Laminate water barrier

Abstract

The present invention relates to a laminate water barrier which is capable of conducting capacitive currents radially out of the cable thus avoiding breakdown due to induced voltage gradients, comprising a laminate structure comprising a metal foil having a lower and an upper surface area, a first layer of a thermoplastic polymer laid onto and covering the lower surface of the layer of metal foil except for a longitudinal uncovered surface area of the layer of metal foil, and a second layer of thermo-plastic polymer laid onto and covering the upper surface of the layer of metal foil except for a longitudinal uncovered surface area of the layer of metal foil, and wherein the laminate structure is wrapped around the cable core such that the first uncovered surface area of the metal foil faces the cable core and the second uncovered surface area of the metal foil faces away from the laminate structure, and the laminate structure is thermally joined by a heat treatment.

Claims

1. A water barrier encapsulating a cable core, wherein the water barrier comprises: at least one layer of a laminate structure being wrapped around the cable core with at least some overlap between opposite edges of the laminate structure, wherein the laminate structure comprises: a layer of metal foil having a lower and an upper surface area, a first layer of thermoplastic polymer laid onto and covering the lower surface of the layer of metal foil except for a first longitudinal uncovered surface area of the layer of metal foil, and a second layer of thermoplastic polymer laid onto and covering the upper surface of the layer of metal foil except for a second longitudinal uncovered surface area of the layer of metal foil, and wherein the laminate structure is wrapped around the cable core such that the first longitudinal uncovered surface area of the metal foil faces the cable core and the second longitudinal uncovered surface area of the metal foil faces away from the laminate structure, and the laminate structure is thermally joined by a heat treatment.

2. The water barrier according to claim 1, wherein the metal foil is either: aluminium, an aluminium alloy of the AA1xxx series, AA5xxx series or the AA6xxx series according to the Aluminium Association Standard, copper a copper-alloy a CuNi-alloy a CuNiSi-alloy, iron a Fe-alloy stainless steel alloy SS316 or stainless steel alloy S32750.

3. The water barrier according to claim 1, wherein the thickness of the metal foil is in one of the following ranges; from 10 to 250 μm, preferably from 15 to 200 μm, more preferably from 20 to 150 μm, more preferably from 25 to 100 μm, more preferably from 30 to 75 μm, and most preferably from 15 to 20 μm.

4. The water barrier according to claim 1, wherein the laminate structure further comprises a first adhesive layer laid in between the metal foil and the first thermoplastic polymer layer and a second adhesive layer in between the metal foil and the second thermoplastic polymer layer, wherein the first and second adhesive layers do not coat the first and second longitudinal uncovered surface areas, respectively.

5. The water barrier according to claim 4, wherein the adhesive layers are chosen from one of: epoxy resins, phenolic resins, polyurethane based glues, cyanoacrylates, acrylic glues, polyester based glues, copolymer of ethylene and ethyl acrylate, copolymer of ethylene and ethyl acrylic acid, methacrylic acid, copolymer of ethylene and glycidyl methacrylate or epoxy-based monomer such as 1,2-epoxy-1-butene, and copolymer of ethylene and maleic-anhydride.

6. The water barrier according to claim 4, wherein the adhesive contains electrically conductive particles.

7. The water barrier according to claim 1, wherein the first and the second layer of a thermoplastic polymer is chosen from one of: a low density polyethylene (LDPE), a linear low density polyethylene (LLDPE), a medium density polyethylene (MDPE), or a high density polyethylene (HDPE), or a copolymer of ethylene with one or more polar monomers of; acrylic acid, methacrylic acid, glycidyl methacrylate, maleic acid, or maleic anhydride.

8. The water barrier according to claim 7, wherein the first and the second layer of thermoplastic polymer contains from 20 to 40 weight % particulate carbon in the polymer mass chosen from one of; comminuted petrol coke, comminuted anthracite, comminuted char coal, carbon black, or carbon nanotubes.

9. The water barrier according to claim 1, wherein one or both of the thermoplastic polymer layers further comprises a second polymer layer of polyethylene terephthalate, PET, or a polyester.

10. The water barrier according to claim 1, wherein the overlap between successive layers of the laminate structure provides a shortest diffusion path, w.sub.3, of at least 10 mm, more preferably at least 15 mm, more preferably at least 20 mm, more preferably at least 25 mm, more preferably at least 30 mm, more preferably at least 35 mm, and most preferably at least 40 mm.

11. The water barrier according to claim 1, wherein the laminate structure is shaped as a rectangular parallelepiped of thickness t.sub.2, width w, and length L, and the first longitudinal uncovered surface area is located along a first side edge of the lower surface of the layer of metal foil, and the second longitudinal uncovered surface area is located along a second side edge opposite the first edge and at the upper surface of the layer of metal foil, and wherein the laminate structure is wrapped helically or longitudinally around the cable core.

12. A power cable, comprising: at least one cable core, where each cable core comprises: an electric conductor, and an electric insulation system electrically insulating the electric conductor, and a water barrier arranged around the electric insulation system, wherein the water barrier is a water barrier according to claim 1, and in that the power cable further comprises a mechanical protection system laid around the at least one cable cores as a group.

13. The power cable according to claim 12, wherein the power cable further comprises an outer sheathing laid onto the water barrier layer by extrusion of a polymer at an extrusion temperature of around 200° C.

