WO2023143686A1

WO2023143686A1 - An offshore wind farm with mooring lines of different lengths

Info

Publication number: WO2023143686A1
Application number: PCT/DK2023/050016
Authority: WO
Inventors: Oliver Tierdad FILSOOF; Mikkel Kiilerich ØSTERLUND; Torben Ladegaard Baun
Original assignee: Vestas Wind Systems A/S
Priority date: 2022-01-26
Filing date: 2023-01-25
Publication date: 2023-08-03

Abstract

An offshore wind farm comprising a plurality of wind turbines (1) is disclosed. Each wind turbine (1) comprises a floating foundation (2) and at least two mooring lines (7), each mooring line (7) interconnecting the floating foundation (2) of the wind turbine (1) and an anchor point (12, 13). At least one of the wind turbines (1) comprises mooring lines (7) having at least two different lengths, thereby causing a movability of the wind turbine (1) from a neutral position to be varying as a function of direction of movement. The invention further relates to a wind turbine (1) for use in such an offshore wind farm and a method for designing a layout for such an offshore wind farm.

Description

AN OFFSHORE WIND FARM WITH MOORING LINES OF DIFFERENT LENGTHS

FIELD OF THE INVENTION

The present invention relates to an offshore wind farm comprising a plurality of wind turbines, each wind turbine comprising a floating foundation and at least two mooring lines, each mooring line interconnecting the floating foundation and an anchor point. The wind turbines of the offshore wind farm according to the invention are able to reduce or cancel wake effects within the offshore wind farm, at several wind directions. The invention further relates to a method for designing a layout for such an offshore wind farm.

BACKGROUND OF THE INVENTION

In offshore wind farms, a plurality of wind turbines are arranged offshore, e.g. at sea, in the vicinity of each other and within a specified site. The wind turbines may be fixedly connected to the seabed, e.g. via a monopile, a tripod foundation, or any other suitable kind of foundation structure. As an alternative, the wind turbines may each comprise a floating foundation which is connected to one or more anchor points, via mooring lines. In this case the wind turbines are not connected to the seabed at a fixed position, but are instead floating in the water surface while being fixed via mooring lines. Due to their floating nature, the floating wind turbines are thereby allowed to move to a certain, limited extent.

Since the wind turbines of an offshore wind farm are arranged in the vicinity of each other, the operation, or even the mere presence, of one of the wind turbines may affect operation of one or more of the other wind turbines, e.g. due to wake effects created by the wind turbines. This may, e.g., lead to reduced total power production of the offshore wind farm and/or increased loads on wind turbines arranged downstream relative to other wind turbines. In order to minimise the adverse consequences of wake effects, the layout of offshore wind farms may be designed in a manner which to the greatest possible extent avoids the wind turbines being positioned in the wake of other wind turbines, when the wind direction is at or near a dominating wind direction at the site of the offshore wind farm. However, such a layout may result in more or less significant wake effects at other wind directions, and this may have the consequence that the annual power production of the offshore wind farm is reduced.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide an offshore wind farm in which the total power production of the offshore wind farm can be increased as compared to prior art offshore wind farms.

It is a further object of embodiments of the invention to provide an offshore wind farm in which wake effects among the wind turbines can be reduced as compared to prior art offshore wind farms.

It is an even further object of embodiments of the invention to provide a method for designing a layout for an offshore wind farm in which the total power production of the offshore wind farm can be increased as compared to prior art offshore wind farms.

It is an even further object of embodiments of the invention to provide a method for designing a layout for an offshore wind farm in which wake effects among the wind turbines can be reduced as compared to prior art offshore wind farms

According to a first aspect the invention provides an offshore wind farm comprising a plurality of wind turbines, each wind turbine comprising a floating foundation, a tower mounted on the floating foundation, a nacelle mounted rotatably on the tower, and a rotor mounted rotatably on the nacelle, each wind turbine further comprising at least two mooring lines, each mooring line interconnecting the floating foundation of the wind turbine and an anchor point, wherein at least one of the wind turbines comprises mooring lines having at least two different lengths, thereby causing a movability of the wind turbine from a neutral position to be varying as a function of direction of movement.

Thus, according to the first aspect, the invention provides an offshore wind farm. In the present context the term 'offshore wind farm' should be interpreted to mean a wind farm which is arranged offshore, e.g. at sea. In the present context the term 'wind farm' should be interpreted to mean a group of a plurality of wind turbines sharing infrastructure, e.g. for power transmission to a power grid, and/or being arranged within a specified geographical site.

Thus, the offshore wind farm comprises a plurality of wind turbines. Each wind turbine comprises a floating foundation, a tower mounted on the floating foundation, a nacelle mounted rotatably on the tower, and rotor mounted rotatably on the nacelle. Accordingly, the wind turbines of the offshore wind farm are of a kind which are not fixedly connected to a fixed position at the seabed, but are instead allowed to move to a certain extent, due to the floating foundations. The wind turbines are typically moved due to wind acting on the rotor of the wind turbines and/or due to waves, current, etc., acting on the floating foundation. The direction of movement of the wind turbines at a given point in time is, thus, typically dependent on the wind direction at that point in time.

The wind turbines may each comprise a single nacelle with a single rotor mounted thereon. Such wind turbines are normally referred to as 'single rotor wind turbines'. In this case the nacelle is normally mounted directly on top of the tower by means of a yaw system which allows the nacelle to rotate relative to the tower in order to position the rotor correctly with respect to the direction of the incoming wind.

