Abstract:
A jacket structure for supporting an offshore wind turbine a jacket assembly guide for assembling such a jacket structure and a method of assembling a jacket structure are described The jacket structure includes a plurality of legs, a plurality of connecting braces and a plurality of node elements arranged on the legs for connecting the braces to the legs. A node-to-leg connection is formed between each node element and a leg; and forming a brace-to-node connection between each brace and a node element. A node-to-leg connection and/or a brace-to-node connection include an adhesive joint.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 11162605.7 EP filed Apr. 15, 2011. All of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    The invention describes a method of assembling a jacket structure, a jacket structure, and a jacket assembly guide. 
       BACKGROUND OF INVENTION 
       [0003]    A wind turbine for an offshore wind park must be mounted securely to withstand the forces exerted on it by water and wind. In shallow offshore locations, the wind turbine tower may be mounted on a concrete plinth or foundation that extends into the sea bed. However, for other offshore locations, such mounting techniques may be unsuitable for various reasons. Usually, a “jacket structure” made of three or four upright steel legs is used to support the wind turbine tower in such offshore locations. The upright legs are interconnected by a mesh of braces in the manner of a pylon. The relatively small surface area of such a jacket structure means that it can withstand the forces exerted on it by the water in which it is submerged. The jacket structure is usually relatively narrow at the top, becoming progressively wider towards the base to provide the necessary structural stability. The base of the jacket structure may be anchored to a concrete foundation on the sea bed, or each leg of the jacket structure may be secured to a separate pile driven into the sea bed. 
         [0004]    A jacket structure for an off-shore wind turbine may be dimensioned so that about two thirds are submerged. For an average water depth of about 45 m, a jacket structure may therefore have a height of about 55 m-56 m. The footprint of such a jacket structure may be in the range of 20 m across. Legs and braces of such a jacket structure are generally made of steel pipes or pipe sections. For the necessary structural stability, the legs of such a jacket structure might be about 1.0 m-1.5 m in diameter, while a brace may have a diameter of about 0.6 m-0.8 m. 
         [0005]    A conventional approach of assembling an underwater jacket structure involves assembling modules corresponding to layers of the jacket structure. Each module comprises identical leg sections interconnected by braces extending between the neighboring leg sections. For a tapered jacket structure that is wider at the bottom and narrower at the top, a series of differently-sized modules is constructed, and the completed modules are then successively hoisted into position, stacked on top of each other, and fastened together. The braces are connected to the legs by means of some suitably shaped node element. For example, a node element may comprise a cast piece of steel of which a part is shaped to fit the leg, and another part is shaped to connect to the end of a brace. In the known construction techniques, the node element is usually connected to the leg and to the brace by welded or bolted connections. Welded connections are usually made by automated welding stations. Assembly of the jacket structure may be carried out in any appropriate location, for example close to a port or harbor. The finished jacket structure may then be skidded to a quayside for transfer to an offshore location. 
       SUMMARY OF INVENTION 
       [0006]    A significant disadvantage of the known method is the associated cost. For example, welded connections are relatively cost-intensive to complete, since automated welding stations are large and expensive. Furthermore, the welding procedure is very time-consuming. Therefore, for an offshore wind park comprising many turbines, the costs of such an assembly procedure can be prohibitive. 
         [0007]    A more economical way of assembling a jacket structure is provided and achieved by a method, jacket structure and jacket assembly guide of the independent claims. 
         [0008]    The method of assembling a jacket structure—which jacket structure in its assembled state comprises a plurality of legs, a plurality of connecting braces and a plurality of node elements arranged on the legs for connecting the braces to the legs—includes forming a node-to-leg connection between each node element and a leg, and forming a brace-to-node connection between each brace and a node element, wherein a node-to-leg connection and/or a brace-to-node connection comprises an adhesive joint. 
         [0009]    Joining a node element to a leg or brace by means of an adhesive joint is much faster and less expensive than, for example, forming a welded connection. Furthermore, an adhesive joint is very strong and can withstand high loads applied to it. 
