Floating cranked spar wind turbine

Abstract

A floating wind turbine assembly 1 comprises a buoyant body 2 forming a spar, a cranked tower 4 with a longitudinal axis making an angle α with a longitudinal axis of the buoyant body and supporting a

Description

The present invention relates to a floating wind turbine installation in deep water in excess of 30m utilising spar type floatation device and tower. To date permanent offshore wind turbine installations have been successfully installed in shallow waters with less than 30m depth. Installations of such devices at greater depth however, will generally result in significant technical problems and costs. Development of floating wind turbine solution allows greater exploitation of the sea area whose depths are greater than 30m. In addition, compared to the permanent offshore wind turbines, the floating wind turbines can be uncoupled from the seabed and brought ashore for repair and work-over in a protected environment. This significantly reduces vessel and repair costs. To date several concepts for floating wind farms are being developed, these include a spar, in which an elongated structure is weighted with significant ballast at the bottom of structure and buoyant tanks near the waterline. Stability is achieved by ensuring centre of gravity is lower than the centre of buoyancy, ensuring that the spar is floating upright at all times. The spar is moored to the sea floor with a number of anchor lines that keep the spar in place. Examples include Sveen et al. US7819073B2 and Molins Borrell et al. US9238896B2. The spar type structures have deep draft which requires deep water shelter for work-over, which means turbine installations and repair to date have been carried out at sea similar to permanent offshore wind farms. To overcome this shortcoming several concepts have been developed using various forms of semisubmersibles, examples include Roddier et al. US2014 / 0196654A1. These can be assembled in dry docks and floated to protected pool for installation of tower, generator and blades while parked against the dock to reduce the height of installation. To assist this, towers are usually placed at one edge of the semisubmersible structure. As the height of the towers are increasing to accommodate longer and larger blades for increased power output from 5MW to 20MW, the costs and availability of the cranes and in turn construction cost of such assemblies are increasing exponentially as these items are built and integrated vertically on the flotation device be it submersible or spar. For offshore wind farm to be competitive, the requirements for the production and assembly of the towers should be such that all current offshore fabrication facilities and dock yards can be utilised without special requirement. Historically a similar problem was faced for fabrication of offshore jackets as these were increasingly fabricated for deeper water. A solution devised was to fabricate and transport these jackets on their sides and then upend them at sea at the intended location. This is not possible for the semisubmersibles. The spar type arrangement while in principle can be assembled on its side, would require being either transported in an inclined manner or requires significant flotation requirements as presented in Sant et al. WO2020 / 165892A to ensure that the blades and generator assembly are sufficiently above the waterline to ensure safe transit of the assembly. While the buoyant compartment comprising the spar is sufficiently buoyant to be transferred, it does not have enough freeboard to ensure that the blades and generator assembly are above the water line. The present invention allows the spar to be fabricated on its side and transported horizontally while keeping the generator and the blades to be above the waterline by providing a crank angle between the longitudinal axis of the tower and the buoyant compartment comprising the spar. Where required, the cranked tower carrying the generator assembly can be supported on an inclined truss to further support fabrication, transportation and installation whereby the said truss also acts as a slide to upend said wind turbine assembly at sea to vertical position. According to one aspect of the present invention there is provided a floating wind turbine assembly fixed to seabed by means of mooring line comprising of: a buoyant body forming a spar; a cranked tower with longitudinal axis making an angle a to longitudinal axis of the buoyant body, wherein the said cranked tower supports a generator assembly including blades while allowing it to freely rotate under wind about a vertical axis parallel to the longitudinal axis of buoyant body formed by the spar. In one embodiment, the said buoyant body forming the spar and the cranked tower are all cylindrical and are connected by means of a cylindrical transition piece in which all parts are either comprised of steel plates or reinforced concrete. In a preferred embodiment the said cylindrical parts forming the spar, tower and transition piece are comprised of a sandwich construction of an inner skin plate and an outer skin plate forming permanent formworks with shear studs along their lengths embedded in concrete infill interposing between the said two skin plates where both the inner and outer skin plates and shear studs may be of steel. To facilitate fabrication, the said cylindrical parts may be constructed in longitudinal segments in which the inner and outer skin plates extend beyond the said interposing concrete infill at the end of each longitudinal segment, forming a cavity with projecting reinforcing bars cast into concrete with adequate lap-joint over the said gap, such that the two segments are stitched together by injection of a nonshrink grout into the said cavity to completely fill