As an alternative, one or more of the wind turbines may comprise two or more rotors, e.g. mounted on separate nacelles, which are in turn mounted on a single tower. Such wind turbines are normally referred to as 'multirotor wind turbines'. In this case the nacelles may be mounted on the tower via arms or other suitable structures, which are capable of performing the required yawing movements relative to the tower.

Each wind turbine further comprises at least two mooring lines. Each mooring line interconnects the floating foundation of the wind turbine and an anchor point. Accordingly, the floating foundation of each wind turbine is connected to at least two anchor points via respective mooring lines. In the present context the term 'anchor point' should be interpreted to mean a structure which is fixed relative to a position within the site of the offshore wind farm, and to which a wind turbine can be moored. The anchor point may, e.g., be in the form of an anchor foundation which is secured to the seabed, a gravity anchor positioned on the seabed, a floating anchor, or any other suitable kind of anchor. For instance, the anchor point may form part of a mooring line grid, where at least some of the wind turbines of the offshore wind turbine are 'anchored' to each other via the mooring line grid, while at least some of the wind turbines, e.g. being arranged at the rim of the offshore wind farm, are anchored to the seabed, thereby ensuring that the offshore wind farm as such remains at a specified position.

The mooring lines determine to which extent the wind turbines are able to move, i.e. the freedom of movement of the wind turbine. When a given wind turbine moves in a certain direction, it will only be allowed to move until at least one of the mooring lines may be considered as fully stretched or extended between the floating foundation and the anchor point to which it is connected, i.e. until there is essentially no slack in that mooring line. This may, e.g., be the case when mooring line forces, and forces acting on the turbine and foundation have reached an equilibrium.

At least one of the wind turbines comprises mooring lines having at least two different lengths, i.e. the mooring lines are not all of the same length. This introduces an asymmetry in the movability, i.e. in the freedom of movement, of the wind turbine, in the sense that the wind turbine is allowed to move further in a direction which is restricted by a long mooring line than in a direction which is restricted by a shorter mooring line. It should be understood that the difference in lengths of the mooring lines is sufficient to introduce a significant asymmetry in movability of the wind turbine. Differences in length which are caused by manufacturing tolerances or variations in the level of the seabed should not be regarded as falling within the scope of the present invention.

Thus, the difference in lengths of the mooring lines causes a movability, i.e. a freedom of movement, of the wind turbine from a neutral position to be varying as a function of direction of movement. In the present context the term 'neutral position' should be interpreted to mean a position of the wind turbine, relative to the associated anchor points, which may be regarded as a design position or a centre position for the wind turbine. The neutral position may, e.g., be a position where the wind turbine would be if no wind, waves or current acted on the wind turbine, and where the slack of the mooring lines would be identical if the mooring lines had the same length. For instance, in the case that the anchor points are positioned on a circle, e.g. evenly positioned on the circle, the neutral position could be the centre of the circle.

Thus, the wind turbine is able to move further away from the neutral position along a direction which is restricted by a long mooring line than in a direction which is restricted by a shorter mooring line. Accordingly, the movability or freedom of movement of the wind turbine from the neutral position is higher along the direction which is restricted by the longer mooring line than along the direction which is restricted by the shorter mooring line. Accordingly, the movability of the wind turbine from the neutral position varies as a function of the direction of movement, i.e. it is dependent on the direction of movement. Since the direction of movement is typically dependent, inter alia, on the wind direction, the movability of the wind turbine from the neutral position is therefore also dependent on the wind direction.

As described above, the wind turbines are allowed to move in a certain direction, determined, e.g., by the wind direction, until it reaches the limit for the movability or freedom of movement in that direction, and then it will stay in this position until the wind direction changes. Accordingly, the positions of the wind turbines within the offshore wind farm are determined by the lengths of the mooring lines in combination with the wind direction. Thus, by carefully selecting the lengths of the mooring lines and the positions of the mooring lines and their associated anchor points, it can be ensured that the wind turbines of the offshore wind farm position themselves relative to each other in a manner which minimises wake effects among the wind turbines at most or all wind directions. For instance, a longer mooring line of a specific wind turbine may be sufficient to move that wind turbine out of the wake of an upstream wind turbine, at a specific wind direction.

It should be noted that the movement of the wind turbines could also be caused primarily by waves and/or current acting on the floating foundation. In this case similar design considerations regarding the lengths of the mooring lines may be applied in order to ensure that the waves and/or current moves the wind turbines to positions which ensure minimal wake effects among the wind turbines at various wind directions.

Accordingly, in the offshore wind farm according to the first aspect of the invention, the wake effects among the wind turbines of the wind farm are reduced at more wind directions than is the case in prior art offshore wind farms, where the mooring lines of the wind turbines have substantially identical lengths. This allows the total power production of the offshore wind farm to be at or close to maximum power production for a larger part of the time, and thereby the annual energy production of the offshore wind farm is increased. Furthermore, the reduced wake effects among the wind turbines may cause a decrease in the loads on the wind turbines.

Each of the at least two mooring lines may have a fixed length. According to this embodiment, the length of a given mooring line is not adjustable, e.g. during operation of the offshore wind farm. Instead, once the lengths of the mooring lines have been selected, their lengths remain the same. Accordingly, the relative positioning of the wind turbines within the wind farm described above, depending on the wind direction, is obtained in a purely passive manner, i.e. without requiring active control or operation of mechanisms for adjusting the length of one or more mooring lines, such as winches or similar devices. This is a very simple and failsafe manner of reducing the wake effects among the wind turbines, and with minimal maintenance requirements. Each wind turbine may comprise at least three mooring lines, and at least one of the wind turbines may comprise at least one mooring line having a first length and at least two mooring lines having a second length, where the second length differs from the first length.

