GB2600927A - Installing offshore floating wind turbines - Google Patents

Installing offshore floating wind turbines Download PDF

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Publication number
GB2600927A
GB2600927A GB2017664.0A GB202017664A GB2600927A GB 2600927 A GB2600927 A GB 2600927A GB 202017664 A GB202017664 A GB 202017664A GB 2600927 A GB2600927 A GB 2600927A
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GB
United Kingdom
Prior art keywords
floating
crane
offshore
wind turbine
floating platform
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
GB2017664.0A
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GB2600927B (en
GB202017664D0 (en
Inventor
Barratt Nysaether Jon
Johansen Bjørn
Byklum Eirik
Ragnar Austreng Knut
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Equinor Energy AS
Original Assignee
Equinor Energy AS
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Publication date
Application filed by Equinor Energy AS filed Critical Equinor Energy AS
Priority to GB2017664.0A priority Critical patent/GB2600927B/en
Publication of GB202017664D0 publication Critical patent/GB202017664D0/en
Priority to PCT/NO2021/050232 priority patent/WO2022098246A1/en
Publication of GB2600927A publication Critical patent/GB2600927A/en
Application granted granted Critical
Publication of GB2600927B publication Critical patent/GB2600927B/en
Active legal-status Critical Current
Anticipated expiration legal-status Critical

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B21/00—Tying-up; Shifting, towing, or pushing equipment; Anchoring
    • B63B21/50—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers
    • B63B21/502—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers by means of tension legs
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B35/00—Vessels or similar floating structures specially adapted for specific purposes and not otherwise provided for
    • B63B35/44—Floating buildings, stores, drilling platforms, or workshops, e.g. carrying water-oil separating devices
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B75/00—Building or assembling floating offshore structures, e.g. semi-submersible platforms, SPAR platforms or wind turbine platforms
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B66—HOISTING; LIFTING; HAULING
    • B66C—CRANES; LOAD-ENGAGING ELEMENTS OR DEVICES FOR CRANES, CAPSTANS, WINCHES, OR TACKLES
    • B66C23/00—Cranes comprising essentially a beam, boom, or triangular structure acting as a cantilever and mounted for translatory of swinging movements in vertical or horizontal planes or a combination of such movements, e.g. jib-cranes, derricks, tower cranes
    • B66C23/18—Cranes comprising essentially a beam, boom, or triangular structure acting as a cantilever and mounted for translatory of swinging movements in vertical or horizontal planes or a combination of such movements, e.g. jib-cranes, derricks, tower cranes specially adapted for use in particular purposes
    • B66C23/36—Cranes comprising essentially a beam, boom, or triangular structure acting as a cantilever and mounted for translatory of swinging movements in vertical or horizontal planes or a combination of such movements, e.g. jib-cranes, derricks, tower cranes specially adapted for use in particular purposes mounted on road or rail vehicles; Manually-movable jib-cranes for use in workshops; Floating cranes
    • B66C23/52—Floating cranes
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
    • F03D—WIND MOTORS
    • F03D13/00—Assembly, mounting or commissioning of wind motors; Arrangements specially adapted for transporting wind motor components
    • F03D13/10—Assembly of wind motors; Arrangements for erecting wind motors
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
    • F03D—WIND MOTORS
    • F03D13/00—Assembly, mounting or commissioning of wind motors; Arrangements specially adapted for transporting wind motor components
    • F03D13/20—Arrangements for mounting or supporting wind motors; Masts or towers for wind motors
    • F03D13/25—Arrangements for mounting or supporting wind motors; Masts or towers for wind motors specially adapted for offshore installation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B35/00—Vessels or similar floating structures specially adapted for specific purposes and not otherwise provided for
    • B63B35/44—Floating buildings, stores, drilling platforms, or workshops, e.g. carrying water-oil separating devices
    • B63B2035/442—Spar-type semi-submersible structures, i.e. shaped as single slender, e.g. substantially cylindrical or trussed vertical bodies
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B35/00—Vessels or similar floating structures specially adapted for specific purposes and not otherwise provided for
    • B63B35/44—Floating buildings, stores, drilling platforms, or workshops, e.g. carrying water-oil separating devices
    • B63B2035/4433—Floating structures carrying electric power plants
    • B63B2035/446—Floating structures carrying electric power plants for converting wind energy into electric energy
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2230/00—Manufacture
    • F05B2230/60—Assembly methods
    • F05B2230/61—Assembly methods using auxiliary equipment for lifting or holding
    • F05B2230/6102—Assembly methods using auxiliary equipment for lifting or holding carried on a floating platform
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2240/00—Components
    • F05B2240/90—Mounting on supporting structures or systems
    • F05B2240/93—Mounting on supporting structures or systems on a structure floating on a liquid surface
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/727—Offshore wind turbines

Abstract

A method of installing a floating offshore wind turbine 8 is provided. Installation of an offshore wind turbine is achieved by providing a crane 1 mounted on a floating structure 3 at an offshore location, the floating structure being tethered to the seabed 5 by one or more taut moorings 6. The crane is used to install an offshore wind turbine on a floating platform 11 at the offshore location. An offshore crane assembly for installing an offshore wind turbine on a floating platform, and an installation assembly including the offshore crane assembly are also provided.

