GB2107422A

GB2107422A - Reinforced pipes

Info

Publication number: GB2107422A
Application number: GB08229519A
Authority: GB
Inventors: Norman C Fawley
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-10-20
Filing date: 1982-10-15
Publication date: 1983-04-27
Also published as: CA1225915A; JPS5874984A; MX164343B; FR2514858A1; GB2107422B; JPH0159474B2; FR2514858B1

Abstract

The invention relates to the treatment of pipes to increase the ductile fracture resistance in the axial direction. According to the invention a plurality of continuous undirectional high-strength fibres are wound around a pipe (10) in the form of a wrapping (14). The fibres are typically tied in bundles (20), (Figures 4-6 not shown) and the fibre wrapping (14) itself may be protected by an outer layer (16), also wrapped around the pipe. The pipe may be steel, aluminium, copper or brass and the fibres may be applied with epoxy, coal tar or asphalt. The outer layer (16) may be coal tar or asphalt-saturated fibre tissue or asbestos felt. The burst strength of the treated pipe in the circumferential direction may be equal to its burst strength in the longitudinal direction. <IMAGE>

Description

SPECIFICATION The treatment of pipes The invention relates to the treatment of pipes to enhance the strength and durability. Particularly, it relates to a treatment which serves to increase the ductile fracture resistance of a pipe.

Pipes, and pipelines formed thereby, are playing increasingly important roles in the transportation of gas, oil, water and other fluids. For the past several decades, pipes have been treated in many manners to improve their serviceability. For example, pipelines installed underground have been coated or wrapped with various materials, such as bituminous materials fiberglass mat, plastic tape and the like for protection from electrolytic and biochemical corrosion, cyclical soil stress, cathodic disbonding and mechanical damage. However, relatively little attention has been directed to improvements in terms of burst strength, fracture prevention, durability, heat resistance, safety factor and weight, and the few attempts at the latter improvements have resulted in additional problems.For example, relatively large diameter wire has been wrapped around the pipe for increasing the radial, or hoop strength of the pipe.

However, these arrangements are susceptible to crevice corrosion caused by the accumulation of moisture, and the resultant formation of corrosion in the spaces between the adjacent sections of the wire, and between the wire and the pipe.

The present invention seeks particularly to provide a treatment for pipes which improves the ductile fracture resistance thereof. According to the invention, a plurality of continuous, unidirectional, highstrength fibres are wrapped around the pipe and adhere to each other and to the pipe. The fibres can thus take up a portion of the circumferential stress in the treated pipe and the pipe itself takes up the longitudinal stress. Further, the wrapping can be controlled so that the burst strength of the treated pipe in the circumferential direction is substantially equal to its burst strength in the longitudinal direction.

By the method of the present invention increased strength and ductile fracture resistance characteristics are imparted to the pipe with a minimum of added labour and materials, and without causing any additional problems, such as the aforementioned crevice corrosion. The method is simple, moreover, by being similar, from a production standpoint, to that used in providing corrosion protection for the pipe.

In preferred embodiments of the invention, the fibres are applied with a viscous substance, such as enamel or epoxy, which forms a homogenous matrix which is bonded to the pipe. Alternatively, the fibres can can be applied with an adhesive backing or pr.e-impregnated with an enamel or an epoxy.

The invention will now be described by way of example and with reference to the accompanying drawing wherein: Figures 1 to 3 are perspective views of a pipe section being treated according to a method of the present invention; and Figures 4, Sand 6 are partial, enlarged, perspective views of configurations of unidirectional, highstrength fibres that can be used in the method of the invention.

The present invention will be described in connection with steel or other metallic pipes of a relatively large diameter which are treated to provide protection against electrolytic and biochemical corrosion, cathodic disbonding, soil stress and mechanical damage, to dramatically increase its strength in the circumferential direction and to increase its ductile fracture resistance in the axial direction.

The metallic pipe is preferably cleaned by either sand or grit blasting or by mechanical scraping and wire brushing to render the pipe surface free from oil, grease, dust, moisture and non-adhering mill scale. A primer can then be applied to the outer surface of the pipe to provide a bonding agent between the steel pipe and a viscous substance to be applied subsequently. The primer can be of any known substance, such as AWWA type B, which is preferred since it has a greatly increased bond factor yet enjoys a fast drying time and is completely compatible with both asphalt and coal tar coatings.

