Method for improving the electrical conductivity of submarine power transmission cables and cable introduction into an offshore structure

Description

[0001] The invention relates to a method for improving the electrical conductivity of submarine cables for power transmission in the area of ​​hotspots of the submarine cables in the area of ​​cable entries on offshore structures and to a corresponding cable entry of an offshore structure.

[0002] The maximum cable current that can be transmitted using submarine power cables is limited by the maximum surface temperature of the conductor. Submarine cables must generally not exceed a specified absolute conductor temperature during operation. The absolute conductor temperature is normally limited to 90°C. This is the sum of the ambient temperature and the so-called conductor overtemperature. Offshore structures with electrical equipment for power generation or transformation, known as "substations," are located above the waterline. The submarine cables are inserted through so-called J-tubes in an offshore structure, for example, in the form of a tower structure with a monopile foundation or in the form of a platform with a jacket foundation. For this purpose, cable entries in the form of guide bushings, cable bushings, or similar devices are provided on the offshore structure.

[0003] J-tubes are tubular devices used to protect submarine cables against external mechanical influences.

[0004] From the article "Thermal Rating of J-tubes using Finite Element Analysis Techniques" by Chippendale Richard (June 25, 2015, XP055916364), a 3D finite element analysis for the evaluation of offshore cables is described. The thermal profile of the cable is examined.

[0005] US 6,225,553 B1 relates to means and methods for increasing the rated power of an offshore high-voltage power cable, which includes passing the cable from the sea to a platform through a bend strain relief (BSR).

[0006] WO 2012 / 167 015 A2 is directed to hybrid systems and methods for generating energy from wind and wave power and in particular to designs of offshore wind wave power plants.

[0007] In the area where the cable enters the offshore structure, submarine cables are typically exposed to ambient temperatures that are higher than those below the waterline. Even at ambient temperatures above 20 °C, measures may need to be taken to limit the conductor temperature, for example, by reducing the electrical power transported through the submarine cable.

[0008] Transition areas of submarine cables where they are exposed to increased temperatures are generally and hereinafter referred to as hotspots.

[0009] Such hotspots lead to a reduction in the maximum permissible current in the conductor in question. Such a reduction is fundamentally undesirable. The invention therefore aims to address this.

[0010] The object is achieved by a method according to claim 1 and by a cable entry of an offshore structure according to claim 2.

[0011] According to one aspect of the invention, a method having the features of claim 1 is provided.

[0012] An offshore structure within the meaning of the present invention can be, for example, a tower structure for a wind turbine or a platform with an electrical installation, for example in the form of a so-called substation. The cable entry can be provided, for example, on a monopile of a tower structure; alternatively, the cable entry can be provided on a transition piece mounted on a jacket foundation or a monopile. The type of foundation and the type of offshore structure are not critical for the present invention.

[0013] As already mentioned above, in the context of the present invention, a hotspot is understood to be an area of ​​a submarine cable which is exposed to an increased ambient temperature and which is particularly critical with regard to the maximum permissible conductor temperature.

[0014] In a variant of the method not included in the scope of protection, passive water cooling is provided in the area of ​​a cable entry into the offshore structure, in particular in a cable bushing of a device for protecting the submarine cable against external mechanical influences. This device is also referred to in technical jargon as a so-called J-tube. It is a tube section of a guide tube for the submarine cable, which is bent, for example, in a J-shape in order to give the submarine cable a predetermined maximum bending radius. Passive water cooling in the area of ​​a cable entry can be ensured, for example, by a guide plug for at least one submarine cable being designed to be permeable to water in the area of ​​the J-tube entry hole. This guide plug can, for example, have holes that allow seawater or saltwater to circulate in the area of ​​the cable entry.Particularly in the area of ​​cable penetrations, clamping the submarine cable would otherwise impede heat dissipation through the corresponding cable guides.