the said gap. In an alternative embodiment, the said inner and outer skin plates forming the permanent formwork are of Fibre Reinforced Polymer (FRP) composite. The said FRP composite plates may be manufactured by filament winding to form an inner and outer tubular shells that are spaced apart by means of spacer bolts forming shear studs with the interposing gap filled with concrete or fibre reinforced concrete which may be of steel fibres. In a preferred embodiment, the buoyant body forming the spar is provided with a variable ballast tank containing sea water near the final water line when the spar is floated vertically, and a fixed ballast tank at its lowest point containing ballast with density greater than water which may be of liquid ballast that can be removed by means of pumping as presented by Haney et al. US2007 / 0221112A1. In another preferred embodiment, the said buoyant body forming the spar is provided with external crank counter weight housing on the plane perpendicular to the crank plane to receive removable counter weights into the said housing to offset the overturning weight of the tower, generator assembly and blades resulting from the eccentricity caused by the crank angle a. The said counter weight being removable to allow manoeuvring of the floating wind turbine assembly during installation at sea. In a preferred method of fabrication the wind turbine assembly is supported horizontally over pedestals and with cranked tower supported by an inclined truss at ground level at the dock yard and fabrication facility so as to allow construction and assembly of the complete floating wind turbine assembly horizontally. In a preferred method of installation, the floating wind turbine assembly is supported horizontally at sea for transportation on flotation tanks on either side of the buoyant body formed by the spar, with the cranked tower supported on an inclined support truss over a flotation barge and towed by a vessel to appropriate location at sea where it is rotated to vertical position by means of flooding the variable tank with sea water and aided by sliding on the said inclined truss, and completion by installation of fixed ballast and crank counter weights to achieve correct free board in the final position and connection to the seabed via mooring lines. At removal the process is reversed to allow transportation of the said wind turbine assembly to shore. Embodiment of this invention will now be described by way of example with reference to accompanying drawings in which: Fig. 1 is the frontal view of the floating offshore wind turbine assembly. Fig. 2     is the side view of the floating offshore wind turbine assembly. Fig. 3     is the sectional view A-A as indicated in Fig. 1 depicting variable and fixed ballast tanks and cranked counter weight. Fig. 4 is the sectional view B-B as indicated in Fig.2 depicting the cylinder forming the buoyant spar hull and the location of cranked counter weight. Fig.5     is an isometric view of the floating offshore wind turbine assembly. Fig.6      is an isometric detail view of location C as depicted in Fig.5. Fig.7 is the side view of the floating offshore wind turbine assembly supported horizontally on pedestals and inclined support truss at ground level at dockyard and fabrication facility. Fig.8 is the side view of the floating offshore wind turbine assembly supported horizontally on flotation tanks with the inclined support frame supported on flotation barge during transportation at sea. Fig.9 is the side view of the floating offshore turbine assembly at installation with variable tank partially flooded and guided by the inclined truss supported on floatation barge. Fig. 10 is an isometric view of typical longitudinal cylindrical segments forming the spar hull, cranked tower and transition piece. Fig. 11 is the sectional view C-C as depicted in Fig. 10 Fig. 12 Is the sectional view D-D depicting the method of stitching the segments making up the longitudinal cylindrical segments forming the spar hull, cranked tower and transition piece. While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of examples. It should be understood that the examples and drawings are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by pending claims. Figs. 1 & 2 depict frontal and side elevation of the proposed floating wind turbine assembly (1) fixed to the seabed (50) via mooring lines (18) and foundations (19) in which the said assembly is comprised of: - a buoyant body (2) forming a spar; - a cranked tower (4) with longitudinal axis making an angle a with the longitudinal axis of the buoyant body (2) such that a is greater than zero; - cranked counter weight (7) to offset the rotational effect of mass of the items supported above the buoyant body (2) resulted by the eccentricity caused by the crank angle a relative to the longitudinal axis of the said buoyant body (2); - a transition piece (3) connecting the said cranked tower (4) and buoyant body (2) - a generator assembly (5) supporting blades (6), the said generator assembly (5) supported on the cranked tower (4) about a vertical axis (Z) such that the said axis is parallel to the longitudinal axis of the buoyant body (2) forming the spar. The said generator assembly being able to rotate freely about axis (Z) to orient the blades to face wind direction. Figs. 3 presents sectional view A-A as indicated in Fig.1, depicting the location of variable ballast tank (10) containing sea water positioned above fixed ballast tank (11) containing ballast with density higher than water within the buoyant module (2) and location of external cranked counter weight (7) and housing (8). The said ballast contained in the fixed ballast tank (11) may be of liquid ballast to allow removal of fixed ballast at sea as presented in prior art. Fig.4 