Description

INSTALLING OFFSHORE FLOATING WIND TURBINES
The present disclosure relates to a method of installing a floating offshore wind turbine and to an offshore crane assembly for installing an offshore wind turbine on a floating platform.
Offshore wind turbines can be designed as fixed-foundation wind turbines. However, for deeper waters, they are more commonly designed as floating structures, comprising a floating platform, such as a floating spar platform, a tower mounted on top of the floating platform, a nacelle mounted to the top of the tower, and a plurality of blades mounted via a rotor hub to the nacelle.
During installation of a floating wind turbine, cranes are often used to lift components of the wind turbine, such as the tower and the nacelle, onto the floating platform, at an offshore location, so that they can be mated with the platform.
The mating process is very sensitive to relative motions between the crane and the floating platform. These relative motions can arise due to the effects of swell or waves on the floating platform and/or crane. This means that the installation process can be very sensitive to weather conditions and time spent waiting for acceptable weather conditions may be considerable.
Fixed cranes are often used during the mating process to minimise relative motion between the crane and the floating platform. These are typically land based cranes or cranes mounted on an offshore structure fixed to the sea floor. However, it is not always possible to utilise fixed cranes. Large floating wind turbines, of the so-called spar type for instance, regularly require water depths of up to 100m or more to accommodate the draft of the structure. This therefore restricts installation to deeper waters, often away from the shore, where it is not practical or may be impossible to provide a fixed crane structure.
In a first aspect, the present invention provides a method of installing a floating offshore wind turbine, the method comprising: providing a crane mounted on a floating structure at an offshore location, the floating structure being tethered to the sea bed by one or more taut moorings; providing a floating platform; and at the offshore location, installing an offshore wind turbine on the floating platform using the crane.
By mounting the crane on a floating structure that is tethered to the sea bed by taut moorings, vertical movement of the floating structure can be limited thereby making the process of installing an offshore wind turbine on a floating platform less sensitive to weather and sea conditions. The method therefore provides a greater range of acceptable weather conditions for offshore wind turbine installation, and can minimise delay caused by waiting for acceptable weather conditions. Accordingly, the term "taut mooring" refers to a mooring arrangement in which one or more mooring lines or members extending between the floating structure and the sea bed remains in tension regardless of the actions of waves, wind or currents on that structure.
As such, the taut moorings mean that the crane can be maintained at an approximately constant vertical position. Hence, relative vertical movement between the crane and the floating platform can be minimised during installation to a level that may be comparable to fixed crane structures. This allows for the necessary precision and sensitivity when installing offshore wind turbines on floating platforms, even at offshore locations having depths too great for a fixed crane structure. This is particularly beneficial when installing large offshore floating wind turbines, which require greater depths to accommodate the draft of the structure.
In comparison, in conventional moorings, such as catenary moorings, the moorings are not pulled taut and are not constantly in tension. That is to say, there is a degree of slackness in conventional moorings which allows the floating structure to move. Hence, conventional moorings do not provide the necessary vertical stability.
The offshore location may be known as an offshore assembly site.
The floating structure and/or the floating platform may comprise a vertically elongate buoyant structure, such as a spar buoy. The buoyant structure preferably has a vertical length of greater than 50m, preferably greater than 100m. The buoyant structure may be circular, or partially circular in cross-section. The diameter of the buoyant structure may be between 5m and 25m, for instance 15m. The floating structure and/or the floating platform may have a draft of greater than 50m, or greater than 100m.
