Subsea power umbilical

Abstract

A dynamic subsea power umbilical 20 for use in the offshore production of hydrocarbon energy or renewals energy comprises one or more power cables 22, each power cable comprising concentrically at lea

Description

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. Background 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. 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. 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 fora 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. API Specification 17E / ISO 13628-5 "Specification for Subsea Umbilicals" provides standards for the design and manufacture of such umbilicals. 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. 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. Figure 2 shows second typical conventional 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 conventional 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, or beneath the sea or even on the sea floor 5 as shown in Figure 2. A conventional prior art power cable 2 in a conventional subsea umbilical is illustrated in the accompanying Figure 3. Going from inside to outside, the power cable 2 comprises a central copper conductor 11, electrical insulation layer 12, a metallic screen 13 and an external polymeric barrier sheath 14. The electric current used to transmit the electric power is conducted by and along the central conductor 11. The primary function of the metallic screen 13 is to accommodate cable charging and circulating currents and to discharge short-circuit currents in the event of a cable failure. Figure 3 shows three power cables 2 in the umbilical, along with an optical fibre cable 15, fillers 16, armour layers 17, and a serving or oversheath 18. However, as some subsea power umbilicals can be 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. Problem to be solved 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 conventionally in a subsea power umbilical generally have limited fatigue resistance. 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. Summary of the invention 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 21 deal with a subsea power umbilical. Claim 22 deals with a subsea power umbilical assembly. Claims 23 to 24 deal with a method of installation of a subsea power umbilical assembly. Claim 25 to 27 deal with a method dissipating capacitive currents arising in a subsea power cable. According to one aspect, there is provided a subsea power umbilical for use in the offshore production of hydrocarbon energy or renewals energy, comprising: (i) one or more power cables, each cable comprising concentrically at least a central conductor, an electrical insulation layer, an external semi-conducting cable sheath (28), but not including a metal screen; (ii) one or more outer armouring layers around the power cable(s), and configured to be earthed to an umbilical termination, and (iii) an electrically-conductive pathway from the power cable(s) to the armouring layer(s). According to another aspect, there is provided a subsea power umbilical assembly, comprising a subsea power umbilical as defined herein and an umbilical termination at each end of the subsea power umbilical, wherein the one or more outer armouring layers of the subsea power umbilical are configured to be earthed to the umbilical termination (32) through an electrically-conductive pathway between the one or more armouring layers and the umbilical termination. According to another aspect, there is provided a method of installing a subsea power umbilical as defined herein with an umbilical termination at each end of the subsea power umbilical, comprising at least the steps of: (a) providing a subsea power umbilical comprising: one or more power cables, each cable comprising concentrically at least a central conductor, an electrical insulation layer, an external semi-conducting cable sheath, but not including a metal screen; one or more outer armouring layers around the power cable(s), and an electrically-conductive pathway from the power cable(s) to the armouring layer(s); (b) locating each end of the subsea power umbilical to an umbilical termination: and (c) earthing each end of the armouring layer(s) to each umbilical termination. According to another aspect, there is provided a method of dissipating capacitive currents arising in a subsea power umbilical as defined herein for use in the offshore production of hydrocarbon energy or renewals energy, comprising at least the step of providing an electrically-conductive pathway from the or each external cable sheath to the armouring layer(s). Brief description of the drawings The disclosure will be further described with reference to examples depicted as schematic illustrations in the accompanying figures in which: Figure 1 is a schematic illustration of an offshore production unit of oil and gas using a subsea power umbilical; Figure 2 is a schematic illustration of an offshore wind farm using subsea power umbilicals; Figure 3 is a cross-sectional view of a prior art subsea umbilical; Figure 4 is a cross-sectional view of a subsea power umbilical according to one embodiment of the present invention; Figure 5 is a schematic view of a subsea power umbilical assembly according to another embodiment of the present invention: and Figure 6 is a cross-sectional view of a power umbilical assembly installed to an umbilical termination according to another embodiment of the present invention. Detailed description 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. Inter-array subsea power umbilicals can be designed for connection of wind turbines with each other and / or with a transformer station located in the offshore wind farm. Inter-array subsea power umbilicals are generally designed for operating at 33kV or above. Export cables are for transmitting power from an offshore wind farm to land, and ‘interconnector’ cables can be for transmitting large loads under water. 