[0015] In a further variant of the method, which is not included in the scope of protection, active cooling of the submarine cable can be provided by seawater circulation through a device for protecting the submarine cable against external mechanical influences. For example, according to the invention, seawater can be actively circulated through the J-tube. If the offshore structure accommodates a so-called substation (transformer station), the substation's cooling system can be used to actively cool the hotspot.

[0016] In another variant of the method, which is not included in the scope of protection, heat dissipation of the submarine cable is achieved in the area of ​​the cable entry by establishing a mechanical heat-conducting connection between the submarine cable and a foundation of the offshore structure as a heat sink.

[0017] The method according to the invention can be applied to both a single submarine cable and multiple submarine cables on an offshore structure.

[0018] According to the invention, the mechanical thermal connection between the submarine cable and the heat sink is achieved by means of a sleeve made of a thermally conductive material that surrounds the submarine cable in the area of ​​the hotspot and is in direct thermally conductive contact with the submarine cable. The heat can then be dissipated via a thermally conductive element, for example, in the form of a rope, cable, or rod, which is connected to the heat sink, for example, to the foundation of the offshore structure.

[0019] According to a further aspect not falling within the scope of protection, it can be provided that heat exchange with seawater is effected in the region of the cable entry using at least one heat exchanger or at least one heat exchanger body. Such a heat exchanger can, for example, be designed as a heat-conducting body with an enlarged surface. The heat-conducting body can be in direct heat-conducting contact with the submarine cable. The submarine cable preferably comprises one or more electrical conductors and at least one insulation enclosing the conductors. When it is mentioned above that the submarine cable is in heat-conducting contact with a heat exchanger body or a sleeve, this means contact between the insulation of the submarine cable and the sleeve or the heat exchanger body.

[0020] The process features described above can be carried out individually or in any combination with each other.

[0021] The invention further comprises an offshore structure with a cable entry having the features of claim 2.

[0022] A cable entry variant not included in the scope of protection is characterized by the fact that the cable entry has at least one guide plug that can be flooded with seawater. This type of cable entry is also referred to as a J-tube entry hole.

[0023] In a variant of the cable entry not falling within the scope of protection, it is provided that the submarine cable has at least one connecting and transition sleeve, by means of which one or more conductors of the submarine cable with a first smaller cross-section and / or made of a material with a first lower conductivity are connected to a section of one or more conductors with a second larger cross-section and / or made of a material with a second greater conductivity.

[0024] For example, it may be provided that the cross-section of the conductors is increased in certain sections, resulting in a lower conductor surface temperature and better conductivity. Alternatively or additionally, it may be provided that the submarine cable is equipped with a material with higher conductivity in certain sections, for example in the hotspot area, so that the electrical resistance of the submarine cable is reduced in the hotspot area and thus the generated heat loss is also lower. For example, the submarine cable may be equipped with a copper cable in the hotspot area, whereas the section of the submarine cable located below sea level may be designed as an aluminum cable.

[0025] Alternatively, a connecting and transition joint can be provided in the area of ​​a hotspot. Within the connecting and transition joint, the conductor cross-section can be doubled, for example, by arranging the conductors not in a butt-joint arrangement in electrically conductive contact, but rather by arranging the conductors one on top of the other within the connecting and transition joint, so that the cross-section is at least doubled. Such connecting and transition joints are also referred to as "factory joints" in technical jargon.

[0026] Furthermore, in an embodiment not covered by the scope of protection, it is possible, for example, to provide a submarine cable section in the area of ​​the hotspot that has a higher conductivity or a larger conductor cross-section. This submarine cable section can be arranged between two connecting and transition joints.

[0027] In the cable entry variant according to the invention, at least one sleeve is provided that thermally conductively surrounds the submarine cable and is connected to the foundation of the offshore structure via a heat conductor. The sleeve itself can have an enlarged surface area and thus be designed as a heat exchanger.

