Manufacturing process for a composite joint, three-dimensional preform for use in said method and structural joint

Abstract

Method of manufacturing a structural joint (1) for use in a multi-membered supporting structure (1010) that is to be subjected to cyclic loading, the method comprising the steps of: - providing for a plurality of structural members, wherein said plurality of structural members comprises at least a first tubular structural member (110); - positioning said plurality of structural members (110, 120, 130) with respect to each other in a predefined arrangement that corresponds to a predefined geometry of the structural joint that is to be manufactured; - forming a fibre-reinforced polymer connecting member (20) onto respective portions of the plurality of structural members in a transitional area covering and thus bonding together those portions of the plurality of structural members that together with the fibre-reinforced polymer connecting member form the structural joint between the respective structural members, wherein the step of forming the fibre-reinforced polymer connecting member comprises the following steps, in any suitable order: o providing a plurality of three-dimensional preforms (21a - 25d), wherein a three-dimensional preform is three-dimensionally formed, preferably prior to arranging said three-dimensional preforms onto the respective portions of the plurality of structural members in the transitional area, from a stack of plies (310) comprising a plurality of at most partially impregnated planar plies of fibre-reinforcement material (311, 312, 313) that are stacked and mutually interconnected, wherein at least one three-dimensional preform of said plurality of three-dimensional preforms comprises at least a first receiving section that is substantially channel-shaped for receiving the first tubular structural member of said plurality of structural members therein, preferably wherein said receiving section has a cross-section that comprises at least a substantially C-, L-, V-, U- and / or J-shaped portion; o covering the respective portions of the plurality of structural members in at least a part of the transitional area by positioning said three-dimensional preforms in a predefined arrangement, such that the first tubular structural member of the plurality of structural members is received in the first receiving section of the at least one three-dimensional preform of said plurality of three-dimensional preforms; o wetting, after the step of covering, the three-dimensional preforms with a curable resin and allowing the wetted predefined arrangement of three-dimensional preforms to cure for forming the structural joint between the respective structural members.

Description

[0001] MANUFACTURING PROCESS FOR A COMPOSITE JOINT, THREE-DIMENSIONAL PREFORM FOR USE IN SAID METHOD AND STRUCTURAL JOINT

[0002] The present invention relates to a method of manufacturing a structural joint for use in a multi-membered supporting structure that is to be subjected to cyclic loading, a method of manufacturing a multimembered supporting structure that is to be subjected to cyclic loading, a three-dimensional preform for use in the method of manufacturing a structural joint, a kit of such three-dimensional preforms, a method of manufacturing the three-dimensional preform and the structural joint.

[0003] Many large supporting structures, such as bridges and foundations for offshore platforms or offshore wind turbines, comprise truss-like structures comprising numerous joints wherein at least two tubular structural members, such a tubular braces and chords, are coupled together. Traditionally, as many of the structures are made from interconnected thin-walled steel tubular members, the structural members are welded together to form the joint. However, the welding process reduces the fatigue resistance of the steel parts and structures. In order to compensate for this effect, larger or, more commonly, thicker members are used to increase the weld area at the connection are used. If thickness is increased this can be done over a certain length, or over the complete member length. Thereby increasing the matenal usage, weight and costs of the overall structure.

[0004] In recent years, structural joints have been introduced wherein the structural members are interconnected by means of applying a fibre reinforced polymer wrap around the respective structural members comprised in the joint, such a joint is, for instance, disclosed in the International Patent Application WO 2019 / 212334 Al. It was shown that such (virgin) joints leads to an increased fatigue resistance when compared to traditionally welded joints. Unpublished International Patent Application PCT / EP2023 / 085629 discloses a wrapped joint that is arranged inside of one of the tubular structural members that are connected, to even further improve the fatigue resistance. Such an internal wrap that effectively acts as an anchor, has been shown to provide a significant improvement of the stiffness and strength of the joint.

[0005] These joint are typically manufactured using a hand-lamination process, wherein plies, fabrics, mats and / or rovings of glass fibre reinforcement are mixed with a resin and manually wrapped or placed around the steel structural hollow sections to form fibre-reinforced polymer wrap joints, or in short FRP or composite joints. Such a process is labour intensive as many man-hours are required, even for laying up a single joint, susceptible to human-induced errors during the hand-laminating process, thereby requiring skilled labourers and due to the fact that a single joint has to be completed in a limited time window, i.e. before the resin starts curing, also requires a lot of hands at the same time. Minor errors in the lay-up of plies may lead to decreased properties of the finalized products. The performance of plies with errors may be deteriorated, and is typically hard to predict on the basis of simulations. Hence, a finalized joint having errors in plies layup typically does not pass the quality control and has to be scrapped. All these constrains and prone to errors of hand layup lead to a process that is hard to scale up, and thus to industrialize in order to be able to produce sufficient numbers of joints, and thereby sufficient number of, for instance, jackets required for a single wind farm comprising tens of offshore wind turbines.

[0006] It is a goal of the present invention to alleviate at least a part of the above-mentioned problem. Specifically, it is an object of the invention to obtain an improved method of manufacturing such joints that can be efficiently industrialized, while also improving the manufacturing quality.

[0007] Thereto, the invention provides for a method of manufacturing a structural joint for use in a multimembered supporting structure that is to be subjected to cyclic loading, the method comprising the steps of: providing for a plurality of structural members, wherein said plurality of structural members comprises at least a first tubular structural member; positioning said plurality of structural members with respect to each other in a predefined arrangement that corresponds to a predefined geometry of the structural joint that is to be manufactured; forming a fibre-reinforced polymer connecting member onto respective portions of the plurality of structural members in a transitional area covering and thus bonding together those portions of the plurality of structural members that together with the fibre-reinforced polymer connecting member form the structural joint between the respective structural members, wherein the step of forming the fibre- reinforced polymer connecting member comprises the following steps, in any suitable order: o providing a plurality of three-dimensional preforms, wherein a three-dimensional preform is three-dimensionally formed, preferably prior to arranging said three-dimensional preforms onto the respective portions of the plurality of structural members in the transitional area, from a stack of plies comprising a plurality of at most partially impregnated planar plies of fibre-reinforcement material that are stacked and mutually interconnected, wherein at least one three-dimensional preform of said plurality of three-dimensional preforms comprises at least a first receiving section that is substantially channelshaped for receiving the first tubular structural member of said plurality of structural members therein, preferably wherein said receiving section has a cross-section that comprises at least a substantially C-, L-, V-, U- and / or J-shaped portion; o covering the respective portions of the plurality of structural members in at least a part of the transitional area by positioning said three-dimensional preforms in a predefined arrangement, such that the first tubular structural member of the plurality of structural members is received in the first receiving section of the at least one three-dimensional preform of said plurality of three-dimensional preforms; o preferably, wetting (e.g. impregnating), after the step of covering, the three-dimensional preforms with a curable resin and allowing the wetted (e.g. impregnated) predefined arrangement of three-dimensional preforms to cure for forming the structural joint between the respective structural members.

[0008] The use of three-dimensional preforms that are specifically shaped for, once arranged in the predefined arrangement onto the respective portions of the plurality of structural members in the transitional area, forming the fibre-reinforcement polymer connecting member, allows arranging the fibre-reinforcements plies onto the (essentially unconnected; i.e. uncompleted) joint before the fibre-reinforcements plies are wetted. It is however noted that, during this process, the structural members may be coupled using a nonload bearing fixation in order for the structural members to remain in the correct geometry. This non-load bearing fixation may be obtained using glue, spotwelding and the like and is sufficient to retain the orientation between the different structural members during the manufacturing process, but is not sufficient to withstand the loading that the joint is to be subjected to once completed. This has a number of benefits; firstly the time-pressure of applying all the wetted plies before the resin starts to cure is removed, such that larger and more complex joints can be manufactured in a more optimized production process, while still enabling to obtain joints having an improved fatigue life when compared to traditionally welded joints. Complex joints can, if needed, be manufactured in a course of multiple days, without negatively affecting the bonding (and thereby its final strength). Furthermore, the placement of pre-forms is a job requiring less skilled labour, which is thereby more readily available, allowing the manufacturing process to be scaled more easily.

[0009] It is noted that the stack of plies comprising a plurality of at most partially impregnated planar plies of fibre-reinforcement material, wherein an “at most partially impregnated ply” is to be interpreted as a ply that comprises too little resin and / or binding agent to provide for sufficient bonding and / or hardening to be able to cure and form the finalized product, and / or can be considered to be substantially nonimpregnated (i.e. substantially dry). In practice, an at most partially impregnated ply will have a percentage by weight of resin and / or binding agent that is in the range of 0% - 15%, preferably in the range of 1% to 10%, more preferably in the range of 2% to 8%, most preferably in the range of 2.5% - 6%. As is explained below, the three-dimensional preform typically comprises a binding agent that temporarily binds the plies of the stack together and enables perform to retain its preformed shape. Such an at most partially impregnated ply is thereby substantially different, i.e. distinguishes, from a so called prepreg (i.e. pre-impregnated fibre) that is already impregnated by a curable resin, such that, after application of the prepreg, it is can be left to cure without having to add a curable resin. As such, in the context of the current disclosure, a three-dimensional preform comprising a binding agent, typically in the above specified ranges, whose (sole) purpose is to temporarily binds the plies of the stack together and enables perform to retain its preformed shape, is considered to be non-impregnated.