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. A conventional prior art power cable 2 in a conventional subsea umbilical is illustrated in the accompanying Figure 3. Going from inside to outside, the power cable 2 comprises a central copper conductor 11, electrical insulation layer 12, a metallic screen 13 and an external polymeric barrier sheath 14. The electric current used to transmit the electric power is conducted by and along the central conductor 11. The primary function of the metallic screen 13 is to accommodate and deal with dissipating cable charging and circulating currents, sometimes terms capacitive currents, and to rapidly discharge short-circuit currents in the event of a cable failure. However, as some 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. In particular, metal screens with a conventional power cable are typically formed from one or more layers of copper, and in the form of thin foil or thin wires. The mechanical strength of copper, especially thin copper, is not significant. With dynamic loading, such thin coppers layers are prone to failure, and any break in the metallic screen in a qualification fatigue test would be deemed a failure. Failure requires a new power cable. Failure after installation requires installation of a new power umbilical. According to one aspect of present invention, there is provided a subsea power umbilical for use in the offshore production of hydrocarbon energy or renewals energy, comprising: (i) one or more power cables, each cable comprising concentrically at least a central conductor, an electrical insulation layer, an external semi-conducting cable sheath, but not including a metal screen; (ii) one or more outer armouring layers around the power cable(s), and configured to be earthed to an umbilical termination, and (iii) an electrically-conductive pathway from the power cable(s) to the armouring layer(s). That is, each power cable does not include the expected and conventional dedicated metal screen. In this way, the subsea power umbilical does not need to rely on the conventional metal screen within the power cable itself to dissipate electrical currents being created within the umbilical during power transmission to an earth. Meanwhile, the armouring layer(s) are still mechanically strong, and still provide the required fatigue resistance to mechanical loads over long periods of time. The central conductor may be made from one or more metals having high electrical conductivity, such as copper, aluminum, aluminum alloy, or copper coated aluminum. The central conductor may be a solid conductor which is a single body, or a stranded conductor which is an assembly of a plurality of thin strands. Optionally, the central conductor can be solid, or can comprise a number of conductive wires, which are typically wound and stranded together. The section of the central conductor can be large, and typically comprises a conductive cross-section between 50mm2 and 2000mm2, such as 50mm2, 70mm2, 95mm2, 120mm2, 150mm2, 240mm2, 300mm2, 400mm2, 630mm2, 800mm2 and 1000mm2. Between the central conductor and the electrical insulation layer there may be an intermediate layer, typically a tape such as a separating tape, a fire resistive tape, a conductive tape or the like. The conductor outer layer may be a semiconductive layer to aid uniform distribution of the electric field. The electrical insulation layer may be formed from one or more insulating materials, such as dielectric materials. Such materials include various grades of polyethylene (PE) such as cross-linked polyethylene (XLPE), rubbers such as ethylene propylene rubber (EPR), thermoplastic elastomers such as High Performance Thermoplastic Elastomers (HPTE), various grades of polypropylene (PP), and fluoropolymers for higher or very high temperatures. The electrical insulating layer may be formed by melt extrusion of the dielectric onto the outer surface of the central conductor. The electrical insulation layer may be formed in one or more layers, serially or simultaneously applied around the central conductor. Optionally, the thickness of the electrical insulation layer is guided by consideration of in-use electrical stress. When a power cable is under tension, the electrical stress, (i.e. the electric field) which is applied to the electrical insulation layer is at a maximum close to the central conductor, and decreases as the radial distance from the central conductor increases. The electrical stress is therefore smaller on the outside diameter of the insulation layer than on its inside diameter. The difference between the inside and outside electrical stress values notably depends on the thickness of the electrical insulation layer, and the difference tends to increase when this thickness increases. EC 60840, Edition 5.1 of 2023-06 sets out an International Standard for designing the extruded electrical insulation layer of power cables for rated voltages comprised between 30kV and 150kV. This standard defines how to calculate the nominal inside and outside electrical stresses as below: Ei = 2 Uo / .(Di x ln(Do / Di)) Eo = 2 Uo / (Do x In (Do / Di)) t = (Do - Di) / 2 where: Ei is the nominal electrical stress on the inside diameter of the electrical insulation layer, in kV / mm; Eo is the nominal electrical stress on the outside diameter of the electrical insulation layer, in kV / mm; Uo is the rated voltage of the power cable in kV; Di is the inside diameter of the electrical insulation layer in mm; Do is the outside diameter of the electrical insulation layer in mm; and t is the thickness of the electrical insulation layer in mm. Optionally, the thickness t of the electrical insulation layer is chosen such that the nominal electrical stress Eo on the outside diameter of the electrical insulation layer is less than 4 kV / mm. Optionally, the nominal electrical stress Ei on the outside diameter of the electrical layer is less than 8 kV / mm. For example, for a rated voltage of 36kV and a conductor diameter of 34.4mm, the insulation thickness is preferably at least 8mm. Optionally, the thickness of the electrical insulation layer used in the present invention is increased by at least 5%, including 10%, 15%, 20% 20%, 25%, 30% or more, compared to the expected thickness in the art, and / or as calculated according to EC 60840, Edition 5.1 of 2023-06 as discussed above. This can further assist by accommodating any local electrical stresses that may occur as a result of removing the conventional metal screen from around the central conductor. I EC 60243-1, Edition 3.0 of 2013-03 also sets out an International Standard for the electric strength of insulating material. Part 1 is Tests methods for the determination of short-time electric strength of solid insulating materials at power frequencies between 48 Hz and 62 Hz. Optionally, the electrical insulation layer has an electric strength of at least 4 kV / mm (as defined by I EC 60243-1), preferably of at least 8 kV / mm, more preferably of at least 10 kV / mm, so that the electrical insulation layer is able to maintain its