Floating wind turbine generator installation

A method of installing a wind turbine generator onto a floating foundation. The floating foundation has variable buoyancy and is pre-ballasted to float at a predetermined vertical position before installation of a wind turbine generator component onto the floating foundation. A wind turbine generator component supported by lifting equipment is brought towards the floating foundation until contact is made with the floating foundation. Ballast is removed from the floating foundation to increase the buoyancy of the floating foundation such that weight of the wind turbine generator component supported by the floating foundation is increased from substantially zero to substantially the entire weight of the wind turbine generator component. The vertical position of the floating foundation is substantially unchanged during transferring weight of the wind turbine generator component onto the floating foundation.

FIELD OF THE INVENTION

The present invention relates to a system and method of installing a wind turbine generator onto a floating foundation.

BACKGROUND OF THE INVENTION

Offshore wind turbine generators for example for deeper water operation can be mounted to a floating foundation. The floating foundation has buoyancy to support the weight of the wind turbine generator mounted upon it. Various types of floating foundation are known which are coupled to the seabed and stabilised in different ways.

One such floating foundation is a buoyancy stabilised floating foundation of a semi-submersible type. The floating foundation typically has a plurality of stabilising columns including internal volumes for containing water ballast for smaller adjustments of tilt of the foundation. The water ballast may be introduced or removed from the body of water in which the floating foundation is floating. The level, or vertical depth, as well as the tilt attitude of the floating foundation can be adjusted by controlling the buoyancy of each of the stabilising columns by adjusting the amount of water ballast in the columns. The wind turbine generator is typically mounted upon one of the stabilising columns. Other buoyancy stabilised floating foundations are known which take the form of a ‘barge’. Buoyancy stabilised floating foundations are typically anchored to the seabed with catenary mooring lines.

Other types of floating foundation are known which are ‘ballast stabilised’ or ‘mooring line stabilised’. These types of floating foundation may still have variable buoyancy for adjusting their floating vertical depth but are typically stabilised in tilt attitude by mooring lines (e.g. tension leg platform with anchors, catenary mooring lines with anchors) and/or by counterweight ballast (‘spar buoy’ type).

Traditionally, during installation of the wind turbine generator upon the floating foundation, e.g. using a crane, the vertical depth and/or tilt attitude of the floating foundation may change as the weight of the wind turbine generator, or components of the wind turbine generator, are transferred from the crane onto the floating foundation as the crane lowers these onto the floating foundation. Movement of the floating foundation in this way may lead to increased safety risks associated with the installation as changing in vertical position of the foundation may lead to angled lifting with a tension line not being vertical and hence submitting the lifting equipment to forces off the intended axis. Furthermore, it may cause damage to the interface between the floating foundation and the wind turbine generator or the interface between components of the wind turbine generator during the installation. This is particularly the situation when the turbine is positioned off the centre of gravity of the floating foundation.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of installing a wind turbine generator onto a floating foundation, comprising: pre-ballasting a floating foundation to float at a predetermined vertical position before installation of a wind turbine generator component onto the floating foundation; bringing a wind turbine generator component towards the floating foundation until direct or indirect contact is made with the floating foundation; and removing ballast from the floating foundation to increase the buoyancy of the floating foundation such that weight of the wind turbine generator component supported by the floating foundation is increased from substantially zero to substantially the entire weight of the wind turbine generator component, wherein the wind turbine generator component is initially supported by lifting equipment and is lowered by the lifting equipment towards the floating foundation and the vertical position of the floating foundation is substantially unchanged during transferring weight of the wind turbine generator component onto the floating foundation.

A further aspect of the invention provides a system for installing a wind turbine generator onto a floating foundation, comprising: a floating foundation having variable buoyancy; lifting equipment for bringing a wind turbine generator component towards the floating foundation until direct or indirect contact is made with the floating foundation; and a controller coupled to a ballast removal device associated with the floating foundation for removing ballast from the floating foundation to increase the buoyancy of the floating foundation such that weight of the wind turbine generator component supported by the floating foundation is increased from substantially zero to substantially the entire weight of the wind turbine generator component, wherein the controller is configured to maintain a vertical position of the floating foundation substantially unchanged during transferring weight of the wind turbine generator component onto the floating foundation.

