Floating offshore support structure for a wind turbine and its operating method using adjustable ballast tanks

Abstract

Floating offshore support structure for a wind turbine and method of operating the same using adjustable ballast tanks. A semi-submersible offshore support structure (3) for a wind turbine (2) supports adjustable ballast tanks (13) above the sea surface that can be filled with water (16) in order to provide an additional load to a part of the support structure. By adjusting the volume of water (16) in the adjustable ballast tanks (13), the wind turbine (2) can be maintained in a vertical orientation despite the wind pressure. A drain pipe (24) is provided to drain water from the tanks (13) to the sea by gravity only to passively empty the tanks (13) in case of a power failure.

Description

Technical Field

[0001] The present invention relates to an active ballast system for a floating offshore support structure for a wind turbine. Specifically, the present invention relates to a system as described in the preamble of the independent claims and a method of operating the same.

Background Art

[0002] A floating offshore support structure for a wind turbine enables the utilization of offshore wind resources over a much larger area than is possible when relying on bottom-fixed foundations. The International Energy Agency estimates that the introduction of floating offshore structures will increase the total amount of commercially exploitable offshore wind resources tenfold.

[0003] A floating offshore support structure for a wind turbine needs to have sufficient buoyancy and stability to support the wind turbine. It is typically a hull structure made of steel or a concrete shell.

[0004] The most common type of floating offshore support structure for wind turbine support is the semi-submersible type. A semi-submersible vessel typically consists of a plurality of columns positioned at a considerable lateral distance. The center of gravity is above the center of buoyancy, and the stability of the structure when exposed to overturning moments caused, for example, by wind loads on the wind turbine rotor and tower is achieved by the restoring moment of the columns when the structure sinks to various degrees as a result of the loads acting on the wind turbine. The floating structure is maintained in place by a mooring system consisting of catenary or taut mooring lines and drag or suction anchors.

[0005] An example of a monopile structure using mooring lines is disclosed in Korean Patent Publication No. 2010-0057550. During installation, a ballast tank provided on the water surface is used to lower the vertical position of the support structure to install the mooring lines, and then the ballast tank is drained again.

[0006] Chinese Patent Application Publication No. 112319691 discloses the general principle regarding the filling and draining of ballast tanks for a floating wind power platform.

[0007] The restoring moment is a function of the inclination of the floating structure. For example, when exposed to the overturning moment caused by the wind loads of the wind turbine rotor and tower, the floating structure tilts until the restoring moment generated by the submergence of different columns becomes equal to and in the opposite direction of the overturning moment. As a result, a specific overturning moment corresponds to a specific inclination angle.

[0008] The inclination of the floating support structure is accompanied by several disadvantages.

[0009] First, in addition to the overturning moment at the bottom of the tower caused by wind loads, the wind turbine rotor and the tower itself generate an additional overturning moment caused by the weight of the wind turbine nacelle and tower. The tower is usually oriented vertically when the wind load does not affect the structure, and the gravitational load from the weight of the wind turbine usually acts near the center of the tower. These gravitational loads are offset as a result of the inclination, and this offset can result in a significant additional load.

[0010] Second, the axis of rotation of the wind turbine rotor does not have its normal direction but results in an angular displacement corresponding to the tilt angle. In modern wind turbines, in order to achieve sufficient blade clearance with respect to the tower, the axis of rotation is usually inclined by about 5 degrees with respect to the horizontal. The power performance of the wind turbine rotor is a function of the cosine of the lifted tilt angle to the 2.5th power. At a rotor tilt angle of 5 degrees, the power performance is reduced by 0.9% compared to the power performance at a rotor tilt angle of zero. This reduction is taken into account when predicting the annual energy output. However, when the tower tilts as a result of the wind load acting on the wind turbine rotor and the tower, as a result, the reduction in power output becomes slightly larger. For example, with a tilt angle of 5 degrees, a total tilt angle of 10 degrees occurs. At this rotor tilt angle, the power performance is reduced by 3.8% compared to the power performance at a rotor tilt angle of zero and by 2.9% compared to the power performance at a standard rotor tilt angle of 5 degrees.

[0011] Considering these disadvantages, it would be advantageous to take measures to ensure that the tilt of the floating structure is minimized.

[0012] The tilt of the floating support structure can be adjusted by the ballast differential of the structure.

[0013] European Patent Application Publication No. 3366567 and US Patent Application Publication No. 2011 / 37264, both assigned to Principle Power Inc, disclose a floating support structure for a wind turbine in which buoyancy tanks at a certain distance from the tower have variable water contents in order to adjust the structure with the wind turbine to balance when pressure is applied to the wind turbine by the wind. Water is repeatedly pumped between the tanks, and two pumps are provided in each connecting pipe to ensure redundancy in the event of a pump failure. Such redundancy is important to maintain the proper functioning of the system.

[0014] However, in the case of a general power failure, the redundant pump may also stop operating. It would be desirable to provide a safety system for such general power failure events as well.

