TECHNICAL FIELD

The present disclosure generally relates to submarine power cables comprising a fibre optic cable.

BACKGROUND

Fibre optical cables are often integrated in submarine power cables for different purposes, including communication, control or for monitoring of e.g. cable temperature and/or vibration.

The optical fibres of a fibre optic cable degrade when exposed to hydrogen. This is known as“hydrogen ageing” or“fibre darkening”. Therefore the optical fibres in a submarine power cable are normally encapsulated into a metal tube made out of copper or stainless steel surrounding the optical fibres. This tube acts as a barrier for hydrogen and hydrogen oxide. The tube prevents the optical fibres from coming in contact with the sea water and from diffusion of hydrogen.

Existing fibre optic cables in high-voltage AC power cables will pick up the alternating electromagnetic field from the power cores. This results in an induced voltage in the fibre optic cable. The fibre optic cable may form a closed electrical circuit, either caused by the saline water normally

surrounding the fibre cable, and/or by deliberate grounding of the fibre- containing tube to sea water in order to reduce the induced voltage. A large induced voltage could, if not properly controlled, cause adverse effects to the submarine power cable.

The current flowing through the fibre optic cable contributes to the unwanted losses in the power cable system. In case the fibre optic cable is broken or damaged causing a reduced cross section area, the current may cause an increase of temperature, current flow in another path causing damage to the submarine power cable.

WO2016/034243 discloses a submarine electrical cable system which has three cores. The submarine cable system comprises an optical cable including a plurality of optical fibres embedded in a water-blocking material and surrounded by a polymeric sheath.

SUMMARY

In view of the above an object of the present disclosure is to provide a submarine power cable which solves or at least mitigates problems of the prior art.

There is hence provided a submarine power cable comprising: a first power core including: a conductor, and an insulation system surrounding the conductor; and a fibre optic cable extending along the first power core, the fibre optic cable including: one or a a plurality of optical fibres, and a watertight dielectric tube housing the plurality of optical fibres; and an outer serving surrounding the first power core and the fibre optic cable.

By using a dielectric tube for housing the optical fibres, the losses from the fibre optic cable can be reduced and essentially eliminated.

The dielectric tube also provides protection from mechanical damage of the optical fibres and may also protect against the influence from hydrogen.

It would appear as if the polymeric sheath of WO2016/034243 is not designed to be watertight, since a water-blocking material inside the polymeric sheath is present.

The dielectric tube may encapsulate the optical fibres.

The dielectric tube is non-metallic. The dielectric tube does hence not contain metal. The submarine power cable may be a low voltage power cable, a medium voltage power cable or a high voltage cable. The submarine power cable may be an AC cable or a DC cable.

According to one embodiment the fibre optic cable comprises a gel with hydrogen absorption properties, wherein the gel is provided in the dielectric tube, and wherein the optical fibres are embedded in the gel.

The gel is hence configured to absorb hydrogen, which may be especially beneficial since polymer materials may emit hydrogen gas, which could otherwise potentially degrade the optical fibres. The hydrogen absorption capability of the gel may be specifically adapted to the hydrogen emission capability of the dielectric material of the dielectric tube.

The gel may additionally act as a water blocking barrier inside the dielectric tube. According to one embodiment the dielectric tube has a diameter which has a large enough magnitude that the volume of the gel is large enough to absorb more hydrogen than that emitted by the dielectric tube.

According to one embodiment the dielectric tube has a solid dielectric body which defines a supporting structure of the fibre optic cable, and thus for the dielectric tube.

According to one embodiment the dielectric tube has a dielectric inner surface.

According to one embodiment the dielectric tube has a resistivity which is greater than to ¹⁰ W m. According to one embodiment the dielectric tube comprises a polymer.

According to one embodiment the dielectric tube comprises a polyamide. The polyamide may for example be nylon, such as nylon 6.6. According to one embodiment the dielectric tube comprises a plurality of solid dielectric layers.

The dielectric tube may for example comprise two solid dielectric layers.

All the solid dielectric layers may according to one example be made of the same dielectric material.

By providing several solid dielectric layers the water-tightness of the dielectric tube may be improved. For example, the risk of a pore in the inner solid dielectric layer being radially aligned with a pore in the outer solid dielectric layer is minimised. According to one embodiment at least two solid dielectric layers are made of different dielectric materials. One solid dielectric layer may for example comprise thermoplastic polyurethane (TPU) and another solid dielectric layer may for example comprise polyethylene (PE).

