Abstract:
Chain link comprising two side limbs at both ends joint by curved end portions. The chain Link comprises at least one endless band of fiber material wound along the perimeter of the chain link. The fiber material follows the longitudinal direction of the limbs and the curvature of the end-portions. In a chain made of interlocking chain links of this type, all tensile loads are absorbed by the fiber material, whereas the bearing load due to interlink contact, plus the link shear and bonding stresses near the contact points are absorbed by the end portions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Application No. PCT/EP2008/061583, filed Sep. 2, 2008, which claims the benefit of European Application No. 07116747.2, filed Sep. 19, 2007, the contents of which is incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to a chain link and a chain, e.g. long, high strength chains of interlocking chain links for mooring lines and tension legs used to connect, offshore platforms or other buoyant marine constructions to the sea floor or anchor chains for yachts or other ships. 
     BACKGROUND OF THE INVENTION 
     Mooring lines and tension legs are generally made from steel link chain cables or polyester ropes having a cross sectional area of up to 750 cm 2 . In service they carry tensile loads for long periods while submerged in sea water. The weight of steel in sea water is 92 percent of its weight in air. Therefore, due to the weight of the steel chains, the buoyancy of the offshore platforms fixed to the sea floor by such chains must be larger than otherwise required so they can buoy the lines. 
     Transport and placement of steel mooring chains and tension legs is difficult due to their length and weight. Typically they are transported by ship or rail to a nearby port, and offloaded to very expensive heavy lift crane vessels or special anchor handling vessels for transportation and offshore installation. If their weight and bulk could be reduced substantially and their ability to be lengthened and shortened readily could be improved, then they could be assembled to a predetermined length and more, easily transported, handled, and more rapidly installed with less expensive and more readily available support vessels. 
     It has been proposed to use ropes of polyethylene fibers, such as the Dyneema® fiber of DSM. The offshore industry is already using polyester ropes for deepwater mooring applications. Such materials are approximately neutrally buoyant in sea water. The tensile strength of such materials is sufficient for long term mooring design. However, ropes have the drawback that they cannot be easily gripped, since their outer covering gets torn off, nor can they be held in place by chain stoppers. Ropes are also sensitive to the abrasive action of mud and sand particles which may penetrate and cause wear between the ropes fibers, thereby weakening of the rope. For these reasons it is often preferred to use metal chain links. 
     As opposed to fiber ropes, chain links can be held in place by chain stoppers. The chain stoppers can be used to secure the chain at a specific lengthy thereby adjusting the tension and optimizing the related station keeping performance. Typically, a chain stopper has two latches holding the chain in place, bearing upon the shoulders of a single link. A chain is pulled through the chain stopper until the desired position, chain angle and chain tension is obtained. An example of a chain stopper is for instance disclosed in U.S. Pat. No. 7,240,633. 
     Under axial load, the individual chain links are subjected to all forms of primary stresses, i.e. bearing, bending, shear and tensile stresses. Near the contact points between links, the bearing load due to axial tension is transformed into complex stress patterns that result in the highest stress in the bar at symmetric locations roughly +/−45 degrees on either side of the crown. Otherwise, for a normal steel chain link, much of the steel structure is highly underutilized. This is because the existing manufacturing processes and machinery using forged round bar stock are well embedded into the traditional chain making industry, resulting in very little advancement in the chain geometry or utilization of hybrid solutions. This is particularly the case when a link is held in a chain stopper. Due to cyclic loads, the chains are also susceptible to fatigue failure. In addition, during transport or installation of the chain the individual links may be subjected to high impact loads. 
     The complicated stress pattern within the individual chain links when the chain is under tensile load hinders a straightforward use of fibers or fiber reinforced material. In fibers, the greatest strength results when the direction of the fibers is in the direction of the load. Unidirectional composite materials have relatively low shear strength parallel to the fiber direction. Link-to-link attachments cause large stresses in the composite matrix in directions having inherently low strength. 
