Patent Description:
More specific the invention relates to a floating offshore support structure, the support structure comprising a plurality of interconnected tubes forming a rigid frame with vertices arranged as a polygon in a horizontal plane, wherein a buoyancy module is mounted to the frame at each of the vertices.

More specific the method relates to assembling a floating offshore support structure, wherein the support structure comprising a plurality of interconnected tubes forming a rigid frame with vertices arranged as a polygon in a horizontal plane, and wherein the method comprises mounting a buoyancy module to the frame at each of the vertices.

Some types of support structures for offshore wind turbines comprise a monopile or multiple columns that are inserted into the sand of a seabed. A typical connection between the monopile and the tower of the wind turbine or between the multiple seabed-imbedded columns and a support frame for the tower comprises a grouted connection between vertically arranged coaxial tubular members, where shear keys are used for providing long-term mechanical stability after hardening of the grout. The frames are often welded structures with multiple tubes arranged as a grid, which are assembled and welded on-shore and then transported to the offshore location.

As an alternative for seabed-grounded offshore wind turbines, floating support structures are provided which often comprise a largely tetrahedral frame where buoyancy modules are provided at the three nodes, and the wind turbine tower is placed on top of a central support column. Examples are disclosed in <CIT>, in this case, tetrahedral frames are assembled and welded on-shore and then transported to the offshore location.

The assembly of the final floating support structure with the buoyancy tanks, however, is challenging, as the tanks are large and add to the size, weight, and complexity. It is therefore desired to find useful methods for such assembly.

<CIT> describes an assembled UHPC pipe-box combination floating structure, including three kinds of prefabricated component of UHPC pipe, UHPC box and UHPC sleeve, form through grafting, welding, gluing. On the basis of the assembled UHPC pipe-box, an ocean operation platform, an ocean net cage, a shoal floating bridge, an underwater sightseeing platform, an ocean leisure platform and the like can be further built.

<CIT> describes a floating offshore support structure which comprises a plurality of interconnected tubes forming a rigid frame with vertices arranged as a polygon in a horizontal plane. The structure comprises a floating body element being mounted to the frame at each of the vertices. There is no disclosure of a sleeve cooperating with a connection member for mounting of the floating body element to the frame.

It is an objective to provide an improvement in the art. In particular, it is an objective to provide an alternative and improved construction and assembly method for a floating offshore support structure, in particular for a wind turbine. These and other objectives are solved by a floating offshore support structure, its assembly method and use as well as a precursor frame structure, as explained in the following.

The floating offshore structure according to the invention is peculiar in that each of the vertices of the frame is provided with a tubular sleeve segment having inserted therein a tubular connection member extending through the sleeve segment, wherein the tubular connection member is secured to the sleeve segment by a portion of pourable but hardened casting material in a void between the tubular sleeve segment and the tubular connection member, wherein a buoyancy module is secured to each of the connection members.

The method according to the invention is peculiar in that he method comprises providing each of the vertices of the frame with a sleeve segment and inserting a connection member through the sleeve segment and securing the connection member to the sleeve segment by filling a portion of pourable casting material, optionally grout, into a void between the sleeve segment and the connection member and hardening the portion and securing a buoyancy module to each of the connection members.

In contrast to fastening buoyancy modules to a support frame by welding or bolting, which is traditionally used in this technical field, the attachment provided by the present invention is based on casted, especially grouted, connections, which eases assembly and results in long-lasting connections even under harsh offshore conditions. The casting method for attaching buoyancy modules to an offshore frame, especially with fine grout, between horizontally aligned coaxial tubular members at vertices of a polygonal frame structure, is a deviation from traditional fastening methods in this field, despite grouted connections being known in general for vertically oriented offshore structure.

A specific advantage achieved by the invention is the reduction of the requirements for frequent inspection as compared to all-steel connections, such as the mentioned bolted and welded solutions. The advantages of the present method are particularly relevant in relation to floating offshore wind turbines in areas with large sea depths.

The floating offshore support structure comprises a plurality of interconnected tubes forming a rigid frame with vertices arranged as a polygon in a horizontal plane, for example a triangle with three vertices. After having produced a precursor frame structure, buoyancy modules are attached to the frame at each of the vertices.

As an example, a precursor frame structure may comprise a rigid frame with a plurality of rigidly interconnected tubes forming a tetrahedral structure with three vertices arranged as a triangle in a horizontal plane and a with a support member as part of the tetrahedral structure and configured for supporting a tower of the wind turbine.

