Support member for a rotor and nacelle assembly

Abstract

A support member for a rotor and nacelle assembly with walls comprising a first portion comprising a first material and a second portion comprising a second material, wherein the first and second portions are connected together; wherein the first portion is located above the second portion; and wherein the first portion has an apparent bending stiffness that is higher than the apparent bending stiffness of the second portion. A wind turbine comprising the support member. A method for manufacturing the support member.

Description

[0001] Support member for a rotor and nacelle assembly

[0002] Field of the invention

[0003] The present invention relates to a support member for a rotor and nacelle assembly, and a wind turbine.

[0004] Background

[0005] Modern wind turbines are usually designed with 3 rotor blades. Every time one of the rotor blade passes in front of the wind turbine tower there will be a slight change in loading on the blade due to a normally slightly reduced wind speed in front of the tower. In other words, the tower is interfering with the free wind. It is also normal, especially for large wind turbines with large rotors such as offshore wind turbines, that the wind speed is different at different positions of the rotor swept area for shorter or longer periods of time. This could for instance be caused by slow changing turbulence effects or surface boundary layer friction effects due to ocean waves, causing the wind speed to be reduced closer to the surface of the sea.

[0006] The effect of the above is that each blade will typically experience a repeating change of wind loads for each revolution of the rotor. This will result in an impulse loading (that can be either a reduction in load or an increase in load) from the blade which is transferred via the rotor hub and further through the nacelle, tower and ultimately into the wind turbine foundation and results in fatigue in different elements of the wind turbine.

[0007] Because there are normally 3 blades in the rotor, this impulse load will “happen” 3 times for every revolution of the rotor. The frequency of this impulse load phenomena is therefore 3 times higher than the rotor frequency and often referred to as the 3 per revolution frequency, or just 3P frequency (given in hertz, or Hz).

[0008] There could also be an unbalance in the rotor caused by different mass distribution in each of the blades or that the blades have different twist or pitch angles resulting in different aerodynamic loading on each individual blade. This will also cause load variations resulting in one per revolution (IP), two per revolution (2P) or three per revolution (3P) load impulses.

[0009] The wind turbine tower will have a natural frequency (also known as eigenfrequency) in bending. Fore-aft or side-to-side motion of the nacelle may cause the tower and as such, the wind turbine to sway (bend) back and forth with its natural frequency. This bending natural frequency will be decided by the tower stiffness and size, and influenced by the stiffness of the foundation to which the tower is mounted on, the mass of the tower and the effected parts of the foundation as well as the mass on top of the tower, i.e. the rotor and nacelle assembly (RNA). This frequency will be referred to as the wind turbine combined natural bending frequency.

[0010] If the wind turbine combined natural bending frequency is close to the 3P blade loading frequency as described above, the tower will experience large structural vibrations which will cause extreme fatigue loading on both the tower and the foundation structure. This may also be the case if the combined natural bending frequency is close to the IP blade loading frequency, or any multiples of IP and 3P frequencies.

[0011] US9651029B2 relates to a self-supporting wind turbine tower with walls comprising an upper portion formed from a composite plastic with a high modulus of elasticity like carbon reinforced epoxy, the upper portion being subdivided into a plurality of segments arranged in a hoop direction of the tower; and a separate, lower portion mounted on a foundation, the upper portion mounted atop the lower portion so as to form the tower, the lower portion formed from a mild steel, wherein the self- supporting tower comprises a reduced weight and an increased natural frequency as compared to a tower of an equivalent size constructed entirely of steel. In this way the natural frequency of the tower may be designed to be higher than the 3P frequency and hence reduce the problem of natural frequency vibrations. However, there will always be some vibrations occurring at the natural frequency and increasing the natural frequency of the tower will increase the number of fatigue cycles and hence increase the fatigue on the tower and reduce the lifespan of the tower and wind turbine. Also, using carbon reinforced epoxy to increase the tower stiffness is expensive.

[0012] In addition, for offshore wind turbines in particular, there is a problem of corrosion of the structural elements at or near the splash zone if made from steel due to the combination of salt water and air.

[0013] Further, for floating wind turbines in particular, it is desired to make the wind turbine tower and foundation as light weight as possible, especially for semisubmersible designs where the tower is placed on top of one of the columns.

[0014] As such, the prior art does not provide a satisfying and cost effective solution for corrosion resistance in the splash zone, reduced weight of the tower and foundation as well as preventing fatigue and ultimately rupture of the wind turbine. The present invention relates to a novel support structure (foundation and tower) for wind turbines and the resulting wind turbines and aims to reduce or resolve the disadvantages of the prior art. SUMMARY OF THE INVENTION

[0015] The present invention is defined by the appended claims and in the following:

[0016] In a first aspect, the invention relates to a support member for a rotor and nacelle assembly with walls comprising: a first portion comprising a first material; and a second portion comprising a second material, wherein the first and second portions are connected together; wherein the first portion is located above the second portion; and wherein the first portion has a bending stiffness that is higher than the bending stiffness of the second portion.