insulation properties and resist the electrical stress when the power cable is operated. Optionally, the external semi-conducting cable sheath is formed from one or more semiconducting materials, including but not limited to polyethylene (PE), polyvinylchloride (PVC), polypropylene (PP) and acrylonitrile butadiene styrene (ABS). optionally with the addition of carbon therewith. Optionally, the external semi-conducting cable sheath comprises more than one layer of the same or different materials. For example, the external semi-conducting sheath comprises an inner layer which is formed from a semi-conducting tape applied on the electrical insulation layer, and an outer layer which is formed by extruding a semiconducting material around the inner layer. Optionally, the subsea power cable comprises at least three such power cables assembled together in helical or S / Z manner. It is a particular feature of the present invention that the or each power cable does not include a metal screen, typically being a series of wires or a foil of copper or other suitable metallic material designed to drain away the capacitive currents that occur within power cables carrying voltages, especially higher voltages. Such metal screens are conventional provided as part of a ‘power cable’, i.e. formed therewith prior to the addition of a final and / or outer protective sheath. Such metal screens are conventionally earthed at each end of the cable to a suitable earthing point, or are able to dissipate current through voids or connection with sea water or the like. The or each armouring layer may be formed from one or more metals. Typically armoured layers for a subsea umbilical are formed form a grade of steel, notably carbon steel, to provide the required strength to the umbilical, especially a dynamic umbilical. The steel armour wires are optionally coated with zinc, (i.e galvanised), to prevent corrosion. Although steel is less conductive than copper, the cross-sectional area of the armouring layer(s) is much greater than the cross-section of any conventional copper metal screen used in the prior art. Thus, one or more steel armouring layers are still able to discharge electrical currents, and to transport electrical currents along the umbilical axis in the event of a cable failure. For example, a short circuit current in the power cables can first circulate along the electrically-conductive pathway until they reach the or each armouring layer, and then along the or each armouring layer until they reach an umbilical termination where they can be discharged to the earth. Optionally, the subsea power cable comprises one or more armoured layers assembled together in helical or S / Z manner around the power cable(s). In one embodiment of the present invention, the subsea power umbilical comprises two, three or four power cables. Such power cables are all within the one or more armouring layers. Optionally, the subsea power umbilical comprises two armouring layers, one armouring layer being assembled around another armoured layer. In addition, optionally both armouring layers are cross-wound. Cross-wound armouring layers torsionally balance the umbilical and limit its rotation under axial load. This is a preferred design for dynamic in-use situations, such as dynamic risers. For static applications, one armouring layer may be sufficient. Optionally, the subsea power umbilical comprises three power cables and two crosswound armouring layers therearound. 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 66 kV or 132kV. Optionally, the subsea power umbilical has a voltage rating between 33kV and 132kV, including 66kV. Optionally, the subsea power umbilical is configured to at least drain a short circuit of at least 3kA, preferably 10kA, more preferably 20kA for 1 second to an umbilical termination. Optionally, the subsea power umbilical comprises at least two power cables, and the external semi-conducting cable sheaths of the power cables directly touch each other. Where the subsea power umbilical comprises three power cables, arranged in a triangular fashion, the external semi-conducting cable sheaths of all three power cables can directly touch each other. In this way, there can be increased electrical conductivity between each of the semiconducting cable sheaths, which can therefore better accommodate, distribute, and subsequently dissipate more evenly undesired electrical current being created within any one power cable, or between any two power cables. The present invention can provide direct capacitive current distribution between all the power cables within the subsea power umbilical. The present invention can also provide better pathways for any short-circuiting issue within any one power cable. Optionally, the one or more outer armouring layers are configured to be earthed to an umbilical termination at each end of the subsea power umbilical. Optionally, the subsea power umbilical further comprises one or more core fillers between the one or more power cables. The core fillers may be formed from any suitable material or materials. Optionally, the or each core filler is a semi-conductive core filler. A semi-conductive core filler may be formed from one or more semi-conducting materials, including but not limited to polyethylene (PE), polyvinylchloride (PVC), polypropylene (PP) and acrylonitrile butadiene styrene (ABS), optionally with the addition of carbon therewith. Optionally, the subsea power umbilical comprises at least two power cables, which form a core assembly. The core assembly may also include one or more fillers as described herein. The core assembly may be formed by the bringing together of its constituents from one or more sources prior to encapsulation as a single entity by the bedding sheath. Optionally the constituents of the core assembly are provided from a number of reels or the like, and wound in a helical or S / Z manner, in a manner known in the art. Optionally, the subsea power umbilical comprises at least two power cables, which power cables form a core assembly, and there is a concentric binding tape around the core assembly. A binding tape can be to assist cohesion of the core assembly prior to the next assembly step, but conventional binding tapes are non-conductive. In one embodiment of the present invention, the binding tape is a semi-conductive tape. Such a tape may include or be made from one or more conducting or semiconducting materials, such as described herein. Such a tape maintains the electrically conductive pathway