CA1225915A

CA1225915A - Method of treating a pipe and product produced thereby

Info

Publication number: CA1225915A
Application number: CA000409572A
Authority: CA
Inventors: Norman C. Fawley
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-10-20
Filing date: 1982-08-17
Publication date: 1987-08-25
Also published as: GB2107422B; FR2514858B1; FR2514858A1; MX164343B; JPS5874984A; JPH0159474B2; GB2107422A

Abstract

METHOD OF TREATING A PIPE AND PRODUCT
PRODUCED THEREBY

ABSTRACT OF THE DISCLOSURE
Disclosed is a method of significantly increasing the ductile fracture resistance in the circumferential direction of a pipe carrying a fluid under pressure, the pipe having adequate ductile fracture resistance in the longitudinal direction which method comprises wrapping a plurality of continuous, unidirectional, high strength, nonmetallic fibers around the pipe to a thickness, uniform along the length of the pipe, sufficient to significantly increase the resistance of the pipe, in the circumferential direction, to ductile fracture as a result of the pressure of the fluid. The invention also pertains to the product of the pipe with the specific wrapping therearound.

Description

~ z ~r~;
METHOD OF TREATING A PIPE AND PRODUCT
PRODUCED THEREBY

BACKGROUND OF THE INVENTION
Pipes, and pipelines formed thereby, are playing increasing-ly important roles in the transportation of gas, oil, water andother fluids. For the past several decades, pipes have been treated in many manners to improve their serviceability. For ex-ample, 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, relative 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 la-tter improve-ments have resulted in additional problems. For example, relativ-ely 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 adjacent sections of the wire, and between the wire and the pipe.

SUMMARY OF THE INVENTION
Accordingly, the present invention seeks to provide an improved pipe which enjoys the protection of the prior art types, yet is dramatically improved from a ductile fracture resistance standpoint.
Further, the present invention seeks to provide an improved pipe of the above type in which increased strength and ductile fracture resistance characteristics are imparted to the pipe with a minimum of added labor and materials, and without causing any additional problems, such as the aforementioned crevice corrosion.

~S ~
In one broad aspect, t~le invention pertains to a method of significantly increasing the ductile fracture resistance in the circumferential direction of a metal pipe carrying a fluid under pressure, the pipe having adequate ductile fracture resist-ance in the longitudinal direction, which method compriseswrapping a plurality of continuous, unidirectional, high strength, nonmetallic fibers around the pipe to a thickness, uniform along the length of the pipe, sufficient to significantly increase the resistance of the pipe, in the circumferential direction, to ductile fracture as a result of the pressure of the fluid, and providing for the fibers a matrix of a viscous substance, wherein the fibers are wrapped under a minimum tension only sufficient for the fibers to adhere to the pipeO
The invention further pertains to a product comprising a metal pipe for carrying a fluid under pressure and having adequate ductile fracture resistance i.n the longitudinal direction and a wrapping around the pipe, the wrapping including a plurality of continuous, unidirectional, high strength, nonmetallic fibers wrapped around the pipe and a matrix of a viscous substance, the fibers being wrapped under a minimum tension only sufricient for the fibers to adhere to the pipe, and the wrapping having a thickness, uniform along the length of the pipe, sufficient to significantly increase the resistance of the pipe to ductile fracture in the circumferential direction as a result of the pressure of the fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
The above description, as well as further aspects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred but nonetheless illustrative embodiment in accordance with the pxesent invention, when taken in conjunction with the accompanying drawings wherein:

Figs. 1 - 3 are perspective views of a pipe section being treated according to the method of the present invention;