The ‘vertical position’ of the floating foundation may be: either with respect to the top surface of the body of water in which the foundation is floating, i.e. a ‘vertical depth’; or may be with respect to a fixed ground surface adjacent the body of water in which the foundation is floating, i.e. a ‘vertical height’. The fixed ground surface may be a seabed, a vessel jacked up on the sea floor or a quayside, for example. In an enclosed body of water with no variation in the water level then the vertical position may be either the vertical depth or the vertical height, as they will be directly related. However in a tidal or other body of water with a varying water level then the vertical position may be selected as: either the vertical depth, where the position of the foundation with respect to the water level is unchanged but that water level is variable; or the vertical height, where the position of the foundation with respect to the water level changes as the water level changes but the position of the foundation with respect to the ground is unchanged.

The invention is advantageous in that risks associated with movement of the floating platform during installation of the wind turbine component vertically and/or tilting relative to horizontal can be avoided or at least ameliorated. In particular, risks associated with geometrical displacement between the lifting equipment and the floating platform, or risks associated with lifting equipment failure, can be avoided or reduced.

Particularly for installation of very tall wind turbine generators, lifting of wind turbine generator component by lifting equipment, such as a crane or a hoist, is safer than sliding movement for example on a slay or a rolling vehicle due to the high position of the centre of gravity. By very tall wind turbine generators is here meant wind turbine generators with tip height of more than 150 m and particularly for wind turbines with tip height of more than 200 m, such as more than 250 m. Wind turbine generators typically has a tip height of less than 300 m but taller wind turbines may be envisioned in the future, and other lifting equipment like for example an airship will be suitable.

The method may further comprise maintaining the floating foundation at a substantially constant attitude, and optionally tilt just prior to and/or during installation of the wind turbine generator component onto the floating foundation.

The method may further comprise maintaining the floating foundation substantially horizontal prior to and/or during installation of the wind turbine generator component onto the floating foundation.

The steps of bringing a wind turbine generator component towards the floating foundation, and removing ballast from the floating foundation, may be repeated for at least two wind turbine generator components to be connected and preferably each wind turbine generator component of a wind turbine generator.

The wind turbine generator component may be the entire wind turbine generator to be installed onto the floating foundation, e.g. in an installation step of bringing the entire wind turbine generator towards the floating foundation and removing ballast from the floating foundation.

Pre-ballasting the floating foundation to float at a predetermined vertical position may include setting the vertical position to an expected final vertical position once all components of the wind turbine generator have been installed onto the floating platform.

The ballast may be water ballast.

The method may further comprise connecting the wind turbine generator component to the floating foundation once contact is made to substantially prevent lateral displacement of the wind turbine generator component relative to the floating foundation before removing ballast from the floating foundation equivalent to at least 5% of the weight of the wind turbine generator component, and preferably before removing any ballast from the floating foundation.

The method may further comprise securing the wind turbine generator component to the floating foundation to substantially prevent vertical displacement of the wind turbine generator component relative to the floating foundation after removing ballast from the floating foundation.

The lifting equipment may be a crane, or a hoist, preferably the lifting equipment includes a tension line, such as a crane cable.

The wind turbine generator component may be initially supported by the lifting equipment by one or more tension lines, and the method may further comprise paying out the one of more tension lines to offset or at least partially compensate a retraction in the tension line(s) as tension in the tension line(s) is reduced during transferring weight of the wind turbine generator component onto the floating foundation.

The mass of ballast removed may not exactly match the mass of the of the wind turbine generator component being installed onto the floating platform. This may account for a difference between the centre of gravity of the ballast and the centre of gravity of the wind turbine generator component being installed.

The vertical position may be taken with respect to either the top surface of the body of water in which the foundation is floating, or with respect to a fixed ground surface adjacent the body of water in which the foundation is floating.

The method may further comprise adding or removing ballast from the floating foundation to account for dynamic water level variation, such as tidal variation and/or wind related variation.

The system may further comprise at least one sensor selected from a tilt sensor and/or a level sensor and/or a pressure sensor coupled to the controller. The floating foundation may include the at least one sensor. Additionally, or alternatively, the lifting equipment or the wind turbine generator component may include the at least one sensor.