[0010] It is furthermore noted that it is in fact the combination of the use of the at most partially impregnated plies (e.g. non-impregnated plies or dry plies as described above) and the three-dimensional preforming allow to obtain an industrialized manner of forming composite joints having a more complex geometry (i.e. any geometry that is more complex than uniaxially connecting a pair of cylindrical tubes). The fibre- reinforced polymer connecting member will feature, for instance, tight comers and / or saddle-shaped geometries at locations where the structural members that are to be interconnected using the fibre- reinforced polymer connecting member meet. In case non-preshaped stacks of (non-impregnated) plies would be arranged on top of the structural members, in particular at locations featuring these complex geometries, in order to be compacted and impregnated for forming the fibre-reinforced polymer connecting member, the non-preshaped stacks will tend to displace with respect to each other when they are forced around the contours of the structural members. This could lead to an uncontrollable forming of fibre-reinforced polymer connecting member as the orientation and location of the fibres will shift. This could additionally lead to air and / or resin pockets that would further diminish the performance of the joint. Additionally, the dry non-preshaped stacks can typically not be pulled into relatively tight comers and / or to follow the saddle-shaped geometries without forming wrinkles in the plies of fibre material. The stmctural behaviour of these wrinkles may also deteriorate the performance of the joint. In fact, in most cases the stmctural behaviour (and the deterioration thereof) cannot be predicted by the use of computer models, such that the obtained load capacity, and fatigue lifetime, of a therefrom resulting joint cannot be determined. Such a joint would not pass a certification process and would thereby be, simply put, unusable.

[0011] The respective portions of the plurality of stmctural members in a transitional area are typically the inner and / or outer circumferential surfaces of the outer ends of tubular stmctural members that are joint together. In most cases the full circumference of the circumferential surface is then covered by (i.e. bonded to) a respective section of the fibre-reinforcement polymer connecting member. For, for instance, tubular stmctural elements having a low L / D (length over diameter) ratio (e.g 1 / 2, 1 / 3, 1 / 4 or smaller), it is also sufficient to cover only a part of the circumference of the inner and / or outer circumferential surface. A partial coverage of the circumference of the inner and / or outer circumferential surface may, for instance, also be sufficient for a first stmctural member (e.g. a first circular tube) having a much larger dimension (such as a diameter) than a second stmctural member (e.g. a second circular tube) it is to be connected to, for instance if the ratio between the respective dimensions of the first stmctural member and the second stmctural member is, for instance, 3 / 2, 2 / 1, 3 / 1 or larger.

[0012] The first tubular stmctural member may also be connected to, for instance, a substantially shell-like (i.e. plate-like) stmctural member, which is then referred to as the second stmctural member. The first tubular stmctural member may then also be inserted through a suitably formed opening in the substantially shelllike stmctural member, such that a fibre-reinforcement polymer connecting member may be formed, using the three-dimensional preforms is described through this disclosure, on both, opposite, sides of the substantially shell-like stmctural member. Such a shell-like stmctural member is not necessarily a flat, or planar, stmctural member, but may also be non-planar (e g. curved). In cases where a first tubular structural member forms a joint with a second tubular structural member having a much larger diameter than (e g. for instance more than 2 times, preferably more than 3 times, more preferably more than 5 times) the first tubular structural member, the joint can be constmcted similar to a connection to a shelllike structural member, such that an opening that is sized to fit the first tubular structural member is arranged in the circumferential wall wherethrough the first tubular member is arranged, such that it extends to an internal space of the second tubular structural member. A respective inner fibrereinforcement polymer connecting member may than be arranged to connect an inner side of the second tubular structural member to the part of the first tubular structural member extending into the internal space and a respective outer fibre-reinforcement polymer connecting member may then be arranged, mutatis mutandis, on the outer side of the second tubular structural member. In these cases is it also typically sufficient to cover only a part of the (i.e. not the full) circumference of the inner and / or outer circumferential surface of the shell-like structural member and / or second tubular structural member.

[0013] The three-dimensional preforms are formed from a stack of individual plies, which are also referred to as mats and / or layers, of fibre-reinforcement material. The individual plies typically comprise high-modulus, high-strength fibres; some typical fibres used comprise cellulose, graphite, glass, boron, and silicon carbide. The preform is obtained, prior to forming the joint, from three-dimensionally forming a stack of plies that comprises a plurality of at most partially impregnated (e.g. substantially dry, i.e. nonimpregnated, not yet wetted by the curable resin) planar plies of fibre-reinforcement matenal that are stacked and mutually interconnected by use of a binder, which is typically a polymeric material, that is applied on, in and / or in between the individual plies of the stack, preferably before the plies are cut in their designed shape. The planar stack can be formed to the pre-form using a method of manufacturing a three-dimensional preform according to a further aspect of the present disclosure.

[0014] Layers of different materials may be used, resulting in a hybrid laminate. The individual layers generally are orthotropic (that is, with principal properties in orthogonal directions) or transversely isotropic (with isotropic properties in the transverse plane) with the laminate then exhibiting anisotropic (with variable direction of principal properties), orthotropic, or quasi-isotropic properties. Quasi-isotropic laminates exhibit isotropic (that is, independent of direction) in-plane response but are not restricted to isotropic out-of-plane (bending) response. Depending upon the stacking sequence of the individual layers, the laminate may exhibit coupling between in-plane and out-of-plane response. The layers of the stack may comprise a woven fabric, non-woven fabric, or a combination of these.

[0015] The three-dimensional preform comprises least a first receiving section that is substantially channelshaped, i.e. having the shape of an open-top channel, for receiving the first tubular structural member of said plurality of structural members therein. Such a channel-shaped receiving section typically has a cross-section that comprises at least a substantially C-, L-, V-, U- and / or J-shaped portion. The crosssection is thus obtaining by cutting along a plane that is substantially perpendicular to a longitudinal axis of the channel-shape. Upon arranging the channel-shape onto the first structural tubular member, the channel-shape effectively surrounds a circumferential section of the outer circumference of the first structural tubular element. Preferably, the inner size and shape of the channel-shaped section exactly fit the outer circumference of the first structural element whereon it is to be positioned, such that a unique (i.e. precise, exact) fit is obtained. This enables a relatively straightforward procedure for arranging the preforms in the predefined arrangement. Furthermore, due to the precise fit, relative movement between fibre-plies is reduced during compacting of the fibre-reinforcement polymer connecting member, such that wrinkle-forming in the formed fibre-reinforcement polymer connecting member is substantially reduced, or even fully prevented. High-quality joints can thereby be obtained.

[0016] The curable resins applied are typically thermosetting polymers, such as benzoxazine resins, bis- maleimide resins (BMI), cyanate ester resins, epoxy (epoxide) resins, phenolic (PF) resins, unsaturated polyester (UP) resins, polyimides, polyurethane (PUR) resins, silicones, and vinyl esters, or may be an injectable reactive thermoplastic resin.

[0017] In a preferred embodiment, said predefined arrangement comprises a plurality of different layers of preforms that are stacked on top of each other, such that a total number of fibre-layers comprised in the fibre-remforced polymer connecting member at a specific location is defined by the number of different layers of respective preforms and the number of plies of each preform at the specific location. By stacking a plurality of different preforms, it is possible to create a relatively thick fibre-reinforced polymer connecting member from stacking a plurality of relatively simple pre-forms.