~2~5~3~5 and;
Figs ~, 5 and 6 are partial, enlarged, perspective views of configurations of unidirectional, high-strength fibers that can be used in the method of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
By way of example, the method of the present invention will be described in connection with steel or other metallic pipes of a relatively large diameter which is 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.
According to a preferred embodiment of the present method, 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.
More particularly, and referring to Fig. 1 of the drawings which shows a section of a pipe 10 being treated according to the method of the present invention, the hot enamel is applied to the surface of the pipe 10 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 1~ can be of a conventional design and ~ r~ r~

is pre-rerably in the form of a weiI, a floodboY., a -lood coater or a curtain coater which receives the enamel at an elevated temperature, preferably approximately 500F, and discharges it onto the outer sur-face 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 dlscharging the enamel onto the outer surface of the pipe.
The aforementioned coal tar enamel is preferably made from coke o oven pitch which is modified and filled, while the asphalt enamel can be produced from selected petroleum crudes that are oxidezed and filled, both in a conven-~ional manner.
While the coating o~ enamel lS still hot, i.e., before it com-pletely drys or bonds, a layer of a plurality of continuous unidirect-ional high-strength inorganic, electrically non-conductive fibers are wrapped around the pipe. More particularly, and referring to Fig. 2, a continuous strip or wrapping 14 of the continuous, unidirectional high-strength fibers are 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., only sufficient tension to insure that the web adequate-1~ adheres to the pipe. The flbers forming the web are preferably in the form of glass, or other materials having properties equivalent to glass, in a configuration to be described in detail later.
; After the wrapping 14 of unidirectional fibers 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 Fig. 3, a wrap-ping 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 fibers) as shown. The outer wrapping 16 may be of any material that will provicie proiection against mechanical ~2~
damage during transportation and, when applicable, burial of the line; as well as protection for the enamel and fibers ~gainst cyc-lical 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 fiber tissue or coal tar or asphalt-saturated, reinforced asbestos felt. Since the outer wrapping 16 is applied whlle the previously applied enamel is still in a hot or unbonded state, it is bonded to the unidirectional fibers and the pipe when the enamel hardens upon bonding.
o The ~ibers 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 w~und in a mann~r -so that the unidirectional fibers extend in a plane substantially perpendicular to the axis of the pipe, with the degree of overlap between each section and its ad~oining section varying from 1% to approximately 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, the ) wrappings 14 and 16 can 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 appiying the wrappings 14 and 16 would transvers 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 Fig. 4. More particularly, the individual, continuous, unidirectional fibers 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 bundies in an alternating "over-under" pattern as shown to provide the separation.

. ~.. __ ~ ...... ......

L~5;91~

A wrapping acco~ding to an alt~rnative el~odi~en~ is depic~ed in Fig. 5. In this embodiment, the unidirectional fibers 1~ are separated into a plurality of bundles 20' which e~.tend for a smaller width than those of the embodiment of Fig. 4 with the string 22 ex-tending 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 manner to hold the bundles 20' in place.
According to the embodiment of Fig. 6, a wrapping 14" is formed by a plura~ity of bundles 20" each of which is formed by a plurality of unidirectional fibers 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 or the like is eliminated and the fibers 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 fibers can be adhered to the pipe hy eliminating the`step of applying the hot enamel and, instead, applying an adhesive to the fibers before they are wrapped onto the pipe. This can be done by forming a wrapping 14 of fibers by any of the techni~ues ~lsclosed above, and then applying an adhesive substance of any con-ventional type to either surface o' 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 ~ig. 2.
Alternatively, an adhesive coating could be applled 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 back-ing material which then is applied to the pipe in the manner discussed above.

