DK201670747A1

DK201670747A1 - Floating wind turbine foundation and method for installation of such foundation

Info

Publication number: DK201670747A1
Application number: DKPA201670747A
Authority: DK
Inventors: Henrik Stiesdal
Original assignee: Stiesdal As
Priority date: 2015-09-21
Filing date: 2016-09-21
Publication date: 2017-04-18

Abstract

A floating wind turbine comprising a hull (1) and a wind turbine (2) mounted on top of the hull (1) is described. Also a method for the assembly and a method for the in-stallation is described. The foundation hull (1) is assembled from pre-fabricated components using bolts or other types of threaded fasteners. The nodes of the hull may be made of cast iron, while the braces may be made of steel. Buoyancy may be established or enhanced using pressurized buoyancy tanks (18). The method for the assembly of the floating wind turbine foundation (1) is carried out using bolting or other threaded fasteners. The method for the installation of the floating wind turbine foundation (1) comprises the steps of fitting temporary ballast tanks (38) to the hull (1) prior to towing to the installation site, at the site partly or completely filling the temporary ballast tanks (38) with a ballast material such as sea water to push the hull (1) to a desired draft, attaching the hull (1) to the tethers (5), de-ballasting the temporary ballast tanks (38) so as tofloat the temporary ballast tanks (38) off the hull (1); and owing the temporary ballast tanks (38) away from the hull (1).

Description

Floating wind turbine foundation and method for installation of such foundation Field of the Invention

The present invention concerns a floating wind turbine foundation and a method for installation of such foundation.

More specific, the invention concerns a floating wind turbine foundation being assembled from pre-fabricated components by bolting.

Moreover, the invention concerns a method for the assembly of a floating wind turbine foundation using bolts.

Moreover, the invention concerns a method for the installation of a floating wind turbine foundation using temporary ballast tanks.

Background of the Invention

Traditionally, offshore wind turbines are installed on bottom-mounted foundations in relatively shallow water. A water depth of 40 to 50 m is normally considered the limit for such bottom-mounted foundations.

In many areas of the world, sufficient suitable offshore areas with water depths of 50 m or less are not available for deployment of offshore wind power to the desired extent. Here, floating foundations for wind turbines will be required. A variety of different floating foundation concepts are possible for use with offshore wind turbines. The three primary concepts are spar buoys, semisubmersibles and TLPs (Tension Leg Platforms). Existing or planned offshore floating foundations are based on one of these primary concepts.

Existing or planned floating foundation concepts generally share a number of characteristics - • They tend to be very heavy - 2000-6000 tons for 6 MW class turbines • They typically apply construction methods from the offshore oil and gas sector • Fabrication typically takes place at the port of floater launch • Build times are typically measured in months • The hydrostatic pressure, which tries to collapse any submerged structure where the internal voids are openly connected to the atmosphere and where the internal pressure is therefore equal to atmospheric pressure, is managed with internal braces and/or stringers • Manufacturing typically requires tens of thousands of manual welding hours These characteristics generally lead to very costly structures.

Summary of the invention

It is the object of the invention to provide a floating offshore foundation and a method for installation of such foundation that avoids the above drawbacks.

This object can be achieved through features described below and as further set out in the claims.

In a first aspect the invention relates to a floating foundation for a wind turbine. The foundation comprises a hull assembled from industrially manufactured components by bolting.

In a second aspect the invention relates to the assembly of a floating foundation for a wind turbine using bolts.

In a third aspect the invention relates to the installation of a floating foundation for a wind turbine using temporary ballast tanks.

The design principles of the project aim at reproducing for floating offshore foundations the design and assembly arrangements which have proven to be efficient for the cost reduction of onshore wind turbines.

The following general design principles are applied: • Keep it simple • Apply industrialization concepts with proven effect - modularization, standardization • Apply technologies and concepts generally in accordance with principles proven to be competitive in the onshore wind industry • Target standard quay facilities and standard installation concepts and vessels, avoid special requirements

The following specific design principles are applied: • No fabrication on site - design must allow all components to be delivered to the assembly site by road transportation • No special processes (as defined in ISO 9001) outside factory environment • The foundation to be assembled by bolting • The structural design to target minimizing of bending moments • The structural design to target minimizing of welded stress risers. This is implemented by using casted nodes, and by having no internal welded stringers

The overall structural design is a triangular frame supporting an internal column that can be seen as a continuation of the wind turbine tower below the transition piece.

