Floating wind turbine generator foundation

Abstract

The present invention relates to a floating wind turbine generator (WTG) foundation for a floating WTG system, the floating WTG foundation comprising: a floater having a generally cylindrical shape, wherein the floater defines a vertical axis, the floater comprising ballast at a lower end thereof, an upper projection extending upward from the floater, the upper projection having a smaller cross-section than the floater, at least three anchors which are connected to the seabed, at least three tendon assemblies, each tendon assembly extending between one of the anchors and a respective mounting position on the floater.

Description

[0001] P36839PC00 / WHA

[0002] Title: Floating wind turbine generator foundation

[0003] BACKGROUND OF THE INVENTION

[0004] The present invention relates to a floating wind turbine generator (WTG) foundation which is a hybrid of a spar and a TLP. Such floating WTG foundations are known. Floating wind farms are considered important for our future energy supply. To date, only a limited number of projects have been successfully installed. The Hywind project which is located off the Scottish coast is one of these. This project is installed in relatively shallow water of 95-120 meter. However, a greater potential for floating wind farms is located in areas with a significantly greater water depth. It was found that floating wind farms are more challenging at greater water depths.

[0005] An important challenge of floating wind farms at greater depths is to keep the costs low. Currently, the costs tend to be a too high to render floating offshore wind turbines financially viable. A cause of the high costs is that the technology is complicated and therefore expensive. The WTG’s are generally the same or substantially the same as WTG’s which are supported by a fixed offshore foundation. Therefore, the WTG’s are generally not the cause of the high costs. The cause of the high costs lies with the floating foundation and its mooring system. The dynamic environment and the high costs of lightweight mooring materials play an important role.

[0006] Many different design criteria and design considerations apply for floating wind turbine foundations. A number of these are discussed below.

[0007] Stability is an important design criterion. Three separate basic concepts to create a stable floating foundation are known: 1) a Tension Leg platform (TLP) 2) a spar and 3) a semisubmersible foundation. TLP’s derive their stability from the tendons. A problem with TLP’s is that the tendons tend to become overly expensive. The stability of the TLP is only provided by the tendons and as a result the pretension forces in the tendons need to be relatively large to prevent slack events in high wave conditions. This requires a substantial amount of tendon material. The tendon material is typically Dyneema (ultra high molecular weight polyethylene) or a similar material and is expensive. In particular in larger water depths in which the tendons need to be long and axially stiff enough, the costs of the tendons becomes burdensome. Another problem of a TLP is that it only becomes stable enough to carry a WTG after the tendons are installed. Therefore, the WTG can only be installed after the TLP is connected to the seabed via the tendons. The installation of the WTG then requires a large crane vessel to install the WTG at the target site. This brings along additional costs.

[0008] Further, it is generally cost-efficient to produce the foundation with the WTG on it at the quayside. This assembly can then be towed to the target site and be moored to the seabed. In this way, an unprecedented, complex lifting and installation operation in which the WTG is lifted as a whole or in parts onto the floating foundation with the help of a large crane vessel is avoided. However, this requires an assembly that is stable enough to carry the WTG during transport and without any connection to the seabed. A TLP cannot provide this stability during transport, which is a disadvantage.

[0009] Additional problems with TLP’s are: they are typically made of steel which is not easily fabricated in some parts of the world.

[0010] TLP’s offer no redundancy when a line fails. Failure of a line will automatically result in loss of the WTG.

[0011] TLP’s require an increasing wire stiffness as water depth increases. This will result in a longer and thicker tendon, resulting in swiftly exploding costs and limited availability. Due to the relative higher natural period, TLP’s see quite high day-to-day accelerations which have a negative impact on the O&M costs due to fatigue issues.

[0012] To date, there are no offshore wind farms which are based on TLP’s, only a number of demonstrators.

[0013] Spars derive their stability from the vertical distance between the COB and COG. A problem with a spar is that for a large WTG, the spar also becomes large and therefore expensive. Since WTG’s tend to become ever larger, this would result in ever larger and more expensive spars. Also, a construction site to produce such spars needs to have considerable water depth to allow production at the quayside. Such a construction site is most often not available near the projected location of a floating offshore wind farms.

[0014] Semi-submersibles derive their stability from the waterplane effect. A problem with semisubmersibles is that they need to be large and have a relatively complicated shape. As a result, they require substantial material and labour to manufacture, resulting in high costs. Also, like TLP’s, semi subs are made of steel which is not always available. Also, they tend to have a relatively high response to waves due to their large water cross sectional area. Further, for both spars and semi-submersibles the mooring lines are generally catenary mooring lines. These require a large footprint on the seabed. The assigned plots for offshore wind farms are generally limited in size. Because of the large footprint, spars and semisubmersible need to keep a significant distance to the circumference of the plot, resulting in a relatively small number of foundations per unit area. TLP’s have vertical or substantially vertical tendons and make better use of the available area.

[0015] The construction material is another important aspect. Because the constructions are rather large for all three types of foundations, the costs of the construction material simply tends to become too high. For spars, TLP’s and semi-subs, steel is often used, but steel is rather expensive and steel fabrication of this scale construction requires a plant such as a shipyard which is not available in some areas around the world. Further, the CO2 associated with steel production and the transport from shipbuilding yards to the windfarm site will result in high emissions.

[0016] Another design consideration for Floating Offshore Wind Turbine Generators are the accelerations of the nacelle. These should be as low as possible, because excessive accelerations result in early fatigue of a number of the mechanical parts in the nacelle with the consequence of loss of lifetime or excessive O&M costs.

[0017] Also, the natural frequencies of the system in relation to the occurring wave frequencies are important. If these frequencies are the same or close to one another, this may result in resonant oscillation, resulting in high accelerations and in damage or early fatigue and loss of lifetime. Therefore, the natural frequency for roll, pitch and heave of a floating offshore WTG should be sufficiently lower or higher than the occurring wave frequencies.

[0018] KR1020130107707A and US9856621B2 each disclose a hybrid foundation of a spar and a TLP. The hybrid foundation combines advantages of a spar and a TLP. The spar provides a portion of the required stability and the tendons provide another portion of the stability. As a result, the tendons can be relatively light, resulting in a cost advantage. The spar can be relatively small, also resulting in a cost advantage. The foundation plus WTG is stable enough to be transported from the quayside to the target site. However, in the present invention it was recognized that the total costs of these systems are still too high. In particular, the costs of the tendons are too high especially when the target site is in deep water. Therefore, the systems of KR1020130107707A and US9856621B2 are still not viable. One of the driving factors in the costs of a spar / TLP hybrid is the costs of the tendons, which may be more than 40 percent of the total costs of the floating wind turbine foundations. The tendons are generally made from high performance synthetic materials, such as Dyneema. These are costly and the costs increase with increasing water depth because a greater water depth requires longer and often also thicker tendons to preserve adequate stiffness.

