"Tower Structure and Method of Raising and Lowering Said Structure "

FIELD OF INVENTION

This invention relates to a tower structure and a method of raising and lowering same, and more particularly to a height adjustable tower structure. The application more particularly relates to the field of turbine towers and the like, their installation, maintenance and repair. In particular, the present invention relates to turbine towers for wind turbines and finds particular application in the field of off-shore wind turbines..

INTRODUCTION

Current wind turbines and the like comprise a tower structure to which the turbine is attached and which raises the turbine above ground or sea level. Such towers are typically manufactured as a large cylindrical structure. These structures may be made of a plurality of smaller cylindrical pieces comprising flanges at each end which are bolted together end on end to produce a static tower which is then moved and installed as one piece.

Turbine towers for wind turbines can be very tall structures. The taller the tower structure the more difficult it is to move to its installation position e.g. if shipped offshore, larger boats are required to transport the tower, larger cranes are needed to lift the tower structure and more man-power may also be required. It is well known that movement of such towers is difficult and time consuming, incurring large costs.

A fundamental problem associated with the development of deep water wind farms is the need for relatively large tower structure support

frameworks. Greater water depths require more substantial foundations with corresponding increases in cost.

Existing offshore wind turbine tower structures have to date been developed mainly in relatively shallow water and use a mono pile driven into the seabed to provide a firm foundation for the tower structure and the turbine. Installation using a mono pile requires further heavy lifting equipment mounted on barges, e.g. jack-up barges, which position the mono pile. As wind farms move further offshore this method of installation becomes increasingly impractical and expensive.

Furthermore, a turbine or the like, when installed, is typically attached at a point along the tower structure, routinely at the (in use) uppermost end portion of the tower structure. The static nature of a conventional tower means maintenance and repair of the tower or the like is difficult, requiring complex structures to be built around the turbine to gain access to the turbine. Health and safety concerns associated with such maintenance and repair add to the cost of maintenance and the amount of time the turbine is non-functional renders the turbine less efficient.

SUMMARY OF INVENTION

In a first aspect, the present invention relates to a tower structure suitable for use with a wind turbine wherein the tower structure comprises at least a first portion and a second portion spaced therefrom, and a floatation device connected to one of the first or second portions whereby one of the first and second portions is moveable relative to the other by fluid introduced to or withdrawn from the space between the first and second portions.

Preferably, one of the first or second portions are adapted to connect to a turbine, for example an offshore wind turbine.

Preferably the first and second portions are telescopically arranged, wherein the first and second portions are arranged co-axially and are longitudinally moveable with respect to one another.

Preferably the second portion is mounted within the first portion.

Preferably, the floatation device is coupled to one of the first and second portions.

Advantageously the tower structure further comprises means for introducing or extracting fluid from the space between the first and second portions.

Preferably the fluid is a liquid, for example water or seawater.

Preferably, the flotation device is connected to the second portion, optionally at the lowermost (in use) end of the second portion. Contact of the flotation device with fluid of higher density than the floatation device results in floating of the flotation device on or in the fluid. Introduction or withdrawal of the fluid into or from the space between the first and second portions therefore results in extension or retraction of the second portion relative to the first portion. Preferably, the first portion provides a vessel into which fluid can be inserted or withdrawn and in which the flotation device and second portion can float and can be raised and lowered depending on the amount of fluid inserted into or withdrawn from the first portion.

Preferably one of the first and second portions is adapted to be placed on or connected to the sea bed.

Optionally, the tower structure further comprises a platform to enable access to the tower structure by personnel such as during maintenance or repair.

Conveniently, the means for introducing or extracting fluid from the space between the first and second portions may comprise pipework or ducting which may be mounted or connected to or accessed or controlled from the platform.

Conveniently the tower structure further comprises stabilising means to control the movement of one portion relative to the other. More preferably the stabilising means comprise guide means, for example rollers such as hydraulically operated rollers which may be mounted on the platform and which preferably act against the outer surface of the second portion.

