Method for reducing oscillations in offshore wind turbines

Abstract

The invention relates to a method of reducing oscillations in an offshore wind turbine comprising one or more thrusters, the method comprising determining an oscillation of the offshore wind turbine and operating the one or more thrusters such that the oscillation is reduced. The invention further relates to an offshore wind turbine comprising one or more underwater thrusters, oscillation determination system for determining an oscillation of the wind turbine and a control system for operating the underwater thrusters in response to signals received from the oscillation determination system.

Claims

1. A method of operating an offshore wind turbine and reducing oscillations, the wind turbine comprising a plurality of blades, a pitch system for rotating the blades and one or more underwater thrusters, the method comprising: above a nominal wind speed, rotating the blades to maintain a substantially constant aerodynamic torque delivered by the rotor and measuring an oscillation of the offshore wind turbine, and operating the one or more thrusters such that the oscillation is reduced, wherein a pitch control rotating the blades to maintain the substantially constant aerodynamic torque is not affected in order to dampen the oscillations.

2. The method according to claim 1, further comprising determining the speed of the oscillation.

3. The method according to claim 1, wherein the oscillation is substantially a fore-aft oscillation.

4. The method according to claim 3, wherein the oscillation is substantially a pitching oscillation.

5. The method according to claim 3, wherein the oscillation is substantially a surging oscillation.

6. The method according to claim 1, wherein the oscillation is substantially a sideways oscillation.

7. The method according to claim 1, wherein the oscillation is substantially a yawing oscillation.

8. The method according to claim 1, wherein the offshore wind turbine is a floating wind turbine.

9. An offshore wind turbine comprising: a plurality of blades, a pitch system for rotating the blades, one or more underwater thrusters, an oscillation measurement system for measuring an oscillation of the wind turbine, a thruster control system for operating the underwater thrusters in response to signals received from the oscillation measurement system, and a pitch control system adapted to rotate the blades to maintain a substantially constant aerodynamic torque delivered by the rotor above a nominal wind speed, wherein the pitch control system operates independently of the operation of the one or more thrusters.

10. The offshore wind turbine according to claim 9, wherein the wind turbine is a floating offshore wind turbine.

11. The offshore wind turbine according to claim 9, wherein the wind turbine is a fixed offshore wind turbine.

12. The offshore wind turbine according to claim 9, wherein the oscillation measurement system comprises at least one accelerometer.

13. The offshore wind turbine according to claim 9, wherein the oscillation measurement system comprises at least one GNSS receiver.

14. The offshore wind turbine according to claim 9, wherein the thruster control system is adapted to determine the speed of an oscillation from the measurements of the oscillation measurement system.

15. The offshore wind turbine according to claim 9, wherein one or more of the underwater thrusters are adapted to assume different orientations with respect to a wind turbine tower of the offshore wind turbine.

16. The offshore wind turbine according to claim 9, comprising a plurality of underwater thrusters that have a fixed orientation with respect to a wind turbine tower of the offshore wind turbine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Particular embodiments of the present invention will be described in the following by way of non-limiting examples, with reference to the appended drawings, in which:

(2) FIG. 1 illustrates an example of a prior art floating offshore wind turbine;

(3) FIG. 2 illustrates a coordinate system for describing the movement of offshore wind turbines;

(4) FIG. 3 illustrates a common pitch strategy in variable speed wind turbines and its effects on the thrust; and

(5) FIGS. 4a-4c schematically illustrate various embodiments of offshore wind turbines according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(6) FIG. 4a schematically illustrates a first embodiment of an offshore wind turbine according to the present invention. The floating wind turbine 10 comprises a wind turbine tower 11, a nacelle 12 arranged on top of said tower and a rotor having a plurality of blades. The water level has been schematically indicated with reference sign 20.

(7) The wind turbine tower 11 may comprise a buoyancy body and a ballast weight. Mooring lines 25 connected at the sea bed 30 serve to substantially keep the wind turbine in place and at least partially stabilize the floating wind turbine. The mooring lines 25 may be connected at stabilization arms 18. Although not clearly distinguishable in the side view provided in FIG. 4a, the stabilization arms 18 and mooring lines are not necessarily provided in the fore-aft direction or only in the fore-aft direction. In embodiments, the stabilization arms may e.g. extend in a radial direction away from the turbine tower. In other embodiments, the stabilization arms may form a circle, square or rectangle or yet other shape around the wind turbine tower.

(8) As explained previously, due to changing loads (which may be caused particularly by pitching of the blades above nominal wind speed, but also by waves and wind gusts), the floating wind turbine may start oscillating e.g. pitching.

(9) An underwater propeller 40 is provided at the bottom of the wind turbine tower. In this embodiment, the underwater propeller 40 is mounted on a shaft 41 which forms an extension of the wind turbine tower. Shaft 41 may be rotated and the propeller may thus change its orientation relative to the tower 11.