By providing each wind turbine with at least three mooring lines it is obtained that the wind turbines are moored in a stable manner. According to this embodiment, at least one of the wind turbines comprises at least one mooring line which is significantly shorter or longer than at least two of the other mooring lines, and which therefore distinguishes from the other mooring lines. This also singles out a direction of movement of the wind in which the movability from the neutral position is significantly higher or lower than other directions.

In the case that the wind turbine, according to this embodiment, comprises exactly three mooring lines, two of the mooring lines have substantially identical lengths, and the third mooring line has a length which is shorter or longer than the length of the other two. Accordingly, in this case the third mooring line defines a direction of movement of the wind turbine along which the movability, i.e. the freedom of movement, of the wind turbine is significantly more or less restricted than along other directions, which are defined by the other mooring lines.

At least two of the wind turbines may each comprise mooring lines having at least two different lengths, and a direction defining a maximum movability from the neutral position for a first wind turbine may differ from a direction defining maximum movability from the neutral position for a second wind turbine.

As described above, a longer mooring line defines a higher movability from the neutral position for the wind turbine along a direction which is restricted by that mooring line, and the direction which is restricted by the longest mooring line will define a maximum movability, i.e. maximum freedom of movement, for the wind turbine. According to this embodiment, this direction with maximum movability is not the same for all the wind turbines of the offshore wind farm. Thereby, for a given wind direction, the movability or freedom of movement of the wind turbines will vary from one wind turbine to another. Accordingly, at a given wind direction, one wind turbine will be allowed to move a first distance from its neutral position before it is stopped by a mooring line, whereas another wind turbine will be allowed to move a second distance from its neutral position before it is stopped, where the first distance is longer than the second distance. At another wind direction, the situation may be reversed, and at yet another wind direction, the two wind turbines may be allowed to move the same distance from their respective neutral positions before being stopped by one of the mooring lines.

Accordingly, the wind turbines of the offshore wind farm position themselves relative to each other, depending on the wind direction. Thereby it can be ensured that the wake effects among the wind turbines are minimised at all or most wind directions. For instance, neighbouring wind turbines may be configured such that they arrange themselves in a staggered manner along the wind direction, regardless of the specific wind direction, thereby ensuring that downstream wind turbines are moved at least partly out of the wake of upstream wind turbines.

For instance, at least two of the wind turbines may each comprise mooring lines having at least two different lengths, and a direction defined by a mooring line of a first wind turbine, the mooring line having a given length, may differ from a direction defined by a mooring line, having a corresponding length, of a second wind turbine.

At least one of the wind turbines may comprise at least one mooring line having a length which is at least 5% longer than at least one of the other mooring lines of the wind turbine.

According to this embodiment, the difference in lengths of the mooring lines is at least 5% of the length of a shorter mooring line. For instance, if one of the mooring lines is approximately 830 m long, then at least one of the other mooring lines should be at least 870 m long. For instance, at least one mooring line may have a length which is at least 10% longer than at least one of the other mooring lines, such as at least 15% longer, such as at least 20% longer.

The different lengths of the mooring lines may cause the movability of at least one of the wind turbines from a neutral position to be variable as a function of direction of movement, between a minimum movability and a maximum movability, and the maximum movability may exceed the minimum movability by a distance which corresponds to at least 5% of a diameter of the rotor of the wind turbine.

According to this embodiment, the movability of the wind turbine varies between a minimum movability and a maximum movability, i.e. between a minimum freedom of movement and a maximum freedom of movement, as a function of direction of movement. The direction of movement which defines the minimum movability is the direction in which the wind turbine is allowed to move the shortest distance from the neutral position before it is stopped by at least one of the mooring lines. Similarly, the direction of movement which defines the maximum movability is the direction in which the wind turbine is allowed to move the longest distance from the neutral position before it is stopped by one of the mooring lines.

According to this embodiment, the maximum movability exceeds the minimum movability by a distance which corresponds to at least 5% of the diameter of the rotor of the wind turbine, such as at least 10%, such as at least 50%, such as at least 100%, i.e. a full rotor diameter. In other words, when the wind turbine moves along the direction which defines maximum movability, it is allowed to move a distance which is longer by at least 5% of the rotor diameter than the distance which it is allowed to move along the direction which defines minimum movability. This may be sufficient to move downstream wind turbines out of the wake of upstream wind turbines to an extent which increases the power production of the downstream wind turbines. This is particularly the case if the wind turbines are arranged in a staggered manner as described above. At least one of the wind turbines may be connected, via the mooring lines, to anchor points which are arranged asymmetrically with respect to the wind turbine.

According to this embodiment, the anchor points are not arranged in a symmetrical manner relative to the wind turbine, in particular relative to the neutral position of the wind turbine. For instance, the anchor positions may be arranged with different distances to the neutral position of the wind turbine and/or with different angular distances between neighbouring anchor positions.

Arranging the anchor points asymmetrically with respect to the wind turbine may further contribute to the asymmetry in movability of the wind turbine.

As an alternative, the anchor positions may be arranged symmetrically relative to the wind turbine, in particular relative to the neutral position of the wind turbine. In this case the anchor positions may be arranged on a circle with the neutral position of the wind turbine at the centre of the circle, and with equal angular distances between the anchor positions. For instance, if the wind turbine comprises exactly three mooring lines, then the anchor points will be arranged on a circle with approximately 120° between the anchor point positions.

