Subsea power umbilical

Abstract

There is disclosed a power cable (11) for use in a subsea power umbilical (2,7), said power cable (11) comprising from inside to outside a central conductor (20), an electrical insulation layer (23), a metallic screen (28) and an external polymeric sheath (29), said central conductor (20) or said metallic screen (20) comprising at least one conductive wire (21, 26, 30), wherein said at least one conductive wire (21, 26, 30) comprises a metal core (31) coated with a copper cladding (32). Also disclosed is a subsea power cable comprising such a power cable, and a method of increasing the slippage of conductive wires (21,26,30) of a metallic screen (28) during bending of a power cable (11 ) for use in a subsea power umbilical (2,7), said power cable (11 ) comprising from inside to outside a central conductor (20), an electrical insulation layer (23), the metallic screen (28) and an external polymeric sheath (29), wherein the method comprises at least the step of increasing the tensile modulus of the conductive wires (21, 26, 30).

Description

[0001] SUBSEA POWER UMBILICAL

[0002] The present invention relates to a subsea power umbilical for transfer of power, and for use in the offshore production of hydrocarbons and / or renewable energy. The invention also relates to a power cable which is a component of the subsea power umbilical.

[0003] Background

[0004] Installations in the sea, such as offshore oil platforms and wind turbines are now commonplace, and subsea power umbilicals are often used to transmit electricity to and from such sea-based installations to those on the land.

[0005] A subsea umbilical comprises a group of one or more types of elongated functional elements such as electric cables, optical fibre cables, or hoses for fluid transportation of, for example, gas, water or chemical products such as methanol. The functional elements can be assembled together in a helical or S / Z manner and over-sheathed and / or over-armoured for mechanical strength and ballast.

[0006] It is desirable for a single umbilical to be able to contain as many functional elements as are required for a particular application, for example, as are required for a particular oil field where the umbilical is intended for use. Umbilicals are typically used for transmitting power, signals and fluids (for example for fluid injection, hydraulic power, gas release, etc.) to and from a subsea installation.

[0007] API Specification 17E I ISO 13628-5 "Specification for Subsea Umbilicals" provides standards for the design and manufacture of such umbilicals.

[0008] Subsea umbilicals which are used for transfer of large amount of electric power are called subsea power umbilicals or power umbilicals. Subsea power umbilicals are notably used in the offshore production of oil and gas, and in the offshore transmission of renewable energy.

[0009] Figure 1 shows a first typical subsea power umbilical 1 for use in the offshore production of hydrocarbon. A floating production unit 10 located at the sea surface 4 is connected to a subsea well tree (not shown) via a pipeline (not shown). The subsea power umbilical 1 is used for transmission of electrical power from the floating production unit 10 to a subsea electric driven equipment 40 located on the sea floor 5, for example a subsea pump. The subsea power umbilical 1 can also be used for transmitting signals between the production unit 10 and the subsea equipment 40.

[0010] Figure 2 shows second typical subsea power umbilicals 1a, 7 for use in the transmission of renewable energy being created. An offshore wind farm comprises a plurality of offshore wind turbines 3. Each wind turbine 3 is capable of generating a large amount of electrical power, for example more than 10 MW. This electrical power needs to be transmitted to a place of storage or usage, typically onshore. Inter-array subsea power umbilicals 1a are designed for connection of wind turbines 3 with each other and / or with a transformer station 6 located in the offshore wind farm. The inter-array subsea power umbilicals 1a are generally designed for operating at up to 66kV or up to 33kV. The transformer station 6 collects the electrical power transmitted by the inter-array subsea power umbilicals 1a of the wind farm. It also increases the voltage to high voltage, for example from 66 kV to 132 kV, before transmitting the electrical power onshore via an export subsea power umbilical 7. The transformer station 6 can be floating as shown in Figure 2, or beneath the sea or even on the sea floor 5.

[0011] Subsea power cables are used for transmitting larger amount of electrical power (typically a few MW). They are generally rated at voltages between 3 kV and 200 kV. Medium voltage cables rated between 6 kV and 35 kV are those most commonly used in the offshore production of hydrocarbons. High voltage cables rated above 35kV and up to around 200kV, are commonly used in the offshore production of renewable energy. Power cables used in the offshore production of renewable energy can be rated at 66kV or 132kV.

