GB2038977A - Joining fibre-reinforced tubular structures - Google Patents

Abstract

This invention is concerned with fibre-reinforced resin pipes intended for applications in which the pipes will be subject to high-axial loadings, for example marine risers used in offshore oil or gas installations. Problems have hitherto been encountered in satisfactorily joining together such pipes, largely because of the often insufficient tensile strength of the jointing means. The present invention provides improved, screw-threaded jointing means and more particularly provides an open-ended tubular structure (3, 6) of fibre-reinforced resin material, at least an end portion thereof having integrally formed therewith an internal or external screw thread (4, 7), a substantial proportion of the fibres comprised in the screw thread extending substantially longitudinally of the structure. <IMAGE>

Description

SPECIFICATION Fibre reinforced resin structures This invention relates to fibre reinforced resin structures.

In the oil industry it is well known to use steel pipes to convey oil and liquid derived therefrom frdm one place to another. In offshore installations, however, the use of steel pipes is not always acceptable because of the highly corrosive environment. In particular, pipes which are used to convey crude oil from an oil bearing zone to a rig at the surface (often referred to as down hole tubing) are subject to harsh conditions and it would be desirable to use pipes made of a substantially non-corrodible material. It has been proposed, for example in the complete specification of our British Patent Application No 41055/75, (Serial No 1553220) to use pipes made of fibre reinforced resin instead of steel for a similar application, namely in a marine riser. The disclosure of that specification is incorporated herein by way of reference thereto.That specification describes, infer alia, methods of joining lengths of fibre reinforced pipes by means of spigot and socket joints. Such joints do, however, have certain disadvantages. In particular, axial displacement of a pipe is necessary in order to interengage an adjacent spigot and socket, which is often inconvenient, and additional steps are necessary in order to render the joint leakproof and to retain the spigot in the socket.

Further, such joints are considered unsuitable for down hole tubing applications for which small diameter pipes (eg about 3" as opposed to typically 18" in the case of marine risers) are used.

Further, it has been proposed to form screwthreaded sections on pipes, formed from dough moulding compounds containing randomly oriented, short fibres, for the purpose of jointing but such joints have, in arduous conditions, suffered from the disadvantage of insufficient tensile strength.

According to the present invention there is provided a tubular structure of a fibre reinforced resin material at least an end portion of said structure having integrally formed internally or externally thereof a screw thread, a substantial, preferably a major (i.e. more than 50%), proportion by volume of the fibres of the reinforcement comprised in the screw thread being preferentially oriented to extend substantially longitudinally of ths structure.

A structure of the invention may have a screwthread along a substantial part or the whole of its length and as such will normally constitute a structure that is subsequently incorporated in an end of a separately formed fibre reinforced resin pipe. In that case the structure will normally be secured to the pipe during the manufacture of the pipe. Alternatively, a screw thread may be formed integrally at both ends of a pipe during the manufacture of the latter.

By "substantially longitudinally" there is meant exactly longitudinally or in a direction that is up to 250, preferably up to 15 , to the longitudinal direction. In addition, some of the fibres may extend circumferentially of the structure to give the structure increased hoop strength. In any given structure there may be any combination of fibre directions provided that a substantial, preferably major, proportion by volume of the fibres lies substantially longitudinally of the structure.

The screw thread may be a parallel screw thread but is preferably a tapered screw thread.

In a preferred embodiment of the invention the structure is of a material made by a method described in our British Patent Specification No 1 434 926. Thus, the structure preferably comprises a plurality of resin impregnated tows (tapes) laid up so as to form a cylinder, the screw thread having been formed externally or internally of the cylinder by a moulding process prior to complete curing of the resin. The tows are laid up in such a manner that a major proportion of the fibres lies substantially longitudinally of the structure and each tow is typically about 8 mm wide and contains from 5000 to 10000 fibres laid in side by side contact.

In offshore oil installation applications we have found that carbon fibres are especially useful in the present invention. Generally, fibres having a tensile modulus within the range 70 to 400GN/m2, preferably 10 to 235GN/m2 will be used. Examples of alternative fibre materials are glass and polyamides (for example Kevlar).