Conveniently, the first portion comprises concrete, metal or metal alloy or alloys, for example steel, or a combination of these.

Conveniently, the second portion comprises concrete, metal or metal alloy or alloys, for example steel, or a combination of these.

Preferably, the tower structure further comprises locking means which may be mounted on one or other or both of the first and second portions. The locking means may be operable to selectively retain the second portion in a fixed position relative to the first portion. In a preferred embodiment the locking means comprise bolts, and preferably apertures are provided in the first and second portions, said apertures aligning to enable the bolts to

be passed therethrough to fix the portions in position relative to one another. In a less preferred but alternative embodiment, the locking means comprises collets mountable in slots which may be machined into one of the first and second

Preferably the tower structure further comprises a base portion. Said base portion preferably comprises a body having a greater width or diameter than the first and second portions and may preferably comprise a flange and preferably a radial flange. Connection points may be provided on the radial flange. . Said base section preferably connects to a lower part of the tower structure.

Optionally, the tower structure is for use with an offshore wind turbine and the base portion acts as a gravity base maintaining the position of the tower structure on the sea bed.

In a second aspect of the present invention there is provided a method of changing the height of a tower structure in accordance with the first aspect of the invention comprising the step of introducing or extracting fluid from the space between the first and second portions to alter the position of the floatation device and thereby alter the relative positions of the first and second portions.

Preferably the method further comprises the step of locking the first and second portions in a selected relative position.

Conveniently, movement of the first or second portion is stabilised, and is preferably vertically stabilised during movement.

Preferably the fluid is pumped into or out of the space between the first and second portions. Preferably also, the height of the tower structure increases as fluid is pumped into the space and decreases as fluid is pumped out of the space.

In a preferred embodiment the fluid may be recycled.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a side view of a tower structure according to one aspect of the present invention in a collapsed configuration comprising a second portion telescopically mounted within a first portion;

Fig. 2 is a side view of the tower structure of Fig. 1 in an erected state showing optional guiding means;

Fig. 3a is a side view of a base for use with the tower structure of

Fig. 1 ;

Fig. 3b is a top view of the gravity base of Fig. 3a;

Fig. 4a is a side view of the gravity base of Figs. 3a and 3b with additional dividing walls;

Fig. 4b is a top view of Fig. 4a;

Fig. 5a is a side view of the gravity base of Figs. 3a and 3b including a deck portion;

Fig. 5b is a top view of Fig. 5a; Fig. 6 is a partial cross-section through a tower structure of a further embodiment of the present invention;

Fig. 7 is a end elevation of the first cylinder of the tower structure of

Fig. 6;

Fig.8 is a schematic elevation view of the support members of Fig. 7.

DETAILED DESCRIPTION

As shown in Fig. 1 , in a first embodiment the tower structure 10 in accordance with the invention is shown in a collapsed non-erected configuration.

The tower structure 10 is shown connected to a base structure 12 which acts to hold and maintain the tower structure 10 in position, e.g. on the seabed, and is described in detail subsequently. The base may be integrally connected to a lower end of the tower structure or may be coupled thereto by any suitable means.

In the embodiment shown in Fig. 1 , the tower structure 10 comprises a first portion in the form of a hollow cylindrical column 14 and a second portion in the form of a second cylindrical column 16 which may be hollow and has an outer diameter which is less than the inner diameter of the first column 14.

The second column 16 is telescopically mounted within the first column 14 and the difference in diameter defines a space 22 between the first and second columns. The second column 16 preferably extends past the (in use) uppermost terminus of the first column 14 when in the collapsed configuration to allow for the attachment of a wind turbine (not shown) prior to installation. The wind turbine is preferably connected to column 16 at its upper end 18.

The upper most terminus of the first column is preferably closed by a cap 19 or cover which may be integrally formed with the column or may be mountable on the column to allow for selected access to the interior of the

first column or the second column for inspection or repair. An aperture 21 is formed in the cap through which the upper end 18 of the second column extends.