[0012] A prior art power cable 2 is illustrated in the accompanying Figure 3. Going from inside to outside, it comprises a central copper conductor 2a, semi-conductor and electrical insulation layers 2b, a metallic screen 2c and an external polymeric sheath 2d. The electric current used to transmit the electric power is conducted by and along the central conductor 2a. The primary function of the metallic screen 2c is to accommodate cable charging and circulating currents and to discharge short-circuit currents in the event of a cable failure. The metallic screen can also be used for electromagnetic shielding and optionally as a water barrier depending on the design and on the voltage rating of the power cable.

[0013] An umbilical can be formed comprising at least one such power cable and is often termed a power umbilical or a subsea power umbilical. A power umbilical may comprise a plurality of power cables, for example, three-phase power can be provided by three power cables bundled together within the power umbilical structure.

[0014] Since subsea power umbilicals are typically subject to constant and dynamic motion generated by for example waves, tides, currents in the sea, and / or the movement of the floating production unit or the wind turbine to which they are connected, the ability of a subsea power umbilical to withstand fatigue is important. And, as the desired lifetime of a subsea power umbilical is now becoming between 20-50 years, a subsea power umbilical is now required to be able to sustain exposure to such mechanical loads over longer periods of time.

[0015] Problem to be solved

[0016] There is a general need to improve the lifetime of subsea power umbilicals which are designed for dynamic applications. It has been found that the metallic screens of the power cables which are currently used in a subsea power umbilical generally have limited fatigue resistance. There is a need to improve the design of the power cables so that their metallic components, notably the metallic screen, can better resist heavy cyclic loadings over longer periods.

[0017] The present invention aims at solving one or more of these problems, and at providing a reliable and efficient solution to increase the service life of subsea power umbilicals which are used in dynamic applications.

[0018] Summary of the invention

[0019] These objectives are achieved by the invention as set forth and characterized in the independent claims, while the dependent claims describe additional details and embodiments of the invention. Claims 1 to 14 deal with a power cable for use in a subsea power umbilical. Claims 15 to 19 deal with a subsea power umbilical comprising at least one power cable as defined in any one of claims 1 to 14. Claims 20 to 22 deal with a method of increasing the slippage of conductive wires of a metallic screen during bending of a power cable.

[0020] According to one aspect of the present invention, the power cable at least comprises, concentrically from its core or inside, to its exterior or outside, a central conductor, an electrical insulation layer, a metallic screen and an external polymeric sheath. The central conductor or the metallic screen of the power cable comprises at least one conductive wire, and said at least one conductive wire comprises a metal core coated with a copper cladding.

[0021] It was found that the copper conductive wires used in conventional power cables were prone to premature failure in very severe dynamic applications, and that replacing these wires by copper-coated metal wires increases the fatigue resistance of the power cable. The consequently increases the service life of the power cable and the subsea power umbilical.

[0022] The term “copper” as used herein includes pure copper and copper alloys comprising at least 60% weight for weight of pure copper, such as beryllium copper alloys or nickel copper alloys.

[0023] The term “metal core” as used herein is a core which is made of a metal different from copper as defined hereabove.

[0024] According to a first embodiment of the present invention, the metal core comprises or is made with steel, which results in the conductive wire being a copper cladded steel wire. A copper cladded steel wire is a composite wire composed of a steel core surrounded by a copper cladding.

[0025] Steel has a larger tensile modulus than the tensile modulus of copper. Therefore, a copper cladded steel wire has a higher axial stiffness than the axial stiffness of a copper wire which has the same outside diameter. It was found that this increase of axial stiffness improves the fatigue resistance of the conductive wire, notably when the wire is helically wound inside the power cable structure, and when the power cable is subjected to cyclic bending and / or axial loadings. The increase of axial stiffness also promotes slip of the conductive wire during bending which reduces the stress range applied to the wire and increases the overall fatigue resistance of the power cable.

[0026] As such, a steel core for a conductive wire in the present invention provides improved fatigue resistance and increased axial stiffness, whilst the copper cladding ensures that the electrical conductivity of the wire remains suitable high or large for the electrical current transmission required.

