GB2407114A

GB2407114A - A method of installing an offshore structure

Info

Publication number: GB2407114A
Application number: GB0324212A
Authority: GB
Inventors: Gordon Jackson
Original assignee: ARUP GROUP Ltd; Ove Arup and Partners International Ltd
Current assignee: Ove Arup and Partners International Ltd
Priority date: 2003-10-15
Filing date: 2003-10-15
Publication date: 2005-04-20
Anticipated expiration: 2023-10-15
Also published as: GB0324212D0; GB2407114B

Abstract

A method of installing an offshore wind turbine is provided. The wind turbine has a base 2 and a tower 4 extending upwardly therefrom and is installed at an installation site on the seabed. The method comprises: transporting the turbine to the installation site supported on a barge 26 such that the tower extends substantially parallel to the barge deck; at the installation site, causing the turbine to rotate under the effect of gravity acting on the base thereof until the tower extends substantially perpendicular to the barge deck and the base is in the sea; and lowering the base to the seabed. The turbine base may be ballasted to cause the turbine to rotate under the affect of gravity.

Description

1 2407114 Method of Installing an Offshore Structure The present invention

relates to a method of installing an offshore structure and, more particularly, to a method of installing an offshore wind turbine.

Offshore wind turbines are well known in the art as a means of harnessing wind power. Due to increasing interest in renewable energy sources, the use of wind farms made up of a large number of wind turbines is becoming more common. Offshore wind farms are particularly desirable as there is limited land available for the installation of onshore wind farms.

To install an offshore wind turbine, the turbine must be transported to an offshore location and secured to the seabed. Traditionally this has been done by transporting a turbine to the installation location as a set of component parts, each part being supported in a vertical orientation above a barge. The turbine base is then lifted from the barge into the sea before being lowered to the seabed. The remaining parts are then assembled on the base. This installation method requires the use of specialist lifting equipment which is relatively expensive to hire and so significantly increases the overall costs of wind farm construction. Thus, alternative installation methods which do not require the use of such specialist equipment have been sought.

In one known method of installing an offshore wind turbine as described in GB 2,327,449 A, the turbine is towed to the installation location with the base of the turbine at least partially supported below a barge and with the tower extending upwardly therefrom. The turbine is then lowered to the seabed using strand jacking means. This method is advantageous in that no specialist lifting equipment is required such that the installation costs are significantly reduced. However, an installation method which allows a turbine to be stably supported above a barge while being transported to the installation site is desirable.

From a first aspect, the present invention provides a method of installing an offshore wind turbine having a base and a tower extending upwardly therefrom, at an installation site on the seabed, the method comprising: transporting the turbine to the installation site supported on a barge such that the tower extends substantially parallel to the barge deck; at the installation site, causing the turbine to rotate under the effect of gravity acting on the base thereof until the tower extends substantially perpendicular to the barge deck; and lowering the base to the seabed.

The invention has the advantage that the wind turbine can be loaded out and transported to the installation site supported above a barge in the horizontal orientation, i.e. in a stable arrangement, and can be installed on the seabed without the need of cranes or other specialist lifting equipment.

The wind turbine could be positioned on the barge such that the base of the turbine was suspended above the water once the tower had been pivoted to extend substantially perpendicular to the barge deck. In a preferred embodiment however, the base is located in the sea once the tower had been pivoted to extend substantially perpendicular to the barge deck.

The weight of the wind turbine base could be designed to be sufficient to cause the turbine to automatically rotate or up-end if the upper end of the turbine is not tied down to the barge. However, the weight of the turbine base required after installation can be significantly less than this and so, the turbine base is preferably ballasted to increase the weight thereof and so cause the turbine to rotate under the effect of gravity acting on the base thereof. In this way, the turbine base design can be optimised for the final installed condition without compromising the installation method.

The base could be lowered to the seabed by a number of methods. For example, the base could be lowered using strandjacking means or hydraulic jacks connecting the barge to the tower. Preferably however, the base is positioned relative to the point about which the tower is caused to pivot such that it is lowered at least a substantial distance towards the seabed by the pivotal movement of the tower. This has the advantage that strand jacking means need not be used to lower the turbine base to the seabed.

