EP3724492A1

EP3724492A1 - Method for installing a subsea cable in an offshore wind farm

Info

Publication number: EP3724492A1
Application number: EP18814658.3A
Authority: EP
Inventors: Jan SELLNER; Antonio RICO RUBIO; Joannes BERQUE; Vincenzo Nava; Alberto Del Pozo Martín
Original assignee: Fundacion Tecnalia Research and Innovation
Current assignee: Fundacion Tecnalia Research and Innovation
Priority date: 2017-12-13
Filing date: 2018-12-12
Publication date: 2020-10-21
Also published as: EP3499517A1; CN111788386A; WO2019115653A1

Abstract

A method for installing a subsea cable (200, 500, 510, 520) in an offshore wind farm (100), the offshore wind farm (100) comprising a substation (101) and a plurality of wind turbines (102, 106-109), the method comprising: laying the subsea cable (200, 500, 510, 520) from a first location to a first wind turbine (102d, 106) of the plurality of wind turbines (102, 106-109) passing through at least a second wind turbine (102b-102c, 107-108) of the plurality of wind turbines (102, 106-109), the first location being a location of the substation (101) or a location of a third wind turbine (102a, 109) of the plurality of wind turbines (102, 106-109).

Description

METHOD FOR INSTALLING A SUBSEA CABLE IN AN OFFSHORE WIND FARM

TECHNICAL FIELD

The present invention relates to the field of subsea cables. Particularly, the invention relates to a method for installing a subsea cable in an offshore wind farm.

STATE OF THE ART

The cost of an offshore wind farm is mainly driven by two factors: the cost of the materials and structures thereof, and the cost of installing the structures. One of the tasks that has a major impact on the latter is the installation of subsea cables.

Generally, an offshore wind farm is organized in strings of wind turbines. Each wind turbine of a string generates electricity that needs to be transferred to a substation that, in turn, transfers it to a facility onshore (e.g. electrical grid). In addition to generating electricity, aside from the last wind turbine of a string (the one farthest away from the substation) each wind turbine also receives electrical energy from other wind turbines in the same string so as to transfer it to the substation, namely, the wind turbines relay the electrical energy provided by other wind turbines until it reaches the first wind turbine of a string (the one closest to the substation); the first wind turbine then relays the electrical energy of the entire string (including the electrical energy provided by the first wind turbine) to the substation. Accordingly, as a string of wind turbines is a plurality of wind turbines connected in series, the closer a wind turbine to the substation is, the more electrical energy that wind turbine has to transfer to the next wind turbine towards the substation.

The submarine cables interconnecting wind turbines must withstand the amount of electrical energy that has to be transferred to the substation, and more specifically, since the voltage is constant throughout the string, the submarine cables must withstand the intensity of the electric current. Therefore, on each string of wind turbines connected in series, cables connecting turbines that are closer to the substation must be able to transfer more electric current than cables connecting turbines that are farther away from the substation.

Current density is defined as the electric current per unit area of cross section. The current density magnitude is the electric current per cross-sectional area at a given location on the cable length.

For a specific conductive material that a submarine cable is made of, there is a maximum value of current density that is allowable; beyond this maximum value, it is more likely that the cable will be damaged by overheating, by failure of the insulation, etc. Therefore, the electrical energy that may be transferred by the electrical conductors is limited.

As the conductive material is a significant part of the cost of a subsea cable, it is often economically suboptimal to equip an entire string with the same cross section of conductive material, as further away from the substation, a smaller cross section is needed than closer to the substation because less electrical energy must be transferred.

Offshore wind farms of the prior art deal with the issue of increasing electrical energy in a string by providing a plurality of subsea cables in the string, each subsea cable connecting two consecutive wind turbines and having an electrical conductor adapted to withstand the maximum electric current that may be available between these two consecutive turbines. Hence, aside from the length (dependent on the distance between consecutive pairs of wind turbines), the subsea cables of a same string are different in that the core conductors of the electrical conductors have different cross-sections or diameters (as the same conductive material is generally used) thereby not exceeding the allowable current density value, and hence the different electric currents may be transferred at each leg of the string. So several subsea cables are produced and installed in a same string of the offshore wind farm.

Even though the core conductors have the adequate size to transfer the electrical energy at each leg, the production thereof (including the production of the corresponding electrical conductors and, hence, subsea cables) is discontinuous as it must be adjusted for each pair of consecutive wind turbines. Moreover, each resulting subsea cable must be installed separately in segments of a length corresponding to the distance between the two turbines it connects. Consequently, for a single string, as many subsea cables must be laid as there are consecutive wind turbines. Both discontinuous processes make installing structures of an offshore wind farm not cost-effective.

There is an interest in reducing the cost of offshore wind farms by optimizing the installation of structures thereof. More specifically, there is an interest in optimizing the production and installation of subsea cables in offshore wind farms.

DESCRIPTION OF THE INVENTION

An aspect of the invention relates to a method for installing a subsea cable in an offshore wind farm, the offshore wind farm comprising a substation and a plurality of wind turbines, the method comprising:

laying the subsea cable from a first location to a first wind turbine of the plurality of wind turbines passing through at least a second wind turbine of the plurality of wind turbines, the first location being a location of the substation or a location of a third wind turbine of the plurality of wind turbines.