The floating platform on which the wind turbine is to be mounted may additionally be tethered to sea bed by one or more moorings, at least for the duration of the installation process. The mooring(s) may be taut moorings, and may be similar to those used to tether the floating structure (on which the crane is mounted) to the sea bed. Alternatively, the floating platform may be free floating during the installation process. 3 -
The mooring(s) are configured to minimise vertical movement of the crane and/or floating platform to a level that facilitates installation of the wind turbine. However, a certain degree of vertical movement is inevitable due to stretch in the moorings and other tolerances. They may limit vertical movement of the floating structure and/or the floating platform to less than lm, preferably less than 500mm, more preferably less than 250mm, most preferably less than 100mm. The mooring(s) may limit a velocity of the floating structure in the vertical direction to less than 0.5 ms-1, preferably less than 0.2 ms-1, most preferably less than 0.1 ms-1.
The mooring(s) may additionally be arranged to limit horizontal movement of the floating structure and/or the floating platform. Horizontal movement of the floating structure and/or the floating platform may be limited to, for example, less than 1m, preferably less than 500mm.
Alternatively or additionally, horizontal stability of the floating structure and/or the floating platform may be provided by one or more thrusters on the floating structure and/or the floating platform. Thus, the thruster(s) may be arranged to limit horizontal movement of the floating structure and/or the floating platform. For instance, propeller(s) and/or pump-jet(s) may be provided on the floating structure and/or the floating platform.
The mooring(s) may comprise one or more of a polyester rope and/or a steel tendon.
Tethering the floating structure and/or the floating platform to the sea bed may comprise adding ballast to the floating structure and/or the floating platform to cause the floating structure and/or floating platform to sink lower in water. The ballast may be water, preferably sea water, for example. The floating structure and/or floating platform may have ballast tank(s) for this purpose and may have associated pumps. The one or more moorings may then be attached between the sea bed and the floating structure and/or the floating platform. At least some of the ballast may be removed from the floating structure and/or the floating platform to increase buoyancy and cause the mooring(s) to be pulled taut.
Additionally, moorings, such as beams, chains or ropes, may be arranged between the floating structure and the floating platform. This can further minimise relative motion between the two structures during installation. The floating structure and/or floating platform may also be tethered to one or more towing vessels, such as tugs. The towing vessel(s) may be used to maintain a constant distance between the floating structure and the floating platform. 4 -
The crane may comprise a crane mast and a lifting arm mounted to the top of the crane mast. The lifting arm is preferably rotatably mounted to the top of the crane mast. The lifting arm may be mounted to the crane mast via a trunnion, for example. The lifting arm may comprise a truss or lifting frame.
The crane may comprise one or more of a winch, a sheave, a power unit and/or a lifting wire. Preferably, the winch is used to pull in or let out the lifting wire.
The crane may be suitable to lift heavy components. For example, the crane may have a lifting capacity of 1000 tonnes or greater.
The crane may include a heave compensation system. This may further reduce the influence of sea conditions, such as waves and swell on the crane. The heave compensation system may be a passive or active system.
Optionally, the vertical position of the floating structure may be adjusted to minimise relative vertical motion between the floating structure and the floating platform. This may be done by operating a winch on each of the one or more moorings to alter the vertical position of the floating structure.
The offshore wind turbine may include a tower, a nacelle mounted to the top of the tower, and one or more rotor blades mounted via a rotor hub to the nacelle. The nacelle is preferably rotatably mounted to the top of the tower. A generator and its associated electronics may be located on or in the nacelle. The generator is preferably configured to be driven by rotation of the rotor hub.
The offshore wind turbine may include (e.g.) three rotor blades. The rotor blades may be rotatably mounted to the rotor hub, for example such that their pitch may be controlled. The rotor hub and rotor blades together may form a rotor of the offshore wind turbine.
The offshore wind turbine may have a rotor diameter of greater than 100m, more preferably greater than 150m and most preferably greater than 200m. Hence, the rotor blades may have a length greater than 50m, greater than 75m or greater than 100m.
The rotor hub is preferably suitably located to accommodate the rotor.
Accordingly, the rotor hub height of the offshore wind turbine may be greater than 100m, greater than 150m or greater than 200m.
