Subsea transmission power cable

Abstract

The subsea power transmission cable 500 comprises at least one electrical conductor 501, said electrical conductor comprising a plurality of strands 502. The cable further comprises at least one condu

Description

The present invention relates to a subsea transmission power cable, particularly but not exclusively for use with an offshore power generating system, and a method of manufacturing same. Background Many subsea applications or system require powerful subsea equipment, such as pumps, which need a supply electricity to operate. Similarly, offshore systems, such as wind energy or power plants, may need power to operate and / or need to be able to transmit the power generated back to land. As a result, subsea electrical cables are needed to run from land, along the seabed, to the equipment or offshore facility to transmit power thereto and therefrom. For subsea applications, there are two types of electrical cables: signal cables and power cables. Signal cables transmit signals and low power (< 1 kW) and are generally rated at a voltage smaller than 3000V. Power cables, in contrast, transmit high electrical power (typically a few MW) needed to operate powerful subsea equipment, such as pumps and generally rated at a medium voltage between 6 kV and 35 kV. Figure 1 shows a typical power cable for use in subsea applications. As shown, the cable comprises a central copper conductor 2a, semi-conductor and electrical insulation layers 2b, a metallic foil screen 2c and an external polymeric sheath 2d. The central conductor 2a has generally a stranded construction and a large section typically comprised between 50mm2 and 400mm2. An umbilical is formed comprising at least one such power cable and is often termed as power umbilical. For example, an umbilical may comprise a power cable and a signal cable. Alternatively, an umbilical may comprise a plurality of power cables, for example, three-phase power is provided by three power cables bundled together within the umbilical structure. For applications where the electric power cable is subjected to a lot of movement and stress, a so-called ‘dynamic electric power cable’ may be used. However, the waves, tide, wind, and movement of an offshore structure not only cause movement of the power cable, but also relative movement between the power cable and the offshore structure. Furthermore, the higher or larger the offshore structure, the more movement there can be. For example, a wind mill or wind turbine generally involves a tower and blades, so that the offshore structure experiences even larger dynamic movement, which will also increase the dynamic movement of the part of the offshore structure at sea level where a power cable is typically attached or connected to. Such movements wear the power cable. Thus, one design challenge is how to improve the dynamic fatigue of such cables. Accordingly, there remains a need for further contributions in this area of technology. The present invention thus aims to provide an improved power cable for use in dynamic subsea environments to provide power to submarine equipment and / or to and from offshore systems and applications which aims to address some of the above referenced problems. An example of a submarine or offshore application is disclosed in more detail in the detailed description. EP2863397 shows a cable that is described as being flexible and watertight. The cable comprises a metallic water resistant barrier with two protective layers of metal tape which have gaps between the turns of the tape. Water resistance is achieved by filling the gaps with a water resistant gel. Summary of the Invention In a first aspect of the invention, a subsea power transmission cable is provided. The subsea power transmission cable comprising at least one electrical conductor, said electrical conductor comprising a plurality of strands, and the subsea power transmission cable further comprises at least one conducting screen, wherein the or each conducting screen comprises two or more overlapping helical windings, wherein the at least one conducting screen is covered on both sides by slip windings. In a second aspect of the invention, a method of manufacturing a subsea power transmission cable is provided, wherein the subsea power transmission cable comprises at least one electrical conductor, wherein the at least one conductor comprises a plurality of strands, and the subsea power transmission cable further comprises at least one conducting screen, wherein each conducting screen comprises two or more overlapping helical windings, wherein the at least one conducting screen is covered on both sides by slip windings, the method comprising at least the steps of: providing the electrical conductor; helically winding the helical windings and slip windings around the electrical conductor. In a third aspect of the invention, use of a subsea power transmission cable in transmitting power extending from or connected to a floating assembly for an offshore power generating system, and optionally connected to a power receiving unit for onward transmission or use of the power is provided, wherein the subsea power transmission cable is the subsea power transmission cable of the first aspect and / or the subsea power transmission cable resulting from the method of the second aspect. In a fourth aspect of the invention, a method of transmitting power from a floating assembly of an offshore power generating system, comprising the step of transmitting the power using a subsea power transmission cable is provided, wherein the subsea power transmission cable is the subsea power transmission cable of the first aspect and / or the subsea power transmission cable resulting from the method of the second aspect. In a fifth aspect of the invention, a method of reducing the dynamic fatigue on a subsea power transmission cable extending from or connected to a floating assembly for an offshore power generating system, comprising the step of providing a subsea power transmission cable, wherein the subsea power transmission cable is the subsea power transmission cable of the first aspect and / or the subsea power transmission cable resulting from the method of the second aspect. In a sixth aspect, a subsea umbilical comprising one or more subsea power transmission cables of the first aspect and / or one or more subsea power transmission cables resulting from the method of the second aspect is provided. The present invention thus aims to provide an improved subsea power transmission cable, a method of manufacturing said subsea power transmission cable, use of the subsea power transmission cable to transmit power and a method of transmitting power via the subsea power transmission cable. The invention also provides a method of reducing dynamic fatigue in subsea power transmission by using the subsea power transmission cable. Brief Description of the Drawings Embodiments of the present invention will now be described by way of examples only, and with reference to the accompanying drawings in which: Figure 1a is a cross-section view of a prior art power cable; Figure 1b is a perspective broken out view of a prior art power cable; Figure 2 is a cross-sectional view of a subsea power transmission cable in accordance with an embodiment of the present invention; Figure 3 is a cross-sectional view of a subsea power transmission cable in accordance with another embodiment of the present invention; Figure 4 is an expanded view of a subsea power transmission cable in accordance with the embodiment of the present invention depicted in Figure 2, wherein each layer is sequentially cut away; Figure 5 is a schematic representation of an apparatus that may be used to manufacture a subsea power transmission cable in accordance with an embodiment of the present invention; Figure 6 is a schematic representation of an apparatus that may be used to manufacture a subsea power transmission cable in accordance with an embodiment of the present invention; and Figure 7 is a schematic representation of part of an offshore assembly. Detailed Description of Embodiments Power cables, such as inter array power cables, are used to transport the energy generated from and to offshore structures. For example a wind mill, or wind turbine, on an offshore structure uses power cables to transfer electric power to shore. Similar power cables can also be used for other renewable energy productions sites. The power cables extend from the offshore structure into the sea. The present invention provides a subsea power transmission cable for transmitting power to subsea electrical equipment and / or to or from an offshore assembly. The present invention further provides a subsea power transmission cable with improved resistance to dynamic fatigue and thus provides an improved subsea power transmission cable for use in environments subject to high stress (e.g. floating installations, submerged or partially submerged installations, other non-fixed installations and deep sea environments). The improved resistance to dynamic fatigue improves the power transmission efficiency across the lifetime of a subsea cable and results in reduced occurrence of the subsea power transmission cable needing to be replaced or repaired. The improved resistance to dynamic fatigue also helps reduce the effect of floating installations and sites more prone to movement fatigue caused by the wind and the weather, for example taller or larger floating assemblies such as wind mills or wind turbines. Within the meaning of the present invention, a subsea power cable may be any suitable and / or desirable type of power cable. For example, the subsea power cable may be an inter-array cable, (e.g. a power cable for direct connection between two wind turbines and / or the transmission of power from turbines to an offshore wind farm,) an export cable, (e.g. a power cable for transmitting electricity from an offshore windfarm to land or to an offshore power consumer,) and / or an interconnector cable (e.g. a power cable for transmitting large amounts of electricity over long distances). The subsea power cable may be an AC power cable or a DC power cable. Within the meaning of the present invention, a slip winding is a layer of slip material in the form of a helical winding that is placed between two layers and functions to provide a water barrier to the ingress of water. The slip material further acts to improve the slip between layers (e.g. reduces frictional resistance) and thus helps to improve fatigue resistance. Optionally, the slip material