from the power cables and towards the one or more armouring layers. In another embodiment of the present invention, the binding tape partly extends around the core assembly. That is, spaces or gaps exist either concentrically or longitudinally along the winding or windings of the tape around the core assembly, for the provision of another conductive material able to provide the electrically conductive pathway from the power cables and towards the one or more armouring layers. This allows for the use of a conventional binding tape, with an additional conductive material being able to locate in the spaces and gaps. Optionally, the additional conductive material is an additional layer. Optionally, the subsea power umbilical further comprises a concentric semi-conductive bedding sheath between the one or more power cables and the one or more outer armouring layers, and wherein he the bedding sheath is configured to electrically connect the or each power cable with the one or more armouring layers. A bedding sheath increases the bundle stability and provides a bedding for the armouring layers. Optionally, the bedding sheath may be formed from any suitable material or materials, and may comprise one or layers of the same or different material. A bedding sheath may be formed from one or more semi-conducting materials, including but not limited to polyethylene (PE), polyvinylchloride (PVC), polypropylene (PP) and acrylonitrile butadiene styrene (ABS), optionally with the addition of carbon therewith. Optionally, the bedding sheath directly touches the or each external semi-conducting cable sheaths of the power cables and / or any binding tape therearound. loptionally, the bedding sheath extends partly through the binding layer in a manner described hereinabove. In this way, there can be direct electrical conductivity between each of the semi-conducting cable sheaths and the bedding sheath, which can therefore better accommodate, distribute, and subsequently dissipate more directly undesired electrical current being created within any power cable. The present invention can provide direct capacitive current direction from any power cable within the subsea power umbilical. The present invention can also provide better pathways for any short-circuiting issue within any one power cable. Thus, in one embodiment of the present invention the subsea power umbilical comprises a binding tape as an inner layer comprising semi-conducting tapes which are wound around the one or more power cables, and a bedding sheath as an outer layer being a semi-conductive sheath extruded around the inner layer, the inner and outer layers being in direct contact with each other, the inner layer being preferably in direct contact with the one or more power cables and / or with the or each semi-conductive core fillers. The semi-conducting binding tapes are used to maintain the core during the lay-up i.e. during the helical or S-Z assembly of the components forming the core assembly of the subsea umbilical. Using semi-conductive tapes to bind the core of an umbilical during a lay-up provides very good electrical contacts between the outer surface of the core assembly and the semi-conductive tapes forming the inner layer of the semi-conductive bedding sheath. This characteristic has the effect of reducing the electrical resistance of the electrically conductive pathway from the power cables to the armouring layer, which improves the ability to safely drain to the earth large short-circuit currents in the event of a cable fault. in another embodiment of the present invention, and where binding tapes used during the core assembly lay-up are not conductive, the binding tape can be wound around the core assembly so that gaps are left in between adjacent windings, and a semiconducting bedding sheath is directly extruded onto the binding tape layer, so that some semi-conductive material can flow through the gaps during the extrusion and come into contact with the power cables and / or the semi conductive core fillers. Optionally, the subsea power umbilical comprises a bedding sheath as described herein, wherein an armouring layer(s) directly touches the bedding sheath. In this way, there can be direct electrical conductivity between the bedding sheath and the armouring layer(s), which can therefore better accommodate, distribute, and subsequently dissipate more directly undesired electrical current being created within any power cable. The present invention can provide direct capacitive current direction from any power cable to the armouring layer(s) within the subsea power umbilical. The present invention can also provide better pathways for any short circuiting issue within any one power cable. Optionally, the subsea power umbilical further comprises one or more sea-water voids between the one or more power cables and the one or more outer armouring layers. Seawater, being electrically conductive, improves the electrically-conductive pathway from the power cables to the outer armouring layers. Optionally, any outer sheath of the subsea power umbilical is preferably permeable or drilled with vent holes to allow sea water to enter inside of the umbilical and to fill voids and channels between the components of the umbilical when the umbilical is installed subsea. Alternatively or in combination with such a permeable outer sheath, the umbilical preferably comprises hollow / tubular fillers which are filled with sea water. Optionally, the one or more outer armouring layers are configured to be earthed to an umbilical termination through an electrically-conductive pathway between the one or armouring layers and the termination. Optionally, the electrically-conductive pathway comprises one or more electrical connections. Such connections may be located outside and / or inside a termination. Optionally, the one or more electrical connections comprise one or more wires connected to the armouring layer(s) by clamping means or welding means. Optionally, the clamping means comprise a metallic tape strapped around the armouring layers. Optionally, the one or more electrical connections comprise a metallic strap which is clamped under tension around and in contact with the one or more outer armouring layers. Optionally, the one or more electrical connections have a collective diameter of at least 20 mm. Optionally, the one or more outer armouring layers comprise a series of steel or galvanized steel wires. Each armouring layer preferably comprises a large number of steel wires - typically 50 to 