The offshore wind turbine may have a rated power output of greater than 3MW, preferably greater than 6MW and more preferably greater than 10MW. Providing the crane may include transporting the crane to the offshore location. The crane may be towed to the offshore assembly site by a vessel, for
-
instance. The offshore location (i.e. the assembly site) may not be the same as a proposed installation site for the offshore wind turbine. That is, the crane may be situated a distance away from where the offshore wind turbine is to be installed. For instance, the offshore assembly site may be situated up to or greater than 100m, 500m or 1km away from the proposed installation site of the offshore wind turbine. By installation site, it is meant the location at which the offshore wind turbine is to be located when operational, e.g. when it is generating electricity.
In one envisaged application, a single assembly site may be used to assemble a number of wind turbines for installation at various locations in a wind farm, or a region of a wind farm. The installation region (e.g. the wind farm) may cover an area of up to several hundred square kilometres, for example 200 km2 or 300 km2. Accordingly, the offshore assembly side may be situated up to or greater than 10km or 20km from the proposed installation site of the offshore wind turbine.
It is also contemplated that the offshore assembly site may be situated up to or greater than 100 km from the proposed installation site of the offshore wind turbine. This could be the case when it is impractical to assemble the offshore wind turbine(s) near to or adjacent to the installation site, for example due to poor weather and/or wave conditions. Accordingly, the assembly site may be situated at a location with more favourable conditions, e.g. near the shore. The offshore wind turbine may be transported to the installation site after assembly.
The method may comprise transporting the offshore wind turbine to the crane. That is to say, the offshore wind turbine may be transported to the offshore location. The offshore wind turbine may be transported on a vessel, for example a barge.
The offshore wind turbine may be transported to the offshore location and/or installed on the floating platform in multiple (i.e. more than one) pieces. Some or all of the components of the offshore wind turbine may be in a partially assembled state during transportation and/or before being installed on the floating platform. The components being transported and/or installed may include the tower, the nacelle, the rotor hub, the blade(s), the generator, a transformer, bearings, a gearbox and/or a pitch actuator etc. In the partially assembled state, at least the rotor hub and the nacelle may be assembled. Alternatively, in the partially assembled state at least the rotor hub, the nacelle and the tower may be assembled. Optionally, in the partially assembled state, the offshore wind turbine may be fully assembled apart from the blades of the offshore wind turbine. 6 -
The components and/or the partially assembled pieces of the offshore wind turbine may be installed separately, i.e. one at a time.
Components and/or partially assembled pieces of the offshore wind turbine may be mounted on, or attached to, components and/or partially assembled pieces already installed on the floating platform. For instance, the tower may be installed on the floating platform and the nacelle subsequently mounted on the tower.
The offshore wind turbine may be transported to the offshore location and/or installed on the floating platform in once piece. That is to say that all of the major parts or components of the offshore wind turbine, including at least the tower, nacelle, rotor hub and blades, are assembled prior to being transported and/or installed.
The crane may be used to lift the offshore wind turbine, components of the offshore wind turbine, and/or partially assembled pieces of the offshore wind turbine into a mating relationship with the floating platform so as to facilitate installation on the floating platform. For instance, the crane may be used to lift the offshore wind turbine, components of the offshore wind turbine, and/or partially assembled pieces of the offshore wind turbine from a vessel and into a mating relationship with the floating platform.
After the offshore wind turbine has been installed on the floating platform, the floating platform may be untethered from the sea bed. This may be achieved by adding ballast to the floating platform to release tension in the mooring(s). The ballast may be water, preferably sea water. The slackened mooring(s) may then be disconnected from the floating platform and/or the sea bed. At least some of the ballast may be removed from the floating platform after the mooring(s) have been disconnected.
The method may include transporting the offshore wind turbine, once installed on the floating platform, to the installation site for permanent installation. The floating platform, along with the offshore wind turbine, may be towed to the installation site by a vessel.
In a second aspect, the invention provides an offshore crane assembly for installing an offshore wind turbine on a floating platform, the offshore crane assembly comprising a crane mounted on a floating structure, the floating structure being tethered to the sea bed by one or more taut moorings. The crane assembly may be suitable for use and/or used in the method of the first aspect, and may include any one or more or all of the optional features discussed above. 7 -
By tethering the crane assembly to the sea bed by taut moorings, vertical movement of the crane assembly can be limited and the crane assembly can be maintained at an approximately constant vertical position. This stability may be comparable to fixed crane structures. This can assist in the installation of an offshore wind turbine on a floating platform, which requires high precision and is sensitive to wave, swell or other weather induced movement.
