PIPE CONSTRUCTION

Abstract: A pipe and method of manufacture of the pipe are described. The pipe comprises a core of helically wound steel strip embedded in a plastics material matrix and lined with inner and outer linings of filament wound fibre reinforced plastics material.

Description: PIPE CONSTRUCTION The Dresent invention relates to a composite construction ot pipe for use in the oil. gas. water ana cnemical industπes It is known to manufacture pipe bv the helical winding 01 a pluralitv of metal stπps and which strips are embedded in a plastics material matrix L S 4 057.049 describes the manufacture of such pipe bv hclicallv winding at least one metallic reinforcing strip onto a mandrel, the strip being coated with and embedded within a polvmeπc bonding material The tube is thus formed of a pluralitv of successive helical convolutions of metal strip, compietelv embedded in the polvmenc material In this construction the polvmenc material provides only a limited degree of corrosion resistance and hence is not suitable for mam pipe applications US 4.351.364 descnbes a similar method of tube construction, but the tube also has inner and outer linings of resin impregnated glass-fibre lavers The glass-fibre layers compπse woven cloth and chopped fibre strand mat The purpose of the inner lining is to provide corrosion resistance and a low flow resistance whilst the purpose of the outer lining is to provide resistance to environmental conditions Whilst the glass-fibre reinforced linings on the inside and outside of the pipe enhance the resistance to corrosion, abrasion and other environmental conoitions. they do not significantly improve the strength of the pipe, nor do they provide optimum corrosion resistance owing to the porositv which is inevitably retained within the glass fibre mat and cloth It is an obiect of the present invention to provide a pipe having a construction which endows it not onlv w ith corrosion and abrasion resistance but also with enhanced strength and stiffness It is a iurther object to provide a pipe construction which when used in high pressure applications will produce a leak failure mode at elevated pressures close to ultimate burst pressure According to a first aspect ofthe present invention there is provided a pipe having inner and outer linings of fibre reinforced plastics material, the inner and outer linings having therebetween a core of helically wound steel strip, the steel strip being embedded in a plastics material matrix wherein the inner and outer linings are filament windings. The filament windings may be mono-filaments or may be wound from multi-filament tows. The filament windings may be laid at an angle of + and - 55° relative to the pipe axis, with an error margin of -*- or - 5" A winding angle of + and - 55 υ is chosen where it is desired to achieve optimum balance of hoop and axial loads, such as in the case where a cylindrical pipe has to withstand full pressure and loads. The precise angie is chosen so as to suit the operating requirements ofthe pipe. Alternative layers or groups of layers of filament windings may be laid at diflFerent positive and negative angles with respect to the pipe axis and to each other. It has been found that the use of filament wound fibre-reinforced inner and outer linings not only further enhances the resistance to corrosion, abrasion and environmental attack, but also imparts additional strength and stiffness to the complete pipe. The degree of additional strength and stiffness may be determined by calculation, but will depend upon the fundamental lining and strip material properties, the ratio of lining to total steel thickness and the helical angle of lining windings to the pipe axis. Because ofthe additional strength imparted by the filament wound linings, it has been found that the number of steel layers may be reduced to produce a pipe of a given strength, thus making the pipe more economic to produce and lighter in construction. Furthermore, the use of filament winding is capable of producing a higher integrity, iower porosity resin matrix than corresponding prior art constructions. Prior an tube constructions employing glass-fibre reinforced plastics (GRP) are unsuitable for high pressure applications due to micro-cracking and porosity as a result ofthe difficulty in fully impregnating woven and chopped strand type fibre reinforcements. The pipe ofthe present invention overcomes these problems because ofthe substantially improved impregnation level of the filament wound linings. The reduced porosity in the lining ofthe pipe ofthe present invention has the advantage of signiricantlv reducing the possibility of corrosive attack ofthe embedded steel stπp. The use of filament wound linings also allows the pipe of the present invention to be used in high pressure applications. The porosity in the prior an constructions lead to local strains under pressure giving nse to local cracking, even at relatively low pressures, causing eventual leakage and failure. At operating pressures, the filament wound linings are subjected onlv to low strain levels due to the presence of the relatively higher modulus strip layers. The pipe pressure containment capability may be maximised by ensuring that the strain capability of the linings is such that they fail at a predetermined pressure onlv after the steel lavcrs have been loaded to a significant proportion of their ultimate strain, preferably greater than their yield strain. An effective means for determining the strain capability of the linings is by appropriate selection of the winding angle ofthe filaments. A significant