Method and structure for lifting and attaching a composite structure to a vertical support
The invention relates to a method of attaching a composite structure, which comprises a first part and a second part, to a vertical support, wherein the first part of the composite structure is lifted to a predetermined height via a lifting device on a carrier arm and is thereafter connected to the vertical support. The invention also relates to a lifting structure for carrying out the method. The method and the lifting structure of the present invention are particularly suitable for installation of wind turbines, in particular in an offshore environment.
Installation of wind turbines is done in most cases by installing the assembly in separate parts. These parts are normally lifted by a crane and comprise amongst others a vertical column or several column sections, a nacelle housing the generator and gear box, a hub for holding the blades which hub is being connected to the shaft of the generator or gear box and rotor blades.
It is known to install wind turbines on land and relatively recently offshore at sea. In order to provide a stable working platform for offshore installation of wind turbines, frequently jack-up barges or jack-up vessels are used. These jack-up structures are provided with vertical legs that can be lowered onto the seabed. The platform can then be jacked-up out of the water along the legs. The jack-up structure is generally equipped with a crane to install the wind turbine parts on a wind turbine foundation that is installed on the seabed. The foundation may partly extend above water level.
A general installation sequence consists of lifting of the column or column sections onto the foundation. After erection of the column, a number of options is open:
1. Install the nacelle separately, followed by the hub with all blades already fixed to the hub. In this case, normally the hub with blades is lying in a horizontal position on the platform deck of the jack-up vessel and needs to be installed in a vertical plane, so that the assembly of hub and blades needs to be rotated. This is a particularly difficult
operation as the size of the blades is increasingly large and may amount to lengths of 50 m or more.
2. Install in one lift the nacelle with the hub and two blades pointing upward, so-called "bunny ears", followed by separate installation of the last blade. In WO 03/100249 a installation of this type is disclosed, in which the last blade is attached to a gripping unit that is attached to a lower section of the crane arm via a number of cables for maintaining a proper orientation of the last blade, when this last blade is lifted towards the hub supported on the column.
3. Any combination of installing the parts and or subassemblies separately on the column.
Due to the variable wave loading on the legs and the flexibility of the jack-up system, the platform in the jacked-up condition moves slightly in the horizontal plane. This, in combination with the flexibility of the crane can cause significant horizontal movement of lifted parts compared to the earth-fixed foundation or other parts already installed on the earth-fixed foundation. Experience has shown that, due to the motions and wind, installation of the last blade (sequence 2 above) may be difficult and that inadvertent impact of the vulnerable blade with already installed parts can cause costly damage to the blade. Installation sequence 1 above avoids the installation of a loose blade, but at the cost of having to rotate an assembly of hub and blades, which is complex due to the large dimensions of the assembly and vulnerability of the blades, especially in windy environmental conditions.
There is also experience with installing assemblies of column, nacelle, hub and blades in one lift by e.g. floating crane units. This however requires large and costly lifting equipment, involves complex logistic for transport of turbines to the installation site, can only be done in a limited weather window and is with the presently available installation equipment not expected to be economically feasible for installing large numbers of turbines.
The known procedures for installing wind turbines, especially offshore, show room for improvement, also in light of the expected growth in number, size and power of the wind turbines that need to be installed in the future.
It therefore is an object of the present invention to provide a method and structure of the above-mentioned type with which safe and controlled installation of composite parts on a vertical support can be effected, in particular of wind turbine parts during offshore installation.
It is a further object of the present invention to provide a method and structure for installation of wind turbines in an offshore environment, in which a turbine blade can be lifted and connected to the hub of a turbine in a stable and secure manner.
Hereto the method according to the present invention comprises the steps of:
lifting the first part of the composite structure to a predetermined height via a lifting device on a carrier arm,
providing a rigid connection between the first part and the carrier arm,
lifting the second part of the composite structure to the rigidly connected first part,
mutually attaching the first and second parts,
disconnecting the rigid connection between the first part and the carrier arm,
lifting the interconnected first and second parts by the lifting device to a predetermined position of the vertical support,
attaching the interconnected first and second parts to the vertical support, and
releasing the lifting device from the interconnected first and second parts.
By providing a rigid connection to the carrier arm of the first part, the second part can be accurately oriented relative to the first part prior to connection. In this way impacting of the parts is prevented and consequent damage is avoided. Furthermore, by assembling the parts at an elevated position on the carrier arm, a large horizontal working area becomes available for installation purposes. By assembling the parts at the elevated position in a substantial vertical plane, it is prevented that complex structures need to be erected from a horizontal assembly orientation to the vertical orientation.