200 wires, for example around 100 wires - which are wound with a helix angle with an absolute value preferably comprised between 5° and 30 °. For dynamic applications, two cross-wound armouring layers are preferably used, both layers preferably having opposite helix angles. This makes the umbilical more torque-balanced, i.e. less likely to rotate when a tensile axial load is applied to it. Optionally, the subsea power umbilical comprises at least two outer armouring layers, and said armouring layers being helically wound around an internal surface of the power umbilical in a countercurrent fashion for each layer. Optionally, the subsea power umbilical further comprises one or more further operable elongate elements from the group comprising: load carrying elements, optical fibres, and core fillers. In one embodiment of the present invention, there is provided a subsea power umbilical comprising: - three power cables, each power cable comprising concentrically at least a central conductor, an electrical insulation layer, an external semi-conducting cable sheath, but not including a metal screen; - three semi-conducting core fillers located between the power cables; - a semi-conductive bedding sheath; and - two outer armouring layers around the power cables, and configured to be earthed to an umbilical termination; wherein the external cable sheaths, semi-conducting core fillers, and semi-conductive bedding sheath provide electrically-conductive pathways from the power cables to the armouring layers. Optionally, the umbilical termination for any of the embodiments described herein is an offshore renewal energy generating platform. Optionally, the subsea power umbilical for any of the embodiments described herein is an inter-array cable, an export cable or an interconnector cable. Optionally, the subsea power umbilical for any of the embodiments described herein is a dynamic inter-array cable. In another aspect of the present invention, there is provided a subsea power umbilical assembly comprising a subsea power umbilical as defined herein, and an umbilical termination at each end of the subsea power umbilical, wherein the one or more armouring layers of the subsea power umbilical are configured to be earthed to each umbilical termination through an electrically-conductive pathway between the one or more armouring layer(s) and the termination. The subsea power umbilical assembly may include one or more of the embodiments of the subsea power umbilical described herein, and the skilled reader understands to achieve such combination. In another aspect of the present invention, there is provided a method of installing a subsea power umbilical as herein to an umbilical termination at each end of the subsea power umbilical, comprising at least the steps of: (a) providing a subsea power umbilical comprising: one or more power cables, each cable comprising concentrically at least a central conductor, an electrical insulation layer, an external semi-conducting cable sheath, but not including a metal screen; one or more outer armouring layers around the power cable(s), and an electrically-conductive pathway from the power cable(s) to the armouring layer(s); (b) locating each end of the subsea power umbilical to an umbilical termination: and (c) earthing each end of the armouring layer(s) to each umbilical termination. The subsea power umbilical of the method of installing may include one or more of the embodiments of the subsea power umbilical described herein, and the skilled reader understands to achieve such combination. Optionally, the armouring layer(s) are earthed to each umbilical termination in the method through one or more electrical connections located inside the termination. In another aspect of the present invention, there is provided a method of dissipating capacitive currents arising in a subsea power umbilical as defined herein for use in the offshore production of hydrocarbon energy or renewals energy, comprising at least the step of providing an electrically-conductive pathway from the or each external cable sheath to the armouring layer(s). The subsea power umbilical of the method of installing may include one or more of the embodiments of the subsea power umbilical described herein, and the skilled reader understands to achieve such combination. Optionally, the umbilical termination is an offshore renewal energy generating platform and the power umbilical is a dynamic inter-array cable, further comprising the step of dissipating capacitive current to the offshore renewal energy generating platform. Optionally, the method further comprises the steps of; - forming a subsea power umbilical comprising one or more power cables, each cable comprising concentrically at least a central conductor, an electrical insulation layer, an external semi-conducting cable sheath, but not including a metal screen; -adding around the power cable(s) one or more outer armouring layers configured to be earthed to an umbilical termination, and -providing an electrically-conductive pathway from the power cable(s) to the armouring layer(s). Turning to the drawings, Figures 1, 2 and 3 are described above. Figure 4 is a cross-sectional view of an example of a subsea power umbilical 20 according to the present invention. Figure 4 shows the subsea power umbilical 20 comprising three power cables 22. Each power cable 22 comprises, concentrically from the centre to the outside, a central conductor 24, an electrical insulation layer 26, and an external semi-conducting cable sheath 28. Each power cable 22 does not include a conventional metal screen. Each power cable 22 may comprises an intermediate semi-conducting layer (not shown) located between the central conductor 24 and the electrical insulation layer 26 in a manner known in the art. The function of this intermediate semi-conducting layer can assist bringing electrical stress closer to the outer surface of the central conductor 24. The central conductor 24 is comprises a series of conductive wires, wound and stranded together. The cross-section of the central conductor 24 can be large, such as 800mm2. The external semi-conducting cable sheaths 28 of the power cables 22 directly touch each other. In this way, there can be increased electrical conductivity between each of the semi-conducting cable sheaths 28 to distribute and subsequently dissipate undesired electrical current being created within any one power cable, or between any two power cables. The subsea power umbilical 20 further comprises three