is semi-conducting. In a first aspect, a subsea power transmission cable is provided. Optionally, the subsea power transmission cable comprises at least one electrical conductor. Optionally, the electrical conductor comprises a plurality of strands. Optionally, the subsea power transmission cable further comprises at least one conducting screen. Optionally, the at least one conducting screen comprises two or more overlapping helical windings. Optionally, the at least one conducting screen is covered on both sides by slip windings. The first aspect (which is then included as part of the second, third, fourth and fifth aspects) provides a subsea power transmission cable that includes a conducting screen that longitudinally extends based on two layers, and which layers exhibit reduced stiffness and reduced material strain during movement such as bending of the power cable (i.e. due to underwater currents). In other words the conductor has improved dynamism (i.e. is more dynamic). This is in part because the layers of the conducting screen are able to better work alongside or otherwise slip with respect to each other, and thus movement such as bending of the power cable may be accommodated with reduced strain. In the present invention, helically winding the helical windings helps to reduce fatigue loading. The conducting screen (i.e. screen layer) is provided radially to the at least one conductor or bundle of conductors. It will be appreciated that the helical windings may be wound in two possible directions: a right-handed winding (Z-lay) or a left-handed winding (S-lay). In some embodiments, the at least two helical windings may have different or alternating S-lay and Z-lay twisting directions. In some embodiments, all helical windings are twisted in the same direction (e.g. S-lay or Z-lay). Indeed, in some embodiments, a plurality of helical windings having the same twisting direction may provide improved fatigue resistance by reducing the trellis effect between layers. However, any suitable and / or desirable winding arrangement is possible. The conducting screen helps to maintain an even electric field across the cable insulation assembly and to prevent this field extending beyond the cable. The screen is may be connected to earth at the cable ends so it is able to dissipate cable charging currents and short circuit currents in the event of insulation failure. Optionally, the helical windings are helical wire windings (i.e. a layer formed by winding a wire helically around the conductor). Optionally, the helical windings are helical tape windings (i.e. a layer formed by winding a tape helically around the conductor). Optionally, the helical tape windings are formed by a metallic tape. Optionally, the helical tape windings are formed of copper tape or copper wire. Optionally, the helical tape windings may be formed from a fabric tape. Optionally, any conducting screen layer can comprise at least three helical windings. Optionally, the first helical winding may be one or more helical fabric tape windings (i.e. a layer formed by helically winding a fabric tape) or other helically wound non-metallic tape windings. Optionally, the helical fabric tape winding may be surrounded by a second winding formed by one or more metal layer(s). Optionally, each metal layers may be formed from helically wound wires (i.e. helical wire windings) or helically wound metallic tape(s) (i.e. helical tape windings). Optionally, the third winding (e.g surrounding the second winding) may be one or more helical fabric tape windings or other helical non-metallic tape winding. Optionally, the metal layers (i.e. forming the second winding) may can be made of round or flat wires. Alternatively, the metal layer is a metallic tape applied spirally around the insulation assembly. The metal layer is for example made of aluminium or copper. Alternatively, the conducting screen can be made of (e.g. a straight, e.g. a continuous) extruded metallic sheath(s). The metallic material for the extruded metallic sheath(s) may comprise (e.g. consist of) aluminium or copper. Copper may be preferred to aluminium in some subsea applications because of heavy corrosion problems observed in the past. Indeed, the copper has a better corrosion resistance than aluminium and, at proper sizes, even able to carry short-circuit currents. In addition, the copper is considered to be very resistant against fatigue phenomena, this is another reason that copper sheath can be used for dynamic power cables which are suspended freely from floating offshore systems and are subjected to repeated bends due to waves. The copper may be preferred to aluminium in some subsea applications because of heavy corrosion problems observed in the past. Copper may be considered to have a better corrosion resistance than aluminium and, at proper sizes, may be able to carry short-circuit currents. In addition, the copper is considered to be very resistant against fatigue phenomena. This makes copper sheaths particularly suitable for dynamic power cables which are suspended freely from floating offshore systems and are subjected to repeated bends due to waves. Optionally, the metallic material for the extruded metallic sheath(s) comprise copper and alloying elements such as beryllium. In some embodiments, the conducting screen layer comprises at least one metal tape helically wound around the conductor or bundle of conductors and the insulation assembly screen layer. For example, each of the helical (e.g. tape) windings forming the conducting screen layer is a metal helical tape winding, advantageously made of such as an aluminium or copper helical tape winding. Preferably, the conducting screen layer is a copper helical tape winding. In the present invention, the first helical (e.g. tape) winding of the two or more overlapping helical (e.g. tape) windings is proximate to, and located at a more radially inward position relative to the conductor than, a second of the two or more overlapping helical (e.g. tape) windings. In other words, the two or more helical (e.g. tape) windings are at least partly layered on top of each other. For example, a first helical (e.g. tape) winding may be provided at a first radial position and a second helical (e.g. tape) winding may be provided over the top of the first helical (e.g. tape) winding such that at least part of the first helical (e.g. tape) winding is covered by the second helical (e.g. tape) winding. In some embodiments, each of the helical (e.g. tape) windings has a winding pitch such that a space or gap is provided between each turn of the helical (e.g. tape) winding. Such an arrangement improves the flexibility of the power cable as the gaps are able to accommodate movement such as bending and resist fatigue loading. Optionally, the gap between each winding (or turn) of the helical (e.g. tape) windings is at least 5% of the thickness of the helical (e.g. tape, e.g. wire) winding, e.g. from 5% to 70% of the thickness of the helical winding, e.g. from 10% to 65% of the thickness of the helical winding, e.g. from 15% to 60% of the thickness of the helical winding, e.g. from 20% to 55% of the thickness of the helical winding, e.g. from 25% to 50% of the thickness of the helical winding, e.g. from 30% to 40% of the thickness of the helical winding. In such embodiments, gaps provided between turns of a first helical (e.g. tape) winding are optionally covered by the second helical (e.g. tape) winding. In other words, the turns of each helical (e.g. tape) winding of the two or more helical (e.g. tape) windings are offset with respect to each other such that the helical windings are overlapping. As such, even though each helical (e.g. tape) winding comprises a gap between each turn, the (outer) conducting screen (layer) is provided as a complete covering layer. In other words, the final form of the conducting screen does not include any gaps extending through the whole thickness of the screen. In some embodiments, the second helical tape overlaps the first helical tape by at least 5%, e.g. from 5% to 70%, e.g. from 10% to 65%, e.g. from 15% to 60%, e.g. from 20% to 55%, e.g. from 25% to 50%, e.g. from 30% to 40%. In some embodiments, each helical (e.g. tape) winding overlaps another helical (e.g. tape) winding in the range of from 10% up to 99% (i.e. percentage of surface area coverage). In other words, at least 99% of a second helical (e.g. tape) winding engages with the material of a first helical (e.g. tape) winding, wherein the remaining 1% corresponds to the part of the second helical (e.g. tape) winding that is overlapping (or covering) the gap between the helical turns of the first helical (e.g. tape) winding. Optionally, each helical (e.g. tape) winding overlaps another helical (e.g. tape) winding in the range of from 10% up to 95%, e.g. from 10% up to 90%, e.g. from 10% up to 80%, e.g. from 10% up to 70%, e.g. from 10% up to 60%, e.g. from 10% up to 50%, e.g. from 20% up to 99%, e.g. from 30% up to 99%, e.g. from 40% up to 99%, e.g. from 50% up to 99%. Optionally, the winding pitch of each of the at least two helical (e.g. tape) windings is the substantially the same. Optionally, the winding pitch of each of the at least two helical (e.g. tape) windings is different. Optionally, the winding direction of each of the at least two helical (e.g. tape) windings is the same. Optionally, the winding direction of each of the at least two helical (e.g. tape) windings is different (i.e. opposite). In some embodiments, each of the helical (e.g. tape) windings have a winding angle that provides gaps between each turn of the helical (e.g. tape) winding. The winding angle may be defined as the angle of the (e.g. tape) winding with respect to the longitudinal extension of the cable (e.g. an axis parallel to the longitudinal axis of the cable proximate to the surface around which the winding is applied). For example, the winding angle of each helical (e.g. tape) winding is 60 degrees or less, e.g. less than 55 degrees, e.g. less than 50 degrees, e.g. less than 45 degrees, e.g. less than 40 degrees, e.g. less than 35 degrees, e.g. less than 30 degrees, e.g. less than 25 degrees. For example, the winding angle of each helical (e.g. tape) winding is greater than 5 degrees, e.g. greater than 10 degrees, e.g. greater than 15 degrees, e.g. greater than 20 degrees, e.g. greater than 25 degrees. Optionally, the winding angle is from 10 degrees to 60 degrees, e.g. from 15 degrees to 55 degrees, e.g. from 20 degrees to 50 degrees, e.g. from 20 degrees to 30 degrees. In some embodiments, the gaps provided