The floating structure may comprise a vertically elongate buoyant structure, such as a spar buoy. The buoyant structure preferably has a vertical length of greater than 50m, preferably greater than 100m. The buoyant structure may be circular, or partially circular in cross-section. The diameter of the buoyant structure may be between 5m and 25m, for instance 15m.
The floating structure may have a draft of greater than 50m, or greater than 100m.
The mooring(s) may be arranged to limit vertical movement of the floating structure to less than 1 m, preferably less than 500mm, more preferably less than 250mm, most preferably less than 100mm. The mooring(s) may be arranged to limit a velocity of the floating structure in the vertical direction to less than 0.5 ms-1, preferably less than 0.2 ms-1, most preferably less than 0.1 ms-1.
The mooring(s) may be arranged to limit horizontal movement of the floating structure. Horizontal movement of the floating structure may be limited to, for example, less than 1m, preferably less than 500mm.
Alternatively, the floating structure and/or the floating platform may include one or more thrusters to limit horizontal movement. For instance, propeller(s) and/or pump-jet(s) may be provided on the floating structure and/or the floating platform. The thruster(s) may be positioned below the waterline.
The mooring(s) may comprise one or more of a polyester rope and/or a steel tendon.
The crane may comprise a crane mast and a lifting arm mounted to the top of the crane mast. The lifting arm is preferably rotatably mounted to the top of the crane mast. The lifting arm may be mounted to the crane mast via a trunnion. The lifting arm may comprise a truss or lifting frame.
The crane may comprise one or more of a winch, a sheave, a power unit and/or a lifting wire. Preferably, the winch is arranged to pull in or let out the lifting wire. 8 -
The crane may be suitable to lift heavy components. For example, the crane may have a lifting capacity of 1000 tonnes or greater.
The crane may include a heave compensation system. This may further reduce the influence of sea conditions, such as waves and swell on the crane. The heave compensation system may be a passive or active system.
A winch may be provided on each of the one or more moorings to enable the vertical position of the floating structure to be adjusted. This could be used to control the vertical positon of the crane during installation of an offshore wind turbine on a floating platform.
The invention also extends to an installation assembly comprising a crane assembly moored to the seabed as discussed above in combination with a floating platform and a wind turbine for mounting thereon. Each of these components is preferably as described above. The assembly is preferably used to perform the method as described above.
Certain embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 illustrates a process of installing an offshore wind turbine on a floating platform using a floating crane assembly; Figures 2A-2D are schematic elevations showing example mooring arrangements for tethering a floating platform to the sea bed; Figure 3 is a schematic elevation showing a series of steps for tethering a floating platform to the sea bed; and Figure 4 is a schematic elevation showing a series of steps for untethering a floating offshore wind turbine from the sea bed.
Figure 1 illustrates a crane assembly 1 for installing an offshore wind turbine on a floating platform. The crane assembly 1 comprises a crane mast 2 mounted on a spar buoy 3 tethered to an anchor 4 on the sea bed 5 by a taut mooring line 6. A lifting arm 7 is rotatably mounted at the top of the crane mast 2. The lifting arm 7 may be configured to rotate about a longitudinal axis of the crane mast 2, which is approximately vertical in operation. The lifting arm 7 may also be configured to pivot about an axis perpendicular to the longitudinal axis of the crane mast 2 to allow the lifting arm 7 to be raised and lowered.
Also shown in Figure 1 is a partially assembled floating offshore wind turbine 8. The offshore wind turbine 8 comprises a tower 9 and a nacelle 10 rotatably mounted at the top of the tower 9. The nacelle 10 may be configured to 9 -rotate about a longitudinal axis of the tower 9, which is approximately vertical in operation, and is controlled in operation to face into oncoming wind. The nacelle 10 is arranged to house or support nacelle-rotor components such as a generator, a hub and a rotor etc. (not shown) in a manner well-known in the art.
The tower 9 is mounted on a spar buoy 11 to allow the offshore wind turbine 8 to float. In Figure 1, the spar buoy 11 is shown tethered to an anchor 12 on the sea bed 5 by a taut mooring line 13. However, the following techniques are applicable to floating offshore wind turbines that are not moored to the sea bed 5 and/or to floating offshore wind turbines using alternative floating platforms or moorings.