[0017] In an embodiment, the first and second portions are connected together to form the support member.

[0018] In an embodiment, the first portion is formed at least of 50%, 60%, 70%, 80%, 90%, 95%, or 99% from the first material.

[0019] In an embodiment, the second portion is formed at least of 50%, 60%, 70%, 80%, 90%, 95%, or 99% from the second material.

[0020] In an embodiment the first portion is directly above of the second portion.

[0021] In an embodiment, the first material may have a modulus of elasticity that is at least 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, 200% or 500% higher than the modulus of elasticity of the second material

[0022] In an embodiment, the second portion may comprise from 5% to 90%, from 5% to 80%, from 5% to 70%, from 5% to 60% from 5% to 50%, from 5% to 40%, from 5% to 30%, or from 10% to 20% of the length of the support member.

[0023] In an embodiment, the second portion may be a unitary component.

[0024] In an embodiment, the second portion may be subdivided into a plurality of segments.

[0025] In an embodiment, the plurality of segments may comprise at least a segment with a first material and at least a segment with a second material, wherein the at least two materials have different moduli of elasticity. In an embodiment, the plurality of segments of the second portion may be arranged in an axial direction of the support member.

[0026] In an embodiment, the support member may further comprise a gasket positioned between the first and second portions.

[0027] In an embodiment, the support member may further comprise a gasket positioned between a second and a third portions.

[0028] In an embodiment, the first portion may be hollow.

[0029] In an embodiment, the second portion may be hollow.

[0030] In an embodiment, the second portion may be a cylinder or a tapered cylinder

[0031] In an embodiment, the first portion may be substantially formed from steel.

[0032] In an embodiment, at least a part of the second portion may comprise composite plastic.

[0033] In an embodiment, at least a part of the second portion may be substantially formed from composite plastic.

[0034] In an embodiment, the composite plastic may comprise a plastic resin such as epoxy, vinylester or polyester.

[0035] In an embodiment, the composite plastic may be a fiber reinforced plastic material. This may be understood as a composite of fibers embedded in a matrix material. The fibers may be of any suitable type, such as glass fibers, polyamid fibers, and others. The matrix material may comprise any suitable plastic material, for instance thermosetting polymers, such as epoxy or vinylester, or a thermoplastic material, such as polyester, PPS, PEEK, PEKK, PEI, PAEK or other.

[0036] In an embodiment, the second portion may have a wall thickness which varies along the length of the second portion.

[0037] In an embodiment, the first portion may have a modulus of elasticity of at least 190 GPA.

[0038] In an embodiment, at least a part of the second portion may have a modulus of elasticity of less than 190 GPA, less than 180 GPA, less than 170 GPA, less than 150 GPA, less than 125 GPA, less than 100 GPA, less than 75 GPA, or less than 50 GPA.

[0039] In an embodiment, the support member may comprise at least a part of a wind turbine tower. In an embodiment, the support member may comprise a wind turbine tower and at least a part of a foundation.

[0040] In an embodiment, the foundation may be a monopile, suction case, multipod, jacket or floating substructure.

[0041] In an embodiment, the monopile, suction case, multipod or jacket may be bottom fixed and at least partly embedded into the seabed.

[0042] In an embodiment, the wind turbine tower may be connected to the foundation using a pipe-in-pipe solution.

[0043] In an embodiment, the second portion is bolted, grouted or molded to the first portion.

[0044] In an embodiment, the support member further comprises a third portion below the second portion and the second portion is bolted, grouted or molded to the third portion.

[0045] In an embodiment, the second portion is bolted, grouted or molded to the third portion.

[0046] In an embodiment, the second portion is bolted grouted or molded to the third portion at least partly below a surface of water.

[0047] In an embodiment, the bolting, grouting or molding is performed at least partly below a surface of water.

[0048] In an embodiment, an epoxy, vinyl ester or polyester resin is used as a grouting material.

[0049] In an embodiment, the third portion may comprise at least part of the foundation.

[0050] In an embodiment, the second portion comprises metal inserts molded into the material of the second portion. These metal inserts allow for the transfer of tension and compression forces between the bolts and said material.

[0051] In a second aspect, the invention relates to a wind turbine comprising a support member according to the first aspect of the invention and a rotor and nacelle assembly. The rotor and nacelle assembly is placed above the support member, in other words the rotor and nacelle assembly will be placed on top of the first portion of the support member.

[0052] In an embodiment of the second aspect, the support member comprises a tuned damping arrangement arranged above said second portion on an outside or an inside of said support member or on an outside or an inside of a wind turbine rotor and nacelle assembly arranged on top of the support member. In an embodiment of the second aspect, the combined natural frequency of the first bending mode shape of the support member is at least 5% lower than the 3P frequency of the wind turbine.

[0053] The combined natural frequency of the first bending mode shape of the support member is the structural frequency of the horizontal movement of the top of the support member in the frame of reference of the foundation of the wind turbine. In other words, the combined natural frequency of the first bending mode shape of the support member is the structural frequency of the horizontal movement of the top of the support member, disregarding the rigid body motions of the wind turbine, due to the movement of the foundation in case of a floating wind turbine.