between turns of a first helical (e.g. tape) winding layer are (e.g. partly or wholly) filled with the material of the next helical (e.g. tape) winding layer applied over the top thereof. . Optionally, the at least one conducting screen is covered on both sides by (e.g. conductive or semi-conductive) slip windings. In other words, the subsea power transmission cable preferably comprises at least two slip windings, wherein two of the at least two slip windings are arranged either side of the at least one conducting screen, such that both sides of the (at least one) conducting screen are covered by the slip windings, i.e. the (at least one) conducting screen is provided between two (e.g. water-swellable) slip windings. In some embodiments, the power transmission cable further comprises a radial water-barrier. The radial water-barrier can be a layer comprising at least one metallic and / or a polymeric sheath. The function of the radial water-barrier is to prevent or significantly diminish ingress of water into the cable insulation assembly and thus to prevent the formation of the water-tree effect, which can lead to cable failure. Optionally, any radial water-barrier layer is separated from the cable core (i.e. the at least one electrical conductor or bundle of conductors, the insulation assembly and the at least one conducting screen(s)). In some embodiments, the radial water-barrier can be formed by longitudinally and / or helically applied metallic tapes. The metallic tape material may be aluminium, copper or stainless-steel. Preferably, the surface of the metallic tapes is smooth (i.e. without any significant defects or corrugations). In some embodiments, a radial water-barrier layer can be made of a (e.g. thin, e.g. from 50 pm to 500 pm, e.g. from 100 pm to 250 pm, e.g. from 150 pm to 200 pm) laminated metallic sheath or laminated metallic tape. Optionally, the laminated metallic sheath(s) may be pre-laminated with a layer of polyolefin polymer(s) or polyolefin copolymer(s). The layer of polyolefin polymer(s) or polyolefin copolymer(s) may have a thickness from 10 pm to 100 pm, e.g. from 20 pm to 80 pm, e.g. approximately 50 pm. In embodiments comprising a laminated metallic tape, the laminated metallic tape may be helically wound (e.g. around the at least one electrical conductor or bundle of conductors, e.g. around the cable core) with the polyolefin or polyolefin copolymer layer(s) facing outwardly (i.e. away from the at least one electrical conductor or bundle of conductors). The edges of the laminated metallic sheath(s) or tape are preferably glued or bonded. Optionally, a polyolefin sheath may be (e.g. simultaneously) extruded (e.g. directly) onto the polyolefin or polyolefin copolymer of the laminate tape to firmly bond the said laminate tape with the polyolefin outer sheath. Such arrangements help to ensure the laminate tape is able to withstand the cable bending without wrinkling or creasing. The laminated metallic sheath(s) may be made from lead, copper or aluminium. The polyolefin polymers may be selected to be one or more of a polyethylene including LDPE, MDPE or HDPE or a polypropylene. The polyolefin copolymers are for example selected among polyethylene copolymers (PEc) or polypropylene copolymers (PPc). Alternatively, when no underlying metallic sheaths (i.e. no conducting screen) are present, the radial water-barrier layer may comprise a polymeric layer or sheath comprising a water-absorbing agent. A watertight polymeric sheath (that reduces the humidity diffusing through the sheath in the vapour phase) comprising the adsorbing agent has sufficient capacity to keep the insulation dry enough for the entire life of the cable and probably beyond. The polymeric sheath comprises a polymeric material. The polymeric material can be for example a polyethylene including low-density polyethylene (LDPE), mediumdensity polyethylene (MDPE) and high-density polyethylene (HDPE), or a polyvinyl chloride (PVC), or a polyamide (PA; Nylon), or a polyurethane (Pll), or even a specialty polymer like polyvinylidene chloride (PVDC). Polyamide has improved mechanical properties than HDPE but polyethylene provides improved chemical and mechanical stability over a long life at moderate costs. HDPE has a rather low vapour permeability while both PVC and polyamide have much higher diffusion rates than HDPE. Specialty polymers such as polyvinylidene chloride (PVDC) have much lower diffusion rates than HDPE but have not been applied in subsea power cables. Optionally, when underlying metallic sheaths (e.g. a conducting screen) are present, the polymeric sheath(s) may be made from semi-conducting polyethylene materials. Optionally, the semi-conducting polyethylene materials may be loaded with fillers such as carbon-black particles. This arrangement helps to provide voltage equalization between layers (e.g. the metallic sheaths and an armouring layer outside). By surrounding the at least one conducting screen with slip windings (e.g. water-swellable slip windings), the prevention of ingress of water through the subsea power transmission cable towards the conductor is improved. For example, the most outwardly slip winding functions to reduce the amount of water that reaches the conducting screen by absorbing the water and preventing its further ingress through the power cable. The conducting screen then provides a physical and / or water blocking barrier through which the transport of water is prevented. Then, should any water successfully pass through the screen layer (e.g. in the event of damage and / or corrosion of the conducting screen), the most inwardly slip winding functions to absorb this water and prevent its further migration towards the conductor. The slip windings also prevent (e.g. salt) water from travelling longitudinally along the cable (i.e. so the insulator layer is prevented from getting wet). This prevents water from reaching the conductor. In some embodiments, the subsea power transmission cable comprises one conducting screen and at least two (e.g. two) slip windings, wherein two of the at least two slip windings are arranged such that there is one slip winding either side of (and engaging with) the conducting (outer) screen. The conducting screen layer and the two slip layers form an assembly of protecting layers for the power transmission cable. In some embodiments, the subsea power transmission cable comprises more than one (e.g. two or more) conducting screens and at least two slip windings. In such embodiments, both sides of each conducting screen may be covered by (at least) two slip windings, i.e. each conducting screen may have at least one slip winding arranged on either side of said conducting screen. For example, the subsea power transmission cable may comprise (when moving outwardly from the conductor) a first slip winding (or first plurality of slip windings), a first conducting screen, a second slip winding(s) (or second plurality of slip windings), a second conducting screen and then a final third slip winding (or third plurality of slip windings). Alternatively, the conducting screens may be arranged such that there is at least one slip winding either side of the plurality of conductive screens, i.e. only the outermost conducting screens of a plurality of conducting screens are covered by the slip windings. For example, the subsea power transmission cable may comprise (when moving outwardly from the conductor) a first slip winding (or first plurality of slip windings), a plurality of conducting screens and then a final second slip winding (or second plurality of slip windings). In some embodiments, the slip windings are formed form a water-swellable (or water-blocking) material, such as a semi-conductive swellable polyester or nonwoven polyester tape. Optionally, the slip windings comprises a fabric tape. Optionally, the fabric tape may comprise swelling powder for longitudinal sealing. The present invention includes at least one electrical conductor formed from a plurality of strands. The present invention is further improved in embodiments where the strands of the electrical conductor have a circular cross-section (i.e. in the plane parallel to the longitudinal axis). For example, cylindrical strands are able to move or slip alongside (i.e. move relative to) each other with reduced radial contact force, thus meaning that the strands are able to move in substantially all three-dimensions with respect to each other, utilising the interstices such that the bending of the cable is accommodated with reduced strain. In comparison, strands of other shapes may have smaller interstices (due to improved packing of the strands together) to accommodate movement and thus reduced slip with greater contact forces and increased strain on bending. The dynamism of the cable of the present invention may be further improved by providing the plurality of strands in a helically wound configuration. In other words, the conductor may comprise a plurality of helical strands such that the conductor may be considered to be helically twisted or wound. It will be appreciated that a helical strand is a strand that has been twisted or wound to provide a substantially continuously curved strand having a winding pitch (i.e. the distance between the start of two consecutive coils). In some embodiments the plurality of strands may be prewound and then assembled to form the conductor. In other embodiments, the strands may be assembled in a linear arrangement and then wound to provide the conductor. In some embodiments, assembling the strands and winding the strands to provide the helical conductor occurs substantially simultaneously. The plurality of strands may be stranded round wires, fibres or yarns. It will be appreciated that the strands may he helically twisted in two possible directions: a righthanded twist (Z-lay) or a lefthanded twist (S-lay). In some embodiments, the conductor may comprise a plurality of layers of strands, wherein the layers have alternating S-lay and Z-lay twisting directions. In some embodiments, all layers of a conductor comprising a plurality of layers of strands are twisted in the same direction (e.g. S-lay or Z-lay). Indeed, in some embodiments, a plurality of layers having the same twisting direction may provide improved fatigue resistance by reducing the trellis effect between layers. However, any suitable and / or desirable twisting arrangement is possible. In some embodiments, the electrical conductor of the subsea transmission cable comprises a plurality of helical strands having interstices therebetween. Optionally, the