The controller may be configured for adjusting the buoyancy of the floating foundation such that the attitude and optionally the tilt of the floating foundation is substantially unchanged during transferring weight of the wind turbine generator component onto the floating foundation.

The wind turbine generator component may be initially connected to the lifting equipment by one or more tension lines. The system may further comprise a load sensor coupled to the tension line(s) and to the controller.

The lifting equipment may be land based, or may be floating, e.g. supported by a floating vessel.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG.1shows a floating foundation10floating on the surface21of a body of water20adjacent a quayside30.

The floating foundation10has an internal volume11for containing a quantity of ballast. In an example the ballast is water ballast. The internal volume11of the floating foundation10is in selective fluid communication with the body of water20via a port12of the floating foundation10. A pump13is arranged to pump water ballast either out from the internal volume11through the port12, or through the port12into the internal volume11. In doing so, the quantity of water ballast in the internal volume11is variable such that the floating foundation10has variable buoyancy.

As shown inFIG.2, a controller40is coupled with the pump13. Under the action of the controller40the pump13is filling the internal volume11of the floating foundation10with water ballast14by drawing water from the body of water20in through the port12such that the amount of water ballast14is increasing and the vertical position of the floating foundation10is decreasing in depth relative to the surface21of the body of water20. This operation is continued until the floating foundation10reaches a predetermined vertical position. In a static body of water with a constant water level the vertical position is taken relative to the surface21of the body of water20but may equally be taken with respect to a fixed ground surface. In a dynamic body of water with a dynamic water level, the vertical position may be taken with respect to either the surface of the body of water, or with respect to the fixed ground surface. The floating foundation is said to be ‘pre-ballasted’ at this predetermined vertical position.

In an example, this predetermined vertical position is selected as the final intended vertical depth of the floating foundation10once a wind turbine generator has been installed upon the floating foundation. However, any vertical position may be selected.

FIG.3shows lifting equipment50mounted upon the quayside30. The lifting equipment50may for example be a crane for movement in multiple directions and for lifting and lowering, or may be a hoist for lifting and lowering vertically only. The lifting equipment50includes a tension line51, such as a crane cable, attached to a distal end of the lifting equipment50and which may be reeled in or paid out so as to raise and lower an attachment point52for coupling to a load. As illustrated inFIG.3the attachment point52of the lifting equipment50is coupled to a wind turbine generator component60.

The wind turbine generator component may be a first one of a plurality of sections of a wind turbine generator to be installed upon the floating foundation10, or may be an entire wind turbine generator to be installed upon the floating foundation10. In the illustrated example the wind turbine component60is the first one of a plurality of wind turbine generator components, e.g. a first tower section. A tension in the tension line51is measured. This measurement may be a direct measurement, e.g. of strain, in the tension line51or may be an indirect measurement through the lifting equipment50more generally. A signal indicative of the tension in the tension line51is communicated to the controller40.

The wind turbine generator component60is lowered towards the floating foundation10until contact is made between the lower end of the wind turbine component60and an upper attachment interface15of the floating foundation10. Contact between the wind turbine component60and the upper attachment interface15of the floating foundation10is sensed and this contact is communicated to the lifting equipment50to stop paying out the tension line51.

The contact between the wind turbine generator component60and the upper attachment interface15of the floating foundation may be signalled in a variety of ways. For example, a contact sensor in the vicinity of the upper attachment interface15of the floating foundation may detect the contact with the wind turbine generator component60. Alternatively, a sensor on the floating foundation10may detect any slight variation in the vertical position of the floating foundation10from the predetermined vertical position. Alternatively, any change in the load supported by the lifting equipment50which deviates from the load of the fully suspended wind turbine generator component60may be detected. These various contact sensing schemes may be used alone or in combination and are given as purely illustrative examples from a non-exhaustive list and further alternatives will be appreciated by those skilled in the art. The lowering of the wind turbine generator component60is stopped once contact is made with the floating foundation10so as to substantially prevent any change in the vertical position of the floating foundation10in the body of water20.

