Floating Wind Turbine Structure with Reduced Tower Height and Method for Optimising the Weight Thereof

Abstract

The present invention relates to a method and a wind turbine structure for optimising the weight of the wind turbine and the offshore foundation. The wind turbine is operated based on the measured wave height which in turn allows the tower height to be reduced so that the ratio between the tower height and the length of the wind turbine blades is greater than 0.5. The rotor is parked in a predetermined position with a maximum or minimum clearance between the tip end of the wind turbine blades and the sea level if the measured wave height exceeds a predetermined threshold. A monitoring unit arranged relative to the wind turbine detects if one or more objects are located within a monitoring area. If an object is located within the monitoring area, the wind turbine is shut down and the rotor is rotated to the parked position.

Claims

1. A method for optimising the weight of a wind turbine structure, the wind turbine structure comprises a wind turbine provided on an offshore foundation, the wind turbine comprises a rotor with at least two wind turbine blades and a rotor hub, the wind turbine further comprises a wind turbine tower, wherein the method comprises the steps of: providing the wind turbine with a distance between a sea level and a lowermost position of a tip end of the at least two wind turbine blades which is equal to or less than 20 metres, measuring at least a wave height, operating the wind turbine according to the measured wave height, positioning the rotor in a parked position with a maximum clearance between the at least two wind turbine blades and the sea level if at least the measured wave height exceeds a predetermined threshold.

2. The method according to claim 1, wherein the method further comprises determining a ratio between a blade length of one of the at least two wind turbine blades and a tower height of the wind turbine tower, wherein the ratio of the wind turbine is greater than 0.5.

3. The method according to claim 1, wherein the wind turbine is operated in a normal operation mode if the measured wave height is equal to or below the threshold.

4. The method according to claim 1, wherein the predetermined threshold is 18 metres or less, preferably between 8 metres and 18 metres.

5. The method according to claim 1, wherein the wind turbine is operated in a shutdown mode if the measured wave height is above the threshold.

6. The method according to claim 5, wherein the method further comprises monitoring a predetermined area relative to the offshore foundation, wherein the wind turbine is further operated in the shutdown mode if at least one moving object is detected within this area.

7. The method according to claim 1, wherein the step of positioning further comprises yawing the nacelle into a parked position in which the rotor is arranged on an opposite side of an outer ladder which provides access to the wind turbine.

8. A wind turbine structure comprising: an offshore foundation configured to be installed at an installation site, the offshore foundation comprises an installation interface for a wind turbine, the wind turbine comprises a wind turbine tower configured to be installed on the offshore foundation, a nacelle provided on top of the wind turbine tower, and a rotor rotatably mounted to the nacelle, the rotor comprises at least two wind turbine blades mounted to a rotor hub, the wind turbine further comprises a control unit configured to control the operation of the wind turbine, wherein at least one measuring unit is configured to measure a wave height, the measuring unit being configured to communicate with the control unit, wherein the wind turbine is further configured to position the rotor in a parked position with a maximum clearance between the at least two wind turbine blades and a sea level if the measured wave height exceeds a predetermined threshold, wherein the distance between the sea level and the lowermost position of a tip end of the at least two wind turbine blades is equal to or less than 20 metres.

9. The wind turbine structure according to claim 8, wherein the offshore foundation is a floating foundation.

10. The wind turbine structure according claim 8, wherein the distance between the sea level and the lowermost position is between 10 metres and 18 metres.

11. The wind turbine structure according to claim 8, wherein the at least one measuring unit is arranged on the wind turbine structure or positioned relative to the offshore foundation.

12. The wind turbine structure according to claim 8, wherein the wind turbine structure further comprises at least one monitoring unit configured to monitor a predetermined area relative to the offshore foundation, wherein the at least one monitoring unit is configured to detect at least one moving object within this area.

13. The wind turbine structure according to claim 12, wherein the wind turbine is configured to operate in a normal operation mode if no control signal is received from the at least one monitoring unit, wherein said control signal either indicative of the at least one moving object being located within this area or indicative of the at least one moving object is not approaching the wind turbine.