[0018] Preferably, the predefined arrangement is such that the number of three-dimensional preforms that are stacked on top of each other at a central section of the fibre-reinforced polymer connecting member are higher than the number of three-dimensional preforms that are stacked on top of each other at outer ends of the fibre-reinforced polymer connecting member; and / or wherein the number of fibre-layers comprised at the central section of the fibre-reinforced polymer connecting member are higher than the number of fibre-layers comprised at the outer ends of the fibre-reinforced polymer connecting member. A fibre- reinforced polymer connecting member having tapered outer ends is thereby obtained. As the highest stresses will typically occur at the root of the joint (i.e. where the various parts and / or outer ends of the structural members meet for forming the joint), such that sufficient composite material is required in the root for obtaining a joint having the desired strength and / or stiffness . Extending this thickness further along the structural members, would , depending on the length and angle of the taper, only have a limited effect on the strength and / or stiffness, but would lead an increase in material usage that is not needed and would add to the amount of manhours required for assembling the predefined arrangement. Hence, the tapering allows for an effective usage of material and manhours. In a preferred embodiment, the plurality of three-dimensional preforms comprises a first set of three- dimensional preforms, wherein each three-dimensional preform of said first set is arranged with a first receiving section that is substantially channel-shaped, and wherein, after arranging the three-dimensional preforms in the predefined arrangement of three-dimensional preforms, the respective first receiving sections of the first set of three-dimensional preforms form a first sleeve around the first tubular structural member of said plurality of structural members in at least a first part of the transitional area, preferably wherein said first sleeve is a first tubular sleeve that surrounds the full outer circumference of the first tubular structural member of said plurality of structural members in at least the first part of the transitional area. The first sleeve, which is preferably tapered towards the distal end of the first sleeve, is thus formed from the first set of three-dimensional preforms that are all arranged in a predefined arrangement. This allows to form a, typically substantially cylindrical, sleeve that is arranged around, and abuts, the first tubular structural member from a set of simpler shaped preforms. In general, it allows to form, in an industrialized manner, a substantially (for instance cylindrical) tubular element of at most partially impregnated fibre-reinforcement material that is arranged to substantially exactly fit around the first tubular structural member from planar stacks of interconnected fibre plies (that are to be preformed) in a controllable manner, such that the previously described errors, such as wrinkles, air- or resin pockets, are prevented, or at least reduced.

[0019] Preferably, the plurality of structural members comprises a second tubular structural member; wherein at least one of said three-dimensional preforms of the plurality of three-dimensional preforms comprises at least a second receiving section that is substantially channel-shaped for receiving the second tubular structural member of said plurality of structural members therein, such that an inner surface of the substantially channel-shaped second receiving section faces an outer surface of the second tubular structural member, or for receiving a second tubular structural member of said plurality of tubular structural members thereon, such that an outer surface of the substantially channel-shaped second receiving section faces an inner surface of the second tubular structural member; wherein the step of covering the respective portions of the plurality of structural members is, respectively, further performed such that the second tubular structural member of the plurality of tubular structural members is received in the second receiving section of the respective at least one three- dimensional preform or such that the second receiving section of the respective at least one three- dimensional preform is received in the second tubular structural member.

[0020] In certain cases, of which an example was given above, a respective part of the fibre-reinforced connecting member may be formed inside of the second tubular structural member. The substantially channel-shaped second receiving section may then be formed such that a portion of the inner circumferential surface of the second tubular structural member faces the an outer surface (i.e. an outer side, closed-side, a surface having substantially diverging normal-vectors) of the channel-shaped second receiving section. Alternatively, the channel-shaped second receiving section may be formed to receive the second tubular structural member therein, such that a portion of the outer circumferential surface of the second tubular structural member faces the an inner surface (i e . an inner side, open side, a surface having substantially converging normal-vectors) of the channel-shaped second receiving section. The advantages of such a second receiving section are similar to the advantages of the first receiving section, described hereabove.

[0021] To further enable to advantages of a sleeve, as was described above, a further preferred embodiment is such that the plurality of three-dimensional preforms comprises a second set of three-dimensional preforms, wherein each three-dimensional preform of said second set is arranged with a second receiving section that is substantially channel-shaped, and wherein, after arranging the three-dimensional preforms in the predefined arrangement of three-dimensional preforms, the respective second receiving sections respectively form a second sleeve around, or in, the second tubular structural member in at least a second part of the transitional area, preferably wherein said second sleeve is a second tubular sleeve that respectively surrounds and connects the full outer, or inner, circumference of the second tubular structural member of said plurality of tubular structural members in at least the second part of the transitional area.

[0022] In a preferred embodiment, the predefined arrangement of three-dimensional preforms of a set respective set of preforms is such that the respective sleeve is formed by:

[0023] - arranging a first subset of three-dimensional preforms such that the respective receiving sections are adjacent to each other and such that side edges of adjacently arranged receiving sections abut and respective inner surfaces of the adjacently arranged receiving sections form a respective circumferential inner, or outer, surface; formed by:

[0024] - arranging a second subset of three-dimensional preforms such that the respective receiving sections are on top of each other such that an inner surface of the receiving section of an above arranged three- dimensional preform abuts an outer surface of the receiving section of a below arranged three- dimensional preform, preferably wherein said above and below arranged three-dimensional preforms are arranged in a staggered arrangement, such that at least one of the side edges of the above arranged three- dimensional preform does not coincide with the side edges of the below arranged three-dimensional preform.

[0025] This enables to form a respective sleeve having a required thickness by stacking a plurality of three- dimensional preforms and / or to form a sleeve covering a large part of the, preferably the full, circumference of the respective structural tubular member by arranging the preforms side-to-side, such that a predefined stiffness and / or strength is obtained for the sleeve and / or full joint. Arranging the various layers of preforms in a staggered arrangement avoids that the interruptions, i.e. the intersections where adjacent preforms meet, are all positioned at the same location, such that stiffness and / or strength is further improved. Preferably, at least one three-dimensional preform of said plurality of three-dimensional preforms comprises a first and a second receiving section, such that the respective at least one three-dimensional preform is comprised in both the first and second set of three-dimensional preforms; and wherein the step of covering the respective portions of the plurality of tubular structural members comprises receiving the first tubular structural member in the first receiving section of the respective three-dimensional preform and, respectively, receiving the second tubular structural member in the second receiving section of the respective three-dimensional preform or receiving the second receiving section of the respective three- dimensional preform in the second tubular structural member. Hereby, the plies of the respective preform overlap both structural members without an interruption and / or discontinuity, such that a joint having a higher stiffness and / or strength is obtained. Additionally, less separate preforms need to be positioned during the lay-up of the predefined arrangement, leading to a more efficient production process.

[0026] In a preferred embodiment, at least one three-dimensional preform comprises the same number of receiving sections as there are tubular structural members that are to be comprised in the structural joint, wherein each receiving section is substantially channel-shaped and arranged for receiving a respective tubular structural member therein or thereon; and wherein the step of covering the respective portions of the plurality of tubular structural members comprises, respectively, receiving the respective tubular structural member in a respective receiving section of the respective three-dimensional preform, or receiving the respective receiving section of the respective three-dimensional preform in the respective tubular structural member. The above described effect is hereby extended to more complex joints interconnecting more than two structural members.

[0027] It is preferred that, when the respective tubular structural member is received in a respective receiving section of the respective three-dimensional preform, an inner surface of a respective receiving section is shaped correspondingly to a portion of the outer surface of the respective tubular structural member, or to an outer surface of a below arranged three-dimensional preform whereon the respective receiving section is to be positioned during the step of covering the plurality of structural members; or wherein, when the respective receiving section of the respective three-dimensional preform is received in a respective tubular structural member, an outer surface of a respective receiving section is shaped correspondingly to a portion of the inner surface of the respective tubular structural member, or to an inner surface of a below arranged three-dimensional preform whereon the respective receiving section is to be positioned during the step of covering the plurality of structural members.

[0028] In short, the cross-section of the abutting surface of the receiving section is has substantially the same shape as the cross-section of the tubular structural member it is to be arranged on. A good fit between the respective receiving section and the tubular structural member, or between successive layers of preforms, are thereby obtained, such that the risk of wrinkles, air- and / or resin-pockets in in the final joint is prevented, or at least reduced, as is also explained in more details below. Preferably, when the respective tubular structural member is received in a respective sleeve, an inner surface of the respective sleeve is correspondingly shaped to a portion of an outer surface of the respective tubular structural member it is to at least partially surround and / or wherein the dimensions, in particular a diameter or radius, of an inner surface of a respective sleeve are / is substantially equal to the dimensions of the outer surface of the respective tubular structural member it is to surround, such that after forming the respective sleeve around the respective tubular structural member, the outer surface of the respective tubular structural member contacts the inner surface of the respective sleeve, in particular contacts substantially the full inner surface of the respective sleeve; or wherein, when the respective sleeve is received in a respective tubular structural member, an outer surface of the respective sleeve is correspondingly shaped to a portion of an inner surface of the respective tubular structural member it is to at least partially cover and / or wherein the dimensions, in particular a diameter or radius, of an outer surface of a respective sleeve are / is substantially equal to the dimensions of the inner surface of the respective tubular structural member it is to cover, such that after forming the respective sleeve in the respective tubular structural member, the inner surface of the respective tubular structural member contacts the outer surface of the respective sleeve, in particular contacts substantially the full outer surface of the respective sleeve.

[0029] The respective receiving sections are thereby shaped to exactly (i.e. precisely) fit the respective parts of the inside, or outside, of the respective structural member whereon they are positioned, or, in case said preform is arranged in a secondary on top of an already positioned layer of preforms, the respective receiving section is shaped to exactly (i.e. precisely) fit the already positioned receiving section(s) of the already positioned layer of preforms. In other words, when the receiving sections are to be arranged around the outside of the structural member, the first layer of preforms (which is thus the below arranged layer for the second layer) can have a receiving section having the same inner diameter as the outer diameter of the tubular structural member it is to be positioned on, whereas the second layer of can have a receiving section having an inner diameter equal to the outer diameter of the tubular structural member plus the thickness of the first layer of preforms. This applies, mutatis mutandis, to the case where the receiving sections are to be arranged around the inside of the structural member, such that, for the first layer, the outer diameter of the receiving section is equal to the inner diameter of the tubular structural member and, for the second layer, the outer diameter of the receiving section is equal to the inner diameter of the tubular structural member minus the thickness of the first layer of preforms. The previously described errors, such as wrinkles, air- or resin pockets, are thereby prevented, or at least reduced, in the ultimately formed fibre-reinforced connecting member (as is explained in more detail below), such that a high-quality joint is obtained.