         [0010]    The jacket structure that supports an offshore wind turbine may comprises a plurality of legs, a plurality of connecting braces and a plurality of node elements arranged on the legs for connecting the braces to the legs, which jacket structure comprises a node-to-leg connection between each node element and a leg; and a brace-to-node connection between each brace and a node element; wherein a node-to-leg connection and/or a brace-to-node connection comprises an adhesive joint. 
         [0011]    A jacket assembly guide for assembling a jacket structure, which jacket structure in its assembled state may comprise a plurality of legs, a plurality of connecting braces and a plurality of node elements arranged on the legs for connecting the braces to the legs—comprises a number of supporting towers for supporting the plurality of legs such that a supported leg is held in a position corresponding to the position of that leg in the assembled state of the jacket structure; and an adhesive handling means for handling adhesive during the forming of an adhesive joint at a node-to-leg connection and/or a brace-to-node connection. 
         [0012]    Because one or more legs are conveniently held in place at the correct angle by a corresponding number of supporting towers, and because the braces may be conveniently held or suspended in place between node elements on the supported legs, for example by a crane, the assembly of a jacket structure may be carried out relatively quickly, so that the overall cost of the structure may be kept favorably low. Also, because workers do not need to physically manipulate heavy legs or leg pieces into place, workplace safety is improved. The adhesive handling means may allow adhesive to be transported in a convenient manner to the level of a joint being formed. For example, an adhesive handling means may comprise a hose and pump arrangement for conveniently transporting or pumping the adhesive to where it is needed. An assembly worker might then only need to manipulate a suitable dispenser, for example a nozzle, to direct the adhesive into the joint being formed or assembled. 
         [0013]    Furthermore, any node elements used to hold the braces in place may be conveniently attached to the legs in advance, for example before one or more of the legs are held in place by supporting towers. In contrast to the alternative known modular method, in which different modules or ‘stacks’ of the jacket structure are separately assembled at ground level by connecting braces between leg segments of a module and then stacking the modules, the method allows the jacket structure to be assembled essentially as a single unit. 
         [0014]    The braces of the jacket assembly guide may effectively be arranged at any appropriate angle to interconnect the legs, so that an arrangement of the braces is not as limited as it would be in a modular design, in which braces may effectively only be connected between the levels bounded by the upper and lower leg segment ends. The supporting towers allow the braces to be conveniently held or suspended in place between the legs, for example by a crane, so that the assembly of a jacket structure may be carried out relatively quickly, and the overall cost of the structure may be kept favorably low. 
         [0015]    A node may comprise a part that is inserted between leg sections of a leg. In such an approach, the node is arranged co-linearly between a lower leg section and an upper leg section. However, forming a node-to-leg connection between a node element and a leg may comprise arranging a node sleeve around a leg or a leg joint. Such a node sleeve may be arranged around an uninterrupted region of the leg, or around a leg joint such as a flanged joint. This allows leg sections to be connected directly to one another, ensuring a more stable leg design. 
         [0016]    A node sleeve may be realized as an essentially cylindrical or conical element which may be slipped over a leg section before the adhesive joint is made between the node and the leg. However, sufficient care would need to be taken in manufacturing such a node sleeve to ensure that it fits snugly enough around the leg while leaving enough space for the adhesive layer. Therefore, arranging the node sleeve around the leg or the leg joint may comprise connecting together at least two node sleeve pieces. The node sleeve pieces may be shaped as half cylinders with two straight sides and two curved, essentially semi-circular sides, and may be joined together using any suitable technique, for example using fasteners such as bolts passed through flanges arranged along abutting edges of the node sleeve pieces. 
         [0017]    Once the node sleeve is arranged on the leg, it should be prevented from slipping from that position. Therefore, an adhesive may be applied between an outside surface of a leg and an inside surface of the node sleeve. 