After the pipe has been primed according to the foregoing, a hot viscous substance, such as coal tar enamel or an asphalt enamel is applied to the primed outer surface of the pipe. Such a step is more particularly illustrated in Figure 1 of the drawings which shows a section of a pipe 10, to the surface of which hot enamel is applied by a discharge head shown, in general, by the reference numeral 12. The head 12 is spaced from the pipe 10 and is adapted to discharge the enamel onto the outer surface of the pipe. The head 12 can be of a conventional design and is preferably in the form of a weir, a flood box, a flood coater or a curtain coater which receives the enamel at an elevated temperature, preferably approximately 500 F (260 C), and discharges it onto the outer surface of the pipe 10.In this coating step, the pipe 10 may be moved longitudinally and rotated relative to the head 12 or a machine can be provided which traverses the fixed length of pipe and includes a head, such as 12, for discharging the enamel onto the outer surface of the pipe.

The aforementioned coal tar enamel is preferably made from coke oven pitch which is modified and filled, while the asphalt enamel can be produced from selected petroleum crudes that are oxidized and filled, both in a conventional manner.

While the coating of enamel is still hot, i.e., before it completely drys or bonds, a layer of a plurality of continuous unidirectional high strength inorganic, electrically non-conductive fibres are wrapped around the pipe. More particularly, and referring to Figure 2, a continuous strip or wrapping 14 of the continuous, unidirectional high-strength fibres is applied to the treated outer surface of the pipe 10 by winding the web around the pipe as shown. The wrapping 14 is wound on the pipe in one or more layers as needed with minimum tension, i.e., just sufficient tension to ensure that the web adequately adheres to the pipe. The fibres forming the web are preferably in the form of glass, or other materials having properties equivalent to glass, in a configura tion to be described in detail later.

After the wrapping 14 of unidirectional fibres has been applied to the pipe, and while the enamel is still in a viscous hot state, an outer warp of protective material is applied to the pipe. In Figure 3, a wrapping of the protective material is shown by the reference numeral 16, and is wound around the pipe (which has been previously covered by the wrapping 14 of unidirectional fibres) as shown. The outer wrapping 16 may be of any material that will provide protection against mechanical damage during transportation and, when applicable, burial of the line; as well as protection for the enamel and fibres against cyclical soil stress and damage by stones or rocks.

The most preferred material for the outer wrapping 16 would be coal tar or asphalt-impregnated, reinforced fibre tissue or coal tar or asphalt-saturated, reinforced asbestos felt. Since the outer wrapping 16 is applied while the previously applied enamel is still in a hot or unbonded state, it is bonded to the unidirectional fibres and the pipe when the enamel hardens upon bonding.

The fibres and the bonded enamel form a homogeneous matrix which is bonded to the pipe and imparts increased strength to the pipe and prevents corrosion as will be described in detail.

It is understood that the wrapping 14 can be wound in a manner so that the unidirectional fibres extend in a plane substantially perpendicular to the axis of the pipe, with the degree of overlap between each section and its adjoining section typically varying from 1% to 50%. The wrapping 16 can be wound in the same manner with the same range of overlap.

As in the case of the application of the viscous substance,thewrappingsl4and l6can be applied by a stationary roller, a payoff head, or the like while the pipe 10 is moved longitudinally and rotated; or alternatively, the pipe could remain stationary and a machine or machines equipped with, or payoff heads, for applying the wrappings 14 and 16 would traverse and rotate about the pipe. Since these techniques are well known in the art, they will not be described in any further detail.

A section of the wrapping 14 is shown in more specific detail in Figure 4. More particularly, the individual, continuous, unidirectional fibres forming the wrapping 14 are shown by the reference numeral 18 and are separated into a plurality of rovings, or bundles, 20 by a transversely extending string (or strings) 22 which extends through the bundles in an alternating "over-under" pattern as shown to provide the separation.

A wrapping according to an alternative embodiment is depicted in Figure 5. In this embodiment, the unidirectional fibres 18 are separated into a plurality of of bundles 20' which extend for a smaller width than those of the embodiment of Figure 4 with the string 22 extending through the bundles as shown.

Although not clear from the drawings, it is understood that the string 22 can be braided, or otherwise tied, in a mannerto hold the bundles 20' in place.

According to the embodiment of Figure 6, a wrapping 14" is formed by a plurality of bundles 20" each of which is formed by a plurality of unidirectional fibres 18 that are stitched in a "Z" pattern by a thread 24 extending through the entire longitudinal length of each bundle.