[0019] For these reasons, no viable system exists to date and to the best of our knowledge, no projects based on a hybrid of a TLP and a spar have been carried out to this date. There is a need for a more cost-effective floating WTG foundation.

[0020] OBJECT OF THE INVENTION

[0021] It is an object of the invention to provide a floating WTG foundation and which has relatively low costs, which makes efficient use of the available space in a wind farm plot and which has a relatively long lifetime. In particular the maintenance costs for the WTG and foundation should be low.

[0022] Further, it is an object that the floating WTG foundation can be made locally.

[0023] SUMMARY OF THE INVENTION

[0024] The present invention provides a floating wind turbine generator (WTG) foundation for a floating WTG system, the floating WTG foundation comprising: a floater having a generally cylindrical shape, wherein the floater defines a vertical axis, the floater comprising ballast at a lower end thereof, an upper projection which extends upward from the floater, the upper projection having a smaller cross-section than the floater, at least three anchors which are connected to the seabed, at least three tendon assemblies, each tendon assembly extending between one of the anchors and a respective mounting position on the floater.

[0025] In some embodiments, each tendon assembly comprises: a main tendon section having a main spring constant (Kmain), a peak shaving spring assembly configured to become active above a peak shaver activation force (Fpeak) on the tendon assembly, the peak shaving spring assembly having a peak shaving spring constant (Kps) , wherein the main tendon section and the peak shaving spring assembly are placed in series, wherein the tendon assembly has a first total spring constant (Ktot,i) below the peak shaver activation force and a second total spring constant (Ktot,2) above the peak shaver activation force, wherein the first total spring constant is greater than the second total spring constant. The peak shaving spring assembly results in lower peak forces in the tendon assemblies and allows a lighter construction of the tendon assemblies, thereby reducing the costs. This applies in particular to sites having high wave heights and high wind speeds.

[0026] In some embodiments, each tendon assembly further comprises a slack prevention spring assembly configured to become active below a lower threshold force (F|OW) on the tendon assembly, the slack prevention spring assembly having a slack prevention spring constant (KSp), wherein the main tendon section, the peak shaving spring assembly and the slack prevention spring assembly are placed in series, wherein the tendon assembly has the first total spring constant (Ktot,i) between the lower threshold force (F|OW) and the peak shaver activation force and the second total spring constant (Ktot,2) above the peak shaver activation force and a third total spring constant (Ktot,3) below the lower threshold force (Flow), wherein the first total spring constant is greater than the second total spring constant and the third total spring constant. Or in other words, the total spring constant (Ktot) of the combined tendon assembly is relatively high in a range between the lower threshold force (Flow) and the peak shaver activation force (Fpeak) and is relatively low below the lower threshold force (F|OW) and above the peak shaver activation force (Fpeak). This prevents the tendon assembly from going slack, which in turn allows a lower pre-tension force and a lighter construction of the tendon.

[0027] In some embodiments, the floating WTG foundation may comprise only a slack prevention spring assembly and not a peak shaving spring assembly.

[0028] In some embodiments, the floater provides sufficient stability to ensure a positive stability for the floating WTG when in free floating mode with the installed WTG on it, allowing transport of the floating offshore WTG from the production site to the target site.

[0029] In some embodiments, a total restoring moment (RMtot) of the floater in case of a roll or pitch movement comprises the following components:

[0030] - a spar restoring moment (SRM) resulting from a vertical distance (28) between the Centre of Buoyancy (COB) and the Centre of Gravity (COG) of the spar,

[0031] - a tendon restoring moment (TRM) resulting from a difference in forces exerted on the floater by the respective tendon assemblies, and wherein 0,40 * RMtot< TRM < 0,8 * RMtot., wherein RMtotis the total restoring moment. This allows construction of a floating WTG system at a production site which is remote from the target site and transport to the target site. The floater provides sufficient stability during transport. It is noted that a spar also has a small waterplane effect but this plays an insignificant role.

[0032] In some embodiments, the peak shaving spring assembly comprises:

[0033] - a pre-compressed peak shaving spring which engages a peak shaving stop when an actual force is below at the peak shaver activation force (Fpeak), and

[0034] - the peak shaving stop, wherein the peak shaving stop prevents the pre-compressed peak shaving spring from elongation beyond a maximum length (Lmax) when an actual force in the tendon assembly is below the peak shaver activation force, and wherein when the actual force in the tendon assembly becomes higher than the peak shaver activation force, the peak shaving spring disengages from the peak shaving stop and becomes further compressed. The peak shaver activation force is the force at which the peak shaving spring assembly starts to participate in the stiffness, thereby reducing the overall stiffness. It was found that this provides a simple and robust solution for the peak shaving spring assembly.

[0035] In some embodiments, the slack prevention spring assembly comprises:

[0036] - a slack prevention spring which is pre-compressed when an actual force in the tendon assembly is above the lower threshold force,

[0037] - a slack prevention stop which prevents the slack prevention spring from becoming pre-compressed more than a threshold pre-compression length when the actual force in the tendon assembly is above the lower threshold force, and wherein when the actual force drops below the lower threshold force, the slack prevention spring extends and becomes active, wherein the slack prevention stop is disengaged, wherein the extending slack prevention spring maintains the actual force in the tendon assembly above zero and prevents the tendon assembly from becoming slack. It is noted that various structural possibilities for the slack prevention stop exist. The stop may be mounted on a rod extending through the slack prevention spring or be mounted on or part of on a housing or frame which surrounds the slack prevention spring.

[0038] It was found that this provides a simple and robust construction for the slack prevention spring assembly.

[0039] In some embodiments, the floater has a vertical circumferential wall and a cross-section which is uniform over the height of the floater, wherein the main longitudinal axis is vertical. This allows a production with slip-forming which is cost-efficient. In some embodiments, the circumferential wall of the floater is manufactured from concrete.

[0040] This is a cheap material that has good resistance against the elements and can be fabricated without the use of experienced welders and shipyards.

[0041] In some embodiments, when seen from above, the floater has a circular cross-section, and wherein in particular the upper projection also has a circular cross section. A circular crosssection is simple and provides a uniform stability in all directions, allowing the nacelle to be oriented in any direction without significant loss of stability.

[0042] In some embodiments, the floating WTG foundation comprises: one or more solid ballast compartments at a lower end of the floater for holding solid ballast, and one or more water ballast compartments for holding a controllable volume of water ballast.

[0043] The solid ballast compartments and the water ballast compartments may be the same compartments. In other words, the water may enter the solid ballast compartment(s).