Additional components 14 of the offshore wind turbine 8 are shown stored on the deck of a barge 15, ready to be coupled to the tower 9 and/or nacelle 10 already mounted on the spar buoy 11. The additional components 14 may include a generator, a rotor hub and/or rotor blades.
Before the crane assembly 1 is used, as will be discussed further below, the mooring line 6 tethering the crane assembly 1 to the sea bed 5 is pulled taut sufficiently for the mooring line 6 to be placed in tension and remain in tension for all wave, swell and load conditions. That is to say, the spar buoy 3 (and the crane assembly 1) is pulled sufficiently towards the sea bed 5 by the mooring line 6 such that the mooring line 6 remains in tension regardless of sea conditions.
The mooring line 6 is preferably made from a material having high axial stiffness and low elasticity, for example a polyester rope or a steel tendon.
With this configuration, variations in buoyancy of the crane assembly 1, for example induced by swell or waves, will only result in limited vertical movement of the crane structure. Instead of inducing vertical movement, changes in buoyancy will cause the tension within the mooring line 6 to vary. Hence, this configuration increases vertical stability and minimises vertical movement of the crane, allowing the crane assembly 1 to remain in an approximately constant vertical position.
The relatively small water plane area of the spar buoy 3 ensures that tension variations within the mooring line 6 remain small. However, this technique may be used with other floating platforms, for instance those having larger water plane areas.
To tether the crane assembly 1 to the sea bed 5, it may first be floated and towed to a suitable location. For example, the crane assembly 1 may be towed to a -10 -position adjacent to a proposed installation site for one or more floating offshore wind turbines.
A process for tethering the spar buoy 3 to the sea bed 5 is shown in Figure 3. After the crane assembly 1 has been towed to the desired offshore location, ballast, such as water, may be added to ballast tanks (not shown) within the spar buoy 3. This will cause the spar buoy 3, and the crane assembly 1, to sink lower in the water. The mooring line 6 may then be attached between the spar buoy 3 and anchor 4 on the sea bed 5. In order to create tension in the mooring line 6, a proportion of the ballast is then removed from the spar buoy 3, e.g. by pumping water from the ballast tanks. This increases the buoyancy of the spar buoy 3, and pulls the mooring line 6 taut.
In Figure 1 the crane assembly 1 is shown as being anchored to the sea bed 5 by a single mooring line 6 attached to the bottom end of the spar buoy 3. However, alternative arrangements are possible. Figures 2A-2D illustrate four alternative mooring line arrangements that may be used in variant embodiments. In addition to limiting vertical movement of the crane assembly 1, these arrangements also increase horizontal stability and limit horizontal movement of the crane assembly 1.
In Figure 2A, the bottom end of the spar buoy 3 is tethered to the sea bed 5 by a first taut mooring line 16a. This is similar to the arrangement shown in Figure 1. Additionally, the upper end of the spar buoy 3 is tethered to the sea bed 5 by a further three taut mooring lines 16b (only two mooring lines are visible in Figure 2A). The three mooring lines 16b attached to the upper end of the spar buoy 3 may be distributed about the perimeter of the spar buoy 3, for example arranged 120° apart from one another.
In Figure 2B, the spar buoy 3 is tethered to the sea bed 5 by three taut mooring lines 17 (only two are visible in Figure 2B). These mooring lines 17 may be arranged about the perimeter of the spar buoy 3 similarly to the mooring lines 16b shown in Figure 2A. However, unlike the mooring lines 16b shown in Figure 2A, the mooring lines 17 in Figure 3A are attached to the spar buoy 3 at a position below the centre of buoyancy of the spar buoy 3.
In Figure 20, the upper end of the spar buoy 3 is tethered to the sea bed 5 by three taut mooring lines 18a (only two are visible in Figure 20), similar to the mooring lines 16b shown in Figure 2A. In addition, the bottom end of the spar buoy 3 is tethered to the sea bed 5 by a further three taut mooring lines 18b (only two are visible in Figure 2C) distributed about the perimeter of the spar buoy 3. For example, the mooring lines 18b attached to the bottom end of the spar buoy 3 may be arranged 1200 apart from one another around the perimeter of the spar buoy 3. In Figure 2D, the upper end of the spar buoy 3 is tethered to the sea bed 5 by three taut mooring lines 19a (only two are visible in Figure 2C), similar to the mooring lines 16b shown in Figure 2A. Three auxiliary mooring lines 19b (only two are visible in Figure 2D) are arranged to tether the bottom end of the spar buoy 3 to the mooring lines 19a. Each of the three auxiliary mooring lines 19b attaches the bottom of the spar buoy 3 to a separate mooring line 19a. Each of the auxiliary mooring lines 19b is arranged taut between the spar buoy 3 and the respective mooring line to limit horizontal movement of the spar buoy 3.
