Technical field This invention relates to node structures for lattice frames and to methods for their fabrication.

Background Lattice frames comprise elongate members, usually tubes, that lie on intersecting longitudinal axes and may be joined by node structures where those axes intersect. Such node structures may be cast in one piece or may be fabricated from parts by welding.

An example of a lattice frame is a 'jacket ^' substructure for a wind turbine or other offshore structure, which will be used to exemplify the invention in the description that follows. Jackets have been used in the oil and gas industry for many years; they have recently also found favour in offshore wind turbine applications where water depth in excess of about twenty metres makes it impractical to use a traditional monopile or gravity-based substructure.

A jacket for an offshore wind turbine is shown in Figure 1 . The jacket 10 shown in Figure 1 is of conventional overall shape that reflects the prior art but includes various node structures 12, 14 in accordance with the invention, which will be described in detail later with particular reference to Figures 3 to 9. in the example shown in Figure 1 , the jacket 10 comprises four upwardly-converging tubular legs 6 that collectively define a truncated four-sided pyramid of square horizontal cross-section. A three-sided pyramid of tripod configuration is also possible, in that case having a triangular horizontal cross-section.

The legs 18 of the jacket 10 will seat into a pre-piled foundation structure (not shown) that is fixed to the seabed, in water whose depth is potentially in excess of thirty metres. The jacket 10 is tail enough to protrude above the surface so that a tubular wind turbine tower may be mounted on top, clear of the water. For this purpose, the jacket 10 is surmounted by a transition piece 20 as shown in Figure 2 for supporting the wind turbine tower and for providing a working platform around its base. Figure 1 shows that each face of the pyramidal jacket 10 comprises a series of cruciform X-braces 22 of downwardly-increasing size, disposed between upper and lower horizontal struts 24 that extend between adjacent pairs of legs 16. Each X-brace 22 comprises four tubular diagonal struts 26 that converge inwardly to connect at a central X-node structure 12 and diverge outwardly toward respective K-node structures 14 that each connect two diagonal struts 26 of a face of the jacket 10 to the legs 16. K-node structures 14 may be described as double-K node structures where they connect four diagonal struts 26 to the associated leg 16, i.e. two struts 26 each from adjacent faces of the jacket 10.

The uppermost and lowermost nodes on each leg 16 may be regarded as Y-node structures 28 as they connect only one diagonal strut 26 to the associated leg 16, although again a Y-node structure may be described as a doubie-Y node structure where two diagonal struts 26 from adjacent faces of the jacket 10 connect to the associated leg 16. However, the Y-node structures 28 are akin to K-node structures 14 as they also connect the horizontal struts 24 to the legs 16.

It is important for a node structure of a lattice frame to minimise stress concentrations. It is also desirable for a node structure to be compact for low material cost and to minimise resistance to water movement due to waves and tides, which imparts lateral loads to the jacket structure. if the node structure is to be fabricated, it is desirable for that node structure to be easy to fabricate in various locations around the world from readily-available materials and with minimal tooling cost. However, whilst preferred aspects of the invention relate to fabricated node structures, other aspects relate to shape features that are independent of the means of manufacture and so may be embodied in a non-fabricated node such as a cast node. Summary of the invention

From one aspect, the invention resides in a node structure for connecting two or more convergent members of a lattice frame to each other and to one or more other members of the lattice frame, the node structure comprising: a pair of opposed spaced-apart faces that are substantially planar and substantially parallel to each other; at least one pair of root formations with respective central longitudinal axes defining an interior angle between them, those axes diverging outwardly for alignment with respective members of the lattice frame and converging inwardly between the faces; and an inner connecting wall disposed between the root formations of the, or each, pair that connects concave-curved inner edges of the faces and extends in a concave curve around the interior angle to join the root formations of that pair, in the case of an X-node structure 12, any two (or three) of the four diagonal struts 26 of an X-brace 22 may be regarded as two or more convergent members of a lattice frame, in which case the other two (or one) of the four diagonal struts 26 of that X-brace 22 may be regarded as one or more other members of the lattice frame. In the case of a K-node structure 14 or a Y-node structure 28, any two or more horizontal struts 24 and/or diagonal struts 26 may be regarded as two or more convergent members of a lattice frame, in which case a leg 16 may be regarded as one other member of the lattice frame. The inventive concept extends to a method of fabricating a node structure for a lattice frame, the method comprising connecting a pair of opposed spaced-aparf faces with a concave-curved connecting wall by joining the connecting wall to concave-curved edges of the faces. The invention also encompasses a lattice frame comprising at least one node structure of the invention or as made in accordance with the method of the invention, a wind turbine installation comprising such a lattice frame, and a wind farm comprising one or more of such a wind turbine installations. Various optional features of the invention are set out in the appended sub-claims.