The laying of a subsea cable from the substation to the first wind turbine with one or more intermediate wind turbines in-between (i.e. the one or more wind turbines between the first and the second ends of the subsea cable laid, which may be, for example, one, two, three, four, five, ten or even more wind turbines) is done continuously, namely, without providing additional subsea cables or without cutting the subsea cable every time a wind turbine is reached while laying the subsea cable, thereby simplifying the installation of the subsea cable and, thus, reducing the cost thereof. The plurality of wind turbines is preferably arranged as a string of wind turbines.

The installation becomes simpler since only one subsea cable needs to be laid between different wind turbines, or between different wind turbines and the substation. Therefore, with this method it can be avoided having to install different subsea cables between each pair of consecutive wind turbines, and/or between the substation and the wind turbine of the string that is closest to the substation.

With the continuous laying of a subsea cable, for example a submersible apparatus or assembly for installing subsea cables has to be loaded once with a single subsea cable and, therefore, it is not surfaced at each intermediate wind turbine so as to finish the laying of the subsea cable, load another subsea cable and lay it. In this way, subsea cable installation is simplified thereby reducing the overall cost of the offshore wind farm.

In the context of the present disclosure, the term“passing through” refers to laying the subsea cable at a close distance from a named facility (e.g. wind turbine), preferably from a foundation thereof, such that a portion of the subsea cable is at close distance therefrom. The distance between a portion of the subsea cable and the facility is less than 50 meters, and preferably less than 25 meters, and even more preferably less than 10 meters. In the context of the present disclosure, the term“substation” refers to any facility where the electrical energy provided by a wind farm, particularly an offshore wind farm, is to be delivered or exported at, for example a substation at an offshore wind farm, a substation onshore close to the coast, etc.

In some embodiments, the subsea cable is laid such that a first portion thereof is laid before laying a second portion thereof; and a cross-section of each core conductor in the first portion of the subsea cable is greater than the cross-section of each core conductor in the second portion of the subsea cable.

In these embodiments, the subsea cable comprises an electrical conductor with a first core conductor adapted to transmit electric current between ends of the subsea cable such that its capacity for transmitting electrical energy varies throughout a length of the same; the subsea cable further comprises insulation and watertight layers. Hence, in these embodiments, the first core conductor has at least two different cross-sections: when observed from a first end to a second end of the subsea cable, the core conductor has at least one stepped increase in the cross-section or diameter thereof. Therefore, from each portion where the cross-section or diameter increases, additional electric current may be transferred to one end thereof (a first end or a second end) thereby making possible to maintain a current density value within the allowable limits of the conductor.

As the subsea cable interconnects several wind turbines, each wind turbine adds the electric power it provides to the subsea cable for its delivery to a substation. So at each wind turbine, the electric current provided by that wind turbine plus the electric current provided by other wind turbines connected thereto needs to be transferred to further wind turbines or to the substation. In the context of the present disclosure, the term“substation” refers to any facility where the electrical energy provided by a wind farm, particularly an offshore wind farm, is to be delivered or exported at, for example a substation at an offshore wind farm, a substation onshore close to the coast, etc.

With the increase in the cross-section or diameter of the first core conductor the subsea cable may withstand all the electric current that has to be transferred, maintaining a current density value within the allowable limits of the conductor. This makes possible not to require different subsea cables, each having one or more core conductors with a different cross- section, for the progressive addition of electric power to be delivered, something which is also more costly since shorter and more intermittent manufacturing processes are necessary in order to provide all the different subsea cables for an offshore wind farm.

The subsea cable is laid starting from the substation, and ending at the last wind turbine (i.e. the first wind turbine) of the plurality of wind turbines (from where less electric current needs be transferred), and the cross-section or diameter of each core conductor within the subsea cable laid has one or more stepped reductions (considering the direction in which the subsea cable is being laid).

Subsea cables of the prior art feature different electrical conductors so that the electric current added at intermediate wind turbines may be transferred to another wind turbine of the plurality of wind turbines (for instance at an end of a string of wind turbines) and/or to the substation of the offshore wind farm. This makes that, in addition to having to manufacture different subsea cables, the subsea cables have to be properly identifiable so that the subsea cables are laid in the correct order.

In some embodiments, the method further comprises: at each wind turbine between ends of the subsea cable laid, cutting the subsea cable thereby providing first and second subsea cables, and connecting an end of each of the first and the second subsea cables to the wind turbine. In these embodiments, the steps of cutting the subsea cable and connecting ends of the first and the second subsea cables are carried out after the step of laying the subsea cable.

Once the subsea cable has been laid, it can be electrically connected to each wind turbine by connecting the ends of the subsea cable to the wind turbines of the plurality of wind turbines, and cutting the subsea cable at each intermediate wind turbine so that resulting ends may be connected to each intermediate wind turbine.