Once contact is made between the wind turbine generator component60and the upper attachment interface15the wind turbine generator component60is connected to the floating foundation10. This connection may be made using locating pins or bolts16. This connection is made to substantially prevent lateral displacement of the wind turbine generator component60relative to the floating foundation10before removal of ballast water14. Preferably the connection is made to substantially prevent lateral displacement of the wind turbine generator component60relative to the floating foundation10before removing water ballast14from the floating foundation10equivalent to at least 5% of the weight of the wind turbine generator component60. Alternatively, this connection may be made before removing any of the water ballast14from the floating foundation10after contact with the wind turbine generator component60.

As shown inFIG.5the buoyancy of the floating foundation10is then increased. The buoyancy of the floating foundation10is increased by reducing the amount of the ballast water14within the floating foundation10. The ballast water14may be removed using the pump13under the action of the controller40so as to displace the ballast water14from the internal volume11through the port12and into the body of water20. As the buoyancy of the floating foundation10is increased the load of the wind turbine generator component60is gradually transferred from the lifting equipment50on to the floating foundation10. In this way, instead of lowering loads onto the floating foundation, the floating foundation is “lifted” until substantially the entire weight of the wind turbine generator component60is supported by the floating foundation10. During this load transfer of the wind turbine generator component60on to the floating foundation10the vertical position of the floating foundation10remains substantially unchanged.

The increase in buoyancy of the floating foundation10by removal of water ballast14from the floating foundation10continues until the floating foundation10supports substantially the entire weight of the wind turbine generator component60, at which point the controller40controls the water pump13to stop removing water ballast14from the floating foundation10.

Completion of the transfer of the load of the wind turbine generator component60onto the floating foundation10may be sensed in a variety of ways. For example, the tension in the tension line51may be monitored until it is judged that the lifting equipment50is no longer supporting substantially any of the weight of the wind turbine generator component60. Alternatively, monitoring the vertical position of the floating foundation10may be used to indicate any slight change in the vertical position of the floating foundation10, indicating that a further offloading of water ballast14will cause an increase in buoyancy to adjust the vertical position of the floating foundation10away from the predetermined vertical position. It will be understood that these examples of determining when transfer of the load of the wind turbine generator component60so as to be fully supported by the floating foundation10are purely illustrative examples of a non-exhaustive list and other determination means will be appreciated by those skilled in the art.

When the tension line51is supporting the weight of the wind turbine generator component60it may be expected that the tension line51may extend elastically under load. As this load is reduced it can be expected that this extension of the tension line51will reduce and so it may be appropriate to cause the lifting equipment50to pay out the tension line51by a small amount just sufficient to offset this retraction in the tension line as tension in the tension line51is reduced during transferring weight of the wind turbine generator component60onto the floating foundation10.

Once substantially the entire weight of the wind turbine generator component60has been transferred onto the floating foundation10the connections16are finally tightened to substantially prevent vertical displacement of the wind turbine generator component60relative to the floating foundation10. Where the connections16were initially locating pins, the locating pins may be removed and replaced with bolts which are subsequently torque tightened. Where the connections16were bolts, these may be left in situ and torque tightened. It is beneficial that the connections16prevent only lateral displacement and not also vertical displacement of the wind turbine generator component60relative to the floating foundation10whilst the buoyancy of the floating foundation10is being increased, and that the connections16prevent both lateral and vertical displacement of the wind turbine generator component60relative to the floating foundation10once some of the water ballast14has been removed from the floating foundation such that the entire weight of the wind turbine generator component60has been transferred onto the floating foundation.

FIG.6illustrates a near repeat of the process above with reference toFIG.3but in which a second wind turbine generator component61is connected to the attachment point52of the lifting equipment50. The second wind turbine generator component61is lowered as before towards the floating foundation until contact is made between a lower end of the second wind turbine generator component61and an upper end of the first wind turbine generator component60already installed on the floating foundation10. As before, the buoyancy of the floating foundation10remains substantially constant such that the vertical position of the floating foundation10is at a predetermined vertical position during lowering of the second wind turbine generator component61. This predetermined vertical position of the floating foundation10may be the same as that selected during the installation of the first wind turbine generator component60, or may be different. Preferably, the same predetermined vertical position of the floating foundation10is selected through installation of all of the wind turbine generator components.

