More particularly, the present invention relates to said flange and fastening of the axial reinforcement of the bending stiffener to the flange.

Traditionally, bending stiffeners of the above type are used in situations where requirements to the bending radius of the cables etc. exist, or where it in general is desirable to protect cables going into a receiving station. The invented bending stiffener may for example be used when guiding flexible risers into the floating construction of a tension leg platform.

Bending stiffeners of the present type are specially known in the offshore industry, and such bending stiffeners traditionally are made of a solid steel flange with welded on axial reinforcement toward the flexible part of the bending stiffener.

Experience shows that the connection between the flange and the axial reinforcement is very important for the properties of the bending stiffener for example in relation to the desired stiffness and design life (fatigue) . Unfortunately, the last mentioned property - fatigue - has in some cases resulted in that bending stiffeners of traditional construction have collapsed.

The bending stiffener of the present invention provides a better distribution of the forces and thereby a significantly increased fatigue strength in relation to the traditional solutions.

According to the invention this is achieved by a bending stiffener designed for mounting to a receiving station, preferably an offshore platform, for guiding cables, flexible risers, umbilicals etc. into an internal axial bore in the bending stiffener, which comprises a termination end arranged to the receiving station, a circular flange including an inner bore, where it to and internal to this is provided a cylindrical insert with an internal bore diameter equal to the bore diameter of the bending stiffener, a composition of a rubber compound with an internal bore, and outer dimensions equal to the bending stiffener is fastened to the end flange/insert and is formed in the longitudinal direction with a substantially conical outer taper toward the flexible end/- cable inlet end of the flexible stiffener. The composition further including layers of axial reinforcement, connected with crossing cord layers therebetween and reinforcement and an outer protection layer wrapped around the outer periphery of the bending stiffener. The bending stiffener device is characterized in that the flange is formed with a number of radial openings (windows) distributed around the flange, and the axial reinforcement runs from the flexible portion of the bending stiffener toward and through a radial opening in the flange and back toward the flexible portion of the bending stiffener through another radial opening in the flange. The invention will in the following be described by means of an embodiment of the invention and with reference to the attached drawings, in which:

Fig. 1 shows a sectional side view of the bending stiffener according to the invention, and Fig. 2 shows a cross section of the end flange of the bending stiffener of Fig. 1.

With reference to the drawings an embodiment of the bending stiffener 1 according to the invention is described. Manufacturing and design of this specific bending stiffener 1 are also described in this embodiment of the invention.

The construction (bending stiffener) 1 is based on an elastic composite construction terminated at a steel flange 2.

The elastic compound consists of an assembly of reinforcement cords 8, 9 and rubber matrix 7. The properties of such compounds are dependent on lay angle and fibers, fraction fill, thickness of rubber between fiber layers, loading direction and material properties. The chosen materials are within the production and manufacturing range of the company of the inventor. Extensive engineering has been carried out in order to develope a suitable design based on the materials available. Detailed analyses of the bending stiffener 1 has been performed by using FEM-techniques.

The bending stiffener 1 has an internal axial bore 6 and is designed for a flexible gas-injection riser with an inner diameter of 203,2 mm (8") and 414 bar internal pressure. The bending stiffener 1 is further designed for a design life of 20 years, and adapted to operate in an offshore environment in air, splash zone, and in submerged state. Structural elements made from polymeric materials, e.g. aramide fibers, shall be utilized to maximum 50% of the tensil braking strength. The design of the bending stiffener 1 is an optimal combination of material and manufacturing technology using elastic composites and advanced calculation techniques. The bending stiffener 1 is an elastic composite construction consisting of the following elements: steel end flange 2, axial aramide reinforcement 8, circumferential aramide reinforcement 11, cross-laid steel cord 9, natural rubber compound 7, rayon pearls 10, and outer layer of neophrene 12.