[0030] In a preferred embodiment, the first tubular structural member is a tubular structural member having a cylindrical cross-section, preferably having a cylindrical cross-section over its entire length; and wherein an inner surface (i.e. an inner side of the cross-section) the first receiving section is substantially concave, preferably wherein an outer surface of the first receiving section is substantially convex, and / or preferably wherein the cross-section of the first receiving section is substantially shaped as an annular sector; and / or wherein the second tubular structural member is a tubular structural member having a cylindrical cross-section, preferably having a cylindrical cross-section over its entire length; and wherein, when the second tubular structural member is received in the second receiving section, an inner surface the second receiving section is substantially concave, or wherein, when the second receiving section is received in the second tubular structural member, an outer surface of the second receiving section is substantially convex, and / or preferably wherein the cross-section of the second receiving section is substantially shaped as an annular sector having a substantially concave inner surface and a substantially convex outer surface; and / or wherein each of the tubular structural members comprised in the structural joint have a cylindrical cross-section, preferably having a cylindrical cross-section over their entire length, and wherein inner surfaces of the respective receiving sections are substantially concave, wherein the respective outer surfaces are substantially convex, and / or wherein the respective cross-sections of the respective receiving sections are substantially shaped as annular sectors. In such a case, a good fit between the respective receiving sections and the respective structural members is obtained, as was also described above. In particular, the annular shape ensures a that the shape of the outer surface does not vary over the various layer, which simplifies the overall production process.

[0031] Preferably, the structural joint is a uniaxial or an at least uniplanar joint of a K, X, T, N, KT, Y type, or a combination of these types in a uniplanar or multiplanar arrangement. These types of joints are commonly used in lattice type structures such as offshore support structures (e.g. jackets) for offshore wind turbines. A single jacket may have dozens of such joints and a wind farm can comprise tens to over a hundred turbines. For a single wind farm multiple-hundreds of such joints may thus be required, that can be reliably and efficiently be manufactured using the above described embodiments of the method of manufacturing.

[0032] The method is highly suitable for manufacturing a newly constructed joint (i.e. virgin joint), in particular wherein the tubular structural members of said structural joint have, prior to forming the newly constructed structural joint, not been part of another structural joint. As the offshore support structures are typically fatigue driven (i.e. limited in their lifetime by the fatigue loading experienced), one cannot use second hand structural members. The method may also be applied, for example in situ e.g. operational phase, for repairing existing joints in a controlled and swift manner.

[0033] It is preferred that the step of wetting the three-dimensional preforms comprises any of the following substeps: arranging the positioned plurality of tubular structural members having the predefined arrangement of three-dimensional preforms thereon and / or therein in a mould; causing a curable resin to flow into the mould and wet the predefined arrangement of three- dimensional preforms with the curable resin, preferably by using a vacuum infusion process, and, preferably, wherein the mould is a flexible mould, such as a vacuum bag, in particular an plastic or elastomeric (e g. rubber, silicone) vacuum bag.

[0034] This enables an effective and relatively simple method for substantially fulling wetting the predefined arrangement of three-dimensional preforms, by applying a pressure differential over the mould, the resin effectively flows through the mould. In particular, when a vacuum infusion process, preferably using a (plastic or elastomeric) vacuum bag (which may be reusable), is employed, there is no need for positively and / or negatively shaped rigid moulds having all different shapes in order to accommodate all the different joints. Rather, the structural members themselves act as a positive mould whereon / wherein the various preforms are arranged, the bag is subsequently arranged thereon and the negative pressure difference “sucks in” the resin and the bag under vacuum forms along the outer surface of the predefined arrangement of three-dimensional preforms for putting a compression force onto the predefined arrangement of three-dimensional preforms prior the infusion process and during the infusion and curing processes. This ensures that the preforms do not shift or slide in the process and a substantially error-free end product can be obtained. A flexible mould may also refer to a semi-rigid (which may also be referred to as semi-flexible) mould that is able to retain a predefined shape, but may (elastically) deform under influence of applied forces. Such a semi-rigid mould may be made from plastic, elastomeric and / or composite materials and may aid in retaining three-dimensional preforms in their correct arrangement. Furthermore, a positive or negative pressure difference may than be applied for drawing in the resin. In a second aspect, the present disclosure relates to a method of manufacturing a multi-membered supporting structure that is to be subjected to cyclic loading, in particular an offshore foundation structure for a wind turbine, wherein the method comprises the step of manufacturing a structural joint according to any of the preceding embodiments. The presented method thereby allows for obtaining multi-membered supporting structures that require less steel (and / or or other construction materials), while having a superior fatigue life, compared to traditionally welded multi-membered supporting structures, in an efficiently and industrialized manner.

[0035] In a third aspect, the present disclosure relates to a three-dimensional preform for use in a method of manufacturing a structural joint according to any of the preceding embodiments according to the first aspect, wherein said structural joint comprises a plurality of structural members that are to be interconnected in said joint, wherein the three-dimensional preform is formed from a stack of plies that comprises a plurality of at most partially impregnated planar plies of fibre-reinforcement material that are stacked, mutually interconnected and formed to obtained a three-dimensional shape; said three- dimensional preform comprise: a plurality of receiving sections that are substantially channel-shaped for receiving, in or on each receiving section, a respective tubular structural member of said plurality of tubular structural members therein, preferably wherein said channel-shaped receiving sections have cross-sections that comprises at least a substantially C-, L-, V-, U- and / or J-shaped portion. The three-dimensional preform according to the third aspect thereby enables the method according to the first aspect and its related advantages. As the three-dimensional preform comprises two receiving sections, a stiff and strong, interconnection between the first and second structural members may be obtained as at least some fibres in the preform will extend over both the first and second structural members.

[0036] Preferably, an inner side of the cross-section of at least a first receiving section is shaped, and preferably sized, correspondingly to a portion of an outer side of the cross-section of the first tubular structural member that is to be received in the first receiving section, or wherein an inner side of the cross-section of the first receiving section is shaped, and preferably sized, correspondingly to a portion of an outer side of a different three-dimensional preform wherein the three-dimensional preform is to be arranged on; and / or wherein an outer side of a second receiving section is shaped, and preferably sized, correspondingly to a portion of an inner side of the cross-section of a second tubular structural member wherein the second receiving section is to be received, or wherein an outer side of the cross-section of the second receiving section is shaped, and preferably sized, correspondingly to a portion of an inner side of a different three-dimensional preform wherein the three-dimensional preform is to be arranged on. This enables a substantially exact fit between with the first tubular structural member and / or underlying layer(s) of perform, such that, for instance during a step of compressing the preforms and / or the vacuum infusion step, no, or at least a reduced number of, the previously described errors form, such that a high- quality joint can be obtained using the method according to the first aspect.

[0037] It is preferred that an inner surface of one or more respective receiving sections are, preferably of all receiving sections are, substantially concave, wherein an outer surface of the respective one or more receiving sections are, preferably of all receiving sections are, substantially convex, and / or wherein a respective cross-section of the respective one or more receiving sections are, preferably of all receiving sections are, substantially shaped as an annular sector. A substantially exact fit can hereby be obtained on structural members having a circular cross section.

[0038] In a preferred embodiment of the three-dimensional preform, the respective receiving sections comprises a respective receiving section longitudinal axis running along a direction that is substantially perpendicular to the plane of the cross-section of the respective receiving section; wherein said three- dimensional preform comprises a parallel set of receiving sections whose respective receiving section longitudinal axes are substantially parallel, in particular substantially coinciding; and / or wherein said three-dimensional preform comprises a non-parallel set of receiving sections whose respective receiving section longitudinal axes are at a non-zero angle, preferably at an angle of at least 15°, preferably at least 30°, more preferably at least 45°, alternatively at approximately 90°, with respect to each other.

[0039] The parallel set of the receiving sections allows to manufacture (uni-)axial joints (or parts of joints) that can enjoy the advantages as previously described. The non-parallel set allows to manufacture angled joints (or parts of joints) that can enjoy the advantages as previously described. The combination of both allows to manufacture joints having more complex geometries, such as joint comprising at least a K, X, T- shape therein, as these comprise both axial parts and angled parts.

