/or substructure. This will induce permanent damage to the tower construction, and the damaged part would have to be replaced along with the mechanical fuse. However, utilizing the plastic

deformation capacity of the tower and/or substructure could prevent damage to additional components, e.g. the rotor and nacelle in case of the tower construction being a wind turbine. Accordingly, the mechanical stop may be viewed as an additional safety measure, in case of severe extreme loads that the fuse cannot even handle.

Figure 4 shows the effect the fuse has on the tower construction (1). In figure 4a the tower construction (1) has been subjected to extreme and/or improbable working conditions resulting in a deformation of the base section (3). In figure 4b is shown the difference between the scenario where the mechanical fuse (10) is present and absent. It is shown that the fuse deforms in a controlled manner and keeps the tower (1) intact. Figure 5 shows an embodiment of the mechanical fuse (10), whereby the fuse is constructed using several vertical elements (12), which is releasable assembled. The vertical elements could be constructed so the cross sectional shape is, in close proximity, similar to the base section of the tower. In this embodiment the cross sectional shape is circular, but could also take other forms polygonal, such as rectangular, elliptical or combinations thereof. In the longitudinal direction of the mechanical fuse, the shape may be conical, straight, tapered, polygonal, hyperboloid or combinations thereof. Further, the mechanical fuse may be made as a lattice structure.

In one embodiment of the mechanical fuse (10), the ratio between the inner and other diameter is within the range of 0.7-0.85.

In one embodiment, the vertical segment (2) is constructed using a ductile material, such as metal, and the mechanical fuse (10) is made from a reinforced or a composite material, such as a ductile reinforced concrete material.

In another embodiment the mechanical fuse is a non-tubular element. By non tubular element is preferably meant that the mechanical fuse an integral part having no voids.

In one embodiment the mechanical fuse is designed so the product of the elastic section modulus times the material strength of the mechanical fuse is smaller than, such as 10% smaller, or equal to the product of the elastic section modulus times the material strength of the vertical segment (2) evaluated at the base section (3) of the vertical segment (2).

Figure 6 shows a cross-section of the mechanical fuse (10) as depicted in the figure. It is shown that, in an embodiment of the fuse, a mechanical stop (20) is incorporated in the mechanical fuse (10), preventing plastic rotation of the fuse to exceed a pre-defined threshold. The mechanical fuse could exceed this pre defined threshold, but then it will be followed by a large increase in the sectional moment, thus the 'deformations' (rotations) will no longer be plastic, which the mechanical stop will prevent. Figure 7 shows an embodiment of the mechanical fuse (10) in relation to an offshore wind turbine. The mechanical fuse is connected to a transitional piece and the offshore wind turbine. In figure 7a the specific embodiment of a tower construction can be seen. The tower construction consists of the offshore wind turbine, the mechanical fuse (19), the transition piece (14) and the monopile (16). In figure 7b it is shown how the mechanical fuse is assembled with the offshore wind turbine and the monopile transition piece.

Reference is made to fig. 8 and in particular figure 8B illustrating a cross sectional view of an embodiment of a mechanical fuse 10 according to a further

embodiment of the invention. As illustrated, the mechanical fuse comprising a tubular shaped part 22. In the shown embodiment, the tubular part 22 is a funnel shaped part 22 with increasing cross section in upward direction. At the lower end of the funnel shaped part 22 an upwardly extending protrusion 23 tapering in upward direction is arranged. In the embodiment shown, the funnel shape part 22 and the protrusion 23 are shaped as truncated cones with the protrusion 23 arranged co-axial with the funnel shaped part 22.

At the upper end of the protrusion, a bearing 24 is provided. In the embodiment shown the bearing 24 is in the form ball.

The fuse 10 also comprises an upper part 25 in mechanical connection with the upwardly extending protrusion. In the embodiment of fig. 8 the mechanical connection is provided by the upper part 25 resting on the bearing 24. In the embodiment shown, the bottom of the upper part 25 is funnel shaped providing a bearing surface for the ball 24. As illustrated, the horizontal dimension of the upper part 25 is smaller than the horizontal dimension of the funnel shaped part 22 so as to accommodate elongate flexible elements 26 as detailed below. In the embodiment shown, the cross section of the upper part 25 as well as the cross sections of the funnel shaped part are circular, but other shapes such as polygonal shapes may be used.

Elongate flexible elements 26 are arranged extending substantially horizontal between the funnel shaped part 22 and the upper part 25 at an elevated position relatively to the lower end of the funnel shaped part 22. In the embodiment shown, the elongate flexible elements 26 are arranged at the upper end of the funnel shaped part 23. In the embodiment of fig. 8, distal ends (relatively to the upper part 25) of flexible elements 26 are received in a groove 27 and proximal ends of the flexible elements 26 are attached to the upper part 25, e.g. by being bolted to the upper part 25. Thus, in the shown embodiment, the flexible elements 26 element 26 are releasable attached to the funnel shaped part 22 and to the upper part 25. The flexible elements 26 are typically arranged edge-by- edge but it is within the scope of the invention to arrange the flexible elements 26 with a distance between neighbouring elements.

As also illustrated, the upper part 25 is bolted to the vertical segment 2 and the funnel shaped part is bolted to the substructure 4.

As can be readily understood from the description and fig. 8, the fuse 10 provides a flexible joint between the vertical segment 2 and the substructure 4, wherein the vertical load is at least partly transferred to the substructure 4 through the bearing 24 and where the flexible elements 26 provide a ductile structure counteracting a moment provided by rotating (tilting) the vertical segment 2 by the flexible elements 26 working in bending.

Thus, the mechanical fuse 10 is also in this embodiment configured to allow rotations in a controlled ductile manner at a smaller load exposure on the tower construction than the design load exposure which would lead to mechanical failure of the vertical segment 2 and/or the substructure 4. Further, in case a mechanical failure arises in the fuse 10 due to load exposures, the fuse is designed so that failures occur in one or more of the flexible elements 26. These flexibles elements 26 are relatively easy to replace, whereby an easy to repair mechanical fuse is provided.

The flexible elements 26 are typically made from metal such as steel, but other material may be used. The flexible elements 26 and the other parts of the fuse is dimensioned according to specific requirements and structural details of the tower, such dimensioning is considered to be available to the skilled person. Kindly observe that the arrow in fig. 8 with no label points at a guide line not forming part of the fuse but is drawn to provide a better 3D representation of the fuse.

List of reference symbols used :

1 Tower construction

2 Vertical segment

3 Base section

4 Substructure

5 Lower end of mechanical fuse

6a, 6b Connecting flange of mechanical fuse

10 Mechanical fuse

12 Vertical elements

14 Transition piece

16 Monopile

20 Mechanical stop

22 Funnel shaped part

23 Protrusion

24 Bearing

25 Upper part

26 Flexible elements

27 Groove

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.