14. The wind turbine structure according to claim 8, wherein the wind turbine is configured to operate in a normal operation mode if the measured wave height is below the threshold.

15. The wind turbine structure according to claim 8, wherein the wind turbine comprises three wind turbine blades.

Description

DESCRIPTION OF THE DRAWING

[0062] The invention is described by example only and with reference to the drawings, wherein:

[0063] FIG. 1 shows a conventional wind turbine installed on an offshore foundation;

[0064] FIG. 2 shows an exemplary embodiment of the wind turbine structure according to the invention;

[0065] FIG. 3 shows an exemplary embodiment of a monitoring unit for detecting objects with a monitoring area;

[0066] FIG. 4 shows a first exemplary embodiment of a measuring unit for measuring the wave height; and

[0067] FIG. 5 shows a second exemplary embodiment of the measuring unit for measuring the wave height.

[0068] In the following text, the figures will be described one by one and the different parts and positions seen in the figures will be numbered with the same numbers in the different figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure.

REFERENCE LIST

[0069] 1 Wind turbine [0070] 2 Offshore foundation [0071] 3 Wind turbine blades [0072] 4 Inner blade section [0073] 5 Outer blade section [0074] 6 Pitch junction [0075] 7 Rotor hub [0076] 8 Nacelle [0077] 9 Wind turbine tower [0078] 10 Sea level [0079] 11 Wind turbine [0080] 12 Offshore foundation [0081] 13 Wind turbine tower [0082] 14 Seabed [0083] 15 Anchoring lines, chains [0084] 16 Monitoring unit [0085] 17 Object, vessel [0086] 18 Measuring unit, buoy [0087] 19 Buoyant element [0088] 20 Anchoring lines [0089] 21 Measuring unit, sonar

DETAILED DESCRIPTION OF THE INVENTION

[0090] FIG. 1 shows a conventional wind turbine 1 arranged on an offshore foundation 2. The wind turbine 1 is here shown as a two-blade wind turbine, but it may comprise three wind turbine blades. The wind turbine blades 3 are here shown as partial-pitch blades, but the wind turbine blades 3 may be full-span blades. The wind turbine blade 3 comprises an inner blade section 4 connected to an outer blade section 5 via a pitch junction 6. The wind turbine blades 3 are mounted to a rotor hub 7 which is rotatably mounted to a nacelle 8. The nacelle 8 is arranged on top of a wind turbine tower 9 having a predetermined tower height.

[0091] In this embodiment, the wind turbine 1 has a ratio between a blade length of the wind turbine blades 3 and the tower height of the wind turbine tower 9 of less than 0.5. The nacelle 8 and rotor hub 7 are placed at a predetermined hub height relative to a sea level 10 so that a sufficient clearance between the lowermost position of the tip end of the wind turbine blades 3 and the sea level 10 is achieved.

[0092] The offshore foundation 2 is here shown as a floating foundation, but may be a different type of offshore foundation, such as a mono-pole, a tripod, a jacket foundation, or a gravity foundation.

[0093] FIG. 2 shows an exemplary embodiment of a wind turbine 11 and an offshore foundation 12 according to the invention. In this embodiment, the wind turbine 11 has a reduced tower height compared to the wind turbine 1.

[0094] The wind turbine 11 comprises a wind turbine tower 13 which has a tower height of less than two times than the blade length of the wind turbine blades 3, thus the ratio is greater than 0.5, preferably between 0.5 and 0.9. The wind turbine blade 3 in this embodiment has the same blade length as the wind turbine blades 3 shown in FIG. 1. This reduces the moment arm and thus the bending moment of the wind turbine 11 which in turn reduces the tilting velocity and acceleration of the nacelle 8. This also saves material and costs of the wind turbine tower 13. The reduced tower height also means that the wind turbine tower 13 has a higher resonance frequency, thus making it less prone to resonate.

[0095] The offshore foundation 12 is here shown as a floating foundation, optionally any type of offshore foundation may be used as mentioned above. The forces and bending moment introduced into the offshore foundation 12 are smaller than those introduced into the offshore foundation 2 due to the reduced tower height and thus the reduced hub height. This means that the offshore foundation 12 in this embodiment has a size and/or weight that is/are smaller than those of the offshore foundation 2, since a smaller mass is needed to provide a stable platform. This in turn saves material and costs of the offshore foundation 12.