[0044] In some embodiments, the peak shaving spring assembly and the slack prevention spring assembly are integrated into a single mechanical system.

[0045] The present further relates to a floating WTG system comprising the floating WTG foundation according to the invention and a WTG installed on the floating WTG foundation.

[0046] The present further relates to a spring mechanism, comprising: a peak shaving spring assembly configured to become active above an peak-shaver activation force (Fpeak), the peak shaving spring assembly having a peak shaving spring constant (Kps), a slack prevention spring assembly configured to become active below a lower threshold force (F|OW), the slack prevention spring assembly having a slack prevention spring constant (Ksp), wherein the peak shaving spring assembly and the slack prevention spring assembly connected to one another and are placed in series.

[0047] In some embodiments, the peak shaving spring constant (Kps) may be non-linear.

[0048] The present further relates to a tendon assembly, comprising: a main tendon section having a main spring constant (Kmain), a peak shaving spring assembly configured to become active above a peak shaver activation force (Fpeak) on the tendon assembly, the peak shaving spring assembly having a peak shaving spring constant (Kps), a slack prevention spring assembly configured to become active below a lower threshold force (F|OW), the slack prevention spring assembly having a slack prevention spring constant (Ksp), wherein the main tendon section, the peak shaving spring assembly and the slack prevention spring assembly are placed in series, wherein the tendon assembly has a total spring constant (Ktot) which is relatively high below a peak shaver activation force and is relatively low above the peak shaver activation force.

[0049] The present invention further relates to a method of manufacturing and installing a floating wind turbine generator, the method comprising the steps: a) manufacturing a floating WTG foundation at a production site, comprising the steps: i. making a floater by making a base, and forming a concrete circumferential wall of the floater on the base by slip forming, ii. installing or making an upper projection, wherein the upper projection extends upward from the floater, iii. providing at least three mounting positions for a tendon assembly on the floater, b) installing a wind turbine generator on the upper projection, thereby forming the floating offshore wind turbine generator, c) installing at least three anchors and in particular four or more anchors at the target site, d) transporting the formed floating offshore wind turbine generator from the production site to the target site, e) installing at least a first, second and third tendon assembly between the respective anchors and respective mounting positions on the floater, wherein steps a) and b) are performed at the production site.

[0050] In some embodiments of the method, step e) comprises ballasting the floater of the floating WTG to a deeper water level prior to connecting the tendon assemblies to the floater, and deballasting the floater after connecting the tendon assemblies.

[0051] Steps c) and d) may be performed simultaneously or in the reverse order as listed.

[0052] In some embodiments of the method, the tendon assemblies are tensioned at a design force (Fdes), and wherein at the design force a total restoring moment (RMtot) of the floater in case of a roll or pitch movement comprises the following components: a spar restoring moment (SRM) resulting from a vertical distance between the COB and the COG of the spar, a tendon restoring moment (TRM) resulting from a difference in forces between the respective tendon assemblies, and wherein 40% RMtot< TRM < 80% RMtot, wherein RMtotis the total restoring moment. It was found that this ratio results in a stable floating WTG system at low costs. The tendon assemblies can be adjusted to the local water depth in a cost efficient manner.

[0053] In some embodiments of the method, each tendon assembly has: a main tendon section having a main spring constant (Kmain), a peak shaving spring assembly configured to become active only above a peak shaver activation force on the tendon assembly, the peak shaving spring assembly having a peak shaving spring constant (Kps), a slack prevention spring assembly configured to become active only below a lower threshold force on the tendon assembly, the slack prevention spring assembly having a slack prevention (SP) spring constant (Ksp), wherein the main tendon section, the peak shaving spring assembly and the slack prevention spring assembly are placed in series, wherein a total spring constant (Ktot) is relatively high in a range between the lower threshold force and the peak shaver activation force and is relatively low above the peak shaver activation force and below the lower threshold force.

[0054] The tendon assemblies may be vertical or inclined. An advantage of inclined tendon assemblies is a better control of the horizontal offset and a stabilisation of the nacelle lateral motions, in particular when the tendon assemblies are oriented towards a point at or near the nacelle.

[0055] In some embodiments of the method, the floating WTG has a natural frequency for heave, roll and pitch which is lower than occurring wave frequencies at the target site. This results in low accelerations of the nacelle and mechanical components in the nacelle, resulting in a relatively long lifetime.

[0056] In some embodiments of the method, the installation is carried out without a transfer of the WTG or any of its large components, i.e. the mast, nacelle or blades, to the floating WTG foundation by a large crane vessel. This advantageously saves costs, because a large crane vessel has a high day rate. Also, a large crane vessel is not always available.

[0057] In some embodiments of the method, a temporary additional buoyancy device is connected to the floater at the production site. Advantageously this may reduce the draft. This can be important in ports having insufficient water depth. In this way, the draft may be reduced by 5- 15 meter.

[0058] In some embodiments of the method, the temporary additional buoyancy device may have a ring shape, U-shape or a C-shape when seen in top view and extends wholly or partially around the floater when seen in top view. This provides a well distributed extra buoyancy. The ring shape provides the best distribution of the additional buoyancy but needs to be splitable in parts or have doors. A U-shape is simple and can do without doors because the floating WTG foundation can be simply inserted into the U-shape when seen from above. A C-shape may need small doors.

[0059] In some embodiments of the method, the temporary additional buoyancy device comprises multiple parts which together form a ring shape or C-shape, or wherein the temporary additional buoyancy device comprises one or two doors which are movable between an open position and a closed position, wherein in the open position the doors provide access to an interior space and allow the floating WTG foundation to enter or exit the interior space and wherein in the closed position the temporary additional buoyancy device surrounds the floating WTG foundation when seen in top view.

[0060] These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

[0061] SHORT DESCRIPTION OF THE FIGURES

[0062] Fig. 1 shows a general view of the floating WTG system of the present invention.

[0063] Fig. 2 shows a diagram of the three main types of stability principles used in floating WTG foundations and the present invention.

[0064] Fig. 3 shows a view of the 6 degrees of freedom of a floating WTG system.

[0065] Fig. 4 shows a side view of the floating WTG foundation of the present invention without the WTG.

[0066] Fig. 5 shows a side view of the floater and the upper projection.

[0067] Fig. 6 shows a section top view of the floater.

[0068] Fig. 7A shows a side view of a peak shaving spring assembly in the active state.

[0069] Fig. 7B shows a side view of the peak shaving spring assembly in the inactive state.

[0070] Fig. 8A shows a side view of a slack-prevention spring in the non-active state.