Upon installation of the subsea cable, since a plurality of subsea cables is finally provided (as a result of cutting the subsea cable for connection to the wind turbines), in case of failure of one of these subsea cables while the offshore wind farm is in operation, said one subsea cable may be replaced by a subsea cable with core conductor/s having a constant cross-section or diameter. That is, only the subsea cable between two consecutive wind farms needs to be replaced in spite of having continuously laid a single subsea cable featuring a greater length than the subsea cable to be replaced.

In some embodiments, each core conductor of the subsea cable has one or more stepped increases of a cross-section thereof along a length of the subsea cable.

In some embodiments, the subsea cable comprises a plurality of portions with as many portions as wind turbines between ends of the subsea cable are upon laying the subsea cable plus one; a length of a first portion of the plurality of portions is equal to or greater than a distance between the first location and the second wind turbine; lengths of remaining portions of the plurality of portions are equal to or greater than corresponding distances between each pair of consecutive wind turbines of the plurality of wind turbines; cross-sections of each core conductor being different in each portion of the plurality of portions; and a cross-section of each core conductor in the first portion being a greatest cross-section with respect to the cross- sections in the remaining portions.

In order to optimize costs, the capacity for delivering electrical energy of the subsea cable are selected according to: the number of wind turbines to be interconnected (if the first location is the location of the third wind turbine) or the number of wind turbines plus the substation to be interconnected (if the first location is the location of the substation), and the maximum electric current that may be available at any wind turbine at some point during operation of the offshore wind farm. To this end, the subsea cable has as many portions (where each core conductor therein has a different cross-section with respect to other portions of the subsea cable) as there are intermediate wind turbines, plus one, so that a different portion is provided between each two consecutive wind turbines, and/or between the second wind turbine and the substation. Therefore, if when laying the subsea cable it is passed through one, two, three, or more wind turbines (i.e. intermediate wind turbines), the number of portions of the subsea cable is two, three, four or more (i.e. the number of intermediate wind turbines plus one), respectively.

The length of each core conductor in each of these different portions (and, thus, the length of the subsea cable in each of these different portions) is at least the distance of the corresponding pair of consecutive wind turbines, and/or the distance of the substation to the second wind turbine, to be interconnected. Lengths greater than these distances are preferable so that the subsea cable has some play while it is laid.

Since the portion of the subsea cable connecting the first location to the second wind turbine carries the highest amount of electric current, the cross-section or diameter of each core conductor therein is the largest among all the cross-sections or diameters of each core conductor along the subsea cable, being the cross-section of each core conductor of this cable portion adapted to support an allowable current density value, according the electric current to be transported.

In some embodiments, a submersible apparatus or assembly for installing subsea cables carries out the step of laying the subsea cable.

In some embodiments, a submersible apparatus or assembly for installing subsea cables carries out or further carries out the step of cutting the subsea cable thereby providing the first and second subsea cables.

In some embodiments, a submersible apparatus or assembly for installing subsea cables carries out or further carries out the step of connecting the end of each of the first and the second subsea cables to the wind turbine.

In some embodiments, the subsea cable comprises:

at least one electrical conductor;

each of the at least one electrical conductor comprises a core conductor, a first portion of the core conductor having a first cross-section, and a second portion of the core conductor having a second cross-section.

The subsea cable comprises one, two, three or more electrical conductors, each electrical conductor comprising a core conductor with a plurality of different cross-sections that makes possible to transport different electric currents from different points or portions of the core conductor. In this regard, the core conductor comprises a plurality of portions, each portion having a substantially constant cross-section or diameter (throughout that portion) but different (in size) to the cross-sections or diameters of the other portions of the plurality of portions.

In some embodiments, the subsea cable comprises one or more core conductors.

In some embodiments, the subsea cable comprises one or more optical fibers.

In some embodiments, the subsea cable has a plurality of different diameters.

In some embodiments, the subsea cable has a substantially constant diameter.

The isolation layer/s of the one, two, three or more electrical conductors may be provided such that a thickness thereof is different in each portion of the plurality of portions of the core conductor. Particularly, the thickness of the isolation layer/s compensates the change in cross-section or diameter of the core conductor so that the diameter of the one, two, three or more electrical conductors is substantially constant between ends thereof, and thus the subsea cable also has a substantially constant diameter between ends thereof. Existing submersible apparatuses or assemblies for installing subsea cables may carry out the laying of the subsea cable without any modification owing to the substantially constant diameter.

In some other embodiments, the subsea cable comprises a plurality of portions, each portion having a substantially constant diameter (throughout that portion) but different (in size) to the cross-sections or diameters of the other portions of the plurality of portions.

The isolation layer/s of the one, two, three or more electrical conductors may be provided such that a thickness thereof is substantially constant between ends of the electrical conductor and, thus, between ends of the subsea cable.

In some embodiments, a cross-section of each core conductor in the subsea cable is constant along a length of the subsea cable.