[0027] Various grades of steel are usable as the metal core of a conductive wire in the present invention. Steel grading systems consider chemical composition, treatment, and mechanical qualities to help fabricators select a suitable grade for the intended use. Steel grades that are regularly used include:

[0028] Carbon steels: S235JR, S275JR, S355, EN3B, EN8, EN9 and EN19

[0029] Alloy steels: EN24, EN31 , EN36, and 14NiCrMo13-4

[0030] Stainless steels: AISI 304, 316, 430, and 440C

[0031] Tool steels: BD2, BH13, BM2 and BO1

[0032] Optionally, the metal core comprises stainless steel. Stainless steel grades are typically alloyed with 10-20% chromium, as well as nickel, silicon, manganese, and carbon. Stainless steels such as grades 304 and 316 stainless steel, can also increase the corrosion resistance of the wire in case the copper cladding would be damaged.

[0033] Optionally, the metal core comprises a non-magnetic stainless steel, which assists to reduces the electromagnetic skin effect around the steel core, and consequently improve the global electrical properties of the wire.

[0034] The term “non-magnetic” as used herein refers to the relative magnetic permeability of the material being between 1 and 1 .5. Carbon steels have a relative magnetic permeability generally larger than 100, therefore they are strongly magnetic. A few stainless steels are non-magnetic, notably austenitic stainless steels such as AISI 316, AISI 316L. Using such non-magnetic stainless steels for the metal core in combination with a copper cladding provides the advantages of increasing fatigue resistance, electrical properties and corrosion resistance.

[0035] It is useful to consider that the diameter (D) of a conductive wire of a metallic screen of a power cable is typically in the range 0.5mm to 4mm; with a radius R being half. Thus, the diameter (D) of a conductive wire of a metallic screen of a power cable may be 0.8mm, 1mm, 1 ,5mm, 2mm, 2.5mm or 3mm.

[0036] In one embodiment, where the present invention includes a conductive wire of a metallic screen of a power cable comprising a metal core coated with a copper cladding, the thickness (T) of the copper cladding can be in the range 10% to 70% of R, including 12.5% to 60%, 15% to 50%, 20% to 50% and 20% to 40%.

[0037] In another embodiment, where the present invention includes a conductive wire of radius R of a metallic screen of a power cable comprising a metal core coated with a copper cladding, the thickness (T) of the copper cladding can be in at least T / R > 0.1 , such as in the range 0.2 to 0.5, including in the range 0.2 to 0.4.

[0038] The present invention is not limited by such dimensions, especially where environmental circumstances require other properties for the power cable. The skilled man can calculate other dimensions of D, R and T based on desired electrical conductivity and fatigue resistance of the power cable in use.

[0039] Steel has larger mechanical properties but smaller electrical conductivity that copper. Therefore, providing a power cable of the present invention having a steel metal core, and increasing T while maintaining R constant, has the technical effect of both increasing the global electrical conductivity of the wire, and decreasing the global axial stiffness of the wire. Where thicker wires are not desired or possible in a power cable, the radius of the metal core may be less than the radius of a conventional allcopper metallic wire, but the thickness T of the copper cladding I sufficient to achieve the conductivity still required for the metallic screen. As such, there can be an efficient trade-off between the mechanical and electrical properties of the conductive wire, for example when the ratio T / R is comprised between at least 10% and 70%. The electrical conductivity of such stainless steels is known to be lower than the electrical conductivity of copper, typically around 5% using the International Annealed Copper Standard (IACS). Thus, the global electrical conductivity of a conductive wire usable in the present invention varies from around 12% IACS when the ratio T / R is around 5% (thin copper coating), to more than 70% IACS when the ratio T / R is around 45% (thick copper coating). Intermediate T / R ratios of 5% and 30% lead to global conductivities of respectively around 30% IACS and 52% IACS.

[0040] For example, a 2mm copper clad stainless steel wire may only have a 40% conductivity of a pure-copper wire, but such a wire can still be sufficient for use in a power cable, along with consideration of the number of wires required to achieve the overall current rating of the metallic screen required for the voltage of the power cable. The current standard for the requirements of an umbilical cables is IEC 60502-2. The current standard for the requirements of offshore wind cables up to 72.5 kV is IEC 63026.