interstices are partly, substantially or wholly filled with a filler material. The filler material may act as either a water-blocking agent or a friction reducing material, or both. A water blocking agent functions to prevent water ingress to the conductor core (e.g. comprised of a plurality of conductor strands). Furthermore, by providing a filler material between the interstices, the friction between the strands may be reduced, and thus movement or slip of the strands with respect to each other is improved. As such, a conductor comprising filler material between the strands provides an improved subsea power transmission cable with reduced material strain on its movement, especially bending, thus improving the lifetime and durability of the cable. In some embodiments the interstices are partly filled with a filler material. In other words, the filler material does not occupy the entire volume of the interstices, i.e. it occupies or fills only part of the volume. For example, the filler material occupies from 0% to 95% of the volume of the interstices, e.g. from 5% to 90% of the volume of the interstices, e.g. from 10% to 80% of the volume of the interstices, e.g. from 20% to 70% of the volume of the interstices, e.g. from 30% to 60% of the volume of the interstices, e.g. at least 50% of the volume of the interstices, e.g. less than 50% of the volume of the interstices, e.g. approximately 50% of the volume of the interstices. In other embodiments, the interstices are substantially or wholly filled with the filler material, i.e. the filler material occupies or fills substantially the whole volume of the interstices. In some embodiments, the filler material occupies or fills the volume between (and around) the conductor strands and a layer enclosing the conductor strands. For example, the filler material may occupy or fill the volume defined between the conductor strands and an insulator layer or semiconductor layer that surrounds the conductor. In some embodiments, the filler material may be a solid, a liquid or a suspension (e.g. a mixture of solid particles or particulates dispersed within a liquid). In some embodiments, the filler material may be in one state (e.g. a liquid state) when the filler material is provided to the interstices (e.g. by pumping the filler material into the interstices or pre-coating the strands with the filler material), and a second state (e.g. a solid state) when the cable is in use (e.g. at the temperatures associated with subsea, e.g. when allowed to cure). The filler material may be a coating or layer provided around each strand. In some embodiments, the plurality of strands forming the electrical conductor are pre-coated with the filler material. For example, the strands may be each dipped or coated in a filler material prior to assembly to form the conductor to provide a coating on the strands. For example, the strands may be provided with a coating via an extrusion process prior to assembly to form the conductor. In some embodiments, the filler material is inserted (e.g. pumped, e.g. injected) into interstices between the plurality of strands as the interstices are being formed. In some embodiments, the filler material is inserted into the interstices after the interstices are formed. In some embodiments, the strands may be pre-coated with a first filler material and then any remaining interstices filled with a second filler material, wherein the first and second filler materials may be the same or different. For example, the (pre-coated or uncoated) strands may be assembled (and wound) to form the conductor and the filler material may be inserted (e.g. pumped) into the interstices, e.g. by dipping or any other suitable or desirable method. The filler material may be inserted either as the interstices are formed (i.e. during the winding process) or after the interstices are formed. In some embodiments, the filler material is a friction-reducing filler and / or a water-resistant material. This reduces the friction between strands and also reduces fatigue loading. For example, the friction-reducing material may comprise a plasticiser, a polymer, a silicone, an oil, grease, wax, such as polyolefin wax, or any other suitable and / or desirable material. In some embodiments, the friction reducing filler material comprises polypropylene. In some embodiments, polypropylene is extruded around each strand to provide a friction reducing coating. In some embodiments, the filler material is semi-conductive. This helps to improve termination by ensuring that the termination lug applies current to all of the conductor strands (rather than just the outer strands). In some embodiments, the at least one conductor comprises at least one aluminium conductor strands. Optionally, all the strands of the at least one electrical conductor comprises (e.g. are) aluminium (e.g. 6000 series aluminium) conductor strands. When compared to copper, aluminium (such as 6000 series aluminium) is typically cheaper, more corrosion resistant, less dense (and thus lighter), harder and more flexible. Thus, although aluminium has a lower conductivity than copper, there are a number of distinct advantages available by using aluminium instead of copper as the conductor of subsea power transmission cables. In particular, the comparative strength of high strength 6000 series aluminium compared to copper allows the conductors comprising high strength 6000 series aluminium strands to assist in the load bearing requirement of the subsea power transmission cable. In some embodiments, the at least one electrical conductor comprises one or more 1000 or 6000 series aluminium conductor strands. For example, the 6000 series of aluminium as defined by the IADS (International Alloy Designation System) is a high tensile series, conventionally only used for uninsulated land-based fixed overhead power lines to transmit high voltages. This series of aluminium is alloyed with magnesium and silicon and it has a lower conductivity than the 1000 series. The 1000 series of aluminium as defined by the IADS (International Alloy Designation System) comprises at least 99% of aluminium and has a lower tensile series than the 6000 series, but has a higher conductivity. Optionally, all of the strands of the at least one electrical conductor comprise one or more 6000 series aluminium conductor strands. In some embodiments, the subsea power transmission cable of the present invention is a medium or high voltage transmission cable. For example, the high power transmission cable is rated at a voltage between 1 kV and 1000 kV, e.g. between 1 kV and 500 kV, e.g. between 1 kV and 250 kV, e.g. between 1 kV and 100 kV, e.g. between 5 kV and 35 kV. Preferably, the high power transmission cable is rated at a voltage between 36 kV and 170 kV according to the I EC standard IEC60840:2020, May 2020 and / or between 7.2 kV and 72.5 kV according to the I EC standard IEC63026:2019, December 2019. It will also be appreciated that due to the softness of copper, conductors for conventional power cables comprising copper strands are compacted together to provide a compacted conductor. Compaction flattens the contact surfaces between adjacent strands which, in turn, reduces and eliminates the interstices between the strands. This provides a conductor with a smaller cross-section due to more densely packed material. However, the rigidity of the compacted conductor is increased, and so the ability of strands to bend cannot occur, resulting in an inflexible conductor and resulting inflexible cable. The extent of compaction may be defined as change or reduction in size of the conductor before and after compaction. For example, a plurality of strands assembled to provide a conductor with a 10 mm diameter that, after compaction, has a 9 mm diameter has been compacted by 10% (i.e. 1-(9 mm / 10 mm). The percentage compaction may therefore defined as the percentage change in the diameter / cross-section upon compaction. Indeed, ISO 13628-5 “Petroleum and natural gas industries — Design and operation of subsea production systems — Part 5: Subsea umbilicals” published in December 2009 by the International Organization for Standardization, API 17E “Specification for Subsea Umbilicals”, 5th Edition - July 2017, established by the American Petroleum Institute, and IEC60183:2015, January 2015, I£060840:2020, May 2020 and / or I£063026:2019, December 2019 , established by the International Electrotechnical Commission (IEC), all provides the standards for the design, material selection, manufacture, design verification, testing, installation and operation of umbilicals and associated ancillary equipment in the field. As a result, traditionally the conductors of the power cables are made from copper. Copper provides the advantage that a plurality of copper strands can be compacted together to form a compact ‘mass’ of copper which reduces the size of the final conductor and creates a smooth uniform surface on top of which further layers (such as one or more insulation layers) may be applied. However, the conventional use of copper has several disadvantages. Due to its low tensile strength, the power cables cannot be used as load bearing components and are often damaged from the high stresses that may arise in dynamic subsea environments, e.g. because of movement caused by the movement of the sea water. The copper conductors therefore need to be suitably protected from possible damage by excessive movement, for example elongation or crushing, and especially under the conditions of being connected to ‘dynamic’ installations, such as floating wind turbines, sea or wave-energy creators, and other floating or non-fixed subsea installations. Conventionally, the electrical conductor in a subsea power cable is also assembled or formed from a plurality of copper strands which are arranged together alongside one another and which are then mechanically semi-compacted or compacted together (depending on the fill factor of the strands) to form a compact and more solid electrical conductor. The electrical conductor of a direct current cable for carrying direct current, e.g. a High Voltage Direct Current, HVDC, cable may be manufactured in a corresponding manner. Within the meaning of the present invention, the term “fill factor” of the strands is defined as the ratio of the metallic cross-section (or as a simplified calculation, the sum of the single wire cross-sections) relative to the area of the smallest circle circumscribing the strands. The fill factor therefore specifies the amount of space within a conductor that