The present disclosure relates to a bearing assembly for a floating offshore wind turbine.

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.

Where fixed-foundation wind turbines are often assembled at their final destination, floating wind turbines are more typically assembled in port and then floated in an upright configuration to their final destination as a complete structure. During transport of a floating wind turbine, the rotor hub of the turbine is locked in place to prevent rotation of the blades. However, the inventors have identified that keeping the rotor hub locked in place results in brinelling of the bearings connecting the rotor hub to the nacelle, which can reduce the life of the wind turbine.

Viewed from a first aspect, the present invention provides an assembly for a wind turbine comprising: a nacelle; a rotor hub configured to rotate with respect to the nacelle; a bearing assembly arranged between the rotor hub and the nacelle; and a controllable loading mechanism configured to selectively control a preload within the bearing assembly, wherein a contact surface of the rotor hub and a contact surface of the nacelle are configured to engage one another to support the rotor hub when the bearing assembly is unloaded.

The described assembly allows the bearings to be unloaded after assembly of the rotor hub and nacelle, whilst still supporting the rotor hub by alternative support means, so as to prevent or reduce damage to the bearing assembly caused by locking the rotor position for a prolonged period of time.

The bearing assembly preferably comprises at least one bearing, and more preferably two bearings. The bearing(s) may be configured to axially and/or radially position the rotor hub with respect to the nacelle. The or each bearing may be a tapered roller bearing, i.e. the rolling elements of the bearing may comprise tapered rollers. The tapered roller bearings may be oriented at approximately 45° with respect to the axis of rotation of the rotor hub, e.g. between 30° and 60°, and preferably between 40° and 50°. The bearings are preferably each substantially annular, and may have a diameter of greater than 2 meters. The at least one bearing may be arranged between a strut of the nacelle and the rotor hub. The strut may be radially inward of the at least one bearing and the rotor hub may be radially outward of the at least one bearing. That is to say, for each bearing, a bearing surface of the nacelle, and particularly of a strut of the nacelle, may be radially inward of a bearing surface on the rotor hub. The or each bearing may comprise a plurality of bearing elements arranged between the bearing surfaces. The or each bearing may also include one or more cage to constrain the bearing elements.

The loading mechanism may comprise an axially-movable element, which may be axially movable in a direction parallel to an axis of rotation of the rotor hub. The axially-movable element preferably provides a bearing surface of at least one bearing of the bearing assembly. The axially-movable element preferably does not rotate with respect to the nacelle. Thus, movement of the axially-movable element controls the preload within the bearing assembly.

The loading mechanism may be configured to apply a load to the axially- movable element, preferably in an axial direction with respect to the axis of rotation of the rotor hub. The load may be applied hydraulically, pneumatically, mechanically, electromechanically, or a combination thereof.

Preferably, the loading mechanism is configured to apply a load hydraulically. Thus, the nacelle may comprise one or more hydraulic chamber adjacent the axially-movable element, which may be positioned to apply a load to the axially-movable element. The assembly may comprise a pump or connections for a pump for controlling the hydraulic pressure within the hydraulic chamber.

The loading mechanism is preferably configured to selectively set a preload within the bearing assembly at least to an operational preload and to an unloaded state. That is to say, the loading mechanism is preferably capable of selectively applying and releasing an operation preload within the bearing assembly. The operational preload is a preload used when the wind turbine is operated, for example to generate power. When the operational preload is applied, there is preferably no play between rolling elements of the bearing assembly and their respective bearing surfaces. The unloaded state preferably has substantially no load on the rolling elements of the bearing.

The contact surfaces of the rotor hub and the nacelle are preferably configured so as not to engage one another when the bearing assembly is loaded to an operational preload. The nacelle may comprise a bracket defining the contact surface of the nacelle. The bracket may be formed integrally with a strut of the nacelle or may be formed as a separate component. The bracket may be substantially annular and may define a substantially annular contact surface. Likewise, the contact surface of the rotor hub may be substantially annular.