(10) If it is measured that the wind turbine is oscillating in an undesirable way, the underwater propeller can be actuated to cause a thrust that attenuates the oscillation. By changing the orientation of the propeller, a forward or rearward thrust, but also a sideways thrust can be provided if needed. Depending on the kind of propeller, the speed of the propeller and/or angle of propeller blades 42 and thereby the amount of thrust may also be adjusted in accordance with circumstances.

(11) In an example, in a fore-aft oscillation, the propeller may be kept substantially in the same orientation and may be operated in a pulsed manner. In another example, the propeller's orientation may continuously or very frequently be changed.

(12) In some embodiments, a propeller may be provided in which the sense of rotation of the propeller blades may be adjustable. In these embodiments, if the propeller blades rotate in a first direction a rearward thrust with respect to the propeller may be established. And if the blades rotate in a second direction, a forward thrust with respect to the propeller may be established. In these embodiments, the propeller's orientation does not need to be changed in order to be able to change the direction of thrust that can be provided.

(13) A system has thus been provided that is adapted to damp oscillations in different directions. To measure an oscillation a receiver of a Global Navigation Satellite System (GNSS), such as a GPS receiver may be provided in the wind turbine. Such a receiver may e.g. be mounted in the nacelle or in the tower. By determining a changing position of such a receiver, an oscillation may be measured. Accordingly, the propeller 40 may be actuated to dampen such an oscillation. The instantaneous position of the receiver (and thus the instantaneous position/orientation of the wind turbine) may be taken into account.

(14) In embodiments of the invention, the changing position of a receiver may be used to determine an oscillation speed and the propeller may be actuated taking the speed of oscillation into account.

(15) In some embodiments of the invention, an augmented GNSS system may be used. In these embodiments, the position of a wind turbine may be determined with respect to a reference point of which the exact location is known. Such a reference point may be located in the wind farm itself or elsewhere.

(16) In alternative embodiments, one or more tri-axial accelerometers may be mounted on the wind turbine. The accelerations that are measured may be used to determine the presence of an oscillation and the oscillation speed. In response to these measurements, the propeller 40 may be actuated.

(17) It should be noted that the pitch control of the blades does not necessarily need to be affected in order to dampen the oscillations. The electrical power output of the wind turbine therefore does not need to be negatively affected.

(18) FIG. 4b shows a second embodiment of a floating wind turbine 10′ in accordance with the present invention. The same reference signs have been used to indicate the same parts or components. In this example, the wind turbine 10′ comprises a platform 16 upon which the wind turbine tower 11 is located. The platform 16 is carried by a plurality of buoyancy tanks 15. In the side view of FIG. 4b, only two buoyancy tanks are shown. In embodiments of the invention, a wind turbine may comprise two, three, four or more of these buoyancy tanks surrounding the platform 16.

(19) Two underwater propellers 40 are mounted on the bottom of the buoyancy tanks 15. Both propellers are adapted to be able to change their orientation, since they are mounted on rotatable shafts 41. The working principle is similar to what was shown before: in response to an oscillation, the propellers may be actuated. Their orientation and/or thrust may be adapted. And they may or may not be actuated at the same time.

(20) In embodiments comprising several buoyancy tanks, one or more of the buoyancy tanks may comprise such thrusters. By proper actuation of the thrusters, various kinds of oscillating motions including surging, pitching, swaying and rolling may be effectively dampened.

(21) Also in these kinds of floating wind turbines, fixed propellers and/or propellers in which the sense of rotation may be changed can be used.

(22) In FIG. 4c, yet a further embodiment of the present invention is illustrated. An offshore wind turbine 10″ is provided that comprises a wind turbine tower that is fixed on a foundation in the sea bed 30. It is clear that this kind of offshore wind turbine is not subject to all the same movements that floating wind turbines are subjected to. However, in circumstances, also the fixed wind turbine may start oscillating e.g. in a fore-aft direction in response to a varying pitch angle.

(23) In this embodiment two fixed underwater propellers 50 have been provided. In response to a measured oscillation, the underwater propellers can be actuated. The orientation of the two fixed propellers is especially suited for damping of fore-aft oscillations. The registration of an oscillation may substantially be the same as hereinbefore described.

(24) In other embodiments, a single fixed propeller 50 may be provided. Such a single fixed propeller may in circumstances be enough to dampen oscillations using a pulsed actuation. In further embodiments, a single fixed propeller in which the sense of rotation (and thus the direction of thrust) can be changed may be employed. In yet further embodiments, the use of e.g. three fixed propellers could be envisaged to be able to dampen oscillations in various directions. It is further possible that not all the propellers are arranged at the same height of the tower.

(25) In all embodiments, alternative thrusters may be provided instead of propellers, such as for example water jets.

(26) Although only a number of particular embodiments and examples of the invention have been disclosed herein, it will be understood by those skilled in the art that other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof are possible. Furthermore, the present invention covers all possible combinations of the particular embodiments described. Thus, the scope of the present invention should not be limited by particular embodiments, but should be determined only by a fair reading of the claims that follow.