[0040] Preferably, the inner sides (i.e. inner surfaces, open sides) and / or outer sides (i.e. outer surfaces, closed sides) of the channel-shaped receiving sections of at least two receiving sections of the three-dimensional preform face towards substantially the same direction. This enables to form pre-forms that can be arranged over the respective structural members in substantially the same plane as the (uni-planar) joint that is to be formed. Alternatively, or additionally, the respective inner, or outer, side of a first channelshaped receiving section faces towards a first direction and wherein the respective inner, or outer, side of a second channel-shaped receiving section faces towards a second direction that is different from the first direction, preferably wherein the first and second directions are at an angle of at least 15°, preferably at least 30°, more preferably at least 45°, alternatively at approximately 90°. This enables to form pre-forms that can be arranged over the structural member in a directions that are at an angle with the plane of the (uni -planar) joint that is to be formed, such as for instance a preform that is to be placed in the axil of two adjoining structural members of the joint. The combination of both features is, for instance, advantageous when forming multi -planar joints (as have been introduced before).

[0041] In a fourth aspect, the disclosure relates to a kit of three-dimensional preforms, comprising a plurality of three-dimensional preforms according to any of the preceding embodiments of the third aspect, for use in method according to any of the preceding embodiments of the first aspect, wherein said three-dimensional preforms are, when arranged in a predefined arrangement, arranged for forming at least a portion of a fibre-reinforced polymer connecting member in and / or on, the plurality of structural members in a transitional area for forming a structural joint for use in a multi -membered supporting structure that is to be subjected to cyclic loading. The kit is thereby applicable in the efficient and industrialized method for forming the structural joint having the improved fatigue lifetime as described before.

[0042] In a fifth aspect, the disclosure relates to a method of manufacturing a three-dimensional preform according to any of the preceding embodiments of the third aspect, the method comprising the steps of:

[0043] - providing a planar preform that comprises a plurality of at most partially impregnated planar plies of fibre-reinforcement material that are stacked and mutually interconnected;

[0044] - forming said planar preform in a mould, preferably using a double diaphragm forming method, for obtaining the three-dimensional preform comprising a plurality of receiving sections that are substantially channel-shaped for receiving in, or on, each receiving section, a tubular structural member therein, or respectively thereon, preferably wherein said channel-shaped receiving sections have cross-sections that comprises at least a substantially C-, L-, V-, U- and / or J-shaped portion.

[0045] The three-dimensional preform and associated benefits can thereby be obtained. The method according to the fifth aspect will be described in more detail in an example described on the basis of figures 4A - 4H.

[0046] In a sixth aspect, the disclosure relates to a structural joint for use in a multi -membered supporting structure that is to be subjected to cyclic loading, wherein said structural joint is obtainable by a method according to any of the preceding embodiments of the first aspect; or wherein said structural joint comprises: a plurality of structural members that are positioned with respect to each other in a predefined arrangement that corresponds to a predefined geometry of the structural joint, wherein the plurality of structural members comprises at least a first tubular structural member; a fibre-reinforced polymer connecting member that is formed onto respective portions of the plurality of structural members in a transitional area covering and thus bonding together those portions of the plurality of structural members that together with the fibre-reinforced polymer connecting member form the structural joint between the respective structural members, the fibre-reinforced polymer connecting member comprising: a plurality of three-dimensional preforms according to any of the preceding embodiments of the third aspect, and / or a kit of three-dimensional preforms according the fourth aspect, wherein the plurality of three-dimensional preforms are arranged in a predefined arrangement for covering the respective portions of the plurality of structural members in at least a part of the transitional area, such that at least the first tubular structural member of the plurality of tubular structural members is received in a first receiving section of the at least one three-dimensional preform of said plurality of three-dimensional preforms; and a cured resin that is bonded, in cooperation with the arrangement of the plurality of three- dimensional preforms, to the plurality of structural members in the transitional area.

[0047] The structural joints according to the fifth aspect, or obtained using a method according to the first aspect, do not require welding, as the fibre-reinforced polymer connecting member acts as a transition member for coupling the respective structural members, such that substantially all (i.e. >95%, preferably >99%, more preferably >99.5%, most preferably >99.9%) of the forces transferred between said structural members are transferred through the fibre-reinforced polymer connecting member. Although said structural members are preferably connected to each other by means of only a, or multiple, fibre- reinforced polymer connecting member, superficial welding, spot welding, or secondary bonding materials, such as glue or resin type of fixators, may still be applied for maintaining the orientation and position of the structural members with respect to each other during applying of, and / or curing of, the fibre-reinforced polymer connecting member. Said structural members are, preferably, not connected by means of fully penetrating welds. In a seventh aspect, the disclosure relates to a multi-membered supporting structure that is to be subjected to cyclic loading, in particular an offshore foundation structure for a wind turbine, wherein the multimembered supporting structure comprises a structural joint according to the sixth aspect.

[0048] The present invention is further illustrated by the following figures, which show preferred embodiments of the various aspects of the disclosure and are not intended to limit the scope of the invention in any way, wherein:

[0049] - Figure 1 schematically shows, in a frontal view, an offshore wind turbine that is founded on a jacket foundation.

[0050] - Figures 2A - 2C schematically show, in various views, an embodiment of the structural joint obtainable using an embodiment of the method of manufacturing according to the first aspect of the present disclosure.

[0051] - Figure 3 schematically shows a side view of an alternative embodiment of the structural joint.

[0052] - Figures 4A - 4H schematically show, as a series of steps, an embodiment of a method of manufacturing a three-dimensional preform according to the fifth aspect of the present disclosure.

[0053] - Figure 5 shows a flow-chart of some of the process-steps of the method of manufacturing the structural joint.

[0054] Figure 1 schematically shows an offshore wind turbine 1001, which comprises a wind turbine 1001 that is supported on a jacket foundation structure 1010. The jacket foundation structure 1010 is anchored in the seabed 1002 and support the turbine to be above the sea 1003, it thereby transfers the forces, that are due to the turbine’s interaction with the wind and the waves acting on the jacket foundation structure 1010, which result in varying forces (i.e. dynamic forces) that excite the jacket foundation structure 1010.

[0055] Such a jacket foundation structure 1010, a more in general multi-membered supporting structures, are composed of a plurality of (elongated) structural members, typically steel cylindrical tubes, that are interconnected at a plurality of structural joints 1011, 1012, 1013. As the jacket foundation structure 1010 is excited by varying dynamic forces, the forces that need to be transferred in the plurality of structural joints 1011, 1012, 1013 are also varying dynamic forces, which lead to varying material stresses and thereby, over the course of its lifetime, to fatigue. Traditionally, the steel cylindrical tubes are welded together for forming the joint 1011, 1012, 1013. The process of welding does not only lead to a localized decrease in fatigue resistance properties (in the so called heat affected zone), but also lead to stress concentrations due to the geometry of the joint 1011, 1012, 1013 itself. This is traditionally addressed by applying tubes having thickened root-sections (i.e. outer end sections), to decrease these effects and to comply with the fatigue life requirements. This leads however to increased welding time (as thicker steel sections need to be welded through) and an increase in (steel) material use. By replacing the traditional welded joint by a structural joint according to the current disclosure, many of these disadvantages can be overwon.

[0056] Figure 2A schematically shows, in a three-dimensional perspective view, a structural joint 1 for use in a multi-membered supporting structure 1010 that is to be subjected to cyclic loading. The joint 1 is in the current example a uniplanar T-shaped joint 1 and comprise first, second and third tubular structural members, or in short first, second and third tubes 110, 120, 130, each having a substantially circular (inner and outer) cross-section. First tube 110 is a longitudinal tube that extends along the primary axis I and second and third tubes 120, 130 are longitudinal tubes that extend, in line with each other, along the secondary axis II. The respective tubes 110, 120 130 meet in a transitional area, which is defined as an area around the intersection point of the primary and secondary longitudinal axis I, II. It is noted that, in an alternative embodiment, the second and third tube 120, 130 can be formed by a single second tube 120 that extends along the secondary axis II.

[0057] A fibre-reinforced polymer connecting member 20 that is onto respective outer ends tubes 110, 120, 130 in a transitional area covering and thus bonding together the outer ends of the tubes 110, 120, 130 that together with the fibre-reinforced polymer connecting member 20 form the structural joint 1 between the respective tubes 110, 120, 130. The fibre-reinforced polymer connecting member 20, which is seen to fully surround (i.e. wrap) the respective outer surfaces 111, 121, 131 of the outer ends of the respective tubes 110, 120, 130.

[0058] The fibre-reinforced polymer connecting member 20 is formed from a plurality of three-dimensional preforms 21a - 25d that are arranged in the transitional area around the respective outer ends of the tubes 110, 120, 130. The plurality of three-dimensional preforms 21a - 25d are arranged in a predefined arrangement such as to coverer the respective portions of the plurality of structural members (i.e. the outer surfaces 111, 121, 131 thereof), such that first tube 110 is received in first receiving sections 21al - 21dl, 22al - 22dl, 24al - 24dl, 25al - 25dl (not all visible) of the at respective three-dimensional preforms 21a - 21d, 22a - 22d, 24a - 24d, 25a - 25d.