         [0018]    Each node may comprise at least one protruding element or ‘node extension’ arranged at an angle to meet the end of a brace. For example, the outer end of the node extension and the corresponding brace end may have complementary shapes that fit together. In a particularly simple and therefore favorable design, the node extension and the brace have simple tubular shapes with essentially the same diameter. Forming a brace-to-node connection between a brace and a node element may simply comprise arranging a brace sleeve to partially cover the node extension and the end of the brace. In practice, the connecting procedure might involve arranging a first (lower) node sleeve in place around a first leg; connecting the lower end of a brace to the node extension of the first node sleeve by means of a first brace sleeve; connecting one piece of a second (upper) node sleeve to the other end of the brace by means of a second brace sleeve and placing that node sleeve piece in place on the other leg; arranging the corresponding node sleeve piece in place and connecting it to the first node sleeve piece. After which the adhesive connection may be made. In this way, geometrical tolerances or imperfections may be compensated for by positioning one or both nodes in an ad hoc manner for each brace. 
         [0019]    Forming a brace-to-node connection may also comprises applying an adhesive between an inside surface of the brace sleeve and outside surfaces of the node extension and the brace. In this way, a secure and robust connection may be formed between the nodes and the braces. 
         [0020]    The method of assembling the jacket structure may comprise connecting at least one leg of the plurality of legs to a supporting tower of a jacket assembly guide such that the leg is held in a position corresponding to the position of that leg in the assembled state of the jacket structure; holding a brace of the plurality of connecting braces in place between node elements; providing connecting access to the node elements and the braces; connecting each brace to the node elements between which it is held; and disconnecting the leg from the supporting tower to release the assembled jacket structure. 
         [0021]    The node-to-leg connections may be carried out prior to the forming the brace-to-node connections. For example, the nodes may conveniently be put into place around each leg before the leg is hoisted into place and held by the supporting tower. To allow ease of connecting the braces to any node extensions, such nodes may be provisionally attached to a leg in preparation for an adhesive joint, but the actual insertion of the adhesive may be postponed for certain of these nodes until after the brace-to-node connections have been formed. 
         [0022]    In contrast to prior art jacket structures, the jacket structure may even comprise one-piece legs, since it is a relatively straightforward procedure to hold such a long element in a supporting tower. 
         [0023]    The adhesive joint may comprise a layer of adhesive between a sleeve and a component enclosed by the sleeve, wherein the adhesive comprises an epoxy resin or grout. An adhesive may be chosen on the basis of its long-term performance, for example the adhesive layer should not shrink or become brittle or porous over time. 
         [0024]    When forces are applied to the legs of the jacket structure, these forces are transferred via the adhesive layer, for example from the leg to the node sleeve. An adhesive such as an epoxy resin or cement grout is relatively hard after curing. Therefore, large forces acting on a leg may potentially result in damage to a node sleeve, since the wall thickness of the node sleeve may be relatively thin. Therefore, the wall thickness of a sleeve or a node sleeve, may decrease towards an outer edge of the sleeve. Using the example of an essentially cylindrical node sleeve, the upper and lower sleeve regions would have a thinner material thickness than a central region of the node sleeve. The wall thickness of the sleeve may decrease gradually, over a relatively wide region, towards the outer edges of the sleeve. The adhesive layer between such a node sleeve and a leg no longer has a constant thickness, but is instead wedge-shaped. Such a tapered or wedge layer may better absorb and deflect the forces acting on the leg, so that the edges of the sleeve are not distorted or bent outward. In this way, the adhesive layer may act as an effective buffer. 
         [0025]    In the assembly of a jacket structure comprising three or more legs, a jacket assembly guide with a single supporting tower for holding one of the legs may already simplify the assembly process. The remaining legs may comprise leg segments which are successively built up while the connecting braces are lifted into place and fastened to the leg. The jacket assembly guide may comprise a supporting tower for each leg of the jacket structure. Thereby for a four-leg jacket structure, the jacket assembly guide may comprise four supporting towers. 
         [0026]    The supporting towers may be erected at a suitable site, for example close to a port and a skidding system, so that a completed jacket assembly may be skidded to a marine transport system for transport to its offshore destination. In this case, the supporting towers may be realized so that the completed jacket structure may pass through a space between the supporting towers. Once a completed jacket structure has been moved on, the legs for the next jacket structure may be hoisted into place, ready for assembly. In this way, a very fast and economical assembly of many jacket structures is made possible, so that the costs of an offshore wind park may be kept low. 