[0054] In an embodiment of the second aspect, the combined natural frequency of the first bending mode shape of the support member is at least 10%, 15%, 20%, 25%, 50% or 75% lower than the 3P frequency of the wind turbine.

[0055] In a third aspect, the invention relates to a method for manufacturing a support member for a rotor and nacelle assembly, the method comprising the steps of providing a first portion of the support member, the first portion comprising a first material, providing a second portion of the support member, the second portion comprising a second material, and connecting the first portion and the second portion to form the support member, wherein the first portion has a bending stiffness that is higher than the bending stiffness of the second portion, and wherein the length of at least the second portion is selected to obtain a desired length of the support member and to determine the combined natural bending and / or torsion mode frequencies of the support member.

[0056] In an embodiment of the third aspect, a 1st combined natural bending mode frequency of the support member is determined by determining the length of the second portion.

[0057] In an embodiment of the third aspect, the length of the second portion is determined such that the 1st combined natural bending mode frequency of the support member is lower than a one per revolution (IP) frequency of a wind turbine rotor at rated wind speed.

[0058] It should be understood that the rated wind speed is the wind speed at which the wind turbine is substantially reaching its full power, i.e it’s rated power, normally around 11-15 m / s.

[0059] In an embodiment of the third aspect, the length of the second portion is determined such that the 1st combined natural bending mode frequency of the support member is below an annual average Tp wave frequency. The Tp wave frequency is here defined as the peak frequency of the wave energy spectrum.

[0060] In an embodiment of the third aspect, the length of the second portion is determined such that the 1st combined natural bending mode frequency of the support member is below a 50-year return period maximum expected Tp wave frequency.

[0061] It should be understood that the 50-year return period maximum expected Tp wave frequency is here defined as the maximum expected peak frequency of the wave energy spectrum during a storm containing the maximum expected wave height over a 50-year period.

[0062] In an embodiment of the third aspect, the length of the second portion is determined such that at least a 3rd or 4th combined natural bending mode frequency of the support member is below an annual average Tp wave frequency. It should be understood that both the first and second bending modes have their maximum deflection at the free end of the support assembly, i.e opposite the foundation end, while the 3rdand 4thbending modes have their maximum deflections below the free end.

[0063] In an embodiment of the third aspect, the method may comprise determining a number of diagonal lay-ups of composite fibers relative to a number of axial lay-ups of composite fibers in the second portion, thereby determining a torsional stiffness of the second portion.

[0064] In an embodiment of the third aspect the torsional stiffness of the second portion may be determined such that at least a 1st combined natural torsion mode frequency of the support member is arranged between a three per revolution (3P) frequency and a six per revolution (6P) frequency of a wind turbine rotor at rated wind speed.

[0065] In an embodiment of the third aspect the bending stiffness and / or the torsional stiffness of said second portion (3) may be adjusted after the support member has been installed at a final location by laminating additional layers of composite material on an outside or an inside of the second portion.

[0066] In an embodiment of the third aspect the method may comprise the step of arranging a tuned damping arrangement above said second portion on an outside or an inside of said support member, or, on an outside or an inside of a wind turbine rotor and nacelle assembly arranged on top of the support member.

[0067] In an embodiment, the invention relates to a support member for a rotor and nacelle assembly with walls comprising: a first portion comprising a first material; and a second portion comprising a second material, where in the first and second portions are connected together to form the support member; wherein the first portion is located above the second portion; and wherein the first material has a modulus of elasticity that is higher than the modulus of elasticity of the second material.

[0068] In an embodiment, the invention relates to a support member for a rotor and nacelle assembly with walls comprising: a first portion comprising a first material; and a second portion comprising a second material, where in the first and second portions are connected together to form the support member; wherein the first portion is located above the second portion; and wherein the first portion has an apparent modulus of elasticity that is higher than the modulus of elasticity of the second portion.

[0069] Apparent modulus of elasticity or apparent bending stiffness will here have the meaning of the effective combined modulus of elasticity or bending stiffness when several materials are mixed or working together, such as for example in a glassfiber reinforced plastic.

[0070] Short description of the drawings

[0071] In the following description this invention will be further explained by way of exemplary embodiments shown in the drawings:

[0072] Fig. l is a front view of a first embodiment of the support member

[0073] Fig. 2 is a front view of a second embodiment of the support member

[0074] Fig. 3 is a perspective view of a wind turbine

[0075] Fig. 4 is a front view of the third embodiment of the support member

[0076] Fig. 5 is a cross section view of a detail of the third embodiment of the support member

[0077] Fig. 6 is a front view of a fourth embodiment of the support member

[0078] Fig. 7 is a front view of a fifth embodiment of the support member Fig. 8 is a front view of a sixth embodiment of the support member

[0079] Fig. 9 is a front view of a seventh embodiment of the support member

[0080] Fig. 10 is a side view of a 15 Megawatt (MW) wind turbine comprising a tower with a second portion of composite material mounted on a third portion comprising a monopile (100)

[0081] Fig. 11 shows the 1st, 2nd, 3rdand 4thbending mode frequencies for the combined system illustrated in fig. 10 related to the axes defined in fig. 10.

