WO1995016160A1

WO1995016160A1 - Field weld protection system

Info

Publication number: WO1995016160A1
Application number: PCT/GB1994/002642
Authority: WO
Inventors: Matthew Thomas Pickett; Douglas Stanley Ford; Michael George Catley
Original assignee: W.R. Grace Limited
Priority date: 1993-12-09
Filing date: 1994-12-02
Publication date: 1995-06-15
Also published as: GB9325245D0; AU1115595A; GB2284785A

Abstract

A method of providing corrosion protection to a joint area in a weight coated marine pipe line and a tape suitable for use in that method. The pipe line comprises a plurality of pipes (2) each with an anti-corrosion coat (4) and a weight coat (6) applied over the anti-corrosion coat (4). The method comprises (a) applying as a circumferential wrap: (i) a first tape member comprising a flexible heat resistant waterproofing backing film (22) such as a high temperature PVC film and a pressure sensitive adhesive layer (28) having a width spanning and adhered to a first joint length of exposed pipe (2a) on either side of a welded joint (4b) and at least partially overlying and adhered to a second joint length adjacent to the first joint length on either side of the joint covered by the anti-corrosion coat, but not the weight coat; and (ii) a second tape member (24) comprising a high melting point abrasion resistant film such as a polyester film having a width less than the width of the first tape member (22) and being laminated at the centre of the outer surface of the first tape so as when applied to overlie the first joint length of exposed pipe (2a) and not to overlie the second joint length (4b), and (b) filling the joint volume (10). The first and second tape members are conveniently provided in the form of a prelaminated tape with the pressure-sensitive adhesive layer (28) protected by a release film or paper (30).

Description

FIELD WELD PROTECTION SYSTEM The invention is directed to a method of protecting weld joints of pipe lines, in particular off-shore pipe lines, from corrosion and to tape for use in such a method. Pipes for use in off-shore pipe lines such as oil or gas transmission pipe lines may be provided with a factory applied anti-corrosion coat and further provided with an outer factory applied concrete weight coat. The weight coat and anti- corrosion coat will generally stop short of each end of the pipe to enable the end to be joined to an end of the adjacent pipe of the pipe line e.g. by welding. The exposed pipe and weld must subsequently be protected from corrosion.

Corrosion protection for off-shore field joints in such concrete weight coated oil or gas transmission pipe lines may be provided by hand wrapping three overlapping bands of self- adhesive tape over the exposed joint area and subsequently bringing the joint to the weight coat level by filling the joint volume with an appropriate joint filling material such as hot mastic asphalt, rapid setting concrete or rapid cure aggregate filled urethane. The tape material used is required to withstand the abrasive effect of the filling material and to exhibit sufficient flexibility to allow complete conformability of the tape to the joint profile, particularly at the edge of the anti-corrosion coating where there is a step between the top of the coating and the exposed pipe. High temperature pvc backed tape, such as Serviwrap M30 from Servicised Limited has been used for this purpose. A first roll of tape, typically 450 mm wide, is applied circumferentially to the exposed steel of the pipe at the welded area. Second and third narrower rolls of tape, typically 150 mm wide, are circumferentially applied each lapping both an edge of the first roll of tape and the edge of the factory applied corrosion coat on one side of the joint. A mold is then placed around the joint, and the joint cavity filled with the filler material. To resist the pouring of the filler material a tough, inflexible film is required which is accordingly difficult to stretch and conform to the joint profile as a single piece, hence the three part application to provide conformity especially at the stepped edge of the corrosion coat. To further assist in resisting pouring of the filler material, the three tapes, which are typically applied so as to provide a wrap with an overlap of 1/10 to 1/3 of a circumference of the pipe, are generally arranged so that the overlap is at the top or crown of the joint where the full impact of the pouring of the filler material is experienced.

Off-shore pipe lines are typically laid using a barge with eight stations: six welding stations; one inspection; and repair station; and one protection (wrap, apply mold and fill) station. Typically the joint will pass from one station to the next at about 8 minute intervals.

A pipe joint between two consecutive lengths of pipe passes through the six welding stations where the welded joint is formed. It is then x-rayed at the inspection and repair station. If the x-ray shows the weld is defective, the defective part of the weld is ground out and repaired. At the protection station the pipe is tape wrapped, a mold applied and the joint volume filled with filler material.