According to still another embodiment, the use of a string orthe like is eliminated and the fibres 18 could be applied to the pipe in bundles by a mechanical winder or the like.

According to an alternate embodiment of the present invention, the unidirectional fibres can be adhered to the pipe by eliminating the step of applying hot enamel and, instead, applying an adhesive to the fibres before they are wrapped onto the pipe. This can be done by forming a wrapping 14 of fibres by any of the techniques disclosed above, and then applying an adhesive substance of any conventional type to either surface of the wrapping 14. Then a backing material of any conventional type could be applied to one adhesive-coated surface of the wrapping 14 and the other adhesive-coated surface could be wrapped around the pipe in a manner similar to that depicted in Figure 2.

Alternatively, an adhesive coating could be applied to one surface of the wrapping 14, a backing applied to said adhesive-coated surface and another adhesive coating applied to the other surface of the backing material which then is applied to the pipe in the manner discussed above.

According to a still further embodiment of the present invention, the fibres can then be impregnated by a liquid substance, such as epoxy, enamel, or other similar type material, before they are wrapped around the pipe. According to this embodiment, the fibres are passed, preferably in the bundle configuration discussed above, through a bath or over a coating roller or the like to impregnate them with the liquid substance. The fibres thus coated could be then immediately wrapped around the pipe as shown in Figure 2 while the liquid substance is still wet, and, when allowed to dry, will adhere to the pipe. Alternatively, the liquid substance can be applied to the fibres in the foregoing manner and then allowed to dry for a predetermined time until it is in a "tacky" state before the coated fibres are wrapped around the pipe in the foregoing manner.

In each of the foregoing embodiments, the diameter of each fibre forming the wrapping 14 is preferably less than 0.001 inches (2.5 microns). This relatively small diameter ensures that each individual fibre will be completely coated so that, when it is wrapped around the pipe 10, a homogenous mixture is formed with no voids between the respective fibres or between the fibres and the pipe itself. Thus a pipe wrapped according to the present invention should be free of any crevice corrosion referred to above.

Also, as a result of the foregoing, the ductile fracture resistance of the pipe in its axial direction is improved considerably. Further, a portion of the stress in the circumferential direction of the pipe 10, i.e., the hoop stress is taken up by the web 14 of continuous unidirectional fibres. Thus, the circumferential strength imparted to the pipe by the unidirectional fibres can be controlled in a manner so that it is equal to the burst strength of the pipes in the longitudinal direction. This, of course, can be regulated by the type, number and size of the fibres used, and the number of layers.

The pipe treated according to this methods of the present invention enjoys several other advantages.

For example, its burst pressure and its safety factor related to a specific working pressure are increased.

Also, the stress value in the metal pipe would be reduced at the same service pressure, thereby significantly reducing its susceptibility to stress corrosion cracking. Of course, the metal content, and therefore the weight, of the pipe can be reduced for a given design service pressure and a given safety factor relative to burst pressure. Also, with sufficient amount of unidirectional fibres on the pipe, a "leak-before-burst" mode of failure is achieved whereby the pipe 10 would leak, rather than burst, when subjected to sufficient internal or external corrosion. Even with a small amount of fibres on the pipe, a propagating ductile crack would be decelerated and arrested within a relatively short distance from its point of origin. These increased fracture resistance characteristics are specifically important for pipes transmitting high pressure gases or volatile liquids such as CO2.

One of the main advantages of the method of the present invention is that it can be easily adaptable to current prior art production techniques for providing a corrosion protection to the pipe since these techniques a fibreglass mat of randomly disposed (as opposed to unidirectional) glass fibres, which have negligible strength characteristics, are provided in conjunction with the hot adhesive substance and the outer protective wrap.

Another advantage of the method of the present invention is that the damage potential of the pipe from any external impact will be greatly reduced.

Many external impacts which may cause bare pipe to rupture, will only cause local denting or gouging in the present pipe. Other impacts which may dent or gouge the bare pipe, will not cause any noticeable damage to the present pipe.

