GB2265200A

GB2265200A - Tube joint

Info

Publication number: GB2265200A
Application number: GB9305067A
Authority: GB
Inventors: Geoffrey Charles Eckold
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1992-03-19
Filing date: 1993-03-12
Publication date: 1993-09-22
Anticipated expiration: 2013-03-12
Also published as: GB9205943D0; GB9305067D0; GB2265200B

Abstract

A joint (10) for connecting a tube to a rigid structure, for example a steel riser tube from a tension leg off-shore oil platform to a seabed wellhead, consists of a steel pipe (12) onto which is wound a layer (18) of fibre-reinforced composite material. The layer (18) tapers over the length of the joint (10), being thick at the end which is to be connected to the rigid structure. Consequently the joint provides a gradual change in stiffness between the tube and the rigid structure, and so overstressing of the tube is less likely. Layer (18) may comprise continuous glass or carbon fibres in an epoxy resin matrix. Layer (18) may adhere to or be slidable on pipe (12). <IMAGE>

Description

Tube Joint This invention relates to a joint for connecting a tube to a rigid structure.

Such a joint is required in off-shore oil platforms, in which a steel riser tube connects a platform to a seabed structure. There is a considerable difference in flexibility between the tubular riser and the sea-bed structure, and the joint experiences considerable stress.

According to the present invention there is provided a joint for connecting a tube to a rigid structure, the joint comprising a steel pipe with at its ends means whereby it may be fixed to the tube and to the structure respectively, and a layer of a fibre reinforced composite material around the pipe, the layer tapering along its length and being thickest adjacent to the means for fixing the pipe to the structure.

Such a tube joint provides a smoother transition between the comparatively flexible tube and the rigid structure, preventing overstressing of the tube, and reducing the bend curvature to which it is subjected.

The joint increases gradually in stiffness towards the rigid structure, which results in a smooth transition in curvature from the tube to the joint. However, in comparison to an all steel tapering joint, the joint of the invention, while providing adequate axial strength, is less stiff so the flange fixed to the structure is subjected to smaller bending moments. Furthermore the joint can be shorter, and also is easier and cheaper to manufacture.

The fixing means at the ends of the pipe may be flanges with apertures for bolts, or alternatively at one or each end the fixing means may be a threaded section of pipe, or may be an exposed part of the steel pipe which may be welded to the tube or to the structure. The composite layer preferably comprises continuous fibres, for example of glass or carbon, which are wound onto the pipe as a mandrel, embedded in a resin matrix.

Preferably the composite layer adheres to the pipe, but alternatively it may be slidable on the pipe.

The invention will now be further described, by way of example only, and with reference to the accompanying drawing which shows a longitudinal sectional view of a joint.

A tension leg platform is a floating platform fixed by vertical tension legs to piled foundations on the sea bed. The structure is compliant, so the platform can undergo lateral movements (sway, surge, and yaw), while the tension legs restrain vertical movement (heave, pitch, and roll). The floating platform is provided with sufficient buoyancy to maintain the legs in tension in all loading and environmental conditions. Such a platform can be used for oil well drilling or production, and can adopt conventional fixed-platform drilling and production technology and equipment. One such item of equipment is a riser, that is to say a steel tube connecting the floating platform to a subsea wellhead.

If, for example, the sea bed is 350 m deep, the riser would be a steel tube of length about 375 m, and might be made up of 9.1 m long steel pipes threaded or welded together, or with flanges bolted together, each of internal diameter 220 mm and external diameter 244 mm.

Where such a riser is joined to the wellhead there is a sudden change in stiffness, and as the floating platform moves problems may arise at the joint resulting in the riser kinking. The joint is therefore designed so the change in stiffness is more gradual. Typically the loads to which the joint is subjected would be: Bottom - Moment 5.65 x 108 N mm Shear force 6.68 x 104 N Tension 1.34 x 106 N Top - Moment 1.36 x 108 N mm Tension 1.34 x 106 N Internal pressure 34 MPa External pressure 3.4 MPa The joint is designed to have the same stiffness at its upper end as the riser tube to which it is connected, and to be sufficiently stiff that if subjected to the above loads its radius of curvature does not become less than 38.1 m (to prevent damage to components within the riser tube, such as umbilicals).The length of the joint is only slightly longer than that of a single steel pipe of which the tube is formed.

Referring to the drawing, the joint 10 comprises a steel pipe 12 of the same dimensions as those forming the tube, that is to say of length 9.1 m, internal diameter 220 mm, and external diameter 244 mm. To its lower end is welded a flanged end portion 14 so it can be bolted to the wellhead, while to its upper end is welded a threaded connector 16 by which it can be connected to the lower end of the riser tube. A layer 18 of a fibre reinforced composite material, comprising carbon fibres and epoxy resin, is wound onto the outer surface of the pipe 12 with the fibre winding angle being at plus or minus 15 degrees to the longitudinal axis 20 of the pipe 12. The thickness of the layer 18 tapers uniformly from zero at the top of the pipe 12 to 36 mm at the bottom of the pipe 12 (adjacent to the flanged portion 14).The thickness of the layer 18 at the bottom end of the pipe 12 is such as to reduce the stresses in the steel pipe 12 to within acceptable limits (less than about 450 MPa). The composite layer 18 extends over part of the flanged end portion 14. When the resin has been cured it will be appreciated that the composite layer 18 adheres to the pipe 12 along its entire length.

The winding of the composite layer 18 can be carried out using conventional equipment, provided it is of sufficient bed length. At the end of each winding layer the winding angle must change from +A through 90 degrees to -A, where A is the desired winding angle, 15 degrees in this case. In making the tapered layer 18 of the joint 10, the tapered composite is built up during the winding process by gradually moving the position at which turnround occurs along the pipe 12. This may be achieved using pin winding, that is by fixing a removable band of pins pointing radially outwards at each turnround position; the pins catch the fibres and restrain them as the angle of winding changes from +A to -A. The pin band is then moved along the length of the joint 10 to provide the required taper. A pin band may similarly be used at the lower end of the joint, and here too it may be desirable to move the pin band during winding so as to taper the layer thickness over the flanged end portion 14.

It will be understood that the nature of the fibres and of the resin, the winding angle, and the thickness and taper of the composite layer can differ from those stated above, and must be chosen to suit the application of concern.

If desired, the composite layer may be arranged to be close-fitting about the pipe but slidable on it, by suitably coating the surface of the pipe before winding the layer, as this can reduce the stress in the pipe.

Furthermore the winding might be performed without use of a pin band, for example by using a thermoplastic matrix for the composite, such that the matrix sets as the tow is wound onto the pipe and the matrix-fibre tow remains in position.

Claims

1. A joint for connecting a tube to a rigid structure, the joint comprising a steel pipe with at its ends means whereby it may be fixed to the tube and to the structure respectively, and a layer of a fibre reinforced composite material around the pipe, the layer tapering along its length and being thickest adjacent to the means for fixing the pipe to the structure.

2. A joint as claimed in Claim 1 wherein the composite layer adheres to the pipe.

3. A joint as claimed in Claim 1 wherein the composite layer is slidable on the pipe.

4. A joint as claimed in any one of the preceding Claims wherein the layer comprises fibres wound at a winding angle of 150 to the longitudinal axis of the joint.

5. A joint for connecting a tube to a rigid structure substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

6. A method for making a joint for connecting a tube to a rigid structure substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.