I. BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to an offshore floating wind turbine system and apparatus and to a method for controlling the wind turbine. The floating wind turbine system may utilize dynamic feedback control of aerodynamic control surfaces and dynamic control of the anchor line force vector to counter heeling and other forces and moments acting on the wind turbine. The resultant wind turbine apparatus, system and method reduce the structural mass and hence cost of the floating wind turbine.

B. Statement of the Related Art

Current technology for wind turbines for commercial power production located at sea is limited to towers permanently installed at shallow depths, i.e., less than approximately 30 meters. Large megawatt-sized wind turbines that require fixed installation and a base that extends to the sea floor are costly due to the mass of material and the commensurate construction cost.

Floating wind turbines allow operation in deeper water. For a floating wind turbine supported by a buoyant body that is attached to the sea floor by one or more anchor lines, the wind pushing on the wind turbine tower, rotor, nacelle and other systems above the waterline and hydrodynamic and hydrostatic forces acting on the buoyant body below the waterline may cause the wind turbine to ‘heel;’ that is, to tilt in the downwind direction. Excessive heel may cause the turbine rotor to strike the tower or may reduce the efficiency and capacity factor of the wind turbine. The ‘capacity factor’ of a wind turbine is the actual output of the turbine compared to the total mechanical ability of the wind turbine to generate power, given sufficient wind. All other things being equal, a wind turbine operating at a higher capacity factor is better utilized and more valuable than a wind turbine operating at a lower capacity factor.

When the floating wind turbine heels, a ‘righting moment’ tends to restore the wind turbine to the upright condition. The righting moment for a floating body is determined primarily by a buoyancy vector acting through the center of buoyancy of the floating body and the equal and opposite mass vector acting through the center of gravity, and by the horizontal distance between those two vectors. For a wind turbine floating on a long, thin buoyant body with the center of buoyancy above the center of gravity, as the heel angle increases, the distance between the buoyancy vector and the mass vector increases, increasing the righting moment. For a conventional floating wind turbine, the buoyancy and the mass must both be relatively large and the center of buoyancy and the center of mass must be separated by a relatively large distance to generate a sufficient righting moment to keep the wind turbine generally upright. As a result, the conventional floating wind turbine will extend deep under the water, all requiring a floating buoyant body structure that is relatively large, heavy and expensive.

Wind turbines are broadly divided into horizontal and vertical wind turbines, with horizontal wind turbines divided into upwind and downwind types. In a horizontal downwind wind turbine, the axis of rotation of the rotor is horizontal and the rotor is located on the downwind side of the tower. In a horizontal upwind wind turbine, the axis of rotation of the rotor is horizontal and the rotor is located on the upwind side of the tower. For a vertical wind turbine, the axis of rotation of the rotor is vertical.

What is needed is a floating wind turbine that has a tower and buoyant body that are lighter than land-based designs and where the wind turbine achieves a relatively large capacity factor and extended life due to reduced inertial and other loads, reduced heel, reduced motion and reduced accelerations caused by variable dynamic motions of wind and water.

II. BRIEF DESCRIPTION OF THE INVENTION

The wind turbine of the Invention comprises a long, low mass, thin buoyant body, a tower arranged over the buoyant body, a generator mounted on the tower and contained within a nacelle, and a rotor operably attached to the generator through a rotor hub. Where the wind turbine is a horizontal wind turbine, the nacelle, generator and rotor are rotatable in pitch about a pitch hinge to compensate for heel of the buoyant body and tower. The tower supports the pitch hinge above the center of gravity of the generator structure, which includes the nacelle, generator, rotor hub and rotor. A pitch actuator system under the control of a control system moves the generator structure about the pitch hinge and maintains the rotor axis of rotation in a horizontal condition perpendicular to the movement of inflowing air. Adjustment of the rotor axis of rotation in pitch allows efficient operation of the rotor when the wind turbine heels and improves the capacity factor of the wind turbine as well as load reduction.

The wind turbine may be a horizontal upwind or downwind wind turbine and may be a vertical wind turbine. The Invention applies to any floating wind turbine design where control of heel, motions, and accelerations of the rotor and other wind turbine components is desirable, which applies to all floating wind turbines.

To control heel angle, motions and accelerations of the rotor hub and other wind turbine components, the Invention may utilize either or both of aerodynamic forces working on aerodynamic control surfaces and an anchor line force applied by one or more anchor lines to the buoyant body. Each of these control strategies is discussed below.

Aerodynamic Control Surfaces

The tower or buoyant body supporting the wind turbine may include one or more aerodynamic surfaces in a spaced-apart relation to the vertical axis of the tower. The aerodynamic control surfaces may be spaced equally from the tower. The spaced-apart relation of the control surfaces and the vertical axis of the tower allows the control surfaces to apply a dynamic and static moment around the center of gravity of the wind turbine. The control surfaces may be supported in the spaced-apart relation by a support structure and may be arrayed around the tower. Any number of control surfaces are contemplated by the Invention. For example, control surfaces may be located at four or more locations distributed about the tower axis. Air moving over the control surfaces acts to counter the moments produced by the wind and waves tending to heel the wind turbine and tending to accelerate and move the wind turbine's hub. The control surfaces are active and under the command of a control system. The control system moves the control surfaces as needed to reduce accelerations and in response to disturbing forces.