semi-conductive core fillers 36 between the power cables 22, able to provide a more overall circular cross-section in a manner known in the art. The subsea power umbilical 20 further comprises a number of sea-water voids 38 between the power cables 22 and the core fillers 36. Sea water being electrically conductive, the sea-water voids 38 reduce the overall resistivity of the electrically-conductive pathway from the power cables 22 to the outer armouring layers 30. Therefore, in the event of a power cable fault, short-circuit currents can better circulate along the electrically-conductive pathway from the power cable to the outer armouring layers which results in a more efficient drainage to the earth. The power cables 22, core fillers 36, and sea-water voids 38 form a core assembly, which can be formed by the bringing together of its constituents from one or more sources prior to encapsulation as a single entity, optionally by a binding tape 37 . Optionally the constituents of the core assembly are provided from a number of reels or the like, and wound in a helical or S / Z manner, in a manner known in the art. The subsea power umbilical 20 further comprises a semi-conductive bedding sheath 34 between the power cables 22 and two outer armouring layers 30. The bedding sheath may be formed from any suitable material or materials. The bedding sheath 34 directly touches the binding tape 37 fully or partly wound around the external semiconducting cable sheaths 28 of the power cables 22. In this way, there can be direct electrical conductivity between each of the semi-conducting cable sheaths 28 and the bedding sheath 34, which can therefore better dissipate more directly undesired electrical current being created within any power cable 22. The subsea power umbilical in Figure 4 also comprises two cross-wound outer armouring layers 30 assembled together in helical manner around the power cables 22. The armouring layers 30 are configured to be earthed to an umbilical termination 32 as discussed hereinafter. The innermost armouring layer 30 directly touches the bedding sheath 34. The outermost armouring layer 30 directly touches the innermost armouring layer 30. Around the armouring layers is an outer protective layer 31. Overall, the subsea power umbilical in Figure 4 shows an electrically-conductive pathway from the power cable 22 to the armouring layers 30. The present invention can provide direct capacitive current direction from any power cable within the subsea power umbilical to the armouring layers 30. The present invention can also provide better pathways for any short circuiting issue within any one power cable 22 to the armouring layers 30. Figure 5 shows a subsea power umbilical assembly 50, comprising a subsea power umbilical 20 as defined above, and an umbilical termination 32 at each end of the subsea power umbilical 20, wherein the one or more armouring layers of the subsea power umbilical are earthed to each umbilical termination through one or more electrical connections located inside the termination 32. The subsea power umbilical 20 can be installed to the umbilical terminations 32 at each end of the subsea power umbilical 20 by locating each end of the subsea power umbilical 20 to an umbilical termination 32, and earthing each end of the armouring layers 30 to each umbilical termination 32 in a manner shown in Figure 6 shows an example of a termination 32 for the present invention The termination 32 comprises a cover 52 and an end plate 54, which are steel parts forming its body. The cover 52 forms a cylindrical bore through which a subsea power umbilical 20 of the present invention having three power cables 22 can extend. There are three holes in the end plate 54 through which the three 3 power cables 22 extend. The extremity of the umbilical outer sheath 31 is removed before the termination is assembled, so that the armouring layers 30 are accessible in the termination. Figure 6 shows electrical connections configured to earth the armouring layers 30 to the umbilical termination 32. The electrical connections comprise a metallic strap 55, earth connection cables 56, earth connection points 59, the earthing cables 56. The metallic strap 55 is clamped under tension around the outer armouring layer 30 to achieve a good electrical contact. The metallic strap 55 is for example made with steel or copper. The metallic strap 55 comprises securing means 57 for connecting the earth connection cable 56 to the strap 55, for example bolt / nut means. The function of the metallic strap 55 is to provide an electrically-conductive pathway for a short-circuit current coming from the armouring layers 30, to the earth connection cables 56. The function of the earth connection cables 56 is to transmit such currents from the metallic strap 55 to the earth connection points 59. The earth connection cables 56 can be standard copper cables used for earthing, and may not necessarily need to be over sheathed with an insulating layer. The earth connection points 59 are typically screw / bolt connecting means able to electrically connect the earth connection cables 56 to the steel body of the termination 54, 52, and to earth the termination 32 via standard earthing cables 58. The extremities of the armouring wires or layers 30 can be folded with hooks and anchored inside a chamber 53 within the termination body, and filled with a hard setting compound such as an epoxy resin. Materials such as epoxy resins are not electrically conductive, which is why the electrical connection pathway provided by the various means 55, 56, 57 is deliberately provided: Conventionally, the electrical resistance between the armouring layers 30 and the termination body 52, 54 is too high to ensure a safe draining of a short circuit current in the event of a power cable fault. Figures 4-6 show a method of dissipating any capacitive currents arising in the subsea power umbilical 20 via an electrically-conductive pathway from the or each power cable 22 to the armouring layers 30, without requiring a conventional metal screen within each power cable, and which is able to accommodate large currents and voltages, especially very large short circuits. The armouring layers 30 provide a large cross-sectional size of conductor, relative to the cross-sectional size of conventional internal metal screens, As the armouring layers are directly earthed, undesired current can be directly conducted away to achieve the same protection of the power cables as conventional metal-screen power cables.