The turbine could be supported above the barge in a number of ways. In one preferred embodiment, the tower is supported on a cradle above the barge deck the contact surface between the cradle and the tower deeming the point about which the turbine pivots.

Preferably the cradle is located adjacent one end of the barge and, more preferably, adjacent the stern of the barge. This allows the base of the turbine to clear the end of the barge and be lowered into the water during pivoting.

Still more preferably, a second cradle is provided adjacent the other end of the barge (more preferably the bow of the barge) and the upper end of the turbine is supported thereon during transportation. This has the advantage that the turbine can be held stably to the two cradles during transportation.

The turbine could be allowed to pivot freely under the action of gravity during installation. Preferably however, means are provided to control the rate of pivoting of the turbine. This avoids the risk of damage to the turbine or other equipment if it pivots too quickly or, in the extreme case, pivots beyond a vertical orientation.

Preferably, the means for controlling the rate of pivoting of the turbine comprise an extensible line connecting the barge to an upper end of the tower. In one particularly preferred embodiment, the extensible line is provided on a winch mechanism.

As discussed above, the base is preferably positioned relative to the point about which the tower is caused to pivot such that it is lowered at least a substantial distance towards the seabed by the pivotal movement of the tower. The barge could remain stationary during this pivotal movement. In one particularly preferred embodiment however, the first end of the barge is ballasted during pivoting of the turbine, so as to lower the end of the barge into the sea. This movement can be used to lower the turbine the remaining distance to the seabed.

The base of the turbine could take any desired configuration. For example, the base could be a gravity base or skirted base. In one preferred embodiment however, a suction base is provided. This has the advantage that the base can be installed on the seabed by applying suction thereto thus penetrating the base accurately into the seabed. A suction base has the further advantage that it can be easily adjusted to ensure that the vertical orientation of the tower is acceptable.

In the method of the invention, the wind turbine for installation may be assembled onshore prior to transportation to the installation site. This has the advantage that time consuming and costly offshore assembly operations are avoided. In the preferred embodiment of the invention, the turbine structure can be assembled onshore in a substantially horizontal orientation. This has the additional advantage of reducing the amount of costly lifting equipment required The assembled turbine structure could be lifted onto the barge prior to transportation.

In an alternative embodiment, the turbine structure could be loaded onto the barge using multi-wheel trailers. Preferably, guide rails are provided extending to the edge of an assembly area at which the barge may dock, and corresponding guide rails are provided on the barge such that the assembled turbine will be loaded onto the barge by moving the turbine along the guide rails.

The invention has been described above in relation to a method of installing offshore wind turbines. It is envisaged that the method could be used to install wind turbines of any size up to the maximum capacity of 5MW currently anticipated in the market.

The method of the invention is particularly applicable to the installation of wind turbines up to a capacity of 3.6MW which is the typical size chosen in most present day wind installations. The invention is however also applicable to the installation of other types of offshore structure. From a further aspect therefore, the present invention provides a method of installing an offshore structure having a base and a tower extending upwardly therefrom, at an installation site on the seabed, the method comprising: transporting the structure to the installation site supported on a barge such that the tower extends substantially parallel to the barge deck; at the installation site, causing the structure to rotate under the effect of gravity acting on the base thereof until the tower extends substantially perpendicular to the barge deck so as to lower the base to the seabed.

A preferred embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure l shows a wind turbine being loaded onto a barge adjacent the shore: Figure 2 shows a first stage in an installation method according to the present invention; Figure 3 shows a second stage in an installation method according to the present invention; Figure 4 shows a wind turbine in its final installed condition; and Figure 5 is a perspective view of an assembled wind turbine for installation by the method of the invention.

A method of installing an offshore wind turbine is shown in Figures 1 to 4. An offshore wind turbine for installation by the method shown is shown in Figure 5 and comprises a base 2 (the substructure) and a tower 4.

The base 2 comprises a central hollow cylindrical portion 6. A first suction bucket 8 is provided at the bottom end of the central portion 6 and is of the same diameter as the central cylindrical portion. First and second tubular frame members 10, 12 extend out radially from the cylindrical portion 6 above the first suction bucket 8, in a plane perpendicular to the longitudinal axis of the cylindrical portion and at an angle to eachother. A further tubular frame member 14 extends between the free ends of the first and second tubular frame members 10, 12. Two further tubular frame members 16, 18 extend from the free ends of the first and second tubular frame members 10, 12 upwardly to a point on the central portion 6 above the first and second tubular frame members 10, 12. Thus, the frame of the base takes the form of a tetrahedron with an equilateral triangle as the base. A suction bucket 8 is located below each of the three corners of the triangular base of the frame.