As noted above, the mooring arrangements illustrated in Figures 2A-2D ensure that horizontal movement, as well as vertical movement, of the spar buoy 3 is minimised, thereby allowing the crane assembly 1 to be maintained in approximately the same vertical and horizontal position regardless of sea conditions.
Alternatively, or in addition, to the mooring arrangements illustrated in Figures 2A-2D, thrusters may be provided on the spar buoy 3 to increase horizontal stability and limit horizontal movement of the crane assembly 1.
The above described arrangements lead to improved vertical stability and limit vertical movement of the crane assembly 1. In addition, it is also possible to improve horizontal stability and vertical movement of the crane assembly 1. Hence, these arrangements make it possible to limit movement of the crane assembly 1, and to maintain its positon relative to another floating structure, such as the floating offshore wind turbine 8.
Referring again to Figure 1, during installation of a floating offshore wind turbine 8, the wind turbine spar buoy 11 may be towed to a suitable position adjacent to the crane assembly 1. The spar buoy 11 may be temporarily tethered to the sea bed 5 using one or more taut mooring lines 13. This may be achieved in a similar process to the process shown in Figure 3 used to tether the crane assembly 1 to the sea bed 5. That is to say, ballast may be added to the spar buoy 11 before the spar buoy 11 is tethered to one or more anchors 12 on the sea bed 5 using one or more mooring lines 13. Ballast may then be removed from the spar buoy 11 to pull the mooring line(s) 13 taut.
-12 -The components 14 of the wind turbine 8, such as the tower 9, nacelle 10 and/or rotor components, may be transported to the installation location on the barge 15. For example, the components 14 may be transported on the deck of the barge 15 at or near sea level.
To install a wind turbine component 14 on the wind turbine spar buoy 11, the component 14 may be lifted from the barge 15 by the crane and moved by the crane into a mating arrangement with the spar buoy 11 or another component already mated to the spar buoy 11. The component 14 may then be mated, i.e. coupled, to the spar buoy 11 or, where appropriate, another component 14 already mated with the buoy.
For example, the tower 9 may be lifted from the barge 15 by the crane assembly 1 and moved into a mating arrangement with the spar buoy 11. Once the tower 9 has been coupled to the spar buoy 11 in the known manner, the crane may then be used to lift the nacelle 10 from the barge 15 and move it into a mating arrangement with the tower 9, where it can be coupled to the tower 9. This process can be repeated until the wind turbine 8 has been fully assembled on the spar buoy 11.
Alternatively, a fully assembled wind turbine 8 (i.e. with most or all the major wind turbine components 14 assembled together) may be transported by the barge 15 to the installation location. In this case, the crane may be used to lift the fully assembled wind turbine 8 from the barge 15 and move it into a mating arrangement with the spar buoy 11 for coupling.
The barge 15 may move considerably in the sea during installation due to, for example, swell and waves. However, extreme precision is not necessary when lifting the wind turbine components 14 or fully assembled wind turbine 8 from the barge 15. Rather, it is the process of mating the wind turbine 8 (or its constituent components 14) with the spar buoy 11 that requires precision. Therefore, it is acceptable for relative motion to arise between the crane and the barge 15.
Once the wind turbine 8 has been installed on the spar buoy 11, the spar buoy 11 may be released from the sea bed 5 so that the floating wind turbine 8 can be towed to its permanent mooring location. Releasing the spar buoy 11 from the sea bed 5 may include adding ballast, such as water, to the spar buoy 11 to release tension in the temporary mooring line(s) 13. The temporary mooring lines 13 may then be disconnected from the spar buoy 11 and/or the anchor(s) 12 on the sea bed -13-to allow the offshore wind turbine 8 to move freely away from its temporary mooring location adjacent the crane. This process is shown in Figure 4.