         [0027]    To hold a leg in place while the braces are being connected, the legs may be temporarily welded into place. However, a supporting tower may comprise a number of temporary connecting means for removably connecting a leg to the tower. For example, a supporting tower may comprise one or more clamps arranged along its length for clamping a leg into place. Such a clamp might be mechanical, for example a ratchet clamp, or might comprise a hydraulic clamp. Supporting a leg by a supporting tower therefore may comprise securely clamping the leg to the supporting tower at one or more points along the leg. 
         [0028]    Once the legs are supported in place, workers may commence connecting the braces between appropriate points on the legs. To this end, the access means of the jacket assembly guide preferably comprises an access platform. Such an access platform may be realized so that a worker may safely stand on the platform and access the relevant parts of the leg to which a brace is to be connected. A platform may be dimensioned so that several workers have room to maneuver. Of course, access may alternatively or additionally be provided by e.g. an external lift e.g. positioned on the ground. 
         [0029]    To connect the braces to the nodes while the legs are held by the supporting towers, the braces may be lifted into place in any manner that is convenient. Holding a brace between node elements may comprise using a lifting apparatus to suspend the brace in position between the node elements. 
         [0030]    Because a supporting tower may hold an entire leg while this is being connected to various braces, that leg may be a one-piece component. This gives provides structural stability, since any joint between two leg segments might be a potential weak spot in the finished structure. However, such a long component may be inconvenient to manufacture. Therefore, a leg of the jacket structure may comprises a plurality of leg sections or pipe pieces, wherein adjacent leg sections of the leg are joined in a leg assembly preceding the assembly of the jacket structure. For example, a pipe piece may have an external flange at each end, and adjacent pipe pieces may be bolted together using a plurality of short bolts inserted through corresponding bolt holes in the flanges. The ends of adjacent leg sections may be shaped to permit a shear key joint between those leg sections. 
         [0031]    As mentioned above, a jacket structure may easily reach or exceed 50-60 m in height. To be able to access all parts of a leg to which a brace is to be connected, the access means of the jacket assembly guide preferably comprises an elevator. Such an elevator may provide rapid access to all platforms of a supporting tower. 
         [0032]    The access means may be realized as a separate entity, for example, as a type of gantry that may be moved around relative to the supporting legs. The access means may be incorporated in a supporting tower, so that a supporting tower comprises one or more access platforms and/or an elevator to provide access to the platform(s). It might suffice if a single supporting tower is equipped with an elevator. For example, if each supporting tower is realized with an appropriate number of access platforms, and these access platforms are connected by gangways, then a single elevator in one of the supporting towers may be sufficient to provide worker access to each access platform. For convenience, it might be preferable if such platforms are arranged at essentially the same levels, so that gangways may be used to connect them. This might be feasible if the nodes at which braces are connected to the legs are arranged uniformly in the jacket structure design. For example, the lowest set of braces might all be of the same length and might therefore all terminate at the same level in the jacket structure. The nodes connecting the brace ends to the legs are therefore also all essentially at the same level. If a more flexible brace arrangement is desired, using braces of different lengths, the position of a gangway may be adjustable, for example using a hydraulic arrangement to raise one end of a gangway so that it may reach the next access platform. 
         [0033]    Alternatively, each supporting tower may be equipped with its own elevator as well as several access platforms. An access platform may be arranged at a level corresponding to a level of a node element of the leg supported by the supporting tower. 
         [0034]    Of course, a supporting tower may comprise a plurality of access platforms and an elevator, whereby the elevator may provide access to each access platform. A combined elevator/access platform design might also involve a funicular track, mounted for example on the exterior of a supporting leg, along which a moveable access platform may travel. 