advantage of the present invention is that the strain capability ofthe pipe construction may be controlled so as to give a leak failure mode at a predetermined pressure before ultimate pressure bursting occurs. In this connection we have found that the contribution of the inner and outer filament wound linings to the ultimate pressure containment capacity of the pipe becomes significant at pressures above which yielding of the steel strip has taken place. It is advantageous therefore for the pipe construction to be so designed that leakage of the liners under internal pressures occurs at a hoop strain level at which the steel strip is in a yielded condition though not liable to ultimate failure. This is illustrated in Figure 1 of the accompanying drawings, which shows a graph comparing typical internal pressure versus hoop strain characteristics for (a) a pipe construction in accordance with the present invention comprising a helically wound steel core with internal and external filament wound fibre reinforced plastics linings; (b) the core without the linings; and ( c) the lininus without the core. It will be seen from Figure 1 that bv ensuπng that the lining strain failure occurs at around 1% le bevond the yield point of the steel core (0 5% strain) but at a strain less than the ultimate steel failure strain, (3%) a leak before burst failure mode is ensured, and at an elevated pressure approaching, but alwavs below the ultimate burst pressure ofthe pipe Tor applications with fluids with which the plastics lining matenal is incompatible, the pipe of the present invention mav also be provided with an impermeable lining such as aluminium, thermosetting piastics matenal or si cone rubber, for example, on which the inner filament wound lining is formed The addition of an impermeable lining or membrane mav alter the leak failure mode unless the strain to failure characteristics ofthe membrane are carefully matched to those ofthe pipe construction The material of the filament windings mav be glass-fibre for most applications However, other continuous fibre matenals such as aramid fibre, eg Kevlar (Trade Mark), or carbon fibre mav be used in combination with or instead of glass-fibre The type of fibre used will depend upon the application in which the pipe is to be employed In some applications, more than one type of fibre may be employed in a singie pipe The thickness ofthe inner and outer linings will vary depending upon the specific requirements for the pipe However, in the case of glass filaments, the inner lining will generallv be of a minimum thickness of 2mm whilst the outer lining will generallv be of a minimum thickness of 1 mm The steel strip core mav be formed of a plurality ofiavers of helically wound stnps which abut along their edges Alternatively, the steel core mav be wound from one or more steel stnps wherein each succeeding turn overlaps the previous turn in the axial and radial directions in the case of strips which abut along their edges at any given section through the pipe wall the winding pattern of the he callv disposed steel strips is so arranged that no axial gap between adiacent edges of the wound strip in anv laver coincides in the radial direction with the axial gap of anv other laver Theretore there is no right line path with respect to the tube axis trom the inside to the outside ofthe tube which passes through more than one ofthe axial heiical gaps In this wav. there is no position in the pipe at which the effective steel thickness is reduced bv more than one strip thickness oi steel For large numbers of steel strip layers. , the winding pattern mav be repeated so that in tact there mav be coincidence ofthe axial gaps ot two or more lavers with onlv modest reduction in axial strength For small helical gaps the local reduction in steel thickness onlv affects the tube axial strength such that for pipes having three or more lavers ot i>teel, the overall pressure vessel strength is substantially unaffected Bv minimising the axial gap. within manufactunng limitations, the hoop strength under internal pressuπsation is unaffected, and as stated above, the axial strength is only reduced to the extent of one steel strip thickness for each coincident axial gap Therelore the steel core of a tube having eight lavers of steel has a hoop strength equivalent to the full eight lavers and an axial strength equivalent to seven lavers When applied to the loading conditions of a closed vessel internally pressurised, it mav be seen that as hoop loading is twice axial loading, the effective reduction in thickness in the axial direction does not detract at all from the ultimate pressure strength of the pipe In terms of stiffness, the effect of the axial gap between the steel strip adjacent edges is the same in both the hoop and axial directions Therefore, the steel strip layers can quite accurately be considered as an isotropic material in which the effective modulus of elasticity is simply reduced by an amount approximately equal to the ratio of the width of the resin gap to that ofthe steel strip Of course, for pipes constructed in accordance with the present invention, there will be a significant additional contπbution to both the axial and hoop strength of the pipe from the filament wound inner and outer linings Pipes ot anv diameter may be produced, but typically thev will lie in the range from about 150mm to about 1000mm The maximum strip thickness and width is determined by the mechanical requirements for winding, le required pipe diameter to strip stiffness and helical windini: angle The strip width is also paπiallv governed