Node structures in accordance with the invention are compact, with low water resistance, and are simple to fabricate from readily-available materials with minimal tooling requirements. Their shape distributes loads effectively, optimises weight and provides an advantageously large weld area without being overly sensitive to welding tolerances, in particular, a large welding area on the jacket leg helps to reduce the thickness of the leg and hence the weight and cost of the jacket as a whole.

Brief description of the drawings In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 is a perspective view of a lattice-type jacket substructure for an offshore wind turbine with its associated foundation structure, the jacket comprising various node structures in accordance with the invention, two of which are shown enlarged in Figure 1 and are further illustrated in Figures 3 to 9;

Figure 2 is a perspective view of a transition piece atop the jacket of Figure 1 ;

Figure 3 is a perspective view of an X-node structure and a K-node structure in accordance with the invention, being part of the jacket of Figure 1 ;

Figures 4(a), 4(b) and 4(c) are a selection of views of a K-node structure like that shown in Figure 3;

Figures 5(a), 5(b), 5(c) and 5(d) are a selection of views of a variant of the K-node structure shown in Figure 4, suitable for use as a Y-node;

Figure 6 is an exploded perspective view of the K-node structure of Figures 4(a), 4(b) and 4(c), showing its component parts;

Figure 7 is an exploded perspective view of a K-node structure corresponding to that shown in Figure 6 but illustrating how certain parts may be integrated;

Figures 8(a), 8(b), 8(c) and 8(d) are a selection of views of a K-node structure like that shown in Figure 3;

Figure 9 is an exploded perspective view of the X-node structure of Figures 8(a), 8(b), 8(c) and 8(d), showing its component parts;

Figures 10 to 12 are a sequence of schematic side views showing a pipe being bent around a former, die or mandrel and then being divided longitudinally along its neutral axis into semi-circular sections: Figure 13 is a perspective view showing a semi-circular section being further divided into quarter-circular sections suitable for use in fabricating the X-node structure shown in exploded form in Figure 9;

Figures 14(a), 14(b), 14(c), 14(d) and 14(e) are a selection of views of a tubular transition section of the X-node structure shown in exploded form in Figure 9, in conjunction with a tool used to hot-forge the cross-sectional shape of that transition section;

Figures 15(a) and 15(b) are schematic plan views showing, respectively, an X- node structure and a K-node structure being variants within the inventive concept; and

Figures 16(a) and 16(b) are schematic plan views showing, respectively, an X- node structure and a K-node structure being further variants within the inventive concept.

Detailed description Reference has already been made to Figures 1 and 2 to put the invention into context. Referring next, then, to Figure 3 of the drawings, this shows a K-node structure 14 and an X-node structure 12 of the lattice-frame jacket 10 shown in Figure 1 .

The K-node structure 14 connects a tubular leg 16 of the jacket 10 to two tubular diagonal struts 26 of X-braces 22 shown in Figure 1 . K-node structures 14 and similar Y-node structures 28 will be described with reference to Figures 4 to 7 of the drawings.

Conversely, the X-node structure 12 connects four diagonal struts 26 of an X-brace 22 in a cruciform arrangement, without connecting those struts 26 to a leg 16 of the jacket 10.

An X-node structure 12 will be described with reference to Figures 8 to 14. Further K-node and X-node variants will be described with reference to Figures 15 and 16,

The K-node structure 14 and the X-node structure 12 shown in Figure 3 and further shown in Figures 4 to 9 have several features in common in conceptual terms. Each node structure 12, 14 has a pair of opposed spaced-apart faces 30 that are substantially planar and substantially parallel to each other. Also, each node structure 12, 14 has one or more pairs of roof formations for alignment with respective struts 26, in this case protruding tubular root formations 32 with respective central longitudinal axes defining an interior angle between them. The faces 30 are substantially parallel to a plane containing the central longitudinal axes of the root formations 32. The central longitudinal axes of the tubular root formations 32 converge inwardly between the faces 30 and diverge outwardly in alignment with respective struts 26. An inner connecting wail 34 between the root formations 32 of each pair connects concave-curved inner edges 36 of the faces 30 and extends in a concave curve around the interior angle to join the root formations 32 of that pair.