The method of the present disclosure also makes possible to install, by continuously laying, subsea cables that have one or more core conductors, each of which has a constant cross-section along the length of the cables. The minimum cross-section of the core conductors must be such that it can withstand the transportation of the maximum electric current that may be available at any intermediate wind turbine. These embodiments are generally less preferable because unnecessary electrically conductive material need be provided, thereby increasing the weight of the subsea cable, and the cost of installing the subsea cable, yet even in these cases the offshore wind farm benefits from the continuous cable laying (in terms of installation costs) with respect to prior art installation methods.

BRIEF DESCRIPTION OF THE DRAWINGS

To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate embodiments of the invention, which should not be interpreted as restricting the scope of the invention, but just as examples of how the invention can be carried out. The drawings comprise the following figures:

Figure 1 shows an offshore wind farm.

Figure 2 shows a cross-section of a subsea cable suitable for offshore wind farms.

Figure 3 shows different layers of an electrical conductor of a subsea cable.

Figures 4-6 show methods for installing a subsea cable in an offshore wind farm in accordance with embodiments of the invention.

Figures 7A-7B, 8A-8B schematically show wire drawing processes.

Figure 9 shows an electrically conductive wire or rod with variable cross-section.

Figure 10-12 show core conductors with variable cross-section.

Figures 13A-13C schematically show a stranding process for producing the core conductor of Figures 1 1 and/or 12.

DESCRIPTION OF WAYS OF CARRYING OUT THE INVENTION

Figure 1 shows an offshore wind farm 100. The offshore wind farm 100 comprises a substation 101 and a plurality of wind turbines 102 arranged in strings (one string 130 is shown with dotted lines for illustrative purposes only). Each string comprises a plurality of wind turbines 102 and has two ends; a first end of each string is the substation 101 and a second end of each string is a wind turbine 102d. Between the two ends of a string there is one or more wind turbines 102a, 102b, 102c (also referred to as intermediate wind turbines within the present disclosure) within the same string that are to be connected to the other wind turbine 102d and the substation 101 . In this sense, a first string of the offshore wind farm 100 may be formed by the wind turbines 102a, 102b, 102c as intermediate wind turbines, by the substation 101 as a first end of the first string and a wind turbine 102d as a second end of the first string.

A first subsea cable 1 1 1 connects the substation 101 to the first wind turbine 102a so as to transfer the electric current provided by the string of wind turbines 102a-102d to the substation 101 . Between the first and a second wind turbines 102a, 102b there is a second subsea cable 1 12 for transferring the electric current provided by the second, a third and a fourth wind turbines 102b-102d to the first wind turbine 102a. A third subsea cable 1 13 connects the second and the third wind turbines 102b, 102c in order to deliver the aggregate electric current that is provided by the third and the fourth wind turbines 102c, 102d. And a fourth subsea cable 1 14 connects the fourth wind turbine 102d to the third wind turbine 102c in order to deliver the electric current provided by the fourth wind turbine 102d.

The closer a subsea cable 1 1 1 -1 14 is to the substation 101 , the greater the cross- section of the core conductors within the subsea cable must be to transmit the electrical energy provided by the wind turbines 102 that are farther away from the substation 101 .

The substation 101 receives the electrical energy provided by each wind turbine 102 of each string 130 of the offshore wind farm 100 and delivers it through a subsea cable 120 to a facility that may be remote from the substation 101 (generally, an onshore electrical grid). In some examples, the substation 101 is onshore and close to a sea.

Figure 2 shows a cross-section of a subsea cable 200 suitable for offshore wind farms. The subsea cable 200 comprises a plurality of electrical conductors 210, an optical fiber 230, and a plurality of layers 221 -224 for protecting the electrical conductors 210 and the optical fiber 230.

A first layer 221 is an armor bedding layer encapsulating each of the electrical conductors 210 and the optical fiber 230. A second layer 222 is a metallic armor layer that surrounds the armor bedding layer with steel wires helically drawn that increase a mechanical resistance of the subsea cable 200. A third layer 223 is a watertight layer for preventing that water gets within the subsea cable 200. A fourth layer 224 is a plastic yarn layer for protecting the subsea cable 200 against abrasion.

Each electrical conductor 210 comprises a core conductor 21 1 , an insulation layer 213 surrounding the core conductor 21 1 , and a watertight layer 215 surrounding the insulation layer 213. In addition to the conductivity of the electrically conductive material forming the core conductor 21 1 , a cross-section of the core conductor 21 1 limits the electric current that the electrical conductors 210 may transfer: for a same electrically conductive material, a core conductor 21 1 having a greater diameter may conduct more electric current than a core conductor having a smaller diameter.

Further, a plastic filling 220 fills the space between the electrical conductors 210 and the optical fiber 230, and the first layer 221 encapsulating them. The plastic filling 220 provides the subsea cable 200 with constant geometry and external stability.

In other embodiments, the subsea cable 200 comprises one electrical conductor 210, two electrical conductors 210 or more than three electrical conductors 210. Similarly, in other embodiments, the subsea cable 200 does not comprise an optical fiber 230, or comprises even more than one optical fiber 230.