Each of the vertices of the frame are provided with a sleeve segment preferably a tubular sleeve segment into which a connection member, preferably a tubular connection member is inserted. The connection member is secured to the sleeve segment by filling a portion of pourable casting material into a void between the sleeve segment and the connection member, after which the material portion is hardened. Once, hardened, the connection member is rigidly fixed to the sleeve segment. Each of connection members at the vertices is then ready for attachment of a buoyancy module.

Potential casting materials are cement-based casting materials, such as grout with very fine sand or other filler material, but typically without stones. Un-sanded grout is a good candidate for the purpose. An alternative casting material is long-term weather-resistant hardening polymer.

Advantageously, in order to provide a rigid mechanical connection between the buoyancy module and the connection member, the sleeve segment has an inner wall provided with shear keys, and/or the connection member has an outer wall provided with other shear keys, and the casting material is provided in the void between the shear keys and the other shear keys for securing the sleeve and the connection member by the hardened casting material against movement relatively to each other. Shear keys are in particularly useful when the casting material is shrinking during hardening.

In practical embodiments, the connection member is longer than the sleeve segment and has a first free part extending a distance out of a first end of the sleeve segment to which the buoyancy module is attached by taking up the free part of the connection member in a further sleeve segment that is attached to the buoyancy module. Similarly, to the securing of the connection member to the sleeve segment of the frame, the tubular sleeve segment is secured to the free part of the connection member by filling a further portion of the pourable casting material into void therein between, which is then hardened.

In the case that the buoyancy module comprises multiple buoyancy members, the connection member comprises a first free part and a second free part that extend a distance out of one of two opposite ends of the sleeve segment. In this case a first buoyancy member, which has attached thereto a first further sleeve segment, is attached to the first free part by taking up the first free part in the first further sleeve segment. Similarly, a second buoyancy member, which has attached thereto a second further sleeve segment, is attached to the second free part by taking up the second free part in the second further sleeve segment. Further portions of pourable casting material are filled into the voids and hardened between the further sleeve segments and the free parts.

If each buoyancy module has more than two buoyancy members, it is also possible to attached more than one buoyancy member at each of the free parts of the connection member.

Advantageously, in order to provide a rigid mechanical connection between the buoyancy members and the connection member, each sleeve segment has an inner wall provided with shear keys, and each free part to which a sleeve segment is mounted has an outer wall provided with other shear keys, and the casting material is provided between the shear keys and the other shear keys for securing each sleeve segment and the connection member by the hardened casting material against movement relatively to each other.

In case that the casting material has a relatively low viscosity, it is an advantage to seal the void between the connection member and the sleeve segment with a seal at either end of the sleeve segment for preventing the casting material from escaping the void during the filling. This is especially useful when the coaxial arrangement has a horizontal longitudinal axis. Similarly, seals may be used for securing proper filling of the casting material into the other voids between the connection member and the further segments.

For example, the seal is an inflatable torus, which is arranged around the connection member and inflated prior to the filling of the casting material into the respective void. The inflation of a torus around the connection member and inside the sleeve segment assists the centration and coaxial alignment of the connection member inside the segment, as well as coaxial alignment of the further segments with the connection member when the buoyancy module is attached to the precursor frame structure.

For offshore wind turbines, triangular arrangements of buoyancy modules are often used. A useful precursor frame structure comprises a rigid frame with a plurality of rigidly interconnected tubes forming a tetrahedral structure with three vertices arranged as a triangle in a horizontal plane and a with a support member as part of the tetrahedral structure and configured for supporting a tower of the wind turbine. Each of the vertices comprises a sleeve segment and a connection member extending through the sleeve segment. As described in more detail above, the connection member is secured to the sleeve segment by a pourable but hardened casting material in a void between the tubular sleeve segment and the connection member. The connection member is coaxially aligned with the sleeve segment in a horizontal plane. As the connection member is longer than the sleeve segment, it has a free part extending a distance out of an end of the sleeve segment, typically both ends of the sleeve segment. Each free part is configured for securing a buoyancy module. However, for ease of transport and storage, the vertices of the precursor frame structure are still free from buoyancy modules, until the precursor is prepared for leaving to be transported to the offshore site. At that stage, the free parts of the three connection members would get buoyancy module attached, as already described above.

For non-triangular polygonal arrangements of the vertices, more than three buoyancy modules are provided and one of these correspondingly attached to each one of the more than three vertices.

For example, the buoyancy members are buoyancy columns.

Typical diameters for the connection member are in the range of <NUM>-<NUM> meters, optionally <NUM>-<NUM> meters.