Preferred resins are epoxy or phenolic resins, but other resins may be used. An example of an especially suitable resin is LZ576/HZ576, an epoxy resin made by Ciba Geigy.

The present invention also provides a method of manufacturing a structure as defined above which comprises forming a cylinder of a fibrecontaining, uncured or only partially cured resin, a substantial, preferably a major, proportion by volume of the fibres of the reinforcement lying in a substantially longitudinal direction, moulding an internal or external screw thread at least on an end portion of said cylinder and then curing the resin.

In a preferred method of the invention the fibres, preferably in the form of tapes as described above, and uncured or partially cured resin are laid up in a rigid, internally cylindrical tool, for example of steel, having a female screw thread formed on its internal surface, thereby forming a cylindrical lining of fibre-containing, uncured or only partially cured resin in the rigid mould. A radially deformable, externally cylindrical tool, for example of a rubber such as silicone rubber, having on its external surface a male screw thread, is then inserted into, and lined up with, the lined rigid tool.

Internal pressure is then applied to the deformable tool by means of, for example, a tapered plug in order to expand the deformable tool into contact with the fibre/resin lining whereupon both internal and external screw threads are formed in the lining. The resin is then fully cured and the structure then freed from the mould. Such a structure may then be further processed into a structure having a male or female thread as described below.

It has been found that in order to be able to use the preferred technique just described, the thickness of the fibre resin lining should preferably not exceed about 1 mm in the case where carbon fibres are used. That is to say that in the case of resin impregnated carbon fibre tows having a typical thickness of about 0.1 5 mm the cylindrical lining should not comprise more than about six layers of such tows and usually there will be from two to five such layers. In order to increase the wall thickness and hence the strength of the structure, in cases where the internally and externally threaded structure just described is eventually to present a female thread, the external surface of the structure may be overlaid, for example overwound, with further fibres which themselves may be impregnated with a resin which is subsequently cured.Such overwinding and impregnation may form part of a process for manufacturing a fibre reinforced resin pipe into which the structure is to be incorporated.

Similarly, where the internally or externally threaded structure is eventually to present a male thread, the internal surface of the structure may be coated with a fibre/resin mixture, the resin being subsequently cured. In the latter case the structure preferably has an unthreaded section which may be overwound by fibres used to manufacture a pipe to which the structure is to be secured. As will be appreciated, mateable male and female structures may be made by the above described method using appropriately dimensioned tools.

It is to be understood that the above described moulding technique, whilst preferred, is given as an example only and other suitable moulding techniques will readily occur to those skilled in the art.

After manufacture of a structure according to the invention, the screw thread is preferably coated with a release agent.

The present invention also provides a tube incorporating a structure as defined above.

A pair of mutually engageable structures of the invention will now be described in more detail by way of example only with reference to the accompanying drawing which is a longitudinal section through the structures which are shown incorporated in respective pipes that can be joined together.

Referring to the drawing, there is shown a pair of fibre reinforced plastics pipes 1 and 2 that are to be screwthreadedly joined together. The pipe 1 is accordingly provided with a female-threaded portion and the pipe 2 is provided with a male threaded portion. The female-threaded portion comprises a substantially cylindrical moulding 3 formed internally with a screw thread 4. The thread does, in fact, taper slightly but it could be a parallel thread.

The moulding 3 was manufactured by the preferred moulding technique described earlier and comprises a plurality of substantially longitudinally extending carbon fibres which, at the thread surface, follow the thread contour and are embedded in expoxy resin LZ576/HX576. The carbon fibres were initially laid up in a steel mould in the form of 8 mm tows of carbon fibres containing from 5000 to 10000 fibres laid side by side and impregnated with uncured resin. Such tows are described in our British Patent Specification No. 1 434 926. The moulding 3 may additionally comprise, for example, a plurality of circumferentially extending fibres in order to increase the hoop strength of the moulding 3. In particular, the root of the screw thread 4 may include one or more tows of carbon or other fibres embedded in the epoxy resin.In any event, a major proportion by volume of the carbon fibres preferably lies in a substantially longitudinal direction.