The concave curvature of the inner connecting wall 34 follows a generally elliptical path between the root formations 32 although the exact curvature is not essential to the invention and could be parabolic or hyperbolic, for example. The concave-curved inner connecting wall 34 of each node structure 12, 14 comprises double-curved parts of convex cross-section that extend around the inferior angle and are bounded by the concave-curved inner edges 36 of the faces 30. More specifically, the K- node structure 4 has an inner connecting wall 34 of semi-circular cross-section extending between the faces 30, with a radius of cross-sectional curvature the same as that of the root formations 32. Conversely, the X-node structure 12 has inner connecting walls 34 each with two portions 38 of quarter-circular cross-section, each portion 38 adjoining a concave-curved inner edge 36 of one of the faces 30.

References in this specification to concave or convex curvature assume that the reader's viewpoint is outside the node structure.

To discuss the K-node structure 14 in more detail, specific reference will now be made to Figures 4 to 7 of the drawings. Figures 4(a) to 4(c) and Figures 6 and 7 show details of the K-node structure 14 shown in Figure 3. Figures 5(a) to 5(d) show a Y-node structure 28 that is like the K-node structure 14 but is used to connect a diagonal strut 26 and a horizontal strut 24 at the top and bottom of the lattice-frame jacket 10 and so has a narrower internal angle between the tubular root formations. The Y-node structure 28 may be regarded as a K-node structure 14 for the purposes of this specification; like numerals are used for like parts. It will be seen that each face 30 in Figures 4 to 7 of the drawings further comprises a straight outer edge 40 opposed to the concave-curved inner edge 36, and straight end edges 42 each diverging from the concave-curved inner edge 36 to the outer edge 40. in addition to the inner connecting walls 34, the faces 30 are also connected by end connecting wails 44 outboard of the root formations 32. Each end connecting wail 44 has a convex semi-circular cross-section extending between the faces 30, between an inner edge 46 adjoining a root formation 32 and an outer edge 48 in continuity with the outer edges 40 of the faces 30. The radius of cross-sectional curvature of the end connecting wails 44 is the same as that of the root formations 32.

Figures 4(b) and 5(b) show that a cylindrical seat recess is defined between the outer edges 40 of the faces 30 and the outer edges 48 of the end connecting wails 44, which recess is shaped to attach the node structure to a leg 16 of the jacket 10. For this purpose, the seat recess has a central longitudinal axis generally parallel with the planes of the faces 30.

A leg 16 of the jacket 10 is shown in cross section in dashed lines in Figure 4(b), received in the seat recess. Figure 4(a) shows that the central longitudinal axes of the tubular root formations 32 intersect on the central longitudinal axis of the leg 16. This is advantageous for efficient load-bearing.

The exploded views of Figures 6 and 7 show the component parts of a K-node structure 14 shown in Figure 4, if that K-node structure 14 is fabricated rather than cast. Figure 6 shows that the tubular root formations 32 may be separate from the inner connecting wall 34 whereas Figure 7 shows that the tubular root formations 32 may be integral with the inner connecting wall 34. The root formations 32 in both cases can simply be cut from a pipe but in Figure 7, the root formations 32 and the inner connecting wail 34 are parts of the same bent pipe, from which an outer convex curve part is removed to fit the concave- curved inner edges 36 of the faces 30.

Turning now to Figures 8 and 9 of the drawings, these show the X-node structure 12 in more detail and, if fabricated, its component parts. The X-node structure 12 connects four convergent diagonal struts 26 of the lattice-frame jacket 10 and so comprises four root formations 32. The root formations 32 are tubular and have respective central longitudinal axes defining interior angles between each adjacent pair of root formations 32. There are four inner connecting walls 34, each of which joins the root formations 32 of a respective adjacent pair.

The inner connecting walls 34 of the X-node structure 12 have portions 38 of quarter- circular cross-section extending along opposite sides of a central concave-curved strip 50 of fiat cross-section.

In the X-node structure 12, the faces 30 are also connected by an internal bulkhead spaced from the inner connecting walls. This bulkhead is defined by a tube 52 that extends between, and is welded, to the faces 30. The tube 52 lies on a central longitudinal axis orthogonal to, and at the intersection of, the central longitudinal axes of the root formations 32. Each face 30 is penetrated by a hole 54 aligned with the tube 52.