According to this embodiment, the asymmetry in movability of the wind turbine arises solely from the difference in lengths of the mooring lines.

According to a second aspect the invention provides a wind turbine for use in an offshore wind farm according to the first aspect of the invention, the wind turbine comprising a floating foundation, a tower mounted on the floating foundation, a nacelle mounted rotatably on the tower, and a rotor mounted rotatably on the nacelle, the wind turbine further comprising at least two mooring lines, each mooring line interconnecting the floating foundation of the wind turbine and an anchor point, wherein at least one of the mooring lines has a length which differs from a length of at least one of the other mooring line(s), thereby causing a movability of the wind turbine from a neutral position to be varying as a function of direction of movement. The wind turbine according to the second aspect of the invention has already been described above with reference to the first aspect of the invention, and the remarks set forth above are equally applicable here.

In particular, each of the at least two mooring lines may have a fixed length.

According to a third aspect the invention provides a method for designing a layout for an offshore wind farm, the offshore wind farm comprising a plurality of wind turbines, each wind turbine comprising a floating foundation, a tower mounted on the floating foundation, a nacelle mounted rotatably on the tower, and a rotor mounted rotatably on the nacelle, each wind turbine further comprising at least two mooring lines, each mooring line interconnecting the floating foundation of the wind turbine and an anchor point, the method comprising the steps of:

- obtaining information regarding site conditions for a site of the offshore wind farm, including information regarding expected wind directions at the site,

- providing layout constraints related to the offshore wind farm,

- supplying the information regarding site conditions and the layout constraints to an optimisation algorithm, the optimisation algorithm applying length of the mooring lines as an optimisation variable, and

- creating a layout for the offshore wind farm, using the optimisation algorithm, the layout specifying optimal lengths of the mooring lines of each of the wind turbines.

It should be noted that a person skilled in the art would readily recognise that any features described in combination with the first aspect of the invention could also be combined with the third aspect of the invention, and vice versa.

The method according to the third aspect of the invention is a method for designing a layout for an offshore wind farm. In the present context the term 'layout' should be interpreted to mean a plan of the site of the offshore wind farm which specifies where the wind turbines should be positioned within the site, including how many wind turbines should be positioned in the offshore wind farm. The layout may further specify design specifications which relate to the individual wind turbines, e.g. the type of wind turbine, the nominal power of the wind turbine, type of foundation, hub height, length of wind turbine blades, number of anchor points and mooring lines, positions of the anchor points, lengths of mooring lines, etc.

The layout of a wind farm is normally designed before the wind farm is erected, i.e. during a planning phase of the wind farm.

The offshore wind farm comprises a plurality of wind turbines. Each wind turbine comprises a floating foundation, a tower mounted on the floating foundation, a nacelle mounted rotatably on the tower, and a rotor mounted rotatably on the nacelle. Each wind turbine further comprises at least two mooring lines, each mooring line interconnecting the floating foundation of the wind turbine and an anchor point. Thus, the offshore wind farm may be a wind farm according to the first aspect of the invention, and the remarks set forth above with reference to the first aspect of the invention are therefore equally applicable here.

In the method according to the third aspect of the invention, information regarding site conditions for a site of the offshore wind farm is initially obtained. The information includes at least information regarding expected wind directions at the site. This may, e.g., include information regarding a dominating wind direction at the site, distribution of wind directions, e.g. as a function of time of year, time of day, etc., and/or any other suitable kind of information related to expected wind directions at the site. The information may, e.g., be in the form of a wind rose. In any event, the information regarding the site conditions for the site of the offshore wind farm at least gives an indication regarding how the wind direction is expected to act at the site of the offshore wind farm once the offshore wind farm has been erected, i.e. what the wind turbines of the offshore wind farm may expect to experience in terms of wind direction. The information regarding site conditions for the site of the offshore wind farm may further include information regarding expected wind speeds, expected current conditions, expected wave conditions, etc.

Next, layout constraints related to the offshore wind farm are provided. In the present context the term 'layout constraints' should be interpreted to mean design parameters for the offshore wind farm which are mandatory, e.g. because they are dictated by certain conditions at the site or because they have been specified by the owner or the constructor of the offshore wind farm. The layout constraints could, e.g., include an outer boundary of the site of the offshore wind farm, zones within the offshore wind farm where wind turbines are not to be positioned, number of wind turbines to be positioned within the site, type(s) of wind turbines, total nominal power of the offshore wind farm, minimum distance between the wind turbines, etc.

Next, the information regarding site conditions and the layout constraints are supplied to an optimisation algorithm. Thereby the optimisation algorithm is in the possession of information regarding expected wind directions at the site of the offshore wind farm as well as information regarding mandatory design parameters for the offshore wind farm.

In the present context the term 'optimisation algorithm' should be interpreted to mean an algorithm which is able to optimise an output, in terms of one or more specified optimisation targets, by adjusting one or more specified optimisation variables, while at all times ensuring that the provided constraints are met.

The optimisation algorithm applies length of the mooring lines as an optimisation variable. Accordingly, the optimisation algorithm can adjust the lengths of the mooring lines of the wind turbines of the offshore wind farm in order to obtain an optimised layout for the offshore wind farm, in terms of one or more specified optimisation targets.