These advantages become especially manifest when installing wind turbines at an offshore site. Offshore installation in parts or subassemblies of wind turbines by use of a crane is currently the most economically feasible solution, also with respect to the overall logistics of transporting the turbine parts to their foundations or to the installation platform situated at the offshore installation site. The invention allows for instance a hub and two blades ("bunny ears" configuration) of a wind turbine to be lifted and rigidly attached to a crane arm, where after a third blade can be connected to the hub without the wave motions imparting deviations that interfere with the assembly of the parts. The third blade may be lifted in a controlled manner towards the rigidly connected sub assembly of hub and two blades, so that the chances of damaging the vulnerable blades are reduced compared to assembling the hub and three blades in a horizontal position on the working deck, followed by erecting the hub and three interconnected blades into a vertical plane. The method according to the invention occupies relatively little deck space of the working vessel (jack up barge) with which the wind turbine parts are erected. Also, the number of handling activities is reduced by the method according to the invention.
Even though the invention has specific advantages in erecting offshore wind turbines, it can also be carried out in other fields of use, such as onshore installation of wind turbines or other vertical structures such as (parts of) masts, buildings, bridges, and the like.
One embodiment of the method according to the invention comprises the step of releasably fixing a connector to the first part, connecting the first part to the carrier arm via the connector, and releasing the connector from the first part after attaching the interconnected first and second parts to the vertical support.
The connector can be a frame that is temporarily connected to a blade, a hub, nacelle or other segment of the wind turbine. The connector is provided with structural members that engage with complementary structural members on the crane arm to establish the rigid connection. The connector may be attachable at various spaced-apart positions along the crane arm, for instance by being provided with fixed bars or flanges, or can be executed in a continuously variable engageable manner, for instance by providing actuated grippers or hydraulic clamps engaging with the crane structure along each desired part of its length. By using a separate connector, direct contact of the assembled parts and the crane arm is prevented hence establishing a rigid connection while avoiding damage which might result from establishing a direct rigid contact with the crane arm.
After connecting the assembled parts (e.g. hub, three blades and nacelle) to the vertical support (e.g. a wind turbine column), the connector is released and lowered towards the working deck. Alternatively, the rigid connection may be disconnected prior to attaching the assembly to the vertical support.
Preferably, the connector is attached to the first lifting device (crane hoist) such that the lifting forces are evenly distributed on the first part via the connector. For connecting a blade of a wind turbine to an assembly of a nacelle, a hub and two blades connected to the hub, the second connector can comprise a frame with substantially horizontal frame members extending along the nacelle, an attachment part of the frame for connecting to the carrier arm being situated at the side of the hub.
The frame may be connected via tugger lines which are attached to a pulling device on the carrier arm, for pulling the first part lifted by first lifting device towards the connector upon rigidly coupling the connector to the carrier arm. These tugger lines
and pulling device may also be used to control the position of the lifted assembly while it is suspending from the first lifting device, or crane hoist, and when the assembly is lifted towards the vertical support.
A second lifting device may be installed on the carrier arm for attaching via on or more lines to the second part, for lifting this part to the rigid connection position in which the first part is held on the carrier arm. In this manner, only a single carrier arm needs to be provided for lifting both first and second parts. For guiding the second lifted part to the assembly that is rigidly connected to the carrier arm, the second part is guided along an upward guide member extending along the carrier arm for guiding the part lifted by the second lifting device along the carrier arm to a connection position on the carrier arm. The guide member may comprise bumpers, rollers and guides generally known in offshore oil and gas installation technology.
For guiding the second lifted structure while it is being lifted from a horizontal to a vertical position, the support surface (such as the deck of the jack up barge), extending substantially transversely to the carrier arm, is provided with a guide track extending along the support surface for guiding the second part transversely to the carrier arm while this second part is lifted in the direction of the carrier arm. In this way, for instance a wing tip of a blade that is up righted and hoisted into a connection position with the subassembly of hub, nacelle and blades, is accurately guided to its connection position.
In one embodiment, the lifting structure comprises an access means situated inside or along the carrier arm for personnel, from the base of the carrier to the position of the connector. The carrier arm may be hollow and have a stair access and personnel support at the connection position for allowing assembly activities . Alternatively, a personnel elevator may be provided at the interior or exterior of the carrier arm.