',~

i~ 3~
According to a still further elr.bodiment of the presen~ invention, the ibers c~n be irnpregnated by a liquid substance, such as epoxy, enamel, or other similar type material, belore they ~e w-apped around the pipe. According to this embodiment, the fibers are passed, prefer-ably 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 fibers thus coated could be then immediately wrapped araund the pipe as shown in Fig. 2 while the iiquid substance is still wet and, when allowed to dry, will adhere to the pipe. Alternatively, O the liquid substance can be applied to the fibers i~ the foregoing manner and then allowed ~o dry for a predetermined time until it is in a "tacky" state before the coated fibers are wrapped around the pipe in the foregoing manner.
In each of the foregoing embodiments, the diameter of each fiber 5~ forming the wrapping 14 is preferably less than .001 inch. This rela-tively small diameter insures that each lndividual fiber will be com-pletely coated so that, when it is wrapped around the pipe 10, a homo-ge~eous mixture is formed with no voids between the respective fibers or between the fibers and the pipe itself. Thus a pipe wrapped accord-O ing to the present invention should be free of any crevice corrosionreferred to above.
Also, as a result of the foregoing, the ductile fracture resis-tance of the pipe in its ~xial 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 contin-uous unidirectional fibers. Thus, the circumferential strength imparted to the pipe by the unid1rectional fibers can be controlled in a manner so that it is equal to the burst strength of the pipes in the longitud-inal direction~ This, o~ course, can be regulated by the type, number O and size of fibers used, and the number of layers.
The pipe treated according to the methods of the present inven-tion enjoys several other advantages. For example, its burst pressure and its safety factor related to ~ specific working pressure are in-creased. Also the stress value in the metal pipe would be reduced at 9~LS

the same servic2 pressure, thereby significantly réducing its sus-ceptibility ~o 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 sa'ety factor relative to burst pressure. Also, with sufficient amount of unidirectional fibers 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 wlth a small amount of fibers on the pipe, a propagating ductile crack would be deceler-o ated and arrested within relatively short distance from its point oforigin. These increased fracture resistance characteristics are specifically important for pipes transmitting high pressure gases or volatile liquids sucn as CO2.
One of the main advantage$ of the method of the present invention is that it can be easily adaptable to current prior art production techniques for providing a corxosion protection to the pipe since these techniques a fiberglass "mat of randomly disposed (as opposed to unldirectional) glass fibers, which have negligible strength chara-cteristics, are provided in coniunction with the hot adhesi~e subs-3 tance and the outer protective wrap.
Another advantage of the method of the present invention is thatthe 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.
It is understood that several variations in the method of the present invention can be made without departing from the scope of the invention. For example, in addition to providing multiple layers of the wrapping 14 of unidirectional fibers 18, multiple layers of the outer wrapping 16 can also be applied in an alternating or unal~ernating sequence with the wrapping 14. It is also understood that the viscous ~-~zs~a~
subs~ance re erred to in the firs~ embodiment ~iscussed 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 ~he filaments and prevent the egress of moisture or other potentially corrosive solutions. These substances can cure or harden by time, heating, cooling, chemical reaction, mois-ture, ultraviolet light or the like and thereby bond the filaments or between the filaments and the pipe itself and, in addition, hola or bond the filaments in place.
0 It is also understood that the technigue of the present method can be applied to any size ~diameter) and type of pipe having character-istics equivalent to metal such as stainless steel, aluminum, copper, or brass to provide the improved characteristics set rorth above, yet retain the individual qualities of these plpes (such as high corrosion resistance ln the case of copper).
The method of the present invention is especially adaptable to pipes that work in a harsh enviro~ment since it will dramatically in-crease the fracture resistance capability and therefore the service life of the pipes. For example, the need for pipe repair or replace-~n ment will be reduced drastically if not eliminated due to much higherstrength and fracture reslstance capability of the pipe.
Further, it is understood that the technique of the method of the .
present invention can be applied to ~ligh stress points along the pipe, such as join~ or longitudinal weld, to effect the advantages deiineated '5 above. Also, it i5 understood that the present invention can be utili-zed 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.
Other modifications, changes and substitutions in the ~oregoing 0 disclosure are intended, and in some instances some featu~es of the invention will be employed without a corresponding use of other features According, it is appropriate that the appended claims be construed broadly and in a manner consistent wi-th the spirit and scope of the in ention therein.

_a~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of significantly increasing the ductile fracture resistance in the circumferential direction of a metal pipe carrying a fluid under pressure, said pipe having adequate ductile fracture resistance in the longitudinal direction, comprising:
wrapping a plurality of continuous, unidirectional, high strength, nonmetallic fibers around the pipe to a thickness, uniform along the length of the pipe, sufficient to significantly increase the resistance of the pipe, in the circumferential direction, to ductile fracture as a result of the pressure of the fluid, and providing for said fibers a matrix of a viscous substance, wherein said fibers are wrapped under a minimum tension only sufficient for said fibers to adhere to said pipe.