Typically, the segments have a diameter <NUM>-<NUM>% larger than the diameter of the connection member in order to provide a sufficiently large void for creating strength of the casted connection, especially when using shear keys.

For example, the floating support structure comprises a support column configured to support a load, such as a wind turbine, and from which a plurality, for example three, braces extend radially to the vertices of the polygonal frame.

Embodiments and further details of the invention are described in the following with reference to the figure, wherein:.

<FIG> shows an exemplary embodiment of a floating support structure <NUM>, typically for offshore use. The support structure <NUM> comprises three buoyancy modules <NUM>, one module <NUM> in each one of three corner nodes <NUM> of a frame <NUM>. The frame <NUM> is exemplified as having three corner nodes <NUM>. Alternatively, the frame <NUM> has more than three nodes <NUM> in a polygonal arrangement. The frame <NUM> is typically made of hollow steel tubes <NUM>, <NUM>, <NUM> that assist the buoyancy modules <NUM> in increasing the total buoyancy of the support structure <NUM>.

Each buoyancy module <NUM> is exemplified as comprising a pair of two buoyancy members 2A, 2B. Alternatively, each module <NUM> comprises only one or more than two buoyancy members. The buoyancy members 2A, 2B are typically air-filled buoyancy tanks. In the following, these are exemplified as cylindrical columns, but they may have alternative shapes.

The frame <NUM> comprises three main braces <NUM>, each one of the main braces <NUM> extending from a support element <NUM>, exemplified as a support column, to only one of the nodes <NUM>. An end <NUM> of each of the main braces <NUM> is fastened to the buoyancy module <NUM> by a connection <NUM>. In the present embodiment the two buoyancy members 2A, 2B are arranged symmetrically on either side of the corresponding end <NUM> of the main brace <NUM>. The connection <NUM> extends horizontally through both buoyancy members 2A, 2B and through the end <NUM> of one of the main braces <NUM>.

The support structure <NUM> is useful for supporting an offshore wind turbine, as illustrated in <FIG>, in which case the support element <NUM> is supporting the tower of the wind turbine <NUM>. In the exemplified support structure <NUM> of <FIG>, a platform <NUM> is shown, which could have a different size, and which could also be used in the case that a wind turbine <NUM> is supported by the structure <NUM>, as illustrated in <FIG>.

The support element <NUM> is potentially centered between the buoyancy modules <NUM>. However, this is not necessary, and the example in <FIG> shows a de-centered support element <NUM>.

At its lower end <NUM>, each buoyancy member 2A, 2B, exemplified a buoyancy column, is provided with a dampening plate <NUM>, which extends laterally relatively to a longitudinal axis X1 through the buoyancy member 2A, 2B.

The frame <NUM> further comprises a number of support braces <NUM>, <NUM> for increased rigidity between the support element <NUM> and the buoyancy modules <NUM>. Illustrated is a tetrahedral frame structure. As the support column <NUM> is not central, the formed tetrahedron is not a regular tetrahedron.

The connection <NUM> that connects the buoyancy members 2A, 2B with one of the main braces <NUM> is tubular and explained in more detail in the following.

<FIG> shows the floating support structure of <FIG>, wherein a cross sectional cut has been made vertically through the connection <NUM> and the buoyancy members 2A, 2B for illustrating the internal structure of the connection <NUM>. The part indicated in the stippled rectangle is shown enlarged in <FIG>.

The connection <NUM> comprises a segmented cylindrical tubular sleeve <NUM> with three sleeve segments, namely a first sleeve segment 12A that is fastened, typically welded, to the first buoyancy column 2A, a second sleeve segment 12B that is fastened to the second buoyancy member 2B, and a third central sleeve segment 12C that is fastened to the end <NUM> of the corresponding main brace <NUM>. A connection member, here shown as a cylindrical connection member <NUM> extends through all three axially aligned sleeve segments 12A, 12B, 12C. As the outer diameter of the cylindrical connection member <NUM> is smaller than the inner diameter of the tubular sleeve <NUM>, an interspace is formed which is filled with a grout material <NUM>. Three portions 16A, 16B, 16C of the grout material <NUM> are provided, namely one portion for each of the segments 12A, 12B, 12C.

<FIG> illustrates a step of the assembly of the support structure <NUM>. The end <NUM> of the main brace <NUM> has been provided with its sleeve segment 12C, and the cylindrical connection member <NUM> has been fastened inside the sleeve segment 12C by the corresponding portion 16C of the casting material, especially grout material <NUM>. As the connection member <NUM> is longer than the sleeve segment 12C, free parts 15A, 15B extend outside the opposite ends <NUM> of the sleeve segment 12C.