[0082] Fig. 12 shows a wind turbine tower 1stbending mode shape related to the axes defined in fig. 10.

[0083] Fig. 13 shows a wind turbine tower 2ndbending mode shape related to the axes defined in fig. 10. The 2ndmode shape is similar to the first mode shape but differs in the azimuth direction.

[0084] Fig. 14 shows a wind turbine tower 3rdbending mode shape related to the axes defined in fig. 10.

[0085] Fig. 15 shows a wind turbine tower 4thbending mode shape related to the axes defined in fig. 10.

[0086] Detailed description of the invention

[0087] Wind turbines have a natural frequency (also known as eigenfrequency) in bending. Fore-aft or side-to-side motion of the nacelle may cause the tower and as such, the wind turbine to sway (bend) back and forth with its natural frequency. This tower bending natural frequency will be decided by the tower stiffness and size, and influenced by the stiffness of the foundation to which the tower is mounted on, the mass of the tower and the effected parts of the foundation, the mass on top of the tower, i.e. the rotor and nacelle assembly (RNA), and, in case of a bottom fixed foundation, seabed interaction with the foundation. This frequency will generally be referred to as the wind turbine combined natural bending frequency, or the wind turbine combined natural bending mode frequency. Hereinafter, this frequency may also be referred to as the support member combined natural bending frequency, or the support member combined natural bending mode frequency.

[0088] The effect of the above is that each blade will typically experience a repeating change of wind loads for each revolution of the rotor. This will cause in an impulse loading (that can be either a reduction in load or an increase in load) from the blade which is transferred via the rotor hub and further through the nacelle, tower and ultimately into the wind turbine foundation and results in fatigue in different elements of the wind turbine. For each revolution of the rotor a particular blade will pass a given point 1 time per revolution. The corresponding blade passing frequency is denoted IP (one per revolution).

[0089] Because there are 3 blades in the rotor, this impulse load will “happen” 3 times for every revolution of the rotor. The frequency of this impulse load phenomena is therefore 3 times higher than the rotor frequency and often referred to as the 3 per revolution frequency, or just 3P frequency (given in hertz, or Hz), or independently of the number of blades, the blade tower passing frequency.

[0090] The person skilled in the art will understand that a wind turbine can have any number of blades and for a 2 bladed wind turbine the corresponding blade tower passing frequency would be 2P frequency.

[0091] A wind turbine rotor is normally designed to spin with a substantially constant operational angular speed (co rated), expressed in radians per second, when the rated wind speed is reached. However, some small variations in the rotor angular speed always, typically a few percent, are to be expected due to rapid variations in the wind speed. The rated wind speed is the wind speed at which the rated (maximum) output power is reached. The rotor angular speed can also be expressed as a frequency, namely the rotor frequency, in rotations per minute (RPM) or rotations per second (Hz).

[0092] If the wind turbine combined natural bending frequency is close to the IP or 3P blade loading frequency as described above, the tower will experience large structural vibrations which will cause extreme fatigue loading on both the tower and the foundation structure.

[0093] Therefore, the 3P blade frequencies and the tower combined natural frequency always will have to be designed with a separation, typically 5-15% separation is regarded as sufficient.

[0094] For large wind turbines, especially offshore, the tower frequencies are normally sought to be designed to be lower than the 3P blade frequency. I.e. the tower must then be “soft enough” to be vibrating slower than the 3P frequency. This type of design is often referred to as “soft-stiff’ design. The alternative is to make the tower stiffer and make sure that the tower vibrates with a higher frequency than the 3P frequency. This is referred to as a “stiff-stiff’ tower design. However, a stiff-stiff tower design has the disadvantage that the tower will then have to be made with thicker steel plates, or alternatively using materials with higher stiffness than steel such as carbon reinforced epoxy laminates, resulting in a considerably heavier tower, for example if only steel is used or a more costly tower, for example if carbon composite is used.

[0095] For very large offshore wind turbines it has proven difficult to make the tower soft enough to maintain the favourable “soft-stiff’ tower design. Since the natural frequency of the structure is scaling with the square root of the structural bending stiffness the tower will have to be made considerably heavier in order to “jump” from a “soft-stiff’ to a “stiff-stiff’ design. It is therefore a desire trying to stay on the “soft-stiff’ region of the tower design.

[0096] For bottom fixed wind turbines on monopiles the natural frequencies of the towers are lower than for floating foundations. In these cases, it may be a desire to even reduce the tower natural frequency to below the IP frequency.

[0097] The inventive support member and wind turbine solve the problem of separating the wind turbine combined natural frequency and the 3P or IP blade frequency by introducing a softer section in the second portion of the tower and / or in the foundation structure below the tower. This softer section can be made for instance of glass fiber reinforced epoxy which has a considerably lower bending stiffness than steel for the same strength and also has a considerable lower cost than carbon reinforced epoxy.

