FIELD OF THE INVENTION

The present invention relates to a method for assembly, launch, and potential recovery inspecting, maintaining, repairing or decommissioning of a floating offshore wind turbine construction. In particular, it relates to a method as described in the preamble of the independent claim.

BACKGROUND OF THE INVENTION

Floating offshore wind turbines are typically launched using one of three methods, drydock launch, semisubmersible barge launch, or slipway launch.

In drydock launch, the floating support structure is constructed in a drydock and is launched by flooding of the dock. Following launch, the floating support structure is towed to quayside where the wind turbine is subsequently installed using an onshore crane located adjacent to the quayside.

In semisubmersible barge launch, the floating support structure is constructed onshore, transferred to one or more semisubmersible barges, and launched by submersion of the barge or barges. Also here, following launch, the floating support structure is towed to quayside where the wind turbine is subsequently installed using an onshore crane located adjacent to the quayside.

In slipway launch, the floating support structure is constructed onshore, transferred to skids and launched by skidding on a slipway. Examples are disclosed in International patent application WO2016/138088. Following launch the floating support structure is towed to quayside where the wind turbine is subsequently installed using an onshore crane located adjacent to the quayside. Examples are disclosed in WO2015/120227. Floating support structures of the single-column spar type are typically installed by other means due to the large draft of the floating support structure. Recent projects based on the single-column spar floating support structure concept have been installed using a large floating crane vessel.

The presentation “Port and Shipyard Requirements for the Installation of Floating Wind Turbines” by Crowle and Thies from the University of Exeter published at RINA London Branch 21 October 2021, London, UK, presents a summary of the above installation methods.

The article “Challenges during installation of floating wind turbines” by Crowle and Thies, published at the 5th International Conference on Offshore Renewable Energy 26-27 August 2021, Online CORE 2021, describes the installation method as follows, noting that this is a high-level procedure that varies somewhat for each substructure type, the strategy chosen by the developer, and the availability of installation vessel and port facilities:

• Load-out of the floating substructure from the fabrication yard. Typically, by either flooding the dry dock, using a slipway, or using a heavy transport vessel to transport the substructure to the water.

• Installing the turbine assembly onto the substructure using onshore cranes (not the case for spar-buoys, see below).

• Spar buoys will be towed to a sheltered area to be ballasted and have the turbine installed onto the substructure (using a crane vessel), before final transit to site.

• Final commissioning of turbine and substructure systems.

The installation methods described above have various disadvantages.

Drydock launch is severely constrained by the dimensions of available drydocks. Floating support structures for present-day wind turbine sizes typically have width of 100 m or more, and the world has very few dry docks of the necessary width. The relevant, large drydocks tend to be heavily utilized for shipbuilding purposes. Semisubmersible barge launch is similarly constrained by the amount of available semisubmersible barges of the required dimensions available on the world market. This constraint resembles the installation vessel constraint known from bottom-fixed offshore wind, potentially eroding one of the key advantages of floating offshore wind relative to bottom-fixed offshore wind, which is the absence of large offshore installation vessels.

Semisubmersible barge launch has the additional disadvantage that the transition of the floating support structure to the semisubmersible barge depends on the sea state in the port of construction. Most ports are tidal, having some variation of water level over the day, and most ports are also to some extent experiencing wave action, in the form of swell from the water body outside the port entrance and/or in the form of wakes from passing vessels. The changes in sea state cause the semisubmersible barge to move relative to the floating support structure being loaded out from its onshore location, often leading to delays in the installation process while waiting for better conditions.

All of the typical installation methods described above have the disadvantage that the wind turbine installation is taking place in a setup where the floating support structure is moored at the quayside, after which the wind turbine is installed using an onshore crane. The fact that most ports are tidal and are to some extent experiencing wave action in the form of swell from the water body outside the port entrance and/or in the form of wakes from passing vessels causes the floating support structure to move relative to the onshore crane. As is the case for the floating support structure launch using semisubmersible barges, such movements often lead to delays in the installation process while waiting for better conditions.