It can therefore be seen that the interior of the first column 14 is selectively sealed from the external environment and particularly from seawater outside. The first column provides a receptacle into which fluids such as water or seawater can be pumped into or withdrawn from.

In this embodiment the first column 14 is mounted within a substantially circular base structure 26 which has a greater diameter than the outer diameter of the first column.

Locking means (not shown) are provided between the first and second columns to selectively lock the columns in a desired relative position.

Preferred locking means are bolts or clamps known to the person skilled in the art. Apertures may be provided at selective positions along the first and second columns and bolts or other fixings may be provided which pass through the co-operating apertures to prevent movement of the second column with respect to the first. Alternatively bores may be provided through the second column and a locking pin may be inserted through one of the bores above the height of the cap 19 of the first column.

In a preferred embodiment, the first column 14 of the tower structure 10 is in the region of 30 to 70 meters in height and preferably is around 55 meters in height, and the second column 16 of the tower structure 10 when extended out of the first column 14 extends in the region of 15 to 75 meters above the first column 14 and more preferably extends around 55 meters above the first column 14.

Tower structure 10 further comprises a mechanism for moving one of the first or second columns and thereby changing the relative positions of the first and second columns and thereby changing the height of the tower structure. The mechanism is shown in the preferred embodiment of Fig. 1 as a flotation device in the form of a flotation can 20. The floatation device is connected to the lower end of the second column 16 or may be integrally formed therewith.

Flotation can 20 comprises a structure which has sufficient buoyancy such that when a fluid is introduced into the space 22 between columns 14 and 16, the flotation can 20 floats on or in the fluid and also supports the weight of the second column 16 and any wind turbine attached thereto.

The flotation can 20 has a diameter in the region of 4 to 10 meters and a height of 7 to 20 meters, preferably a diameter of 8 meters and a height of 15 meters to provide a volume of approximately 750 cubic meters.

As will be readily understood by the person skilled in the art, the buoyancy of the flotation can is dependent on the weight and density of the flotation can 20 relative to the fluid placed within space 22. Flotation can 20 must have sufficient buoyancy to move the weight of column 16 and the wind turbine (not shown). In the preferred embodiment the fluid is water and the flotation device is a hollow steel drum. In order to increase buoyancy the steel drum may be filled with a fluid of a lower density than the fluid introduced into space 22, and conveniently is filled with air.

Other suitable materials may be used in the manufacture of the floatation can 20, for example but not limited to high strength plastics, metals and metal alloys.

Guide means 24 are mounted at the upper end of the first column 16 to assist in stabilising and controlling movement of the second column 14 relative to the first. The guide means comprise rollers 25 which are mounted on or in a housing or on arms or actuators 27 and preferably hydraulic actuators connected to the first column. The rollers 25 are held against the outer surface of the second column by any suitable method such as springs (not shown) or hydraulic pressure or may be held against the outer surface under tension to hold the second column at a selected height.

A platform 40 may be provided on the tower structure for access by personnel for inspection, maintenance or repair. The platform may be a supported around the exterior of the first column at a selected height. Means (not shown) may be provided for adjusting the height of the platform.

A fluid supply may be provided on the platform for controlling raising or lowering of the second column with respect to the first. The fluid supply may be comprise a tank or other storage facility. Alternatively fluid may be drawn from the surrounding environment, e.g. seawater when the tower is installed in an off-shore location.

Pipework or ducting (not shown) is provided between the fluid supply and the space 22 between the first and second columns and pumps, gauges and/or other apparatus required to control the flow of fluid into or out of the space as will be fully understood by the person skilled in the art will also be provided on the platform and controllable by an operator on the platform or from a remote location.

The tower is erected by selective movement of second column 16 relative to first column 14 such that the tower structure 10 moves between the collapsed configuration in Fig. 1 and the extended or operating configuration shown in Fig. 2.

As fluid is introduced into space 22, the flotation can 20 held captive within the interior of the first column rises within the first column. As the flotation can 20 floats in or on the fluid and rises, the second column 16 which is connected thereto rises by a corresponding distance. The upper end of the second column rises through the cap 19 of the first column and the second column moves away from the base 26 of the tower structure upwards moving end point 18 further from the base structure 26.