[0071] Fig. 8B shows a side view of a slack-prevention spring in the active state. Figs. 9A, 9B and 9C show an assembly comprising a peak shaving spring assembly and a slack-prevention spring in three different states.

[0072] Fig. 10 shows a graph of the spring constants of the main tendon section, the peak shaving spring assembly and the slack prevention as a function of the actual force.

[0073] Fig. 11 shows a graph of the total spring constant as a function of the actual force.

[0074] Fig. 12 shows a graph of the forces in the tendon assembly as a function of the length of the tendon assembly.

[0075] Fig. 13 shows the various ballasting states during the installation of the floating WTG system.

[0076] Fig. 14 shows the difference between vertical tendon assemblies and inclined tendon assemblies.

[0077] Figs. 15A and 15B show a top view and a sectional side view of a temporary additional buoyancy device.

[0078] Figs. 15C and 15D show top views of another embodiment of the temporary additional buoyancy device.

[0079] Figs. 15 to 21 show various stages in another embodiment of the invention.

[0080] DETAILED DESCRIPTION OF THE FIGURES

[0081] Turning to figure 1, the present invention relates to a floating Wind Turbine Generator (WTG) system 1 comprising a floating WTG foundation 10.

[0082] Turning to fig. 2, three types of floating WTG foundations exist. A spar derives it stability from a vertical distance between the centre of buoyancy (COB) and the centre of gravity (COG). A TLP derives its stability from the force in the tendons (mooring stiffness). A semi-submersible derives it stability from the water plane effects. The present invention is a hybrid of a spar and a TLP and combines the stability from a vertical distance between the centre of buoyancy (COB) and the centre of gravity (COG) with the stability from the forces in the tendons.

[0083] Turning to fig. 3, the six degrees of freedom which play a role in a floating WTG system 1 are: roll, pitch, yaw, surge, sway and heave. The floating WTG 1 comprises a WTG 11 and a floating WTG foundation 10. The WTG comprises a mast 101, a nacelle 102 and blades 103.

[0084] Resistance to yaw is important and is correlated to resistance to pitch. If the resistance to pitch is relatively low, the WTG will tilt backwards at relatively high wind speeds. As a result, the rotor axis will tilt and will not be horizontal. This results in a yaw moment. As consequence, the resistance to yaw needs to be greater when the resistance to pitch is low. Turning to fig. 4, the floating WTG 1 of the present invention is shown with only a portion of the mast 101 of the WTG 11. The floating WTG system 1 comprises the floating WTG foundation 10 and a WTG 11 installed on the floating WTG foundation.

[0085] The floating WTG foundation 10 comprises: a floater 14 having a generally cylindrical shape, wherein the floater defines a vertical axis 16, the floater comprising ballast 18 at a lower end thereof, an upper projection 20 extending upward from the floater, the upper projection having a smaller cross-section than the floater, at least three anchors 22 which are connected to the seabed 29, at least three tendon assemblies 24, each tendon assembly extending between one of the anchors and a respective mounting position 35 on the floater.

[0086] The ballast 18 may be solid ballast. The upper projection may be a cylinder having a uniform diameter, but may also be conical or have a different shape. The upper projection may extend to inside the floater as is shown, but may also not extend to inside the floater and have a lower end which is fixed to or integral with a roof 90 of the floater.

[0087] Typically there will be four or more anchors 22 and an equal number of tendon assemblies. The anchors 22 can be of various type and the type is not essential: dead weight (also called gravity anchor), suction cup, pile or a different type of suitable anchor. Therefore the term ’’connected to the seabed” is broad and covers a dead weight lying on the seabed.

[0088] The upper projection may extend into the floater so that the connection between the upper projection and the floater is very rigid. In operation the floater is submerged and the upper projection 20 intersects the water level 15. In this way, the effects of the waves on roll, pitch and heave are relatively low.

[0089] The floater 14 allow for system stability in the unlikely event of a failed tendon assembly 24.

[0090] Each tendon assembly 24 comprises a main tendon section 26 having a main spring constant Kmain and a peak shaving spring assembly 30 configured to become active above a peak shaver activation force Fpeak on the tendon assembly. The main tendon section 26 is typically made of Dyneema or a similar material and is an expensive part. The peak shaving spring assembly has a peak shaving spring constant Kps. The main tendon section 26 and the peak shaving spring assembly 30 are placed in series. The tendon assembly has a first total spring constant Ktot,i below the peak shaver activation force and a second total spring constant Ktot,2 above the peak shaver activation force, wherein the first total spring constant is greater than the second total spring constant. The peak shaver activation force is higher than the design force Fdes of the tendon assembly 24. This is explained more in detail below.

[0091] Each tendon assembly further comprises a slack prevention spring assembly 31 configured to become active below a lower threshold force F|OWon the tendon assembly. The slack prevention spring assembly has a slack prevention spring constant Ksp. The main tendon section, the peak shaving spring assembly and the slack prevention spring assembly are placed in series. A total spring constant Ktotis relatively high in a range between the lower and peak shaver activation force and is relatively low below the lower threshold force and above the peak shaver activation force.

[0092] Each tendon assembly may further comprise an upper section 38 which extends between the floater 14 and the spring assembly 30 or 31 . The upper section may be a chain or made from similar material as the main section 26. The spring assemblies 30, 31 are placed relatively high to allow for inspection and maintenance. Either of the spring assemblies 30, 31 can be the upper spring assembly.

[0093] The chain has a significant higher stiffness than other sections and is relatively short and therefore no contribution to the overall stiffness.

[0094] The tendon assemblies 24 can be configured and tuned for the water depth at the installation site and for wave and wind conditions which prevail at the installation site. . The present invention is in particular suitable for water depths between 250m and 1500m.

[0095] The tendon assemblies are placed under a pre-load by providing sufficient buoyancy to the floater. In other words, when there are no forces of waves and / or wind on the floating WTG, the tendon assemblies 24 are under tension. This is important because the tendon assemblies add to the stability of the floating WTG.

[0096] In cases where the wave conditions are relatively mild, the peak shaving spring assembly and the slack prevention spring assembly are not active. In some locations having calm weather, one or both may not be necessary. This saves costs.

[0097] The dimensions of the floater and the tendon assemblies are chosen such that a total restoring moment RMtotof the floating WTG 1 in case of a roll or pitch movement comprises the components: - a spar restoring moment (SRM) resulting from a vertical distance (28) between the Centre of Buoyancy (COB) and the Centre of Gravity (COG) of the spar,

[0098] - a tendon restoring moment (TRM) resulting from a difference in forces exerted on the floater by the respective tendon assemblies, and wherein 0,40 * RMtot< TRM < 0,8 * RMtot.