The contact surfaces preferably correspond to one another. The contact surfaces may be configured to restrict relative rotation between the rotor hub and the nacelle when engaged and/or to maintain concentricity between the rotor hub and the nacelle when engaged. The contact surfaces are preferably flat surfaces or frustoconical surfaces, i.e. such that relative rotation between the rotor hub and the nacelle is prevented by friction. This minimises a distance that the rotor hub needs to move for the surfaces to engage. However, the surfaces may comprise one or more locking structures, such as teeth, to prevent relative rotation between the rotor hub and the nacelle.

The nacelle preferably comprises a generator configured to be driven by rotation of the rotor hub.

The rotor hub may be configured to receive one or more rotor blades, and preferably three rotor blades. Optionally, the assembly may comprise the one or more rotor blades mounted to the rotor hub. 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 wind turbine.

The assembly is preferably for use with a wind turbine having a rotor diameter of greater than 100m, more preferably greater than 150m and most preferably greater than 200m. The assembly is preferably for use with a wind turbine having a rated power output of greater than 3MW, more preferably greater than 6MW and most preferably greater than 10MW. The assembly may be for use with an offshore wind turbine.

Viewed from a second aspect, the present invention further provides a wind turbine comprising a tower and the assembly described above, wherein the nacelle is mounted to the top of the tower. The nacelle is preferably rotatably mounted to the top of the tower.

The wind turbine may be an offshore wind turbine, and may be a floating offshore wind turbine. Thus, the turbine may be mounted to a floating platform, which may be a spar buoy platform. In other embodiments, the floating platform may be a semi-submersible platform. The floating platform may be tethered to the seabed by one or more moorings, which may be catenary moorings.

In other embodiments, the offshore wind turbine may be a fixed-foundation offshore wind turbine. In yet further embodiments, the wind turbine may be an onshore wind turbine.

The wind turbine may have a rotor diameter of greater than 100m, more preferably greater than 150m and most preferably greater than 200m. The wind turbine may have a rated power output of greater than 3MW, more preferably greater than 6MW and most preferably greater than 10MW.

Viewed from a third aspect, the present invention provides a method of installing an offshore wind turbine, comprising: transporting the offshore wind turbine to an offshore location, wherein a bearing assembly arranged between a rotor hub of the wind turbine and a nacelle of the wind turbine is unloaded during the transporting, and wherein the rotor hub is supported during the transporting by engagement between a contact surface of the rotor hub and a contact surface of the nacelle; and at the offshore location, applying a preload to the bearing assembly such that the contact surfaces disengage from one another and the bearing assembly supports the rotor hub with respect to the nacelle.

The rotor hub and nacelle may be part of an assembly as described above and/or the wind turbine may be a wind turbine as described above. The assembly or wind turbine may include any one or more of the optional features described thereof.

The offshore wind turbine is preferably transported in a fully or partially assembled state.

The fully assembled state is intended to mean that all major parts of the offshore wind turbine are assembled, including at least the floating platform tower, nacelle, rotor hub and blades. In the partially assembled state, preferably at least the rotor hub and the nacelle are assembled, and more preferably at least the rotor hub, the nacelle and the tower are assembled. Optionally, in the partially assembled state, the wind turbine may be fully assembled apart from the platform and/or the blades of the wind turbine.

The offshore wind turbine may be transported in a substantially horizontal configuration. For example, the offshore wind turbine may be transported in a substantially horizontal configuration on a ship. A horizontal configuration is intended here to mean that a tower of the offshore wind turbine (that would normally extend substantially vertically during operation) extends substantially horizontally. Substantially horizontal may be, for example, an angle of less than 20°, preferably less than 10° and more preferably less than 5° with respect to true horizontal.

Certain preferred embodiments of the present invention will now be described in greater detail by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows a floating offshore wind turbine;

Figure 2 shows a cross-section through a nacelle and a rotor hub of the wind turbine in a first configuration; and

Figure 3 shows a cross-section through the nacelle and the rotor hub in a second configuration.

