Temporary damper assembly

The invention describes a temporary damper assembly for use during vertical storage and/or vertical transport of a tower.

Background

A complete installed wind turbine may be provided with a permanent damper, for example a tuned mass damper that is mounted in the upper reaches of the tower. The damper is constructed to counteract oscillations at the natural frequency of the tower (its "eigenfrequency"). When the wind turbine tower oscillates, a tuned mass damper will also oscillate, but phase-shifted with respect to the tower oscillations. A liquid damper permanently mounted in the upper tower level may be partially filled with a sloshing liquid, and the quantity of liquid is chosen on the basis of the tower eigenfrequency. The effect of a permanent damper is to suppress the tower oscillations, keeping the amplitude within safe levels.

An offshore wind turbine can have a tower height in the order of 80 - 100 m. Such a tower generally comprises two or more essentially cylindrical hollow tower sections with circular flanges at either end. A tower section can be bolted to another tower section and/or a further structure such as a transition piece using bolts inserted through the flanges.

A wind turbine tower may be pre-assembled and temporarily stored before being loaded onto a ship for transportation to the installation site. While it is possible to store and transport a tower horizontally, the own weight can result in significant deformation or ovalization. Furthermore, an upending manoeuvre of a heavy and long tower can be hazardous. For these reasons, it may be preferred to store and transport a pre-assembled tower in its vertical or upright orientation.

However, there are certain problems associated with the vertical storage and transport of a tall tower. A main problem is vortex-induced cross-wind vibrations. This is particularly relevant during transport by ship, since the tower stands upright on deck and cannot be sheltered from the wind. A tower may be structurally compromised by vortex- induced cross-wind vibrations. Furthermore, the resulting loads must be borne by the mounting fittings on deck, and these can become damaged. Vortex-induced cross-wind vibrations during storage and transport can therefore lead to high costs. The permanent damper intended for installation in the wind turbine tower cannot be used to suppress such vibrations during storage and transport, because it is explicitly tuned to the eigenfrequency of the complete installed wind turbine including the nacelle, generator, aerodynamic rotor etc.

Therefore, it is necessary to provide a way of suppressing or damping vortex-induced cross-wind vibrations of a pre assembled tower during storage and transport. In one approach, helical configurations of vortex strakes are attached to the outside of a tower prior to storage or transport in an exposed environment. The vortex strakes alter the airflow over the tower surface in such a way to prevent the development of vortices, and therefore prevent vortex- induced vibration. However, equipping a tower with vortex strakes can take many hours, and a similar length of time is required again to remove them. Furthermore, the work associated with mounting vortex strakes is largely dependent on weather conditions, and generally cannot be carried out in high winds or stormy conditions on account of the hazardous nature of the work.

It is therefore an object of the invention to provide an improved way of storing and transporting an upright tower. Description

The object is achieved by the temporary damper assembly of claim 1; and by the method of claim 10 of suppressing vortex- induced vibration in a tower.

According to the invention, the temporary damper assembly is intended for use during storage and/or transport of a tower, and comprises a liquid damper that is tuned to the natural frequency of vibration of the tower; a tower cover realised to cover an annular upper opening of the tower during storage and/or transport; a mounting interface configured to suspend the liquid damper from the annular upper opening of the tower; and a load transfer means for the transfer of loads between the liquid damper and the tower.

An advantage of the inventive temporary damper assembly is that offshore handling effort is reduced, thereby reducing the overall costs. This is because the inventive temporary damper can be installed at (and later removed from) the upper level of a tower in a relatively brief procedure. In contrast, it can take many hours to prepare a tower with vortex strakes, and a similar length of time to detach the vortex strakes at the final destination of the tower. Furthermore, handling of the inventive temporary damper is relatively independent of weather conditions, compared to the handling of vortex strakes, which generally cannot be done in stormy conditions. A further advantage of the inventive temporary damper assembly is that it reduces the hazards involved in preparing a tower for storage and/or transport, since it involves less manual handling steps.

According to the invention, the method of suppressing vortex- induced vibration in a tower during storage and/or transport comprises the steps of determining the natural frequency of vibration (i.e. the eigenfrequency) of the tower; tuning a liquid damper to that natural frequency; preparing a temporary damper assembly by attaching the tuned liquid damper to a tower cover and providing a load transfer means for the transfer of loads between the liquid damper and the tower; and mounting the temporary damper assembly onto an annular upper opening of the tower prior to storage and/or transport of the tower.

Particularly advantageous embodiments and features of the invention are given by the dependent claims, as revealed in the following description. Features of different claim categories may be combined as appropriate to give further embodiments not described herein.