The control surfaces may comprise box wings composed of both horizontal control surfaces and vertical control surfaces. Each of the horizontal and vertical control surface may be moved independently by an actuator. One of the functions of the vertical control surfaces is to keep the box wing aligned to the wind direction by pivoting the box wing on a vertical axis. Because the control surfaces are distributed around the tower body, the control surfaces can apply moments and forces to the wind turbine in vertical and horizontal directions. The moments and forces applied by the control surfaces to the wind turbine can affect the motion of the wind turbine in any or all of the six degrees of freedom for a floating object; namely, roll, pitch and yaw (rotation in the x, y and z axes) and surge, sway and heave (translation in the x, y and z directions).

Sensors may be located at a spaced apart relation to the tower, such as at the location of each of the box wings, to detect wind speed and direction at the control surfaces. The control system thus may detect a wind gust or acceleration at or near the tower or control surfaces.

The control system described below may select an angle of attack of each of the control surfaces with respect to the wind passing over the control surface and may direct actuators to move the control surfaces to the selected angles of attack to counteract the heeling angle, accelerations and forces acting on the wind turbine. When the detected environmental and other conditions approach allowable operating limits, the control system also may determine that the wind turbine may not operate safely and may command actuators to feather and stop the rotor blades.

The use of aerodynamic control surfaces allows the wind turbine to maintain heel angles within acceptable limits in weather conditions beyond what would be the case without the aerodynamic control surfaces and limits unwanted accelerations to the rotor by wind and wave action. The aerodynamic control features and the anchor line actuators, discussed below, may be used together or separately.

Anchor Line Force Vector

An anchor line, which may be a single anchor line or multiple anchor lines, holds the floating wind turbine in place against the forces of wind and water. The anchor line applies an anchor line force vector to the wind turbine at the buoyant body. The connection of the anchor line to the buoyant body may be adjusted using one or more anchor line actuators. The anchor line actuators change the location of the anchor line force vector with respect to the buoyant body in the vertical and lateral directions. Changing the location of the anchor line force vector can increase the righting moment applied to the wind turbine by the anchor line force vector, decrease the heeling moment applied to the wind turbine by the anchor line force vector, and adjust the motion of the wind turbine in yaw, all as discussed below.

The anchor line force vector has two components—a horizontal component and a vertical component. The horizontal component is equal to the net force of wind and water on the wind turbine and keeps the wind turbine at its selected location. The vertical component is equal to the weight of the anchor line that is not resting on the sea floor, less the weight of water displaced by the anchor line. Because the wind turbine of the invention may operate in deep water, the anchor line may be long and may have substantial weight. The anchor line actuators selectably move the vertical component of the anchor line force vector away from the center of buoyancy of the buoyant body in the upwind direction when the wind turbine is experiencing excessive heel, so that the vertical component of the anchor line force vector applies a righting moment to the wind turbine and reduces heel.

If the anchor line is attached to the wind turbine below the centers of gravity and buoyancy, the horizontal component of the anchor line force vector applies a heeling moment to the wind turbine that is additive to the heeling moment caused by the forces of wind and current. By raising the anchor line attachment location with respect to the centers of gravity and buoyancy, an anchor line actuator may reduce and even reverse the heeling moment caused by the horizontal component of the anchor line force vector.

The anchor line actuator may both raise the anchor line attachment location and move the attachment location away from the center of buoyancy in the upwind direction.

The connection of the anchor line to the buoyant body may comprise a rotatable beam that is movable by the anchor line actuators with respect to the buoyant body to change the location of the anchor line force vector laterally and vertically. The rotatable beam may comprise a single link that is hinged to move about a pin from a first position, in which the anchor line attachment is located proximal to the longitudinal axis of the tower, to a second position in which the anchor line attachment location is located distal to the longitudinal axis of the tower in the upwind direction. The rotatable beam also may occupy any position between the first and second positions. Any suitable mechanism known in the art may be used to move the rotatable beam between the first and second positions, including one or more hydraulic cylinders, cables powered by electric motors, hydraulic motors, or hydraulic cylinders, lead screws turned by electric or hydraulic motors, screw jacks, or by any other suitable mechanism.

Alternately, an extendable beam may comprise a cantilever beam that selectably extends laterally from the buoyant body, as by a hydraulic ram, by a rack-and-pinion, by a screw thread, by the operation of cables on a capstan, or by any means known in the art for extending one object from another object.

An extendable knee may move the anchor line attachment location and may be defined by a scissor linkage having two or more links operated hydraulically, by a screw, or by any other means known in the art, including the means described elsewhere in this document.