     U.S. Pat. No. 5,269,129 discloses a chain formed of links made of fiber-reinforced resin composite material. Each link has a terminal loop located at each axial end of a long strap. Loops, located at adjacent ends of successive links, are joined by relatively short connecting links that overlap bushings located within each of the loops. The bushings and connecting links are held in position at each lateral side of the links by pins and washers. A ring surrounds each link where the strap flares to form each terminal loop. The loops may be unitary or spaced laterally to receive within the space a unitary loop of an adjoining link aligned axial Iy with the other loop. A pin located within the loops supports washers at each lateral side of the links to maintain the position of the links and to transfer load between the links. Such chains have the drawback that they have only moderate impact resistance. The links comprise a number of washers, pins and other separate parts resulting in an elaborate to assembling of the chain. Moreover, the strength of the chain is determined by the strength of the pins linking the chain links. The chain links are shaped rather differently from the traditional interlocking toroid steel chain links, so their use requires modification of existing equipment and facilities, such as chain stoppers. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a light-weight link chain which overcomes these problems and combines high impact resistance and high tensile strength with traditional link shape so that existing facilities, in particular chain jacks and chain stoppers, can be used with it. 
     The object of the invention has been achieved by designing a chain link comprising two side limbs at both ends joined by curved end portions characterized in that the chain link comprises at least one endless band of fiber material wound along the perimeter of the chain link. The fiber material follows the longitudinal direction of the limbs and the curvature of the end-portions. This way, all tensile loads are absorbed by the fiber material, whereas the bearing load due to interlink contact, plus the link shear and bending stresses near the contact points are absorbed by the end portions. The link can be shaped in a similar configuration as traditional oval toroid link shapes, so that existing facilities such as chain stoppers can be used, or may be elongated to ultimately reduce the cost per unit length of the chain. The length to width ratio of the chain links can be made larger, e.g., to utilize fewer end pieces, while using longer limbs, e.g., limbs interconnected with one or more studs to form H-shaped or ladder shaped sections. Ladder shaped sections having multiple studs can be used to control global torsion and/or assembly. The studs can have a small diameter, e.g., a diameter smaller than the diameter of the limbs. Such extended designs can for example be used to replace steel tendons currently used with tension leg platforms. 
     The endless band of fiber material can be a band of a woven or unidirectional fiber material or combinations thereof, e.g. in different layers. To secure the fiber material and to give it extra strength the fiber material can be embedded in a polymeric matrix, such as an epoxy or polyester matrix. This is especially the case when the fiber material has been wound around the chain link more than once. 
     A suitable material for the end portions could be steel as used in existing offshore mooring chains, or, alternatively, specialty metals in the areas of high contact loads and stress, combined with other synthetic materials in the non-load bearing structure. Under tensile load, the end-portions form contact points between the various links. In use these contact points are heavily subjected to wear. By making the end portions of steel, the wear resistance of steel and the shear strength is combined with the high tensile strength of the fibers. The center-section can also be made of steel, but since the mechanical tensile stresses are carried by the fiber material, the limbs can be made with a smaller steel cross section or of a light weight material such as aluminum, or a plastic material, such as polyurethane, polyepoxy or polyester. 
     The end-portions—and optionally also the limbs—can be fitted with a recess along their outline in which the band of reinforcement fiber material is disposed. This way, the fiber material is effectively protected from impact and wear loads, and suffers less from impact fatigue. 
     In a specific embodiment the end-portions and the limbs can be formed by separate parts. The contact faces between those parts can for example be fitted with a pin and a corresponding pin hole or a similar joint to allow the pieces to interlock. 
     It is possible to tit the inside contact areas of the end-portions with engineered surfaces, much like the way the human shoulder works. The end-portions would come in two varieties: male and female. The male variety has an extrusion, the female a recess. The male piece is able to rotate and slide in the female recess which reduces the wear normally seen between the contact areas of two chains. This way, the service life of the chain link can be extended. 
     Optionally, the limbs can be linked by a stud to form an H-shaped center-section. This way, the chain becomes a studlink chain, which is less likely to get tangled than a studless chain, for example when in a chain locker or a bundle. 
     The fiber material can for example be installed in a predetermined tension, e.g. with a tension designed to most effectively mobilize the available strength of the different load bearing materials considering the geometry, and the different ultimate strength and moduli of elasticity. 
     Suitable fiber materials are for instance carbon fibers, polyethylene fibers, aramide fibers and glass fibers. Suitable polyethylene fibers are for instance the Dyneema® fibers commercially available from DSM. Suitable aramide fibers are for instance Twaron®, available from Teijin, or Kevlar®, available from DuPont. 