As the second column 16 rises out of the cap 19 of the first column, the progress of the second column is guided by the rollers 25 mounted at the upper end of the first column. This provides stability to the second column and prevents tipping or tilting of the second column relative to the central axis of the tower. This is very important given the weight of a wind turbine that may be mounted on the upper end of the second column.

In the illustrated embodiment, once second column 16 has been moved to a desired height it is fixed in place by operation or activation of the locking means.

When it is desired to decrease the height of the tower structure, for example for maintenance or repair of a wind turbine mounted thereon, fluid can be withdrawn from the space 22 between the first and second columns. As the level of fluid within the first column falls, the floatation can 20 falls in a controlled manner within the first column and the second

column 16 is gradually retracted back into the first column thereby preventing damage to either the tower structure or the wind turbine.

In the preferred embodiment the tower structure 10 of the invention is stabilised by a base structure 26 in the form of a gravity base 12. As shown in Figs 3a and 3b, gravity base 12 is comprised of a concrete circular base 26 with side walls 28, although the skilled artisan would understand that a base could be manufactured from any suitable material capable of withstanding environmental pressures, for example, for offshore wind turbines where the base is placed underwater. The base structure 12 could be manufactured from concrete or any metal resistant to corrosion from seawater and resistant to high pressures.

The base functions to stabilise columns 14 and 16 which are mounted therein by any suitable means. As described below, the base is made of heavy materials, for example concrete and steel and may be also be filled with ballast such as sand, grit or further concrete to add weight to the base and keep columns 14 and 16 stable.

It will also be understood that although the illustrated base of Figs. 3a and 3b is cylindrical, any shape may be used e.g. the base could be square, oval, hexagonal etc in cross section.

The base may also be of any suitable size. The size of the base is dependent on the dimensions of the tower structure 10 to which it is connected. In some embodiments the base covers a surface area of approximately 800 square meters and includes 2000 to 6000 tonnes of concrete or other materials. The internal volume of the base can be in the region of 2000 to 3000 cubic meters.

Pile guides 30 which may be in the form of elongate hollow rods are provided on the base. The guides are vertically mounted around the inner circumference of the base and may be permanently fixed in position. The guides may be connected to the base 26. The upper open ends of the rods provide lifting sockets 32 into which lifting equipment (not shown) can be attached to move the base either in an incomplete or completed form as shown in Fig. 1.

The base is divided into a plurality of chambers or compartments 36 by walls 34 that extend substantially radially from the side walls. The radial walls extend from the side walls 28 of the base and terminate at an inner ring 35 which is of sufficient diameter to surround the lower end of the first column 14.

The end of each radial wall 34 adjacent the side wall 28 has a region of increased thickness. In the embodiment shown, the radial walls terminate in a triangular support 37 which abuts or may be integrally formed with the side wall.

A pile guide 30 extends through each triangular support 37 and in the illustrated embodiment pile guides also extend through the compartments defined between radial walls.

The compartments 36 may be filled with concrete, water, sand, grit or the like to add weight and stability to the base if required.

If required a deck element or cover member 38 can be fixed over the top of the dividing walls 34 and sides walls of the base to seal off the base and to maintain any grit or sand or other weighting means within the base to add weight and stability to the base.

As will be understood by the skilled man the base can be manufactured around the tower structure 10 or added to the tower structure 10 after manufacture.

In some embodiments the base sits on the seabed. In other embodiments the base has leg portions (not shown) which contact the seabed. In other embodiments the base is bolted to the seabed using techniques well know to the person skilled in the art.

As will be understood by the person skilled in the art, columns 14 and 16 can be manufactured from any suitable rigid material which can be moved by the flotation can 20. Suitable materials include but are not limited to concrete, metals and metal alloys, e.g. steel. The suitable strength of the material depends on the function to which the tower structure 10 will be used. For example, column 16 must be capable of supporting the weight of the wind turbine.