Whereas the known three part tape system using high temperature pvc backed tape such as Serviwrap M30 from Servicised Limited operates very successfully over a range of conditions, it may not be suitable where extreme elevated temperatures would be experienced such as the very high residual weld temperatures encountered when large diameter, thick walled pipe is used. Also when weld repairs are carried out at the inspection and repair station immediately before the protection station where wrapping takes place, the residual weld temperature will be high. The high residual weld temperature tends to soften the applied tape rendering it prone to abrasion by the abrasive application of filler material.

An alternative material commercially available for use in the above application is standard PVC tape with a heat laminated polyester film. Whilst this is less affected by high temperatures, the film is of poor flexibility and generally difficult to apply and offers poor conformability, in particular at the edge of the factory applied anti-corrosion coating and especially at low ambient temperatures.

According to the present invention there is provided a method of providing corrosion protection to a joint area in a weight coated pipe line; said pipe line comprising a plurality of pipes having an anti-corrosion coat applied circumferentially thereto and a weight coat applied over said anti-corrosion coat said pipes being co-axially joined by a plurality of pipe joints wherein a joint area comprises to either side of a said pipe joint: a first joint length of exposed pipe to which neither said anti-corrosion nor said weight coat is applied; and a second joint length adjacent to said first joint length to which said anti-corrosion coat is applied but said weight coat is not applied; said method comprising: (a) applying as a circumferential wrap a first tape member comprising a flexible heat resistant waterproofing backing film and a pressure sensitive adhesive layer having a width spanning and adhered to said first joint length and at least partially overlying and adhered to said second joint length on either side of said joint; a second tape member comprising a high melting point abrasion resistant film having a width less than the width of the first tape member and being laminated at the centre of the outer surface of the first tape so as when applied to overlie said first joint length of exposed pipe and not to overlie said second joint length;

(b) filling the joint area to the weight coat level with a filler material.

Preferably the first and second tapes are laminated prior to application and applied in a single operation.

In a second aspect of the present invention there is provided a tape for use in the above method comprising:- a flexible heat resistant waterproofing backing film; a strip of high melting point abrasion resistant film laminated at the centre of a first major surface of the flexible backing film; the second major surface of the flexible backing film being coated with a layer of pressure sensitive adhesive.

Preferably the heat resistant waterproofing backing film is a high temperature pvc film and the high melting point abrasion resistant film is a polyester film.

The invention will be more clearly understood from the following description given by way of example only by reference to the Figures in which:-

Figure 1 - Prior Art - shows a partial cross section through a concrete weight coated submarine pipeline joint using a prior art field weld protection system;

Figure 2 - exemplary - shows a partial cross section through a concrete weight coated submarine pipeline joint using a weld protection method of the present invention;

Figure 3 - exemplary - shows a cross-section through a length of tape according to the present invention and for use in the weld protection method of the present invention. As shown in Figures 1 and 2, a length of pipe 2 of, for example, steel, is coated with a circumferential external anti- corrosion coating 4 which may be, for example, a bitumen enamel, a coal tar enamel, or a fusion bonded epoxy layer. The anti-corrosion coating 4 stops short of each end of the length of pipe 2 to leave a stepped edge 4a and an exposed pipe portion 2a.

A factory applied concrete weight coat 6 circumferentially surrounds the anti-corrosion coating 4 stopping short of the edge 4a of the anti-corrosion coating 4, thereby leaving an exposed section 4b of anti-corrosion coating 4. The weight coat ensues negative buoyancy of a pipeline formed from a plurality of such pipes.

In use consecutive lengths of such pipe 2 are placed concentrically end to end and butt welded to form a pipe line. The outside of the pipe line around each welded joint thus comprises a welded bead 8 and, on either side of the welded bead 8, a first joint length, comprising the length of exposed pipe 2a. Adjacent the first joint length of exposed pipe 2a is the stepped edge 4a of the anti-corrosion coating and a second joint length, comprising the length of exposed anti-corrosion coating 4b. Adjacent the exposed anti-corrosion coating is a length of concrete weight coating 6 extending the length of the pipe line to the next adjacent joint. Upon making the joint, an approximately annular shaped empty joint volume 10 is formed between the ends of the concrete weight coat 6 on either side of the welded bead 8.