Each tubular root formation 32 of the X-node structure comprises a transition section 56 whose cross-section varies in an outward direction to terminate in a circular cross-section. As best shown in Figure 9, each transition section 56 has outwardly-widening portions 58 of convex-curved cross-section that blend in to portions 38 of quarter-circular cross- section of the inner connecting walls 34. The outwardly-widening convex-curved portions 58 are joined by outwardly-narrowing, fiat outer face portions 60.

Figures 10 to 14 illustrate manufacturing steps that may be employed to build a fabricated X-node structure 12 as shown in Figures 8 and 9. it will be apparent to the skilled reader where similar manufacturing steps can be taken when building a fabricated K-node structure 14 as shown in Figures 4 to 7.

Where the node structures 12, 14 are fabricated from metal parts, the faces 30 and the strips 50 may be defined simply by sheet or plate parts and the double-curved parts 34, 38 may be defined simply by being cut longitudinally from an inner concave curve of a bent pipe, as the manufacturing sequence of Figures 10 to 13 will now show.

Figure 10 shows a pipe 62 about to be bent around a curved former, die or mandrel 64 and Figure 1 1 shows the pipe 62 after bending. Figure 12 shows the pipe divided longitudinally along its neutral axis to produce an element 66 of semi-circular cross- section. Figure 13 shows one such element 66 being further divided into elements of quarter-circular cross-section, both being suitable for use as an inner connecting wall portion 38 when fabricating the X-node structure 12 shown in exploded form in Figure 9. Figures 14(a) to 14(e) show a tubular transition section 56 of the X-node structure 12 shown in exploded form in Figure 9, containing a tool 68 used to hot-forge the cross- sectional shape of that transition section 56. The tool 68 comprises a head 70 on a tapering central shaft 72, the head 70 having four iobes 74 equi-angularly spaced about the shaft 72 in cruciform cross-section.

The circumferential width of each lobe 74 tapers distally along the shaft 72. Specifically, the Iobes 74 are relatively wide and hence close together at a proximal end of the head 70 where they define together a circular cross-section. Conversely, the Iobes 74 are relatively narrow and hence further apart at a distal end of the head 70 where they define together a generally square cross-section with rounded corners. Each lobe 74 fits into a respective corner of the generally square cross-section, to define the outwardly-widening portions 58 of convex-curved cross-section of the tubular transition section 56, joined by the outwardly-narrowing, fiat outer face portions 60.

Figures 5(a) and 15(b) show, respectively, simplified X-node structure and K-node structure variants 76, 78. Figures 16(a) and 16(b) show, respectively, further-simplified X- node structure and K-node structure variants 80, 82. As before, each node structure variant 76, 78, 80, 82 shown in Figures 15(a), 15(b), 16(a) and 16(b) has a pair of opposed spaced-apart faces 30 (only one of which is visible in these views) that are substantially planar and substantially parallel to each other. Also, each node structure 76, 78, 80, 82 has one or more pairs of root formations whose central axes diverge outwardly and converge inwardly between the faces 30.

In the case of the node structures 76, 78, 80, 82, the root formations are holes 84 with respective central axes defining an interior angle between them, for alignment with respective struts (not shown here) to be received in the holes. An inner connecting wall between each pair of holes 84 connects concave-curved inner edges 36 of the faces 30 and extends in a concave curve around the interior angle to join the root formations of that pair.

In Figures 15(a) and 15(b), an inner connecting wall 34 of semi-circular cross-section extends between the faces 30, with a radius of cross-sectional curvature the same as that of the holes 84 defining the root formations. These concave-curved inner connecting walls 34 are bounded by the concave-curved inner edges 36 of the faces 30. In the even simpler variant shown in Figures 16(a) and 16(b), concave-curved inner connecting wails 86 have a flat cross-section to extend straight between concave-curved inner edges 36 of the opposed faces 30. The resulting holes 84 are rectangular, more specifically square, but transition sections could be added to the holes 84 for end-on welding to tubular struts of circular cross-section. Such transition sections could be similar to those shown in Figure 9, but would not be identical to them because one end of the transition section would need to be of square cross-section to match the associated hole 84.

Many other variations are possible within the inventive concept. For example, a node structure of the invention may be integral with a leg portion and/or one or more strut root portions to which further leg sections and strut sections may be welded end-on to build a lattice frame.

Where the node structures are fabricated from metal parts, double-curved parts may be defined not only by cutting along a pipe or tube but instead by a bent or forged U- or C- section or by being cut longitudinally from an inner concave curve of a bent or forged U- or C-section.