bv the need to maintain an overlap pattern such that there is minimum coincidence ol axial strip edge gaps from the inside to the outside of the pipe, and such that the axial interlaminar shear loads do not cause failure In practice for pipes having diameters between 1 0mm and 1000mm. the steel strip mav have a width Ivinu in the range from 50mm to 250mm. and a thickness lying in the range from 0 12mm to 1 mm The axial gap between adjacent stπp edges of successive helical turns will preferably be no more than 5mm. and may generally lie in the range from 1mm to 3mm. The steel is prepared prior to winding to provide a suitably clean surface for bonding with the resin Suitable preparation techniques include grit blasting and/or various known methods of chemical cleaning The resin mav be an epoxy resin or any type suitable for the intended application ofthe pipe The resin may also include fillers According to a second aspect of the present invention there is provided a method of making a pipe, the method comprising the steps of forming an inner lining by filament winding of a fibre material onto a mandrel at a predetermined angle with respect to the pipe axis, providing a resin matrix for the fibre layer, helically winding a steel strip core onto said fibre material layer, providing said steel strip core with a resin matrix, forming an outer lining by filament winding of a fibre material onto the outside ofthe pipe at a predetermined angle with respect to the pipe axis, providing said outer layer of fibre material with a resin matrix, at least partially curing said resin matrix and removing said pipe from said mandrel. Preferablv, the steel strip core may have at least three layers of helically wound steel strip. The inner and outer linings may be formed by passing giass or other filaments through a resin bath prior to winding onto the mandrel or outside ofthe tube which may be rotated by suitable drive means The method mav further include the step of providing an initial non-filament wound, fibrous layer on the mandrel which is also provided with a resin matrix In order that the present invention mav be more fully understood may be more fully understood, an example will now be described bv wav of illustration only with reference to Figures 2 and 3 of the accompanying drawings, of which - Figure 2 shows a schematic axial cross section through a tube according to the present invention, and Figure 3 which shows a schematic representation o the manufacture of the tube of Fig. 2 Referring now to Figures 2 and 3 of the drawings ere the same features have common reference numerals A length of pipe indicated generallv at 10 is produced by winding a pluralitv of layers onto a heated rotating mandrel 1 The mandrel is coated with a known release agent ( not shown) A resin rich inner surface 14 is provided bv helically wrapping a "C" glass or polyester veil onto the mandrel and impregnating with a suitable epoxy resin such as MY 750 supplied by the Ciba-Geigy company with a suitable hardener system Successive helical turns ofthe veil material are overlapped in the axial direction, the width of the veil material being about 150mm A thickness of GRP inner lining 16 is then built up by helically applying filament windings from a creel 17 of "E" giass fibre filaments 18 laid at 55 to the tube axis. The filaments 18 are passed through a resin bath (not shown) immediately prior to winding so that the windings are effectively provided with a resin matrix as they are being wound. A number of filament rovings are laid such that the minimum thickness is about 2mm Prepared steel stπp 20 from coils 22 is then helically wound onto the still uncured wet resm of the inner lining 16, successive helical turns lying adiacent each other with a maximum axial gap 24 of 5mm An epoxv resin 26 containing an appropriate filler, is simultaneously applied by known means to the steel strip as it is wound onto the pipe such that in the finished pipe, each laver of steel is coated with and bonded to the next laver bv the resm system. The number steel lavers and the total thickness is determined bv the required pressure, stiffness rating and diameter ofthe pipe and the combined mechanical properties of the steel and linings The helical windings ofthe steel strips are axially offset to each other so that there is no right line path from the inside to the outside of the tube through more than one ofthe axial gaps 26, and as indicated by the dashed line 28. An outer lining 30 is applied by a number of rovings of 55° helical windings of an "E" glass fibre filament 32 from a creel 34. As with the inner iining, the filament 32 is passed through a resin bath (not shown) immediately prior to winding. The thickness ofthe outer lining 30 is a minimum of 1mm. Whilst still rotating, heat is applied to the thus constructed pipe to raise the temperature for the minimum time which will satisfactorily solidify or cause the resin to gel. The mandrel with gelled or cured pipe assembly is removed from the winding machine (not shown) and allowed to cool to ambient temperature whereupon the mandrel is removed from the pipe. The pipe may then be post-cured in a free standing position if required. After cooling to ambient temperature, the pipe is cut to finished length by grinding through the total wall thickness, ie GRP and steel, to remove a minimum of two pitch lengths of steel from each end. Although the method described above involves heating ofthe mandrel 12 during the winding process, this may be unnecessary with certain types of resin materials, particularly those having a short curing time.