Additionally, depending on the relative dimensions of the first column 14 and the diameter of the flotation can 20, flotation can 20 can act to stabilise the second column 16 when extended or extending out of the first column 14. For example, where the floatation can diameter and the inner diameter of the first column 14 are such that the flotation can fits into the first column leaving little space between the first column 14 and the outer surface of flotation can 20, the flotation can vertically stabilises second column 16.

Preferably, the first column 14 extends at least 20 meters above the high water mark when installed.

A further embodiment of the present invention is shown in Figs. 6 and 7. In this embodiment like elements have been identified with like reference numbers but increased by 100.

In this embodiment an annular support member 142 is mounted on the upper end of the first column 114. The support member is shown in Fig. 7 as a ring 144 and preferably a concrete ring which has an outer diameter similar to that of the first column. The inner diameter of the ring is slightly larger than the outer diameter of the second column to allow the second column to pass through the ring as the height of the tower structure is increased or decreased. Such a concrete ring is particularly advantageous from a maintenance aspect.

The upper surface 146 of the ring provides a substantially horizontal platform 147 for access to the upper part of the second cylinder which this is in the retracted position.

The inner surface 148 of the ring, adjacent to the outer surface of the second column in use has a lip 150 which is substantially perpendicular to the horizontal platform leading to a tapered surface 152 which slopes back towards the outer circumference of the ring, the purpose of which will be described below. Securing means 154 such as bolts or tendons pass vertically through apertures (not shown) in the ring and the first column and may be secured at the base of the tower structure (not shown).

The lower surface of the ring 156 adjacent the tapered surface overhangs the upper end of the first cylinder.

Additional securing means 158 which may also be in the form of bolts or tendons pass vertically through apertures (not shown) in the ring which

open in the overhang of the lower surface 156 of the ring and extend into the space between the first and second columns and are secured to the base of the second column 116.

A steel ring 160 is cast into the inner surface of the first cylinder at a location which represents the position of the lower end of the floatation device 120 when the second column is in the extended position. The steel ring provides hoop strength to resist pressure applied thereto when the second column is in the elevated position.

In this embodiment the lower end of the second cylinder 116 comprises a steel cylinder to provide structural strength and partial buoyancy. A steel flange 162 is mounted around the outer surface of the second cylinder. The steel flange may be welded into position. Angle brackets 164 are provided between the flange and the outer surface of the second column. In the preferred embodiment 8 brackets are used although other numbers are envisaged. The outer surface 166 of the brackets are tapered inwardly from the lower surface to the upper surface, the taper matching that of the tapered surface 152 of the concrete ring.

The brackets 164 provide strength and form a taper lock when they mate with the concrete ring 144 when the second cylinder is raised to its elevated position.

Guides, rollers and /or anchor points 168 may be provided at the outer edge of the flange. In the embodiment shown such an anchor point is provided at the base of each of the brackets. The additional securing means from the concrete ring are secured to the anchor points. Rollers provided at the anchor points may assist in smooth movement of the second column with respect to the first.

The base of the second column is secured against the inner surface of the first column by guides, rollers and/or a jacking mechanism 170 which works through water tight glands (not shown). In this embodiment it is envisage that there are 8 such glands although the numbers may vary. In another embodiment a hydraulic ram or threaded rod may be controlled to force a shoe or connector outwardly from the base of the second column.

Buoyancy aids such as polystyrene blocks 172 are moulded or mounted around the lower end of the second column to provide additional buoyancy if this is required.

In this embodiment a fluid is introduced into the space between the first and second columns and the buoyancy of the lower end of the second column raises the lower end of the second column with the first column and out of the concrete ring.

As the second column rises through the first column, the tapered surface of the brackets 166 comes into abutment with the tapered inner surface 152 of the concrete ring and this spreads the load around the concrete ring and acts to ensure that the second column is centralised within the first column and cushions the impact between the concrete ring 144 and the brackets 164 to prevent damage occurring. Additionally the load is safely spread from the second column to the concrete ring.

