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EP-4232709-B1 - SPAR PLATFORM FOR A FLOATING OFFSHORE WIND TURBINE

EP4232709B1EP 4232709 B1EP4232709 B1EP 4232709B1EP-4232709-B1

Inventors

  • SKAARE, Bjørn

Dates

Publication Date
20260506
Application Date
20211022

Claims (8)

  1. A floating spar platform (7) for supporting an offshore wind turbine, the spar platform (7) comprising: at least one first ballast tank (15) for holding adjustable ballast; and at least one second ballast tank (16) for holding adjustable ballast, wherein the second ballast (16) tank is arranged vertically higher than the first ballast tank (15), characterised in that the first ballast tank (15) is fluidly connected to the second ballast tank (16) through a flow regulating device, and the flow regulating device is arranged such that liquid can flow from the second ballast tank (16) to the first ballast tank (15) under gravity when the flow regulating device is open or partially open.
  2. A floating spar platform (7) according to claim 1, wherein the spar platform (7) comprises one or more pumps (9) arranged to pump ballast into and/or out of the first and/or second ballast tanks (15, 16).
  3. A floating spar platform (7) according to any of claims 1 or 2, wherein the flow regulating device is a control valve (22).
  4. A floating spar platform (7) according to any preceding claim, wherein the bottom of the second ballast tank (16) slopes downwardly towards the flow regulating device.
  5. A floating spar platform (7) according to any preceding claim, wherein the spar platform (7) is arranged such that when the first and second ballast tanks (15, 16) are empty, the centre of gravity (31) of the spar platform (7) is vertically below the centre of buoyancy (30) of the spar platform (7).
  6. A floating spar platform (7) according to any preceding claim, wherein the cross sectional area of the spar platform (7) varies along its length, preferably wherein the cross sectional area of the spar platform (7) decreases from the bottom of the spar platform (7) to the top.
  7. A floating spar platform (7) according to any preceding claim, wherein the spar platform (7) includes an internal compartment, the internal compartment being separated into the first and second ballast tanks (15, 16) by a divider (14) arranged across the internal compartment.
  8. A floating offshore wind turbine (1) comprising a wind turbine mounted on a floating spar platform (7) according to any preceding claim, wherein the wind turbine comprises: a tower (2) mounted on the spar platform (7); a nacelle (3) mounted at the top of the tower (2); one or more rotor blades (6) rotatably mounted to the nacelle (3) by a rotor hub (5); and a generator arranged to be driven by rotation of the rotor hub (5).

Description

The present disclosure relates to the design of a spar platform for a floating offshore wind turbine. Offshore wind turbines can be designed as fixed-foundation wind turbines that are fixed to the sea bed. However, for deeper waters where the wind is often stronger and more consistent, it may be difficult or uneconomical to fix a wind turbine to the sea bed. It has therefore been proposed to provide floating wind turbines in order to enable the wind turbine to be located in deeper waters. One example of a floating wind turbine has a conventional wind turbine structure mounted on a floating foundation, such as a "spar buoy" type structure. A spar buoy is an elongate, typically cylindrically shaped, buoyant structure and provides a stable support for offshore wind turbines. When a floating wind turbine is acted on by forces, such as those caused by changes in wind speed or waves, the whole structure moves about in the water. These motions may have a large amplitude but relatively low frequency, i.e. they are large slow motions. The motions experienced are described as "heave" which is the linear vertical (up/down) motion, "sway" which is the linear lateral (side-to-side) motion, "surge" which is the linear longitudinal (front/back) motion, "roll" which is the rotation of the body about its horizontal (front/back) axis, "pitch" which is the rotation of the body about its transverse (side-to-side) axis, and "yaw" which is the rotation of the body about its vertical axis. Floating wind turbines are typically assembled by mounting a wind turbine onto a spar buoy at an offshore location, i.e. whilst the spar buoy is floating in a body of water. There is a period of time when the spar buoy is floating in the body of water separate from the wind turbine structure, during which the spar buoy can be heavily influenced by wave motions, leading to large undesirable motions of the spar buoy. These motions can make it difficult to install the wind turbine components onto the spar buoy. In order to minimise the wave-induced motion of the spar buoy, it has been common to carry out installation in calm, sheltered waters, such as deep water fjords. In practice, this limits the application of floating wind turbines to regions with access to sheltered waters, where the water is deep enough to accommodate the draft of the spar buoy. WO 2010/120186 A1 discloses a floating platform for supporting an offshore wind turbine. The platform includes a ballast system comprising several ballast tanks and pumps for adding ballast to and removing ballast from the tanks. During installation and maintenance of a wind turbine, the amount of ballast in the tanks is added or removed to lower the floating structure to a desired depth. WO 2010/093259 A2 discloses an offshore wind turbine comprising a vertical turbine unit that can be removably attached to a docking station. The turbine unit includes an elongate shaft having multiple compartments at its lower end which may be filled will sea water in order to regulate buoyancy of the shaft. ES 2524491 A1 discloses a floating spar platform having two ballast tanks positioned at its lower end. A first ballast tank contains concrete and a second ballast tank is provided to hold ballast water. During installation, ballast water is added to the second ballast tank to lower the centre of gravity of the platform and increase its stability prior to anchoring the platform with mooring lines. In a first aspect, the present invention provides a floating spar platform for supporting an offshore wind turbine, the spar platform comprising: at least one first ballast tank for holding adjustable ballast; and at least one second ballast tank for holding adjustable ballast, wherein the second ballast tank is arranged vertically higher than the first ballast tank. The first ballast tank is fluidly connected to the second ballast tank through a flow regulating device, and the flow regulating device is arranged such that liquid can flow from the second ballast tank to the first ballast tank under gravity when the flow regulating device is open or partially open. The phrase "higher than" as used herein means that when a quantity of ballast is held in the two tanks, the second, higher, ballast tank will have a higher centre of mass than the first, lower, ballast tank. This may be achieved, for example, by arranging the second ballast tank vertically above the first ballast tank, or by virtue of the second ballast tank having a bottom or lower end that is arranged higher than the bottom or lower end of the first ballast tank. In the absence of any damping or driving forces, systems will tend to oscillate at their natural frequencies. A system driven by an external force at its natural frequency will show a maximum resonant response, and will oscillate at a higher amplitude compared to when the same force is applied at another frequency. The period of the oscillations corresponding to the natural frequency is known as the