Claims: CLAIMS 1 A pipe having inner and outer iinings of fibre reinforced plastics material, the inner and outer linings having therebetween a core of helically wound steel strip, the steel strip being embedded in a plastics material matrix wherein the inner and outer linings are are filament windings A pipe according to claim 1 wherein the filament windings are monofilaments. A pipe according to claim 1 wherein the filament windings are multi-filament tows A pipe according to any one preceding claim wherein alternate layers of filament windings are laid at different positive and negative angles with respect to the pipe axis A pipe according to any one preceding claim from 1 to 3 wherein alternate layers of filament windings are laid at different positive and negative angles with respect to each other A pipe according to any one preceding clai from 1 to 3 wherein the filament windings are laid at an angle of + and - 55 ' relative to the pipe axis. A pipe according to claim 6 wherein the angle of the filament windings have an error margin of + or -5° A pipe according to any one preceding claims further including an impermeable lining inside the filament wound inner lining A pipe according to claim 8 wherein the impermeable lining is selected from a material in the group comprising aluminium, thermosetting plastics materials and silicone rubber 10 λ pipe according to any one preceding claim wherein the filament wound lining material is at least one selected from the group comprising glass fibre, aramid fibre and carbon fibre 1 1 A pipe according to any one preceding claim wherein the thickness ofthe inner filament wound lining is a minimum of 2mm. 1 . A pipe according to any one preceding claim wherein the thickness ofthe outer filament wound lining is a minimum of 1mm. 13 A pipe according to any one preceding claim wherein the core of helically wound steel strip comprises a plurality of layers of helically wound strips which abut along their edges. 14 A pipe according to any one preceding claim from 1 to 12 wherein the core of helically wound steel strip comprises at least one strip having successive turns overlapping in the axial and radial directions. 15. A pipe according to any one preceding claim from 1 to 13 wherein the winding pattern of the steel strips is so arranged that no axial gap between adjacent edges ofthe wound strip in any layer coincides in the radial direction with the axial gap of any other layer. 16. A pipe according to any one preceding claim from 1 to 13 wherein the winding pattern of steel strips is repeated in the radial direction. 17 A pipe according to any one preceding claim from 1 to 13, 5 and 16 wherein the gap between adjacent edges of successive turns is a maximum of 5mm. 18 A pipe according to any one preceding claim from 1 to 13 and 15 to 17 wherein the gap between adjacent edges of successive turns lies in the range from 1mm to 3mm. 19. A pipe according to any one preceding claim wherein the plastics material matrix is an epoxy resin. 0 A pipe according to any one preceding claim wherein the plastics material resin also includes a filler. A pipe according to anv one preceding claim wherein the steel stπp ofthe core has a width lying in the range from 50mm to 250mm A pipe according to anv one preceding claim wherein the steel stπp ofthe core has a thickness iying in the range from 0 12mm to 1 mm A pipe according to any one preceding claim wherein it has a diameter lying in the range from about 150mm to about 1000mm A pipe according to anv one preceding claim, wherein the strain capability of the linings in response to increasing internal pressure within the pipe is such that the linings fail, and therefore provide a leak failure mode, only after the steel core has been loaded to a significant proportion of its ultimate strain A pipe according to Claim 24, wherein the pressure strain capability ofthe linings is such that they provide a leak failure mode at a predetermined internal pressure after the steel core has been loaded beyond its yield strain A method of making a pipe, the method comprising the steps of forming an inner lining by filament winding of a fibre material onto a mandrel at a predetermined angie with respect to the pipe axis, providing a resin matrix for the fibre layer, helically winding a steel strip core onto said fibre material layer, providing said steel strip core with a resin matrix, forming an outer lining by filament winding of a fibre material onto the outside ofthe pipe at a predetermined angle with respect to the pipe axis, providing said outer layer of fibre material with a resm matrix, at least partially cuπng said resin matπx and removing said pipe from said mandrel A method according to claim 26 wherein said steel core compπses at least three layers A method according to either claim 26 or claim 27 wherein the inner and outer linings are formed by passing said fibre material through a res bath pπor to winding ~ 1»c9. A pipe substantially as hereinbefore described with reference to the accompanying description and Figures 1. 2 and 3 ofthe drawings. A method of making pipe substantially as hereinbefore described with reference to the accompanying description and Figures 1, 2 and 3 ofthe drawings.

Inventors: DABINETT JOHN, MILLWARD WILLIAM ANTHONY, DABINETT, JOHN, MILLWARD, WILLIAM, ANTHONY
Applicants: DABINETT JOHN, MILLWARD WILLIAM ANTHONY, ROYAL ORDNANCE PLC, DABINETT, JOHN, MILLWARD, WILLIAM, ANTHONY
Application year: 1993
Main classification: F16L9
IPC codes: B29D23, F16L9, F16L23, F16L37, F16L58