A method for assembling a wind turbine tower
The method of using cover elements to block flange holes during alignment addresses the safety and cost issues in wind turbine tower assembly, facilitating efficient and safe alignment of tower sections.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- VESTAS WIND SYSTEMS AS
- Filing Date
- 2026-01-15
- Publication Date
- 2026-07-23
Smart Images
Figure DK2026050008_23072026_PF_FP_ABST
Abstract
Description
[0001] A METHOD FOR. ASSEMBLING A WIND TURBINE TOWER
[0002] FIELD OF THE INVENTION
[0003] The present invention relates to a method for assembling a wind turbine tower, the wind turbine tower comprising at least two tower sections. The method according to the invention allows for safe and easy assembly of the wind turbine tower.
[0004] BACKGROUND OF THE INVENTION
[0005] When assembling or erecting wind turbine towers, multiple tower sections are often mounted one at a time, so that each tower section is mounted on top of the tower section mounted immediately previously. To this end, the next tower section may be hoisted to a position above the previously mounted tower section, lowered into abutment or contact with the previously mounted tower section, and attached thereto.
[0006] The tower sections may be provided with flanges, and in this case they are attached to each other by attaching an upper flange of the previously mounted tower section to a lower flange of the next tower section by means of suitable fastening elements, such as bolts. In this case the fastening elements are each passed through holes formed in the respective flanges, and it is therefore necessary that the tower sections are aligned, in the sense that these holes need to be positioned in alignment with each other, in order to allow the fastening means to pass through holes in both flanges. Moreover, other parts of the tower sections, such as tower ladders mounted in the interior of the tower sections, may need to be aligned.
[0007] In order to ensure the correct aligning of the tower sections, it will often be necessary to adjust the position of the subsequent tower section relative to the previously mounted tower section, after the subsequent tower section has been hoisted to the mounting position above the previously mounted tower section.For instance, the subsequent tower section may be rotated relative to the previously installed tower section until alignment has been obtained. This may require the presence of personnel at or near the position where the flanges are being attached to each other. In this case it is important that it is safe for the personnel to be present. For instance, it may be required that the distance between the flanges during manipulation of the subsequent tower section does not exceed a certain safety distance, thus preventing that the personnel is caught between the flanges.
[0008] In order to facilitate the attachment of the flanges to each other, it may be desirable to arrange the fastening elements in the lower flange of the subsequent tower section before hoisting the subsequent tower section to the installing position. However, in this case the fasteners may extend below the lower flange of the subsequent tower section by a longer distance than the specified safety distance between the flanges. In this case, when the position of the subsequent tower section is adjusted into alignment with the previous tower section, the fasteners may fall into the holes formed in the upper flange of the previous tower section, thus essentially preventing the alignment of the tower sections.
[0009] DESCRIPTION OF THE INVENTION
[0010] It is an object of embodiments of the invention to provide a method for assembling a wind turbine tower in which the assembly of two tower sections can be performed in a safe, easy and cost effective manner.
[0011] It is a further object of embodiments of the invention to provide a wind turbine tower assembly which allows for safe, easy and cost effective assembly of tower sections.
[0012] According to a first aspect the invention provides a method for assembling a wind turbine tower, the wind turbine tower comprising at least two tower sections, the method comprising the steps of:- mounting a first tower section on a wind turbine foundation structure or on a previously installed tower section, the first tower section comprising an upper flange provided with a plurality of holes, each configured to receive a fastening element, the upper flange being arranged at or near an upper end of the first tower section,
[0013] - arranging at least one cover element adjacent to the upper flange of the first tower section, the at least one cover element covering the plurality of holes of the upper flange,
[0014] - providing a second tower section, the second tower section comprising a lower flange provided with a plurality of holes, each configured to receive a fastening element,
[0015] - arranging fastening elements in at least some of the holes of the lower flange of the second tower section,
[0016] - hoisting the second tower section to a position above the first tower section with the lower flange of the second tower section facing the upper flange of the first tower section, and arranging the fastening elements in abutment with the at least one cover element,
[0017] - moving the second tower section into alignment with the first tower section,
[0018] - removing the at least one cover element, allowing the fastening elements to enter the holes of the upper flange of the first tower section, and attaching the second tower section to the first tower section by means of the fastening elements.