As an alternative, the buoyant body may define a track that extends in a lateral direction with respect to the longitudinal axis of the buoyant body. The anchor line may be attached to a shuttle that is selectably movable on the track. The control system controls the location of the shuttle on the track to change the location of the anchor line vector with respect to the buoyant body. The trolley may be moved by a screw thread, by a rack and pinion or other gear train, by cables actuated by one or more hydraulic cylinders or motors, by wedges, by gravity, or by any means known in the art or described in this document.

Movement of the rotatable beam described above may be aided by a ratchet and pawl. In normal operation, the control system keeps the ratchet and pawl in a locked condition so that the rotatable beam does not move. When the control system detects heel angles above a pre-determined threshold, the control system releases the pawl. The heel angle of the buoyant body increases and decreases as the wind and waves ebb and flow. Each time the heel angle decreases, the angle of the anchor line changes with respect to the buoyant body and the pawl advances on the ratchet, moving the anchor line vector with respect to the buoyant body. When the heel angle drops below the pre-determined threshold for a pre-determined period of time, the control system locks the pawl, fixing the rotatable beam in place in the second position. When the control system detects that conditions such as wind speed, wave height and heel angle have subsided consistent with normal operation, the control system releases the pawl and the weight of the rotatable beam and anchor line pull the rotatable beam to the first position.

The rotatable beam either with or without the ratchet and pawl may be equipped with a variable flotation body. When the control system detects heel angles above the pre-determined threshold, the control system may both release the pawl and pump air into flotation chambers on the rotatable beam to assist in raising the rotatable beam. If the buoyancy of the variable flotation body exceeds the weight of the anchor line above the sea floor less the weight of the displaced water, the variable flotation body alone may lift the anchor line and rotatable beam without the use of the pawl. When the rotatable beam is in the raised position, the control system locks the rotatable beam in place, as by a pin inserted by a hydraulic cylinder or solenoid. When the heel angle is below the pre-determined threshold for a pre-determined period of time, the control system locks the rotatable beam in the second position. When the control system detects conditions consistent with normal operation, the control system may release the rotatable beam. The weight of the rotatable beam and the anchor lines return the rotatable beam to the first position.

The adjustable connection of the anchor line to the buoyant body allows the control system to change the location of the anchor line force vector acting on the wind turbine. The adjustable connection may allow the wind turbine to resist wind forces tending to heel the wind turbine and to maintain a heel angle within acceptable operating limits in conditions that would otherwise require shutdown of the wind turbine. Changing the location of the anchor line connection to the buoyant body also allows the wind turbine to operate at a higher torque input than would be the case without changing the location of the anchor line connection.

Because the anchor line is attached to the buoyant body at a single location, the floating buoyant body is free to move passively in yaw like a weathervane in response to the wind and water. Selectably moving the anchor line force vector in the upwind or downwind direction with respect to the centers of buoyancy and gravity allows the control system to passively trim the yaw position of the wind turbine and to reduce or eliminate yawing oscillation of the wind turbine. The result is that the floating wind turbine of the Invention can eliminate the heavy and expensive yaw bearing required by land-based wind turbines. The anchor line vector actuators are effective to restrain the motion of the wind turbine in yaw and to keep the plane of rotation of the rotor orthogonal to the incoming wind and as such to reduce rotor blade flapping and high cyclic loads.

The control system controls the operation of the generator, the rotor pitch, the generator actuator system, the anchor line actuator system and the aerodynamic control surfaces. The control system may autonomously control startup, operation, rotor blade pitch angle, shutdown, and parking of the rotor. The control system may be in communication with a remote operator, as by radio, may receive instructions from the remote operator, and may execute the instructions from the remote operator. The control system may receive information from a remote location, such as weather information and wind turbine location information, and may autonomously act upon that information, as by shutting down the wind turbine and parking the rotor blades in anticipation of a predicted storm.

The control system may detect environmental conditions, such as wind direction and speed, water current direction and speed and wave height. The control system may also detect the heel angle of the tower, heel angle rate of change, the nacelle pitch angle, nacelle pitch angle rate of change, nacelle frequency, the flapping angle of each rotor blade, the deflection of each rotor blade, accelerations acting on the rotor hub, the rotational speed of the rotor, the radial location of the rotor blades, nacelle pitch angle, and any other parameter that a wind turbine operator may find useful. From this information, the control system may select independently the pitch angle of each rotor blade at each radial location as each rotor blade rotates about the rotor hub.

The rotor may be foldable and parkable under the control of the control system, as described by U.S. Pat. No. 7,821,148 issued Oct. 26, 2010, which is incorporated by reference as if set forth in full herein. The blades of the rotor may be constructed as described by U.S. Pat. No. 8,192,169 issued Jun. 5, 2012, and U.S. Pat. No. 8,863,382 issued Oct. 21, 2014, which are incorporated by reference as if set forth in full herein.

The wind turbine of the Invention offers the advantages of reduced construction cost and an increased capacity factor compared to prior art designs. The net result is that the wind turbine uses less material and is hence less expensive than would otherwise be the case.