The triangular frame may be a simple tetrahedron, or a tetrahedral frame “extended” with a lower section in order to establish a double-sided support of the central columns in the buoyancy tank assemblies. Alternatively, it may be e.g. an inverted tetrahedron supported by vertical columns.

The central column can be designed using the same principles as applied for the wind turbine tower - tapered design with varying wall thickness, split into one or more sections as dictated by the overall cost of manufacturing and transportation, sections to be assembled with bolted flange joints.

The braces can be designed as constant-diameter tubes, preferably having constant wall thickness.

All flanges can be oriented towards internal bolt joints. Flanges can be forged “nose-flanges”, i.e. with a forged continuation of the wall so the weld can be executed as a butt weld.

In a preferred embodiment, no welding of brackets or similar are permitted inside or outside the central column and the braces. The only welds permitted are longitudinal butt welds to make rolled plate into cans (tube elements), circumferential butt welds to join cans into tubes, and circumferential butt welds to join tubes with flanges.

Castings can preferably have minimum 30 mm wall thickness. All castings can be machined on the mating surfaces.

All subsea bolt joints can be designed for life, i.e. so as not to require re-torqueing during the design life. As baseline, this can be achieved by dimensioning bolt connections for 85% of lowest probable pretensioning, and by selecting lengths of bolts and bolt extenders so that removal of 10 μ of surface roughness from each surface in the complete bolt assembly will not lead to more than 15% loss of pretension. In addition, the design for life can be verified by suitable testing.

Ancillary systems (platforms, walkways, boat landings, anodes, etc.) can generally be mounted on the cast part of the structure (transition piece, nodes). Such systems may be supported on braces using straps, avoiding the need for welding on braces

The following overall transport and installation arrangements are envisaged: 1. Components are delivered to the assembly area and stored 2. The foundation is assembled at the quay 3. The foundation is lifted into the water 4. The wind turbine is installed on the foundation at the quayside 5. The anchors and tethers are pre-installed 6. Temporary tanks are fitted to the foundation 7. The complete wind turbine and foundation assembly is towed to site 8. The foundation is lowered to the desired depth and is hooked up to the tethers 9. The temporary tanks are floated off and are towed back to the port

The following assembly sequence for the foundation itself is envisaged: 1. The buoyancy tanks are assembled onto the buoyancy tank support 2. The bottom node is placed on the ground 3. The radial braces are bolted to the bottom node 4. The lateral nodes are bolted to the radial braces 5. The central column is bolted to the bottom node 6. The TP is bolted to the central column 7. The lateral braces are bolted to the TP and the lateral nodes 8. The bottom braces are bolted to the lateral nodes 9. The completed tetrahedron is lifted onto the tank assemblies 10. The ancillary systems (platforms, walkways, boat landings, anodes, etc.) are mounted

It is expected that the foundation will be lifted into the water using a mobile crane.

Following installation of the turbine the foundation will be fitted with temporary ballast tanks.

The foundation is lowered to the desired depth by ballasting of the temporary tanks. Ballasting is done with sea water pumped into the tanks.

When the desired depth has been reached the tethers are connected to the lateral nodes.

Following tether connection, the temporary tanks are de-ballasted and floated off the foundation. Then they are towed back to port.

In the following, examples of preferred embodiments are described, visualized in the accompanying drawings, in which:

Figure 1 illustrates a floating wind turbine foundation according to the invention;

Figure 2 illustrates the floating wind turbine foundation depicted in Figure 1 in more detail;

Figure 3 illustrates variants of the floating wind turbine foundation according to the invention;

Figure 4 illustrates variants of the flex joint used as connection to the tethers applied for station keeping of a floating wind turbine foundation according to the invention;

Figure 5 illustrates a gravity base used for station keeping of a floating wind turbine foundation according to the invention;

Figure 6 illustrates an assembly method used to assemble a floating wind turbine foundation according to the invention;

Figure 7 illustrates a set of temporary ballast tanks used to install a floating wind turbine foundation according to the invention; and

Figure 8 illustrates an installation method used to install a floating wind turbine foundation according to the invention.