[0015] In the specification of Chinese Patent Application Publication No. 107685838, a semi-submersible crane platform with a crane near one end of the platform is disclosed. To balance, a ballast system with a plurality of ballast tanks of the tubular columns is used. During crane lifting, to counteract the tilt, water flows by gravity from the upper tank filled on the water surface on the crane side of the platform to the empty lower tank under the water surface on the other side. During load release, to counteract the tilt in the opposite direction, water flows by gravity from the filled upper tank on the opposite side of the platform to the empty lower tank on the crane side. Alternatively, water from the upper tank is discharged by gravity, and water flows by gravity from the outside into the lower underwater tank. The system is configured to flow water once into the empty lower tank on one side during crane lifting and once into the empty lower tank on the other side when releasing the load. The lifting process can be controlled over a period of one hour. After the lifting and release processes, before lifting the next crane, it is necessary to empty the lower tank and refill the upper tank. The valve that determines the flow rate is remotely controlled. This system is not disclosed for dynamic adjustment during wind load fluctuations. Also, no technical solution for the case of power failure is disclosed.

Prior Art Documents

Patent Documents

[0016]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0017] The object of the present invention is to provide an improvement in the art. Specifically, the present invention aims to provide an improvement in the structure and operation of a floating offshore structure for a wind turbine, particularly a semi-submersible structure with ballast tanks at nodes that can be filled with water ballast to various degrees, and by this improvement, for example, when a general power failure occurs, safety is improved against events of pump failure. This object is similarly achieved by an offshore wind turbine system and a method of its operation as described below and in the claims.

[0018] In short, a semi-submersible offshore support structure for a wind turbine is located above the sea surface and supports one or more adjustable ballast tanks that can be filled with water to provide an additional load to each part of the support structure. By adjusting the water volume in the adjustable ballast tanks, the wind turbine can be maintained in a vertical orientation despite the wind pressure. The drainage system is provided to drain water from the tank to the sea by gravity only to passively empty the tank in case of a power failure. [Means for Solving the Problems]

[0019] The system is described in more detail below.

[0020] A floating offshore wind turbine system comprises a wind turbine combined with a semi-submersible support structure for floating on a water surface in a body of water.

[0021] For example, the offshore location is in seawater, but it can also be in a lake or other marine waters. For the sake of simplicity, seawater is exemplified below without distinguishing the general principles of use in other offshore waters such as lake water.

[0022] The semi-submersible floating body support structure includes a tower support for supporting the tower of the wind turbine and includes at least one buoyancy member, and usually includes a plurality of buoyancy members that provide buoyancy to the support structure when normally in water. For example, the buoyancy members are arranged at a lateral distance with respect to the central axis of the tower, usually the vertical axis. Usually, each buoyancy member includes one or more buoyancy columns fixed to the nodes of the support structure. Such buoyancy columns can usually be made of steel concrete or reinforced concrete.

[0023] The buoyancy members and portions of the support structure are optionally configured to be partially fillable with permanent ballast during installation of the system to balance the structure and ensure achieving the desired operating draft. Such permanent ballast is optionally in the form of seawater. Using seawater provides the advantage that the permanent ballast can be removed again. This can be the case, for example, when the floating body structure needs to be towed back to a maintenance port and such a port does not have a water depth required to dock the floating body structure at its operating draft. However, during operation of the wind turbine in the sea, the amount of such seawater of the permanent ballast in the corresponding permanent ballast tank usually does not change. Usually, at least most, if not all, of the permanent ballast seawater is below the water surface for reasons of stability.

[0024] According to the present invention, unlike the optional permanent ballast, the support structure includes at least one tank with an adjustable amount of water as ballast, hereinafter referred to as an adjustable ballast tank as appropriate. The present invention has an effective volume whose effective part is arranged distal to the tower and is configured to be filled with water by gravity to provide an additional load to the support structure as a restoring force against the inclination of the tower with respect to a predetermined direction despite changes in wind pressure.

[0025] Specifically, the adjustable ballast tank has a water-fillable volume or at least an effective part of the volume arranged above the water surface. The effective part is arranged to maintain a position above the water surface when the system is in operation, even in the tilted state of the support structure and the wind turbine tower.

[0026] In this regard, it is necessary to understand that the adjustable ballast tank generally has a water-fillable volume or at least an effective part of the volume arranged above the water surface. As a result, due to a part of the high waves that change the water surface level while passing through the tank, even though the water-fillable volume or at least the effective part of the volume is immediately below the water surface, the function of the system, especially drainage, is maintained. However, on a time-average basis, the volume or at least the effective part of the volume is above the water surface, even though some waves potentially change the state at short moments. What is important in this context is that the water-fillable volume or at least the effective part of the volume is above the water surface long enough to function without significant disruption despite the waves. For example, the volume or at least the effective part of the volume is arranged to maintain a position above the water surface for at least 90% or even further at least 95% of the operating time. In other words, the influence of the passing waves is less than 10% or even further less than 5% of the normal operating time.

[0027] The term "normal operation" is used here to distinguish from abnormal weather conditions such as storms where extremely high waves can cause a large and sudden change in water level in a short time.

[0028] To fill the adjustable ballast tank, the system comprises a suction port for sucking water from the water surrounding the support structure below the water surface, and a pump system for pumping water, such as seawater, through a conduit from the suction port to the adjustable ballast tank when the water area is the sea. A drainage system is provided to drain water back into the seawater of the water area surrounding the adjustable support structure from the adjustable ballast tank, and it is of a floating type.