During the manufacture of moulding 3 by the aforesaid method, internal pressure is applied to an ^externally threaded, silica o rubber tool by means of a tapered plug after heatir r the assembly of tools and epoxy resin 'fi re mixture to about 1 000C. The temperature is then increased to about 1 400C at which temperature resin gelation rapidly occurs. After curing, the moulding is freed from the mould tools and is post-cured at about 1 800C. The moulding is then incorporated into the tube 1.The moulding 3 may, however, be incorporated into the tube 1 before the resin of the moulding 3 has fully cured provided that the partcured moulding is held rigid by the use of supporting tooling of matching shape and dimensions. The moulding 3 is incorporated in the tube 1 by bonding it to the machine-tapered internal surface 5 of the tube 1. As an alternative, the moulding 3 could be incorporated in the tube 1 during the manufacture of the latter, the fibres used in the manufacture of the tube being wound over the moulding 3.

The male threaded portion of pipe 2 comprises a moulding 6 having an external screw thread 7.

The moulding 6 was produced in a manner similar to that of moulding 3 and is shown to be secured in the end of tube 2 by helically wound fibres/ resin used in the manufacture of that tube.

Alternatively, however, the moulding 6 may merely be bonded to the external surface of the pipe 2.

Claims (1)

1. An open-ended tubular structure of fibrereinforced resin material, at least an end portion of said structure having integrally formed internally or externally thereof a screw-thread, a substantial proportion of the fibres of the reinforcement comprised in the screw thread extending substantially longitudinally of the structure.

2. A structure according to Claim 1 wherein more than 50% by volume of said fibres comprised in the screw thread extend substantially longitudinal of the structure.

3. A structure according to Claim 1 or Claim 2 wherein a proportion of said fibres comprised in the screw thread extend substantially hoopwise.

4. A structure according to any one of Claims 1 to 3 wherein the screw thread is a tapered screw thread.

5. A structure as claimed in any one of Claims 1 to 4 comprising a plurality of laid-up, resin inpregnated tows, each tow comprising a plurality of fibres laid in the same plane in side to side contact.

6. A structure according to Claim 5 wherein each tow contains from 5000 to 10000 fibres.

7. A structure according to any one of Claims 1 to 6 wherein said fibres comprise carbon fibres, glass fibres and/or polyamide fibres.

8. A structure according to any one of Claims 1 to 7 wherein said resin is an epoxy or phenolic resin.

9. A structure as claimed in Claim 1 substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.

10. A method of manufacturing a structure as claimed in Claim 1 which comprises forming a hollow cylinder of a fibre-containing, uncured or only partially cured flowable resin, a substantial proportion by volume of the fibres comprised in the screw thread lying in a substantially longitudinal direction, moulding an external or internal screw thread at least on an end portion of said cylinder and then curing the resin.

1 A method according to Claim 10 wherein said fibres are in the form of tows as specified in Claim 5 or Claim 6.

12. A method according to Claim 10 or Claim 1 1 including the steps of laying up said fibres and flowable resin in a rigid, internally cylindrical female tool having a screw thread formed on at le,ast part of the length of its internal surface thereby forming a cylindrical lining of fibrecontaining resin in the mould, co-axially locating in the lined tool a radially expansible, externally cylindrical male tool having a screw thread on at least a part of the length of its external surface, expanding said male tool into contact with the fibre/resin lining and then curing the resin thereby yielding a structure having both internal and external screw threads.

13. A method according to Claim 12 wherein said radially expansible male tool is made of a rubber.

14. A method according to Claim 12 or Claim 13 wherein the male tool is expanded by inserting a tapered plug thereinto.

1 5. A method according to any one of Claims 12 to 14 comprising the further step of overlaying the external or internal surface of the structure with fibres optionally impregnated with a resin which is subsequently cured so as to form a female or male threaded structure respectively.

16. A method as claimed in Claim 10 conducted substantially as hereinbefore described with reference to the accompanying drawing.

17. A structure whenever manufactured by a method as claimed in any one of Claims 10 to 1 6.

18. A pipe adapted to withstand high axial loading having at one or both ends thereof a structure as claimed in any one of Claims 1 to 9 and 17 to enable the pipe to be screw threadedly coupled to a further pipe or other element.

19. A pipe as claimed in Claim 19, the remainder of which is made of a fibre reinforced resin material.

21. A pipe as claimed in any one of Claims 18 to 20 which is a marine riser.