[0099] Turning to fig. 14, the tendon assemblies 24 may be vertical but preferably extend at an angle of 2-12 degrees to a vertical. In particular the tendon assemblies 24 may be oriented towards a point at or near the nacelle 101 This results in a center of rotation of the WTG foundation 10 which is about at a level of the nacelle 101 which will reduce wave frequent nacelle lateral accelerations as indicated in dashed lines. This consequently reduces fatigue of mechanical components in the nacelle. The vertical or near-vertical orientation allows for an efficient use of the available plot area.

[0100] Returning to fig. 4, the floater 14 provides sufficient stability to ensure a positive stability for the floating WTG 1 when in free floating mode with the installed WTG 11 on it, allowing transport of the floating WTG 1 from the production site to the target site. The term production site is intended to be interpreted broadly and covers any location that is protected against the wind, waves, tide and / or current which may occur at full sea. This can be a protected bay, a drydock, an in-port location, a location on land near a quayside, a floating dock in a harbour.

[0101] The floater 14 comprises:

[0102] - one or more solid ballast compartments 42 at a lower end 43 of the floater for holding solid ballast 18,

[0103] - one or more water ballast compartments 44 for holding a controllable volume of water ballast.

[0104] The solid ballast compartments and the water ballast compartments may be the same compartments. In other words, the water may enter the solid ballast compartment(s).

[0105] The floater may have a pump for ballasting or de-ballasting with seawater, but this is optional because the pump is only needed at installation and de-installation.

[0106] The tendon assemblies 24 can be made of robust and durable materials. The spring assemblies 30 and 31 can be made from available materials. Specialist parts - which tend to be expensive - are not required or only required to a limited extent.

[0107] Turning to figs. 5 and 6, the floater 14 is made from concrete by slip forming. The concrete will generally be reinforced concrete. This allows a cost-effective production and provides an advantage over TLP’s, spars and semi-subs that are made from steel. Not only the material is cheaper, but the production method with slip-forming also allows more production with less labour and therefor lower costs. The floater comprises a circumferential wall 23 which is circular. The floater may have internal walls 17 which are vertical and extend radially. A central column 19 may be provided in the floater for connection with the upper projection 20. The internal walls 17 and the central column may be manufactured together with the circumferential wall 23 by slip-forming. Mounting positions 35 for the upper ends of the tendon assemblies 24 may be integrated with a junction between an internal wall 17 and the circumferential wall. Alternatively, the floater may be provided with consoles 39 which project outwardly over a horizontal distance from a lower portion of the circumferential wall and the mounting positions 35 can be provided on the free ends of the consoles. Fig. 4 shows small consoles 39, but these may be larger or not present at all.

[0108] The circumferential wall 23 is vertical and the floater has cross-section which is uniform over a height of the floater. The main axis of the floater is vertical.

[0109] When seen from above the floater 14 has a circular cross-section, and wherein in particular the upper projection also has a circular cross section.

[0110] Turning to figs. 7A and 7B, the peak shaving spring assembly 30 comprises a precompressed peak shaving spring 60 which is pre-compressed at the peak shaver activation force Fpeak. The peak shaving spring assembly further comprises a peak shaving stop 61 . The peak shaving spring assembly 30 further comprises an upper eye 62 which is connected via a rod 66 to a lower end 67 of the peak shaving spring 60. A plate 167 may be provided at the lower end 67. The rod 66 extends through a bore 65 in the peak shaving spring 60. The peak shaving spring assembly 30 further comprises a lower eye 63 which is connected via a casing or frame 64 to the stop 61 at an upper end 68 of the peak shaving spring 60. The upper eye is connected to an upper section of the tendon and the lower eye will be connected to a lower section of the tendon.

[0111] As shown in fig. 7B, the peak shaving stop 61 prevents the peak shaving spring 60 from elongation beyond a maximum length (Lmax) when an actual force in the tendon assembly 24 is below the peak shaver activation force. The actual force in the tendon assembly 24 is lower than the peak shaver activation force and the peak shaving spring 60 engages the peak shaving stop 61 and is not active. The peak shaving spring assembly 30 is rigid when an actual force is below the peak shaver activation force. As shown in fig. 7 A, when the actual force becomes higher than the peak shaver activation force, the peak shaving spring assembly 30 becomes active. The peak shaving spring 60 is compressed further and the upper end 68 disengages from the peak shaving stop 61.

[0112] The peak shaving stop 61 may have various constructions. It may be mounted on the rod 66 or on the casing 64 or have a different construction. What is important is that it prevents the peak shaving spring 60 from elongation beyond the maximum length (Lmax). The maximum length may be adjustable.

[0113] Turning to fig. 8A (on the right), the slack prevention spring assembly 31 comprises a slack prevention spring 70 which is pre-compressed when an actual force in the tendon assembly is above the lower threshold force. The slack prevention spring assembly comprises a slack prevention stop 71 which prevents the slack prevention spring 70 from becoming precompressed more than a threshold pre-compression length (or minimum length) when the actual force in the tendon assembly is above the lower threshold force. The slack prevention spring assembly further comprises an upper eye 72, a lower eye 73, a casing or frame 74 and a rod 76 which extends through a bore 75 in the slack prevention spring 70. The stops 71 may be projections which project inwardly from the casing or frame 74. Alternatively, the end stop 71 may be mounted on the rod 76. Both alternatives are shown in figs. 8A and 8B, resulting in some redundancy. The end stop 71 which is mounted on the rod 76, engages an end plate 174 of the casing. A plate 267 may be provided at an end of the slack prevention spring and may connect the end 79 of the slack prevention spring to the end of the rod 76. It is noted that various constructions for the slack prevention stop 71 are possible. What is important is that it prevents further compression of the slack prevention spring 70 beyond the minimum length when the force is greater than lower threshold force (F|OW).

[0114] Turning to fig. 8B, when the actual force drops below the lower threshold force, the slack prevention spring 70 disengages from the slack prevention stop 71 and becomes active, thereby keeping the actual force in the tendon assembly above zero and preventing the tendon assembly from becoming slack. The slack prevention spring assembly 31 is rigid when an actual force is above the lower threshold force (Flow).

[0115] The springs 60, 70 can be made of rubber, cellular plastics or another polymer. The other main parts of the spring assemblies 30, 31 can be made of steel.