Several other variations in the method of the present invention can be made, for example, in addition to providing multiple layers of the wrapping 14 of unidirectional fibres 18, multiple layers of the outer wrapping 16 can also be applied in an alternating or unalternating sequence with the wrap ping 14. It is also understood that the viscous substance referred to in the first embodiment discussed above is not necessarily limited to the enamels just described, but can be in the form of other viscous materials, such as resin, urethane, epoxy or paint, which will completely coat the filaments and prevent the egress of moisture or other potentially corrosive solutions.These substances can cure or harden by time, heating, cooling, chemical reaction, moisture, ultraviolet light of the like and thereby bond the filaments or between the filaments and the pipe itself and, in addition, hold or bond the filaments in place.

It is also understood that the technique of thb present method can be applied to any size (diameter) and type of pipe having characteristics equivalent to metal such as stainless steel, aluminium, copper, or brass to provide the improved characteristics set forth above, yet retain the individual qualities of these pipes (such as high corrosion resistance in the case of copper).

The method of the present invention is especially adaptable to pipes that work in a harsh environment since it will dramatically increase the fracture resistance capability and therefore the service life of the pipes. For example, the need for pipe repair or replacement will be reduced substantially, if not eliminated, due to much higher strength and fracture resistance capability of the pipe.

Further, it is understood that the technique of the method of the present invention can be applied to high stress points along the pipe, such as joint or longitudinal weld, to effect the advantages delineated above. Also, it is understood that the present invention can be utilized to arrest or to deaccelerate ductile cracks that form in the pipes. In certain cases, the cracks will be prevented from any propagation at all.

Claims

1. A method of treating a pipe to increase its ductile fracture resistance in the axial direction comprising wrapping a plurality of continuous, unidirectional, high-strength fibres around the pipe, and adhering the fibres to each other and to the pipe.

2. A method according to Claim 1 wherein the fibres take up a portion of the circumferential stress in the treated pipe and the pipe itself takes up the longitudinal stress.

3. A method according to Claim 1 or Claim 2 wherein the wrapping is controlled so the burst strength of the treated pipe in the circumferential direction is substantially equal to its burst strength in the longitudinal direction.

4. A method according to any preceding Claim wherein the fibres are wrapped around the pipe such that they extend in a plane substantially perpendiculay to the pipe axis.

5. A method according to any preceding Claim wherein the fibres are formed into a plurality of bundles before they are applied to the pipe.

6. A method according to any preceding Claim wherein the fibres are glass.

7. A method according to any preceding Claim wherein the diameter of each individual fibre is not greater than 0.001 inches (2.5 microns).

8. A method according to any preceding Claim wherein the fibres are wrapped around the pipe with minimum tension.

9. A method according to any preceding Claim wherein the fibres are wrapped around the pipe in such a manner as to preclude voids between adjacent fibres and between the fibres and the pipe.

10. A method according to any preceding Claim including the step of applying an outer layer of protective and corrosion preventing material over the fibres.

11. A method according to Claim 10 wherein the protective and corrosion preventing material is a felt material.

12. A method according to Claim 11 wherein the material is a coal tar or asphalt impregnated with reinforced glass fibre tissue.

13. A method according to Claim 11 wherein the material is coal tar or asphalt saturated reinforced asbestos felt.

14. A method according to any preceding Claim wherein a sufficient thickness of fibres is applied to arrest or prevent ductile crack propagation in the axial direction of the pipe.

15. A method according to any preceding Claim wherein the pipe is metallic.

16. A method according to any preceding Claim wherein the fibres increase the strength of the pipe in a circumferential direction and protect the pipe from corrosion.

17. A method according to any preceding Claim wherein a sufficient thickness of fibres is applied to protect the treated pipe from damage due to external impact.

18. A method according to any preceding Claim wherein the adhering step comprises applying a viscous substance to the pipe which forms a homogenous matrix with the fibres which bonds to the pipe.

19. A method according to Claim 18 wherein the viscous substance is in the form of one of a hot coal tar enamel and hot asphalt enamel.

20. A method according to Claim 19 wherein the enamel is applied at a temperature of approximately 5000F (260 C) and bonds the fibres to each other and to the pipe upon cooling to a predetermined temperature.

21. A method according to Claim 18 wherein the viscous substance is in the form of one of a resin, a urethane, a paint and an epoxy.

22. A method according to any of Claims 18 to 21 including the step of applying a primer to the pipe before applying the viscous substance, the primer serving as a bonding agent between the pipe and the viscous substance.

23. A method according to any of Claims 18 to 22 wherein the viscous substance is applied before the fibres.

24. A method of treating a pipe to increase its ductile fracture resistance in the axial direction substantially as described herein with reference to the accompanying drawing.