is occupied by a strand (and thus the quantity of metal (e.g. copper or aluminium) within the conductor). Conventionally, power cable electrical conductors formed from compressed or compacted round wires can achieve a filling factor of 0.92, meaning that 92% of the conductor cross-section is made up of metallic strand material. However, it has been found that conductors formed from compressed or compacted strands have a reduced electric conductivity, as the round wires forming strands are cold worked by the compression. The extent of compaction may be defined as the change in the fill factor before and after the compaction. For example, a plurality of strands assembled to provide a conductor having a fill factor of 0.7 before compaction, and a fill factor of 0.9 after compaction, would have been compacted by 22% (i.e. 1 - (0.7 / 0.9)). The percentage compaction may therefore defined as the percentage change in the fill factor upon compaction. The percentage compaction is therefore defined as the percentage change in the diameter / cross-section upon compaction. Thus, in some embodiments of the present invention, the strands are wholly or substantially not compacted together to form the electrical conductor. For example, the extent of compaction of the conductor is less than 10%, e.g. less than 8%, e.g. less than 5%, e.g. less than 4%, e.g. less than 3%, e.g. less than 2%, e.g. less than 1.5%, e.g. less than 1%, e.g. less than 0.5%, e.g. substantially 0%. In some embodiments, the subsea power transmission cable further comprises at least one inner conductive or semi-conductive screen or layer around the at least one electrical conductor, also called conductor screen. The conductive or semi-conductive layer or conductor screen function serves to equalize stresses around the conductor strands. In some embodiments, a first inner conductive or semi-conductive layer (i.e. screen or screen layer) is provided to cover each conductor of the at least one conductors. In some embodiments, a first inner conductive or semi-conductive layer (i.e. screen layer) is provided to cover a bundle of conductors, wherein the bundle of conductors comprises at least two conductors. In some embodiments, each inner conductive or semi-conductive layer may be a continuous layer (e.g. provided by extrusion or dipping). In some embodiments, the inner conductive or semi-conductive layer may be a winding layer (e.g. a conductive or semi-conductive tape wound to provide a substantially continuous layer, i.e. with no gaps between turns of the windings). Optionally the at least one inner conductive or semi-conductive screen is made from a semi-conductive material, such as polyethylene including low-density polyethylene (LDPE), medium-density polyethylene (MDPE) and high-density polyethylene (HDPE), or cross-linked polyethylene (XLPE). Optionally, the semi-conductive material is loaded with fillers such as carbon-black particles. In some embodiments, the subsea power transmission cable comprises a first inner conductive or semi-conductive screen and a second outer conductive or semi- conductive screen also called insulation screen. The nature and the function of the second outer conductive or semi-conductive screen is identical to the nature and the function of the first inner conductive or semi-conductive screen. For example, the second conductive or semi-conductive screen may be separated from the at least one conductor or bundle of conductors by at least the first conductive or semi-conductive screen. In some embodiments, the subsea power transmission cable further comprises at least one radial insulator layer. The insulator layer is responsible for the reliable longterm operation of the subsea cable. In some embodiments, an insulator layer is circumferential to the at least one electrical conductor. For example, the insulator layer may be provided around each conductor of the at least one conductors. In some embodiments, an insulator layer is provided around a bundle of conductors, wherein the bundle of conductors comprises at least two conductors. Optionally, an insulator layer is separated from the at least one electrical conductor or bundle of electrical conductors by at least the first conductive or semi-conductive screen. Optionally, an insulator layer is provided between first and second screens described herein. In some embodiments, an insulator layer may be a continuous layer (e.g. provided by extrusion or dipping). In some embodiments, an insulator layer may be a winding (e.g. an insulating tape wound to provide a substantially continuous layer, i.e. with no gaps between turns of the windings). Optionally, an insulator layer is made from an insulating material, vulcanized materials like cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR), or high-performance thermoplastic elastomer (HPTE) like polypropylene copolymer (PPc) (natural or synthetic) rubber, polyvinyl chloride (PVC), polyether ether ketone (PEEK), silicones, low density polyethylene (LDPE), high density polyethylene (HDPE), hard grade ethylene propylene rubber (HEPR), polyurethane(s) (PUR) and / or polypropylene (PP). The assembly formed by the conductor screen, the insulating layer and the insulation screen is commonly named insulation assembly. Optionally, the conducting screen is separated from the at least one electrical conductor or bundle of conductors by at least one (e.g. two) conductive or semi-conductive screen(s) and / or an insulator layer. In some embodiments, the subsea power transmission cable comprises an outer (e.g. outside, e.g. external, e.g. outermost) layer or sheath. Optionally the outer layer or sheath is a protective layer as it protects the cable against mechanical damage and performs the function of insulation even to a short circuit. The outer layer may be made of any suitable and / or desirable material, such as a polymeric (e.g. thermoplastic or thermoset) material like for example a polyethylene including medium-density polyethylene (MDPE) or high-density polyethylene (HDPE). In some embodiments, the sheath or outer layer covers (i.e. is proximate to and engages) a water-swellable tape layer. In some embodiments, the subsea power transmission cable comprises: - an electrical conductor comprising a plurality of strands having interstices thereinbetween, wherein the interstices are partly, substantially or wholly filled with a filler material, - a covering first inner semi-conductive screen layer - a covering insulator layer; - a covering second inner semi-conductive screen layer; - a covering first slip winding; - a covering conducting screen comprising: - a first helical (e.g. tape) winding; - a second helical (e.g. tape) winding; and - optionally, a slip winding arranged between the first helical (e.g. tape) winding and the second helical (e.g. tape) winding; - a covering second slip winding; and - an outer protective sheath. For example, the subsea power transmission cable comprises: - an electrical conductor comprising a plurality of strands having interstices thereinbetween, wherein the interstices are partly, substantially or wholly filled with a filler material, - a first semi-conductive screen layer arranged to cover the electrical conductor; - a insulator layer, arranged to cover the insulator layer; - a second semi-conductive screen layer, arranged to cover the insulator layer; - a first slip winding layer, arranged to cover the second semi-conductive screen winding; - a conducting metal tape screen, arranged to cover the first slip winding layer, wherein the metal tape screen comprises at two or more (e.g. two) helical (e.g. tape) windings; - a second slip winding layer arranged to cover the metal tape screen; and - an outer protective sheath arranged to cover the second slip winding layer, (e.g. and provide the outer surface of the subsea power transmission cable. In another aspect of the invention, an umbilical is provided comprising a plurality (e.g three) of subsea power transmission cables (the features of which are disclosed above) covered by a protective sheath. For example, a three-core power transmission cables can be arranged in a one three-core cable configuration or in a three one-core cable configuration. In a second aspect of the invention, a method of manufacturing a subsea power transmission cable is provided. Optionally, the subsea power transmission cable comprises at least one electrical conductor. Optionally, the at least one conductor comprises a plurality of strands. Optionally, the subsea power transmission cable further comprises at least one conducting screen. Optionally, each conducting screen comprises two or more overlapping helical (e.g. tape) windings. Optionally, the at least one conducting screen is covered on both sides by slip windings. Optionally, the method comprises providing the electrical conductor. Optionally, the method comprises helically winding the helical (e.g. tape) windings around the electrical conductor. Optionally, the method comprises helically winding the slip windings around the electrical conductor. In some embodiments, the method of manufacture comprises the steps of - providing a plurality of strands; - helically winding said strands to form the electrical conductor and forming interstices thereinbetween; - filling the interstices with a filler material; - adding a first semi-conductive screen layer covering the electrical conductor; - adding an insulator layer covering the first semi-conductive screen layer; - adding a second semi-conductive screen layer covering the insulator layer; - adding a first slip tape winding covering the second semi-conductive screen layer; - adding a conducting screen covering the first slip tape winding, wherein the screen covering comprises a first helical winding and a second helical winding; - adding a second slip tape winding covering the conducting screen covering; and - adding an outer protective sheath covering the second slip tape winding. It will be appreciated that the subsea power transmission cable resulting from the method of the second aspect corresponds to the subsea power transmission cable of the first aspect. As such, the features of the first aspect disclosed above apply equally to the second aspect in addition to the following. In some embodiments, the method further comprises pre-coating the plurality of strands with the filler material. For example, the method may comprise extruding a filler material (e.g. a thermoplastic or thermoset material) around each of the plurality of strands before assembling the strands to form the conductor. For example, the