When the second column reaches its maximum height within the first cylinder, the jacking mechanism 170 at the lower end of the second column is operated and bears outwardly against the steel ring 160 cast into the outer column.

In one embodiment the concrete ring 144 is mounted on top of the upper end of the first column 114, however it is envisaged that the concrete ring may be extended and may replace the first column in some embodiments. In this case, the heavy steel ring 160 is provided at the appropriate height within the concrete ring to provide resistance to the jacking pressure established when the lower end of the second column 116 is secured in the elevated position.

In the embodiment shown in Figure 7, the outer surface of the concrete ring is provided with a plurality of pairs of radial flanges 174 which may extend for the height of the concrete ring. In this embodiment there are 4 pairs of flanges and each pair of flanges is provided at approximately 90 degrees to the next. Each of the flanges of a pair are spaced apart to provide a sheltered access area 176 between each flange pair. Remotely controlled lifts and emergency ladders (not shown) can be provided between each flange pair. This allows personnel working on the platform to be able to approach the platform from a number of different directions depending on the weather or sea conditions and to gain access the platform form any of a number of approaches and further to be able to gain access to the platform from a sheltered location. Should the weather conditions change, personnel may leave the platform from a different access area than they arrived.

In this embodiment the securing means 154 extend through the radial flanges and connect the concrete ring to the base 112 of the tower.

Furthermore in this embodiment it is envisaged that the lower end of the second column 116 acts as a floatation device to lift the second column within the first column to erect the tower. It will be apparent to the person

skilled in the art that such a separate floatation device as described above could alternatively be used with this embodiment.

In a further modification of the present invention illustrated in Fig. 8 one or more independently height adjustable supports 200 may be provided on the upper surface 146 of the concrete ring. A first support 200a may be provided at or adjacent the outer surface of the ring and a further support 200b may be spaced from the first support, between the first support and the inner surface 148 of the ring. The supports may be elongate rods or legs. Each support may have a guide, wedge or shoe 202 mounted on the upper end thereof which is adapted to receive, hold, clamp or secure a rotor blade B of a wind turbine. Preferably the guides are clamps and more preferably hydraulic clamps. Therefore the guides may be U or C shaped in form and may comprise a channel 204 within which the rotor blade can be received and retained. Fixing means (not shown) may be provided to hold the blade in position. Each guide may be independently adjustably mounted on the end of the support. For example, the guides may swivel, rotate, and or slide laterally upon the support. The angle of the guides on the supports may also be independently adjustable. The supports 200 may be retractable into or below the surface of the concrete ring in order that when not in use they do not restrict personnel working on the ring and furthermore may be telescopically arranged. Locking means may be provided to lock the supports at a plurality of different heights.

When a rotor is to be inspected, replaced or repaired, the second column 116 is lowered as described above. The supports 200a, 200b are extended above the surface 146 of the concrete ring 144 at the required location beneath the descending rotor blade B. The inner support 200b is raised to a greater height than the outer support 200a, the relative heights depending upon the angle of the rotor blade to be supported. As the rotor

blade descends towards the supports 200 it is received within the guides 202 on the upper end of the supports. This allows the rotor blade to be disconnected from the nacelle of the rotor whilst it is supported above the surface of the ring and also provides stability to the turbine and the blade. The blade can then be removed and a replacement blade mounted in the guides. The guides 202 can then be moved towards the rotor to bring the free end of the blade into the required position for connection whilst the weight of the rotor blade is fully supported on the ring. For example the guides may be slide laterally along the top or upper part of the supports 200 to position the blade as required. Alternatively sockets may be provided in the upper surface of the platform and the height adjustable supports may be mounted in the sockets when required.

This provides a significant safety feature for personnel seeking to maintain or replace the rotor blades.

It is envisaged that such a support arrangement could equally be provided on any of the described embodiments of the present invention or could be used on other platforms where safe and secure handling of a rotor blade is required to carry out inspection, maintenance or repair.