FIG.7shows the step in the installation sequence substantially the same as that described above with reference toFIG.4but in which the second wind turbine generator component61just makes contact with the first wind turbine generator component60already installed on the floating foundation10. Once contact is made between the second wind turbine generator component61and the first wind turbine generator component60connections17are installed between the first and second wind turbine generator components60,61in a substantially identical manner to the installation of the connections16described previously.

FIG.8illustrates a step in the installation sequence similar to that described above with reference toFIG.5but in which the load of the second wind turbine generator component61is transferred from the lifting equipment50onto the floating foundation10by increasing the buoyancy of the floating foundation10by removing water ballast14from the floating foundation10. As before, the controller40controls the water pump13to eject some of the water ballast14from the internal volume11of the floating foundation10through the port12and into the body of water20. This continues until substantially the entire weight of the second wind turbine generator component61has been transferred onto the floating foundation10. The vertical position of the floating foundation10is substantially unchanged during transferring of weight of the second wind turbine generator component61onto the floating foundation10, as before.

FIG.9illustrates a step in the installation sequence similar to that described above with reference toFIG.8but in which the final wind turbine generator component62is being installed onto the floating foundation10to complete the installation of a wind turbine generator65onto the floating foundation10. As before, the buoyancy of the floating foundation10is increased by removal of water ballast14until substantially the entire weight of the final wind turbine generator component62is supported by the floating foundation10. The vertical position of the floating foundation10is substantially unchanged during transferring weight of the final wind turbine generator component62onto the floating foundation10.

As shown inFIG.10once the complete wind turbine generator65has been installed upon the floating foundation10the floating foundation10having the wind turbine generator65installed thereon may be towed by a vessel from the quayside30to a desired wind turbine generator operating location. The floating foundation10may be secured to the seabed18by catenary cables17. It is to be noted that the vertical position of the semi-submersible floating foundation10may be the same or different to the vertical position of the floating foundation10during installation of the wind turbine generator65onto the floating foundation.

It will be appreciated that although in the examples described above with reference toFIGS.1-10the floating foundation is described in relation to a semi-submersible platform, the floating foundation may be fully submersed in the body of water20. Furthermore, whilst the floating foundation10is shown inFIG.10secured to the seabed18with catenary mooring lines17, the floating foundation may be secured and stabilised using a variety of known systems, e.g. by a tension leg platform with anchors.

Furthermore, in an example where the body of water in which the foundation is floating is tidal, the water level will be dynamic. It may be desirable to maintain the vertical position of the floating foundation substantially constant with respect to the quayside30, or other fixed ground surface, during installation of the wind turbine generator where the water level is dynamic. Ballast may be added or removed from the foundation to account for the variation in the dynamic water level. This ballast variation may be performed alongside the ballast removal to account for the transfer of load during installation of the wind turbine generator. An advantage of keeping the vertical position of the floating foundation constant with respect to the fixed ground surface to account for the dynamic water level is that the lifting equipment50then does not need to be adjusted to accommodate for the dynamic water level. Alternatively, the vertical position of the floating foundation can be maintained substantially constant with respect to the water level, even though the water level is dynamic with respect to the fixed ground surface. The lifting equipment50can then be controlled to accommodate for the dynamic water level, e.g. by adjusting the length of the tension line/crane cable length. If the dynamic water level is due to tidal variation, a look up table of tidal height or dynamic water level modelling may be used in either case. Alternatively, a vertical position sensor system may be used for establishing the dynamic water level with respect to the fixed ground surface to determine the tidal height. The sensor system may be preferable as this could accommodate water level variations due to weather, e.g. storms, as well as predictable tidal effects. In one embodiment, the need for dynamic water level compensation could be overcome by keeping the water level constant by using a dam or lock arrangement.