[0029] A system for repeatedly adjusting the inclination should consider receiving water with a floating structure from a water area and returning the water to the water area. Therefore, for an adjustable ballast tank, the floating body support structure does not require a storage tank. Also, when using such a plurality of adjustable ballast tanks, there is no flow between such adjustable ballast tanks.

[0030] Advantageously, the drainage system is configured to drain water from the ballast tank by gravity alone to passively empty the adjustable ballast tank in case of a power failure and return it to the water surrounding the adjustable support structure.

[0031] In an offshore wind turbine system held by a single point mooring, the support structure itself with the wind turbine is directed towards a downstream location in the wind direction, and the inclination changes only in the wind direction depending on the wind speed. In this case, a single ballast adjustment tank is sufficient to achieve the restoring effect.

[0032] Regarding a fixed system fastened to the seabed by several mooring lines, such a plurality of adjustable ballast tanks are used to appropriately counteract and adjust the inclination, and usually at least three adjustable ballast tanks are used, but in some cases, two adjustable ballast tanks may be sufficient for that purpose.

[0033] The adjustable ballast tanks are distributed at different azimuth angles around the axis of the tower in a plane lateral to the axis to adjust the inclination of the tower in different lateral directions by individually adding or draining water from the ballast tanks. For use as a counterweight, the adjustable ballast tank, or at least its main part, is arranged at a certain distance to the tower.

[0034] In some embodiments, for adjusting the inclination, the plurality of adjustable ballast tanks can be filled individually and independently of each other, and correspondingly, can also be emptied individually.

[0035] For example, the floating body support structure includes a plurality of buoyancy members, and each buoyancy member has, for example, an adjustable ballast tank mounted on its upper part above the sea surface. The adjustable ballast tank can be filled with water in order to provide an additional vertical gravitational load to each buoyancy member of the floating body support structure.

[0036] This additional vertical gravitational load is used to balance the structure and maintain the inclination of the floating body support structure at a predetermined preferred level, for example, at a level that brings the vertical orientation of the wind turbine tower, despite changes in the wind pressure of the wind turbine structure.

[0037] The buoyancy members of the floating body support structure on which the adjustable ballast tank is mounted above the sea surface are optionally columns that provide a restoring moment to the floating body support structure as a result of different immersions of the columns. Other members of the floating body support structure, for example, braces that form a grid structure, can also have adjustable ballast tanks mounted above the sea surface.

[0038] Hereinafter, the system used to fill and drain the adjustable ballast tank is an active ballast system.

[0039] The active ballast system includes one or more pumps for pumping water from the suction inlet through a conduit into the adjustable ballast tank. The suction inlet reaches the seawater around the structure below the water surface.

[0040] Furthermore, each adjustable ballast tank is provided with a drain pipe for draining water from the adjustable ballast tank to the sea outside the support structure by gravity alone. In this way, in the event of a power failure, the adjustable ballast tank can passively empty the water, which also reduces the risk of the system capsizing and is important for returning the wind turbine tower to a safe orientation for access to the system for repair.

[0041] For example, an adjustable ballast tank has a water-fillable volume or at least an effective part of that volume. It is entirely above the water surface and at a certain distance from the water surface when the support structure is oriented at a predetermined preferred inclination angle. Due to the positioning of the effective part of the volume above the water surface, it can be emptied completely or mostly by gravity drainage, and active pump drainage is not necessary for discharge. Advantageously, even in the case of a power failure, the tank always has a volume that is entirely above the water surface or at least an effective part of that volume during offshore operations, even when the structure is inclined, in order to empty the effective part of the volume.

[0042] In these cases, the water-fillable volume or at least an effective part of that volume is advantageously always entirely above the water surface during normal offshore operations and even above typical waves expected during normal operation. Deviations from normal operation are due to the occurrence of unexpectedly high waves under abnormal weather conditions. Such waves can be of low probability or rare, so they are not considered in the sizing of normal operation. This is especially because the water flowing above the effective part under such unexpectedly high wave conditions is for a very short period of time, possibly over or onto the tank, and thus does not substantially affect the overall function of the system, especially the drainage of water. In other words, even in the special case of abnormal weather conditions with particularly high waves that can suddenly and significantly change the water level and even wash out the entire flushing tank, the functional principle of the system is maintained, especially the drainage system, because such extreme waves occur for a relatively short period of time.

[0043] In some embodiments, individual pumps are provided for each of the adjustable ballast tanks, such that each of the single pumps pumps seawater to only one of each of the adjustable ballast tanks.

[0044] In some embodiments, the system has no interconnectivity of adjustable ballast tanks by means of a water conduit. Water does not flow from one tank to another. Water can be filled into an adjustable ballast tank and drained from this adjustable ballast tank independently of other adjustable ballast tanks.

[0045] In some practical embodiments, each ballast tank has a drain pipe that is always open even during filling of the ballast tank. The drain pipe is configured to empty the volume of the adjustable ballast tank or an effective part of its volume within a predetermined time, for example, less than 6 hours, optionally less than 1 hour, provided that seawater is not added to the adjustable ballast tank in the meantime. In a simple version, the drain pipe is an always-open opening and always drains the instant water is present inside the adjustable ballast tank as long as water exists inside the adjustable ballast tank.