[0041] The effect of T / R on the global axial stiffness is opposite. Indeed, T / R ratios of 5%, 15%, 20%, 25%, 30% and 45% respectively lead to global stiffnesses of around 95%, 87%, 84%, 80%, 77% and 68% of the stiffness of a steel wire that would have the same outer diameter D (i.e. corresponding to T / R = 0%).

[0042] The skilled person can see that a balance between electrical conductivity and axial stiffness may be required to achieve the properties of the power cable desired. A good trade-off between electrical conductivity and axial stiffness is obtained when T / R is comprised between 10% and 70%.

[0043] The manufacture of a conductive wire comprising a metal core coated with a copper cladding can be carried out in a number of known ways, including starting with a metal wire, and cladding copper in the form of a strip or the like around the core, optionally and / or drawing and / or shaping, in various orders, until the final shape and diameter is achieved.

[0044] The manufacture of a power cable according to the present invention can be carried out in the same or similar manner known in the art. Typically, the or each conductive wire of a metallic screen are wound around the electrical insulation layer using rotating supplies of wire. Where the conductive wires of the present invention have the same or very similar diameter to conventional pure-copper wires, little or no change is required in the manufacturing process to lay the conductive wires comprising a metal core coated with a copper cladding onto the electrical insulation layer. Where the conductive wires of the present invention have a larger or smaller diameter to conventional pure-copper wires, the skilled person can adjust the conventional manufacturing process to lay the conductive wires comprising a metal core coated with a copper cladding onto the electrical insulation layer. Where the present invention requires more or less conductive wires to the number of conventional pure-copper wires, the skilled person can adjust the conventional manufacturing process to lay the different number of conductive wires comprising a metal core coated with a copper cladding onto the electrical insulation layer.

[0045] According to a second preferred embodiment of the present invention, the metal core is made with one or more grades of aluminium, including the 6000 series, which results in the conductive wire being a copper cladded aluminium wire. Aluminium core solutions primarily benefit from the good electrical conductivity of aluminium and from improved fatigue properties compared with copper. Contrary to steel, aluminium does not have a high modulus, but this drawback can be compensated by increasing the diameter of the aluminium core, thereby increasing the global axial stiffness of the wire, with a minimised increase in weight, which is also an advantage.

[0046] The embodiments described herein in relation to the conductive wire having a steel core apply equally to the conductive wire having an aluminium core.

[0047] Where the power cable of the present invention comprises more than one conductive wire for the central conductor, or more than conductive wire for the metallic screen, the present invention is not limited by any of:

[0048] -the number of conductive wires;

[0049] -the laying angle of the conductive wires on the electrical insulation layer;

[0050] -the uniformity of the conductive wires, such as their diameter, shape or configuration; and - the constitution of the conductive wires.

[0051] Thus, the present invention includes the use of different arrays of conductive wires, and / or some being coated with copper cladding, and / or some being regular and some being irregular, and / or different diameter conductive wires.

[0052] A central conductor of a power cable requires a conductivity to achieve its power transmission function, and typically comprises a plurality of conductive wires as strands, sometimes in circular concentric arrays or patterns. By way of example, outside stands of a central conductor (those being the most distant from the cable central axis) could have large T / R ratios, i.e. thicker copper cladding to maximize conductivity, for example 30% < T / R < 50% to achieve an equivalent electrical conductivity comprises between 52% IACS and 75% IACS.

[0053] In another embodiment of the present invention, the at least one conductive wire comprising a metal core coated with a copper cladding is or is part of a metallic screen. It was found that the metallic screen is more prone to fatigue than the central conductor and that the preferred location for using the copper-cladded metal wires is the metallic screen.

[0054] Optionally, all conductive wires of the metal screen are copper-cladded metal wires. Optionally, a metallic screen in the present invention comprises a plurality of conductive wires separated with gaps around electrical insulation layer. The gaps may be regular or irregular, and allow movement of the conductive wires during bending of the power cable. That is, the plurality of conductive wires can be configured to have lateral movement around electrical insulation layer during bending of the power cable.

[0055] In this way, the present invention includes increasing the lateral mobility of the wires not only by having separation gaps wherein wires can move, but also by increasing the axial stiffness of the wires compared to copper wires.

[0056] Optionally, the gaps between the conductive wires are larger than the diameter of the conductive wires, for example being in the range >1 to 4 times the diameter of the conductive wires. Such gaps allow the conductive wires the room to slip during bending of the power cable.