Figure 1 illustrates a floating offshore wind turbine.

The wind turbine comprises a tower 2, a nacelle 4 rotatably mounted at the top of the tower 2, and a rotor comprising a rotor hub 6 and a plurality of blades 8 rotatably mounted to the nacelle 4.

The nacelle 4 is configured to rotate about a longitudinal axis of the tower 2, which is approximately vertical in operation, and is controlled to face into oncoming wind.

The rotor is configured to rotate about a substantially horizontal axis so that the blades 8 are driven to rotate by the oncoming wind. The rotor is coupled to a drive shaft of a generator housed within the nacelle 4.

In the illustrated embodiment, the tower 2 is mounted to a spar buoy 10, which is moored to the sea bed by three catenary bridle moorings. However, the following techniques are applicable to wind turbines using alternative floating platforms or moorings.

Floating offshore wind turbines are usually designed with large rotor diameters to generate a high output power to maximise cost efficiency. Smaller offshore wind turbines might be rated at around 3-4MW and have rotor diameters of only about 100m. Typically, however, floating offshore wind turbines have power outputs rated at greater than 8MW and today such wind turbines are commonly designed to achieve a rated power output of around 12-13 MW with a rotor diameter of about 220m.

Figure 2 shows a detailed, cross-sectional view through an upper part of a bearing assembly 12 rotatably coupling the rotor hub 6 to the nacelle 4. The bearing assembly 12 in this embodiment comprises a pair of tapered roller bearings 14, 16 arranged radially between the rotor hub 6 and a non-rotating strut 18 of the nacelle 4, so as to support and position the rotor hub 6. Within a large wind turbine having a rotor diameter of over 200m, each of the bearings 14,

16 in the illustrated configuration may have a diameter of up to around 4m.

A forward bearing 14 comprises a first forward bearing surface 20 provided by the strut 18 of the nacelle 4, a second forward bearing surface 22 provided by the rotor hub 6, and a plurality of tapered rollers 24 provided between the first and second forward bearing surfaces 20, 22.

A rearward bearing 16 comprises a first rearward bearing surface 26 provided by the nacelle 4, a second rearward bearing surface 28 provided by the rotor hub 6, and a plurality of tapered rollers 30 provided between the first and second rearward bearing surfaces 26, 28.

The forward and rearward bearings 14, 16 are each oriented at approximately 45° with respect to the axis of rotation of the rotor. Accordingly, by applying axial loading to the bearings, a radially outward load is applied to the rotor hub 6 by the bearings. Whilst not shown, the rollers 24, 30 of each of the bearings 14, 16 will typically be constrained by respective cages.

In accordance with the present disclosure, the first rearward bearing surface 26 is provided on an axially movable element 32, which is an annular element that is movable along the direction of the axis of rotation of the rotor. The position of the axially-movable element 32 can be controlled hydraulically, by supplying pressurised fluid to a hydraulic chamber 34 adjacent to the axially movable element 32. This allows the loading on the bearings 14, 16 to be controlled.

During operation, a predetermined hydraulic pressure is applied to the hydraulic chamber 34 to apply a predetermined preload to each of the bearings 14, 16. The bearing preload is a sustained load applied to the bearings 14, 16 that continues to act, independent of external forces, so as to ensure contact between the tapered rollers 24, 30 and the corresponding first and second bearing surfaces 20, 22, 26, 28 during operation.

As discussed above, it has been found that this preloading can cause brinelling of the components of the bearings 14, 16 when the rotor hub 6 is locked against rotation with respect to the nacelle 4 for a sustained period of time, for example during storage and transport of the wind turbine after assembly but before final deployment. The present disclosure proposes to avoid this problem by unloading the bearings 14, 16 during this time.

With reference to Figure 3, the nacelle 4 is further provided with an annular brace 36 defining a support surface 38 configured to engage a corresponding support surface 40 on the rotor hub 6 when the bearings 14, 16 are unloaded. In this case, that is when the hydraulic pressure has been removed from the hydraulic chamber 34 and the axially movable element 32 has been moved to the axially rearward position.