Once the completed floating offshore wind turbine 8 has been towed away from the crane assembly 1, another spar buoy can be towed adjacent to the crane assembly 1. The process of assembling a floating offshore wind turbine can then be repeated. Temporarily mooring spar buoys adjacent to the crane in this way allows for quick serial assembly of wind turbines.
Whilst in the process discussed above the wind turbine spar buoy 11 is temporarily tethered to the sea bed 5 during installation of the wind turbine 8 on the spar buoy 11, the installation process may also be carried out without the wind turbine spar buoy 11 being tethered to the sea bed 5. In this case, the spar buoy 11 may be allowed to float freely in the water adjacent the crane. This may be acceptable where relative motion between the crane and the wind turbine spar buoy 11 is expected to be small, for example in calm waters.

Claims (20)

  1. -14 -CLAIMS1. A method of installing a floating offshore wind turbine, the method comprising: providing a crane mounted on a floating structure at an offshore location, the floating structure being tethered to the sea bed by one or more taut moorings; providing a floating platform; and at the offshore location, installing an offshore wind turbine on the floating platform using the crane.
  2. 2. A method according to claim 1, wherein the floating structure and/or the floating platform comprises a vertically elongate buoyant structure
  3. 3. A method according to claim 2, wherein the floating structure and/or the floating platform comprises a spar buoy.
  4. 4. A method according to any of claims 1, 2 or 3, wherein the floating platform is tethered to sea bed by one or more moorings,
  5. 5. A method according to claim 4, wherein the mooring(s) tethering the floating platform to the sea bed are taut moorings.
  6. 6. A method according to any preceding claim, wherein the mooring(s) are arranged to limit horizontal movement of the floating structure and/or the floating platform.
  7. 7. A method according to any preceding claim, wherein tethering the floating structure to the sea bed comprises: adding ballast to floating structure to cause the floating structure to sink lower in water; attaching the mooring(s) between the sea bed and the floating structure; and removing ballast from the floating structure to increase buoyancy and cause the mooring(s) to be pulled taut.-15 -
  8. 8. A method according to any preceding claim, wherein tethering the floating platform to the sea bed comprises: adding ballast to floating platform to cause the floating platform to sink lower in water; attaching the mooring(s) between the sea bed and the floating platform; and removing ballast from the floating platform to increase buoyancy and cause the mooring(s) to be pulled taut.
  9. 9. A method according to any preceding claim, wherein the offshore wind turbine is installed on the floating platform in multiple pieces.
  10. 10. A method according to any of claims 1 to 8, wherein the offshore wind turbine is installed on the floating platform in once piece.
  11. 11. An offshore crane assembly for installing an offshore wind turbine on a floating platform, the offshore crane assembly comprising a crane mounted on a floating structure, the floating structure being tethered to the sea bed by one or more taut moorings.
  12. 12. An offshore crane assembly according to claim 11, wherein the floating structure comprises a vertically elongate buoyant structure
  13. 13. An offshore crane assembly according to claim 12, wherein the floating structure comprises a spar buoy.
  14. 14. An offshore crane assembly of claims 11 or 13, wherein the floating structure has a draft of greater than 50m, preferably greater than 100m.
  15. 15. An offshore crane assembly of any of claims 11 to 14, wherein the mooring(s) are arranged to limit a velocity of the floating structure in the vertical direction to less than 0.5 ms-1.
  16. 16. An offshore crane assembly of any of claims 11 to 15, wherein the mooring(s) are arranged to limit horizontal movement of the floating structure, preferably to limit horizontal movement to less than 1m.-16 -
  17. 17. An offshore crane assembly of any of claims 11 to 16, wherein the mooring(s) comprise one or more of a polyester rope and/or a steel tendon.
  18. 18. An offshore crane assembly of any of claims 11 to 17, wherein the crane comprises a crane mast and a lifting arm mounted to the top of the crane mast.
  19. 19. An offshore crane assembly of any of claims 11 to 18, wherein the crane has a lifting capacity of 1000 tonnes or greater.
  20. 20. An installation assembly comprising: the crane assembly of any of claims 11 to 19; a floating platform; and a wind turbine for mounting on the floating platform.
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WO2012066790A1 (en) * 2010-11-18 2012-05-24 三菱重工業株式会社 Ship for installing offshore wind turbines, and method for installing offshore wind turbines using same

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