The fibre optic cable may comprise a plurality of inner dielectric tubes arranged inside the watertight dielectric tube. Each inner dielectric tube may house a plurality of optical fibres. Each inner dielectric tube may encapsulate a respect bundle of optical fibres. The watertight dielectric tube may encapsulate the inner dielectric tubes. The inner dielectric tubes may be stranded inside the watertight dielectric tube, or they may be arranged without being stranded.

According to one embodiment the dielectric tube is provided with a coating comprising an electrically conducting material. The electrically conducting material may for example be a metallic material in the form of e.g. a thin metal layer, a semiconducting material, or carbon, for example amorphous carbon. The thin metal layer may for example be deposited onto the dielectric tube by means of chemical or physical vapour deposition. The coating prevents or reduces diffusion of hydrogen. The optical fibres may thereby be protected from fibre darkening. The metal layer may for example have a thickness in the range of too nm to too pm, for example too nm to 80 pm, such as loo nm to 60 mhi, for example 100 nm to 40 pm, such as 100 nm to 20 pm, e.g. 100 nm to 10 pm.

The coating of electrically conducting material may be provided on the inner surface of the dielectric tube and/or on the outer surface of the dielectric tube.

According to one embodiment the coating is provided intermittently along the axial direction of the dielectric tube. Hereto, the dielectric tube may have axially spaced-apart regions that are provided with the coating.

The dielectric tube may alternatively or additionally be made of a dielectric material or dielectric materials that prevents or reduces diffusion of hydrogen. The optical fibres may thereby be protected from fibre darkening.

Alternatively, or additionally, according to one embodiment the optical fibres are coated with a hydrogen protecting layer. Protection against fibre darkening may thus be provided regardless of the configuration of the dielectric tube. The hydrogen protection layer may for example comprise or consist of amorphous carbon.

According to one embodiment the hydrogen protecting layer comprises a conducting material. The conducting material, in particular an electrically conducting material, may for example comprise a metal, or carbon. The electrically conducting material may for example comprise or consist of amorphous carbon.

One embodiment comprises a second power core of the same type as the first power core, wherein in cross-section of the submarine power cable the fibre optic cable is arranged in an interstice, or space, between the first power core and the second power core.

One embodiment comprises a second power core of the same type as the first power core, and a hollow filler device extending along the first power core and the second power core, which filler device in cross-section of the submarine cable is arranged between the first power core and the second power core, wherein the fibre optic cable extends in the filler device.

One embodiment comprises an armour layer arranged around the first power core, wherein the armour layer includes a plurality of metal wires, and wherein the fibre optic cable forms part of the armour layer.

According to one embodiment the submarine power cable is a DC submarine power cable. Electromagnetic fields, causing losses in a traditional metal tubed fibre optic cable, can also occur for DC submarine power cables in special cases, for example in the event of transients due to switching or because of cable faults. The dielectric tube is hence also advantageous for DC submarine power cables.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 schematically shows a cross-section of an example of a multi-core submarine power cable;

Fig. 2 schematically shows a cross-section of an example of a fibre optic cable; and Fig· 3 schematically shows a cross-section of an example of a single-core submarine power cable. DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying

embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Fig. l depicts an example of a submarine cable comprising a fibre optic cable.

The exemplified submarine cable l is an AC cable, in particular a three-phase submarine cable. The exemplified submarine cable l comprises a first power core 3a, a second power core 3b, and a third power core 3c.

The first power core 3a comprises a conductor 5a for conducting an electric current. The first power core 3a also comprises an insulation system 7a surrounding the conductor 5a. The insulation system 7a comprises an inner semiconducting layer 9a, a solid insulation layer 11a, for example a cross- linked polyethylene (XLPE) layer, arranged radially outside of the inner semiconducting layer 9a, and an outer semiconducting layer 13a arranged radially outside of the solid insulation layer 11a. The inner semiconducting layer 9a and the outer semiconducting layer 13a may for example comprise XT. PR mixed with carbon black.

The second power core 3b and the third power core 3c also have

corresponding conductors 5b, 5c and insulation systems 7b, 7c. The three cores 3a-3c are arranged in a stranded manner.