As can be seen in Figure 3, in the unloaded configuration, the rollers 24, 30 of the bearings 14, 16 are not engaged with both of their respective first and second bearing surfaces 20, 22, 26, 28. Instead, the rotor hub 6 is supported and centred via the engagement between the contact surface 40 of the rotor hub 6 and the contact surface of the nacelle 4.

The engagement between the contact surfaces 38, 40 is maintained by a force 42 applied to the rotor in a substantially axially rearward direction. The force 42 may be applied by gravity, for example by transporting the wind turbine in a horizontal configuration as will be discussed later. Alternatively, for example if it is desired to transport the wind turbine in a vertical configuration, the force 42 may be applied by other means, such as mechanically using an external device fastened to the nacelle 4 and the rotor hub 6.

In the exemplary embodiment, the annular brace 36 is provided as a separate component of the nacelle 4 that is locked against rotational and axial movement. The annular brace defines the hydraulic chamber 34 and the support surface 38 of the nacelle 4. However, in other embodiments, this function may be provided by an annular brace 36 that is formed integrally with other, non-rotating components of the nacelle 4, such as the strut 18.

Furthermore, whilst the axially movable element 32 in the illustrated embodiment is hydraulically movable, it will be appreciated that in other embodiments the axially movable element 32 may be mechanically, pneumatically or electromechanically movable, or it may employ a combination of means. For example, in one embodiment, the preload may be applied to the axially movable element 32 by hydraulic means such as those described above and then locked mechanically such as by means of one or more bolts, allowing the hydraulic pressure to be released whilst maintaining the preload on the bearings 14, 16.

A method of deploying the offshore wind turbine will now be described. The offshore wind turbine is first assembled in port into an operational condition. Here, the wind turbine functions will be tested to ensure that the wind turbine is fully operational.

Once the wind turbine has been tested and approved for deployment, it will be laid down in a horizontal position on top of a cargo ship. Optionally, the blades may be removed at this point, but it is expected that they will be kept in place.

When the wind turbine is in the horizontal position, with the forward side of the turbine facing upwards, the hydraulic pressure in the hydraulic chamber 34 is released to remove the preload on the bearings 14, 16. This causes the rotor to move downwards (rearwardly in the axial direction) and the contact surfaces 38, 40 of the rotor hub 6 and nacelle 4 engage one another, locking the rotor hub 6 against rotation and supporting the weight of the rotor. The frustoconical shape of the contact surfaces 38, 40 maintains concentricity between the rotor hub 6 and the strut 18 of the nacelle 4.

With the bearings 14, 16 unloaded, the ship then carries the wind turbine in the horizontal position with the bearings 14, 16 unloaded to a deployment location. The preload is then re-applied to the bearings 14, 16 by supplying pressurised hydraulic fluid to the hydraulic chamber 34. Then, the wind turbine is set in the vertical orientation by a crane or the like on the ship, and is deployed in the water.

Once deployed, the wind turbine will be secured by moorings or the like, and will be connected by subsea cabling to an offshore electrical substation, which transports the electricity to shore by subsea cabling.

In the above example, the wind turbine is transported in a horizontal configuration. However, as discussed previously, it may alternatively be transported in a vertical configuration. If the wind turbine is transported in the vertical configuration, then gravity cannot be used to engage the contact surfaces 38, 40 when the bearings 14, 16 are unloaded, and instead the engagement is maintained by, for example, a bracket attached to the rotor hub 6 and nacelle 4.

Whilst the described technique of unloading the bearings 14, 16 and supporting the rotor hub 6 using a separate brace 36 after assembly of the rotor hub 6 and nacelle 4 is particularly applicable to floating, offshore wind turbines, it can also be applied to other wind turbine structures that suffer from the same problem. This might include fixed-foundation offshore wind turbines, or indeed onshore wind turbines.