[0057] Optionally, the plurality of conductive wires is configured in a helical pattern around the electrical insulation layer.

[0058] According to a second aspect of the present invention, there is provided a subsea power cable comprising at least one power cable as defined herein.

[0059] Optionally, the subsea power cable comprises at least three such power cables assembled together in helical or S / Z manner, optionally over armoured and / or over sheathed. In addition, optionally the subsea power cable comprises at least one filler element and / or at least on optical fibre cable.

[0060] Optionally, the subsea power cable is an inter-array cable. According to a third aspect of the present invention, there is provided a method of increasing the slippage of conductive wires of a metallic screen during bending of a power cable for use in a subsea power umbilical, said power cable comprising from inside to outside a central conductor, an electrical insulation layer, the metallic screen and an external polymeric sheath , wherein the method comprises at least the step of increasing the tensile modulus of the conductive wires.

[0061] Increasing the tensile modulus of the conductive wires allows the conductive wires to better slip (relative to each other and around the electrical insulation layer) during bending of the power cable, and therefore to more easily accept dynamic loading on the power cable (and therefore a subsea power umbilical having such a power cable) in use. This leads to a longer service life for the power cable, and so for the subsea power umbilical.

[0062] In one embodiment of the present invention, the method of increasing the tensile modulus of the conductive wires comprises the step of forming the conductive wires with a metal core coated with a copper cladding.

[0063] Optionally, the method further comprises the step of laying the conductive wires on the electrical insulation layer with gaps between the conductive wires larger than the diameter of the conductive wires. The possible embodiments, alternatives, properties and parameters for the conductive wires, metallic screen, power cable, subsea power umbilical, gaps et al that can be involved in the method of the present invention are described elsewhere herein, and apply equally to a method of increasing the slippage of the conductive wires of a metallic screen during bending of a power cable.

[0064] Detailed description

[0065] Preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:-

[0066] Figure 1 is a schematic illustration of an offshore production unit of oil and gas using a subsea power umbilical.

[0067] Figure 2 is a schematic illustration of an offshore wind farm using subsea power umbilicals.

[0068] Figure 3 is a perspective broken out view of a power cable according to the prior art.

[0069] Figure 4 is a cross-sectional view of a subsea power umbilical according to the present invention.

[0070] Figure 5 is a perspective broken out view of a power cable according to the present invention.

[0071] Figure 6 is a cross-sectional view of a conductive wire used in a power cable according to the present invention.

[0072] Figure 1 a subsea power umbilical 1 according to the present invention for use in the offshore production of hydrocarbon. A floating production unit 10 located at the sea surface 4 is connected to a subsea well tree (not shown) via a pipeline (not shown). The subsea power umbilical 1 is used for transmission of electrical power from the floating production unit 10 to a subsea electric driven equipment 40 located on the sea floor 5, for example a subsea pump. The subsea power umbilical 1 can also be used for transmitting signals between the production unit 10 and the subsea equipment 40.

[0073] Figure 2 shows a subsea power umbilicals 1a, 7 for use in the production of renewable energy. An offshore wind farm comprises a plurality of offshore wind turbines 3. Each wind turbine 3 is capable of generating a large amount of electrical power, for example more than 10 MW. This electrical power needs to be transmitted back to land. Inter-array subsea power umbilicals 1a are designed for direct connection of wind turbines 3 with each other and / or with a transformer station 6 located as part of the offshore wind farm. The inter-array subsea power umbilicals 1a are generally designed for 66kV and 33kV with the later becoming less common. The transformer station 6 collects the electrical power transmitted by the inter-array subsea power umbilicals 1a of the wind farm. It also increases the voltage to high voltage, for example from 66 kV to 132 kV, before transmitting the electrical to the land via an export subsea power umbilical 7. The transformer station 6 can be floating as shown in Figure 2, or beneath the sea or even on the sea floor 5.

[0074] Figure 3 is previously discussed.

[0075] Figure 4 shows a subsea power umbilical 17 according to one example of the present invention. The subsea power umbilical 17 comprises three power cables 11 , a plurality of filler elements 12 and an optical fibre cable 13 which are assembled together in a helical manner and form a central assembly. Going from inside to outside, this central assembly is surrounded by an external armouring layer 14 and by an external protection sheath 15.