[0028] Alternatively, it can be provided that the connecting and transition sleeve itself takes on the function of a heat-conducting sleeve and that it is connected via a heat conductor to a heat sink or to the foundation of the offshore structure as a heat sink.

[0029] According to another aspect not falling within the scope of protection, it may be provided that the device for protecting the submarine cable against external mechanical influences is connected to a preferably open seawater cooling circuit.

[0030] The above-described features of the cable entry according to the invention can be provided individually or in combination with one another.

[0031] An embodiment not falling within the scope of protection is explained below with reference to the accompanying drawings. They show: Figure 1 a schematic representation of a connecting and transition sleeve of a submarine cable of a cable entry according to a first embodiment not belonging to the scope of protection, Figure 2 a schematic representation of a submarine cable according to a second embodiment of the cable entry not belonging to the scope of protection and Figure 3 a schematic representation of a submarine cable according to a second embodiment of the cable entry.

[0032] The figures only show the connecting and transition joint 1 of the submarine cable 2. In the illustrated embodiments, the connecting and transition joint 1 is provided between two submarine cable sections 3 of the submarine cable 2. The connecting and transition joint 1 is located in the area of ​​a cable entry (not shown) of an offshore structure.

[0033] The submarine cable 2 comprises a plurality of conductors 4, which in the case of the embodiment according to Figure 1are joined together in a butt-like manner to create an electrical connection between the two submarine cable sections 3. The transition and connecting sleeve 1 comprises, from the outside inward, a sheath 5 made of cross-linked polyethylene, a first sleeve 6 made of silicon carbide, and a second sleeve 7 made of copper. The conductors 4 are preferably made of aluminum. By means of the second sleeve 7, which directly encloses the butt-joined conductors 4, both the electrical conductivity and the thermal conductivity of the conductors 4 formed by the transition and connecting sleeve 1 are improved.

[0034] In the embodiment according to Figure 1 the ladders 4 are provided with individual butt points arranged at different locations, so that the individual butt points are arranged offset from one another in the manner of a wall connection.

[0035] The embodiment according to Figure 3corresponds to that according to Figure 1 , with the difference that the individual butt points of the conductors 4 are all arranged in one cross-sectional plane. Otherwise, identical elements are provided with the same reference numerals. The embodiment according to Figure 2 differs from those according to the Figures 1 and 3 by twisting the conductors 4 with offset individual contact points, which increases the electrical conductivity. Otherwise, the embodiment according to Figure 2 those according to the Figures 1 and 3 . Identical parts are provided with the same reference symbols. List of reference symbols

[0036] 1Connection and transition joint 2 Submarine cable 3Submarine cable sections 4Conductor 5Sheath 6First sleeve, 7Second sleeve

Claims

1. Method for improving the electrical conductivity of submarine cables for transmission of energy in the region of hotspots of the submarine cables in the region of cable entries at offshore structures, wherein the method comprises limiting the maximum surface temperature of one or more conductors of the submarine cable by connection of a heat sink in the region of a hotspot, characterized in that, in the region of the cable entry, removal of heat from the submarine cable is brought about in that a mechanical thermally conductive connection of the submarine cable to a foundation of the offshore structure is produced, and in that the mechanical thermally conductive connection is brought about by means of a sleeve composed of a thermally conductive material that engages around the submarine cable and that is in direct thermally conductive contact with the submarine cable.

2. Offshore structure with a cable entry comprising at least one submarine cable (2), a cable leadthrough into the structure and a device for protecting the submarine cable (2) against external mechanical effects, with a mechanical thermally conductive connection to a foundation of the offshore structure, as a remotely arranged heat sink, as a means for limiting the maximum surface temperature of one or more conductors (4) of the submarine cable, characterized by at least one sleeve composed of a thermally conductive material that surrounds the submarine cable (2) and that is in direct thermally conductive contact with the submarine cable (2) and that is connected to the foundation of the offshore structure via a heat conductor.