presents sectional view B-B as indicated in Fig.2, depicting the location of the cranked counter weight (7) external to buoyant body (2) on a plane perpendicular to the crank angle plane. As the buoyant body (2) forming the spar is circular the cranked counter weight (7) is of semicircular form on the outer body of the said buoyant body (2) presented therein. The said cranked counter weight (7) is provided with counterweight housing (8) for accommodating removable counterweights (9) as presented in Figs. 5 and 6. The said counterweights (9) are installed to balance the rotational moment caused by the mass of items supported above the buoyant body (2) resulted by the eccentricity caused by the crank angle a relative to the longitudinal axis of the said buoyant body (2). The counterweights (9) are removable to assist installation and removal of the floating wind turbine assembly (1) at sea. Fig. 7 presents a method of supporting the floating wind turbine assembly (1) at dockyard and fabrication facility at ground level (40) allowing assembly to be carried out in a horizontal position to reduce reliance on tall cranes. The buoyant body (2) is supported on pedestals while the cranked tower (4) is supported on pedestals via an inclined support truss (13). Fig.8 presents a method of transporting the floating wind assembly (1) at sea level (60), wherein the buoyant body (2) may be additionally supported on flotation tanks (14) on both side, with the cranked tower (4) supported on an inclined truss (13) on a floatation barge (15) connected to a vessel (17) by means of tie (16) to allow the whole assembly to be towed by the said vessel (17) to installation location. Fig.9 depicts a method of installation where by the floatation tanks (14) indicated in Fig.8 are removed, and the variable tank (10) is partially flooded to rotate the floating wind assembly (1) towards the vertical position while assisted by inclined support truss (13) acting as a slide. Once said assembly is near vertical position, the fixed ballast tank (11) depicted in Fig. 3 is ballasted, ensuring that floating wind assembly (1) is floating upright with desired free board for the blades (6) above sea level. The crank counter weights (9) as depicted in Fig.6 are installed to ensure correct inclination of the buoyant body (2) and in turn the floating wind assembly (1). Installation is completed by connecting said floating wind assembly to the seabed (50) via the foundations (19) and the mooring lines (18) as presented in Figs. 1&2. In a preferred arrangement at least 3 mooring lines (18) and foundations (19) are provided, spaced radially at 120deg apart about the buoyant body (2). In an alternative installation method, the said crank counter weights (9) are installed at fabrication facility before transportation to reduce vessel time at sea during installation. At removal from the seabed (50), the said process in Fig. 9 is reversed. The mooring lines (18) are removed; the variable tank (10) is unflooded and the ballast is removed from the fixed ballast tank (11) to allow the wind turbine assembly to float and rotate to the horizontal position and the crank tower (4) to be supported on inclined support truss (13) and floatation barge (15). Finally the floatation tanks (14) are attached as per Fig.8 to ensure stability during transfer. To reduce manufacturing time and costs, the said buoyant module (2), transition piece (3) and cranked tower (4) may be of longitudinal cylindrical segments (30), that are stitched together to form the final tubular geometry as presented in Fig. 10. Traditionally these segments (30) are of solid steel plates with bulkheads and stiffeners. The segments are then welded together to form the final tubular geometry. To reduce the quantity of steel as well as the required bulk head and stiffeners, Fig. 11 depicts an alternative method for the manufacture of the segments in which each segment (30) is made up of an inner and outer skin plate (20&21) respectively, forming a permanent formwork with shear studs (23) embedded in the concrete infill (24) filling the interposing space between the said skin plates (20&21) creating a composite steel-concrete sandwich section. In this way the need for internal bulkheads and stiffeners are removed and quantity of steel is reduced, resulting in a more cost effective method of construction. In a preferred embodiment the said skin plates (20&21) and shear studs (23) are of steel. In an alternative embodiment the said skin plates (20&21) can be of Fibre Reinforced Polymer (FRP) composite material with the shear studs (23) formed of stud bolts. The said FRP composite skin plates can be manufactured using filament winding technique. Fig. 12 depicts a method for stitching together the said longitudinal segments (30). To facilitate the stitching of longitudinal segments together the inner and outer skin plates (20& 21) extend beyond the said interposing concrete infill (24) at the end of each longitudinal segment, forming a cavity (26) with projecting reinforcing bars (25) cast into concrete infill (24) with adequate lap-joint over the said cavity (26), such that the two segments are stitched together by injection of a non-shrink grout at injection point (28) into the said cavity (26) and vented at location (29) to completely fill the said cavity (26). In a preferred embodiment the said inner and outer skin plates (20&21) are of steel and welded at (27) to form continuous skin plates that seal the cavity. In an alternative embodiment where the said skin plates are of FRP composite, this is achieved by provision bonded lap joints at these locations (27). 5 In a preferred embodiment, the said concrete infill (24) is fibre reinforced to increase its stiffness and strength in which the fibre reinforcement may be of steel fibres.