Figure 1 shows the use of the known weld joint corrosion protection provided by hand wrapping three overlapping bands of self adhesive tape 1(a) and 1(b) over the exposed joint area 8 and subsequently filling the joint volume 10 with a filler material such as hot mastic asphalt, rapid setting concrete or rapid cure aggregate filled urethane, all as further described above. In accordance with the invention, as shown in Figure 2, a flexible heat resistant waterproofing backing film 22 such as a high temperature pvc backing film is adhered via a pressure sensitive adhesive backing layer 28 so as to span the exposed area of pipe 2a either side of the weld bead 8 and to overlap the edge 4a and at least part of the exposed portion 4b of the anti-corrosion coating 4 on either side of the weld bead 8. An abrasion resistant high melting point film 24 is laminated by adhesive layer 26 to the outer surface of the flexible heat resistant waterproofing backing film 22 and overlies the exposed area of pipe 2a, but does not overlap the edge 4a of the anti-corrosion coating 4. The joint volume 10 is filled with a filler material as above.

Advantageously the wrapping may be applied in the form of a prelaminated tape. Such a tape for use in the present invention is shown in Figure 3. A flexible heat resistant waterproofing backing film 22 has a width sufficient to span the exposed area 2a of pipe 2 to either side of weld bead 8 with each edge of the film 22 overlapping the corresponding edge 4a of the exposed part of the factory applied pipe anti- corrosion coating 4.

A strip of high melting point abrasion resistant film 24 which is narrower than the flexible heat resistant waterproof backing film 22, having a width sufficient to overlie substantially the whole of the exposed area 2a of pipe 2 but not to overlie the edge 4a of the exposed part of the factory applied anti-corrosion coating 4, is laminated by an adhesive layer 26 to the upper surface of the PVC backing film 22.

The lower surface of the flexible heat resistant waterproofing backing film 22 is coated with a layer of pressure sensitive adhesive 28. Advantageously the tape may be supplied in the form of the laminate and on a roll with the adhesive 28 protected with a layer 30 of release film or paper.

The tape 20 is applied by wrapping under tension covering the entire joint area centred on the circumferential weld line 8. The tape is preferably applied so^' as to create an overlap of one tenth to one third of the pipe circumference. Preferably the overlap is arranged to lie at the crown of the pipe joint. Once the tape laminate is in place the joint area may be enclosed by a mold and the joint volume 10 filled with a filler material as above. Preferably the filler material is marine mastic, applied at for example a temperature of 170 to 220^βC. The flexible heat resistant waterproofing backing film 22 should be sufficiently flexible to allow conformity to the joint profile. It should be heat resistant, by which is meant that it should be able to withstand the high residual weld temperatures that may be experienced in using the present invention, typically up to 200°C. It should further be waterproofing, by which is meant it must resist the passage of water which could otherwise lead to corrosion of the pipe joint.

The flexible heat resistant waterproofing film 22 may for example be a vulcanised EPDM (ethylene-propylene-diene rubber) , a polychloroprene, an extruded or calendered thermoplastic rubber or an elastomeric polyurethane film. Preferably the flexible film 22 is a high temperature PVC film. By high temperature PVC film is meant a film made from PVC formulated to provide mechanical stability at temperatures higher than a standard PVC grade of comparable flexibility. By standard PVC grade is meant a PVC formulated from PVC resin, a monomeric plasticiser such as dioctyl phthalate, a filler such as calcium carbonate, heat stabilisers such as lead phthalate, and lubricant such as calcium stearate. Preferably the high temperature PVC film will retain mechanical stability and resist thermal degradation when subjected to temperatures of at least 150°C, preferably 200°C to 220°C for at least ten minutes. Such a high temperature PVC backing film 22 may typically have a thickness of 0.5 to 1 mm.

The preferred high temperature PVC film 22 is more preferably a flexible PVC formulation, comprising a high Fikentscher K value (65 to 75 measured in accordance with DIN 53 726) PVC resin, reinforcing fillers, heat stabilisers and high temperature polymeric plasticisers or extenders such as highly chlorinated polyethylene, ethyl/vinyl acetate copolymers or elastomeric extenders. Preferably the high temperature PVC film should pass the Drumskin Test for assessing the mechanical stability of plastic tapes in applications where high temperatures are encountered for short periods.