For floating support structures carrying wind turbines in the 2-3 MW class, these problems have sometimes been mitigated by installing the wind turbine on the floating support structure while in the drydock or while on land prior to barge launch. However, these methods are not really realistic when it comes to modem wind turbines of 12-15 MW rating and above. Other installation methods that try to overcome these challenges are described in the literature.

US 2011/0119889 describes a method whereby a spar buoy floating support structure is gripped by a structure attached to a floating crane. This method ensures uniform and synchronized motions by the floating support structure and the crane, but at the significant cost associated with purpose-built vessels operating at sea.

US 2022/0316446 describes a method whereby a semisubmersible floating support structure is launched from an extended deck carried by two vessels. This method ensures uniform and synchronized motions by the floating support structure and the crane, but at the significant cost associated with purpose-built vessels operating at sea.

None of the above installation methods offer easy opportunities for genuine “conveyor belt” installation of large-scale floating offshore wind projects. Dry dock and semisubmersible barge constraints restricts installation windows, and day-to-day changes to the sea state when installing the wind turbine at quayside further introduces stops and delays in the installation processes.

Similar difficulty applies in case the floating support structure has to be taken out of the water for inspection, maintenance or repair. Here, conventional methods suffer from all the above-mentioned disadvantages, making it very costly and time-wise uncertain to be able to take a floating support structure back to an onshore location.

It would be desirable to have a method for launching and recovering a floating support structure for modem wind turbines of 12-15 MW rating and above that avoids the disadvantages of the known methods. In particular, it would be desirable to have a method that permits true “conveyor belt” installation of large-scale floating offshore wind projects. DESCRIPTION / SUMMARY OF THE INVENTION

It is therefore an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide an improved launch method for floating offshore wind turbines of 12-15 MW rating and above where the support structure is launched into water while the wind turbine is already mounted on the support structure.

It is also an objective to provide an improved recovery method for floating offshore wind turbines of 12-15 MW rating and above where the support structure is recovered to an onshore position for inspection, maintenance, repair or decommissioning while the wind turbine remains mounted on the support structure.

These objectives and further advantages are achieved with a method and system as described below and in the claims.

A floating offshore wind turbine construction comprises a wind turbine in combination with a floating support structure. The floating support structure may be a semisubmersible or tension leg support structure, or it may be a spar support structure using a separate keel. Irrespective of the type of floating support structure the floating offshore wind turbine construction is intended for floating in a water body at a water surface.

For example, the wind turbine is supported on a semisubmersible floating support structure that comprises a tower support carrying the tower of the wind turbine. The support structure comprises at least one buoyancy member, but typically a plurality of buoyancy members, providing buoyancy to the support structure when in water. For example, the buoyancy members are arranged at lateral distances relatively to a central axis of the tower, typically vertical axis. Typically, each buoyancy member comprises one or more buoyancy columns fixed to a node of the support structure. Such buoyancy columns may typically be made of steel or reinforced concrete.

For example, the offshore location is in sea water but can also be a lake or other offshore body of water. For simplicity, it is exemplified in the following for sea water without delineating from the general principle of use in other offshore waters, such as lake water.

The invention is based on the use of an installation site comprising a combination of one or more of the following elements: i) one or more onshore cranes, ii) one or more onshore stations for assembly, repair and maintenance, and disassembly, iii) an onshore transport system, and iv) a slipway.

The onshore crane may be a local crane available in the relevant port. It may also be a ring crane, e.g. a Mammoet PTC 200-DS or similar, or one or more crawler cranes, e.g. Liebherr LH 11350 or similar. The crane or cranes may be fitted with boom extension to allow for turbine nacelle and blade installation at hub height. Combinations of the above and other types of cranes are also possible.

The onshore station or stations for assembly, repair and maintenance, and disassembly may be specific locations suitable for crane work and for other relevant completion works.

An assembly station may be established adjacent to a fixed crane, facilitating standardized operations in assembly, repair and maintenance, and disassembly. One or more assembly stations may also be established at one or more locations suitable for crane works using moveable cranes, such as crawler cranes.