[0096] The clearance between the tip end of the wind turbine blades 3 and the sea level 10 is in this embodiment equal to or less than 25 metres, preferably between 5 and 20 metres.

[0097] FIG. 3 shows the wind turbine 11 arranged on the offshore foundation 12 where the offshore foundation 12 is secured to a seabed 14 by means of one or more anchoring lines 15, e.g. anchor chains. One or more monitoring units 16 are arranged on the offshore foundation 12. The monitoring unit 16 is configured to detect any moving objects 17 within a monitoring area, such as vessels. The monitoring unit 16 comprises a radar configured to transmit an electromagnetic signal, e.g. radio waves, and receive the reflected signal from an object 17 located in the monitoring area. The monitoring unit 16 further comprises an electronic circuit, e.g. a microprocessor, configured to analyse the received signal and determine if the object 17 is located within the monitoring area or not, optionally also the direction and/or speed of the object 17. This defines a safety zone around the wind turbine structure 11, 12.

[0098] The monitoring unit 16 monitors the monitoring area and sends a signal or command to a control unit located in the wind turbine 11 indicating that an object 17 is located inside the monitoring area. The control unit then switches the wind turbine 11 to a shutdown mode in which the rotor is parked in a predetermined position. In the parked position, the rotor and thus the wind turbine blades 3 are rotated to a horizontal position as shown in FIG. 3 and optionally located in this position using a rotor locking system. This reduces the risk that the wind turbine blades 3 accidently hit the object 17. When the monitoring unit 16 determines that no objects 17 are located within the monitoring area, a second signal or command is send to the control unit. The control unit then switches the wind turbine 11 and the rotor into a normal operation mode or normal start-up mode depending on the measured wind speed.

[0099] FIG. 4 shows a first exemplary embodiment of a measuring unit 18 arranged relative to the foundation 12. The measuring unit 18 is configured to measure the wave height relative to a nominal sea level, e.g. using a wave sensor, a GPS receiver or one or more accelerometers. The measuring unit 18 comprises a buoyant element 19, e.g. a buoy shaped element, configured to be placed at the sea level 10 so that it follows the wave movements. The buoyant element 19 is secured to the seabed 14 via one or more anchoring lines 20. Here only one measuring unit 18 is shown, but two or more measuring units 18 may be used.

[0100] The measuring unit 18 measures the wave height continuously or periodically and transmits the measured signals to the control unit in the wind turbine 11 via a wired or wireless connection. The control unit then determines the wave height and compares it to a predetermined threshold. Alternatively, the measuring unit 18 comprises an electronic circuit, e.g. a microprocessor, which determines the wave height and sends a signal indicating the wave height to the control unit. The threshold value may be 18 metres or less, preferably between 5 and 15 metres. If the measured wave height exceeds the threshold, then the control unit switches the wind turbine into the shutdown mode and the rotor is positioned in the parked position. When the measured wave height drops below the threshold, the control unit switches the wind turbine 11 and the rotor into the normal operation mode or normal start-up mode depending on the measured wind speed. This reduces the risk that the waves hit the wind turbine blades 3 during rotation.

[0101] FIG. 5 shows a second exemplary embodiment of the measuring unit. The measuring unit 21 is in this embodiment arranged at the seabed 14 and connected to the control unit in the wind turbine 11 via a wired or wireless connection. The measuring unit 21 is configured to measure the wave height relative to the nominal sea level 10, e.g. using a sonar in the form of an upwards looking sonar.

[0102] The measuring unit 21 transmits an acoustic signal, e.g. acoustic pulses, towards the sea level 10 and receives a reflected signal from the sea level 10 as indicated by dotted lines in FIG. 5. The measuring unit 21, or the control unit, then determines the wave height. The measured wave height is then compared to the threshold as described above.

[0103] The present invention is not limited to the illustrated embodiment or the described embodiments herein, and may be modified or adapted without departing from the scope of the present invention as described in the patent claims below.