[0098] By tuning the length of the soft section the combined natural frequency can be tailored to be soft enough to suite any needs for staying in the “soft-stiff’ region, or even the “soft-soft” region.

[0099] The bending stiffness of a cross section of any kind is dependent on both the material’s modulus of elasticity (Young’s modulus) as well as the second moment of area of the cross section in question, The bending stiffness (BS) of a hollow circular pipe, i.e. of a typical wind turbine tower, can be written as;

[0100] BS = E*I, where E is the modulus of elasticity (Young’s modulus) of the material and I is the second moment of area of the cross section,

[0101] For a pipe 1= (?r / 4)*(Ro4-Ri4), where;

[0102] Ro = outer diameter and Ri = inner diameter of the pipe.

[0103] The natural frequency of a vertical cantilevered beam rigidly connected to the ground in one end and free in the other is given as; f=kn*[sqrt(E*I / (m*L4)] where, kn is a number associated with each of the natural mode shapes, m is the mass per unit length and L is the beam length. A mode shape is the shape of the deflected structural member at its maximum deflection during vibration at one of its natural frequencies. The first mode is here defined as the mode with the lowest natural frequency, the second mode is the mode with the second lowest natural frequency etc. Mode shapes of a pipe (or wind turbine tower with a wind turbine placed on top) can vary slightly based on the mass and stiffness distribution. Fig. 10 illustrates a wind turbine comprising a tower with a second portion 3 of composite material mounted on a third portion 100 comprising a monopile. The 1st, 2nd, 3rdand 4thbending mode frequencies for the combined system in fig. 10 are illustrated in fig. 11, and related to the axes defined in fig. 10. The different bending mode shapes of the wind turbine in fig. 10 are illustrated in figs. 12 to 15.

[0104] As can be seen from this formula, reducing the bending stiffness (BS) will reduce the natural frequency of the beam. Moreover, the natural frequency is proportional to the square root of the bending stiffness El. A wind turbine placed on top of a tower will show a similar behaviour. Reducing the bending stiffness of the tower or the foundation will reduce the bending natural frequency of the tower, which may be desirable if a lower combined natural bending frequency is desired in order to avoid coinciding with the load impulse frequencies from the wind turbine rotor or the wave frequencies which act on the tower or the foundation.

[0105] In order to lower the bending stiffness, it is therefore possible to either lower the modulus of elasticity, by choosing a less stiff material, or lower the second moment area, or both. However, if the second moment of area is reduced, for instance by reducing the wall thickness of the pipe, the bending stresses will increase, and the pipe will possibly not be able to withstand the loads.

[0106] Bending stress is given by; Sigma=M*I / y , where;

[0107] Sigma is bending stress, M is bending moment and y is the distance from the structural neutral axis to the stress point in question, for a pipe equal to the pipe radius.

[0108] If for instance attempting using concrete to reduce the bending stiffness this would not work even though the modulus of elasticity is lower for concrete than steel. This is because a concrete pipe will need a considerably larger wall thickness than steel to sustain the same loads. In addition, concrete does not resist tension well and needs to be reinforced with steel. Therefore, although the modulus of elasticity of concrete is less than steel, the second moment of area will typically be considerably larger for concrete than for steel, and steel reinforcement needs to be used inside the concrete as well, and hence the bending stiffness (El) is still larger for a concrete pipe than a steel pipe of equal structural strength. Therefore, when increased bending stiffness is desired for wind turbine towers a concrete solution is preferred, and not preferred when less bending stiffness is desired.

[0109] Similarly as for the bending stiffness, the torsional stiffness will influence the torsion mode frequencies of a tower and foundation. The polar second moment of area of a hollow pipe is given as;

[0110] K=l / 27t(Ro4-Ri4)

[0111] The torsional 1st mode natural frequency of a hollow pipe with an end point mass (for instance a rotor and nacelle assembly of a wind turbine) can be estimated by; fl = l / 27t (GK / (J+Js / 3)*L)

[0112] Where G is the shear modulus, J is the mass moments of inertia of the point mass and Js is the mass moment of inertia of the pipe and L is the length of the pipe (or tower in a wind turbine installation).

[0113] Similarly as for the bending frequencies explained above it can be seen that reducing the torsional stiffness GK will reduce the natural torsional frequency of the pipe. Moreover, the natural frequency is proportional to the square root of the torsional stiffness GK.

[0114] The shear modulus can be tailored in a composite structure by choosing the amount of 45 degrees layers versus the 0 degrees layers (axial direction) during the lay-up of the reinforcement fibers before laminating with epoxy or other resin types.

[0115] In a first embodiment shown on Fig. 1, a support member 5 of a wind turbine is illustrated. The support member 5 comprises a tower 6 and a foundation 4. The support member comprises a first portion 2 and a second portion 3. The wind turbine further comprises a rotor and nacelle assembly (see Fig. 3), placed on top of the first portion 2 of the support member 5. In this embodiment, the first and second portions 2,3 are connected together, for example by bolts.