The diameter of the cylindrical portion and the tubular frame members of the substructure is chosen to match the size required for connection to the tower and to ensure that the structure's fundamental mode of vibration is compatible with the blade and turbine rotational periods. The spacing of the suction buckets provided at each of the three corners of the base of the frame is also chosen to ensure that the structure's fundamental mode of vibration is compatible with the blade and turbine rotational periods.

Although the substructure of the wind turbine is configured as a suction bucket base in the embodiment shown, it could also be a gravity base or skirted base The suction bucket foundation shown is believed to be the most suitable for a water depth range of 8 to 25m and a seabed mainly comprising stiffclay.

The tower 4 of the offshore wind turbine is secured to the base at the lower end thereof prior to installation and comprises a tapered support arm 20 which gets narrower towards the upper end thereof. A nacelle 22 is attached at the free end of the support arm 20 and three rotor blades 24 extend out from the nacelle.

The parts of the offshore wind turbine for installation using the method of the invention may be manufactured separately offside at any convenient location and may be transported to the assembly site by any chosen method. The complete turbine structure may then rapidly be assembled and precommissioned at an onshore assembly site which is ideally located as close as possible to the offshore installation site.

The assembly and installation of an offshore wind turbine is described below with reference to Figures 1 to 4. Once the various component parts of the wind turbine have been delivered to the assembly site, the substructure and the tower are assembled on site. Both the tower and the base are then lifted onto respective cradles 23, 25 to be supported in a horizontal orientation. The cradles 23, 25 are the supports on which the structure is eventually transported to the installation site and so are dimensioned appropriately Moveable skid shoes are provided on the base of each cradle to cooperate with skid rails on which the cradles move as will be described below. The rear cradle 25 which supports the turbine substructure includes a trunnion 27 which provides a pivotal support for the structure during installation. The trunnion on the rear cradle also provides a restraint against rolling motion of the structure during assembly and transportation.

The tower 4 is then joined to the base 2 in the horizontal orientation by moving the cradles on guide rails or skid rails. The joint between the tower and the base is formed by stressing the tower to the base using hydraulic bolt-tensioning equipment such as the Hydratight system. Cabling for transferring power created by the turbine to the base is also connected. Some pre-commissioning activities are also carried out with the completed turbine structure supported on the cradles in the horizontal orientation.

For installation, the completed turbine structure is loaded onto a barge 26. The barge is an offshore transportation barge having minimum dimensions of 91 m length, 27m width and 6m depth. The barge is provided with a temporary ballast system. Skid rails 28 are welded to the deck 30 of the barge. The skid rails 28 comprise two guide rails extending parallel to and spaced from one another along the length of the deck.

Skid rails 31 having corresponding spacing are provided on the shore such that when the barge is moored stern-on to the shore and de-ballasted to the appropriate height, the skid rails on the shore and on the barge form a continuous track.

A winch 32 is provided on the barge deck 30 at the front thereof, located in between the two skid rails 28.

To load the turbine structure onto the barge, the structure is brought to the edge of the shore supported on the cradles which can be moved along the skid rails. Strand jacking 34 is attached to the barge deck and to the cradles supporting the turbine structure and the cradles are pulled along the skid rails 28 until both cradles are located on the barge. The moveable shoes on the cradles 23, 25 are then locked to the skid rails 28. A line 36 is attached between the upper end of the tower 4 and the winch 32. This line is tightened to hold the tower down to the deck 30 during transportation. - 8

During loading, the barge could be set on a prepared subsea bemm. This allows the barge to be loaded at any stage in the tidal cycle. As the turbine structure is skidded out onto the barge, the barge is deballasted to match the increasing load Trim ballast is then added to keep the barge horizontal as the turbine structure traverses the barge to its fmal transportation position.

Once loading onto the barge has been completed as shown in Figure 1, the barge is towed to the offshore installation site. At the installation site, the barge is located exactly using subsea transponders and stem mooring lines are then attached to the barge to maintain its position.