One or more stations may be established for completion of the floating offshore wind turbine, including the floating support structure. Such completion may comprise internal completion of the wind turbine after assembly, e.g. connection of cabling and final tensioning of bolts, mounting of accessories such as navigation lights, energization and pre-commissioning, and even trial operation within specific operating conditions that leave sufficient margins on the stability of the structure while standing upright onshore.

One or more stations may be established for inspection, maintenance or repair of the floating support structure or the wind turbine using smaller cranes that may not be capable of lifting main parts of the structure such as the entire rotor, the nacelle, the tower or similar, but are capable of lifting relevant main components, such as blades, gearboxes, generators, etc.

One or more stations may be established for the dry storage of completed floating offshore wind turbines awaiting launch, or for floating offshore wind turbines having been recovered and awaiting inspection, maintenance or repair, or for floating offshore wind turbines awaiting disassembly at the end of useful life.

An onshore transportation system may be established based on skidding. The skidding tracks may be permanently installed, or they may be removable. Movements of components of the floating support structure and of the completed floating support structure, also with the wind turbine mounted, may take place through sliding of the components or structures on the skidding tracks, or it may take places using wheel-based trolleys or other friction-reducing equipment. Alternative transportation systems may be established based on SPMTs (Self-Propelled Modular Transporters), rollers or other systems that are not based on tracks but rely on the rolling or sliding on top of large flat or inclined surfaces.

A particular advantage may be established with an arrangement of skidding tracks suitable for a given floating support structure type. For example, a triangular semisubmersible floating support structure may advantageously be transported using two skidding tracks, where one tracks supports two of the tree legs at the vertices of the triangular structure or using three skidding tracks where each track supports one of the three legs at the vertices of the triangular structure.

A slipway may be constructed that forms an inclined plane connecting one or more largely flat and horizontal areas used for assembly, completion, inspection, maintenance or repair, or disassembly of floating offshore wind turbines with the sea or with any other relevant body of water. The slipway may be made of concrete or steel, and it may be fitted with skids to facilitate the launch process.

A particular advantage is established with the slipway arrangement comprising three parallel tracks. In this arrangement each of three legs at vertices of a triangular structure is supported by a corresponding track at launch, for example on corresponding trolleys, and moved along the tracks down into the water, until the support structure floats off the tracks by the lift of buoyancy members.

A further particular advantage is established with a slipway arrangement comprising three parallel tracks, where the middle track is offset away from the quayside relative to the two lateral tracks, and where each of three legs at vertices of a triangular structure is supported by a corresponding track at launch. Such offset will cause the floating offshore wind turbine to have an inclination during launch down the slipway that is less than the inclination of each of the tracks. This will improve stability of the floating offshore wind turbine during launch.

A further particular advantage is established with an offset of the middle track that is equal to the distance from the line connecting the centre of the two legs that are moving on the lateral tracks to the centre of the third leg that is moving on the middle track. At this particular offset the floating offshore wind turbine will remain vertical during launch down the slipway. This will further improve the stability of the floating offshore wind turbine during launch.

Accordingly, the support structure, advantageously while carrying the wind turbine, is moved from the platform onto the three tracks such that the first, second and third leg is supported by the first, second and third track, respectively, at the same time. The support structure is moved along the three tracks in parallel with the tracks down into the water until the support structure floats off the tracks by the lift of buoyancy members. In order for the support structure floating off the track, the tracks extend into the water to a water depth DI that is sufficiently deep under the water surface for maintaining the orientation of the support structure until it floats off the tracks. As a floating support structure, in particular semisubmersible, the support structure extends to a depth D2 under water when floating off the tracks, which is less than the water depth DI.

The improved installation and launch method for floating offshore wind turbines of 12-15 MW rating and above according to the invention has a number of steps.

Firstly, the floating support structure is made ready for wind turbine installation. The floating support structure may be assembled at the installation site, or it may be assembled elsewhere.

An advantageous assembly arrangement at the installation site can be arranged by the assembly from prefabricated modules at an assembly station.

If assembled elsewhere, the floating support structure may be brought to the installation site using the onshore transportation system. Alternatively, it may be brought to the installation site by sea transport, being skidded up the slipway onto the transportation system, or being lifted onto the transportation system using one or more cranes.