[0116] In this first embodiment the first portion 2 of the support member 5 is a hollow cylinder substantially made of steel. The second portion 3 of the support member 5 is a hollow cylinder substantially formed from a glass fiber reinforced epoxy which has a considerably lower bending stiffness than steel for the same strength. The foundation 4 is substantially made of steel.

[0117] Steel has a modulus of elasticity that is higher than the modulus of elasticity of the glass fiber reinforced epoxy. Since the second portion 3 of the support member 5 has a lower modulus of elasticity than the first portion 2 of the support member 5, by tuning the length of the first and second portions 2,3, the support member and the wind turbine combined natural frequency can be tailored to suite any needs, especially to be lower than the blade frequency of the wind turbine.

[0118] It should be understood that increasing the length of the second portion will result in a softer tower with a lower combined natural bending frequency, even if the total height of the wind turbine tower and foundation is kept unchanged. I.e. as long as the second portion has a lower bending stiffness than the first portion above it, the length of the second portion can be used to reduce, and therefore tune, the combined natural bending frequency of the tower and foundation to a lower frequency.

[0119] Fig.3 illustrate a wind turbine 1 comprising a floating foundation 4, a tower 6 and a rotor and nacelle assembly 7.

[0120] In a second embodiment shown on Fig. 2, a support member 5 of a wind turbine is illustrated. The support member 5 comprises a tower 6 and a foundation 4. The support member comprises a first portion 2 and a second portion 3. The wind turbine further comprises a rotor and nacelle assembly (see Fig. 3), placed on top of the first portion 2 of the support member 5. In this embodiment, the first and second portions 2,3 are connected together, for example by bolts.

[0121] In this embodiment, the tower 6 has a pipe-in-pipe connection to the foundation 4.

[0122] In this second embodiment the first portion 2 of the support member 5 is a hollow cylinder substantially made of steel, and the second portion 3 of the support member 5 is a hollow cylinder substantially formed from a glass fiber reinforced vinyl ester which has a considerably lower bending stiffness than steel for the same strength. Steel has a modulus of elasticity that is higher than the modulus of elasticity of the glass fiber reinforced vinyl ester. The foundation 4 is substantially made of steel.

[0123] Since the second portion 3 of the support member 5 has a lower modulus of elasticity than the first portion 3 of the support member 5, by tuning the length of the first and second portions 2,3, the natural stiffness the support member and the wind turbine combined natural frequency can be tailored to suite any needs, especially to be lower than the blade frequency of the wind turbine

[0124] In a third embodiment shown on Fig. 4 and 5, a support member 5 of a wind turbine is illustrated. The support member 5 comprises a tower 6 and a foundation 4. The support member comprises a first portion 2 and a second portion 3. The wind turbine further comprises a rotor and nacelle assembly (see Fig. 3), placed on top of the first portion 2 of the support member 5. In this embodiment, the first and second portions 2,3 are connected together, for example by bolts. In this embodiment, the foundation 4 is a floating structure.

[0125] Figure 5 illustrates the cross section of A in figure 4, and shows that in this third embodiment the first portion 2 of the support member 5 is a hollow cylinder substantially having a wall 12 made of steel and the second portion 3 of the support member 5 is a hollow cylinder having a wall 13 comprising inner and outer layers 14,15 made of glass fiber reinforced epoxy and a core 16, made of a material such as balsa, encased in outer 15 and inner 14 layers made of glass fiber reinforced epoxy. This wall 13 has a considerably lower bending stiffness than steel for the same strength. Steel has a modulus of elasticity that is higher than the modulus of elasticity of the glass fiber reinforced epoxy.

[0126] The foundation 4 is substantially made of steel.

[0127] The wall 12 of the first portions 2, is welded to a gusset plate 22, at a weld 18. The gusset plate 22 being also welded to a deck bracing 21.

[0128] The gusset plate 22 is bolted to the second portion by bolts 23 into inserts 17, encased to the outer and inner layers 15,14.

[0129] Since the second portion 3 of the support member 5 has a lower modulus of elasticity than the first portion 2 of the support member 5, by tuning the length of the first and second portions 2,3, the natural stiffness the support member and the wind turbine combined natural frequency can be tailored to suite any needs, especially to be lower than the blade frequency of the wind turbine

[0130] In a fourth embodiment, illustrated in figure 6, the first portion 2 of the support member 5 is a hollow cylinder substantially made of steel, and the second portion 3 of the support member 5 is a hollow cylinder substantially formed from a glass fiber reinforced epoxy which has a considerably lower bending stiffness than steel for the same strength. The foundation 4 is also substantially made of glass fiber reinforced epoxy. Steel has a modulus of elasticity that is higher than the modulus of elasticity of the glass fiber reinforced epoxy.

[0131] The first and second portions 2,3 and the foundation 4 are connected together to form the support member 5.