Claims

1. A subsea power umbilical (20) for use in the offshore production of hydrocarbon energy or renewals energy, comprising:(i) one or more power cables (22), each power cable comprising concentrically at least a central conductor (24), an electrical insulation layer (26), an external semi-conducting cable sheath (28), but not including a metal screen;(ii) one or more outer armouring layers (30) around the power cable(s), and configured to be earthed to an umbilical termination (32), and(iii) an electrically-conductive pathway from the power cable(s) to the armouring layer(s) (30).

2. A subsea power umbilical (20) as claimed in claim 1, having a voltage rating between 30kV and132kV.

3. A subsea power umbilical (20) as claimed in claim 1 or claim 2, configured to at least drain a short circuit of at least 10kA for 1 second to the umbilical termination (32).

4. A subsea power umbilical (20) as claimed in any one of the preceding claims, comprising at least two power cables (22), and wherein the external semi-conducting cable sheaths (28) of the power cables directly touch each other.

5. A subsea power umbilical (20) as claimed in any one of the preceding claims, wherein the one or more armouring layers (30) are configured to be earthed to an umbilical termination (32) at each end of the subsea power umbilical (20).

6. A subsea power umbilical (20) as claimed in any one of the preceding claims, wherein the electrical insulation layer (26) has an electric strength of at least 4 kV / mm (as defined by IEC 60243-1).

7. A subsea power umbilical (20) as claimed in any one of the preceding claims, further comprising one or more semi-conductive core fillers (36) between the one or more power cables (22)8. A subsea power umbilical (20) as claimed in any one of the preceding claims comprising at least two power cables (22), which power cables form a core assembly, and a concentric binding tape (37) around the core assembly.

9. A subsea power umbilical (20) as claimed in claim 8, wherein the binding tape (37) is a semi-conductive tape10. A subsea power umbilical (20) as claimed in claim 8 or claim 9, wherein the binding tape partly extends around the core assembly.