[0046] Despite always draining water from the tank, the pump system has the capacity to pump water into the tank at an inflow rate into the tank that exceeds the outflow rate from the tank through the drainage system in order to fill the adjustable ballast tank with water.

[0047] In some practical embodiments, each adjustable ballast tank can drain through the pump when the rotational speed of the pump is less than the speed required to maintain the output pressure of the pump at a level that counteracts the hydrostatic pressure from the water in the adjustable ballast tank.

[0048] Despite the hydrostatic pressure from the water in the adjustable ballast tank, the pump can operate at a speed less than the speed required to maintain the output pressure of the pump at a level that counteracts the hydrostatic pressure from the water in the adjustable ballast tank in order to fill the adjustable ballast tank with water.

[0049] Alternatively, or additionally, an active drain valve can be provided to drain the adjustable ballast tank.

[0050] The active drain valve can be configured as an electrically controlled valve driven, for example, by a solenoid or an electric motor. The active drain valve is optionally configured to actively close under remote control, but is normally open to automatically drain the adjustable ballast tank through the active drain valve in the event of a power failure. For example, an individual active drain valve is provided for each adjustable ballast tank. Alternatively, a single active drain valve is used and all adjustable ballast tanks are connected by conduits through the single active drain valve to empty all the tanks.

[0051] Alternatively, or in addition, the active drain valve can be configured as a pilot-controlled valve to use the pressure from each pump to determine the opening and closing of the valve. When the pump supplies water through a conduit to the adjustable ballast tank, the pressure in the conduit is higher than the hydrostatic pressure from the water in the adjustable ballast tank. This pressure difference can be used to operate the pilot-controlled active drain valve. When the pressure in the conduit is higher than the hydrostatic pressure from the water in the adjustable ballast tank, the pilot-controlled active drain valve is configured to close, and when the pressure in the conduit is below the hydrostatic pressure from the water in the adjustable ballast tank, it is configured to be open. The pilot-controlled active drain valve is optionally located in the adjustable ballast tank, but can be located in any other suitable location.

[0052] During operation, the actual inclination of the floating body support structure is measured by the sensors of the control system. In this regard, the inclinometer is a useful sensor. Optionally, the control system may use an accelerometer in combination or use only the accelerometer. When the inclination deviates from a predetermined preferred orientation, typically the vertical orientation of the wind turbine tower, for example, when at least the first buoyancy member is raised high out of the water and the remaining second buoyancy members are submerged deeper in the water, water is pumped into one or more of the adjustable ballast tanks supported by one or more of the buoyancy members that rise high out of the water to counteract the inclination, and the additional weight of the water pumped into such adjustable ballast tank(s) acts as a counterweight. Alternatively, or additionally, water may be drained from an adjustable ballast tank or tanks supported by one or more members that are submerged deeper in the water, and the reduced weight of the water drained from such adjustable ballast tank(s) acts in the opposite direction of the counterweight.

[0053] The system is generally used for adjustment during normal operation. When the wind increases and the inclination changes so as to deviate from a predetermined optimal inclination, the system is programmed to restore the optimal conditions.

[0054] In some embodiments, the pump operates intermittently at a constant pump speed. Next, the filling rate of water into each adjustable ballast tank is adjusted by stopping and starting the pump.

[0055] In some embodiments, the pump operates intermittently or constantly at a variable pump speed. Next, the filling rate of water into each adjustable ballast tank is adjusted by varying the pump speed and, optionally, also by stopping and starting the pump.

[0056] In some embodiments, the drainage rate is a simple function of the hydrostatic pressure of the adjustable ballast tank and drainage occurs through a fixed drainage pipe, for example, through the drainage hole of the adjustable ballast tank or through the pump.

[0057] In some embodiments, the drainage rate depends on the drainage through the pump and is adjusted by varying the pump operation. Variations in the pump operation can occur by starting and stopping the pump or by varying the speed of the pump.

[0058] In some embodiments, the drainage rate is adjusted by controlling the drainage volume through the active drainage valve. The drainage rate of such an active drainage valve can be directly adjusted, for example, by solenoid control or motor control of an electric active drainage valve, or can be indirectly adjusted by varying the pump pressure to activate a pilot-controlled active drainage valve.

[0059] In some embodiments, a constant operation of the pump is necessary to maintain the inclination of the floating body support structure at a predetermined preferred level. For example, this applies when a fixed drain pipe that is always open is installed in the adjustable ballast tank or when drainage through the pump is applied. Although this may seem like a waste of power at first glance, it should be noted that the power consumption by a pump that is always running is negligible compared to the power production, and is acceptable considering the high operational passive safety achieved by such a simple system. However, more notably, the fact is that a system that constantly adjusts the optimal inclination of the tower results in an increase in power production that far exceeds the power consumption for the pump operation.

[0060] In the following, some numbers are given in a concise form as specific numerical examples of a possible wind turbine system.