[0019] Thus, the method according to the first aspect of the invention is a method for assembling a wind turbine tower comprising at least two tower sections.
[0020] In the method according to the first aspect of the invention, a first tower section is initially mounted on a wind turbine foundation structure of on a previouslyinstalled tower section. In the present context the term 'wind turbine foundation structure' should be interpreted to mean a wind turbine foundation, a part of a wind turbine foundation or a part being mounted on or attached to a wind turbine foundation, and onto which a lowermost tower section is mounted. For instance, the wind turbine foundation structure may be a transition piece or a similar structure interconnecting a wind turbine foundation, e.g. an offshore wind turbine foundation, and the wind turbine tower.
[0021] The first tower section comprises an upper flange provided with a plurality of holes, each configured to receive a fastening element. In the present context, the term 'upper flange' should be interpreted to mean a flange arranged at an end of the first tower section, where the end forms the uppermost portion of the first tower section when the first tower section has been mounted on the wind turbine foundation structure or the previously installed tower section, and thus forms part of the wind turbine tower being assembled. Accordingly, the upper flange is arranged at or near an upper end of the first tower section. The upper flange may, e.g., be an inner flange, an outer flange or a T-flange.
[0022] Next, at least one cover element is arranged adjacent to the upper flange of the first tower section. The at least one cover element covers the plurality of holes of the upper flange. Accordingly, the at least one cover element essentially blocks the plurality of the holes of the upper flange in the sense that it prevents objects from entering the holes. All of the plurality of holes may be covered by a single cover element. Alternatively, two or more cover elements may be applied.
[0023] Next, a second tower section is provided. The second tower section comprises a lower flange provided with a plurality of holes, each configured to receive a fastening element. In the present context the term 'lower flange' should be interpreted to mean a flange arranged at an end of the second tower section, where the end forms the lowermost portion of the second tower section when the second tower section forms part of the wind turbine tower being assembled. The lower flange may, e.g., be an inner flange, an outer flange or a T-flange.
[0024] Next, fastening elements, e.g. in the form of bolts, such as stud bolts, are arranged in at least some of the holes of the lower flange of the second towersection. Fastening elements may be arranged in each of the holes. Alternatively, only some of the holes may be provided with fastening elements, while other holes may be left empty.
[0025] The second tower section is then hoisted to a position above the first tower section with the lower flange of the second tower section facing the upper flange of the first tower section. Accordingly, the upper flange of the first tower section and the lower flange of the second tower section, with the fastening elements mounted therein, are arranged immediately opposite each other.
[0026] Moreover, the fastening elements mounted in the holes of the lower flange of the second tower section are arranged in abutment with the at least one cover element. Accordingly, the fastening elements, e.g. free ends of the fastening elements, touch or rest on the at least one cover element. However, since the at least one cover element covers the plurality of holes of the upper flange of the first tower section, the fastening elements are prevented from entering the holes of the upper flange of the first tower section.
[0027] Next, the second tower section is moved into alignment with the first tower section. Thus, the second tower section is moved relative to the first tower section until the holes of the lower flange of the second are positioned adjacent to, i.e. in alignment with, the holes of the upper flange of the first tower section. For instance, the second tower section may be rotated relative to the first tower section until the respective holes are aligned.
[0028] Due to the at least one cover element covering the holes of the upper flange of the first tower section, the fastening elements are prevented from entering the holes of the upper flange of the first tower section during the movement of the second tower section relative to the first tower section, even if they are moved past the holes. Accordingly, the second tower section can be moved essentially unrestricted and easily. Moreover, this can be done while the distance between the upper flange of the first tower section and the lower flange of the second tower section is smaller than the length of the part of the fastening elements protruding below the lower flange of the second tower section, and for instance does not exceed a specified safety distance.Finally, when the first tower section and the second tower section are aligned, the at least one cover element is removed, thus allowing the fastening elements to enter the holes of the upper flange of the first tower section. Thus, each fastening element now protrudes through a hole of the lower flange of the second tower section as well as a hole of the upper flange of the first tower section. The second tower section is then attached to the first tower section by means of the fastening means.