     In environments that are less extreme than offshore another embodiment of a chain link according to the present invention can be used. With this design, the end portions and limbs are surrounded by a sleeve of the fiber material. The end portions and limbs can for example be formed by a foam core comprising two mirrored shapes both forming half a side of a chain link. This foam can be polyurethane, although any material would be usable. The fibers can be embedded in a polymeric matrix, e.g. an epoxy or polyester matrix. Such a design can for example be attractive for the market of pleasure yachts and the like, where chains are generally subjected to much lower mechanical loads. The chains can be designed to be neutrally buoyant and could be marketed based on a fashionable and trendy outlook. 
     Such a chain link can be constructed by slipping a fiber sleeve over one of the mirrored shapes, then, the next shape would be placed next to the first one and the fiber would continue to be pulled or rolled from the first mirrored shape over the second mirrored shape. Preferably, the sleeve is somewhat longer than the perimeter of the core, so that the first end of the sleeve is slipped over its other end. This construction allows the chain to be made without any weak spots, since the fiber sleeve covers both the cores and essentially creates a one piece chain. The fiber sleeve can be made out of normal or pre-impregnated carbon fiber. However, pre-impregnated carbon fiber would have the advantage that the sleeve would not have to be treated with epoxies during application, thereby simplifying the construction process. 
     The chain links as described can form a chain by having the curved end portion of a chain link grip around a curved end portion of an adjacent chain link. These chains are particularly useful for anchoring a floating structure, such as a ship or an offshore platform, wherein at least the chain is used to link the floating structure, e.g., to a seabed. 
     The chain can for example be assembled by assembling a first chain link having a recess along its outer fiber, wherein subsequently a band of fiber material is wound around the chain link within its recess, then a second link is assembled having one curved end portion gripping around a curved portion of the first link, then the second link is rotated while a fiber supply winds a fiber material around the chain link, then these steps are repeated assembling further interlocking chain links until a chain of a desired length is obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing, wherein: 
         FIG. 1  shows in perspective view of a first embodiment, of a cable link according to the present invention; 
         FIG. 2A  shows a plan view of a second embodiment of a cable link according to the present invention; 
         FIG. 2B  shows a cross section along the line B-B′ in  FIG. 2A ;  FIG. 2C  shows a cross section along the line C-C′ in  FIG. 2A ; 
         FIG. 3A  shows a plan view of a third embodiment of a cable link according to the present invention; 
         FIG. 3B  shows a cross section along line B-B′ in  FIG. 3A ; 
         FIG. 3C  shows the body of the cable link according to  FIG. 3A ; 
         FIG. 4A  shows a plan view of a third embodiment of a cable link according to the present invention; 
         FIG. 4B  shows a cross section over line B-B′ in  FIG. 4A ; 
         FIG. 4C  shows a longitudinal cross section over line C-C′ in  FIG. 4B . 
         FIG. 5  shows a chain according to the present invention; 
         FIG. 6 : shows a device for assembling a chain according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a stud link  1  comprising a body  2  of two side limbs  3 ,  4  of a light weigh plastic material, such as polyurethane. At both ends the side limbs  3 ,  4  are mutually joined by curved steel end portions  5 ,  6 . The end portions  5 ,  6  have the shape of a circular segment, the first end portion  5  being of a smaller curvature radius than the other end portion  6 . The body  2  spans a band  7  of pre-tensioned unidirectional reinforcement fiber material, such as carbon fibers wound along the outer perimeter of the body  2 . The end portions  5 ,  6  and the side limbs  3 ,  4  comprise a recess  8  extending along the outer perimeter of the body  2  to receive the band of reinforcement fiber material. A crossbar or stud  9  spaces the two limbs  3 ,  4 . The crossbar  9  and the limbs  3 ,  4  are made of one single piece of a light weight plastic material, such as polyurethane foam. The end portions are made of steel. 
       FIG. 2A  shows a chain link  11  without a stud. The link  11  comprises a body  12  with side limbs  13 ,  14  and end portions  15 ,  16 . In this particular embodiment the end portions are of equal size. The link  11  is shown in cross section along line B-B′ in  FIG. 2B . In  FIG. 2C , the link  11  is shown in cross section along line C-C′. An endless band  17  of fiber material is sunk in an endless recess  18  extending along the outer perimeter of the body  12 . The limbs  13 ,  14  and the end portions  15 ,  16  are all provided with a recess. When assembled, these recesses are in line to form the endless recess  18 . 