The submarine power cable 1 may also comprise an armour layer 17, comprising a plurality of helically wound wires, surrounding the three cores 3a-3c. The submarine power cable 1 also comprises an outer serving 15 surrounding the three cores 3a-3c. The outer serving 15 may for example comprise a sheath, such as a polymeric sheath, or wound yarn layer(s). The submarine power cable l furthermore comprises a fibre optic cable 19. The fibre optic cable 19 may for example be used for communication, for control or for monitoring of e.g. cable temperature and vibration.

The fibre optic cable 19 extends along the three power cores 3a-3c in the axial direction of the submarine power cable 1. The fibre optic cable 19 is arranged adjacent to two of the three power cores. In the example in Fig. 1, the fibre optic cable 19 is arranged in a space adjacent to the first and the third power core 3a and 3c. This region may contain filler material or a hollow filler device 4 in which the fibre optic cable 19 is arranged.

The fibre optic cable 19 comprises a plurality of optical fibres 19a and a watertight dielectric tube 19b. The fibre optic cable could alternatively comprise only a single optical fibre. The dielectric tube 19b houses the optical fibres or optical fibres 19a and protects them from sea water and mechanical stress. The dielectric tube 19b typically only houses the optical fibre(s) 19a and no other component of the submarine power cable 1. The dielectric tube 19b thus forms a housing or encapsulation only for the optical fibre(s) 19a.

The dielectric tube 19b preferably has a resistivity p>io ¹⁰ W m, for example p>io ¹⁰ W m.

The dielectric tube 19b has a solid dielectric body. The solid dielectric body is tubular and defines the dielectric tube 19b. The solid dielectric body forms the supporting or carrying structure of the dielectric tube 19b and for the entire fibre optic cable 19. The inner surface and the outer surface of the dielectric tube 19b is dielectric. The dielectric tube 19b is hence dielectric throughout.

The dielectric tube 19b may for example comprise polymer, or consist of polymer. The dielectric tube 19b may for example comprise polyamide, or consist of polyamide.

In some examples of the fibre optic cable 19, the inner surface and/or the outer surface of the dielectric tube 19b may be coated with an electrically conducting material. The electrically conducting material forms a conductive layer. Any increased hydrogen diffusion capability of the dielectric tube 19b compared to a traditional metallic tube may thus be mitigated.

In order to minimize the impact of an electrically conducting material, the coating may be thin relative to the thickness of the dielectric tube 19b. The conductive layer may for example have a thickness in the range of 100 nm to 100 pm, for example 100 nm to 80 pm, such as 100 nm to 60 pm, for example 100 nm to 40 pm, such as 100 nm to 20 pm, e.g. 100 nm to 10 pm. The thickness of the conductive layer may depend on the degree of hydrogen diffusion protection needed.

According to one example, the coating may be provided intermittently in the axial direction of the fibre optic cable 19. The dielectric tube 19b is hence provided with regions of coating that are disconnected in the axial direction, i.e. the regions do not form a single continuous coating along the fibre optic cable 19. Alternatively, in case the conductive layer is provided continuously along the length of the dielectric tube 19b, a semi-conductive polymeric layer may be provided on top of the conductive layer.

In case a stronger hydrogen protection barrier is required, either on the optical fibres or on the dielectric tube, the impact of any conductive layer should be minimised. In addition to, or as an alternative to the above considerations concerning thinness or intermittent provision of the conductive layer, the position of the fibre optic cable could be varied inside the submarine power cable. For example, the fibre optic cable could cross one of the power cores. In particular, the fibre optic cable could extend in parallel to the power cores 3a and 3c for a defined length, then extend in parallel to power cores 3a and 3b for the defined length, and then extend in parallel to power cores 3b and 3c for the defined length. This pattern may be repeated throughout the length of the submarine power cable. In order to accomplish the above, a space should be provided for the fibre optic cable to pass between the three different interstices. This could be accomplished by placing a central element in the submarine power cable, which separates the power cores, or by allowing the fibre optic cable to pass a power core on the outside between the power core and the outer serving.

According to one example, the optical fibres 19a may be selected to be less affected by hydrogen ageing. Examples of optical fibres of this type are G.652.C/D and G.654.