[0029] Accordingly, the tower sections are attached to each other in an easy and cost effective manner, since the second tower section can be hoisted to the installing position with the fastening elements mounted in the holes of the lower flange thereof, without risking that the fastening elements enter the holes of the upper flange of the first tower section during alignment of the first tower section and the second tower section. Moreover, the alignment can be performed while the distance between the flanges of the tower sections is small, thus ensuring that the assembly can be performed in a safe manner.
[0030] The step of removing the at least one cover element may comprise slidingly moving the at least one cover element along a substantially sideways direction.
[0031] According to this embodiment, once the second tower section has been aligned with the first tower section, the at least one cover element is pushed or pulled away from the flanges along a sideways direction, i.e. along a direction which is substantially perpendicular to a length direction of the tower sections. For instance, the at least one cover element may be moved along a direction being substantially parallel to surfaces defined by the flanges facing each other. This is an easy way of removing the at least one cover element, so as to allow the fastening elements to enter the holes of the upper flange of the first tower section, and it will not require any movement of the second tower section. In particular, it is not necessary to lift the second tower section, and it can therefore remain within a safety distance from the first tower section during this step.
[0032] In order to allow the at least one cover element to be moved slidingly along the substantially sideways direction, the at least one cover element may bearranged along the upper flange of the first tower section in such a manner that it does not protrude into the holes of the upper flange. This will allow the at least one cover element to slide along an upper surface of the upper flange of the first tower section.
[0033] Each of the at least one cover element may be in the form of a plate like element. In the present context the term 'plate like element' should be interpreted to mean an element with a substantially planar shape, i.e. an element with a small thickness compared to the extension of the element parallel to a plane being perpendicular to a direction defined by the thickness of the element. Such an element is very suitable for being positioned along a surface of a flange, so as to cover holes formed in the flange without protruding into the holes. Furthermore, such an element is very suitable for being removed slidingly along a substantially sideways direction.
[0034] The at least one cover element may have a low friction surface. In the present context the term 'low friction surface' should be interpreted to mean a surface which gives rise to limited friction when moving along another surface. For instance, the low friction surface of the at least one cover element may be relatively smooth, i.e. having a low surface roughness, e.g. corresponding to stainless steel. For instance, the coefficient of kinetic friction between the at least one cover element and the upper flange of the first tower section may be small, such as below 0.5.
[0035] Thus, according to this embodiment, the at least one cover element can be moved slidingly along the upper surface of the upper flange of the first tower section with limited friction, and the force required to remove the at least one cover element is therefore minimised. Moreover, the fastening elements can easily slide along the at least one cover element during the aligning movement of the second tower section.
[0036] Each of the at least one cover element may be configured to cover at least two of the holes of the upper flange of the first tower section. For instance, a single cover element may cover all of the holes of the upper flange of the first towersection. As an alternative, two or more cover elements may be applied, each covering at least two of the holes of the upper flange of the first tower section.
[0037] Thus, according to this embodiment, a separate cover element is not provided for each of the holes of the upper flange of the first tower section. This limits the number of cover elements that need to be removed in order to allow the fastening elements to enter the holes of the upper flange of the first tower section. Accordingly, this facilitates the step of removing the at least one cover element, thus allowing for a faster and more cost effective process.
[0038] The step of arranging the at least one cover element adjacent to the upper flange of the first tower section may comprise temporarily fixing the at least one cover element to the upper flange of the first tower section. According to this embodiment, the at least one cover element is prevented from accidently moving or shifting during the step of moving the second tower section into alignment with the first tower section. Thus, it is ensured that the holes of the upper flange of the first tower section remain covered as long as this is needed. However, since the fixation of the at least one cover element is temporary, the at least one cover element can still be easily removed when the step of moving the second tower section into alignment with the first tower section has been completed.
[0039] For instance, the at least one cover element may be temporarily fixed to the upper flange of the first tower section by means of at least one magnet. This is an easy and cost effective way of fixing the at least one cover element to the upper flange of the first tower section, and it requires no tools or modifications to the flange. Moreover, the at least one cover element can easily be released from the flange, simply by applying a slightly larger force to the at least one cover element than would be required if the at least one cover element was not temporarily fixed to the flange.
[0040] As an alternative, the at least one cover element may be temporarily fixed to the upper flange of the first tower section by means of other suitable fixation means, such as clamps, brackets, straps, hooks, etc.The step of hoisting the second tower section and arranging the fastening elements in abutment with the at least one cover element may cause the fastening elements to be pushed in an upwards direction.