2. The method of Claim 1 wherein said fibers take up a portion of the circumferential stress in said treated pipe and said pipe itself takes up the longitudinal stress.

3. The method of Claim 2 wherein said step of wrapping is controlled so the burst strength of said pipe in the circumferential direction is substantially equal to its burst strength in the longitudinal direction.

4. The method of Claim 1 wherein said fibers are wrapped around said pipe in a manner so that they extend in a plane substantially perpendicular to the axis of said pipe.

5. The method of Claim 1 wherein said fibers are formed into a plurality of bundles before they are applied to said pipe.

6. The method of Claim 1 wherein said fibers are glass.

7. The method of Claim 1 wherein the diameter of each individual fiber is less than .001 inches.

8. The method of Claim 1 wherein there are no voids between adjacent fibers and between said fibers and said pipe.

9. The method of Claim 1 further comprising the steps of applying an outer layer of protective and corrosion preventing material over said fibers.

10. The method of Claim 9 wherein said protective and corrosion preventing material is a felt material.

11. The method of Claim 10 wherein said felt material is coal tar or asphalt impregnated with reinforced glass fiber tissue.

12. The method of Claim 10 wherein said felt material is coal tar or asphalt saturated reinforced asbestos felt.

13. The method of Claim 1 wherein the thickness of said fibers is such that they prevent ductile crack propagation in the axial direction of said pipe.

14. The method of Claim 1 wherein said fibers increase the strength of said pipe in a circumferential direction and protect said pipe from corrosion.

15. The method of Claim 1 wherein the thickness of said fibers is such that they arrest ductile crack propogation in the axial direction.

16. The method of Claim 1 wherein the thickness of said fibers is such that they protect said treated pipe from damage due to external impact.

17. The method of Claim 1 wherein said viscous substance is in the form of a hot coal tar enamel or hot asphalt enamel.

18. The method of Claim 17 wherein said enamel is applied at a temperature of approximately 500°F and bonds said fibers to each other and to said pipe upon cooling to a predetermined temperature.

19. The method of Claim 1 wherein said viscous substance is in the form of a resin, a urethane, a paint or an epoxy.

20. The method of Claim 1 further comprising the step of applying a primer to said pipe before said step of applying said viscous substance, said primer serving as a bonding agent between said pipe and said viscous substance.

21. The method of Claim 1 wherein said step of applying said viscous substance is before said step of applying said fibers.

22. The method of Claim 1 further comprising the step of applying an adhesive to said fibers before said step of wrapping.

23. The method of Claim 22 wherein said step of applying an adhesive further comprises the step of applying an adhesive to both surfaces of a wrapping formed by a plurality of fibers and applying a backing material to one of said adhesive-coated surfaces.

24. The method of Claim 23 wherein said step of applying an adhesive to said fibers further comprises the step of applying a backing material to the adhesive-coated fibers and applying an additional coat of adhesive to said backing material.

25. The method of Claim 1 wherein said step of applying an adhesive to said fibers comprises the step of applying a liquid epoxy to said fibers before said step of wrapping.

26. The method of Claim 25 wherein said step of wrapping is done while said liquid epoxy is still wet.

27. The method of Claim 25 further comprising the steps of allowing said liquid epoxy to partially dry before said step of wrapping.

28. A product comprising:
a metal pipe for carrying a fluid under pressure and having adequate ductile fracture resistance in the longitudinal direction; and a wrapping around the pipe, the wrapping including a plurality of continuous, unidirectional, high strength, non-metallic fibers wrapped around the pipe and a matrix of a viscous substance, said fibers being wrapped under a minimum tension only sufficient for said fibers to adhere to said pipe, and said wrapping having a thickness, uniform along the length of the pipe, sufficient to significantly increase the resistance of the pipe to ductile fracture in the circumferential direction as a result of the pressure of the fluid.