Secondly, once the floating support structure is ready for turbine installation, it is placed in a turbine assembly station. The turbine assembly station may be the same station as that where a floating support structure assembled at the installation site is assembly from prefabricated modules, or it may be a different assembly station.

Next, the wind turbine is installed on the floating support structure using one or more cranes.

A particular advantage may be achieved by ballasting the floating support structure prior to the installation of the wind turbine. Such pre-ballasting will improve the onshore stability of the floating offshore wind turbine construction comprising the floating support structure and the wind turbine, and it will save ballasting work after the launch of the floating offshore wind turbine.

Next, the floating offshore wind turbine construction comprising the floating support structure and the wind turbine is completed. Such completion may comprise internal completion of the wind turbine after assembly, e.g. connection of cabling and final tensioning of bolts, mounting of accessories such as navigation lights, energization and pre-commissioning, and even trial operation within specific operating conditions that leave sufficient margins on the stability of the structure while standing upright onshore. The completion may be carried out at the turbine assembly station, or it may be carried out at one or more different stations. A particular advantage may be achieved by ballasting the floating support structure prior to any trial operation of the wind turbine if such ballasting has not been carried out prior to the installation of the wind turbine. Such pre-ballasting will improve the onshore stability of the floating offshore wind turbine comprising the floating support structure and the wind turbine, and it will save ballasting work after the launch of the floating offshore wind turbine.

Next, if not already located at the slipway, the floating offshore wind turbine construction is transported to the slipway. Prior to the transport to the slipway, the floating offshore wind turbine construction may be temporarily stored at an onshore storage station.

Next, the floating offshore wind turbine construction is launched using the slipway. The movement down the slipway is continued until the floating offshore wind turbine construction floats off the slipway, whether off the slipway itself, off skids on the slipway, or off supports used to support the floating support structure on the slipway or the skids.

A particular advantage can be established by providing the floating offshore wind turbine construction with pivoted supports that permit the transfer from the horizontal plane to the inclined plane of the slipway without causing edge loads on the underside of the floating support structure.

A particular advantage can be established by providing the floating offshore wind turbine construction with additional ballast prior to launch using the slipway, thereby ensuring stability also on the inclined plane of the slipway.

Following launch, the floating offshore wind turbine construction may be moored in the port for final completion, for awaiting the availability of installation vessels or similar, or it may be towed directly from the slipway to the offshore installation site. In a similar way, the improved method for the recovery of floating offshore wind turbine constructions of 12-15 MW rating and above according to the invention has a number of steps.

Firstly, the floating offshore wind turbine construction is towed to the slipway. Here, it is transferred onto supports facilitating the use of the transportation system of the installation site. The supports may be pivoted to permit the transfer from the inclined plane of the slipway to the horizontal plane of the installation site.

The floating offshore wind turbine may be de-ballasted partly or completely before being transported up the slipway.

Next, the floating offshore wind turbine construction is transported up the slipway onto the horizontal surface of the installation site.

Next, the floating offshore wind turbine construction is transported to one or more stations for inspection, maintenance, repair or decommissioning of the floating support structure or the wind turbine. The station may be located adjacent to the slipway.

Following completion of the inspection, maintenance or repair works the floating offshore wind turbine may again be launched using the slipway.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, where FIG. l is a drawing in three-dimensional perspective view of a floating offshore wind turbine;

FIG. 2 is a side view of the semisubmersible offshore wind turbine in floating conditions,

FIG. 3 illustrates an alternative semisubmersible offshore wind turbine;

FIG. 4 illustrates a further alternative example of a semisubmersible offshore wind turbine;

FIG. 5 A-D are perspective illustrations of installation sites;

FIG. 6 is a perspective illustration of a slipway extending from a platform into water; FIG. 7 is a head-on illustration of a slipway extending from a platform into water;

FIG. 8 is a side view of a launch situation.