[0041] According to this embodiment, the length of the part of the fastening elements which extends below the lower flange of the second tower section exceeds the maximum desirable or allowable distance between the upper flange of the first tower section and the lower flange of the second tower section. Accordingly, when the second tower section is hoisted into position above the first tower section, the free ends of the fastening elements are initially brought into contact with the at least one cover element, while the second tower section is still arranged at a higher level than a maximum desirable or allowable level. As the second tower section is lowered further towards the first tower section, so as to further reduce the distance between the flanges, the free ends of the fastening elements are retained or supported by the at least one cover element, thus preventing the fastening elements from moving further downwards. Therefore, the fastening elements are pushed upwards inside the holes of the lower flange of the second tower section.
[0042] Accordingly, during the step of moving the second tower section into alignment with the first tower section, the free ends of the fastening elements truly rest on the at least one cover element. Moreover, when the at least one cover element is subsequently removed, the fastening elements will automatically move in an opposite, i.e. downwards direction, and fall into the holes of the upper flange of the first tower section due to gravity.
[0043] The step of hoisting the second tower section may comprise positioning the lower flange of the second tower section at a distance from the upper flange of the first tower section which is lower than a maximum safety distance. As described above, this ensures that it is safe for personnel to be present at or near the flanges of the tower sections, at least during the step of moving the second tower section into alignment with the first tower section.
[0044] The maximum safety distance may, e.g., be dictated by regulations, and it may be selected in such a manner that the risk of injuries to personnel is minimisedwhile still allowing the second tower section to move relative to the first tower section in order to bring the tower sections into alignment. For instance, the maximum safety distance may be within the interval 25 mm to 75 mm, such as within the interval 30 mm to 70 mm, such as within the interval 40 mm to 60 mm, such as approximately 50 mm.
[0045] The step of moving the second tower section into alignment with the first tower section may be performed while sliding the fastening elements along the at least one cover element.
[0046] According to this embodiment, a part of each fastening element, e.g. free ends thereof, rest upon or are supported by the at least one cover element once they have been brought into contact therewith during the step of hoisting the second tower section. During the subsequent step of moving the second tower section into alignment with the first tower section, the fastening elements remain in contact or abutment with the at least one cover element. Since they are mounted in the holes of the lower flange of the second tower section, they will be moved along with the second tower section, relative to the first tower section, and thus relative to the at least one cover element. Accordingly, the fastening elements will slide along a surface of the at least one cover element.
[0047] The step of moving the second tower section into alignment with the first tower section may be performed at least partly manually. According to this embodiment, the presence of personnel at or near the flanges of the tower sections is required during the step of moving the second tower section into alignment with the first tower section, and it is therefore particularly important that safety distances between the flanges of the tower sections are observed.
[0048] The step of mounting a first tower section may comprise mounting the first tower section on a transition piece, the transition piece comprising an upper flange provided with a plurality of holes, each configured to receive a fastening element, and may further comprise the steps of:arranging at least one cover element adjacent to the upper flange of the transition piece, the at least one cover element covering the plurality of holes of the upper flange,
[0049] - arranging fastening elements in at least some of a plurality of holes provided in a lower flange of the first tower section,
[0050] - hoisting the first tower section to a position above the transition piece with the lower flange of the first tower section facing the upper flange of the transition piece, and arranging the fastening elements in abutment with the at least one cover element,
[0051] - moving the first tower section into alignment with the transition piece,
[0052] - removing the at least one cover element, allowing the fastening elements to enter the holes of the upper flange of the transition piece, and attaching the first tower section to the transition piece.
[0053] According to this embodiment, the first tower section is the lowermost tower section of the wind turbine tower being assembled, and it is therefore mounted on a transition piece rather than on a previously installed tower section. In the present context the term 'transition piece' should be interpreted to mean an element which is mounted on or attached to the foundation of the wind turbine, and onto which the wind turbine tower is mounted. Accordingly, the transition piece forms a connection between the wind turbine foundation and the wind turbine tower, and it may be regarded as forming part of the wind turbine foundation structure. Transition pieces are often applied in offshore wind turbines.