[0132] Since the second portion 3 of the support member 5 has a lower modulus of elasticity than the first portion 3 of the support member 5, by tuning the length of the first and second portions 2,3, the natural stiffness the support member and the wind turbine combined natural frequency can be tailored to suite any needs, especially to be lower than the blade frequency of the wind turbine In a fifth embodiment, illustrated in figure 7, the first portion 2 of the support member 5 is a hollow cylinder substantially made of steel, and the second portion 3 of the support member 5 is a hollow cylinder substantially formed from a glass fiber reinforced epoxy which has a considerably lower bending stiffness than steel for the same strength. The first and second portions 2,3 are connected together to form the support member 5.

[0133] In this embodiment, the wind turbine tower is the first portion 2 of the support member 5 and the foundation 4 is the second portion 3 of the support member 5. Steel has a modulus of elasticity that is higher than the modulus of elasticity of the glass fiber reinforced epoxy.

[0134] Since the second portion 3 of the support member 5 has a lower modulus of elasticity than the first portion 3 of the support member 5, by tuning the length of the first and second portions 2,3, the natural stiffness the support member and the wind turbine combined natural frequency can be tailored to suite any needs, especially to be lower than the blade frequency of the wind turbine

[0135] In a sixth embodiment, illustrated in figure 8, the first portion 2 of the support member 5 is a hollow cylinder substantially made of steel, and the second portion 3 of the support member 5 is a hollow cylinder substantially formed from a glass fiber reinforced epoxy which has a considerably lower bending stiffness than steel for the same strength. The upper part of the foundation 4 is part of the second portion of the support member, and as such also substantially made of glass fiber reinforced epoxy. Steel has a modulus of elasticity that is higher than the modulus of elasticity of the glass fiber reinforced epoxy.

[0136] The first and second portions 2,3 and the foundation 4 are connected together to form the support member 5.

[0137] Since the second portion 3 of the support member 5 has a lower modulus of elasticity than the first portion 3 of the support member 5, by tuning the length of the first and second portions 2,3, the natural stiffness the support member and the wind turbine combined natural frequency can be tailored to suite any needs, especially to be lower than the blade frequency of the wind turbine

[0138] In a seventh embodiment, illustrated in figure 9, the first portion 2 of the support member 5 is a hollow cylinder substantially made of steel, and the second portion 3 of the support member 5, here the lower part of the foundation 4 is a hollow cylinder substantially formed from a glass fiber reinforced vinyl ester which has a considerably lower bending stiffness than steel for the same strength. The upper part of the foundation 4 is part of the first portion 2 of the support member, and as such also substantially made of steel. Steel has a modulus of elasticity that is higher than the modulus of elasticity of the glass fiber reinforced vinyl ester.

[0139] The first and second portions 2,3 and the foundation 4 are connected together to form the support member 5.

[0140] Since the second portion 3 of the support member 5 has a lower modulus of elasticity than the first portion 3 of the support member 5, by tuning the length of the first and second portions 2,3, the natural stiffness the support member and the wind turbine combined natural frequency can be tailored to suite any needs, especially to be lower than the blade frequency of the wind turbine

[0141] The inventive support member and wind turbine illustrated throughout the different embodiments allow for the design of a support member and wind turbine having a reduced wind turbine combined natural frequency, especially lower than the 3P blade frequency of the wind turbine, which will in turn reduce the fatigue of the support member or wind turbine over time, and as such increase the lifespan of the support member or wind turbine.

[0142] The foundation may comprise a second portion bolted or grouted or molded to a third portion wherein at least one bolted or grouted or molded connection is arranged below the surface of the water by installing a seal between a monopile (third portion) and the composite section (second portion) at the lower part of an overlapping interface between them, and further injecting an epoxy resin in the void in said interface. Because the epoxy has higher density than water, the water will be displaced upwards as the resin fills up the void between the monopile and the composite section, hence a glued or grouted section can be made under water.

[0143] Even though the load frequencies and the combined natural bending mode frequencies can be separated most of the time with the proposed invention, it may still be possible that some loads frequencies can still be present during rare events. In this case a tuned damper can be placed at the upper part of the tower to reduce possible vibrations at any natural frequencies.

Claims

CLAIMS1. A support member (5) for a rotor and nacelle assembly with walls comprising: a first portion (2) comprising a first material; and a second portion (3) comprising a second material, wherein the first and second portions (2,3) are connected together; wherein the first portion (2) is located above the second portion (3); and wherein the first portion (2) has an apparent bending stiffness that is higher than the apparent bending stiffness of the second portion (3).

2. A support member according to claim 1, wherein the second portion (3) comprises from 5% to 50% of the length of the support member (5).

3. A support member (5) according to any one of the previous claims, wherein the second portion (3) is a unitary component.

4. A support member (5) according to any one of the previous claims, wherein the second portion (3) is subdivided into a plurality of segments.

5. A support member (5) according to claim 4, wherein the said plurality of segments comprises at least a segment with a first material and at least a segment with a second material, wherein the at least two materials have different moduli of elasticity.

6. A support member (5) according to claim 4 or 5, wherein the plurality of segments are arranged in an axial direction of the support member7. A support member (5) according to any one of the previous claims, further comprising a gasket positioned between the first and second portions.