Modifications and improvements to the invention may be envisaged without departing from the scope of the invention.

The person skilled in the art will however readily realise that the base structure 26,126 could be omitted and the rest of the tower structure 10 could be restrained in a set position within a body of water by any other suitable means, e.g. steel ropes and bolts and or other types of moorings attached to the seabed.

Alternatively the base structures as described above may be modified to provide further advantages. For example, the base structure may be formed of a concrete body with a depending skirt provided around the outer edge of the body. A pipe or conduit may be cast around the inner surface of the skirt with means to pump a fluid such as air into the pipe provided either on a working platform and operable from the platform or from a remote location by suitable control means. Apertures may be provided in the pipe at appropriate locations.

As the skirt penetrates the seabed air pumped through the pipe and out of the apertures provides for a cushioned landing of the base on the seabed and also provides for self levelling of the base. Once the base is in position on the seabed the skirt will close off the area beneath base and a pump which may be provided either on the base or on a working platform can be operated to maintain a vacuum within the skirt thereby holding the base on the seabed.

This further improves the installation and stability of a tower structure of an offshore wind turbine.

It will be understood by the person skilled in the art that the second column 16, 116 could extend further than shown or alternatively the second column could not extend past the terminus of the first column 14, 114, when in the collapsed configuration, depending on the use to which the tower structure is to be adapted.

Control means may be provided to facilitate remote control of the floatation device in order to alter the height of the tower structure for example in response to adverse weather conditions. The control means may be remotely operated for example by an operator on a support vessel.

Other arrangements of the tower structure may be used. For example, in a less preferred embodiment (not shown) the first column 14, 114 may be telescopically mounted on the second column and the second column 16,116 fixed to the base structure. In such an embodiment the central column wraps around the second column in a telescopic arrangement which is the reverse of that illustrated in Fig. 1. In this embodiment the tower structure operates in a similar manner to a piston, the space between the two columns being sealed by a seal and fluids which are less dense than air being used to erect the tower structure.

In another less preferred but alternative embodiment the two columns are connected via a hinge mechanism such that when the hinge mechanism is open the tower structure is not fully erected. In such an embodiment one column is capable of moving through 180 degrees via operation of the hinge mechanism to be fully extended or fully collapsed.

Other designs of columns may also be used, for example but not limited to square, oval or hexagonal columns in cross section.

The base structure can be any suitable structure to balance, support weight the tower structure on the seabed floor. As will be appreciated by the skilled man, steel piling and other designs of base or gravity base structures can be used.

Additionally, the overall stability of the tower structure and base structure can be improved by the addition of ropes, cables, guide means or stays connected via any suitable connecting means to the base structure. The stays extend upwards and are connected to the platform. At the low water mark the stays may be guided by one or more rollers connected to the first

column by any suitable means, for example concrete and steel rings disposed around the first column and extending substantially horizontally outwards from the first column. It will be appreciated that the one or more rollers may be positioned at a height other than the low water mark. Tension in the stays may be increased by any suitable means known to the person skilled in the art.

Advantageously, if the tower structure and base structure are towed offshore as one piece, the stays may be in place and tensioned following installation at the offshore site.

Conveniently, the stays may be used to attach a floating walkway onto which boats, ships and other marine vessels may dock.

Preferably 8 stays are used but the skilled man will appreciate that more or less stays may be used.

The working platform 147 on the first column may be raised above the upper surface of the first column in a jacking operation to provide a raisable platform from which inspection, maintenance or repair could be carried out. In such an embodiment jacking means would be provided to raise the platform on the first column to the required height and may be operated by control means provided on the platform.

It will be appreciated that embodiments of the present invention and particularly as shown in Figs.6 and 7 provide a tower structure which benefits from a positive connection from the installation, working platform or turbine attached to the tower to the base of the tower. This provides improved stability to the structure.

Furthermore, the tower structure 10 hereinbefore described could also be used in an onshore wind farm as well as an offshore wind farm as the selectively collapsible structure tower structure 10 will also provide benefits during overland transportation.