Finally, a layout for the offshore wind farm is created, using the optimisation algorithm, the layout specifying optimal lengths of the mooring lines of each of the wind turbines. As described above with reference to the first aspect of the invention, applying different lengths to the mooring lines of the wind turbines introduces an asymmetry in the movability, i.e. the freedom of movement, of the wind turbines, with respect to direction of movement. Since the information regarding site conditions for the site of the offshore wind farm, including information regarding expected wind direction at the site, was supplied to the optimisation algorithm, the expected wind directions are taken into account when creating the layout. Thereby the lengths of the mooring lines of the wind turbines are also adjusted while taking the expected wind directions into account. Accordingly, in the optimised layout for the offshore wind farm, the lengths of the mooring lines are optimised in the sense that they are selected in such a manner that the wind turbines will position themselves relative to each other, at all or most wind directions, in a manner which minimises wake effects between the wind turbines, e.g. resulting in optimised annual total power production of the offshore wind farm.

The optimisation algorithm may apply fixed length of the mooring lines as an optimisation variable, and the layout may specify optimal fixed lengths of the mooring lines of each of the wind turbines. This has already been described above with reference to the first aspect of the invention.

The optimisation algorithm may apply levelized cost of energy (LCoE) of the offshore wind farm as an optimisation target, and the created layout for the offshore wind farm may specify lengths of the mooring lines of each of the wind turbines which results in a minimised LCoE.

According to this embodiment, the layout of the offshore wind farm is optimised in the sense that the LCoE is minimised. LCoE is a measure for the cost of producing a unit of energy by means of a wind turbine or, in the present case, of an offshore wind farm. LCoE takes the costs involved in erecting and operating the wind farm, as well as the expected energy production of the wind farm into account. Accordingly, when the LCoE is applied as an optimisation target, a balance needs to be found between the cost of a change in design and the expected increase in power output caused by the change in design. The cost of a change in design may include an increase in direct manufacturing cost, e.g. due to additional use of material, e.g. including cost of power cable/transmission line and/or mooring line materials, change in material, impact on manufacturing process of components of the wind turbines, etc., as well as an increase in maintenance cost, e.g. due to increased loads and/or wear on the wind turbines caused by the change in design.

Alternatively or additionally, the optimisation algorithm may apply loads on the wind turbines and/or energy production of the offshore wind farm and/or wake effects among the wind turbines as an optimisation target. According to this embodiment, the layout of the offshore wind farm is optimised in terms of minimising loads on the wind turbines, maximising energy production of the offshore wind farm and/or minimising wake effects among the wind turbines of the offshore wind farm.

Alternatively or additionally, other suitable optimisation targets may be applied, e.g. cost of valued energy, net present value of investment, etc.

The optimisation algorithm may further apply positions of anchor points for the wind turbines as an optimisation variable.

According to this embodiment, the optimisation algorithm may adjust the positions of the anchor points, in addition to the lengths of the mooring lines, in order to obtain the specified optimisation target. As described above, positioning the anchor points asymmetrically with respect to the wind turbines may further contribute to the asymmetry in movability or freedom of movement of the wind turbines.

The layout constraints related to the offshore wind farm may include a number of wind turbines and/or one or more types of the wind turbines to be arranged in the offshore wind farm.

According to this embodiment, the number of wind turbines to be positioned within the site of the offshore wind farm and/or the type(s) of wind turbines which are to be erected have already been selected, and therefore these parameters can not be adjusted or changed when the layout for the offshore wind farm is created, using the optimisation algorithm.

The type of wind turbine may, e.g., specify the manufacturer of the wind turbine, the model of the wind turbine, the nominal power, the hub height, the length of the wind turbine blades, the type of foundation, etc.

The method may further comprise the step of supplying an initial layout for the offshore wind farm to the optimisation algorithm, and the step of creating a layout for the offshore wind farm, using the optimisation algorithm, may comprise applying the initial layout as a starting point for the optimisation.

According to this embodiment, the optimisation process is started from an initial 'guess' for a layout, and one or more parameters, including the lengths of the mooring lines, are adjusted, starting from the values specified in the initial layout, in order to obtain an optimised layout. The initial layout may, e.g., be a layout which positions the wind turbines relative to each other in an optimal manner with respect to a dominating wind direction at the site, and with standard, identical lengths of the mooring lines. Starting from an initial layout may allow an optimal layout to be reached faster and with less processing power.

It should be noted that two or more initial layouts may be used as input to the optimisation process.

The method may further comprise the step of erecting an offshore wind farm in accordance with the created layout. According to this embodiment, once the layout for the offshore wind farm has been created, the actual offshore wind farm is erected in accordance with the created layout. In particular, the offshore wind farm is erected with mooring lines with lengths which match the optimal mooring line lengths specified by the created layout. Thereby the resulting offshore wind farm is optimal in terms of the specified optimisation target(s). It is noted that the method according to the third aspect of the invention may also be applied to existing offshore wind farms, where existing mooring lines may be exchanged by mooring lines of different lengths.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1 is a perspective view of a wind turbine according to an embodiment of the invention,

Fig. 2 shows side views of a prior art wind turbine and a wind turbine according to an embodiment of the invention,

Fig. 3 illustrates movability for a prior art wind turbine and for a wind turbine according to an embodiment of the invention,

Figs. 4 and 5 illustrate movement of wind turbines in a prior art offshore wind farm and in an offshore wind farm according to an embodiment of the invention at two different wind directions,

Fig. 6 illustrates movability for a prior art wind turbine and a wind turbine according to an alternative embodiment of the invention, and

Figs. 7 and 8 are flow diagrams illustrating methods according to two embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a perspective view of a wind turbine 1 according to an embodiment of the invention. The wind turbine 1 is positioned at sea and forms part of an offshore wind farm. The wind turbine 1 comprises a floating foundation 2, a tower 3, a nacelle 4 and a hub 5 carrying three wind turbine blades 6. The hub 5 and the wind turbine blades 6 form a rotor of the wind turbine 1. The wind turbine 1 further comprises three mooring lines 7. Each mooring line 7 interconnects the floating foundation 2 and an anchor point (not shown).