The bending stiffener 1 is shown in Fig. 1. The end flange 2 is bolted at 5 to the flexible pipe termination 14. The bending stiffener 1 is fixed to the end flange 2 by three mechanisms: 1) the rubber 7 is bonded to the steel 2,

2) the axial aramide reinforcement 8 is made by strips running from the flexible end 13 of the stiffener 1, through a

radial opening 3 in the flange 2 and back into the stiffener 1 through another radial opening 3. The area between the radial openings 3 is machined to a cylindrical shape to avoid stress concentrations. The cylinder diameter is more than 20 fiber- rope diameters. (20 rope diameters is used for obtaining full strength efficiency of aramide ropes 8) , and

3) a cylindrical steel insert 4 with a bulb at the outer end is used internally. This insert stiffener 4 is the innermost part of the construction 1 and provides additional anchoring of the stiffener 1 to the end flange 2.

The axial aramide reinforcement 8 is the main mechanism for terminating the forces. In addition, the axial aramide reinforcement 8 gives sigificant contribution to the bending stiffness of the construction 1. The circumferential aramide reinforcement 11 is wrapped on the outside of the stiffener 1. This is done to ensure that the compression side of the stiffener 1 can take the compressive stresses. With this circumferential reinforcement 11 the radial expansion is restricted at the compression side. The compression will hence results in pressure build-up in the rubber 7. This pressure will be a major part of the compression stiffness of the stiffener 1.

For high load designs additional stiffness is required. In the design this is achieved by inclusion of cross-laid steel cord 9 in the inner part of the stiffener 1. These cord layers 9 are bonded to the axial aramide reinforcement 8. The number of steel cord layers 9 and aramide layers 8 are selected to give adequate stiffness and design stresses. To avoid excessive shear stresses in the rubber 7, the effective axial stiffness of the cross-laid steel cord 9 is similar to the axial stiffness of the aramide 8.

The matrix selected is a natural rubber compound 7 due to the excellent bonding to the steel 2, 4, the excellent fatigue properties and that the material 7 is well suited for manufacturing of such devices. A clorophrene outer layer 12 is used to provide UV- and ozon-resistance of the stiffener 1.

The design analysis has been performed using the Finite Element Program NISA.

The bending stiffener 1 is built up in layers with natural rubber 7 and cord 8 on a steel mandrel with flanges that can be fastened in each end. The termination flange 7 is fastened in one end of the mandrel. Layers of rubber 7 are built on the mandrel. When the external rubber diameter match the diameter of the inner aramide cord layer 50 mm wide sheets with axial aramide cords 8 are applied. The aramide sheet (Kevlar) 8 is laid through the windows 3 of the termination flange 2, rolled flat and adhered to the rubber 7 in the specified length. Between the aramide sheets 8 corresponding rubber subjects 7 will be filled up. Two cross-laid layers of steel cord 9 are added at 37,5° angle with the aramide 8. Rubber 7 is laid until the diameter match the next aramide layer 8. The free ends of aramide layer one 8 are drawn through window 3 circle two, rolled out and then fastened. The operation is repeated until all the required axial aramide 8 and steel cord layers 9 are built in. One layer of circum- ferential aramide 11 is wrapped around the stiffener 1 to provide hoop strength of the stiffener l. Finally, a rubber layer of neophrene 12 is placed over the stiffener 1 and the terminating flange 2. Additionally, Neophrene 12 is applied on the back side of the flange 2 to protect the aramide. To ensure metal-to-metal contact in the bolting circle 5 the back side of the flange 2 is machined down in the area where rubber is applied.

The results of the FEM-analysis were compared with a full scale test, and the two test results were in good agreement to each other. The base case for the design 1 has been a 203,2 mm (8") with 414 bars gas riser used on a production vessel in 300 m water depth. This corresponds to a top tension load of 700 kN and 30° angle with a radius of curvature of 3,1 m in the bending stiffener 1. The weight of the bending stiffener 1 is about 1500 kg, the axial length is 1,8 m and the diameter at the end flange is 1,2 m.

The analysis has verified that the bending stiffener design 1 can meet the specified loads and the utilization of different components are below the acceptable values. The termination of the elastic composite to the steel flange 2 is achieved by mechanical termination of the reinforcing members 8 in a manner reducing the stress concentration. The termination does not include additional components or other additives. The design is based on standard materials available at the inventor, and there is a potensial for optimalization for selecting stiffer polymer materials which include short fiber reinforced rubber.