In the figures, similar or corresponding elements are denoted with the same reference numerals.

Figure 1 shows a floating wind turbine according to the invention. A floating foundation hull 1 supports a wind turbine 2 for electric power production. A set of anchors 3 on the sea bed 4 are connected to the hull 1 using tethers 5.

The wind turbine 2 comprises a rotor 6 for extracting kinetic energy from the wind, a nacelle 7 containing the equipment needed to support the rotor and to convert the rotational energy delivered by the rotor 4 into electric energy, and a tower 8 supporting the nacelle and the rotor.

The floating foundation hull 1 is partially submerged under the water plane 9, and it is kept in position by the tethers 10.

In this embodiment the foundation is functionally a TLP (Tension Leg Platform). It may also be constructed as a semisubmersible, in which case the taut tethers 5 will be replaced with catenary mooring lines.

Figure 2 shows one embodiment of the floating foundation hull 1 in more detail. The floating foundation hull 1 may be implemented as a tetrahedral structure comprising a central column 11, three diagonal braces 12, three radial braces 13, and three lateral braces 14. At the apex of the tetrahedron the three diagonal braces 12 are connected to the central column 11 at a transition piece 15 which also serves to connect the wind turbine tower 8 to the floating foundation hull 1. At the center of the bottom place of the tetrahedron the three radial braces 11 are connected to the central column 9 at a bottom node 16. At each comer of the bottom plane of the tetrahedron the diagonal brace 12, the radial brace 13, and the lateral brace 14 are all connected at a lateral node 17. At each lateral node 17 a set of hull buoyancy tanks 18 are arranged.

The central column 11 and the braces 12, 13 and 14 may be manufactured from steel, using conventional manufacturing methods for tubular and tapered steel members, such as applied in the onshore wind turbine industry.

The transition piece 15, the bottom node 16 and the lateral nodes 17 may be manufactured from nodular cast iron using conventional manufacturing methods for casting and machining of large castings, such as applied in the wind turbine industry.

The buoyancy tanks 18 may be manufactured from e.g. glass-fiber reinforced plastics or from steel. If manufactured from glass-fiber reinforced plastics the manufacturing method may be filament winding; if manufactured from steel the manufacturing method may be conventional manufacturing methods for tubular and tapered steel members, such as applied in the onshore wind turbine industry.

When submerged the buoyancy tanks 18 are subject to an external, hydrostatic pressure, and in order to ensure structural stability it would be normal to reinforce the walls with internal stringers and/or braces. However, in the embodiment shown here the tanks are totally encapsulated and are filled with a suitable gas, e.g. air or nitrogen, which is kept at a pressure that is higher than the external, hydrostatic pressure at all relevant depths of submersion of the buoyancy tanks. In this way the buoyancy tanks can be manufactured as thin-walled structures without any need for internal stringers and/or braces.

Figure 3 shows various embodiments of the hull 1. These embodiments have various relative advantages when it comes to support of the buoyancy tanks 18.

In Figure 3.a, the buoyancy tanks 18 are arranged above the lateral nodes 17, i.e. in the opposite vertical direction of what is shown in Figure 2. This arrangement has the advantage that the connection between the buoyancy tanks 18 and the lateral nodes 17 can be made as compliant or flexible suspensions, thereby avoiding the transfer of bending moments, arising from hydrodynamic forces acting on the buoyancy tanks 18, from the buoyancy tanks 18 to the lateral nodes 17.

In Figure 3.b, the hull 1 has an extra set of radial braces 19 and lateral braces 20, supporting a set of lateral columns 21. The buoyancy tanks 18 are arranged around the lateral columns 21. This arrangement has the advantage that the hydrodynamic forces acting on the buoyancy tanks 18 do not give rise to bending moments in the structure of the hull 1 but are transferred largely as transversal forces.

In Figure 3.c, the hull 1 has a different arrangement of the structural members than shown in Figure 3.b but generally achieves the same advantages. Here the buoyancy tanks 18 are arranged symmetrically around the lateral nodes 17, which further serves to reduce bending moments.

Figure 4 shows various embodiments of the flex joints that are applied to connect the tether 5 to the lateral node 17 and to the anchor 3.