[0059] The predefined arrangement is arranged such that the three-dimensional preforms are arranged in a plurality of (in this example four) different layers A - D, wherein the first layer A is (in particular the three-dimensional preforms 21a - 25a of the first layer A are) arranged onto, and around, the respective circumferential outer surfaces 111, 121, 131 of the respective outer ends of the tubes 110, 120, 130. The second layer B is arranged onto, and around, the first layer A; the third layer C is arranged onto, and around, the second layer B; and the fourth layer D is arranged onto, and around, the third layer C; and this is continued, mutatis mutandis, in case more (or less) than four layers are applied. The individual layers A - D are also composed of a plurality of different three-dimensional preforms 21a - 25a, 21b - 25b, 21c - 25c, 2 Id - 25d that are arranged adjacent to each other, such that that the respective outer edges of adjacent preforms face each other and, preferably, touch (i.e. abut) each other. As such, each layer A - D also has a specific predefined arrangement of three-dimensional preforms 21a - 25a, 21b - 25b, 21c - 25c, 2 Id - 25d, that, once positioned in the predefined arrangement, have an outer shape that is substantially similar to the outer shape of the finalized fibre reinforcement connecting member 20.

[0060] The first receiving sections 21al - 2 Id 1, 22al - 22dl, 24al - 24dl, 25al - 25dl of the respective three- dimensional preforms 21a - 2 Id, 22a - 22d, 24a - 24d, 25a - 25d thereby form a first sleeve 201 of the fibre-reinforced polymer connecting member 20. The sleeve 201 effectively surrounds the full outer circumferential surface of the outer end the first tube 110. As the tubes 110, 120, 130 are cylindrical tubes, the respective receiving sections 2 lai - 25d3 all have a substantially concave inner surface and substantially convex outer surface (as best seen in figure 2C, and indicated as 23do for the outer surface of preform 23d and 23di for the inner surface of preform 23d).

[0061] The respective inner sides of the cross-sections of the receiving sections 2 lai - 25a3 of the first layer A are shaped, and in the current example also sized, correspondingly to the portion of the respective outer side of the cross-section of (i.e. the respective inner surfaces have the same shape and dimensions as the portion of the respective outer surface of) the respective tubular structural member 110, 120, 130 that is to be received in the respective receiving section 21al - 25a3 of the first layer. For layer B, that is arranged on top of layer A, the inner side of the cross-section of the respective receiving sections 2 lb 1 - 25b3 are shaped, and in the current example also sized, correspondingly to the portion of the respective outer sides of the receiving sections 2 lai - 25a3 of the first layer A. In particular, the respective inner surfaces of the three-dimensional preforms 21b - 25b of the second layer B are arranged to be closely fit, e.g. be identical to, the respective outer surfaces of the three-dimensional preforms 21a - 25a of the first layer A. This is applied, mutatis mutandis, to further layers, in this case the third and fourth layer C, D. And, in case a respective receiving section of the three-dimensional preforms is to be arranged inside of the a structural member, this may be applied mutatis mutandis.

[0062] The cross-sections of the receiving sections 2 lai - 25d3 are seen to have a substantially concave inner surface and a substantially convex outer surface, thereby having a cross-section that is substantially shaped as an annular sector. The cross-sections of the receiving sections 2 lai - 25d3 of increasing layers (as seen in an outward radial direction R with respect to the primary and secondary longitudinal axes I, II) thereby feature annular sectors having an increased sized, as the outer diameter of a lower layer substantially corresponds to the inner diameter of the directly there above arranged layer.

[0063] The respective sleeves 201, 202, 203 are further seen to taper from a maximum thickness at the respective root sections of the respective sleeves 201, 202, 203, that arranged near a central section 204 of the fibre reinforced polymer connecting member 20, towards a minimum thickness (in this example corresponding to the thickness of the preforms 2 la - 25a of the first layer A) at the opposing distal end of the respective sleeves 201, 202, 203. The taper can be easily obtained as a longitudinal size (i.e. as seen along the respective longitudinal axis I, II) of the three-dimensional preforms 21a -25d, in particular of the respective receiving sections 21al -25d3 thereof, decrease for increasing layers. In other words, the three- dimensional preforms 21a -25a of the first layer A, in particular of the respective receiving sections 21al - 25a3 thereof, are longer than the three-dimensional preforms 21b -25b of the second layer B, in particular of the respective receiving sections 2 Ibl -25b3 thereof, and so on.

[0064] The first embodiment 1 of figures 2A - 2C is further seen to comprise a staggered arrangement of preforms 21a - 25d, such that (for instance) the respective edges of the three-dimensional preforms 21al - 25a3 of the first layer A do not overlap with the respective edges of the three-dimensional preforms 2 Ibl -25b3 of the second layer B. In figure 2C it is clearly seen that this is achieved by having the sector size (i.e. a width), as seen in an angular direction around the secondary longitudinal axis, of the three- dimensional preforms 23a -23d, in particular of the respective receiving sections thereof increase with increasing layers. In particular the relative increase of the sector size is greater than the ratio of two times the thickness with respect to the outer diameter. In the current example, a strict staggering is applies, such that none of the edges of directly adjacent layers overlap, however, in certain case the edges may overlap, in particular in less critically loaded areas. The second embodiment 2 of figure 3, which is differs in the first embodiment only in the strictly non-staggered arrangement shown, shows the other extreme of the strict non-staggering. In this particular case, as the diameter increased with increasing layers, the sector size of the preforms 26a - 26d also increase in order to correct for this. It is noted however, that any combination of local staggered and non-staggered arrangements can be achieved.

[0065] Figures 4A - 4H schematically show, as a series of steps, an example for manufacturing a three- dimensional preform. Figure 4A shows that a stack of plies 310 comprising a plurality of at most partially impregnated, in particular substantially dry, planar plies of fibre-reinforcement material 311, 312, 313, which are interconnected by means of a binder (typically a material that is arranged to soften above a predefined temperature), is placed on a lower flexible diaphragm 320.

[0066] In figure 4B an upper flexible diaphragm 330 is arranged on top of the lower flexible diaphragm 320, thereby forming an interior space 340 in between said diaphragms 320, 330 wherein said stack of plies 310 is situated. Both diaphragms 320, 330 are formed from substantially gas-impermeable material, in particular a elastomeric material, and are arranged for jointly sealing said interior space 340. The lower and upper diaphragms 320, 330 are seen to be arranged in a respective lower and upper frame 321, 331 having respective lower and upper sealing means 322, 332 arranged at their lower ends. The upper sealing means 322 abut the lower frame 321 for providing a substantially gas-tight seal. The next step, as shown in figure 4C, is to remove air from the interior space 340 between the diaphragms to establish a vacuum pressure (i e . < less than atmospheric pressure, preferably less than lOOmbar), such that the stack of plies 310 is compressed between the respective diaphragms.

[0067] The compressed stack of plies 310 is subsequently heated, for instance by means of a (double-sided) infrared heater 350 as is shown in figure 4D. The stack of plies 310 is heated to a temperature above the predefined temperature, such that the binder softens.

[0068] The compressed and heated stack of plies 310 is subsequently moved (figure 4E) over a positive mould 360, forced to attain the shape of the positive mould 360 and left to cool (figure 4F). This is performed by causing a relative movement of the compressed and heated stack of plies 310 and the positive mould 360 towards each other. The diaphragms 320, 330 are thereby stretched over the positive mould and the therebetween held compressed and heated stack of plies 310 is forced to attain the shape of the positive mould 360. The lower frame 321 comprises the lower sealing means 322 that thereby seal a second interior space between the positive mould 360 and / or the thereto connected mould base 361 and the lower diaphragm 320. By removing air from the second interior space between the mould 360 / mould base 361 and the lower diaphragm 320 to establish a vacuum pressure, the lower diaphragm 320, and thereby the compressed and heated stack of plies 310, is forced to follow the exact contour of the positive mould 360.

[0069] The positive mould 360 is shaped to correspond to (a portion of) the arrangement of the plurality of structural members 110, 120 ,130 that are to be interconnected in the joint 1. The heated and compressed stack of plies 310 is thereby formed as a respective preform that is to be used in the method for forming the structural joint. After the stack of plies 310 is sufficiently cooled, the binder hardens again, such that the shape forced onto the stack of plies 310 is retained.

[0070] At this point, as is shown in figure 4G, the upper diaphragm 330 is removed and the manufactured three- dimensional preform 314 is obtained and can be removed from the lower diaphragm 320 that is still stretched over the positive mould (figure 4H). Using a variety of positive moulds 360, or an adjustable mould, the full set of three-dimensional preforms required for forming the fibre-reinforced polymer connecting member 20 of the structural joint 1 can be obtained. In some embodiment the actual arrangement of structural members 110, 120, 130 may be used as the positive mould, such that the three- dimensional preforms may be manufactured in-situ, i.e. at the site where, for instance, a multi -membered supporting structure is manufactured / assembled.