method may comprise dipping the strands in container comprising the filler material before assembling the strands to form the conductor. In some embodiments, the method comprises inserting (e.g. pumping) the filler material into interstices between the plurality of strands as the interstices are being formed. In some embodiments, the method comprises inserting (e.g. flooding, e.g. filling) filler material into the interstices after the interstices are formed. For example the method may comprise dipping the (pre-coated or uncoated) strands in a container of a filler material such that the filler material infiltrates or floods the interstices. In some embodiments, the method may comprise heating the filler material. For example, heating the filler material may provide the filler material in a more pliable state (e.g. more pliable than at room temperature, e.g. a liquid or fluid state) which may make it easier to fill the interstices with the filler material. For example, to extrude a filler material (e.g. a thermoplastic or thermoset material) around a strand, the filler material may be heated to provide a fluid state, e.g. heated to a temperature above its glass transition temperature, e.g. heated to a temperature above its melting temperature. Similarly, it may be easier to pump the filler material into the interstices if the filler material is in a fluid state. It will be appreciated that the subsea power transmission cable of the present invention may be formed using conventional manufacturing steps and processes. As such, the manufacture and addition of the or each conductive and semi-conductive screen winding, the insulator layer, the slip winding layers (e.g. the water-swellable tape layers), the tape screen layer and the outer protective sheath, may be formed by any suitable and / or desirable method known in the art. For example, the method may comprise any one of extruding, coating, winding, layering, applying and / or attaching any one of these layers. For example, the step of providing any one of these layers may include an initial step of coating (e.g. the preceding layer) with an adhesive or bonding layer. In some embodiments, the method involves using an apparatus comprising a plurality of bobbins arranged to rotate to wind or twist, continuously, the plurality of strands to provide the at least one conductor. In some embodiments, the method involves using an apparatus comprising at least one bobbin arranged to rotate to wind or twist, continuously, a conductive or semi-conductive tape material around (i.e. circumferential to) the conductor (or bundle of conductors) to provide an inner conductive or semi-conductive screen layer. It will be appreciated that being wound “around” is intended to include where the tape material is not immediately proximate to the conductor and therefore does not engages the conductor strands. Instead there may be other layers or materials between the conductor and the tape material, but the material is still being wound “around” the conductor. In some embodiments, the method involves using an apparatus comprising at least one bobbin arranged to rotate to wind or twist, continuously, a slip winding (e.g. the water-swellable tape) around the conductor (or bundle of conductors) to provide the slip windings. It will be appreciated that being wound “around” is intended to include where the tape material is not immediately proximate to the conductor and therefore does not engages the conductor strands. Instead there may be other layers or materials between the conductor and the tape material, but the material is still being wound “around” the conductor. In some embodiments, the method involves using an apparatus comprising at least one bobbin arranged to rotate to wind or twist, continuously, a conducting tape around the conductor (or bundle of conductors) to provide a helical (e.g. tape) winding of the conducting tape screen layer. It will be appreciated that being wound “around” is intended to include where the tape material is not immediately proximate to the conductor and therefore does not engages the conductor strands. Instead there may be other layers or materials between the conductor and the tape material, but the material is still being wound “around” the conductor. Optionally, the method includes adding the helical (e.g. tape) windings at an angle in the range of from 10 degrees to 60 degrees, e.g. from 15 degrees to 55 degrees, e.g. from 20 degrees to 50 degrees, e.g. from 20 degrees to 30 degrees. In some embodiments, the method involves using an apparatus comprising at least one bobbin arranged to rotate to wind or twist, continuously, an insulator material (e.g. an insulator tape) around conductor (or bundle of conductors) to provide an insulator layer. It will be appreciated that being wound “around” is intended to include where the tape material is not immediately proximate to the conductor and therefore does not engages the conductor strands. Instead there may be other layers or materials between the conductor and the tape material, but the material is still being wound “around” the conductor. Optionally, the apparatus is able to vary one of more of the: number of bobbins; angle of the bobbins relative to the conductor (or bundle of conductors); and / or speed of the winding or braiding to modify the properties of the resulting layers. I.e. the angle of the bobbins may be varied to provide winding of different pitch. In a third aspect of the invention, use of a subsea power transmission cable is provided, optionally to transmit power between a floating assembly for an offshore power generating system such as a wind power generating system, and a power receiving unit for onward transmission or use of the power. Preferably the subsea power transmission cable is the cable of the first aspect of the invention. As such, the above mentioned features and embodiments of the first aspect apply equally to this third aspect. Optionally, the subsea power transmission cable of the present invention is useable as part of an offshore system comprising a floating assembly for generating power. Optionally, the subsea power transmission cable can be connected to an offshore power consumer. Optionally, the subsea power transmission cable of the present invention is connectable between a floating assembly for generating power and an offshore power consumer. An offshore power consumer may be any system, structure, installation or apparatus, which consumes power. The offshore power consumer may be fixed or floating. Where the offshore power consumer is floating, such floating structure or installation may also be subject to dynamic fatigue as described herein. That is, the floating part or portion of the consumer to which a power cable is typically attached or connected, is subject to particular dynamic movement. Such movements wear the power cable. The improved resistance to dynamic fatigue provided by the present invention improves the power transmission efficiency across the lifetime of a subsea power transmission cable and thus the offshore system, and results in reduced occurrence of maintenance, repair or replacement of all or parts of the offshore system. The improved resistance to dynamic fatigue also helps reduce the effect of floating installations and sites more prone to movement fatigue caused by the wind and the weather. In a fourth aspect of the invention, a method of transmitting power from a floating assembly of an offshore power generating system. Optionally, the method comprises the step of transmitting the power using a subsea power transmission cable. Preferably the subsea power transmission cable is the cable of the first aspect of the invention. As such, the above mentioned features and embodiments of the first aspect apply equally to this third aspect. Such a floating assembly may comprise a base support unit at sea level, a windmill unit extending upwardly therefrom, and a power generating unit on or within the floating assembly between the windmill unit and the subsea power transmission cable of the present invention. A floating assembly may be anchored or tethered to the sea bed. The skilled reader can see other floating assembly arrangements for the production of renewable energy, with which the subsea power transmission cable of the present invention is useable. In a fifth aspect of the invention, a method of reducing the dynamic fatigue on a subsea power transmission cable is provided. Optionally the subsea power transmission cable is located between a floating assembly for an offshore power generating system and a power receiving unit for onward transmission or use of the power. Optionally, the method comprises the step of providing a subsea power transmission cable. Optionally the subsea power transmission cable is the cable of the first aspect of the invention. As such, the above mentioned features and embodiments of the first aspect apply equally to this third aspect. Turning to the drawings, Figure 2 shows a cross-section of a subsea power transmission cable 300 in accordance with an embodiment of the present invention. The subsea power transmission cable 300 includes a plurality of metal (e.g. copper or aluminium) strands 302 arranged axially alongside each other to provide a conductor 301 and with a filler material 304 in the interstices between the strands. The conductor 301 (with filler material 304) is then covered by, from the inside out, a first semi-conductive screen layer 306, an insulator layer 308, a second semi-conductive screen layer 310, a first slip (e.g. water-swellable) winding 312, a metal conducting screen 314, a second slip (e.g. water-swellable) winding 316 and, finally, an outer sheath layer 318. The metal tape screen layer 314 and first and second slip tapes 312, 316 form a conducting screen, which may be described as a metallic diffusion or fluid barrier that prevents or reduces the ingress of water or gas toward the conductor which may corrode, e.g. oxidize the metal conductor, and thus result in electrical breakdown or disruptive discharge. Preferably, any water that reaches the insulation will be deionised. The slip tapes 312, 316, arranged either side of the metal tape screen layer 314, comprise a water-swellable tape material that absorbs water. By positioning a slip tape 312 outward of the metal tape screen layer 314, the slip tape 312 acts as a “first line of defence” and ensures minimal water reaches the metal tape screen layer 314. The second slip tape 316 thus acts to prevent salt-water from travelling longitudinally along the cable so the