[0061] Power required to compensate for leakage · Overturning moment 350,000 kNm · Radius 45 m · Required restoring force 8000 kN · Mass required for restoration m = 800 tons · Head h = 10 m (above sea level) · Potential energy E = mgh = 800,000 kg * 10 m / s² * 10 m = 80,000,000 J · Drainage time T = 6 h = 21,600 s · Net required power (assuming no losses) Po = E / T = 80,000,000 / 21,600 W = 3.7 kW · Total required power assuming 50% total losses P = 2Po = 7.5 kW Empirical rule regarding power output from offshore wind turbines: a. 1 / 3 of the total time has no power generation b. 1 / 3 of the total time linearly increases power from zero to rated power, with the average power equal to 50% of the rated power and the energy output equal to 1 / 3 of the total energy output c. 1 / 3 of the total time is at rated power Energy losses due to ballast · It is assumed that two of the three pumps are always operating · During operation in mode a, there are no losses and no power output · During operation in mode b, there is a 15 kW loss and a 7.5 MW power output = 0.2% · During operation in mode c, there is a 0 kW loss (because the turbine automatically compensates by generating an additional 15 kW of power), and a 15 MW power output · Total losses 0.2% * 1 / 3 = 0.06% Energy gains due to ballast · During operation in mode a, there are no losses and no power output · During operation in mode b, assuming an average tilt angle of 5 degrees and a relationship of the 2.5th power of cos, the power is 2.9% higher · During operation in mode c, there is a 0 kW loss (because the turbine automatically compensates) · Total gains 2.9% * 1 / 3 = 1.0% Total effect of ballast · Total losses 0.2% * 1 / 3 = 0.06% · Total gains 2.9% * 1 / 3 = 1.0% · Resulting net gain 1.0% - 0.06% = 0.9%

[0062] At a power price of 50 EUR / MWh, the resulting power gain corresponds to a monetary value exceeding 300,000 EUR over the project's lifespan.

[0063] The present invention will be described in more detail with reference to the drawings.

Brief Description of the Drawings

[0064]

Figure 1

Figure 2A

Figure 2B

Figure 2C

Figure 3A

Figure 3B

Figure 3C

Figure 4A

Figure 4B

Figure 4C

Figure 4D

Figure 5

Figure 6A

Figure 6B

Figure 7A

Figure 7B

Figure 7C

Figure 8A

Figure 8B

Mode for Carrying Out the Invention

[0065] FIG. 1 shows an offshore wind turbine facility 1. The facility 1 includes a wind turbine 2 and an offshore support structure 3 with a tower support 8. The wind turbine 2 is mounted to operate on the tower support 8, and the wind turbine 2 is supported under offshore conditions by the tower support 8. The wind turbine 2 includes a rotor 5 and a tower 7, and a nacelle 6 connecting the rotor 5 to the tower 7.

[0066] The offshore support structure 3 is a semi-submersible floating offshore structure with buoyancy members 9A, 9B, 9C. The buoyancy members 9A, 9B, 9C assist in maintaining the support structure 3 partially above water and provide stability to the floating structure. An example of the water surface 4 with respect to the vertical extension lines of the buoyancy members 9A, 9B, 9C is shown in FIG. 2A. It is observed that the buoyancy members 9A, 9B, 9C are semi-submerged in water.

[0067] The semi-submersible support structure is typically used with mooring lines (not shown) fastened to the seabed to maintain the support structure 3 in place. As a vertically damping effect of waves on the floating support structure 3, the heave plate 14 extends horizontally from the bottom of the buoyancy members 9A, 9B, 9C.

[0068] The illustrated structure 3 has a tetrahedral shape and includes a first radial brace 11A extending from the lower part of the tower support 8 to the first buoyancy module 9A at the node farthest from the tower support 8, and two further radial braces 11B extending from the lower part of the tower support 8 to each of the other two remaining buoyancy modules 9B, 9C at the nodes on the opposite side of the tower support 8. Further stability is achieved by two additional braces 10A extending from the farthest buoyancy member 9A to the other two buoyancy members 9B, 9C. The two additional side braces 10A form a planar triangular shape and include the two short radial braces 11B and the buoyancy members 9A, 9B, 9C at each of the three nodes of the tetrahedron.

[0069] The term "radial brace" is used with respect to braces 11A, 11B that extend radially away from the tower support 8, and the term "diagonal brace" is used with respect to braces 12A, 12B on the diagonal side of the vertical triangle formed by the tower support 8, one of the radial braces 11A, 11B, and one of the diagonal braces 12A, 12B.

[0070] It is pointed out that the form of the tetrahedral structure is only an example, and the support structure can alternatively be provided as any other suitable structure with a plurality of columns located at a significant lateral distance.

[0071] The tower support 8 is illustrated as a support column with a central cylindrical axis 25 also serving as the central axis of the tower 7. However, the tower support 8 can have other shapes. As shown in FIG. 2A, the tower support 8 extends to a position above the water surface 4 and also has the characteristics of such a floating body support structure type.

[0072] In FIG. 2A, a situation where the wind speed is zero is shown. By the support structure 3 balancing the wind turbine, the tower 7 of the wind turbine 2 becomes vertical.