[0116] Turning to figa. 9A, 9B, 9C, the peak shaving spring assembly 30 and the slack prevention spring assembly 31 are integrated into a single assembly 33 which is shown in three different states. Fig. 9A shows the assembly 33 when the actual force is below the lower threshold force (Flow). The slack prevention spring assembly 31 is active (disengaged from the stop) and the peak shaving spring assembly 30 is inactive (engaged against the stop). Fig. 9B shows the assembly 33 when the actual force is greater than the lower threshold force (F|OW) but below the peak shaver activation force (Fpeak). The slack prevention spring assembly 31 is inactive (engaged against the stop) and the peak shaving spring assembly 30 is inactive (engaged against the stop). In the tendon assembly, the sitffness is now only provided by the tendon material itself, in any case not by the slack prevention spring assembly 31 or the the peak shaving spring assembly 30. Fig. 9C shows the assembly 33 when the actual force is greater than the peak shaver activation force (Fpeak). The slack prevention spring assembly 31 is inactive (engaged against the stop) and the peak shaving spring assembly 30 is active (disengaged from the stop). Any further elongation of the tendon assembly will result in a reduced increase in the actual force, thereby limiting the peak load.

[0117] The single assembly 33 of the slack prevention spring assembly 31 and the peak shaving spring assembly 30 allows for easy installation.

[0118] Turning to fig. 10, indicative numbers for the stiffness and force are provided. The numbers depend on WTG capacity, water depth etc..

[0119] The present invention further relates to a spring mechanism 25 configured to be part of the floating WTG system 1 , the spring mechanism comprising:

[0120] • the peak shaving spring assembly 30 configured to become active above an peak shaver activation force Fpeak, the peak shaving spring assembly having a peak shaving spring constant Kps,

[0121] • the slack prevention spring assembly 31 configured to become active below a lower threshold force Flow, the slack prevention spring assembly having a slack prevention spring constant KsP, wherein the peak shaving spring assembly and the slack prevention spring assembly connected to one another and are placed in series.

[0122] The present invention further relates to a tendon assembly 24, comprising:

[0123] • a main tendon section 26 having a main spring constant Kmain,

[0124] • a peak shaving spring assembly 30 configured to become active above an peak shaver activation force Fpeak on the tendon assembly, the peak shaving spring assembly having a peak shaving spring constant Kps, • a slack prevention spring assembly 31 configured to become active below a lower threshold force F|OW, the slack prevention spring assembly having a slack prevention spring constant KsP.

[0125] The main tendon section, the peak shaving spring assembly and the slack prevention spring assembly are placed in series, wherein a total spring constant (Ktot) is relatively high in a range between the lower and peak shaver activation force and is relatively low below the lower threshold force and above the peak shaver activation force.

[0126] Method of manufacturing and installing

[0127] A method of manufacturing and installing a floating wind turbine generator (WTG) system 1 comprises the steps: a) manufacturing a floating WTG foundation 10 at a production site, comprising the steps: i. making a floater 14 by making a base 15, and forming a concrete circumferential wall 23 of the floater on the base by slip forming, ii. installing or making an upper projection 20, wherein the upper projection extends upward from the floater, iii. providing at least three mounting positions 35 for a tendon assembly 24 on the floater, b) installing a WTG 11 on the upper projection, thereby forming the floating WTG system 1, c) installing at least three anchors 22 and in particular four or more anchors at the target site 62, d) transporting the formed floating offshore wind turbine generator from the production site to the target site, e) installing at least a first, second and third tendon assembly 24 between the respective anchors and respective mounting positions 35 on the floater, wherein steps a) and b) are performed at the production site.

[0128] Turning to figures 10, 11 and 12, fig. 10 shows the different stiffnesses an example. The peak shaving spring assembly 30 limits the forces in the tendon assemblies at high loads, for instance in a storm. As a result, the main sections 26 of the tendons can be constructed lighter and therefore cheaper. The floating WTG system 1 can absorb dynamic loads.

[0129] The slap prevention spring assembly 31 prevents the tendon assemblies 24 from going slack when the floating WTG system undergoes significant roll, pitch or heave movements. As a result, the pretension can be lower. These two effects are cumulative and result in significantly lighter and therefore cheaper main sections 26 of the tendons. Because the main sections 26 of the tendon assemblies may contribute as much as 40 percent to the total costs, this results in a significant saving.

[0130] The natural roll and pitch and heave periods of the floating WTG system 1 may be between 22 and 35 seconds which is above the natural periods of most wave conditions. The floating WTG system 1 has a natural frequency for heave, roll and pitch which is lower than occurring wave frequencies at the target site. This results in low accelerations, in particular of the nacelle, and a long lifetime of the mechanical components in the nacelle. This results in less fatigue loads on nacelle components which in turn results in lower Operation and Maintenance (O&M) costs during the lifetime. It is noted that O&M is expected to account for the largest single contributor of the Levelized Cost Of Electricity (LCOE) of floating windfarms.

[0131] The mooring configuration with system response below the wave frequency is not stiffness dominated as other TLP’s, making it more water depth independent. This allows for significant cost savings compared to traditional stiffness dominated mooring in deep water. This results in a significant cost saving and mooring material availability advantage compared to other pure TLP solutions. The advantage increases significantly with water depth.

[0132] Fig. 12 shows the force in the tendon assembly as a function of the length. Between F|OWand Fpeak, the force in the tendon assembly varies strongly with varying length. Below F|OWand above Fpeak, the force varies less.

[0133] Turning to fig. 13, various stages in the installation procedure are shown. On the left, the stage in which the floating WTG system 1 is finished and lifted from the seabed at the quayside. Use is made of an extra floater 150 in case of shallow water at the production site. More on the extra floater further below. Second from left, the draft is increased by adding water ballast. This ensures sufficient stability for transportation. Third from left, when at the target site, the draft is further increased by adding water ballast. Second from right, prior to connecting the tendon assemblies 24 to the floater 14, an extra amount of water ballast is added, resulting in extra draft, to allow attachment of the tendon assemblies 24 in a slack state. On the right, after the tendon assemblies have been connected, the floater is deballasted to raise the floating WTG system 1 to the operational draft with the tendon assemblies 24 at the design tension force.

[0134] The installation is carried out without a transfer of the WTG or any of its large components, i.e. the mast, nacelle or blades, to the floating WTG foundation by a large crane vessel. Also, the floating WTG system may be towed back to port without any crane vessel in case of maintenance or repair which cannot be carried out at the target site.

[0135] The tendon assemblies 24 are tensioned at a design force Fdes. At the design force a total restoring moment (RMtot) of the floater in case of a roll or pitch movement comprises the components:

[0136] - a spar restoring moment (SRM) resulting from a vertical distance between the COB and the COG of the spar,

[0137] - a tendon restoring moment (TRM) resulting from a difference in forces between the respective tendon assemblies, and wherein 40% RMtot< TRM < 80% RMtot.

[0138] The floating WTG system 1 has passive components to avoid excessive maintenance requirements.