[0071] Figure 5 shows a flow-chart of some of the process-steps S101 - S106 of the method of manufacturing the structural joint. The first step S 101 is providing for a plurality of structural members, wherein said plurality of structural members comprises at least a first tubular structural member and providing for a providing a plurality of three-dimensional preforms. These preforms have previously been formed in a separate process, for instance using the process shown in figures 4A - 4H. Subsequently, in step S 102, the plurality of structural members is positioned with respect to each other in a predefined arrangement that corresponds to a predefined geometry of the structural joint that is to be manufactured.

[0072] In order to form the fibre -re inforced polymer connecting member onto respective portions of the plurality of structural members in that transitional area that covers and thus bonds together those portions of the plurality of structural members that together with the fibre-reinforced polymer connecting member for forming the structural joint between the respective structural members, a number of further steps are performed. In step S 103, the respective portions of the plurality of structural members in at least a part of the transitional area are covered by positioning said three-dimensional preforms in a predefined arrangement, such that at least the first tubular structural member of the plurality of structural members is received in the first receiving section of the at least one three-dimensional preform of said plurality of three-dimensional preforms. In step SI 04 the assembly of preforms and structural members is prepared for a resin infusion process, for instance by arranging a flexible mould around the assembly obtained in step SI 03. After the preparations are done, the resin is infused, in step SI 05, for wetting the three- dimensional preforms with a curable resin. Finally, in step S106, the wetted predefined arrangement of three-dimensional preforms is allowed to cure for forming the structural joint between the respective structural members.

[0073] The present invention is not limited to the embodiment shown, but extends also to other embodiments falling within the scope of the appended claims.

Claims

Claims1. Method of manufacturing a structural joint for use in a multi -membered supporting structure that is to be subjected to cyclic loading, the method comprising the steps of providing for a plurality of structural members, wherein said plurality of structural members comprises at least a first tubular structural member; positioning said plurality of structural members with respect to each other in a predefined arrangement that corresponds to a predefined geometry of the structural joint that is to be manufactured; forming a fibre-reinforced polymer connecting member onto respective portions of the plurality of structural members in a transitional area covering and thus bonding together those portions of the plurality of structural members that together with the fibre-reinforced polymer connecting member form the structural joint between the respective structural members, wherein the step of forming the fibre- reinforced polymer connecting member comprises the following steps, in any suitable order: o providing a plurality of three-dimensional preforms, wherein a three-dimensional preform is three-dimensionally formed prior to arranging said three-dimensional preforms onto the respective portions of the plurality of structural members in the transitional area from a stack of plies comprising a plurality of at most partially impregnated planar plies of fibre-reinforcement material that are stacked and mutually interconnected, wherein at least one three-dimensional preform of said plurality of three- dimensional preforms comprises at least a first receiving section that is substantially channel-shaped for receiving the first tubular structural member of said plurality of structural members therein, preferably wherein said receiving section has a cross-section that comprises at least a substantially C-, L-, V-, U- and / or J-shaped portion; o covering the respective portions of the plurality of structural members in at least a part of the transitional area by positioning said three-dimensional preforms in a predefined arrangement, such that the first tubular structural member of the plurality of structural members is received in the first receiving section of the at least one three-dimensional preform of said plurality of three-dimensional preforms; o wetting, after the step of covering, the three-dimensional preforms with a curable resin and allowing the wetted predefined arrangement of three-dimensional preforms to cure for forming the structural joint between the respective structural members.

2. Method according to claim 1, wherein said predefined arrangement comprises a plurality of different layers of preforms that are stacked on top of each other, such that a total number of fibre-layers comprised in the fibre-reinforced polymer connecting member at a specific location is defined by the number of different layers of respective preforms and the number of plies of each preform at the specific location.

3. Method according to claim 2, wherein the predefined arrangement is such that the number of three-dimensional preforms that are stacked on top of each other at a central section of the fibre- reinforced polymer connecting member are higher than the number of three-dimensional preforms that are stacked on top of each other at outer ends of the fibre-reinforced polymer connecting member; and / or wherein the number of fibre-layers comprised at the central section of the fibre-reinforced polymer connecting member are higher than the number of fibre-layers comprised at the outer ends of the fibre- reinforced polymer connecting member.

4. Method according to any of the preceding claims, wherein said plurality of three-dimensional preforms comprises a first set of three-dimensional preforms, wherein each three-dimensional preform of said first set is arranged with a first receiving section that is substantially channel-shaped, and wherein, after arranging the three-dimensional preforms in the predefined arrangement of three-dimensional preforms, the respective first receiving sections of the first set of three-dimensional preforms form a first sleeve around the first tubular structural member of said plurality of structural members in at least a first part of the transitional area, preferably wherein said first sleeve is a first tubular sleeve that surrounds the full outer circumference of the first tubular structural member of said plurality of structural members in at least the first part of the transitional area.

5. Method according to any of the preceding claims, wherein said plurality of structural members comprises a second tubular structural member; wherein at least one of said three-dimensional preforms of the plurality of three-dimensional preforms comprises at least a second receiving section that is substantially channel-shaped for receiving the second tubular structural member of said plurality of structural members therein, such that an inner surface of the substantially channel-shaped second receiving section faces an outer surface of the second tubular structural member, or for receiving a second tubular structural member of said plurality of tubular structural members thereon, such that an outer surface of the substantially channel-shaped second receiving section faces an inner surface of the second tubular structural member; wherein the step of covering the respective portions of the plurality of structural members is, respectively, further performed such that the second tubular structural member of the plurality of tubular structural members is received in the second receiving section of the respective at least one three- dimensional preform or such that the second receiving section of the respective at least one three- dimensional preform is received in the second tubular structural member.

6. Method according to claim 5, wherein said plurality of three-dimensional preforms comprises a second set of three-dimensional preforms, wherein each three-dimensional preform of said second set is arranged with a second receiving section that is substantially channel-shaped, and wherein, after arranging the three-dimensional preforms in the predefined arrangement of three-dimensional preforms, the respective second receiving sections respectively form a second sleeve around, or in, the secondtubular structural member in at least a second part of the transitional area, preferably wherein said second sleeve is a second tubular sleeve that respectively surrounds and connects the full outer, or inner, circumference of the second tubular structural member of said plurality of tubular structural members in at least the second part of the transitional area.

7. Method according to at least claim 4, wherein the predefined arrangement of three-dimensional preforms of a set respective set of preforms is such that the respective sleeve is formed by:- arranging a first subset of three-dimensional preforms such that the respective receiving sections are adjacent to each other and such that side edges of adjacently arranged receiving sections abut and respective inner surfaces of the adjacently arranged receiving sections form a respective circumferential inner, or outer, surface; and / or, when also dependent on at least claim 2, formed by:- arranging a second subset of three-dimensional preforms such that the respective receiving sections are on top of each other such that an inner surface of the receiving section of an above arranged three- dimensional preform abuts an outer surface of the receiving section of a below arranged three- dimensional preform, preferably wherein said above and below arranged three-dimensional preforms are arranged in a staggered arrangement, such that at least one of the side edges of the above arranged three- dimensional preform does not coincide with the side edges of the below arranged three-dimensional preform.

8. Method according to at least claim 5 or at least claim 6, wherein at least one three-dimensional preform of said plurality of three-dimensional preforms comprises a first and a second receiving section, such that the respective at least one three-dimensional preform is comprised in both the first and second set of three-dimensional preforms; and wherein the step of covering the respective portions of the plurality of tubular structural members comprises receiving the first tubular structural member in the first receiving section of the respective three-dimensional preform and, respectively, receiving the second tubular structural member in the second receiving section of the respective three-dimensional preform or receiving the second receiving section of the respective three-dimensional preform in the second tubular structural member.

9. Method according to any of the preceding claims, wherein at least one three-dimensional preform comprises the same number of receiving sections as there are tubular structural members that are to be comprised in the structural joint, wherein each receiving section is substantially channel-shaped and arranged for receiving a respective tubular structural member therein or thereon; and wherein the step of covering the respective portions of the plurality of tubular structural members comprises, respectively, receiving the respective tubular structural member in a respective receiving section of the respective three-dimensional preform, or receiving the respective receiving section of the respective three-dimensional preform in the respective tubular structural member.

10. Method according to any of the preceding claims, wherein, when the respective tubular structural member is received in a respective receiving section of the respective three-dimensional preform, an inner surface of a respective receiving section is shaped correspondingly to a portion of the outer surface of the respective tubular structural member, or to an outer surface of a below arranged three-dimensional preform whereon the respective receiving section is to be positioned during the step of covering the plurality of structural members; or wherein, when the respective receiving section of the respective three-dimensional preform is received in a respective tubular structural member, an outer surface of a respective receiving section is shaped correspondingly to a portion of the inner surface of the respective tubular structural member, or to an inner surface of a below arranged three-dimensional preform whereon the respective receiving section is to be positioned during the step of covering the plurality of structural members.