insulator layer 308 is prevented from getting wet. The insulator layer 308 prevents water from reaching the conductor. As discussed above, the metal conducting screen 314 may comprise two helical (e.g. tape) windings, wherein the helical turns are arranged such that there is a gap or space between each of the turns and the helical turns of the two windings are offset with respect to each other such that there are no gaps that extend all the way through the metal conducting screen 314. As discussed above, one or more of the layers may be provided as sheaths or extruded polymeric layers. For example, the insulator layer 308 and the first 306 and second 310 semi-conductive screen layers may be extruded around the plurality of strands 302 forming the conductors. For high-quality insulation assembly, the three layers 306, 308, 310 are advantageously manufactured simultaneously in a tripleextrusion system. Figure 3 shows a cross-section of a subsea power transmission cable 400 in accordance with another embodiment of the present invention. The subsea power transmission cable 400 includes a plurality of metal (e.g. copper or aluminium) strands 402 arranged axially alongside each other to provide a conductor 401. Each strand is coated in a filler material 404 which at least partially fills the interstices 403 between the strands. Optionally, a filler material (not shown) may further fill the interstices between the strands 402. The conductor of the subsea power transmission cable 400 is then covered by, from the inside out, a first semi-conductive screen layer 406, an insulator layer 408, a second semi-conductive screen layer 410, a first slip (e.g. water-swellable) winding 412, a metal conducting screen 414, a second slip (e.g. water-swellable) winding 416 and, finally, an outer sheath layer 418. Figure 4 is an expanded view of a subsea power transmission cable 500 in accordance with another embodiment of the present invention, wherein each layer is sequentially cut away for clarity. The subsea power transmission cable 500 includes a plurality of metal (e.g. copper or aluminium) strands 502 arranged axially alongside each other to provide a conductor 501 having interstices between the strands 502. Although not shown, the interstices are filled, partly or wholly, by a filler material. The conductor 501 is then covered by, from the inside out, a first semi-conductive screen layer 506, an insulator layer 508, a second semi-conductive screen layer 510, a first slip (e.g. water-swellable) winding 512, a metal conducting screen 514, a second slip (e.g. water-swellable) tape 516 and, finally, an outer sheath layer 518. As shown in Figure 4, the first 512 and second 516 slip windings and the metal conducting screen 514 are formed by winding a tape material (e.g. a slip or water-swellable tape and a metal tape respectively) around the conductor 501 to form a conducting screen as described herein. Although the direction of the turns of the slip windings 512, 516 are shown to be opposite in direction to the turn direction of the metal conducting screen 514, this is for exemplary purposes only. Indeed, the turn direction of the metal conducting screen 514 may be the same or different to that of the slip windings 512, 516. The metal tape screen 514 is formed by a first helical tape winding 514a and a second helical tape winding 514b. As shown in Figure 4, both the first 514a and the second 514b helical tape windings are wound such that there are gaps 520 provided between each turn of the helical tape. The two helical windings 514a, 514b are then offset from each other such that the tape of the second helical tape winding 514b covers the gaps formed by the first helical tape winding 514a. As such, there is no continuous gap provided through the metal tape screen 514. The gaps 520 between the windings provides flexibility to the cable and helps to reduce fatigue loading and allows the subsea power transmission cable 500 to flex. The gaps 520 may then be filled with a water-resistant or water-repellent material (such as oil or grease) to prevent the ingress of water through the subsea power transmission cable 500 towards the conductor 501. The strands 302, 402, 502 of the electrical conductor 301,401, 501 may be made of copper or aluminium, or any other suitable electrically conductive material or material composition. The embodiments of the subsea power transmission cable 300, 400, 500 described above may be incorporated into an umbilical for a subsea power line. The electrical conductor, the subsea power transmission cable are advantageously adapted for AC and for High Voltage for AC. Figure 5 shows an exemplary apparatus 600 that may be used to manufacture an embodiment of the subsea power transmission cable of the present invention. As shown, the apparatus includes a reel 620 from which either individual strands 602 or the conductor 601 (made from a plurality of strands 602) can be reeled off. The strands 602 or conductor 601 is then pulled and directed, via guiding rollers 650, into a bath comprising filler material 604. By passing a strand 602 through the bath, the strand 602 is coated in a layer of the filler material 604. By passing the conductor 601 through the bath, the conductor is both coated in a layer of filler material 604 as well as the interstices between the strands being impregnated with filler material 604. In the embodiment shown, the filler material is a curable material such that, when the coated strand 602 or conductor 601 is passed through a curing means 660 (e.g. a UV light source or thermal treatment) the layer of filler material 604 is cured to provide a coating 604’. As a result, the strand 602 or conductor 601, on output from the apparatus 600 is a coated strand 602’ or conductor 60T. Figure 6 shows an exemplary apparatus 700 that may be used to manufacture an embodiment of the subsea power transmission cable of the present invention. This apparatus 700 may be used in conjunction with the apparatus 600. For example, the stands being fed into the apparatus 700 may be coated strands 702’, or alternatively, the apparatus 600 may be incorporated as part of post-winding system 740, i.e. after the conductor 701 has been formed. With reference to Figure 6, strands 702 (or coated strands 702’) are reeled off of a reel into a bobbin wheel 730 comprising a plurality of bobbins. Each bobbin has a single strand 702, 702 fed therethrough and, upon rotation of the bobbin wheel 730, the strands are helically wound around each other to provide the helical strands of the conductor 701, 701’. Once the conductor 701, 701’ has been formed, the conductor 701, 701’ passes into the post-winding system 740. This system may include a plurality of extruders 741, 742, 743 which are arranged to provide different extruded layers around the conductor 701, 70T. For example, in the embodiment shown, the post-winding system includes three extruders 741, 742, 743 which provide a first semi-conductive layer 706, an insulating layer 708 and a second semi-conductive layer 710 in a triextrusion process. In some examples, the apparatus 600 may also be included in this stage upstream from the extruders to fill the interstices of the conductor 701 with filler material. After passing through the post-winding system 740, the conductor 701, 701’ (comprising various extruded layers) is then passed through several winding reels which are arranged to provide different winding layers to the cable. For example, the embodiment shown has four winding reel. When moving from left to right, the first reel is arranged to provide a first slip winding layer; the second reel is arranged to provide a first helical tape winding 714a of a conducting screen, where the winding includes gaps 760 between each turn of the winding; the third reel is arranged to provide the second helical tape winding 714b of the conducting screen such that the gaps 760 are covered by the second helical tape winding 714b; and the fourth is arranged to provide a second slip winding 716. The resulting cable may then pass through a further extrusion stage to provide an outer sheath to the cable. Figure 7 schematically shows part of an offshore system 100 comprising a subsea power transmission cable or line 200 extending between land (not shown) and an offshore floating assembly 140, including a windmill unit 150, a power generating unit 151, and a support unit 152. The subsea power transmission line 200 includes a relatively static subsea power transmission section 204, located at the bottom of the sea and resting on the seabed, and a dynamic subsea power transmission section 206 which extends from the seabed to the floating assembly 140. The dynamic subsea power transmission section 206 is clearly subject to a range of stresses (e.g. due to currents 212, gravity 214, hydrodynamic drag 216 and / or pressure). The subsea power transmission cable of the present invention (as described herein and as exemplified in relation to figures 2-4) may be used as at least the dynamic subsea power transmission section 206 of the transmission line 200 shown in Figure 7. Optionally, the subsea power transmission cable of the present invention is able to transmit power from a floating assembly of an offshore wind power generating system or the like such as the floating assembly 140 shown in Figure 7, for onward transmission or use of the power. In the example shown, the dynamic subsea power transmission cable 206 is connected to the floating assembly 140 by a termination or connector (not shown). The cable 206 is typically terminated directly into encapsulated switchgear by means of gas-insulated switchgear (GIS) terminations, polymeric plug-in connectors, or transformer terminations. In addition, to help the structural integration and provide a safe fixation to the floating assembly 140, the power transmission cable 206 is typically guided through a J-tube, carried at its upper extremity by a hang-off system which can withstand the gravity weight of its vertically suspended section and, protected of overbends and fatigue by a bending stiffener or bending restrictor 208 because the area at the connection point is subjected to repeated dynamic efforts. The dynamic subsea power transmission section 206 and the static subsea power transmission section 204 may be connected by a flexible joint. Buoyancy members or a mid-water arch 210 are / is attached to the dynamic subsea power transmission cable 206 to reduce the load and strain exerted on the dynamic subsea power transmission cable 206 as it extends from the seabed to the surface of the sea.