Figure 3 shows, in more detail (in a side view on the right), different layers of one electrical conductor 210 of the subsea cable 200 of Figure 2. The electrical conductor 210 comprises the core conductor 21 1 , the insulation layer 213, the watertight layer 215, a screening layer 212 interposed between the core conductor 21 1 and the insulation layer 213, and additional screening and insulation layers 214 interposed between the insulation layer 213 and the watertight layer 215.

Figure 4 shows a method for installing a subsea cable 500 in an offshore wind farm in accordance with an embodiment of the invention. In Figure 4 there is shown a string of wind turbines 106-109 in a sea 150.

The method comprises laying the subsea cable 500 from a first wind turbine 106 to a second wind turbine 109 passing through third and fourth wind turbines 107, 108 in a continuous manner. That is, the subsea cable 500 is continuously laid so that portions thereof are proximate to each wind turbine 106-109 of the string; preferably, each of these portions is at a distance that is less than 100 meters from the foundation of a wind turbine 106-109, more preferably at a distance that is less than 50 meters, and/or less than 25 meters, and/or less than 10 meters.

The method also comprises cutting the subsea cable 500, once it has been laid, at the portions that are proximate to intermediate wind turbines 107, 108 (the term intermediate referring to wind turbines that are between ends of the subsea cable 500), that is at a first position 507 where a portion of the subsea cable 500 is close to the third wind turbine 107, and at a second position 108 where a portion of the subsea cable 500 is close to the fourth wind turbine 108. Precisely at the first and second positions 507, 508 a cross-section or diameter of each core conductor within the subsea cable 500 varies so that the subsea cable 500 is adapted to transport additional electric current provided by the intermediate wind turbines 107, 108. The subsea cable 500 may be first lifted to platforms (not illustrated) of the wind turbines 107, 108 so that it may be cut outside of the water. Further, the subsea cables resulting from cutting the subsea cable 500 may be connected to the wind turbines 106-109.

The subsea cable 500, which may be produced with methods such as, for example, those described with reference to Figures 7-12, comprises three electrical conductors with core conductors having a plurality of different cross-sections so that more electric current may be transferred by the subsea cable 500 upon connection to the intermediate wind turbines 107, 108. In this sense, the subsea cable 500 has a plurality of different diameters, particularly it comprises a first portion 501 with a diameter d1 (each core conductor having a first cross- section or diameter 504), a second portion 502 with a second diameter d2 greater than the first diameter d1 (each core conductor having a second cross-section or diameter 505 greater than the first cross-section or diameter 504), and a third portion 503 with a third diameter d3 greater than the second diameter d2 (each core conductor having a third cross-section or diameter 506 greater than the second cross-section or diameter 505). At the first and the second positions 507, 508 the diameters of the subsea cable 500 vary (and so does the cross-sections and diameters of each core conductor) from the first diameter d1 to the second diameter d2, and from the second diameter d2 to the third diameter d3, respectively.

In other embodiments, the method comprises laying the subsea cable 500 from a substation (not illustrated) to the first wind turbine 106 passing through the second, third and fourth wind turbines 107-109 in a continuous manner.

Figure 5 shows a method for installing a subsea cable 510 in an offshore wind farm in accordance with an embodiment of the invention. The offshore wind farm at least comprises the string of wind turbines 106-109 in the sea 150.

The installation process is similar to the one described with reference to Figure 4, the difference being that the subsea cable 510 installed has a substantially constant cross-section throughout a length thereof even though it comprises one or more electrical conductors 515, each electrical conductor 515 comprising a core conductor with a plurality of different cross- sections 521 -523.

In a first portion 51 1 of the subsea cable 510, the core conductor of each electrical conductor 515 has a first cross-section or diameter 521 . Further, each electrical conductor 515 also comprises an insulation layer that in the first portion of the subsea cable 510 has a first thickness 521 (computed as the difference between its outer and inner diameters). In a second portion 512 of the subsea cable 510, the core conductor of each electrical conductor 515 has a second cross-section or diameter 522 greater than the first cross-section or diameter 521 , whereas the insulation layer has a second thickness 532 smaller than the first thickness 531 . In a third portion 513 of the subsea cable 510, the core conductor of each electrical conductor 515 has a third cross-section or diameter 523 greater than the second cross-section or diameter 522, and the insulation layer has a third thickness 533 smaller than the second thickness 532. By decreasing the thickness 531 -533 of the insulation layer as the cross-section or diameter 521 -523 of the core conductor increases, the cross-section of each electrical conductor 515 is kept constant throughout the length of the subsea cable 510.

Figure 6 shows a method for installing a subsea cable 520 in an offshore wind farm in accordance with an embodiment of the invention. The offshore wind farm at least comprises the string of wind turbines 106-109 in the sea 150.

The installation process is similar to the one described with reference to Figure 4, the difference being that the subsea cable 520 installed has a substantially constant cross-section throughout a length thereof. In this embodiment, the subsea cable 520 comprises an electrical conductor 540 comprising three core conductors having a substantially constant cross-section 541 along the length of the subsea cable 520. In other embodiments, the electrical conductor 540 comprises a single core conductor with substantially constant cross-section 541 along the length of the subsea cable 520.

Figures 7A-7B schematically show a wire drawing process. A plurality of drawing dies 321 -324 and pulling discs 330a, 330b are used in the wire drawing process. An electrically conductive wire or rod 310 is pulled by the pulling discs 330a, 330b; the cross-section of the electrically conductive wire or rod 310 is altered due to the drawing dies 321 -324 through which the electrically conductive wire or rod 310 goes through. In this respect, the drawing dies 321 - 324 progressively reduce the cross-section of the electrically conductive wire or rod 310 as the pulling discs 330a, 330b pull the electrically conductive wire or rod 310. To this end, an inner drawing diameter of the drawing dies 321 -324 is always smaller than a diameter of the electrically conductive wire or rod 310 to be drawn; further, the inner drawing diameter of a drawing die 321 -324 is always smaller than that of a previous drawing die (e.g. the inner drawing diameter of the last drawing die 324 is smaller than the inner drawing diameter of the drawing die 323 before the last drawing die 324) according to the order in which the wire or rod is being drawn in.

When the electrically conductive wire or rod 310 has gone through the last drawing die 324, an electrically conductive wire or rod 315 with a cross-section smaller than an initial cross- section thereof is provided. When the electrically conductive wire or rod 310 is wire drawn, a length 340 of a portion of the resulting electrically conductive wire or rod 315 comprising the smallest cross-section (owing to the last drawing die 324) must be provided such that it extends at least a distance between two facilities (e.g. wind turbines, substations, etc.) that are to be connected with that portion.

After having the necessary length 340, the resulting electrically conductive wire or rod 315 is wire drawn again as shown in Figure 7B. Another plurality of drawing dies 321 -323 (a subset of the plurality of drawing dies 321 -324 of Figure 4A since the last drawing die 324 has been removed therefrom in Figure 7B) is provided in the wire drawing process of Figure 4B so that the length 340 remains unchanged. Hence, another portion having a greater cross-section can be provided throughout a length 341 owing to the plurality of drawing dies 321 -323. The length 341 needs to extend at least a distance between two facilities that are to be connected with the corresponding portion of the resulting electrically conductive wire or rod 316. By repeating such wire drawing processes modifying the drawing dies provided, the electrically conductive wire or rod is provided with more than two different cross-sections, each of these cross-sections extending a length of a corresponding portion.

In some non-illustrated examples, the plurality of drawing dies 321 -323 of Figure 7B results from removing more than one drawing die from the plurality of drawing dies of the first wire drawing process. In some other non-illustrated examples, the plurality of drawing dies 321 -323 of Figure 7B comprises one or more drawing die/s that is/are different from the plurality of drawing dies of the first wire drawing process.

Figures 8A-8B schematically show a wire drawing process. A first one or more drawing dies 321 -322 and pulling discs 330a, 330b are used in the wire drawing process. An electrically conductive wire or rod 310 is pulled by the pulling discs 330a, 330b; the cross-section of the electrically conductive wire or rod 310 is altered due to the first one or more drawing dies 321 - 322 through which the electrically conductive wire or rod 310 goes through. In this example, the first one or more drawing dies 321 -322 comprises two drawing dies 321 -322 so as to progressively reduce the cross-section of the electrically conductive wire or rod 310 as the pulling discs 330a, 330b pull the electrically conductive wire or rod 310, but it is readily apparent that one or more than two drawing dies could be used instead.

When the electrically conductive wire or rod 310 has gone through the first one or more drawing dies 321 -322, an electrically conductive wire or rod 31 1 with a cross-section smaller than an initial cross-section thereof is provided. In this example, an entire length of the electrically conductive wire or rod 310 is drawn so as to provide the electrically conductive wire or rod 31 1 with a smaller cross-section.

The resulting electrically conductive wire or rod 31 1 is wire drawn again as shown in Figure 8B through a second one or more drawing dies 323. The second one or more drawing dies 323 further reduce the cross-section of the electrically conductive wire or rod 31 1 . Therefore, by partially wire drawing (i.e. wire drawing only a portion or a length, so not the entire electrically conductive wire or rod 31 1 is wire drawn) the electrically conductive wire or rod 31 1 , the resulting electrically conductive wire or rod 312 is provided with different portions, each portion having a different cross-section (a first portion having a cross-section resulting from the drawing through the first one or more drawing dies 321 , 322 of Figure 8A, and a second portion having a cross-section resulting from the drawing through the second one or more drawing dies 323 of Figure 8B). By repeating such wire drawing processes with additional third, fourth and further one or more drawing dies, the electrically conductive wire or rod is provided with more than two different cross-sections since each time it is wire drawn a further portion with a smaller cross-section is provided.

Figure 9 shows an electrically conductive wire or rod 350 made by wire drawing processes such as those described with reference to Figures 7A-7B, 8A-8B. The electrically conductive wire or rod 350 has a plurality of portions 351 -354, each of which features a different cross-section. Particularly, a first portion 351 of the electrically conductive wire or rod 350 has a first cross-section or diameter d1 ; a second portion 352 of the electrically conductive wire or rod 350 has a second cross-section or diameter d2; a third portion 353 of the electrically conductive wire or rod 350 has a third cross-section or diameter d3; and a fourth portion 354 of the electrically conductive wire or rod 350 has a fourth cross-section or diameter d4. The electrically conductive wire or rod 350 has a cross-section or diameter with stepped reductions along a length thereof with respect to a first end thereof (i.e. d1 > d2 > d3 > d4), or with stepped increases along a length thereof with respect to a second end thereof (i.e. d4 < d3 < d2 < d1 ).

The electrically conductive wire or rod 350 is provided with the plurality of different cross-sections by wire drawing an electrically conductive wire or rod a plurality of times through drawing dies. After the first wire drawing, each time the electrically conductive wire or rod 350 is to be wire drawn, a different one or more drawing dies is provided. For example, at least one drawing die of a plurality of drawing dies may be withdrawn each time the electrically conductive wire or rod 350 is to be wire drawn, particularly the drawing die/s providing the electrically conductive wire or rod with the smallest cross-section or diameter (as shown in Figures 7A-7B); or at least one drawing die is added or replaces another at least one drawing die previously used so that a smaller cross-section or diameter may be provided to the electrically conductive wire or rod (as shown in Figures 8A-8B). A length of each portion of the plurality of portions 351 -354 is made longer or shorter each time the electrically conductive wire or rod 350 is wire drawn, and is preferably equal to or greater than a distance between two consecutive wind turbines of an offshore wind farm that are to be connected with a subsea cable, and/or equal to or greater than a distance between a substation and a wind turbine of the offshore wind farm that are to be connected with a subsea cable.

Figure 10 shows a core conductor 360 made by stranding a plurality of electrically conductive wires resulting from wire drawing processes such as those described with reference to Figures 7A-7B, 8A-8B. The core conductor 360 comprises a plurality of electrically conductive wires. Each electrically conductive wire has a plurality of portions, each portion having a different cross-section (for example each of these wires may be the electrically conductive wire or rod 350 of Figure 9).

The plurality of electrically conductive wires is stranded so as to provide the core conductor 360. The core conductor 360 has a plurality of portions 361 -364 each of which features a different cross-section. Particularly, a first portion 361 of the core conductor 360 has a first cross-section or diameter d1 ; a second portion 362 of the core conductor 360 has a second cross-section or diameter d2; a third portion 363 of the core conductor 360 has a third cross-section or diameter d3; and a fourth portion 364 of the core conductor 360 has a fourth cross-section or diameter d4. The core conductor 360 has a cross-section with stepped reductions along a length thereof with respect to a first end thereof (i.e. d1 > d2 > d3 > d4), or with stepped increases along a length thereof with respect to a second end thereof (i.e. d4 < d3 < d2 < d1 ).

In some non-illustrated examples, transitions from one portion to another portion may be provided such that a reduction or an increase in cross-section is progressive; such transitions may improve the sturdiness of the core conductor. This may be carried out by spacing the change in cross-section of each wire, that is, some wires have a change in cross- section occurring at a length greater than some other wires, for example.

In some non-illustrated examples, a plurality of electrically conductive wires with a constant cross-section is provided on an outermost part or layer of the core conductor, and a plurality of electrically conductive wires with variable cross-sections is provided on an innermost part or layer of the core conductor. In some non-illustrated examples, a plurality of electrically conductive wires with a constant cross-section is provided on an innermost part or layer of the core conductor, and a plurality of electrically conductive wires with variable cross- sections is provided on an outermost part or layer of the core conductor. Both core conductor configurations may improve the sturdiness of the core conductor.

Figure 1 1 shows a core conductor 370 made by a stranding process such as that described with reference to Figures 13A-13C. The core conductor 370 comprises a plurality of electrically conductive wires with a constant cross-section or diameter. In some other examples, each of the electrically conductive wires has a plurality of portions, each portion having a different cross-section. In this example, different wire lengths have been coiled on stranding drums (exemplary stranding drums are shown in Figures 9A-9C). The stranding process is started by portion 371 , producing the biggest core cross-section. When the stranding drums coiled with the shorter wires are finished, by further stranding with the stranding drums coiled with the longer wires the portion 372 is provided. This process is repeated for portion 373 and, if it is the case, for further portions.

Therefore, by adjusting the wire length loaded on each stranding drum, each different portion (371 , 372 and 373) is easily performed. Thus, automatically and without stranding process interruption, as each drum ends, a core shown on Figure 1 1 is provided.

The core conductor 370 has a plurality of portions 371 -373 each of which features a different cross-section. Particularly, a first portion 371 of the core conductor 370 has a first cross-section or diameter d1 ; a second portion 372 of the core conductor 370 has a second cross-section or diameter d2; and a third portion 373 of the core conductor 370 has a third cross-section or diameter d3. The core conductor 370 has a cross-section with stepped reductions along a length thereof with respect to a first end thereof (i.e. d1 > d2 > d3), or with stepped increases along a length thereof with respect to a second end thereof (i.e. d3 < d2 < d1 ).