The Drumskin Test method may be conducted as follows:-

1) prepare a 125 mm x 125 mm test piece of the tape by cutting from a sample; 2) drill 2 mm x 10 mm holes in the sides of an aluminium can 63 mm diameter. The holes allow air to escape during the test. Suitable cans are available from Delapak Ltd. , Kershaw, Worcestershire, England (63 x 80 mm seamless aluminium extruded screw cap containers) ; 3) place the test piece on the open top of the can; carefully folding it down over the can side. The test piece is wired in position so that a continuous "drumskin" is formed across the open top of the can;

4) place this test assembly quickly into an oven which is stabilised at 220°C as measured by a thermometer adjacent to the test position. The oven temperature should not drop by more than 20^βC immediately after loading the assembly and the temperature should recover to 220°C plus or minus 3°C between 3 and 7 minutes after commencement of the test. After 10 ± 0.1 minutes the test assembly is removed from the oven and allowed to cool to room temperature. Standard PVC grades will melt and sag or drip to the bottom of the can. High temperature grades should not melt or drip and should sag very little if at all.

Representative high temperature PVC grades are given in Examples 1 to 5 below.

Example 1

High temperature PVC formulation using high temperature polymeric plasticiser.

Component Parts bv weight

70 k-value PVC resin: 100

High molecular weight polyester plasticiser (1) 35 to 70 Filler (2) 20 to 100

Heat stabiliser (3) 1 to 10

Lubricants/processing aids (4) 0.5 to 2

Example 2

High temperature PVC formulation using highly chlorinated polyethylene:

Component Parts bv weiσht

70 k-value PVC resin: 100

Chlorinated polyethylene (5) 40 to 80

Plasticiser (6) 20 to 50 Filler (2) 10 to 50

Heat stabilisers: (3) 1 to 10

Lubricants/processing acids (4) 0.5 to 2

Example 3

High temperature PVC formulation using ethyl/vinyl acetate copolymer:

70 k-value PVC resin: 100

Ethyl/Viny Acetate copymer (7) 50 to 80

Plasticiser (6) 20 to 50 Filler: (2) 10 to 100

Heat stabilisers: (3) 1 to 10

Lubricants/processing acid (4) 0.5 to 2 Example 4 High temperature PVC formulation using nitrile rubber elastomeric modifier:

70 k-value PVC resin: 100

Nitrile rubber (8) 20 to 50

Plasticiser (6) 20 to 50

Filler: (2) 20 to 100 Heat stabilisers: (3) 1 to 10

Lubricants/processing acids (4) 0.5 to 2 Example 5

High temperature PVC formulation using solid polyurethane elastomeric modifier: 70 k-value PVC resin: 100 polyurethane elastomer (9) 60 to 100

Plasticiser (6) 0 to 50

Filler: (2) 20 to 100

Heat stabilisers: (3) 1 to 10 Lubricants/processing acids (4) 0.5 to 2

In the above Examples 1 to 5, representative components 1 to 9 are as follows:

1. High molecular weight polyester plasticiser e.g. Santicizer 429 from Mansanto Co. 2. Filler e.g. precipitated CaC03, talc, carbon black.

3. Heat stabiliser e.g. lead phthalate.

4. Lubricants/processing aids e.g. calcium stearate.

5. Chlorinated polyethylene e.g. XO 2243.49 from DOW Chemicals.

6. Plasticiser e.g. dioctyl phthalate, a standard monomeric plasticiser available from Ciba Geigy.

7. Ethyl/vinyl acetate copolymer e.g. Elvaloy 742 from DuPont Co. 8. Nitrile rubber e.g. Chemigum from Goodyear Chemicals. 9. Polyurethane elastomer e.g. Baymod PU from Bayer. The laminated abrasion resistant film 24 must resist the abrasive effect of the filler material used to fill the joint volume 10, even when subjected to high residual weld temperature or the application of marine mastic at temperatures of as high as 220°C. By "high melting point" is meant a melting point of over 220^βC as measured by ASTM E794-85. Abrasion resistance for the purposes of the present invention requires a tensile modulus preferably above that of standard PVC (15 N mm"²) and good tear resistance even when subjected to elevated temperatures of 150^βC and above, especially 200°C to 220°C. For example, the abrasion resistant film 24 may be: a polyester film; a cellulose acetate film; a polyamide film such as Nylon 6 or Nylon 6,6; a poly chloro-tetrafluoro ethylene film; or a polyimide film. The abrasion resistant film 24 is preferably a polyester film, more preferably polyethylene terephthalate, and may typically be 15 to 75 μm thick. Where the heat and abrasion resistant film 24 is a polyester film, a degree of shrinkage is available at temperatures around 200°C. This enhances the conformability and adhesion of the laminated tape 20 around the weld bead where the temperature upon application of the tape 20 may be at or around 200°C.