[0073] In Figure 2B, a situation is shown where the wind indicated by arrow 15 is pushed against the wind turbine 2. Due to the pressure from the wind 15, the tower 7 is tilted away from the wind 15, and while the distal buoyancy member 9A is pushed upward from the water, the two opposing buoyancy members 9B, 9C provided at the nodes closer to the tower support 8 are pushed deeper into the water.

[0074] In Figure 2C, in order to restore the proper balance, water 16 is pumped into the adjustable upper ballast tank 13 provided on the upper part of the distal first buoyancy member 9A. Due to the weight of the water 16 in the effective part 13B of the adjustable ballast tank 13, the distal buoyancy member 9A is pushed downward by the gravity that restores the balance. As a result, the tower 7 achieves a vertical orientation again despite the wind pressure 15. The partial filling and draining of the adjustable ballast tank 13 are continuously performed during operation to assist in balancing the equipment 1.

[0075] When the preferred orientation of the wind turbine tower 7 deviates from the vertical, for example, by several degrees, the system can be correspondingly controlled with respect to such a preferred orientation.

[0076] An adjustable ballast tank 13 is provided on top of the water line 4 for each of the three buoyancy members 9A, 9B, 9C, and using these buoyancy members, when the pressure of the wind 15 or some other force acting on the wind turbine and / or floating structure, such as waves and currents, has a component acting laterally on the tower 7, a balance can be created and maintained. Such a situation is shown in Figure 3.

[0077] Figure 3A shows a situation where the tower 7 of the wind turbine becomes vertical by the support structure 3 balancing the wind turbine.

[0078] In Figure 3B, a situation is shown where the wind is being pushed from the right against the wind turbine 2. Due to the pressure from the wind, the tower 7 is tilted away from the wind, causing one of the buoyancy members 9C to be pushed more upward from the water, while the opposing buoyancy member 9B, also provided at a node closer to the tower support 8, is pushed deeper into the water.

[0079] In Figure 3C, to create balance, water 16 is pumped into the adjustable upper ballast tank 13 provided at the upper part of the third buoyancy member 9C. Due to the weight of the water 16 in the volume of the adjustable ballast tank 13, the distal buoyancy member 9C is pushed downward by the gravitational force that restores balance, and as a result, the tower 7 becomes vertical again. The partial filling and draining of the adjustable ballast tank 13 are continuously carried out during operation to assist in balancing the equipment 1.

[0080] The filling and draining of the adjustable ballast tank are usually carried out under computer control.

[0081] Since all three buoyancy members 9A, 9B, 9C are equipped with adjustable ballast tanks 13, the balance with the vertical tower orientation can be established in any direction regardless of the wind, wave, and water flow acting on the wind turbine 2 and / or the support structure 30.

[0082] Figure 4 shows various configurations of the support structure using adjustable ballast tanks as described in this specification. Figure 4A shows a support structure where the tower support is provided at the corners of a horizontal triangle and the buoyancy members are arranged at the corners of the triangle as a single column. In Figure 4B, the tower support is centrally located between the buoyancy members, and each of the buoyancy members comprises two buoyancy columns. Figure 4C shows the position of the tower support centrally between three buoyancy members arranged in a triangle and connected by a horizontal bottom bar, and the bottom bar optionally acts as a tank for seawater as permanent ballast. In Figure 4D, the support structure is mainly rectangular with obtuse corners. Usually, the buoyancy members extend on the surface and there is permanent ballast below the surface.

[0083] Figure 5 shows an example of controlling the level 17 of water 16 in an adjustable ballast tank 13 supported by a buoyancy member 9A of a semi-submersible support structure 3. It should be noted that the adjustable ballast tank 13 is located above the sea surface 4. The water 16 in the adjustable ballast tank 13 is provided from the surrounding sea through an inlet 18 below the water surface 4, and the seawater is sent to the tank 13 through a conduit 23 by a pump 20.

[0084] In some alternative embodiments, the water 16 can be drained from the tank 13 through the conduit 23 and an active control valve 21, and the control valve 21 is controlled by a motor 22, for example, for the discharge 19 of water back into the sea again. Advantageously, the valve 21 is always an open valve, so that in case of an abnormal current, the valve 21 opens and empties the tank 13.

[0085] In some embodiments, the pump flow rate of the pump 20 is adjusted according to the required amount of water in the adjustable ballast tank 13. In other embodiments, the pump 20 operates constantly, and the valve 21 is adjusted by the motor 22. As a result, it is the adjustment of the valve 21 rather than the adjustment of the pump 20 that determines whether the water 16 is pumped into the tank 13 and the amount of the water 16. This embodiment has the advantage of minimizing electronic control and increasing the robustness of the system.

[0086] In some embodiments, as an additional option or as an alternative to the valve 21, in the event of a power failure, to ensure that the tank 13 is emptied, the tank 13 potentially has a passive drain pipe 24 that is always open and always discharges water 16' from the tank 13. This implies that the pump 20 needs to pump new water 16 into the tank 13 to replenish the drained water 16' and maintain a specific predetermined water level in the tank 13. The water pumping capacity of the pump 20 is higher than the drainage rate of the water 16' through the drain pipe 24, and changing the level of the water 16 in the tank 13, when such a valve 21 is provided, is achieved by adjusting the pump speed and / or adjusting the drainage 19 through the valve 21 during pump operation. Constant pump operation implies that there is always power consumption when there is water 16 in the tank 13. However, with respect to the drain pipe 24 configured to drain the tank 13 within a time frame in the range of 1 to 12 hours, for example within the range of 1 to 6 hours, the additional power consumption due to pump operation is relatively small and is justified compared to avoiding the simplicity of the safety system and the potential risk that mechanical and electrical components may fail and prevent the drainage of the tank and the wind turbine from overturning.