DETAILED DESCRIPTION / PREFERRED EMBODIMENT

FIG. 1 illustrates a floating offshore wind turbine construction 1. The floating offshore wind turbine construction 1 comprises a wind turbine 2 and a floating offshore support structure 3 with a tower support 8 on which the wind turbine 2 is mounted for operation and by which it is supported in offshore conditions. The wind turbine 2 comprises a rotor 5 and a tower 7 as well as a nacelle 6 that connect the rotor 5 with the tower 7.

The offshore support structure 3 is a semisubmersible floating offshore structure with buoyancy members 9A 9B, 9C that assist in keeping the support structure 3 partially above water and which give the semisubmersible floating structure stability. An example of a water surface 4 relatively to the vertical extension D2 of the buoyancy members 9A 9B, 9C under the water surface 4 is illustrated in FIG. 2. It is observed that the buoyancy members 9A 9B, 9C are semi-submersed in the water. When floating, the support structure is extending a structure depth D2 under the water surface 4.

Semisubmersible support structures are typically used with mooring lines (not shown) fastened to the seabed in order to maintain the support structure 3 at the location. For vertically damping influence of waves on the floating support structure 3, heave plates 14 extend horizontally from the bottom of the buoyancy members 9 A, 9B, 9C. Each buoyancy member 9 A, 9B, 9C, including its heave plate 14 functions as a leg 13 while on land or other horizontal construction platform.

The exemplified structure 3 has a tetrahedral shape that comprises a first radial brace 11 A that extends from a lower part of the tower support 8 to the first buoyancy member 9A at the most distal node, relatively to the tower support 8, and two further radial braces 1 IB, 11C that extend from the lower part of the tower support 8 to each of the other two remaining buoyancy members 9B, 9C at nodes on opposite sides of the tower support 8. Further stability is achieved by two additional braces 10A that extend from the most distal buoyancy member 9A to the two other buoyancy members 9B, 9C. The two additional side braces 10A form a planar triangular shape with the two shorter radial braces 1 IB, 11C. As exemplified in FIG. 2, the buoyancy members 9A, 9B, 9C are located at the three nodes of the tetrahedron.

The term radial braces is used for braces 11 A, 1 IB, 11C that extend radially away from the tower support 8, and the term diagonal brace is used for a brace 12 A, 12B, 12C that is a diagonal side of a vertical triangle formed by the tower support 8, one of the radial braces 11 A, 1 IB, 11C and one of the diagonal braces 12A, 12B, 12C.

The tower support 8 is exemplified as a cylindrical support column with a central axis 15 that is also a central axis of the tower 7. However, the tower support 8 could have other shapes. As illustrated in FIG. 2, the tower support 8 extends to a position above the water surface 4, which is characteristic for semisubmersible floating support structures.

In the triangular arrangement, as illustrated in FIG. 1 and 2, the corners of the tetrahedron are arranged according to an elongate isosceles triangle having a width B at the base and an altitude A from the base to the vertex.

However, optionally, the triangle is an equilateral triangle.

As illustrated in FIG. 1, the tower support 8 is located midway on the base of the triangle. In other embodiments, the tower support 8 is in the centre of a triangle, for example an equilateral triangle.

FIG. 3 illustrates and alternative structure, where the tower support is arranged centrally between buoyancy members, each of which comprises a pair of buoyancy columns. The pairs of buoyancy columns are arranged at the comers of an equilateral triangle.

FIG. 4 illustrates a further example, where the tower is arranged in the centre of the triangle and the support structure is not a tetrahedron, but where the tower support is connected to the buoyancy members by horizontal bottom bars, which optionally serves as tanks for sea water as permanent ballast. These are examples for illustration, and other examples of triangles and positions of the tower support are possible.

FIG. 5 A is a perspective illustration of an installation site 100. The installation site 100 has a number of stations, an assembly station 200, a completion station 300, a storage section 400, and a launch station 500. As seen in FIG. 5A, a floating offshore wind turbine construction 600 has been launched and is in the process of being towed to the installation site.

As shown in more detail in FIG. 5B, a ring crane 201 is placed at the assembly station 200. It is connected with skid tracks 202 to two floating support structure assembly stations 203 and 204. Floating support structures 3 are in partial stages of assembly using smaller cranes not shown. A wind turbine 2 is being mounted by the ring crane 201 on a floating support structure 3.