         [0035]    At least a significant if not a majority portion of the node-to-leg and brace-to-node connections is made using adhesive connections. However, it may be desirable to make some connections in an alternative manner. For example, a node may be connected between leg sections using an internal or external flange connection. A node may be connected to a brace by means of a suitable clevis or flange connection. Alternatively, a brace and a tubular node extension may be shaped to simply fit one into the other, for example by shaping the tubular extension to fit inside the brace pipe piece, or vice versa. These and other connecting means will be described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    Other objects and features will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. 
           [0037]      FIG. 1  shows a jacket structure supporting an offshore wind turbine; 
           [0038]      FIG. 2  shows a cross-section through a first embodiment of an adhesive joint in a jacket structure according to an embodiment; 
           [0039]      FIG. 3  shows a detail of a cross-section through a second embodiment of an adhesive joint in a jacket structure according to an embodiment; 
           [0040]      FIG. 4  shows further adhesive joints in a jacket structure according to an embodiment; 
           [0041]      FIG. 5  shows an embodiment of a jacket assembly guide according to an embodiment; 
           [0042]      FIG. 6  shows a jacket structure being assembled using the jacket assembly guide of  FIG. 5 ; 
           [0043]      FIG. 7  shows a number of node elements for a jacket structure according to an embodiment; 
           [0044]      FIG. 8  shows a number of brace-to-node connections in a jacket structure according to an embodiment; 
           [0045]      FIG. 9  shows an expansion bolt for connecting a brace to a node element in a jacket structure according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0046]    In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale. 
         [0047]      FIG. 1  shows a simplified rendering of a jacket structure  1  used in an offshore location. The jacket structure  1  comprises several legs  10  interconnected by a plurality of braces  11 , and is held in place (in this example) by piles  2  driven into the sea bed  3 . The jacket structure  1  serves to support a wind turbine  4  in an offshore location. The wind turbine  4  itself may be quite large, for example about 80 m in height with a rotor diameter in the range of 100-140 m, particularly when housing a powerful generator in the Megawatt range. To provide the necessary structural stability, the jacket structure  1  may be made of steel/iron legs and braces, securely fastened to each other at nodes along the legs. The largely ‘empty’ structure therefore presents a relatively small surface area to the forces acting on it above and below the water level. 
         [0048]    A significant number of the node-to-leg connections and the brace-to-node connections may be made using adhesive joints. A grouted or glued node-to-leg connection is shown in cross-section in  FIG. 2 . Here, two node sleeve pieces  21  of a node sleeve  20 A are shown, shaped to fit over a shear key joint  41  between leg pieces  102  (a flanged connection between the sleeve pieces  21  is not shown). In the shear key joint  41 , the ends of the leg sections may abut directly. Any gaps between the node  20 A and the leg pieces  102  are sealed using a seal  24  arranged around the circumference of the leg sections  102 , and then an adhesive (e.g. an epoxy resin)  23  or grout  23  is pumped through a suitable channel (not shown) into the space between the node  20 A and the leg pieces  102 . The adhesive  23  or grout  23  is subsequently allowed to cure or harden. An advantage of such an adhesive joint  230  for a node sleeve  20 A is that it may be mounted onto the leg while this is being held in a jacket assembly guide, and its position and/or orientation may be adjusted to accommodate for any intolerances or deviations regarding the length/position of the brace to which it is to be connected. 
         [0049]      FIG. 3  shows a detail of a cross-section through a node  20 A and a leg section  102  (which may be any type of leg section or part of a one-piece leg). Here, the node  20 A has a tapered wall thickness so that the wall of the node  20 A becomes progressively thinner towards the upper edge. Of course, the same tapered wall may be present at the lower region of the node  20 A. The wall thickness decreases over a length L, which may be a significant portion of the node height, for example one fifth or one sixth of the node height. 