[0076] The plurality of filler elements 12 fills the interstitial voids between the various umbilical elements in order to obtain the desired circular configuration. The optical fibre cable 13 is used for data and signal transmission.

[0077] The external armouring layer 24 withstands axial loads applied to the subsea power umbilical 17, notably those due to the weight and the movement of the subsea power umbilical 17. The external armouring layer 24 comprises steel or composite armours which are helically wound around the central assembly. The three power cables 11 are preferably identical and form a 3-phase power supply circuit. Each power cable 11 comprises from inside to outside a central conductor 20, an electrical insulation layer 23, a metallic screen 28 and an external polymer sheath 29. The metallic screen 28 of each power cable 11 comprises at least one conductive wire which comprises a metal core coated with a copper cladding. Figure 5 shows a power cable 11 according to another example of the present invention. From inside outside, it comprises a central copper conductor 20, semiconductor and electrical insulation layers 25, a metallic screen 28 and an external polymeric sheath 29.

[0078] The central conductor 20 comprises conductive wires 21 which are wound and stranded together. The section of the central conductor 20 is large and typically comprised between 50mm2and 400mm2.

[0079] From inside outside, the central conductor 20 is surrounded by an inner semi- conductive sheath 22, by an electrically insulating sheath 23 and by an outer semi- conductive sheath 24.

[0080] The metallic screen 28 comprises conductive wires 26 which are helically wound and achieve the function to discharge short-circuit in the event of cable failure. These conductive wires 26 are preferably round but flat wires could also be used. The wires 26 are all wound with the same helix angle on the same cylindrical support with separation gaps in between adjacent wires. Due to these separation gaps, each wire 26 can move aside when the power cable is bent, thereby accommodating the curvature changes. Conductive tapes 27 are cross wound above and in contact with the wires 26, which created electrical bridge between the conductive wires 26, thereby improving the global electrical functions of the metal screen.

[0081] When the power cable is of the semi-wet type, the metallic screen 28 is not watertight and is limited to electrical functions. When the power cable is of the semidry type, the metallic screen 28 is watertight which is generally obtained by having the tapes 27 wound with overlap and bonded or welded together.

[0082] The fatigue loading on the wires 26 in the metallic screen 28 is generated by its contact, position and geometry in the power cable structure and is dramatically affected by whether the wire is stuck in position or free to move independently. This effect is governed by the axial stiffness of the wire (rather than strength) with increased stiffness promoting early onset slip and hence an improved performance. As shown in Figure 5, the wires 26 in the metallic screen 28 are separated by gaps 35. The gaps 35 provide room for the wires 26 in the metallic screen 28 to lateral movement of the wires 26 during bending. In this way, the present invention includes increasing the lateral mobility of the wires not only by having separation gaps 35 wherein wires can move, but also by increasing the axial stiffness of the wires compared to copper wires.

[0083] By way of example only, where the wires 26 have a diameter of 2mm, the gaps 35 are at least 2mm, typically 3mm or more.

[0084] As shown by Figure 6, the conductive wires used in the metallic screen comprise a metal core 31 coated with a copper coating 32 with an interface 33 in between the core 31 and the coating 32. The coating 32 preferably has a uniform and almost constant coating thickness equal to T as shown by Figure 6. The present invention could also work with a copper coating of irregular thickness. The conductive wire 30 is preferably round with an outside diameter equal to R.

[0085] The metal core 31 is made with steel, preferably stainless steel, more preferably non-magnetic stainless steel such as for example AISI 316.

[0086] The present invention provides means and apparatus to increase axial stiffness and improve fatigue resistance of a power cable for use in a subsea power cable, and therefore of the subsea power umbilical itself. Such power cables and umbilicals are subjected to cyclic bending and / or axial loadings in use. The increase of axial stiffness promotes slip of the conductive wire during bending, which reduces the stress range applied to the wire and increases the overall fatigue resistance of the power cable. The present invention leads to an increased service life for the power cable and therefore to the subsea power umbilical, with little or no change to the manufacturing process of the power cable.