       FIG. 3A  shows in plan view a third embodiment of a cable link according to the invention. Cable link  21  comprises a body  22  (see  FIG. 3B ) surrounded by a sleeve  27  of a fiber material embedded in a matrix of a cured polymeric resin, such as an epoxy resin. The sleeve  21  has its outer ends joined to each other to form a closed loop. The cable link has two straight sides  23 ,  24 , and two circularly curved end portions  25 ,  26 . As shown in  FIG. 3C , the body  22  is made of two C-shaped sections  28 ,  29 , each forming a curved end portion  25 ,  26  at both ends extended with a half section of the straight edges  23 ,  24 . Both C-shaped sections  28 ,  29  have one free end provided with a projection  30  and another free end provided with a correspondingly shaped recess  31 . The sections  28 ,  29  are glued together to form the body  22 . The body  22  is made of a single piece of a light weight plastic material, such as foamed polyurethane. 
       FIG. 4A  shows a further possible main link  41  according to the invention, shown in  FIG. 4B  m cross section along line B-B′.  FIG. 4C  shows the same chain link in longitudinal cress section along line C-C′ in  FIG. 4B . As in the embodiment of  FIG. 2A , a pre-tensioned fiber band  47  is sunk in a recess  48  extending over the outer perimeter of the body  42 . In this embodiment, one of the end portions  45  is provided with a bulge  49  on the inner perimeter of its curvature. The other end portion  46  is provided with a corresponding socket  50 . In an assembled chain, the bulge  49  of each cable link  41  is shaped to cooperate with the socket  50  of an adjacent cable link  41  to form a ball joint or articulation. 
       FIG. 5  shows a chain  51  made of interlocking oval toroid chain links  52 . Each chain link  52  comprises two side limbs  53  at both ends joint by curved end portions  54 . The curved end portion  54  of a chain link  52  grip around a curved end portion  54  of an adjacent chain link  52 . Each one of the chain links  52  is provided with an endless band  55  of fiber material wound along the perimeter of the chain link  52 . The endless bands  55  lay sunk within a recess  56  extending along the perimeter of the chain link  52 . The curvature of the curved end portions of the toroid links  52  have an inner diameter corresponding to the diameter of the side limbs  53 . Accordingly, the distance between the side limbs  53  corresponds to the diameter of the side limbs  53 . As a result, a chain link  52  can only slide in one direction relative to an interlocking adjacent link  52 , and the contact surface between two interlocking links  52  is maximized. Optionally, the side limbs  53  can bulge inwards, so that the movement of a link  52  relative to an interlocking adjacent link  52  is restricted to two degrees of freedom of rotational movement and the links  52  can only hinge in two directions relative to the respective adjacent link. 
     The chain links  52  in  FIG. 5  are of the type as shown in  FIG. 2A-2C , but they can also be of the type shown in  FIG. 1 ,  FIGS. 3A-3C  or  FIGS. 4A-AC  or any other suitable type of chain link according to the present invention. 
       FIG. 6  shows schematically a plan view of a device  60  for assembling a chain according to the present invention. The chain is made of oval toroid chain links  61  comprising two side limbs  63 ,  64  linked at both sides by C-shaped curved end portions  65 ,  66 . The device  60  comprises two parallel supply lines  67  for the simultaneous supply of two side limbs  63 ,  64  and a curved end portion  65 . A third supply line  68  extends in a direction perpendicular to the other two and serves to supply further curved end portions  66 . The three supply lines  67 ,  68  come together at a platform  69  with a U-shaped opening  70 , where an assembled chain link  61  is positioned in a vertical position, with its side limbs  63 ,  64  extending horizontally. The supply lines  67  transport the side limbs  63 ,  64  until they lay at opposite sides of the vertical chain link in the U-shaped opening  70 . A curved end portion  65  follows the side limbs  63 ,  64  to be attached to these at one end, while the third supply line  68  supplies the other curved end portion  66  which passes the open inner area of the vertical chain link  61  in the U-shaped opening  70  and is then linked to the outer ends of the present side limbs  53 ,  63 , thus forming a new chain link  61  interlocking the vertical chain link in the U-shaped opening  70 . The assembled chain link  61  on the platform  69  is then rotated while a spinner  71  spins a fiber material  72  from a roll  73  of fiber material around the chain link  61 . The fiber material is received in a recess  74  extending along the outer fiber of the chain link  61 . After winding the fiber material, the chain is moved further over a distance corresponding to the length of a single chain link  61 , and the newly assembled chain link  61  on the platform  59  is turned to a vertical position, taking the place of the chain link positioned in the U-shaped opening  70  and the stops described above are repeated until a chain of a desired length is obtained.