Claims

1. A subsea power transmission cable comprising at least one electrical conductor, said electrical conductor comprising a plurality of strands, the cable further comprising at least one conducting screen, wherein the or each conducting screen comprises two or more overlapping helical windings, and wherein the at least one conducting screen is covered on both sides by slip windings.

2. A subsea power transmission cable as claimed in claim 1 wherein the overlapping helical windings are formed of copper tape or copper wire.

3. A subsea power transmission cable as claimed in claim 1 or claim 2 wherein each overlapping helical winding is wound at an angle in the range 20°to 50°.

4. A subsea power transmission cable as claimed in any one of claims 1 to 3, wherein the two or more overlapping helical windings are offset relative to each other, and comprise gaps between each winding.

5. A subsea power transmission cable as claimed in claim 6 wherein the gap is in the range of from 15% to 60% of the thickness of the helical winding.

6. A subsea power transmission cable as claimed in any one of the preceding claims wherein the slip windings are formed from one or more of the group comprising a semi-conductive swellable polyester or non-woven polyester tape.

7. A subsea power transmission cable as claimed in any one of the preceding claims wherein the slip windings are formed from a water-swellable material.

8. A subsea power transmission cable as claimed in any one of the preceding claims wherein one helical tape winding overlaps another helical winding in the range of 10% to 50%.

9. A subsea power transmission cable as claimed in any one of the preceding claims wherein the two or more slip windings are offset relative to each other10. A subsea power transmission cable as claimed in any one of the preceding claims wherein the plurality of strands are a plurality of helical strands having interstices thereinbetween, wherein the interstices are partly, substantially or wholly filled with a friction-reducing filler material.

11. A subsea power transmission cable as claimed in as claimed in any one of the preceding claims wherein the at least one electrical conductor comprises one or more 1000 or 6000 series aluminium conductor strands.

12. A subsea power transmission cable as claimed in any one of the preceding clams wherein all the strands of the at least one electrical conductor comprise aluminium conductor strands.

13. A subsea power transmission cable as claimed in claim 12 wherein all the strands of the at least one electrical conductor comprise one or more 6000 series aluminium conductor strands.

14. A subsea power transmission cable as claimed in any one of the preceding clams wherein the power transmission cable is a medium or high voltage transmission cable.

15. A subsea power transmission cable as claimed in any one of the preceding clams wherein the strands are not compacted together to form the electrical conductor.

16. A subsea power transmission cable as claimed in any one of the preceding clams wherein the plurality of strands forming the electrical conductor are pre-coated with the filler material, or the filler material is pumped into interstices between the plurality of strands as the interstices are being formed.

17. A subsea power transmission cable as claimed in in any one of the preceding clams wherein the power transmission cable comprises radially outwardly:- an electrical conductor comprising a plurality of strands having interstices thereinbetween, wherein the interstices are partly, substantially or wholly filled with a filler material,- a covering first inner semi-conductive screen layer;- a covering insulator layer;- a covering second inner semi-conductive screen layer;- a covering first slip winding;- a covering conducting screen comprising:- a first helical winding; and- a second helical winding;- a covering second slip winding; and- an outer protective sheath.

18. A subsea power transmission cable as claimed in claim 17, wherein the covering conducting screen further comprises a slip winding arranged between the first helical winding and the second helical winding.

19. A method of manufacturing a subsea power transmission cable comprising at least one electrical conductor, said comprising a plurality of strands, the cable further comprising a conducting screen comprising two or more overlapping helical windings, wherein the at least one conducting screen is covered on both sides by slip windings, the method comprising at least the steps of:providing the electrical conductor;helically winding the helical windings and slip windings around the electrical conductor.

20. A method of manufacturing as claimed in claim 19 further comprising the step of providing a gap between each winding of each helical winding.

21. A method of manufacturing as claimed in claim 20 wherein the gap is in the range of from 15% to 60% of the thickness of the helical winding.

22. A method of manufacturing as claimed in any one of claims 19 to 21 further comprising adding the helical tape windings at an angle in the range 20° to 50°.

23. A method of manufacturing as claimed in any one of claims 19 to 22 comprising the steps of:- providing a plurality of strands;- helically winding said strands to form the electrical conductor and forming interstices thereinbetween;- filling the interstices with a filler material;- adding a first semi-conductive screen layer covering the electrical conductor;- adding an insulator layer covering the first semi-conductive screen layer;- adding a second semi-conductive screen layer covering the insulator layer;- adding a first slip tape winding covering the second semi-conductive screen layer;- adding a conducting screen covering the first slip tape winding, wherein the screen covering comprises a first helical winding and a second helical winding;- adding a second slip tape winding covering the conducting screen covering; and- adding an outer protective sheath covering the second slip tape winding.

24. A method of manufacturing a subsea power transmission cable as defined in any one of claims 1 to 18.

25. Use of a subsea power transmission cable as defined in any one of claims 1 to 18 in transmitting power from a floating assembly for an offshore power generating system for onward transmission.

26. A method of transmitting power from a floating assembly of an offshore power generating system, comprising the step of transmitting the power using a subsea power transmission cable as defined in any one of claims 1 to 18.

27. A method of reducing the dynamic fatigue on a subsea power transmission cable extending from or connected to a floating assembly for an offshore power generating system, comprising the step of providing a subsea power transmission cable as defined in any one of claims 1 to 18.

28. A subsea umbilical comprising one or more subsea power transmission cables as defined in any one of claims 1 to 18 and / or the subsea power transmission cable resulting from the method of manufacture as defined in any one of claims 19 to 24.

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