[0087] For example, each adjustable ballast tank 13 is provided with an independent pump 20 with an individual conduit 23, and optionally, each is provided with an individual suction port 18 and an outlet 19. This avoids connections between the tanks 13. In other words, the system has no interconnection of the adjustable ballast tanks 13 by water conduits. Water is not pumped from one tank 13 to another.

[0088] The pump 20 is preferably located on the tower support 8, but this is not strictly necessary. Usually, the pump 20 is located at or below the sea level 4.

[0089] The control and operation of the pump 20 and / or the valve 21 are automatically adjusted, for example, by using an inclinometer. As a further option, an accelerometer is used.

[0090] Referring generally to the reference numerals in FIG. 4, although only a few of them are used in FIG. 6, it can be seen that they are equally applicable. FIG. 6A shows an embodiment in which the pump 21 is used to fill the adjustable ballast tank 13, indicated by the thick arrow, with seawater. As shown in FIG. 6B, the water is drained from the tank 13, flows in the reverse direction through the pump, and returns to the surrounding sea. This occurs when the operation of the pump 21 stops or, alternatively, when the rotational speed of the pump 21 is less than the speed required to maintain the output pressure of the pump against the hydrostatic pressure from the water in the adjustable ballast tank 13.

[0091] Referring generally to the reference numerals in FIG. 4, although only a few of them are used in FIG. 7, it can be seen that they are equally applicable. FIG. 7A shows an embodiment in which the pump 21 is used to fill the adjustable ballast tank 13, indicated by the thick arrow, with seawater. As shown in FIG. 7B, the water is drained from the tank 13 and returns to the surrounding sea through the drain pipe 24 provided in the tank 13. Also, when the pump 21 is in operation, the drain pipe is always open. Therefore, when it is necessary for the water to rise in the tank 13, the pump needs to pump water into the tank 13 at a speed higher than the drainage speed through the drain 24, as shown in FIG. 7A. To maintain the water level in the tank 13, the pump 21 needs to operate at a speed equal to the drainage speed, as shown in FIG. 7C. In this embodiment, referring to FIG. 7B, a passive drain 24 provided as a drain opening that is always open is shown, and it should be noted that the effective volume 13B of the adjustable ballast tank 13, which can drain water only up to the lower edge of the opening of the drain 24, is defined by the lower edge of the opening of the drain 24 so that water can be drained to adjust the ballast. However, the drain pipe 24 can be arranged at various levels as required, for example, provided at the bottom 13A of the tank 13.

[0092] Referring generally to the reference numerals in FIG. 4, although only a few of them are used in FIG. 8, it can be seen that they are equally applicable. FIG. 8A shows an embodiment in which water is drained from the tank 13 and returns to the surrounding sea through the drain pipe 24 provided in the tank 13. As shown in FIG. 8B, also, when the pump 21 is operating, the drain pipe 24 is always open. In order to maintain the water level in the tank 13, the pump 21 needs to pump at a rate equal to the drainage rate through the drain pipe 24, as shown in FIG. 8B. Optionally, the pump speed of the pump 20 is adjusted by adjusting the speed of the pump 20 in accordance with a high or low drainage rate through the drain pipe 24. Alternatively, the pump 21 is started at a constant speed, and the rate at which water is added to the tank 13 is adjusted by the valve 26, which is connected such that fluid flows not only to the tank 13 but also to an additional drain pipe 24', so that seawater pumped by the pump 21 but not flowing into the tank 13 is again discharged to the sea. The additional drain pipe 24' optionally comprises a valve that can also be used to drain the tank faster than through the drain pipe 24 alone.

Claims

1. An offshore wind turbine system (1) comprising a wind turbine (2) combined with a semi-submersible support structure (3) configured to float on a water surface (4), wherein the support structure (3) comprises a tower support (8) for supporting a tower (7) of the wind turbine (2), and at least one buoyancy member (9A, 9B, 9C) for providing buoyancy to the support structure (3), and the support structure comprises at least one adjustable ballast tank (13) having a water-fillable volume with an effective portion (13B) of the volume, the effective portion (13B) of the volume being located distal to the tower and configured to receive water by gravity as a restoring force against the inclination of the tower (7) with respect to a predetermined direction despite changes in wind pressure (15) to provide an additional load to the support structure (3). The effective portion (13B) of the volume of the adjustable ballast tank (13) is arranged above the water surface (4) so as to maintain a position above the water surface (4) during operation even in an inclined state of the support structure (3) and the wind turbine tower (7), and the system (1) comprises a water inlet (18) for sucking water from the water surrounding the support structure (3) below the water surface (4) during operation, and a pump system (22) for pumping water from the inlet (18) through a conduit (23) to the effective portion (13B), and the system (1) comprises a drainage system (19, 20, 21, 22, 24) configured to drain water from the effective portion (13B) and passively return it to the water surrounding the support structure (3) by gravity only in case of a power failure. An offshore wind turbine system (1) characterized by this.