[0054] The first tower section is mounted on the transition piece in a manner which is very similar to the manner in which the second tower section is mounted on the first tower section, and as described above. In particular, the step of mounting the first tower section on the transition piece includes arranging at least one cover element adjacent to an upper flange of the transition piece, so that the at least one cover element covers a plurality of holes formed in the flange.Moreover, the first tower section is hoisted into position, along with fastening elements mounted in a lower flange thereof, and moved into alignment with the transition piece, essentially as described above with reference to the second tower section. Finally, the at least one cover element is removed, thus allowing the fastening elements to enter the holes of the upper flange of the transition piece, also essentially as described above with reference to the second tower section. Accordingly, the remarks set forth above with reference to the mounting of the second tower section are equally applicable here.
[0055] The fastening elements may be stud bolts. Alternatively, the fastening elements may be of any other suitable kind, such as other kinds of bolts, e.g. HV bolts (i.e. high-strength or 'Hochfest vorgespannt' bolts), threaded rods being provided with separate nuts, etc. For instance, bolts as used and disclosed in EP 3 973 178 Bl may be used.
[0056] According to a second aspect the invention provides a wind turbine tower assembly comprising:
[0057] - a first tower section comprising an upper flange provided with a plurality of holes, each configured to receive a fastening element, the upper flange being arranged at or near an upper end of the first tower section,
[0058] - a second tower section comprising a lower flange provided with a plurality of holes, each configured to receive a fastening element, at least some of the holes of the lower flange of the second tower section having a fastening element mounted therein,
[0059] - at least one cover element arranged adjacent to the upper flange of the first tower section, the at least one cover element covering the plurality of holes of the upper flange,
[0060] wherein the second tower section is arranged at a position above the first tower section with the lower flange of the second tower section facing the upper flange of the first tower section, and with the fastening elements arranged in abutment with the at least one cover element.The wind turbine tower assembly according to the second aspect of the invention is very suitable for being assembled by means of a method according to the first aspect of the invention. The remarks set forth above with reference to the first aspect of the invention are therefore equally applicable here.
[0061] BRIEF DESCRIPTION OF THE DRAWINGS
[0062] The invention will now be described in further detail with reference to the accompanying drawings in which
[0063] Figs. 1-5 illustrate method steps of a method according to an embodiment of the invention, and
[0064] Fig. 6 is a top view of a tower section forming part of a wind turbine tower assembly according to an embodiment of the invention.
[0065] DETAILED DESCRIPTION OF THE DRAWINGS
[0066] Figs. 1-5 illustrate method steps of a method according to an embodiment of the invention. Fig. 1 illustrates a tower section 1 with a lower flange 2, the tower section 1 being arranged in an upright position, i.e. with its longitudinal direction arranged substantially vertically. The tower section 1 is held by a crane 3 and lifted slightly off the ground. A number of fastening elements 4, in the form of bolts, have been arranged in holes 5 formed in the lower flange 2. The fastening elements 4 extend below the lower flange 2 by a distance which exceeds 50 mm. The tower section 1 forms a second tower section in the sense of the claimed invention, and it is about to be installed in a wind turbine tower assembly.
[0067] In Fig. 2, the second tower section 1 of Fig. 1 has been hoisted, by means of the crane 3, to a position above a first tower section 6 which has already been mounted on a wind turbine foundation structure (not shown) or on a previously installed tower section (not shown).The first tower section 6 comprises an upper flange 7 which is provided with a plurality of holes 8, each configured to receive a fastening element 4. At least one cover element 9, in the form of at least one plate like member, has been arranged adjacent to the upper flange 7, in such a manner that the holes 8 formed in the upper flange 7 are covered.
[0068] A tower ladder 10 is mounted on an inner wall surface of the first tower section 6, and another tower ladder 11 is mounted on an inner wall surface of the second tower section 1. The tower ladders 10, 11 are not aligned.
[0069] The at least one cover element 9 prevents the fastening elements 4 mounted in the holes 5 of the lower flange 2 of the second tower section 1 from entering the holes 8 formed in the upper flange 7 of the first tower section 6. Furthermore, the second tower section 1 is arranged above the first tower section 6 in such a manner that the distance between an upper surface of the at least one cover element 9 and a lower surface of the lower flange 2 is less than 50 mm, i.e. less that the distance at which the fastening elements 4 extend below the lower flange 2. This is in order to ensure the safety of personnel that might be present at or near the tower sections 1, 6. As can be seen, the mutual positions of the tower sections 1, 6 causes the fastening elements 4 arranged in the holes 5 of the lower flange 2 of the second tower section 1 to be pushed back through the holes 5 in an upwards direction.