The ring crane 201 may advantageously be located adjacent to the quayside so as to be available for unloading of incoming goods from ships and barges.

As illustrated in FIG. 5B, a floating offshore wind turbine construction 1 is located at the completion station 300. The floating offshore wind turbine construction 1 has been transported to the completion station 300 using skidding on skid tracks 302. A medium-voltage cable is connected to the floating offshore wind turbine construction 1, enabling not only mechanical completion (final tightening of bolts, mounting of cables, mounting of the last equipment, etc.) but also electrical completion (checking of all electrical connections) and trial operation.

As better seen in FIG. 5C, a number of floating support structures 3 and a number of floating offshore wind turbines constructions 1 are located in the storage station 400. They have been transported to the storage station 400 on skidding tracks 405.

As illustrated in FIG. 5D, a floating offshore wind turbine construction 1 is located at the launch station 500 comprising a horizontal or near horizontal platform 16 and a slipway 23. A second floating offshore wind turbine construction 1 is in the process of being launched down the slipway 503. FIG 6 and FIG. 7 show a situation where a wind turbine installation 1 is positioned on a platform 16 at the end of a slipway 23, shown as a ramp, that has been built with offset of the tracks 17A, 17B, 18. Typically, the support structure 3 would be pushed on trolleys 21 to the slipway 23 or lifted by a crane to this position.

The slipway 23 comprises a first track 17A, a second track 17B and third track 18 extending in parallel to each other from the platform 16 and into the water below the water surface 4. The first track 17A and the second track 17B are arranged symmetrically on opposite sides of the third track 18, which is a central track. The first track 17A and the second track 17B are horizontally offset from the third track 18 by a distance equal to the altitude A of the triangle, which is illustrated in more detail in FIG. 7.

As illustrated in FIG. 8, floating offshore wind turbine construction Imoves along the three tracks 17 A, 7B, 18 down into the water until the support structure 3 including the wind turbine 2 floats off the tracks 17A, 7B, 18 by the lift of buoyancy members 9 A, 9B, 9C, which are shown in more detail in FIG. 1. The tracks 17 A, 7B, 18 extends into the water to a water depth DI below the surface 4, which is sufficiently deep under water for maintaining the orientation of the support structure 3, until the wind turbine installation floats off the tracks 17 A, 7B, 18 due to its buoyancy members 9 A, 9B, 9C. As mentioned above, when starting floating, the support structure 3 extends a depth D2 underneath the water surface 4, which is less than the depth DI to which the tracks 17A, 7B, 18 extends under water at their ends. This is illustrated in more detail in FIG. 7, which shows the situation where the wind turbine installation 1 is about to float and not any more supported by the tracks 17 A, 7B, 18.

For ease of motion along the tracks 17A, 7B, 18, illustrated by double arrow 22, each leg 13 of the support structure 3 is carried by a wedge-shaped trolley 21, of which there are provided one for each track 17 A, 7B, 18 and typically with rollers on its underside. Optionally, the trolleys have brakes in order to control the speed of the launch. This allows use of relatively steep slipways/ramps 23, which is useful for narrow waters at the end of the slipway/ramp 23, such as in a harbour, river, or canal. Optionally, the tracks at least one, but advantageously at least two of the tracks 17 A, 17B, 18 is provided with rails for better guidance of the trolleys during launch.

Typical dimension ranges for the system outlined above for turbines in the 12-15 MW range are as follows:

• The installation site 100 may have a length of around 300 m or more in a direction parallel to the quayside and a width of around 500 m or more in a direction perpendicular to the quayside. • The wind turbine 2 may have a rotor diameter of 220 m or more and a distance from the tower bottom flange to the rotor centre of 115 m or more.

• The floating support structure 3 may have a length of 100 m and a width of 100 m.

• The slipway 503 may have a width of around 120 m or more and an inclination of 3-10 degrees. • The tacks 17A, 17B, 18 may have a width of 15 m or more and an inclination of 3-

30 degrees.