         [0050]      FIG. 4  shows a detail of a node-to-leg connection and a brace-to-node connection using adhesive joints. Here, the node  20 A is shown in place around a shear key  41  joint (indicated by the dotted lines) between leg pieces  102 . Node sleeve pieces  21  are joined by node flanges  25  and short bolts  42 . A tubular extension  26  has the same diameter as the brace  11  and is shaped to meet the brace  11  at a butted joint  43  as indicated by the dashed lines. A brace sleeve  112  is arranged around this joint  43  and held in place by a layer of adhesive. For both sleeves  20 A,  112 , the adhesive is prevented from escaping by seals  24  arranged around the perimeters of the sleeves  20 A,  112  until the adhesive is hardened or cured. Here, a single tubular extension  26  is shown on one sleeve piece  21 . Of course, two such tubular extensions  26  may be cast on a single sleeve piece, so that the vertical node flanges  25  flank the tubular extensions  26 . Alternatively, each sleeve piece  21  may be cast with tubular extensions  26 , so that the node flanges  25  run vertically between the tubular extensions  26 . 
         [0051]      FIG. 5  shows a jacket assembly guide  5  according to an embodiment. Here, the jacket assembly guide  5  comprises four supporting towers  51  for each of four legs of a jacket structure. In this embodiment, each supporting tower  51  is tilted at an angle corresponding to the angle of a leg of an assembled jacket structure. The supporting towers  51  may themselves be held in place by some other module such as a crane or gantry. In this example, the supporting towers  51  are secured at ground level by anchoring to a suitable foundation  54 . Each supporting leg  51  has a number of clamps  56  arranged at appropriate intervals for holding a leg of a jacket structure to be assembled. Access platforms  52  are arranged on the supporting towers  51 . These access platforms  52  may be fixed at strategic positions along the supporting towers  51  to provide convenient access to node elements of the legs, or may be movable along the supporting towers  51 . An elevator  55  (indicated by the dashed line) in a supporting tower  51  provides access to the access platforms  52 . A gangway  53  provides access to node elements on legs held by supporting towers that do not have an access platform or elevator. The access platforms  52  and any gangways  53  are arranged on the jacket assembly guide  5  so that the assembled jacket structure may be skidded out from the jacket assembly guide  5  along tracks  6  of a skidding system. 
         [0052]      FIG. 6  shows a jacket structure being assembled using the jacket assembly guide  5  of  FIG. 5 . Here, the legs  10 ,  100  are clamped in place by the clamps  56  arranged on the supporting towers  51 . The legs  10 ,  100  are therefore already held at the angle required to obtain the desired geometry of the finished jacket structure. A leg may be a one-piece leg  100  made of a single pipe piece, or may comprise several leg parts connected together by a shear key, by internal or external flanges, or by any other suitable joint. A crane (not shown) is used to hoist braces  11  into place between the legs  10 . 
         [0053]    Workers  8  on the access platforms  52  may maneuver the braces  11  so that each end of a brace  11  is correctly positioned relative to a corresponding node element previously mounted on the leg  10 ,  100 , for example a node sleeve  20 A. Here, a brace  11  is shown suspended in place by crane tackle  7  of a crane, so that workers  8  on two access platforms may connect each end of the brace  11  to the node  20 A on a leg  10 ,  100 . Once the brace ends are connected to the nodes, the crane may be used to fetch the next brace  11 . The workers need only form the brace-to-node connections at each end of the brace  11 , and may then use the elevators  55  or gangways  53  to move to the next node in preparation for connecting the next brace  11 . In this way, a fairly quick assembly process is made possible. Once the jacket structure I has been assembled, it may be released from the assembly guide  5  by skidding it along the skidding tracks  6 , and the legs  10 ,  100  for the next jacket structure may be hoisted into place and clamped to the supporting towers  51 . 
         [0054]      FIG. 7  shows various other realizations of node elements for connecting a leg to one or more braces of a jacket structure according to an embodiment. In a first embodiment shown in part A of the diagram, a node  20 B is a cast node with two tubular extensions  26  arranged at appropriate angles for connecting to braces. In the example shown, the cast node  20 B is connected to the leg parts  101  by means of long bolts  40  through flanges in the interior of the leg. Of course, such a node  20 B may be realized with only one tubular extension  26 , and two such nodes may be bolted, one above the other, between successive leg parts  101  of a leg. 