[0139] The floating WTG system 1 has a natural roll, pitch and heave frequency which is lower than the occurring wave frequencies. As a result, the overall stiffness can be low and the pretension in the tendon assemblies can be relatively low.

[0140] The floater 14 will remain stable with WTG fully installed due to it’s positive free floating GM value. This allows for quayside installation and commissioning of the WTG. This allows for offshore installation without the use of very expensive or not available heavy life crane vessels. The floater with WTG also remains stable when one of the four mooring lines fails. This redundancy ensures that the unit is not lost provided that remedial action is undertaken before severe storm conditions occur.

[0141] Turning to figs. 15A and 15B, some production sites may be too shallow to allow the floating WTG foundation to float. In that case, a temporary additional buoyancy device 115 is connected to the floater at the production site. In this way, the draft may be reduced by 5-15 meter. In the embodiment of figs. 15A and 15B, the temporary additional buoyancy device 115 has a ring shape. The ring shape is composed of two parts 122A and 122B. The temporary additional buoyancy device 115 has water ballast compartments. Winches 116 are provided on the temporary additional buoyancy device 115. In this case four winches are shown but a different number is possible. When seen in top view, the temporary additional buoyancy device 115 has a ring shape which can be split in two parts. The temporary additional buoyancy device 115 surrounds the floating WTG foundation when seen in top view.

[0142] Turning to figs. 15B and 15C, in another embodiment, the temporary additional buoyancy device 115 has a hinge 130 which interconnects the two parts 122A, 122B and allows the two parts 122A, 122B to open and close by a pivoting movement. In the open state, the floating WTG system 10 may enter or exit the inner space 117 of the temporary additional buoyancy device 115 as indicated by the circles in in fig. 15B. In the closed state, the combination of the the temporary additional buoyancy device 115 and the floating WTG system 10 is ready for transport.

[0143] Alternatively, a ring shape with doors, A U-shape, or a C-shape with doors is also possible. In an open position, the doors provide access to the interior space 117 and allow the floating WTG foundation to enter or exit the interior space. The temporary additional buoyancy device 115 defines an inner space 117 in which the floating WTG foundation 10 can be positioned.

[0144] Turning to fig. 16, when the floater is positioned in the inner space 117, the temporary additional buoyancy device 115 is connected to the floater by cables 118 which extend downward from the winches through a gap between the temporary additional buoyancy device 115 and the floater and are connected to a lower side of the floater 14. The draft may be for instance 2m at this stage. Turning to fig. 17, next the WTG may be installed on the upper projection 20. This increases the draft, for instance to 8 m. Turning to fig. 18, next solid ballast may be added. This increases the draft, for instance to 15m. The system is now ready for transport to the target site.

[0145] Turning to figs. 19 to 21 , when the combination of the temporary additional buoyancy device 115 and floating WTG system 1 arrives at the target site, the winches 116 are paid out and the floating WTG system is gradually lowered. The draft increases, for instance to 50-52 meters. Next, the temporary additional buoyancy device 115 is disconnected by disconnecting the cables. When the temporary additional buoyancy device 115 has a splitable ring shape, the ring shap is split in two parts and the temporary additional buoyancy device 115 can be removed. In case of doors, the doors are opened. Next, the tendon assemblies may be installed as previously indicated. This operation may in principle also be carried earlier, as soon as the available water depth is sufficient. The floating WTG system can then be transported without the temporary additional buoyancy device 115. The temporary additional buoyancy device 115 can then be returned to the production site earlier, but it may require an extra tug vessel.

[0146] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

[0147] The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and / or having, as used herein, are defined as comprising (i.e. , open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

Claims

- 23 -CLAIMS1 . Floating wind turbine generator (WTG) foundation (10) for a floating WTG system (1), the floating WTG foundation (10) comprising: a floater (14) having a generally cylindrical shape, wherein the floater defines a vertical axis (16), the floater comprising ballast (18) at a lower end thereof, an upper projection (20) which extends upward from the floater, the upper projection having a smaller cross-section than the floater, at least three anchors (22) which are connected to the seabed, at least three tendon assemblies (24), each tendon assembly extending between one of the anchors and a respective mounting position (35) on the floater.

2. Floating WTG foundation according to the preceding claim, wherein each tendon assembly (24) comprises: a main tendon section (26) having a main spring constant (Kmain), a peak shaving spring assembly (30) configured to become active above an peak shaver activation force (Fpeak) on the tendon assembly, the peak shaving spring assembly having a peak shaving spring constant (Kps) , wherein the main tendon section and the peak shaving spring assembly are placed in series, wherein the tendon assembly has a first total spring constant (Ktot,i) below the peak shaver activation force and a second total spring constant (Ktot,2) above the peak shaver activation force, wherein the first total spring constant is greater than the second total spring constant.

3. Floating WTG foundation according to preceding claim, wherein each tendon assembly further comprises a slack prevention spring assembly (31) configured to become active below a lower threshold force (Flow) on the tendon assembly, the slack prevention spring assembly having a slack prevention spring constant (Ksp), wherein the main tendon section, the peak shaving spring assembly and the slack prevention spring assembly are placed in series, wherein the tendon assembly has the first total spring constant (Ktot.i) between the lower threshold force (F|OW) and the peak shaver activation force and a second total spring constant (Ktot,2) above the peak shaver activation force and a third total spring constant (Ktot,3) below the lower threshold force (F|OW), wherein the first total spring constant is greater than the second total spring constant and the third total spring constant.

4. Floating WTG foundation (10) according to any of the preceding claims, wherein a total restoring moment (RMtot) of the floater in case of a roll or pitch movement comprises the following components:- a spar restoring moment (SRM) resulting from a vertical distance (28) between the Centre of Buoyancy (COB) and the Centre of Gravity (COG) of the spar,- a tendon restoring moment (TRM) resulting from a difference in forces exerted on the floater by the respective tendon assemblies, and wherein 0,40 * RMtot< TRM < 0,8 * RMtot.

5. Floating WTG foundation (10) according to any of the preceding claims, wherein the peak shaving spring assembly (30) comprises:- a pre-compressed peak shaving spring (60) which engages a peak shaving stop (61) when an actual force is below at the peak shaver activation force (Fpeak), and- the peak shaving stop (61), wherein the peak shaving stop prevents the pre-compressed peak shaving spring (60) from elongation beyond a maximum length (Lmax) when an actual force in the tendon assembly is below the peak shaver activation force, and wherein when the actual force in the tendon assembly becomes higher than the peak shaver activation force, the peak shaving spring disengages from the peak shaving stop and becomes further compressed.