14. The power cable according to claim 13, wherein the outer sheathing is one of; a polyolefine based material, HDPE, LDPE, LLDPE, MDPE, polyvinyl chloride (PVC), polypropylene (PP), or thermoplastic polyurethane (TPU).

15. The power cable according to claim 14, wherein the polymer material of the outer sheathing contains until 40 weight % particulate carbon in the polymer mass, and where the particulate carbon is one of; comminuted petrol coke, comminuted anthracite, comminuted char coal, carbon black, or carbon nanotubes.

Description

BRIEF DESCRIPTION OF FIGURES

[0056] FIG. 1a) is a cut view drawing as seen from the side taken along the stapled line marked as A-A′ on FIG. 2, and which schematically illustrates the stratigraphic structure of an example embodiment of the laminate structure according to the invention.

[0057] FIG. 1b) is a drawing as seen from above of the example embodiment of the laminate structure shown in FIG. 1b).

[0058] FIG. 1c) is drawings of example embodiments of the location and configuration of the longitudinal uncovered surface areas of the laminate structure according to the invention.

[0059] FIG. 2) is a drawing schematically illustrating as seen from the side of a cable core being covered with the example embodiment of a laminate structure shown in FIGS. 1a) and 1b).

[0060] FIG. 3 is a vertical cut view drawing as seen from the side of a section of an example embodiment of the water barrier according to the invention comprising three windings of the laminate tape.

[0061] FIGS. 4a) and 4b) are cross-sectional cut view drawings of an example embodiment of the laminate structure according being longitudinally wrapped around a cable core.

[0062] FIG. 4c) is a drawing illustrating an excerpt of the formation of an example embodiment of the water barrier according to the invention applying two longitudinally wrapped laminate structures.

[0063] FIG. 5 is cut view drawings as seen from the side of an example embodiment of the water barrier according to the invention applied to simulate the water blocking capacity.

DETAILED DESCRIPTION

[0064] The water blocking effect of the water barrier according to the invention is verified by simulation of water intrusion through the water barrier. The simulation applied an embodiment of laminate structure comprising an adhesive layer between the metal foil and the first and second thermoplastic polymer layers. The simulation is based on determination of the diffusion of water through the thermoplastic polymer layer and the adhesive layer of the laminate structure. The metal foil was assumed impenetrable for water.

[0065] The calculations were made on an example embodiment shown in FIG. 5. The water barrier was assumed to consist of two partly overlapping layers of the laminate structure according to the invention laid onto an insulation layer 20 of cross-linked polyethylene (XLPE). The laminate structure consisted of a first 5 and a second 7 layer assumed to be cross-linked polyethylene (XLPE). Both layers are 50 μm thick. The first and second adhesive layers (6) were assumed to be continuous layers of cross-linked polyethylene (XLPE) of 2.5 μm thickness. The metal foil 4 was assumed to be 18 μm thick. The lower edge of domain 20 below the water barrier was assumed to always be completely dry, i.e. a relative humidity of 0%. In the simulation, domain 20 was assumed to be XLPE. Above the water barrier, there was assumed an outer sheathing 21 of high density polyethylene where the top edge is always saturated with water, i.e. constantly having a relative humidity of 100%. Furthermore, the diffusion over the boundaries on the left and right sides of FIG. 5 is always set to zero.

[0066] The simulations are based on Fick's law of diffusion and Henry's law to determine the saturation and diffusion of moisture through the outer sheathing 21, adhesive layers 6 and the thermoplastic polymer layer 5, 7, and the inner domain 20. The simulation method and the diffusion and solubility parameters applied in the calculations are taken from reference [1].

[0067] The diffusion coefficient D was calculated using the Arrhenius parameters D.sub.0 and E.sub.D. D.sub.0 and ED was 3.30.10.sup.4 m.sup.2/s respectively 55.7 kJ/mol for the adhesive layers 6 and the polymeric parts of the laminate structure 5,7 as well as the inner domain 20. The corresponding parameters for the outer sheathing 21 were 1.40.Math.10.sup.2 m.sup.2/s and 81.37 kJ/mol.

[0068] The solubility coefficient S was calculated using the Arrhenius parameters S.sub.0 and E.sub.S. S.sub.0 and E.sub.S was 1.80.Math.10.sup.−7 kg/(m.sup.3Pa) respectively 9.90 kJ/mol for the adhesive layers 6 and the polymeric parts of the laminate structure 5,7 as well as the inner domain 20. The corresponding parameters for the outer sheathing 21 were 7.21.Math.10.sup.−11 kg/(m.sup.3Pa) and −35.92 kJ/mol.

[0069] The temperature was assumed to be 40° C.

[0070] With these assumptions and parameters, the calculations gave that the time needed for 1 gram of moisture entering into the low-density polyethylene insulation layer 20 of the cable core was 320 years with an overlap (length w.sub.1) of 10 mm, 768 years with an overlap of 25 mm, and 1216 years with an overlap of 40 mm.

REFERENCE

[0071] 1. S. M. Hellesø, S. Hvidsten, G. Balog, and K. M. Furuheim (2011), “Calculation of water ingress in a HV subsea XLPE cable with a layered water barrier sheath system”, Journal of Applied Polymer Science 121(4):2127-2133 DOI: 10.1002/app.33568