         [0055]    In a second embodiment shown in part B of the diagram, a node  20 C is realized as two halves  21  or shell pieces  21  shaped to fit over a shear key joint  41  or shear lock  41  between leg pieces  102 . Instead of a tubular extension, one or more devises  30  may be arranged onto a shell piece  21 , for example by casting. Such a clevis  30  comprises a clevis tray  31  and side walls  33  with through holes  32  for connecting to a brace. The node sleeve pieces  21  may be bolted together using short bolts  42  on flanges along the vertical edges of the node sleeve pieces  21 . 
         [0056]    In a third embodiment shown in part C of the diagram, a node  20 C is realized as two shell pieces  22  shaped to fit over an external flange joint  44  of two leg pieces  103 , as indicated by the broken lines. This embodiment might be favorable if leg pieces  103  with external flanges, for bolting together using short bolts, are to be used. The node  20 C may be cast with one or more tubular extensions or devises for connecting to a corresponding number of braces. 
         [0057]    A node may connect to more than one brace. For example, for a four-leg jacket, two tubular extensions or clevises may be arranged on a node at essentially right angles to provide connecting points for two braces arranged between two leg pairs, whereby one leg is common to each leg pair, and the node is arranged on that leg. For a three-leg jacket, two extensions may be arranged at an essentially  120 ° angle to each other on a single node. 
         [0058]    A brace  11  may comprise a pipe piece  110  with a cast gusset  12  at either end (only one is shown here) for connecting to a clevis of a node, as shown in  FIG. 8 . The cast gusset  12  is connected to the brace pipe piece  110  by means of a flange connection. The cast gusset  12  is solid with a through hole  122  that is dimensioned to sit between the through holes of a clevis. Part A of the diagram shows a brace  11  connected at its upper end to a downward-facing clevis  30  of a node. Here, node  20 C is used as an example. The clevis tray  31  and the gusset  12  form a relatively close fit to prevent an upward movement of the brace  11 . A suitable pin bolt may be passed through the through holes  32 ,  122  in order to connect the node and the brace  11 . In part B of the diagram, a brace  11  is connected at its lower end to an upward-facing clevis  30  of a node  20 C. Fastening the brace to the node is carried out while the leg is held in the jacket assembly guide. Part C of the diagram shows a flanged connection between a further node  20 E and a brace  11  in a jacket structure according to an embodiment. Here, the node  20 E and brace  11  are joined by means of a flange-to-flange joint. The node  20 E comprises a cast tubular extension  26  arranged at a suitable angle to meet the brace  11 . Here, the tubular extension  26  is shown to point upward, for connecting to a lower end of the brace  11 , and comprises a flange at its outer end. The brace  11  also has a corresponding flange at its lower end. The flanges are connected by means of suitable short bolts. 
         [0059]      FIG. 9  shows a pin bolt  70 , in this case an expansion bolt  70 , for connecting a brace to a clevis node in a jacket structure according to an embodiment. The expansion bolt  70  comprises an inner cylinder  72  with a constant internal diameter and a tapering external diameter, and an outer cylinder  71  with a constant external diameter, a tapering internal diameter, and a longitudinal split. The cylinders  71 ,  72  are arranged around a threaded shaft  73 . The external diameter of the outer cylinder  71  fits relatively closely inside the gusset through hole. The expansion bolt  70  is assembled between the clevis walls  33  and inside the gusset through-hole. The bolt  70  is then tightened by means of nuts  74  at the outer ends of the threaded shaft  73 , so that the outer cylinder expands and is forced against the inside of the gusset through-hole. In this way, a sturdy connection is assured between a brace and a node, requiring relatively few fastening parts. 
         [0060]    Although preferred embodiments and variations thereon have been disclosed, it will be understood that numerous additional modifications and variations may be made thereto without departing from the scope of the invention. For example, any of the bolted node connections shown in  FIG. 7  may be augmented by an adhesive layer when used in the assembly of a jacket structure. Braces that are used to connect the same two legs and that are arranged to “cross over” each other at an intersection may comprise brace sections connected at that intersection using any suitable joint, for example using an X-shaped sleeve piece which may be connected to the brace sections by an adhesive joint. 
         [0061]    For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other elements.