According to one example, the dielectric tube 19 may comprise a plurality of solid dielectric layers. The dielectric material of at least two solid dielectric layers may be different from each other. Each solid dielectric layer may for example be different from the dielectric material of the other solid dielectric layer(s). Alternatively, all solid dielectric layers may be made of the same dielectric material.

According to one example, the fibre optic cable 19 may comprise a hydrogen scavenging material. The hydrogen scavenging material may be a gel, and the dielectric tube 19b and/or the optical fibres 19a may be coated with the gel. According to one example, the optical fibres 19a may be coated with a hydrogen protecting layer. The hydrogen protecting layer may for example comprise an electrically conducting material. An example of an electrically conducting material for this purpose is amorphous carbon.

Fig. 2 shows an example of the fibre optic cable 19. The exemplified fibre optic cable 19 comprises a gel 19c which is contained inside the dielectric tube 19. The gel 19c is a hydrogen-absorbing or hydrogen-scavenging

gel/compound. The gel 19c hence has hydrogen absorption properties or hydrogen-scavenging properties. According to one variation of the gel 19c, the gel 19c has a hydrogen-absorption capability which is adapted to the specific material of which the dielectric tube 19b is made. In particular, the gel 19c may be composed such that it absorbs essentially all of hydrogen emitted by the dielectric tube 19b.

The gel 19c may according to one variation be composed such that it is able to absorb not only hydrogen emitted by the dielectric tube but also externally generated hydrogen which diffuses into the dielectric tube 19b. For example, the amount of gel 19c inside the dielectric tube 19b may be increased, which thereby increases the hydrogen absorption capability thereof. By designing a dielectric tube 19b with a larger inner diameter, the inner volume of the dielectric tube 19b will increase quadratically while the envelope surface of the dielectric tube increases linearly. For example, a doubling of the diameter doubles the envelope surface while the volume is quadrupled. More gel 19c may thereby be provided in the dielectric tube 19b, whereby the absorption capability is increased. In particular, the absorption capability will increase more than the additional hydrogen emitted by the larger envelope surface of the dielectric tube 19b. The diameter of the dielectric tube 19b may hence be configured/designed such that the internal volume of the dielectric tube is large enough that the gel 19c can absorb more hydrogen than that emitted by the dielectric tube 19b. The gel 19c may have water-blocking properties. The optical fibres 19a are embedded in the gel 19c. The gel 19c may fill the majority of the available space left by the optical fibres 19a in the dielectric tube 19b, for example at least 85% of the space, such as at least 90% of the space. The gel 19c may be in direct contact with the inner surface of the dielectric tube 19b. Fig. 3 shows another example of a submarine power cable comprising a fibre optic cable. The submarine power cable 1’ comprises only one power core, namely first power core 3a’. The first power core 3a’ includes a conductor 5a’ for conducting current, and an insulation system 7a’ surrounding the conductor 5a’. The insulation system 7a’ comprises an inner semiconducting layer 9a’, a solid insulation layer 11a’ arranged radially outside of the inner semiconducting layer 9a’, and an outer semiconducting layer 13a’ arranged radially outside the solid insulation layer 11a’.

The exemplified submarine power cable 1’ further includes an armour layer 17 comprising a plurality of helically wound wires, surrounding the first power cores 3 a’. The submarine power cable T also comprises a fibre optic cable 19. The fibre optic cable 19 is identical to the one described with reference to Fig. 1. In the present example, the fibre optic cable 19 forms part of the armour layer 17. To this end, dielectric tube 19b of the fibre optic cable 19 forms one of the armour wires. The remaining armour wires are typically made of metal. The exemplified fibre optic cable 19 extends along the first power core 3a’, in particular in a helical manner concurrently with the remaining armour wires. The submarine power cable 1’ furthermore includes an outer serving 15 surrounding the armour layer 17. The submarine power cable 1’ could alternatively be provided without the armour layer 17, but still comprise the fibre optic cable 19. In this case, the fibre optic cable 19 extends along the first power core 3a’, radially inside of the outer serving 15.

In use of the submarine power cable 1, 1’, wavelengths that are most affected by hydrogen aging may be avoided in those examples where the fibre optic cable 19 is not provided with any hydrogen diffusion protection.

Alternatively, or additionally, the communication/control/sensing-system used with the fibre optic cable 19 may be designed to allow for increased attenuation caused by hydrogen ageing. The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.