Claims

CLAIMS1 . A power cable (11 ) for use in a subsea power umbilical (2,7), said power cable (11 ) comprising from inside to outside a central conductor (20), an electrical insulation layer (23), a metallic screen (28) and an external polymeric sheath (29), said central conductor (20) or said metallic screen (20) comprising at least one conductive wire (21 , 26, 30), wherein said at least one conductive wire (21 , 26, 30) comprises a metal core (31) coated with a copper cladding (32).

2. A power cable (11 ) as claimed in claim 1 , wherein the metal core (31 ) comprises steel.

3. A power cable (11 ) as claimed in claim 2, wherein the metal core (31 ) comprises stainless steel.

4. A power cable (11 ) as claimed in any one of claims 2 to 3, wherein the metal core (31) is made with a non-magnetic steel.

5. A power cable (11 ) as claimed in claim 1 , wherein the metal core (31 ) comprises aluminium.

6. A power cable (11 ) as claimed in any one of the preceding claims, wherein the thickness T of the copper cladding (32) is between 10% and 70% of the outside radius R of the conductive wire (21 , 26, 30).

7. A power cable (11 ) as claimed in claim 6, wherein the thickness T of the copper cladding (32) is between 12.5% to 60%, optionally between 15% to 50%, more optionally between 20% to 50%, and more optionally between 20% to 40%, of the outside radius R of the conductive wire (21 , 26, 30).

8. A power cable as claimed in any of the preceding claims, wherein the metallic screen (20) comprises at least one conductive wire (26, 30) which comprises a metal core (31) coated with a copper cladding (32).

9. A power cable as claimed in claim 8, wherein all conductive wires (26, 30) of the metallic screen (20) comprise a metal core (31) coated with a copper cladding (32).

10. A power cable as claimed in any one of the preceding claims wherein the metallic screen (28) comprises a plurality of conductive wires (21 , 26, 30) separated with gaps (35) around electrical insulation layer (23).

11. A power cable as claimed in claim 10 wherein the plurality of conductive wires are configured to have lateral movement around electrical insulation layer (23) during bending of the power cable (11 ).

12. A power cable as claimed in claim 10 or claim 11 wherein the gaps (35) between the conductive wires (21 , 26, 30) is larger than the diameter of the conductive wires.

13. A power cable as claimed in any one of claims 10 to 12 wherein the plurality of conductive wires are configured in a helical pattern around the electrical insulation layer (23).

14. A power cable as claimed in any one of the preceding claims having a rating of 66kV or 132kV.

15. A subsea power umbilical (1 , 7, 17) for use in the offshore production of hydrocarbon or renewable energy, said subsea power umbilical (1 , 7, 17) comprising at least one power cable (11 ) as defined in any of the preceding claims.

16. A subsea power umbilical (1 , 7, 17) as claimed in claim 15, said subsea power umbilical (1 , 7, 17) comprising at least three power cables (11 ) assembled together in a helical or S / Z manner.

17. A subsea power umbilical (1 , 7, 17) as claimed in any one of claims 15 to 16, said subsea power umbilical (1 , 7, 17) comprising at least one filler element (12) and / or at least one optical fibre cable (13).

18. A subsea power umbilical (1 , 7, 17) as claimed in any one of claims 15 to 17, said subsea power umbilical (1 , 7, 17) comprising an external armouring layer (14) and / or an external protection sheath (15).

19. A subsea power umbilical (1 , 7, 17) as claimed in any one of claims 15 to 18, said subsea power umbilical (1 , 7, 17) is an inter-array cable.

20. A method of increasing the slippage of conductive wires (21 ,26,30) of a metallic screen (28) during bending of a power cable (11 ) for use in a subsea power umbilical (2,7), said power cable (11 ) comprising from inside to outside a central conductor (20), an electrical insulation layer (23), the metallic screen (28) and an external polymeric sheath (29), wherein the method comprises at least the step of increasing the tensile modulus of the conductive wires (21 , 26, 30).21 . A method as claimed in claim 20 wherein the step of increasing the tensile modulus of the conductive wires (21 , 26, 30) comprises forming the conductive wires (21 , 26, 30) with a metal core (31) coated with a copper cladding (32).

22. A method as claimed in claim 20 or claim 21 further comprising the step of laying the conductive wires (21 , 26, 30) on the electrical insulation layer (23) with gaps between the conductive wires (21 , 26, 30) larger than the diameter of the conductive wires.

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