[0070] The free ends of the fastening elements 4 are arranged in abutment with the at least one cover element 9, i.e. they rest on the at least one cover element 9. This will allow the second tower section 1 to be moved relative to the first tower section 6 in order to align the tower sections 1, 6, without risking that the fastening elements 4 fall into the holes 8 formed in the upper flange 7 of the first tower section 6. Instead, the fastening elements 4 may slide along the at least one cover element 9 during the movement of the second tower section 1. For instance, the second tower section 1 may be rotated relative to the first tower section 1 in order to align the tower ladders 10, 11 of the respective tower sections 1, 6.The at least one cover element 9 may be temporarily fixed to the upper flange 7 of the first tower section 6, e.g. by means of magnets (not shown). This ensures that the at least one cover element 9 is not accidentally moved or shifted during the aligning movement of the second tower section 1.
[0071] In Fig. 3, the second tower section 1 has been rotated relative to the first tower section 6 in the manner described above. Thus, the tower ladder 10 mounted on the first tower section 6 and the tower ladder 11 mounted on the second tower section 1 are now aligned. Furthermore, the rotation of the second tower section 1 has brought the holes 5 formed in the lower flange 2 of the second tower section 1 into alignment with the holes 8 formed in the upper flange 7 of the first tower section 6. The free ends of the fastening elements 4 are still resting against the at least one cover element 9.
[0072] In Fig. 4, the at least one cover element has been removed upon completion of the rotation of the second tower section 1. This could, e.g., be done by slidingly moving the at least one cover element 9 along a substantially sideways direction, i.e. along a direction being substantially perpendicular to the length direction of the tower sections 1, 6. Due to the alignment of the holes 5, 8, the removal of the at least one cover element 9 has caused the fastening elements 4 to enter the holes 8 formed in the upper flange 7 of the first tower section 6, due to gravity.
[0073] In Fig. 5, the second tower section 1 has been lowered, and the lower flange 2 of the second tower section 1 is now arranged in abutment with the upper flange 7 of the first tower section 6. Furthermore, the tower sections 1, 6 have been attached to each other, via the flanges 2, 7, by applying nuts 12 to the fastening elements 4. Finally, the crane 3 has been detached from the second tower section 1, and the mounting of the second tower section 1 may now be considered as completed.
[0074] Fig. 6 is a top view of a first tower section 6, e.g. of the kind illustrated in Figs.
[0075] 2-5. The upper flange 7 provided with a plurality of holes 8 can be clearly seen, and a tower ladder 10 is mounted on an inner wall of the tower section 6.Six cover elements 9 are arranged adjacent to the upper flange 7, in such a manner that all of the holes 8 are covered by one of the cover elements 9.
[0076] Moreover, each cover element 9 covers several holes 8. The cover elements 9 may be temporarily fixed to the upper flange 7, e.g. by means of magnets (not shown).
[0077] The cover elements 9 are provided with handles 13 allowing the cover elements 9 to be easily removed by pulling the cover elements 9 along a substantially sideways direction, via the handles 13, when the aligning movement of the second tower section (not shown) has been completed. In the case that the cover elements 9 are temporarily fixed to the upper flange 7 by means of magnets, the removal of the cover elements 9 merely requires a slight additional pulling force in order to overcome the magnetic force between the magnets and the upper flange 7.