In the following, it may be assumed that a tower is intended for use in an offshore wind turbine. The tower of such a wind turbine generally has the form of a narrow cone with an essentially circular cross-section. The tower is widest at its base, where it can be mounted to a transition piece of an offshore foundation. Such a tower may be assumed to comprise two or more tower sections, and the tower is pre-assembled before transport to the offshore installation site.

The liquid damper is preferably realised as a tank or other vessel that can be suspended in the interior of the tower. In a preferred embodiment of the invention, the liquid damper is realised as an essentially cylindrical tank with a diameter of approximately 40% - 50% of the tower interior diameter.

The tank can be partially filled with a fluid or sloshing liquid such as BASF Glysantin ^® G30 ^® which is primarily intended for use as an antifreeze. The liquid damper can comprise a fluid inlet/outlet so that the fill level of fluid can be adjusted. Preferably, the liquid damper is dimensioned for use with tower with natural frequencies in the range 0.45 Hz to 0.9 Hz. The step of tuning the liquid damper comprises adjusting the volume of liquid in the liquid damper. The volume of liquid present in the tank can be determined by weighing the liquid damper (on its own or as part of the assembled temporary damper assembly), for example.

The liquid damper can be installed using any suitable type of mounting interface. In a preferred embodiment of the invention, the mounting interface comprises a plurality of brackets, for example removable adapter brackets. Each bracket extends between an outer surface of the liquid damper and the annular upper opening of the tower. The brackets are preferably realised so that the liquid damper is centrally positioned in the tower interior. In a preferred embodiment of the invention, the mounting interface comprises at least three brackets, evenly distributed at 120° intervals about the liquid damper.

The brackets can be attached to the liquid damper in any suitable manner. In a preferred embodiment of the invention, shoes are provided on the outer surface of the liquid damper, and each shoe is shaped to receive a vertical side of a bracket. For example, a bracket can be slotted into a shoe and then bolted to the shoe using bolts, screws or other fasteners.

To suspend the liquid damper in the tower, the mounting interface preferably extends to the upper rim of the tower where it can be attached to the tower flange. As explained above, the flange has a bolt circle through which bolts are passed when the tower is being attached to another tower or further component. In a preferred embodiment of the invention, the mounting interface comprises a number of radial arms that extend outward to the upper rim. In a further preferred embodiment of the invention, each radial arm can terminate in an arc segment. The curvature of an arc segment preferably matches the curvature of the upper rim of the tower. Preferably, the radial arms are adjustable in length and/or the arc segments are detachable, so that the assembly can be adapted to a range of tower diameters. For example, the radial arm lengths can be lengthened or shortened as required, and a set of arc segments with suitable curvature can be chosen. In this way, the temporary damper assembly can be used for a variety of tower diameters.

The temporary damper assembly is provided by mounting the liquid damper to the top cover in such a way that the liquid damper is suspended from the underside of the top cover. When the top cover is placed onto the tower, the liquid damper will then be positioned in an upper level of the interior of the tower.

To be able to suppress vortex-induced vibration or other oscillations in the tower, a load transfer bridge is required between the liquid damper and the tower wall. In one approach, the liquid damper and/or its mounting interface could be constructed in such a way to achieve a form fit, i.e. to achieve physical contact between the tower interior wall and one or more surfaces of the liquid damper and/or its mounting interface. For example, each bracket can have an outer face that lies against the tower interior surface. However, such a form-fit construction would restrict the temporary damper assembly to a specific tower diameter. Therefore, in a preferred embodiment of the invention, a maximum diameter of the mounting interface can be smaller than the inner diameter of the tower, and the difference is bridged by a plurality of adjustment bolts. For example, an adjustment bolt can be inserted into an essentially horizontal threaded bushing attached to or arranged in a bracket of the mounting interface, so that an outer end of the adjustment bolt can make contact with the tower interior surface. Of course, the outer end of the adjustment bolt can terminate in a flat pad or plate so that forces are spread over a larger area, thus avoiding damage to the tower surface. There are various other ways of bridging a gap between the mounting interface and the tower, for example using an arrangement of hydraulic cylinders to apply pressure to the tower interior surface. The load transfer interface is adjusted or completed from the interior of the tower. To this end, a technician obtains access to the tower interior. When the temporary damper assembly has been lowered into place so that the top cover rests on the upper annular face of the tower, the technician can for example turn the adjustment bolts so that their outer ends apply sufficient pressure against the inner surface of the tower. The technician can access the upper tower level by an elevator that is part of the tower assembly, for example.