The floating foundation 2 allows the wind turbine 1 to move to a certain extent, i.e. within a certain region along the water surface. How far the wind turbine 1 is allowed to move in a given direction is determined by the mooring lines 7, since the wind turbine 1 will only be allowed to move until one of the mooring lines 7 is fully stretched or extended between the floating foundation 2 and the anchor point to which it is connected, i.e. until there is essentially no slack in that mooring line 7.

The mooring lines 7 of the wind turbine 1 have at least two different lengths. For instance, two of the mooring lines 7 may have substantially identical lengths, while a third mooring line 7 is longer or shorter than the other two.

Alternatively, none of the lengths of the mooring lines 7 may be identical, i.e. the length of each mooring line 7 may differ from the length of each of the other mooring lines 7.

The difference in length of the mooring lines 7 creates an asymmetry in the movability, i.e. the freedom of movement, of the wind turbine 1, in the sense that the movability of the wind turbine 1 from a neutral position depends on the direction of movement. Thus, the wind turbine 1 is allowed to move further away from the neutral position along a direction which is restricted by a long mooring line 7 than along a direction which is restricted by a shorter mooring line 7.

The upper part of Fig. 2 is a side view of a prior art wind turbine 1, and the lower part of Fig. 2 is a side view of a wind turbine 1 according to an embodiment of the invention. The wind turbines 1 illustrated in Fig. 2 could, e.g., be identical to the wind turbine 1 of Fig. 1.

The wind direction is indicated by arrow 8. The wind acts on the rotor 5, 6 of the wind turbine 1 and thereby pushes the wind turbine 1 in the same direction 8. The wind turbine 1 is allowed to move until one of the mooring lines 7a is fully stretched or extended, and when this occurs the wind turbine 1 stops moving and stays in this position. In Fig. 2 the wind turbines 1 have reached this position.

In the prior art wind turbine 1, all of the mooring lines 7a, 7b, 7c have identical lengths. In the wind turbine 1 according to the invention, mooring lines 7b and 7c have identical lengths, and the same length as the mooring lines 7a, 7b, 7c of the prior art wind turbine 1. However, mooring line 7a is longer than mooring lines 7b and 7c, thereby allowing the wind turbine 1 to move further along a direction which is restricted by mooring line 7a. It is noted that the length of each of the respective mooring lines 7a, 7b, 7c is fixed, i.e. the lengths are not adjustable, e.g. during operation of the wind turbine 1.

Accordingly, it can be seen from Fig. 2 that the wind turbine 1 according to the invention has moved further along the direction 8 than the prior art wind turbine 1 before it was stopped by the mooring line 7a. This is also illustrated by a further slack introduced in the mooring lines 7b and 7c of the wind turbine 1 according to the invention.

Fig. 3 illustrates movability or freedom of movement for a prior art wind turbine and for a wind turbine according to an embodiment of the invention. The wind turbines may be the wind turbines illustrated in Figs. 1 and 2.

Contour 9 represents the boundaries for movements of the prior art wind turbine and contour 10 represents the boundaries for movements of the wind turbine according to the invention. The arrows 8 represent various wind directions, and the dots 11 represent the positions where the wind turbine is stopped, in the manner described above with reference to Fig. 2, when the wind direction is as illustrated by the arrow 8 ending at the respective dots 11.

The shown wind directions 8 are spaced apart with 10° between neighbouring arrows 8. It can be seen that for some wind direction intervals, a large change in the end position 11 of the wind turbine occurs when the wind direction 8 changes, whereas this change is significantly smaller for other wind direction intervals, notably at or near the 'corners' of the contour 10. In the prior art wind turbine, the mooring lines have identical lengths, whereas in the wind turbine according to an embodiment of the invention, one of the mooring lines is significantly longer than the other two mooring lines, as described above with reference to Fig. 2. The anchor points are arranged at identical positions for the prior art wind turbine and for the wind turbine according to an embodiment of the invention. The different lengths of the mooring lines of the wind turbine according to an embodiment of the invention allows the wind turbine to move significantly further along directions which are restricted by the longer mooring line, corresponding particularly to the wind directions 8 in the upper part of the contour 10.

It can be seen from Fig. 3 that the wind turbine according to an embodiment of the invention has a significantly higher movability or freedom of movement, in most directions, than the prior art wind turbine. This has an impact on how wind turbines position themselves relative to each other in an offshore wind farm at various wind directions 8. This will be described in further detail below with reference to Figs. 4 and 5.

Figs. 4 and 5 illustrate two wind turbines 1 arranged in an offshore wind farm at two different wind directions 8. The wind turbines 1 drawn with dashed lines represent prior art wind turbines 1, and the wind turbines 1 drawn with solid lines represent wind turbines according to an embodiment of the invention. The contours 9, 10 for each wind turbine, described above with reference to Fig. 3, are also shown. It can be seen that the contours 10 for the two wind turbines according to an embodiment of the invention are oriented differently. This is because the orientation of the long mooring line differs from one wind turbine 1 to the other. Thereby a direction defining maximum movability, i.e. maximum freedom of movement, for one wind turbine 1 also differs from a direction defining maximum movability for the other wind turbine 1.