Some embodiments of the method and lifting structure according to the invention will by way of example be explained in detail with reference to the accompanying drawings. In the drawings:
Fig. 1 shows a schematic side view of one embodiment of a lifting structure according to the present invention for offshore installation of wind turbine generators, Fig. 2 shows a perspective view of a subassembly of nacelle, hub and two blades in "bunny ears" configuration, prior to lifting Fig. 3 shows a top perspective view of the lifted subassembly including a lifting frame and coupling elements on the crane arm and lifting frame,
Fig. 4 shows a close up perspective view of the rigidly connected coupling means on the lifting frame and on the crane arm, Figs. 5 and 6 show perspective views of the positioning of a third blade using a protective casing, relative to the rigidly connected hub and blades, via a second lifting device,
Fig. 7 shows the third blade in its connected position and
Fig. 8 shows the assembled blades, hub and nacelle prior to attaching to the wind turbine column.
Figure 1 shows a jack-up vessel 1 for offshore installation of wind turbine generators. The Jack-up vessel 1 comprises a deck structure 2 and is supported on the sea bed 3 via vertical columns 4, 5. On the deck 2, a crane arm 7 is placed comprising a first hoist 8 with a winch 9 from which via a hoisting wire 10 a nacelle 12 and a hub 13 are supported. Via a mechanical connector 15, the subassembly of hub 13 and nacelle 12 are rigidly connected with the crane arm 7. Via a second hoist 17 and lifting cable 18, a wind turbine blade 19 is lifted from its horizontal position on the deck structure 2, to a vertical orientation. In this particular embodiment, the upper end 22, or root of the wind turbine blade 19 is guided along upward guide member 21 along the crane arm 7 to the position in which it can be connected to the hub 13. The lower end, or tip, 22 'of the wind turbine blade 19 is guided along horizontal guide member 23 extending along the deck 2, during up righting and lifting of the blade 19. After attaching one or more blades to the hub 13, the assembly of nacelle 12, hub 13 and connected blades 19 is released from the crane arm 7 by releasing the connector 15, and the assembly can be attached to vertical column 25, which has been pre-installed on foundation 24 on the sea bed 3.
Fig. 2 shows an embodiment in which the nacelle 12 and hub 13 have been attached to two blades 26, 27 in a so-called "bunny-ears" configuration. A frame 33 is connected to the nacelle. The frame 33 has a guide member 34 and a connector bar 35 for rigidly engaging with a complementary connector or receptacle on the crane arm 7. The hoisting wire 10 is connected to the frame 30. The blade 19 remains horizontally supported on the deck 2, and is protected by a casing 32.
Fig. 3 shows the assembly of nacelle 12, hub 13 and blades 26, 27 lifted to the height of the attachment position in which a complementary connector in the form of a receptacle 37 on the crane arm 7 can rigidly engage with the connector bar 35 of the frame 33. For this purpose a pulling device or winch 38 on the crane arm 7 can shorten the length of tugger wire 39 that is attached via the receptacle 37 to the frame 33. Hereby the frame 33 is guided to its attachment position, wherein the guide members 34 position the frame relative to the crane arm 7 such that the connector bar 35 falls into the recess of the complementary connector receptacle 37.
Fig. 4 shows the position in which the connector bar 35 is locked into the receptacle 37 on the crane arm 7 at the moment the tugger wire 39 has been pulled taut by winch 38. The guide members 34 bear against both sides of the crane arm 7 for properly aligning the frame 33 with respect to the crane arm 7.
Fig 5. shows the lifting of the blade 19 by the second hoist 17, the upper end 22 of the blade being attached to the lifting line 18, while the lower end 22 slides along guide member 23 on the deck 2. The blade 19 is encased in the protective casing 32, to which the lifting cable 18 is attached. After reaching the up righted position shown in figure 6, the root 22 of the blade 19 is attached to the hub 13 and the protective casing 32 can be released from the blade, as it is shown in figure 7. Thereafter, the connector bar 35 is released from the receptacle 37, such that the assembly of nacelle 12, hub 13 and three blades 19, 26, 27 can be lifted to be placed on top of column 25. During the installation of the assembly, the tugger wire 39 is maintained at the proper tension to control the position of the assembly relative to the crane arm 7.