The temporary damper assembly is preferably lifted by crane to the level of the upper end of the tower. A suitable lifting fitting can be provided, to hold the temporary damper assembly steady while it is suspended from the crane lifting tackle. The mounting interface preferably comprises a plurality of connectors - e.g. eyelets - for connecting to cables of a lifting fitting. The tower cover can have openings through which such cables can pass, so that the cables can be attached to the brackets of the mounting interface. In a preferred embodiment, the top cover comprises a plurality of hatches that can be opened to accommodate cables of a lifting fitting. Once the temporary damper assembly is in place, the cables are detached, and the hatches can be closed again so that the tower interior is sealed off from the environment. In a further preferred embodiment, the tower cover comprises an elastomer rim arranged to rest on the annular upper end of the tower. An elastomer or rubber rim can act as a seal to prevent water from entering the tower interior.

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. Figure 1 shows an embodiment of the inventive temporary damper assembly;

Figure 2 shows an embodiment of the inventive temporary damper assembly being lowered onto a tower;

Figure 3 shows an embodiment of the inventive temporary damper assembly in place on a tower;

Figure 4 shows a perspective view into an embodiment of the inventive temporary damper assembly in place on a tower;

Figure 5 shows embodiments of the inventive temporary damper assembly parked on a nacelle transport frame.

In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

Figure 1 shows an embodiment of the inventive temporary damper assembly 1, and shows a liquid damper 10 suspended from a tower top cover 11. In one exemplary embodiment, the top cover 11 can have an arrangement of panels or fittings on its underside, to which the damper 10 can be secured using bolts.

The diameter D10 of the damper 10 is significantly less than the interior diameter of a tower. In this exemplary embodiment, a mounting interface 12 comprises three angular brackets 120 (two are visible in the drawing) evenly distributed about the damper 10, and mounted in shoes 121 provided on the outer face of the damper 10. An elastomer seal or rim 112 is provided at the outer edge of the top cover 11. One of several lifting brackets 16 is shown. These are provided for attaching to cables of a lifting fitting during a lifting manoeuvre. Figure 2 shows a temporary damper assembly 1 being lowered towards a tower 2 so that the damper 10 will be positioned in the interior space at the upper level of the tower 1. A lifting fitting 3 is being used, and is suspended from lifting tackle of a crane (not shown). Several cables 30 extend from the lifting fitting 3 to the lifting brackets 16. During a lifting manoeuvre, hatches 110 of the top cover are opened, and the cables 30 extend through the opened hatches.

The upper level of the tower 2 may be 80 - 100 m above ground so that it is difficult or even impossible to assess the accuracy of the manoeuvre from a position on the ground. Therefore, to avoid damage to the temporary damper assembly 1 or the tower 2, a number of cameras and/or sensors (not shown) can be deployed to provide feedback about the lifting manoeuvre so that position adjustments can be made as necessary.

Figure 3 shows an embodiment of the inventive temporary damper assembly 1 in place on a tower 2. An interior floor or platform 20P is indicated. A tower may generally be constructed to include a platform 20P near its upper end so that service personnel can work at this level. The arc segments 122 at the outer ends of the radial arms 123 rest on the circular rim of the tower 2. For simplicity, the diagram shows two brackets 120 at diametrically opposite sides of the damper 10. However, it shall be understood that at least three brackets 120 would be deployed, preferably evenly spaced about the cylindrical body of the liquid damper 10.

The diagram indicates the load transfer bridge comprising adjustment bolts 13 that are tightened so that their outer ends are pressed against the tower interior surface 20S. The diameter D12 defined by the outer ends of the brackets 120 can therefore be smaller than the inner diameter D20 of the "smallest" tower (of a number of different-sized towers), and the adjustment bolts 13 can bridge the gap. In this way, one type of temporary damper assembly 1 can be prepared for use with towers of different diameters. Each of these towers can be stored and/or transported vertically using a single type of temporary damper assembly 1.

Figure 4 shows a perspective view into an embodiment of the inventive temporary damper assembly 1 in place on a tower 2. The top cover 11 is not shown but shall be assumed to be present. The diagram shows the radial arms 123 (six in this embodiment), terminating in exchangeable arc segments 122.

The arc segments 122 are chosen to match the curvature of the tower flange. The diagram also shows several adjustment bolts 13 in retracted position (not pressed against the tower wall).

Figure 5 shows embodiments of the inventive temporary damper assembly 1 "parked" on a nacelle transport frame 4. After unloading a tower at an offshore installation site, mounting the tower onto a foundation and mounting the nacelle onto the tower, the temporary damper assembly 1 that was used with the tower can be parked on a yaw support part 40 of the nacelle transport frame, since the diameters correspond.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements .