BACKGROUND

The invention relates to a cooling system and method for cooling a power cable.

Power cables, in particular electric power cables, are used to transport electric energy between an offshore location such as a wind farm and an onshore land station. At the landfall location a casing is drilled at a certain depth below the sea defense to protect it from wave impact and seabed erosion. The cable is guided through the casing from the land station. Due to the deep burial of the electric export cable and the enclosure by the casing, the cable locally has a relatively high insulation. As a result the cable cannot dissipate as much heat at this location compared to the remainder of the cable and therefore limits the transport capacity of the entire cable.

In order to remove this bottleneck the landfall section of the cable is typically replaced by a section with a larger cross section or a material with higher conductivity. During the replacement, the cable production process is interrupted and the cores of the cable are carefully joined at the replacement section. As a result, the production of cables with varying sections is complex, time consuming and therefore expensive. In addition, the joints form weak spots in the cable which are prone to failure.

Alternatively, it has been known to provide the power cables with one or more cooling channels, integrated into the cable design, as for example disclosed in WO 2013/125962 A1.

SUMMARY OF THE INVENTION

In WO 2013/125962 Al, the cooling channels are integrated into the cable design. The cooling channels extend along the entire cable length, thus requiring considerable pressure to obtain the forced flow of cooling medium through the cable. The cooling channels are also provided in sections of the cable that do not necessarily require cooling. When water is used as a cooling medium, WO 2013/125962 Al proposes that the water is simply discharged into the sea. The cable may also be provided with laterally extending drain channels to discharge the cooling medium in to the sea and slits or openings to flood the internal channels of the cable with water to further dissipate heat. When glycol is used, WO 2013/125962 Al proposes to use a closed circuit with a return channel, also integrated in the cable design, and a crossover between the supply channel and the return channel at a predetermined distance into the sea.

In all of the known cable designs, the cable is relatively complex and requires interruption of the cable production process and joining of cable sections of different design. The interruptions or joints cause weak spots in the cable that may considerably reduce its lifetime .

DE 10 2015 101 076 Al discloses device for cooling a power cable. The device comprises an outer tube and an inner tube defining an outer volume and an inner volume, respectively. At one end of the outer tube, the device is provided with an end part that seals the outer volume with respect to the inner tube. An end part of the inner tube extends beyond said seal and is no longer surrounded by the outer volume. External conduits are provided to connect the inner volume and the outer volume to a pump unit. Alternatively, when conduits may be short- circuited to feed the cooling medium from the inner volume back into the outer volume. The known device according to DE 10 2015 101 076 A1 does not appear to be used in subsea conditions at a landfall location. A disadvantage of the known device is that the external conduits are easily damaged by the harsh subsea conditions, i.e. due to the constant motion of the sea and the effect of salt on the connections. This problem is even bigger when the known device is at least partially buried at the landfall location. These systems have to be durable up to twenty- five years. The known external conduits will fail well before that .

The known device also appears to be inherently unsuitable to be drawn through a horizontal drill hole, as is typical practice when installing electrical power cables between offshore and landfall locations. The external conduits of the known device would thus have to be installed after pulling the cable through the horizontal drill hole, which makes installation unnecessarily complex.

It is an object of the present invention to provide a cooling system and a method for cooling a power cable, wherein at least one of the aforementioned problems is addressed.

According to a first aspect, the invention provides a cooling system for cooling a continuous power cable, wherein the cooling system comprises a first sleeve that extends in a longitudinal direction along a longitudinal axis for receiving an intermediate section of the continuous power cable at or along said longitudinal axis, wherein the first sleeve defines a first volume that in use extends around the intermediate section, wherein the cooling system is provided with a second sleeve that defines a second volume separate from the first volume, wherein the first volume and the second volume are arranged for receiving a cooling medium for cooling the intermediate section, wherein the cooling system has a first end and a second end opposite to the first end in the longitudinal direction, wherein the cooling system is provided with a first end sealing member at the first end, wherein the first end sealing member comprises a first sealing part and a second sealing part for sealing the first volume and the second volume, respectively, at the first end, wherein the cooling system further comprises a connector that connects the first volume and the second volume in fluid communication at or near the first end, wherein the second sleeve is arranged to surround the first sleeve about the longitudinal axis, wherein the connector is formed by an aperture in the first sleeve at or near the first end where the second sleeve surrounds the first sleeve, wherein the aperture connects the first volume and the second volume in fluid communication.

By providing the cooling system around the intermediate section of the continuous power cable, the power cable can be effectively cooled at said intermediate section, without requiring modifications to the continuous power cable itself. As a result, the continuous power cable can be uniform throughout its length. In particular, the continuous power cable can have a uniform cross section throughout, in dimensions and/or in material. Moreover, because the first volume is connected to the second volume at the first end, the first end can effectively return the cooling medium towards the second end. This can be particularly useful when the first end is hard to reach, e.g. when the first end is buried in the seabed at a landfall location. The connection between both volumes can be made by a connector that extends between both volumes. Hence, said connector can be kept on the inside of the cooling system where it is not exposed to harsh subsea conditions. Therefore, the lifetime of the cooling system can be increased significantly.

In a preferred embodiment the first volume and the second volume are arranged to be connected in fluid communication to a pump at or near the second end of the cooling system for together with the connector at the first end forming a closed recirculation circuit for the cooling medium. Hence, the cooling medium can be supplied from and returned to the second end. Again, this can be particularly useful when the first end is hard to reach, e.g. when the first end is buried in the seabed at a landfall location. Moreover, by forming a closed recirculation circuit at the intermediate section only, between the first end and the second end, the length of said recirculation circuit can be kept relatively short compared to the total length of the continuous power cable.

In a further embodiment the intermediate section is a landfall section that has a seaside and a landside in the longitudinal direction, wherein the first end is the seaside and the second end is the landside. Hence, the pump can be conveniently arranged at the land side while the cooling medium can be returned via the connector at the first end.

In another embodiment the continuous power cable has a sea section extending outside of the cooling system from the first end, wherein the first sealing part comprises a first opening for passage of the continuous power cable through the first sealing part in the longitudinal direction at the transition from the intermediate section to the sea section, wherein the first sealing part is arranged for sealing the first volume in cooperation with the power cable at the first opening. Hence, the continuous power cable is allowed to pass through and out of the cooling system at the first end while the first volume can be effectively sealed, and thus terminated, at the first end.

In a preferred embodiment thereof the first sealing part comprises a first flange that is connected to the first sleeve and that is arranged to sealingly abut the continuous power cable at the first opening. The first flange can thus effectively seal the first volume in cooperation with the continuous power cable.

In a further embodiment thereof the first opening is concentrically positioned relative to the first sleeve. It can thus be ensured that the continuous power cable exit the first sleeve concentrically, at least at the first end of the cooling system.

In another embodiment the cooling system comprises spacers for spacing the intermediate section apart from the first sleeve to form the first volume. By spacing the intermediate section from the first sleeve, the space between the continuous power cable and the first sleeve can be set, thus defining the first volume. In particular, the spacers can provide a uniform first volume in the longitudinal direction along the continuous power cable. Preferably, the spacers are arranged for keeping the intermediate section concentric or substantially concentric with respect to the first sleeve. In particular, the spacers are arranged to keep the continuous power cable away from the first sleeve. In this way, heat spots can be prevented .

In another embodiment the second sleeve is arranged to surround the first sleeve about the longitudinal axis. Hence, the second volume can be formed between the first sleeve and the second sleeve.

In a preferred embodiment thereof the second sealing part comprises a second opening for passage of the first sleeve through the second sealing part in the longitudinal direction. The first sleeve can thus extend through the second sealing part .

In a further embodiment thereof the second sealing part comprises a second flange that is connected to the second sleeve and that is arranged to sealingly abut the first sleeve at the second opening. The second flange can thus effectively seal the second volume in cooperation with the first sleeve.

In a further embodiment thereof the second flange is connected to both the first sleeve and the second sleeve. The first sleeve, the second sleeve and the second flange can thus form an interconnected and/or rigid assembly, like an end cap, that can be mounted more easily to the continuous power cable.

In a further embodiment thereof the second opening is non-concent rically positioned relative to the second sleeve. The second opening may for example be located at or near the bottom of the second sleeve. Consequently, the first sleeve may be positioned off- center, e.g. at the bottom of the second sleeve. This means that no spacers are required between the first sleeve and the second sleeve. The first sleeve can simply rest on the bottom of the first sleeve, with the second volume being formed above the first sleeve.

In a further embodiment the cooling system is provided with a second end sealing member at the second end, wherein the second end sealing member comprises a third sealing part and a fourth sealing part for sealing the first volume and the second volume, respectively, at the second end, wherein the cooling system further comprises an inlet that is connected in fluid communication with one of the first volume and the second volume at or near the second end for connection to the output of a pump and an outlet that is connected in fluid communication with the other of the first volume and the second volume at or near the second end for connection to the input of the pump. Hence, a closed recirculation circuit can be formed that extends only along the intermediate section between the first end and the second of the cooling system.

In a preferred embodiment thereof the continuous power cable has a land section extending outside of the cooling system from the second end, wherein the third sealing part comprises a third opening for passage of the continuous power cable through the third sealing part in the longitudinal direction at the transition from the intermediate section to the land section, wherein the third sealing part is arranged for sealing the first volume in cooperation with the power cable at the third opening. Hence, the continuous power cable is allowed to pass through and out of the cooling system at the second end while the first volume can be effectively sealed, and thus terminated, at the second end.

In a further embodiment thereof the third sealing part comprises a third flange that is connected to the first sleeve and that is arranged to sealingly abut the continuous power cable at the third opening. The third flange can thus effectively seal the first volume in cooperation with the continuous power cable.

In a further embodiment thereof the fourth sealing part comprises a fourth opening for passage of the first sleeve through the fourth sealing part in the longitudinal direction. The first sleeve can thus extend through the fourth sealing part.

In a further embodiment thereof the fourth sealing part comprises a fourth flange that is connected to the second sleeve and that is arranged to sealingly abut the first sleeve at the fourth opening. The fourth flange can thus effectively seal the second volume in cooperation with the first sleeve.

In a further embodiment thereof the first sleeve protrudes outside of the second sleeve at the second end over a second protrusion distance. Hence, the first sleeve can be exposed at the second end, e.g. for connection to the pump .

In a further embodiment thereof the third sealing part is spaced apart from the fourth sealing part over the second protrusion distance. Hence, the first volume and the second volume can be sealed at spaced apart positions.

In a further embodiment thereof one of the inlet and the outlet connects in fluid communication to the first volume where the first sleeve protrudes outside of the second sleeve at the second end. Hence, the connection of the first volume to one of the inlet and the outlet can be made outside of the second volume.

According to a second aspect, the invention provides a method for a continuous power cable with the use of the cooling system according to any one of the aforementioned embodiments, wherein the method comprises the steps of : receiving the intermediate section of the continuous power cable in the first sleeve; forming the first volume that extends around the intermediate section; sealing the first volume at the first end with the first sealing part; arranging the second sleeve outside of the first sleeve; forming the second volume separate from the first volume; sealing the second volume at the first end with the second sealing part; connecting the first volume in fluid communication to the second volume; and circulating the cooling medium through the first volume and the second volume to cool the intermediate section.

The method and its embodiments relate to the practical implementation of the cooling system according to any one of the aforementioned embodiments and thus have the same technical advantages, which will not be repeated hereafter .

Preferably, the method further comprises the step of connecting the first volume and the second volume in fluid communication to a pump at or near the second end of the cooling system for together with the connector at the first end forming a closed recirculation circuit for the cooling medium.

In another preferred embodiment of the method, the continuous power cable comprises a sea section and a land section extending outside of the cooling system from the first end and the second end, respectively, wherein the first volume and the second volume are sealed at the intermediate section with respect to the sea section and the land section.

Preferably, the continuous power cable has a uniform cross section in the sea section, the intermediate section and the land section. In another embodiment the cooling medium is a fluid, in particular a fluid with a freeze-point of minus twenty degrees Celsius or less, more in particular a glycol-water mixture.

The various aspects and features described and shown in the specification can be applied, individually, wherever possible. These individual aspects, in particular the aspects and features described in the attached dependent claims, can be made subject of divisional patent applications .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be elucidated on the basis of an exemplary embodiment shown in the attached schematic drawings, in which:

figure 1 shows a cross section of a landfall location and a cooling system according to a first embodiment of the invention for cooling an intermediate section of a power cable at said landfall location;

figure 2 shows a side view of the cooling system in more detail;

figure 3 shows a cross section of the cooling system according to figure 2;

figure 4 shows a cross section of the cooling system according to line IV - IV in figure 2;

figure 5 shows a detail of the cooling system according to the circle V in figure 3; and

figure 6 shows a side view of an alternative cooling system according to a second embodiment of the invention for cooling a power cable at a substation; and figure 7 shows an isometric view of a detail of the alternative cooling system according to the circle VII in figure 6. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows cooling system 1 according to a first exemplary embodiment of the invention. The cooling system 1 is arranged around an intermediate section 90 of a continuous power cable 9, in particular an electric power cable. In this particular example, the intermediate section 90 is a landfall section of the continuous power cable 9 at a landfall location. The landfall location is the location where the sea S meets the land T. In particular, the landfall location comprises a shoreline between the sea S and the land T. The intermediate section 90 of the continuous power cable 9 has a length that is preferably in the range of a few hundred meters up to or exceeding one thousand meters. The continuous power cable 9 further has a sea section 91 that is arranged on or in the seabed in the sea S and a land section 92 that surfaces at or inland of the shoreline.

The continuous power cable 9 as shown in figure 1 has a uniform cross section, both in material and dimensions. Hence, the continuous power cable 9 can be made from a single length, e.g. without joints. In this exemplary embodiment, the continuous power cable 9 is an offshore copper cable for transporting electricity from an offshore windfarm to a land-based station. Alternatively, other suitable electrically conducting materials may be used. Such an offshore copper cable typically has a diameter of approximately at least two hundred and fifty millimeters may comprise one or more cores. Optionally, the cable may comprise additional conductors, i.e. one or more fiber optic cables. Such a cable may reach temperatures of sixty to seventy degrees Celsius or even higher, especially when buried deep in the ground at the landfall location.

As shown in figures 2, 3 and 4, the cooling system 1 comprises a first sleeve 11 that extends in a longitudinal direction L along a longitudinal axis A. The first sleeve 11 is arranged for receiving the intermediate section 90 at or parallel to said longitudinal axis A. The first sleeve 11 defines a first volume VI that in use is partially occupied by and/or extends around the intermediate section 90. As best seen in figure 4, the cooling system 1 comprises a plurality of spacers 8 for spacing the intermediate section 90 from the first sleeve 11. In particular, the plurality of spacers 8 are arranged for keeping the continuous power cable 9 in a concentric position relative to the first sleeve 11. Hence, in this example the first volume VI is an annularly shaped volume extending completely around the continuous power cable 9. This prevents direct contact between the relatively hot continuous power cable 9 and the first sleeve 11, which contact would otherwise result in heat spots and damage to the first sleeve 11 and the continuous power cable 9 itself .

The cooling system 1 is further provided with a second sleeve 12 that defines a second volume V2 separate from the first volume VI. In this exemplary embodiment, the second sleeve 12 extends around the first sleeve 11 about the longitudinal axis A and thus forms a second volume V2 that is located outside of the first volume VI.

In this example, both the first sleeve 11 and the second sleeve 12 are constructed from the same material, e.g. HOPE. Alternatively, the first sleeve 11 may comprise additional thermal insulation, e.g. by increasing the thickness of the material or by adding an additional, thermally insulating layer. Hence, heat transfer from the first volume VI to the second volume V2 can be reduced.

The first volume VI and the second volume V2 are arranged for receiving a cooling medium M for cooling the intermediate section 90 in a manner that will be described hereafter in more detail. In this example, the cooling medium M is a fluid, in particular a fluid with a freeze- point of minus twenty degrees Celsius or less, more in particular a glycol-water mixture.

As shown in figures 2 and 3, the cooling system 1 has a first end El and a second end E2 opposite to the first end El in the longitudinal direction L. Note that, for the purpose of this invention, the first end El and the second E2 are not necessarily limited to the end surfaces of the cooling system 1. Instead, the first end El and the second end E2 indicate an area at or near said end surface of the cooling system 1, e.g. an area of a few meters at respective ends El, E2 of the cooling system 1. The first end El faces towards the seaside while the second end E2 faces to towards the landside. In this exemplary embodiment, a pump P is provided on or at the land. This may be a hydraulic, electric or mechanical pump. The second end E2 of the cooling system 1 may conveniently be connected to the pump P for recirculation of the cooling medium M while the first end El is arranged for returning the flow from one of the first volume VI and the second volume V2 to the other of the first volume VI and the second volume V2 in a manner that will be described hereafter in more detail. The pump P may comprise one or more pumps, e.g. centrifugal pumps, to pump the cooling medium M into and draw the cooling medium M out of the respective volumes VI, V2. A cooling unit (not shown) may be provided, e.g. with air coolers, to cool the cooling medium M before recirculation.

As shown in detail in figure 5, the cooling system 1 is provided with a first end sealing member 3 at the first end El. The first end sealing member 3 comprises a first sealing part 31 and a second sealing part 32 for sealing the first volume VI and the second volume V2, respectively, at the first end El.

The first sealing part 31 comprises a first opening 33 for passage of the continuous power cable 9 through the first sealing part 31 in the longitudinal direction L at the transition from the intermediate section 90 to the sea section 91. The first sealing part 31 is arranged for sealing the first volume VI in cooperation with the continuous power cable 9 at the first opening 33. In particular, the first sealing part 31 comprises a first flange 34 that is connected to the first sleeve 11 and that is arranged to sealingly abut the continuous power cable 9 at the first opening 33. The first flange 34 may be a dividable flange that can be mounted in sealing abutment around the continuous power cable 9 once said cable 9 is in place within the first sleeve 11. In this exemplary embodiment, the first opening 33 is concentrically positioned relative to the first sleeve 11. Hence, the first opening 33 can concentrically guide the continuous power cable 9 through the first sealing part 31. In combination with the aforementioned spacers 8, the continuous power cable 9 can be guided concentrically or substantially concentrically through the first end El of the cooling system 1.

The second sealing part 32 comprises a second opening 35 for passage of the first sleeve 11 through the second sealing part 32 in the longitudinal direction L. Like, the first sealing part 31, the second sealing part 32 comprises a second flange 36. This second flange 36 is connected to the second sleeve 12 and is arranged to sealingly abut the first sleeve 11 at the second opening 35. Preferably, the second flange 36 is connected to both the first sleeve 11 and the second sleeve 12, so as to form a rigid assembly. Unlike the first opening 33, the second opening 35 is non-concentrically positioned or positioned off-center relative to the second sleeve 12. Hence, the first sleeve 11 can be fitted through the second sealing part 32 in an off-center position, e.g. at the bottom of the second sleeve 12. Consequently, no spacers are required as the first sleeve 11 is allowed to simply rest on the bottom of the second sleeve 12, with the second volume V2 being formed in the space between the first sleeve 11 and the second sleeve 12 above the first sleeve 11.

As shown in figure 2, the first sleeve 11 protrudes outside of the second sleeve 12 at the first end El over a first protrusion distance Dl . As a result, the first sealing part 31 is spaced apart from the second sealing part 32 over the same first protrusion distance Dl. Hence, the first sleeve 11 is at least partially exposed, i.e. not surrounded by the second sleeve 12, at the first end El. The first sleeve 11 and the first sealing part 31 are therefore exposed and easily accessible, e.g. for mounting or connecting purposes.

As shown in figure 5, the cooling system 1 further comprises a connector 6 for connecting the first volume VI and the second volume V2 in fluid communication to each other at or near the first end El. Because of the connector 6, the first end El effectively becomes a return cap or recirculation cap for the cooling medium M. The cooling medium M may be pumped into the first volume VI at the second end E2, cools the intermediate section 90 and then flows through the connector 6 at the first end El into the second volume V2 before it returns to the second end E2 through the second volume V2.

In this exemplary embodiment, the connector 6 is formed by at least one aperture 6 in the first sleeve 11 at or near the first end El where the second sleeve 12 surrounds the first sleeve 11. At that location, the first sleeve 11 is the only thing that separates the first volume VI from the second volume V2. Hence, providing the first sleeve 11 with the aperture 6 effectively creates a direct connection between the first volume VI and the second volume V2. The aperture 6 may be prefabricated or created in situ. More than one apertures 6 may be provided. Preferably, the aperture 6 is formed as a slob hole. Said aperture 6 connects the first volume VI and the second volume V2 in fluid communication. In this example, the aperture 6 is spaced apart from the second sealing part 32 in the longitudinal direction L, e.g. over a distance of at least ten centimeters. This prevents that the aperture 6 lies to close to the second sealing part 32, which could cause leaking. As shown in figures 2 and 3, the cooling system 1 is provided with a second end sealing member 4 at the second end E2. The second end sealing member 4 comprises a third sealing part 41 and a fourth sealing part 42 for sealing the first volume VI and the second volume V2, respectively, at the second end E2. The first sealing part 41 and the second sealing part 42 are substantially the same in construction as the first sealing part 31 and the second sealing 32, respectively, and will therefore be described only briefly.

The third sealing part 41 comprises a third opening 43 and a third flange 44 similar in configuration to the first opening 33 and the first flange 34, respectively, yet at the second end E2. Consequently, the third opening 34 is provided for passage of the continuous power cable 9 through the third sealing part 41 in the longitudinal direction L at the transition from the intermediate section 90 to the land section 92.

The fourth sealing part 42 comprises a fourth opening 45 for passage of the first sleeve 11 through the fourth sealing part 42 in the longitudinal direction L. The fourth sealing part 42 comprises a fourth opening 45 and a fourth flange 46 similar in configuration and function as the second opening 35 and the second flange 36, respectively, yet at the second end E2.

As shown figure 2, the first sleeve 11 protrudes through the fourth opening 45 to a position outside of the second sleeve 12 at the second end E2 over a second protrusion distance D2. As a result, the third sealing part 41 is spaced apart from the fourth sealing part 42 over the same second protrusion distance D2. Hence, the first sleeve 11 is at least partially exposed, i.e. not surrounded by the second sleeve 12, at the second end E2. The first sleeve 11 and the third sealing part 41 are therefore exposed and easily accessible, e.g. for mounting or connecting purposes .

The cooling system 1 further comprises an inlet 71 that is connected in fluid communication with the first volume VI at or near the second end E2 for connection to the output of the pump P and an outlet 72 that is connected in fluid communication with the second volume V2 at or near the second end E2 for connection to the input of the pump P. In particular, the inlet 71 connects in fluid communication to the first volume VI where the first sleeve 11 protrudes outside of the second sleeve 12 at the second end E2, i.e. at the first sleeve 11 within the second protrusion distance D2.

In the previously discussed embodiments, the sleeves 11, 12 extend continuous, i.e. without joints, from the respective sealing parts 31, 32 at the first end El to the respective sealing parts 33, 34 at the second end E2. It will however be apparent to one skilled in the art that the sleeves 11, 12 may alternatively be provided in two more sections. In particular, the flanges 34, 36, 44, 46 may be directly connected to or integral with a small section of the respective sleeves 34, 36, 44, 46. Consequently, the aperture 6 of the first embodiment of invention may be provided in a small sleeve section of the first sleeve 11 directly connected to or integral with the first flange 34. This allows for the main section of the first sleeve 11 to be completely continuous, i.e. with said aperture 6.

A method for cooling the intermediate section 90 of the continuous power cable 9 with the use of the aforementioned cooling system 1 will be described hereafter with reference to figures 1-5.

At the landfall location as shown in figure 1, a hole is drilled in the ground underneath the shoreline to accommodate the intermediate section 90 of the continuous power cable 9. For the purpose of this invention, the drill hole is larger than the diameter of the continuous power cable 9. In particular, the drill hole is large enough to accommodate the outer diameter of the second sleeve 12.

The first sleeve 11, the second sleeve 12, the first end sealing member 3 and the second end sealing member 4 can be assembled prior to insertion into the drill hole. Alternatively, the second sleeve 12 may already be inserted into the drill hole and the first sleeve 11 can be inserted in situ. The second sealing part 32 and the fourth sealing part 42 can already be mounted to the second sleeve 12 at the first end El and the second end E2, respectively, to provide guidance for the pulling in of the first sleeve

11. In particular, the second sealing part 32 and the fourth sealing part 42 can already be mounted to first sleeve 11 and the second sleeve 12, respectively. The first sleeve 11 is temporarily provided with a pulling means to pull the first sleeve 11 through the second sleeve 12. Once the first sleeve 11 is in place, the temporary pulling means may be removed. The second volume V2 between the first sleeve 11 and the second sleeve 12 is now already substantially sealed. Optionally, further sealing means, e.g. elastomeric material or glue, may be provided to increase the effectiveness of the sealing.

After the first sleeve 11 and the second sleeve 12 are assembled, the continuous power cable 9 can be pulled through the first sleeve 11 in the same manner as the first sleeve 11 was pulled through the second sleeve

12. The continuous power cable 9 may be provided with the aforementioned spacers 8 at regular intervals to space the cable 9 apart from the first sleeve 11. Optionally, a bell- mouth (not shown) may be temporarily mounted to the end El, E2 that receives the continuous power cable 9 to guide said cable 9 more easily into the opening 33, 43 of the respective sealing part 31, 41.

Figures 2 and 3 shows the cooling system 1 with the continuous power cable 9 already in place, meaning that the intermediate section 90 is inside the first sleeve 11 and the sea section 91 and the land section 92 are at opposite ends El, E2 of the cooling system 1 outside of said cooling system 1. Now, the first volume VI may be sealed with respect to the continuous power cable 9. This means that the first sealing part 31 and the third sealing part 41 are mounted to the first end El and the second end E2, respectively. More in particular, the first flange 34 and the third flange 44, which are preferably formed as dividable flanges, are mounted to the first sleeve 11 at the first end El and the second end E2, respectively, in sealing abutment with the continuous power cable 9. Optionally, further sealing means, e.g. elastomeric material or glue, may be provided to increase the effectiveness of the sealing.

With both volumes VI, V2 sealed, the inlet 71 and the outlet 72 at the second end E2 of the cooling system 1 may be connected in fluid communication to the pump P. The aperture 6, as shown in figure 5, already provides a direct connection between the first volume VI and the second volume V2 at the second end E2.

Thus, a recirculation circuit for the cooling medium M is provided that extends locally and/or exclusively along the landfall section between the first end El and the second end E2. The pump P may be activated to pump the cooling medium M through the inlet 71 into the first volume VI at the second end E2 to cool the continuous power cable 9. The cooling medium M then travels through the connector 6 at the first end El from the first volume VI into the second volume V2 and is returned through the outlet 72 at the second end E2 back to the pump P. The cooling medium M may be treated in the cooling unit (not shown) to cool the cooling medium M back to an acceptable cooling temperature before recirculation of the cooling medium M. The cooling unit may be integral to the pump or a separate unit.

Alternatively, the inlet 71 is connectable to the inlet of the pump P and the outlet 72 is connectable to the outlet of the pump P such that the flow of the cooling medium is reversed.

Preferably, the intermediate section 90 of the continuous power cable 9 is to be cooled such that its temperature remains below ninety degrees Celsius or another limit associated with the cable design.

Figures 6 and 7 show an alternative cooling system 101 according to a second embodiment of the invention for cooling a power cable 9 at a substation 108. Such a substation is typically used for offshore linking of power cables from various sources to various destinations, i.e. for connection of power cables originating from a windfarm to power cables that lead from the wind farm to the land. For this purpose, the substation may be provided with infrastructure, i.e. frameworks, connectors, housing, etc. to handle the network of the power cables.

As shown in figure 6, the substation 108 is supported on the seabed. One or more power cables 9 extend along said seabed towards the substation 108. The substation 108 comprises a top or a deck 180 extending above the water level and one or more tubes 181, 182, 183, 184 extending down from the deck 180 towards and/or up to the seabed for receiving the one or more power cables 9. Said one or more tubes 181, 182, 183, 184 may be so-called J-tubes or I-tubes. In this example, J-tubes are used because of the characterising deflection at the ends thereof to easily received and/or guide the power cables 9 along a bend from the seabed orientation into a more upright orientation towards the deck 180 of the substation 108.

The alternative cooling system 101 is provided with a pump assembly P that pumps water from the sea or from another water source up towards the deck 180. As best seen in figure 7, the alternative cooling system 101 further features one or more connection flanges 102 for connecting the pump assembly P to a respective one of the one or more tubes 181-184 at the deck 180. Preferably, the connection flange 102 is provided with one or more supply openings 121, 122, 123 distributed circumferentially around the respective tube 181-184 for directing the water supply by the pump assembly P into the respective tube 181-184 from different sides of the respective tube 181-184. Alternatively, when the respective tube 181-184 has already been installed and connected to the infrastructure on the substation 108, it may no longer be possible to install a dedicated connection flange 102 to the tube 181-184. Instead, a hole may be drilled into the respective tube 181-184 and the pump assembly.

When operational, the pump assembly P pumps water into the respective tube 181-184 via the connection flange 102 or the drilled hole and the water travels through the tube 181-184 towards the seabed to cool the power cable 9 extending therein. As the water is pumped into the respective tube 181-184 at a significant height above the water level, it may be convenient to restrict the outflow of water from the respective tube 181-184 at or near the seabed to ensure that a substantial part, preferably the entire height, of the respective tube 181-184 can be filled with water.

The alternative cooling system 101 may be combined with the aforementioned cooling system 1 according to the first embodiment of the invention to create a combined cooling system 1, 101 that cools both temperature critical areas of a power cable, i.e. both at the landfall section and the substation section of a power cable.

It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the scope of the present invention.