In Fig. 4, the wind direction 8 is directly from the right in the Figure. Accordingly, the wind turbines 1 have been yawed to a position where their rotors face the incoming wind. Furthermore, the wind turbines 1 have been pushed to the respective end positions corresponding to this wind direction 8. It can be seen that for the prior art wind turbines 1 this causes one of the wind turbines 1 to be arranged almost directly behind the other prior art wind turbine 1. Accordingly, it must be expected that the downstream wind turbine 1 is highly affected by wake effects created by the upstream wind turbine 1. This significantly reduces the power production of the downstream wind turbine 1, and may also increase the loads on the downstream wind turbine 1.

On the other hand, for the wind turbines 1 according to an embodiment of the invention, the increased movability caused by the difference in lengths of the mooring lines, and the different orientations of the contours 10 for the two wind turbines 1, ensure that the downstream wind turbine 1 is moved out of the wake of the upstream wind turbine 1. Accordingly, the downstream wind turbine 1 is essentially unaffected by the wake effects created by the upstream wind turbine 1, and thereby the total power production of the offshore wind farm can be increased as compared to the prior art scenario.

Fig. 5 shows the wind turbines 1 of Fig. 4. However, in Fig. 5 the wind direction 8 is directly from the left in the Figure. However, it can be seen that essentially the effect is obtained as in the situation illustrated in Fig. 4, i.e. for the wind turbines 1 according to an embodiment of the invention, the downstream wind turbine 1 is moved out of the wake of the upstream wind turbine 1.

Fig. 6 illustrates movability, i.e. freedom of movement, for a prior art wind turbine 1 and a wind turbine 1 according to an alternative embodiment of the invention. The wind turbine 1 drawn with a dashed line represents a prior art wind turbine 1, and the wind turbine 1 drawn with a solid line represents a wind turbine 1 according to an embodiment of the invention. Contours 9, 10 representing movability for the wind turbines 1 are shown, similar to Figs. 4 and 5.

For the prior art wind turbine 1, three anchor points 12 are arranged symmetrically with respect to the wind turbine 1, i.e. the anchor points 12 are arranged on a circle, with a neutral position for the wind turbine 1 in the centre, and with approximately 120° between the anchor points 12. For the wind turbine 1 according to an embodiment of the invention, three anchor points 13 are arranged asymmetrically with respect to the wind turbine 1. More particularly, two of the anchor points 13 are arranged at positions corresponding to the positions of the prior art anchor points 12, whereas the third anchor point 13 is arranged with a longer distance to the neutral position of the wind turbine 1. Furthermore, the angular distance between this anchor point 13 and its neighbouring anchor points 13 differs from 120°.

The asymmetrical arrangement of the anchor points 13 adds to the asymmetry in the movability of the wind turbine 1, which was already caused by the difference in length of the mooring lines. It can also be seen that the asymmetrical arrangement of the anchor points 13 affects the shape of the contour 10 representing the movability of the wind turbine 1.

Fig. 7 is a flow chart illustrating a method for designing a layout for an offshore wind farm according to a first embodiment of the invention.

Information regarding site conditions 14 for the site of the offshore wind farm and layout constraints 15 are supplied to an optimisation algorithm 16. The information regarding site conditions 14 includes information regarding expected wave conditions, expected current conditions and expected wind conditions, including expected wind directions, for the site. The layout constraints 15 include total nominal power for the offshore wind farm, type of floating foundations for the wind turbines and the area and boundaries of the offshore wind farm.

Furthermore, a cost model 17 is supplied to the optimisation algorithm 16. The cost model 17 specifies the costs arising from lost power production due to wake among the wind turbines and costs arising from manufacturing of the wind turbines, the floating foundations, the mooring system and the power cables, as a function of variations in appropriate design parameters for the offshore wind farm.

A shape database 18 stores a plurality of pre-calculated floating wind turbine movement contours and/or position patterns for wind turbines within offshore wind farms. The pre-calculated movement contours and/or position patterns have been computed based on arbitrary, but realistic, conditions with respect to wind, wave, current, mooring line lengths and anchor positions, etc.

Based on the information regarding site conditions 14, the layout constraints 15 and the cost model 17, the optimisation algorithm 16 seeks to create an optimal layout for the offshore wind farm, with Levelized Cost of Energy (LCoE) as optimisation target. Accordingly, available optimisation variables are adjusted in order to minimise the LCoE of an offshore wind farm established in accordance with the created layout. One of the available optimisation variables is the lengths of the mooring lines of the wind turbines.

To this end, the optimisation algorithm 16 consults the shape database 18 in order to identify a layout which to the greatest possible extent matches the site conditions 14 and the layout constraints 15, and which, in accordance with the cost model, minimises the LCoE.

Finally, the optimisation algorithm 16 outputs an optimal layout 19 for the offshore wind farm. The layout 19 specifies at least the lengths of the mooring lines, the anchor positions, the types of floating foundations and the configuration of the power cables.

Fig. 8 is a flow chart illustrating a method for designing a layout for an offshore wind farm according to a second embodiment of the invention. The method illustrated in Fig. 8 is very similar to the method illustrated in Fig. 7, and it will therefore not be described in further detail here.

In the method illustrated in Fig. 8, instead of consulting a shape database, the optimisation algorithm 16 interacts with a shape generator 20 in order to create an optimised layout for the offshore wind farm, based on the site conditions 14, the layout constraints 15 and the cost model 17. The optimisation target is still LCoE, and the optimisation variables include length of the mooring lines, positions of the anchor points and type of floating foundation.