8. A support member (5) according to any one of the previous claims, wherein the first portion (2) is hollow.

9. A support member (5) according to any one of the previous claims, wherein the second portion (3) is hollow.

10. A support member (5) according to any one of the previous claims, wherein the first portion (2) is substantially formed from steel.

11. A support member (5) according to any one of the previous claims, wherein at least a part of the second portion (3) is substantially formed from composite plastic.

12. A support member (5) according to claim 11, wherein the composite plastic is a composite plastic resin such as epoxy or polyester.

13. A support member (5) according to claim 11 or 12, wherein the composite plastic is a fiber reinforced composite plastic, such as glass fiber reinforced composite plastic.

14. A support member (5) according to any one of the previous claims, wherein the second portion (3) has a wall thickness which varies along the length of the second portion (3).

15. A support member (5) according to any one of the previous claims, wherein the support member (5) comprises at least a part of a wind turbine tower (6).

16. A support member (5) according to any one of the previous claims, wherein the support member comprises a wind turbine tower (6) and at least a part of a foundation (4).

17. A support member (5) according to claim 16, wherein the foundation (4) is a bottom fixed foundation.

18. A support member (5) according to claim 17, wherein the foundation (4) comprises a monopile at least party embedded into the seabed.

19. A support member (5) according to claim 16, wherein the foundation (4) is a floating platform.

20. A support member (5) according to any one of the previous claims, wherein the second portion (3) is bolted, grouted or molded to the first portion (2).

21. A support member (5) according to claim 20, wherein support member further comprises a third portion below the second portion (3) and the second portion (3) is bolted, grouted or molded to the third portion.

22. A support member (5) according to claim 21, wherein the second portion (3) is bolted, grouted or molded to the third portion below a surface of water.

23. A support member (5) according to claims 20-22, wherein an epoxy or vinyl ester is used as a grouting material.

24. A support member (5) according to claim 20 or 21, wherein the second portion (3) comprises metal inserts molded into the material of the second portion (3).

25. A wind turbine (1) comprising a support member (5) according to any one of the previous claims and a rotor and nacelle assembly.

26. A wind turbine (1) according to claim 25, wherein the combined natural frequency of the support members first bending mode shape is at least 5% lower than the 3P frequency of the wind turbine.

27. A method for manufacturing a support member (5) for a rotor and nacelle assembly, the method comprising the steps of;- providing a first portion (2) of the support member (5), the first portion (2) comprising a first material,- providing a second portion (3) of the support member (5), the second portion (3) comprising a second material, and- connecting the first portion (2) and the second portion (3) to form the support member (5), wherein the first portion has a bending stiffness that is higher than the bending stiffness of the second portion, and wherein the length of at least the second portion (3) is selected to obtain a desired length of the support member and to determine the combined natural bending and / or torsion mode frequencies of the support member (5).

28. The method according to claim 27, wherein a 1st combined natural bending mode frequency of the support member (5) is determined by determining the length of the second portion (3).

29. The method according to claim 27 or 28, wherein the length of the second portion (3) is determined such that the 1stcombined natural bending mode frequency of the support member (5) is lower than a one per revolution (IP) frequency of a wind turbine rotor at rated wind speed.

30. The method according to any one of claims 27 to 29, wherein the length of the second portion (3) is determined such that the 1st combined natural bending mode frequency of the support member (5) is below an annual average Tp wave frequency.

31. The method according to any one of claims 27 to 30, wherein the length of the second portion (3) is determined such that the 1st combined natural bending mode frequency of the support member (5) is below a 50-year return period maximum expected Tp wave frequency.

32. The method according to any one of claims 27 to 31, wherein the length of the second portion (3) is determined such that at least a 3rdor 4thcombined natural bending mode frequency of the support member (5) is below an annual average Tp wave frequency.

33. The method according to any one of claims 27 to 32, comprising determining a number of diagonal lay-ups of composite fibers relative to a number of axial lay-ups of composite fibers in the second portion (3), thereby determining a torsional stiffness of the second portion (3).

34. The method according to claim 33, wherein the torsional stiffness of the second portion (3) is determined such that at least a 1stcombined natural torsion mode frequency of the support member (5) is arranged between a three per revolution (3P) frequency and a six per revolution (6P) frequency of a wind turbine rotor at rated wind speed.

35. The method according to any one of claims 27 to 34, wherein the bending stiffness and / or the torsional stiffness of said second portion (3) is adjusted after the support member (5) has been installed at a final location by laminating additional layers of composite material on an outside or an inside of the second portion (3).

36. The method according to any one of claims 27 to 35, comprising the step of arranging a tuned damping arrangement above said second portion (3) on an outside or an inside of said support member (5), or, on an outside or an inside of a wind turbine rotor and nacelle assembly (7) arranged on top of the support member (5).

37. The support member according to any one of the claims 1 to 24, comprising a tuned damping arrangement arranged above said second portion (3) on an outside or an inside of said support member (5), or, on an outside or an inside a wind turbine rotor and nacelle assembly (7) arranged on top of the support member (5).

38. The support member (5) according to any one of the previous claims, further comprising a gasket positioned between the second and third portions.

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