6. Floating WTG foundation (10) according to any of the preceding claim, wherein the slack prevention spring assembly (31) comprises:- a slack prevention spring (70) which is pre-compressed when an actual force in the tendon assembly is above the lower threshold force,- a slack prevention stop (71) which prevents the slack prevention spring assembly from becoming pre-compressed more than a threshold precompression length when the actual force in the tendon assembly is above the lower threshold force, and wherein when the actual force drops below the lower threshold force, the slack prevention spring (70) extends and becomes active and the slack prevention stop (71) is disengaged, wherein the extending slack prevention spring maintains the actual force in the tendon assembly above zero and prevents the tendon assembly from becoming slack.

7. Floating WTG foundation (10) according to any of the preceding claims, wherein the floater has a vertical circumferential wall and a cross-section which is uniform over the height of the floater, wherein the main longitudinal axis is vertical.

8. Floating WTG foundation (10) according to any of the preceding claims, wherein the circumferential wall of the floater is manufactured from concrete.

9. Floating WTG foundation (10) according to any of the preceding claims, wherein when seen from above the floater has a circular cross-section, and wherein in particular the upper projection also has a circular cross section.

10. Floating WTG foundation (10) according to any of the preceding claims, wherein the floater (14) provides sufficient stability to ensure a positive stability for the floating WTG (1) when in free floating mode with the installed WTG (11) on it, allowing transport of the floating offshore WTG from the production site to the target site.

11. Floating WTG foundation (10) according to any of the preceding claims, comprising:- one or more solid ballast compartments (42) at a lower end (43) of the floater for holding solid ballast, and- one or more water ballast compartments (44) for holding a controllable volume of water ballast.

12. Floating WTG foundation (10) according to any of the preceding claims, wherein the peak shaving spring assembly and the slack prevention spring assembly are integrated into a single mechanical system (46).

13. Floating WTG system comprising the floating WTG foundation (10) according to any of the preceding claims and a WTG (1) installed on the floating WTG foundation.

14. Spring mechanism (25), comprising: a peak shaving spring assembly (30) configured to become active above an peak shaver activation force (Fpeak), the peak shaving spring assembly having a peak shaving spring constant (Kps), a slack prevention spring assembly (31) configured to become active below a lower threshold force (F|OW), the slack prevention spring assembly having a slack prevention spring constant (Ksp), wherein the peak shaving spring assembly and the slack prevention spring assembly are placed in series.

15. Tendon assembly (24), comprising:- 26 - a main tendon section (26) having a main spring constant (Kmain), a peak shaving spring assembly (30) configured to become active above an peak shaver activation force (Fpeak) on the tendon assembly, the peak shaving spring assembly having a peak shaving spring constant (Kps), a slack prevention spring assembly (31) configured to become active below a lower threshold force (F|OW), the slack prevention spring assembly having a slack prevention spring constant (Ksp), wherein the main tendon section, the peak shaving spring assembly and the slack prevention spring assembly are placed in series, wherein the tendon assembly has a total spring constant (Ktot) which is relatively high in a range between the lower threshold force (Flow) and the peak shaver activation force (Fpeak) and is relatively low below the lower threshold force (F|OW) and above the peak shaver activation force (Fpeak)16. Method of manufacturing and installing a floating wind turbine generator (1), the method comprising the steps: a) manufacturing a floating WTG foundation (10) at a production site, comprising the steps: i. making a floater (14) by making a base (15), and forming a concrete circumferential wall (23) of the floater on the base by slip forming, ii. installing or making an upper projection (20), wherein the upper projection extends upward from the floater, iii. providing at least three mounting positions (35) for a tendon assembly (24) on the floater, b) installing a wind turbine generator (WTG, 40) on the upper projection, thereby forming the floating offshore wind turbine generator (1), c) installing at least three anchors (22) and in particular four or more anchors at the target site (62), d) transporting the formed floating offshore wind turbine generator from the production site to the target site, e) installing at least a first, second and third tendon assembly (24) between the respective anchors and respective mounting positions (35) on the floater, wherein steps a) and b) are performed at the production site.

17. Method according to the preceding claim, wherein step e) comprises ballasting the floater of the floating WTG to a deeper water level prior to connecting the tendon assemblies to the floater, and de-ballasting the floater after connecting the tendon assemblies.- 27 -18. Method according to any of the preceding method claims, wherein the tendon assemblies are tensioned at a design force (Fdes), and wherein at the design force a total restoring moment (RMtot) of the floater in case of a roll or pitch movement comprises the components:- a spar restoring moment (SRM) resulting from a vertical distance between the COB and the COG of the spar,- a tendon restoring moment (TRM) resulting from a difference in forces between the respective tendon assemblies, and wherein 40% RMtot< TRM < 80% RMtot.

19. Method according to any of the preceding method claims, wherein each tendon assembly has: a main tendon section (26) having a main spring constant (Kmain),- a peak shaving spring assembly (30) configured to become active only above an peak shaver activation force on the tendon assembly, the peak shaving spring assembly having a peak shaving (PS) spring constant (Kps),- a slack prevention spring assembly (31) configured to become active only below a lower threshold force on the tendon assembly, the slack prevention spring assembly having a slack prevention (SP) spring constant (Ksp), wherein the main tendon section, the peak shaving spring assembly and the slack prevention spring assembly are placed in series, wherein a total spring constant (Ktot) is relatively high in a range between the lower threshold force and the peak shaver activation force and is lower above the peak shaver activation force and below the lower threshold force.

20. Method according to any of the preceding method claims, wherein the floating WTG has a natural frequency for heave, roll and pitch which is lower than occurring wave frequencies at the target site.

21. Method according to any of the preceding method claims, wherein the installation is carried out without a transfer of the WTG or any of its large components, i.e. the mast, nacelle or blades, to the floating WTG foundation by a large crane vessel.

22. Method according to any of the preceding method claims, wherein a temporary additional buoyancy device (115) is connected to the floater (14) at the production site.- 28 -23. Method according to the preceding method claim, wherein the temporary additional buoyancy device (115) has a ring shape, U-shape or a C-shape when seen in top view and extends around the floater when seen in top view.

24. Method according to any of the two preceding method claims, wherein the temporary additional buoyancy device comprises multiple parts which together form a ring shape or C-shape or wherein the temporary additional buoyancy device comprises one or two doors which are movable between an open position and a closed position, wherein in the open position the doors provide access to an interior space and allow the floating WTG foundation to enter or exit the interior space and wherein in the closed position the temporary additional buoyancy device surrounds the floating WTG foundation when seen in top view.

25. Method according to any of the three preceding method claims, wherein the temporary additional buoyancy device comprises a plurality of winches (116) for connection to the floater (14) and wherein the temporary additional buoyancy device is connected to the floater via cables which extend through a gap between the temporary additional buoyancy device and the floater to a lower region of the floater.

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