Claims

1. An offshore wind farm comprising a plurality of wind turbines (1), each wind turbine (1) comprising a floating foundation (2), a tower (3) mounted on the floating foundation (2), a nacelle (4) mounted rotatably on the tower (3), and a rotor (5, 6) mounted rotatably on the nacelle (4), each wind turbine (1) further comprising at least two mooring lines (7), each mooring line (7) interconnecting the floating foundation (2) of the wind turbine (1) and an anchor point (12, 13), wherein at least one of the wind turbines (1) comprises mooring lines (7) having at least two different lengths, thereby causing a movability of the wind turbine (1) from a neutral position to be varying as a function of direction of movement.

2. An offshore wind farm according to claim 1, wherein each of the at least two mooring lines (7) has a fixed length.

3. An offshore wind farm according to claim 1 or 2, wherein each wind turbine (1) comprises at least three mooring lines (7), and wherein at least one of the wind turbines (1) comprises at least one mooring line (7) having a first length and at least two mooring lines (7) having a second length, where the second length differs from the first length.

4. An offshore wind farm according to any of the preceding claims, wherein at least two of the wind turbines (1) each comprises mooring lines (7) having at least two different lengths, and wherein a direction defining a maximum movability from the neutral position for a first wind turbine (1) differs from a direction defining maximum movability from the neutral position for a second wind turbine (1).

5. An offshore wind farm according to any of the preceding claims, wherein at least one of the wind turbines (1) comprises at least one mooring line (7) having a length which is at least 5% longer than at least one of the other mooring lines (7) of the wind turbine (1).

6. An offshore wind farm according to any of the preceding claims, wherein the different lengths of the mooring lines (7) cause the movability of at least one of the wind turbines (1) from a neutral position to be variable as a function of direction of movement, between a minimum movability and a maximum movability, and wherein the maximum movability exceeds the minimum movability by a distance which corresponds to at least 5% of a diameter of the rotor (5, 6) of the wind turbine (1).

7. An offshore wind farm according to any of the preceding claims, wherein at least one of the wind turbines (1) is connected, via the mooring lines (7), to anchor points (13) which are arranged asymmetrically with respect to the wind turbine (1).

8. A wind turbine (1) for use in an offshore wind farm according to any of the preceding claims, the wind turbine (1) comprising a floating foundation (2), a tower (3) mounted on the floating foundation (2), a nacelle (4) mounted rotatably on the tower (3), and a rotor (5, 6) mounted rotatably on the nacelle (4), the wind turbine (1) further comprising at least two mooring lines (7), each mooring line (7) interconnecting the floating foundation (2) of the wind turbine (1) and an anchor point (12, 13), wherein at least one of the mooring lines (7) has a length which differs from a length of at least one of the other mooring line(s) (7), thereby causing a movability of the wind turbine (1) from a neutral position to be varying as a function of direction of movement.

9. A wind turbine (1) according to claim 8, wherein each of the at least two mooring lines (7) has a fixed length.

10. A method for designing a layout for an offshore wind farm, the offshore wind farm comprising a plurality of wind turbines (1), each wind turbine (1) comprising a floating foundation (2), a tower (3) mounted on the floating foundation (2), a nacelle (4) mounted rotatably on the tower (3), and a rotor (5, 6) mounted rotatably on the nacelle (4), each wind turbine (1) further comprising at least two mooring lines (7), each mooring line (7) interconnecting the floating foundation (2) of the wind turbine (1) and an anchor point (12, 13), the method comprising the steps of: obtaining information regarding site conditions (14) for a site of the offshore wind farm, including information regarding expected wind directions at the site,

- providing layout constraints (15) related to the offshore wind farm,

- supplying the information regarding site conditions (14) and the layout constraints (15) to an optimisation algorithm (16), the optimisation algorithm (16) applying length of the mooring lines (7) as an optimisation variable, and

- creating a layout (19) for the offshore wind farm, using the optimisation algorithm (16), the layout (19) specifying optimal lengths of the mooring lines (7) of each of the wind turbines (1).

11. A method according to claim 10, wherein the optimisation algorithm (16) applies fixed length of the mooring lines (7) as an optimisation variable, and wherein the layout specifies optimal fixed lengths of the mooring lines (7) of each of the wind turbines (1).

12. A method according to claim 10 or 11, wherein the optimisation algorithm (16) applies levelized cost of energy (LCoE) of the offshore wind farm as an optimisation target, and wherein the created layout (19) for the offshore wind farm specifies lengths of the mooring lines (7) of each of the wind turbines (1) which results in a minimised LCoE.

13. A method according to any of claims 10-12, wherein the optimisation algorithm (16) applies loads on the wind turbines (1) and/or energy production of the offshore wind farm and/or wake effects among the wind turbines (1) as an optimisation target.

14. A method according to any of claims 10-13, wherein the optimisation algorithm (16) further applies positions of anchor points (12, 13) for the wind turbines (1) as an optimisation variable.

15. A method according to any of claim 10-14, wherein the layout constraints (15) related to the offshore wind farm include a number of wind turbines (1) and/or one or more types of the wind turbines (1) to be arranged in the offshore wind farm.

16. A method according to any of claims 10-15, further comprising the step of supplying an initial layout for the offshore wind farm to the optimisation algorithm (16), and wherein the step of creating a layout (19) for the offshore wind farm, using the optimisation algorithm (16), comprises applying the initial layout as a starting point for the optimisation.

17. A method according to any of claims 10-16, further comprising the step of erecting an offshore wind farm in accordance with the created layout (19).