The grout material <NUM> is potentially provided with a relatively low viscosity and without sand or stones. In order for it to be filled into the interspace between the sleeve segment 12C and the connection member <NUM>, seals, of which an example is illustrated in <FIG>, are arranged around the connection member <NUM> to fill the interspace in the void at the ends <NUM> of the sleeve segment 12C. For example, such seals are inflatable torus-shaped tubes, which are comparable in principle with inflatable tire tubes, only much larger, for example in the range of <NUM>-<NUM> meters in diameter.

In the situation illustrated in <FIG>, the sleeve segment 12C and the connection member <NUM> are coaxially aligned in a horizontal direction. At this stage of the construction, the unfinished support structure may be transported as a precursor to the final assembly location, where the buoyancy modules <NUM> are fastened to the free parts 15A, 15B of the connection member <NUM>.

In <FIG>, the space <NUM> of such seal is seen between adjacent portions 16B, 16C of the grout material <NUM>. A toroidal seal <NUM> that is useful at the ends <NUM> in such space <NUM> is illustrated in <FIG>.

As the cylindrical connection member <NUM> is secured to the end <NUM> of the main brace <NUM>, the assembly with the buoyancy members 2A, 2B is simplified. These are pushed with their further sleeve segments 12A, 12B onto the first and second end parts 15A, 15B of the already fixed connection member <NUM> from either end. After positioning and adjusting the orientation of the buoyancy members 2A, 2B relatively to the main brace <NUM> with the third segment 12C at its end <NUM>, the corresponding portions 16A, 16B of the grout material are filled into the voids between the connection member and the respective segments 12A, 12B. Alternatively the first buoyancy member 2A is positioned and adjusted at the first end 15A of the connection member where after grout is filled into the void between the first further sleeve segment 12A and the connection member <NUM> and hardened to secure the first buoyancy member 2A to the connection member <NUM>, where after the second buoyancy member 12B is attached in similar steps.

The seals that were provided in the space <NUM> is typically left, and corresponding seals are provided at the opposite ends of the sleeve <NUM> when the casting material, especially grout material, is pumped into the void between the corresponding further segment 12B, 12C and the connection member <NUM>. After hardening, all three segments 12A, 12B, 12C are rigidly secured to the connection member <NUM>.

Typically, the cylindrical connection member <NUM> is a hollow tube, advantageously steel tube. If the ends of the hollow cylindrical connection member <NUM> are tightened, the inner void is providing additional buoyancy.

Typically, the grout material <NUM> itself does not provide sufficient rotational and axial stability between the sleeve <NUM> and the connection member <NUM>. However, by using shear keys, which are provided on the outer side of the connection member <NUM> and to the inner surface of the segments 12A, 12B, 12C of the sleeve <NUM>, a high degree of stability is achieved. This is illustrated in <FIG>, which is a simplified cross-sectional view to illustrate the principle. For example, first shear keys 22A that extend partially or entirely around the connection member <NUM> and along the inner side of the sleeve <NUM> secures the connection against axial displacement off the segments 12A, 12B, 12C of the sleeve <NUM> relatively to the connection member <NUM>. Second shear keys 22B that extend in the axial direction of the connection member <NUM> on the outer side of the connection member <NUM> and on the inner side of the sleeve <NUM> secure the connection <NUM> against rotational forces. Typically, shear keys are metal profiles, bent or straight, that are welded to the steel surface of the corresponding component and optionally forma shear keys grid.

Claim 1:
A method of assembling a floating offshore support structure (<NUM>), wherein the support structure (<NUM>) comprising a plurality of interconnected tubes (<NUM>, <NUM>, <NUM>, <NUM>) forming a rigid frame (<NUM>) with vertices (<NUM>) arranged as a polygon in a horizontal plane, and wherein the method comprises mounting a buoyancy module (<NUM>) to the frame (<NUM>) at each of the vertices (<NUM>), characterised in that the method comprises providing each of the vertices (<NUM>) of the frame (<NUM>) with a sleeve segment (12C) and inserting a connection member (<NUM>) through the sleeve segment (12C) and securing the connection member (<NUM>) to the sleeve segment (12C) by filling a portion (16C) of pourable casting material (<NUM>), optionally grout, into a void (<NUM>) between the sleeve segment (12C) and the connection member (<NUM>) and hardening the portion (16C) and securing a buoyancy module (<NUM>) to each of the connection members (<NUM>).