Claims
CLAIMS1. A method for assembling a wind turbine tower, the wind turbine tower comprising at least two tower sections (1, 6), the method comprising the steps of:- mounting a first tower section (6) on a wind turbine foundation structure or on a previously installed tower section, the first tower section (6) comprising an upper flange (7) provided with a plurality of holes (8), each configured to receive a fastening element (4), the upper flange (7) being arranged at or near an upper end of the first tower section (6),- arranging at least one cover element (9) adjacent to the upper flange (7) of the first tower section (6), the at least one cover element (9) covering the plurality of holes (8) of the upper flange (7),- providing a second tower section (1), the second tower section (1) comprising a lower flange (2) provided with a plurality of holes (5), each configured to receive a fastening element (4),- arranging fastening elements (4) in at least some of the holes (5) of the lower flange (2) of the second tower section (1),- hoisting the second tower section (1) to a position above the first tower section (6) with the lower flange (2) of the second tower section (1) facing the upper flange (7) of the first tower section (6), and arranging the fastening elements (4) in abutment with the at least one cover element (9),- moving the second tower section (6) into alignment with the first tower section (1),- removing the at least one cover element (9), allowing the fastening elements (4) to enter the holes (8) of the upper flange (7) of the firsttower section (6), and attaching the second tower section (1) to the first tower section (6) by means of the fastening elements (4, 12).
2. A method according to claim 1, wherein the step of removing the at least one cover element (9) comprises slidingly moving the at least one cover element (9) along a substantially sideways direction.
3. A method according to claim 1 or 2, wherein each of the at least one cover element (9) is in the form of a plate like element.
4. A method according to any of the preceding claims, wherein the at least one cover element (9) has a low friction surface.
5. A method according to any of the preceding claims, wherein each of the at least one cover element (9) is configured to cover at least two of the holes (8) of the upper flange (7) of the first tower section (6).
6. A method according to any of the preceding claims, wherein the step of arranging the at least one cover element (9) adjacent to the upper flange (7) of the first tower section (6) comprises temporarily fixing the at least one cover element (9) to the upper flange (7) of the first tower section (6).
7. A method according to claim 6, wherein the at least one cover element (9) is temporarily fixed to the upper flange (7) of the first tower section (6) by means of at least one magnet.
8. A method according to any of the preceding claims, wherein the step of hoisting the second tower section (1) and arranging the fastening elements (4) in abutment with the at least one cover element (9) causes the fastening elements (4) to be pushed in an upwards direction.
9. A method according to any of the preceding claims, wherein the step of hoisting the second tower section (1) comprises positioning the lower flange (2) of the second tower section (1) at a distance from the upper flange (7) of the first tower section (6) which is lower than a maximum safety distance.
10. A method according to any of the preceding claims, wherein the step of moving the second tower section (1) into alignment with the first tower section (6) is performed while sliding the fastening elements (4) along the at least one cover element (9).
11. A method according to any of the preceding claims, wherein the step of moving the second tower section (1) into alignment with the first tower section (6) is performed at least partly manually.
12. A method according to any of the preceding claims, wherein the step of mounting a first tower section (6) comprises mounting the first tower section (6) on a transition piece, the transition piece comprising an upper flange provided with a plurality of holes, each configured to receive a fastening element, and further comprises the steps of:- arranging at least one cover element adjacent to the upper flange of the transition piece, the at least one cover element covering the plurality of holes of the upper flange,- arranging fastening elements in at least some of a plurality of holes provided in a lower flange of the first tower section,- hoisting the first tower section (6) to a position above the transition piece with the lower flange of the first tower section facing the upper flange of the transition piece, and arranging the fastening elements in abutment with the at least one cover element,- moving the first tower section (6) into alignment with the transition piece,- removing the at least one cover element, allowing the fastening elements to enter the holes of the upper flange of the transition piece, and attaching the first tower section (6) to the transition piece.
13. A method according to any of the preceding claims, wherein the fastening elements (4) are stud bolts.
14. A wind turbine tower assembly comprising:- a first tower section (6) comprising an upper flange (7) provided with a plurality of holes (8), each configured to receive a fastening element (4), the upper flange (7) being arranged at or near an upper end of the first tower section (6),- a second tower section (1) comprising a lower flange (2) provided with a plurality of holes (5), each configured to receive a fastening element (4), at least some of the holes (5) of the lower flange (2) of the second tower section (1) having a fastening element (4) mounted therein,- at least one cover element (9) arranged adjacent to the upper flange (7) of the first tower section (6), the at least one cover element (9) covering the plurality of holes (8) of the upper flange (7),wherein the second tower section (1) is arranged at a position above the first tower section (6) with the lower flange (2) of the second tower section (1) facing the upper flange (7) of the first tower section (6), and with the fastening elements (4) arranged in abutment with the at least one cover element (9).