To commence installation, the temporary barge ballasting system is connected to the central cylindrical portion 6 of the base 2. The central portion 6 is filled with water from the ballasting system at a rate of 500m3/hour so as to move the centre of gravity of the turbine structure towards the stern. The turbine begins to up-end (i.e. to pivot about the trunnion 27) within about an hour of the ballasting commencing. The rate at which the structure rotates is controlled by the bow winch 32 releasing line 36. The position of the rear cradle 25 on the barge is set to obtain a 0. 5m sea-bed clearance at lowest astronomical tide (LAT) when the structure has been pivoted to vertical.

Figure 2 shows the installation process at approximately mid-point. Once the process has reached about this stage and as it continues, the temporary ballast system adds water at a rate of 1000m3/hour to the barge stern such that the stern is lowered by about 3m in 1.5 hours. This allows the base to be set down on the seabed and for the barge to clear the rear cradle 25. If necessary a jacking system can be provided on the rear cradle 25 such that the cradle can be lowered to provide a quick release of the barge from the structure on installation. The position of the structure and barge as installation is completed is shown in Figure 3.

Once in the vertical orientation, the structure is secured to the seabed. To do this, individual suction can pumps are activated leading to each of the suction cans, thus drawing water out of the cans and allowing them to penetrate the seabed. The line 3 6 is then released from the top of the tower. Finally as shown in Figure 4, the barge is released from the structure and returns to shore.

It will be appreciated that the installation method described above with reference to the drawings is a preferred embodiment only. Various changes could be made to the embodiment which fall within the scope of the invention which is defined by the appended claims.

Claims

Claims I. A method of installing an offshore wind turbine having a base

and a tower extending upwardly therefrom, at an installation site on the seabed, the method comprlsmg.

transporting the turbine to the installation site supported on a barge such that the tower extends substantially parallel to the barge deck; at the installation site, causing the turbine to rotate under the effect of gravity acting on the base thereof until the tower extends substantially perpendicular to the barge deck, and lowering the base to the seabed.
2. A method as claimed in claim 1, wherein the turbine base is ballasted to cause the turbine to rotate under the effect of gravity acting on the base thereof.
3. A method as claimed in claim I or 2, wherein the base is positioned relative to the point about which the tower is caused to pivot such that it is lowered at least a substantial distance towards the seabed by the pivotal movement of the tower.
4. A method as claimed in claim 1, 2 or 3, wherein the tower is supported on a cradle above the barge deck, the contact surface between the cradle and the tower defining the point about which the turbine pivots.
5. A method as claimed in claim 4, wherein the cradle is located adjacent a first end of the barge
6 A method as claimed in claim 5, wherein a second cradle is provided adjacent the other end of the barge, and the upper end of the turbine is supported thereon during transportation.
7. A method as claimed in any of claims 4, 5 or 6, wherein the cradle is lowered relative to the barge during rotation of the turbine.
8. A method as claimed in any preceding claim, wherein means are provided to control the rate of pivoting of the turbine.
9 A method as claimed in claim 8, wherein the means for controlling the rate of pivoting of the turbine comprise an extensible line connecting the barge to an upper end of the tower.
10. A method as claimed in any preceding claim, wherein the first end of the barge is ballasted during pivoting of the turbine, so as to lower the first end of the barge into the sea.
11. A method as claimed in any preceding claim, wherein the wind turbine is provided with a suction base.
12. A method as claimed in any preceding claim, wherein the wind turbine for installation is assembled onshore prior to transportation to the installation site.
13. A method as claimed in any preceding claim, wherein guide rails are provided extending to the edge of an assembly area at which the barge may dock, and corresponding guide rails are provided on the barge such that the assembled turbine is loaded onto the barge by moving the turbine along the guide rails.
14. A method of installing an offshore structure having a base and a tower extending upwardly therefrom, at an installation site on the seabed, the method comprlsmg: transporting the structure to the installation site supported on a barge such that the tower extends substantially parallel to the barge deck; and at the installation site, causing the structure to rotate under the effect of gravity acting on the base thereof until the tower extends substantially perpendicular to the barge deck so as to lower the base to the seabed.