FIG.11illustrates a further example of a floating foundation110having a plurality of stabilising columns including internal volumes111for containing ballast water114. Each stabilising column has a water pump113for controlling an amount of water ballast114in the respective internal volumes111of the stabilising columns. The stabilising columns are connecting by cross beams119. The water pumps113are coupled to a controller140. The floating foundation110has a level sensor118for sensing the vertical position of the floating foundation110in the body of water20and a tilt attitude sensor117for sensing the tilt attitude of the floating foundation110. The sensors117,118are coupled to the controller140. It is preferred that the components of the wind turbine generator are mounted upon one of the stabilising columns. Preferably the floating foundation110has three stabilising columns although any number of stabilising columns may be used.

The components of the wind turbine generator160are installed upon the floating foundation110in a substantially identical manner to that described above with reference toFIGS.1-10. Beneficially, since the floating foundation110has a plurality of internal volumes111for containing the water ballast114not only the vertical position but also the tilt attitude of the floating foundation110can be controlled during installation of the wind turbine generator components onto the floating foundation110. Removing or increasing the amount of water ballast114in any of the plurality of the internal volumes111under the action that the controller140and the water pumps113enables control of both the vertical position and tilt attitude of the floating foundation110. In this way the floating foundation110may be maintained at a substantially constant tilt attitude just prior to and/or during installation of the wind turbine generator component (s) onto the floating foundation. For example, the floating foundation110may be maintained at a substantially horizontal attitude prior to and/or during installation of the wind turbine generator components (s) onto the floating foundation110.

It is commonly found that wind turbine generators are designed with a rotor tilt angle such that the rotor shaft is inclined with respect to the horizontal. This rotor tilt angle may typically be around 6°. It may therefore be beneficial to set the floating foundation110at a tilt attitude offset from the horizontal for installation of the rotor, nacelle or other components of the wind turbine generator. It may be desirable to keep the floating foundation110substantially horizontal during installation of the tower components and then incline the floating foundation at a tilt angle for installation of the nacelle or rotor components before returning the floating foundation to a substantially horizontal attitude, e.g. before towing the floating foundation out to its desired operating location.

Whilst in the example described above with reference toFIG.11the level sensor118and the tilt attitude sensor117are shown mounted on the floating foundation110it will be appreciated that these and other sensors may be provided, and the sensors need not be located on the floating foundation. In a preferred example a plurality of pressure sensors any be provided on the floating foundation or integrated into the floating foundation, e.g. at an interface with the base of the wind turbine generator to be installed on the floating foundation. Alternatively, the sensor may be installed or integrated in the lifting equipment or in the wind turbine generator component to be installed.

As can be seen fromFIG.11, broken line ‘M’ indicates generally the centre of mass of the wind turbine generator160and this may be offset or significantly offset from a centre of buoyancy of the floating foundation110alone when the amount of water ballast114in each of the internal volumes111is identical. Therefore, it is to be expected that in order to maintain the floating foundation110substantially horizontal, or at some other preferred tilt attitude, the amount of water ballast114in the internal volumes111of the floating foundation110may be dissimilar. The mass of ballast water114removed may therefore not exactly match the mass of the wind turbine generator component being installed on to the floating platform to account for a difference between the centre of gravity of the ballast and the centre of gravity of the wind turbine generator component being installed.

FIG.12illustrates a further example in which the installation of the wind turbine generator or wind turbine generator components onto the floating foundation10is performed with lifting equipment50mounted, not to a quayside30as inFIG.1, but to a floating vessel80positioned adjacent the floating foundation10. In this way, installation of the wind turbine generator onto the floating foundation may be performed nearer the site of intended operation of the wind turbine generator, e.g. away from shore. In a further alternative example, the lifting equipment50may be mounted to a jack-up vessel.

Whilst in the above examples the wind turbine generator is described as being installed as a plurality of wind turbine generator components, in a further example an entire wind turbine generator may be constructed and brought into contact with the floating foundation as an entire wind turbine generator for installation on the floating foundation the steps of the installation however remain unchanged regardless of whether one component, a plurality of components, or an entire wind turbine generator are being installed onto the floating foundation.

In the above described examples, the floating foundation has variable buoyancy using water ballast, however it will be appreciated that other types of ballast may be used so long as a ballast may be added and removed from the floating foundation. For example, solid ballast weights, e.g. concrete or stones, may be loaded and offloaded to/from the floating foundation to vary the buoyancy of the floating foundation. However, water is preferred as it allows for a smooth change in buoyance.