11. Method according to any of the preceding claims 4 - 10, wherein, when the respective tubular structural member is received in a respective sleeve, an inner surface of the respective sleeve is correspondingly shaped to a portion of an outer surface of the respective tubular structural member it is to at least partially surround and / or wherein the dimensions, in particular a diameter or radius, of an inner surface of a respective sleeve are / is substantially equal to the dimensions of the outer surface of the respective tubular structural member it is to surround, such that after forming the respective sleeve around the respective tubular structural member, the outer surface of the respective tubular structural member contacts the inner surface of the respective sleeve, in particular contacts substantially the full inner surface of the respective sleeve; or wherein, when the respective sleeve is received in a respective tubular structural member, an outer surface of the respective sleeve is correspondingly shaped to a portion of an inner surface of the respective tubular structural member it is to at least partially cover and / or wherein the dimensions, in particular a diameter or radius, of an outer surface of a respective sleeve are / is substantially equal to the dimensions of the inner surface of the respective tubular structural member it is to cover, such that after forming the respective sleeve in the respective tubular structural member, the inner surface of the respective tubular structural member contacts the outer surface of the respective sleeve, in particular contacts substantially the full outer surface of the respective sleeve.

12. Method according to any of the preceding claims, wherein the first tubular structural member is a tubular structural member having a cylindrical cross-section, preferably having a cylindrical cross-section over its entire length; and wherein an inner surface the first receiving section is substantially concave, preferably wherein an outer surface of the first receiving section is substantially convex, and / or preferably wherein the cross-section of the first receiving section is substantially shaped as an annular sector; and / or wherein the second tubular structural member is a tubular structural member having a cylindrical cross-section, preferably having a cylindrical cross-section over its entire length; and wherein, when the second tubular structural member is received in the second receiving section, an inner surface the secondreceiving section is substantially concave, or wherein, when the second receiving section is received in the second tubular structural member, an outer surface of the second receiving section is substantially convex, and / or preferably wherein the cross-section of the second receiving section is substantially shaped as an annular sector having a substantially concave inner surface and a substantially convex outer surface; and / or wherein each of the tubular structural members comprised in the structural joint have a cylindrical cross-section, preferably having a cylindrical cross-section over their entire length, and wherein inner surfaces of the respective receiving sections are substantially concave, wherein the respective outer surfaces are substantially convex, and / or wherein the respective cross-sections of the respective receiving sections are substantially shaped as annular sectors13. Method according to any of the preceding claims, wherein the structural joint is a uniaxial or an at least uniplanar joint of a K, X, T, N, KT, Y type, or a combination of these types in a uniplanar or multiplanar arrangement.

14. Method according to any of the preceding claims, wherein said joint is a newly constructed joint (i.e. virgin joint), in particular wherein the tubular structural members of said structural joint have, prior to forming the newly constructed structural joint, not been part of another structural joint.

15. Method according to any of the preceding claims, wherein the step of wetting the three- dimensional preforms comprises any of the following substeps: arranging the positioned plurality of tubular structural members having the predefined arrangement of three-dimensional preforms thereon and / or therein in a mould; causing a curable resin to flow into the mould and wet the predefined arrangement of three- dimensional preforms with the curable resin, preferably by using a vacuum infusion process, and, preferably, wherein the mould is a flexible mould, such as a vacuum bag.

16. Method of manufacturing a multi-membered supporting structure that is to be subjected to cyclic loading, in particular an offshore foundation structure for a wind turbine, wherein the method comprises the step of manufacturing a structural joint according to any of the preceding claims.

17. Three-dimensional preform for use in a method of manufacturing a structural joint according to any of the preceding claims 1 - 15, wherein said structural joint comprises a plurality of structural members that are to be interconnected in said joint, wherein the three-dimensional preform is formed from a stack of plies that comprises a plurality of at most partially impregnated planar plies of fibrereinforcement material that are stacked, mutually interconnected and formed to obtained a three- dimensional shape; said three-dimensional preform comprise: a plurality of receiving sections that are substantially channel-shaped for receiving, in or on each receiving section, a respective tubular structural member of said plurality of tubular structural memberstherein, preferably wherein said channel-shaped receiving sections have cross-sections that comprises at least a substantially C-, L-, V-, U- and / or J-shaped portion.

18. Three-dimensional preform according to claim 17, wherein an inner side of the cross-section of at least a first receiving section is shaped, and preferably sized, correspondingly to a portion of an outer side of the cross-section of the first tubular structural member that is to be received in the first receiving section, or wherein an inner side of the cross-section of the first receiving section is shaped, and preferably sized, correspondingly to a portion of an outer side of a different three-dimensional preform wherein the three-dimensional preform is to be arranged on; and / or wherein an outer side of a second receiving section is shaped, and preferably sized, correspondingly to a portion of an inner side of the cross-section of a second tubular structural member wherein the second receiving section is to be received, or wherein an outer side of the cross-section of the second receiving section is shaped, and preferably sized, correspondingly to a portion of an inner side of a different three-dimensional preform wherein the three-dimensional preform is to be arranged on.

19. Three-dimensional preform according to claim 17 or 18, wherein an inner surface of one or more respective receiving sections are, preferably of all receiving sections are, substantially concave, wherein an outer surface of the respective one or more receiving sections are, preferably of all receiving sections are, substantially convex, and / or wherein a respective cross-section of the respective one or more receiving sections are, preferably of all receiving sections are, substantially shaped as an annular sector.

20. Three-dimensional preform according to any of the preceding claims 17 - 19, wherein the respective receiving sections comprises a respective receiving section longitudinal axis running along a direction that is substantially perpendicular to the plane of the cross-section of the respective receiving section;- wherein said three-dimensional preform comprises a parallel set of receiving sections whose respective receiving section longitudinal axes are substantially parallel, in particular substantially coinciding; and / or- wherein said three-dimensional preform comprises a non-parallel set of receiving sections whose respective receiving section longitudinal axes are at anon-zero angle, preferably at an angle of at least 15°, preferably at least 30°, more preferably at least 45°, alternatively at approximately 90°, with respect to each other.

21. Three-dimensional preform according to any of the preceding claims 17 - 20, wherein the inner sides of the channel-shaped receiving sections of at least two receiving sections of the three-dimensional preform face towards substantially the same direction; and / or wherein the inner side of a first channel-shaped receiving section faces towards a first direction and wherein the inner side of a second channel-shaped receiving section faces towards a second directionthat is different from the first direction, preferably wherein the first and second directions are at an angle of at least 15°, preferably at least 30°, more preferably at least 45°, alternatively at approximately 90°.

22. Kit of three-dimensional preforms, comprising a plurality of three-dimensional preforms according to any of the preceding claims 17- 21, for use in method according to any of the preceding claims 1 - 15, wherein said three-dimensional preforms are, when arranged in a predefined arrangement, arranged for forming at least a portion of a fibre-reinforced polymer connecting member in and / or on, the plurality of structural members in a transitional area for forming a structural joint for use in a multimembered supporting structure that is to be subjected to cyclic loading.

23. Method of manufacturing a three-dimensional preform according to any of the preceding claims 17- 21, the method comprising the steps of:- providing a planar preform that comprises a plurality of at most partially impregnated planar plies of fibre-reinforcement material that are stacked and mutually interconnected;- forming said planar preform in a mould, preferably using a double diaphragm forming method, for obtaining the three-dimensional preform comprising a plurality of receiving sections that are substantially channel-shaped for receiving in, or on, each receiving section, a tubular structural member therein, or respectively thereon, preferably wherein said channel-shaped receiving sections have cross-sections that comprises at least a substantially C-, L-, V-, U- and / or J-shaped portion.

24. Structural joint for use in a multi-membered supporting structure that is to be subjected to cyclic loading, wherein said structural joint is obtainable by a method according to any of the preceding claims 1- 15; or wherein said structural joint comprises: a plurality of structural members that are positioned with respect to each other in a predefined arrangement that corresponds to a predefined geometry of the structural joint, wherein the plurality of structural members comprises at least a first tubular structural member; a fibre-reinforced polymer connecting member that is formed onto respective portions of the plurality of structural members in a transitional area covering and thus bonding together those portions of the plurality of structural members that together with the fibre-reinforced polymer connecting member form the structural joint between the respective structural members, the fibre-reinforced polymer connecting member comprising: a plurality of three-dimensional preforms according to any of the preceding claims 19- 23, wherein the plurality of three-dimensional preforms are arranged in a predefined arrangement for covering the respective portions of the plurality of structural members in at least a part of the transitional area, such that at least the first tubular structural member of the plurality of tubular structural members is received in a first receiving section of the at least one three-dimensional preform of said plurality of three- dimensional preforms; anda cured resin that bonds, in cooperation with the arrangement of the plurality of three-dimensional preforms, the plurality of structural members in the transitional area.

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