11. A subsea power umbilical (20) as claimed in any one of the preceding claims, further comprising a concentric semi-conductive bedding sheath (34) between the one or more power cables (22) and the one or more armouring layers (30), and wherein the bedding sheath is configured to electrically connect the or each power cable (22) with the one or more armouring layers (30).

12. A subsea power umbilical (20) as claimed in claim 11, wherein the bedding sheath (34) directly touches an armouring layer (30).

13. A subsea power umbilical (20) as claimed in any one of the preceding claims, further comprising one or more sea-water voids (38) between the one or more power cables (22) and the one or more outer armouring layers (30).

14. A subsea power umbilical (20) as claimed in any one of the preceding claims, wherein the one or more armouring layers (30) are configured to be earthed to an umbilical termination (32) through an electrically-conductive pathway between the one or more armouring layers (30) and the termination (32).15 A subsea power umbilical (20) as claimed in claim 14, wherein the electrically-conductive pathway comprises at least one or more or more electrical connections (55, 56, 57) and a metallic strap (55) around the one or more outer amouring layers (30).

16. A subsea power umbilical (20) as claimed in any one of the preceding claims, wherein the one or more armouring layers (30) comprise a series of steel wires.

17. A subsea power umbilical (20) as claimed in claim 16 comprising at least two armouring layers (30), said armouring layers being helically wound around an internal surface of the power umbilical in a countercurrent fashion for each layer.

18. A subsea power umbilical (20) as claimed in any one of the preceding claims, further comprising one or more further operable elongate elements from the group comprising: load carrying elements, optical fibres, and core fillers.

19. A subsea power umbilical (20) as claimed in any one of the preceding claims, comprising:- three power cables (22), each power cable comprising concentrically at least a central conductor (24), an electrical insulation layer (26), an external semiconducting cable sheath (28), but not including a metal screen;- three semi-conducting core fillers located between the power cables (22);- a semi-conductive bedding sheath (34); and- two outer armouring layers (30) around the power cables, and configured to be earthed to a umbilical termination (32);wherein the external cable sheaths, semi-conducting core fillers, and semi-conductive bedding sheath provide electrically-conductive pathways from the power cables to the armouring layers.

20. A subsea power umbilical (20) as claimed in any one of the preceding claims, wherein the umbilical termination (32) is an offshore renewal energy generating platform.

21. A subsea power umbilical (20) as claimed in claim 20 wherein the power umbilical is a dynamic inter-array cable.

22. A subsea power umbilical assembly (50) comprising a subsea power umbilical (20) as claimed in any one of claims 1 to 21 and an umbilical termination (32) at each end of the subsea power umbilical (20), wherein the one or more armouring layers (30) of the subsea power umbilical are configured to be earthed to the umbilical termination (32) through an electrically-conductive pathway between the one or more armouring layers (30) and the umbilical termination (32).

23. A method of installing a subsea power umbilical as defined in any one of claims 1 to 21 to an umbilical termination (32) at each end of the subsea power umbilical (20), comprising at least the steps of:(a) providing a subsea power umbilical (20) comprising: one or more power cables (22), each cable comprising concentrically at least a central conductor (24), an electrical insulation layer (26), an external semi-conducting cable sheath (28), but not including a metal screen; one or more outer armouring layers (30) around the power cable(s), and an electrically-conductive pathway from the power cable(s) to the armouring layer(s) (30);(b) locating each end of the subsea power umbilical to an umbilical termination: and(c) earthing each end of the armouring layer(s) to each umbilical termination.

24. A method as claimed in claim 23, wherein the armouring layer(s) (30) are earthed to each umbilical termination (32) through one or more electrical connections (55, 56, 57) located inside the termination (32).

25. A method of dissipating capacitive currents arising in a subsea power umbilical (20) as defined in any one of claims 1 to 21 for use in the offshore production of hydrocarbon energy or renewals energy, comprising at least the step of providing an electrically-conductive pathway from the or each external cable sheath (28) to the armouring layer(s) (30).

26. A method as claimed in claim 25, wherein the umbilical termination (32) is an offshore renewal energy generating platform and the power umbilical is a dynamic inter-array cable, further comprising the step of dissipating capacitive current to the offshore renewal energy generating platform.

27. A method as claimed in claim 25 or claim 26, further comprising the steps of; - forming a subsea power umbilical (20) as defined in any one of claims 1 to 21; - installing and earthing the subsea power cable to one or more umbilical termination(s) (32); and-providing an electrically-conductive pathway from the power cable(s) to the umbilical termination(s).

Images