The abrasion resistant film 24 may be laminated to the flexible backing film 22 using any suitable adhesive 26. Suitable adhesives 26 for forming the laminate may include known curing, pressure sensitive or hot melt adhesives. Where a hot melt adhesive is used it is preferable that the adhesive have a softening point of 80° to 120°C. This has the advantage that during production of the laminated tape 20 excessive stress is not introduced into the flexible backing film 22. Additionally at application temperatures, typically up to 220°C for hot mastic filling material, the viscosity modulus of such a hot melt adhesive allows a degree of differential movement in the laminate to prevent stress concentrations at the laminate edges.

The pressure sensitive adhesive 28 used to adhere the tape 20 to the pipe joint is typically a bituminous adhesive and may typically be in a layer 0.5 to 1.0 mm thick. The invention enables single piece application, compared with the three piece tape application of the prior art, allowing a reduction in joint coating time and in material used. The central, reinforcing laminate 24 allows wrapping with post-weld residual temperatures of up to 200°C and subsequent application of hot mastic filling material for filling the joint volume at temperatures as high as 220° without distortion or loss of integrity. The flexible, non- laminated margins allow full conformability even to steep coating cutbacks at low ambient temperature.

Claims

CLAIMS 1. A method of providing corrosion protection to a joint area in a weight coated pipe line; said pipe line comprising a plurality of pipes having an anti-corrosion coat applied circumferentially thereto and a weight coat applied over said anti-corrosion coat said pipes being co-axially joined by a plurality of pipe joints wherein a joint area comprises to either side of a said pipe joint: a first joint length of exposed pipe to which neither said anti-corrosion nor said weight coat is applied; and a second joint length adjacent to said first joint length to which said anti-corrosion coat is applied but said weight coat is not applied; said method comprising: (a) applying as a circumferential wrap a first tape member comprising a flexible heat resistant waterproofing backing film and a pressure sensitive adhesive layer having a width spanning and adhered to said first joint length and at least partially overlying and adhered to said second joint length on either side of said joint; a second tape member comprising a high melting point abrasion resistant film having a width less than the width of the first tape member and being laminated at the centre of the outer surface of the first tape so as when applied to overlie said first joint length of exposed pipe and not to overlie said second joint length;

(b) filling the joint area to the weight coat level with a filler material.

2. A method according to claim 1 wherein said flexible backing film of said first tape is a high temperature PVC film.

3. A method according to claim 1 or claim 2 wherein said first and second tapes are laminated prior to application and applied in a single operation.

4. A method according to claim 3 wherein said tape is applied by wrapping under tension.

5. A method according to claim 3 or claim 4 wherein said tape is applied so as to create an overlap of one tenth to one v third of the pipe circumference at the crown of the pipe.

6. A method of assembling an off-shore weight coated pipe line from a plurality of lengths of pipe each length of pipe: having a factory applied anti-corrosion coat and an outer factory applied concrete weight coat; and comprising at each end thereof a length of pipe adjacent the end thereof to which neither said anti-corrosion coat nor said weight coat is applied and adjacent thereto a length of pipe to which said anti-corrosion coat is applied but said weight coat is not applied said method comprising; concentrically joining a first pipe and a second pipe by welding; providing corrosion protection in accordance with claim 1.

7. A tape for use in the method of any preceding claim comprising: a flexible, heat resistant, waterproofing backing film a strip of high melting point abrasion resistant film laminated at the centre of a first major surface of the flexible backing film the second major surface of the flexible backing film being coated with a layer of pressure sensitive adhesive.

8. A tape according to claim 7 wherein said flexible backing film is a high temperature PVC film.

9. A tape according to claim 7 or claim 8, wherein the layer of pressure-sensitive adhesive is protected with a release layer.

10. A tape according to any one of claims 7 to 9, wherein said lamination is by means of a hot melt adhesive with a softening point of 80° to 120^βC.

11. A tape according to any one of claims 7 to 10, wherein said high melting point abrasion resistant film is a polyester film.