Figure 4.a shows a classical spherical joint. The tether 5 is connected to the inner part 22, and the outer part 23 is connected to the lateral node 17 or to the anchor 5. Flexibility is obtained with an elastomeric element 24 inserted between the inner part 22 and the outer part 23 of the flex joint.

Figure 4.b shows an alternative joint. The joint is made as a gimbal. One part 25 of the gimbal is connected to the tether 5, and another part 26 of the gimbal is connected to the lateral node 17 or to the anchor 5. Flexibility is obtained with an elastomeric element 27 inserted between the one part 25 or 26 and a central, cross-shaped gimbal element 28.

Figure 4.c shows another alternative joint. In this arrangement the tether 5 comprises a set of tension members 29. Each tension member is at either end connected to the lateral node 17 or to the anchor 5 using a gimbal 30 for each tension member 29.

Figure 5 shows a gravity anchor 3 suitable for use as a TLP anchor. The anchor is made as a concrete slab 31 surrounded by a circumferential skirt 32 made of interlocking steel such as used for sheet piling. The concrete slab comprises a base plate 33 and a plinth 34. The skirt 32 is arranged so as to act both as a suction skirt (reaching 0.5 -1.0 m deeper than the bottom of the concrete slab) at the lower side 35 and as a floatation freeboard (reaching minimum 4 m above the top of the concrete slab) at the upper side 36. After installation the anchor can be filled with ballast in the void 37.

Figure 6 shows a mounting sequence for the hull 1.

The foundation shall be assembled on flat, level terrain adjacent to the quayside. The following assembly sequence is envisaged: 1. The buoyancy tanks are assembled onto the buoyancy tank support 2. The bottom node is placed on the ground 3. The radial braces are bolted to the bottom node 4. The lateral nodes are bolted to the radial braces 5. The central column is bolted to the bottom node 6. The TP is bolted to the central column 7. The lateral braces are bolted to the TP and the lateral nodes 8. The bottom braces are bolted to the lateral nodes 9. The completed tetrahedron is lifted onto the tank assemblies 10. The ancillary systems (platforms, walkways, boat landings, anodes, etc.) are mounted

The assembly sequence is illustrated in Figures 6 a-d.

Figure 7 shows temporary a temporary ballast tank set 38 mounted on the lateral node 17 of the hull 1. The ballast tank set comprises a set of tanks 39 mounted on a central column 40.

Figure 8 shows the foundation after it has been towed to the installation site.

In Figure 8.am the foundation is in place. The temporary ballast tanks 38 are not submerged.

In figure 8.b, the foundation has been lowered to the desired depth by ballasting of the temporary ballast tanks 38. Ballasting is done with sea water pumped into the tanks. When the desired depth has been reached the tethers are connected to the lateral nodes.

In Figure 8.c, following tether connection the temporary tanks 38 have been de-ballasted and are floated off the foundation. From here they will be towed back to port.

The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts disclosed. With the benefit of the present disclosure, it will be appreciated that details described with respect to one embodiment disclosed herein can be combined with or used on other embodiments disclosed herein, even though such combination or use may not be explicitly shown or recited herein. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.

Claims

1. A floating wind turbine comprising a hull (1) and a wind turbine (2) mounted on top of the hull (1), characterized in that the foundation hull (1) is assembled from pre-fabricated components using bolts or other types of threaded fasteners.

2. A floating wind turbine according to Claim 1,characterized in that the nodes of the hull (1) are made of cast iron.

3. A floating wind turbine according to any of the preceding claims, characterized in that buoyancy is established or enhanced using pressurized buoyancy tanks (18).

4. A method for the assembly of a floating wind turbine foundation (1), characterized in that the assembly method comprises bolting or otherwise assembling using threaded fasteners the foundation hull (1) from pre-fabricated components.

5. A method for the installation of a floating wind turbine foundation (^.characterized in that the installation method comprises the steps of Fitting temporary ballast tanks (38) to the hull (1) prior to towing to the installation site; Following towing to the installation site partly or completely filling the temporary ballast tanks (38) with a ballast material such as sea water to push the hull (1) to a desired draft; Attaching the hull (1) to the tethers (5); De-ballasting the temporary ballast tanks (38) so as to float the temporary ballast tanks (38) off the hull (1); and Towing the temporary ballast tanks (38) away from the hull (1).