Claims

1- A floating wind turbine assembly comprising of a buoyant body forming a spar, and a cranked tower with longitudinal axis making an angle a greater than zero to longitudinal axis of the said buoyant body carrying the generator assembly; a generator assembly supported on to the said cranked tower and rotating freely under wind about a vertical axis parallel to the longitudinal axis of the said buoyant body; and blades connected to the said generator.2- A floating wind turbine assembly according to claim 1, wherein the said buoyant body, cranked tower and the transition piece are cylindrical.3- A floating wind turbine assembly according to claim 1, wherein the said buoyant body is provided with variable ballast tank.4- A floating wind turbine assembly according to claim 1, wherein the said buoyant body is provided with fixed ballast tank.5- A floating wind turbine assembly according to claim 4, wherein the said fixed ballast tank contains liquid ballast with density higher than water.6- A floating wind turbine assembly according to claim 1, wherein the said buoyant body is provided with external crank counter weight on a plane perpendicular to the crank plane of said cranked tower and buoyant body.7- A floating wind turbine assembly according to claim 6, wherein the said external crank counter weight is provided with series of counter weight housing to accept removable counter weights.8- A floating wind turbine assembly according to claim 1, wherein the said complete wind turbine assembly is supported horizontally over pedestals and inclined support frame over ground level at the dock yard so as to allow construction and assembly of the complete wind turbine assembly horizontally.9- A floating wind turbine assembly according to claim 1, wherein the said complete wind turbine assembly is supported horizontally at sea level for transportation on flotation tanks on either side of the said buoyant body, with the said cranked tower supported on inclined support frame over flotation barge with whole assembly towed by a vessel via connection means.10- A floating wind turbine assembly according to claim 1, wherein the said complete wind turbine assembly is rotated to the vertical position by means of flooding of the said variable tank with sea water.11- A floating wind turbine assembly according to claim 1, wherein the said fixed ballast tanks are flooded with ballast as of claim 4 and removable counterweight cylinders are installed as per claim 7 to achieve the correct free board above water level for wind turbine assembly.12- A floating wind turbine assembly according to claim 1, wherein the said complete wind turbine installation is secured to the seabed by means of mooring lines to seabed foundations.13-14-15-16-17-18-19-20-A floating wind turbine assembly according parts are of reinforced concrete.A floating wind turbine assembly according parts are of steel plate.A floating wind turbine assembly accordingto claimto claimto claim2 where2 where2 wherein the cylindricalin the cylindricalin the cylindricalparts are comprised of an inner skin plate and an outer skin plate both carrying shear studs along their lengths embedded in concrete infill interposing between the said two skin plates.A floating wind turbine installation according to claim 15 wherein the inner and outer skin plates extend beyond the said concrete infill at the end of each longitudinal segment to form a cavity with projecting reinforcing bars with ends cast into concrete infill at one end and forming a lap joint over the said cavity on the other, such that the two segments are stitched together by injecting a nonshrink grout to completely fill the said cavity.A floating wind turbine assembly according to claim 15 &16 wherein the inner and outer skin plates are of steel.A floating wind turbine installation according to claims 15 to 17, where in the said inner and outer skin plates are welded together to create a sealed cavity with the exception of the grouting and venting locations.A floating wind turbine assembly, according to claims 15 &16 wherein the said inner and outer skin plates are of Fibre Reinforced Polymer composite.A floating wind turbine assembly according to claim 15 wherein the infill concrete is of fibre reinforced concrete.21- A floating wind turbine assembly as claimed in any proceeding claims wherein the fibre reinforcement to concrete is of steel fibres.

Images