2. The system according to claim 1, wherein the effective portion (13B) of the volume is, regardless of inclination, in normal offshore operations, for at least 90% of the time, optionally always, completely above the water surface (4).

3. The system according to claim 1 or 2, wherein the water-fillable volume of the ballast tank (13) is always completely above the water surface (4) in normal offshore operations, regardless of inclination.

4. The drainage system (19, 21, 22, 24) comprises a drain pipe (24) in the ballast tank (13), the drain pipe (24) being always open even during filling of the ballast tank (13), the drain pipe (24) being configured to empty the effective part (13B) of the volume of the adjustable ballast tank (13) in less than 6 hours when no seawater is added to the ballast tank (13), the pump (20) having a capacity for pumping water into the ballast tank (13) at a pump speed exceeding the drainage speed in order to fill the ballast tank (13) despite drainage from the ballast tank, the system according to any one of claims 1 to 3.

5. The drainage system (19, 20, 21, 22, 24) comprises an actively controlled drain valve (21) for draining the ballast tank (13) through the drain valve (21), the drain valve (21) being configured to close during filling of the adjustable ballast tank (13), but normally open for automatic drainage from the ballast tank (13) through the drain valve (21) in the event of a power failure, the system according to any one of claims 1 to 4.

6. The pump system comprises a pump (22) having a reverse flow function as part of the drainage system (19, 20, 21, 22, 24), configured to reverse the flow of water from the ballast tank (13) and flow it in the reverse direction through the pump (22) back to the water surrounding the support structure (3) when the pump (22) is not in operation, the system according to any one of claims 1 to 5.

7. The support structure comprises a plurality of ballast tanks (13) that can be individually and mutually independently filled by the pump system, and water can be individually drained from the ballast tanks (13) by the drainage system (19, 20, 21, 22, 24). The ballast tanks (13) are distributed at different azimuth angles about the axis (25) of the tower in a plane lateral to the axis (25) in order to adjust the inclination of the tower in different lateral directions by reciprocally adding or draining water individually to or from the ballast tanks (13). The system according to any one of claims 1 to 6.

8. The system according to claim 7, comprising at least three of such adjustable ballast tanks (13) without water flowing from one of the adjustable ballast tanks (13) to the other.

9. The pump system comprises individual pumps (20) for each of the ballast tanks (13), and each of the individual pumps (20) pumps water from the water surrounding the support structure (3) to only one of the ballast tanks (13). The system according to claim 7 or 8.

10. The drainage system comprises individual drain pipes (19, 20, 21, 22, 24) for each of the ballast tanks (13), and each of the drain pipes (19, 20, 21, 22, 24) is configured to drain water from only one of the ballast tanks (13) to the water surrounding the support structure (3). The system according to any one of claims 7 to 9.

11. The support structure (3) comprises a plurality of buoyancy members (9A, 9B, 9C) that provide buoyancy to the support structure (3). The buoyancy members (9A, 9B, 9C) are arranged at a constant distance from nodes of a polygonal configuration and with respect to the tower support (8), and each of the buoyancy members (9A, 9B, 9C) supports one of the adjustable ballast tanks (13). The system according to any one of claims 7 to 10.

12. The system according to claim 11, wherein each adjustable ballast tank (13) is provided above the corresponding buoyancy member (9A, 9B, 9C). **Claim 13** A method for operating an offshore wind turbine system (1) according to any one of claims 1 to 12, the method comprising measuring the inclination of the tower (7) deviating from a predetermined preferred orientation, for example a vertical orientation, and counteracting the inclination by pumping water from the water surrounding the support structure (3) into the ballast tank (13). **Claim 14** The system comprises a plurality of such adjustable ballast tanks (13), the adjustable ballast tanks (13) being fillable individually and independently of each other and arranged to empty individually, the adjustable ballast tanks (13) being distributed at different azimuth angles about the axis (25) of the tower (7) in a plane lateral to the axis (25), the method comprising adjusting the inclination of the tower (7) in different lateral directions by reciprocally adding or draining water to the adjustable ballast tanks (13) individually. The method according to claim 13. **Claim 15** The support structure (3) comprises a plurality of buoyancy members (9A, 9B, 9C) that provide buoyancy to the support structure (3), the buoyancy members (9A, 9B, 9C) being arranged at a constant distance from nodes of a polygonal configuration and from the tower support (8), each of the buoyancy members (9A, 9B, 9C) supporting one of the adjustable ballast tanks (13), the method comprising measuring the inclination of the tower (7) that is deviated from a predetermined preferred orientation, by raising at least a first buoyancy member (9A) of the buoyancy members (9A, 9B, 9C) higher out of the water and sinking a second member of the remaining buoyancy members (9B, 9C) deeper into the water, counteracting the inclination by pumping water into the adjustable ballast tank (13) supported by the first buoyancy member (9A) or only by gravity draining water from the adjustable ballast tank (13) supported by the second buoyancy members (9B, 9C) or by both pumping the water and gravity draining the water, according to the method of claim 14.

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