The first portion 371 comprises a first plurality of electrically conductive wires that may be stranded as described next with respect to Figure 13A. The second portion 372 comprises a second plurality of electrically conductive wires that has fewer electrically conductive wires than the first plurality and that may be stranded as described next with respect to Figure 13B; in this respect, the second plurality of electrically conductive wires comprises a subset of electrically conductive wires of the first plurality, that is, the first portion 371 comprises the electrically conductive wires of the second portion 372 and one or more additional electrically conductive wires that provide the first portion 371 with the cross-section or diameter d1 larger than the cross-section or diameter d2 of the second portion 372. The third portion 373 comprises a third plurality of electrically conductive wires that has fewer electrically conductive wires than the second plurality (the third plurality comprises a subset of electrically conductive wires of the second plurality of the second portion 372) and that may be stranded as described next with respect to Figure 13C.

Figure 12 diagrammatically shows a section of a core conductor 380 made by a stranding process such as that described with reference to Figures 13A-13C. The core conductor 380 comprises a plurality of electrically conductive wires 391 -393 with a constant cross-section or diameter.

The stranding process is started by first portion 381 , producing the biggest core cross- section with first wires 391 (in an inner-most part of the first portion 381 ), second wires 392 and third wires 393 (in an outer-most part of the first portion 381 ). When the stranding drums coiled with the first wires 391 are finished (or the first wires 391 are cut), by further stranding with the stranding drums coiled with the second wires 392 (in an inner-most part of a second portion 382) and the third wires 393 (in an outer-most part of the second portion 382) the second portion 382 is provided (a length thereof depending upon a length of the second wires 392 as coiled in the stranding drums, or a position at which the second wires 392 are cut). This process is repeated at least for a third portion 383 in which only the third wires 393 remain as the second wires 392 have been already stranded in the first and the second portions 381 , 382.

The stranding process with which the core conductor 380 is manufactured may be, for example, the one described next with reference to Figures 13A-13C.

Figures 13A-13C schematically show a stranding process 400a-400c of a method for producing the core conductor 370 of Figure 1 1 and/or the core conductor 380 of Figure 12.

Figure 13A shows a first stage of the stranding process 400a in which a stranding machine 405 takes a first plurality of electrically conductive wires 404a from first, second and third sets of electrically conductive wires 401 -403 and strands it to produce a core conductor with a first cross-section 410a. Figure 13B shows a second stage of the stranding process 400b in which the stranding machine 405 takes a second plurality of electrically conductive wires 404b from the first and second sets of electrically conductive wires 401 , 402 and strands it to provide the core conductor of Figure 13A with a second cross-section 410b that is smaller than the first cross-section 410a. Figure 13C shows a third stage of the stranding process 400c in which the stranding machine 405 takes a third plurality of electrically conductive wires 404c from the first set of electrically conductive wires 401 and strands it to provide the core conductor of Figures 13A, 13B with a third cross-section 410c that is smaller than the first and second cross-sections 410a, 410b. By stranding the electrically conductive wires 404a-404c during the stranding process 400a-400c more or less times, the lengths of each portion of the core conductor having a different cross-section 410a-410c are adjusted.

In some cases, the electrically conductive wires that, during the stranding process (in the second and third stages of the stranding process 400b, 400c), are not to be stranded due to the reduction in the number thereof, are cut so that the stranding machine 405 does not strand them. In some other cases, lengths of the electrically conductive wires are adjusted in advance (they are provided with longer or shorter lengths depending on a length that a corresponding portion of the core conductor must have) so that the entire lengths may be stranded by the stranding machine 405 and result in a core conductor with variable cross- section.

To achieve greater stability and sturdiness of the outer geometry of the core conductor, the stranding process may be configured so that the longest wires remain on the surface of the core conductor. In this way, the outer surface of the core conductor will not present irregularities that may affect the correct application of the rest of the conductor layers which encapsulate the core conductor.

In this text, the terms “core conductor” and “electrical core conductor” are interchangeably used.

In this text, the term“comprises” and its derivations (such as“comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

The invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.

Claims

1 . A method for installing a subsea cable (200, 500, 510, 520) in an offshore wind farm (100), the offshore wind farm (100) comprising a substation (101 ) and a plurality of wind turbines (102, 106-109), the method comprising:

laying the subsea cable (200, 500, 510, 520) from a first location to a first wind turbine (102d, 106) of the plurality of wind turbines (102, 106-109) passing through at least a second wind turbine (102b-102c, 107-108) of the plurality of wind turbines (102, 106-109), the first location being a location of the substation (101 ) or a location of a third wind turbine (102a, 109) of the plurality of wind turbines (102, 106-109).

2. The method of claim 1 , further comprising:

at each wind turbine (102b-102c, 107-108) between ends of the subsea cable laid (200, 500, 510, 520), cutting the subsea cable (200, 500, 510, 520) thereby providing first and second subsea cables (1 1 1 -1 14), and connecting an end of each of the first and the second subsea cables (1 1 1 -1 14) to the wind turbine (102b-102c, 107-108);

wherein steps of cutting the subsea cable (200, 500, 510, 520) and connecting ends of the first and the second subsea cables (1 1 1 -1 14) are carried out after the step of laying the subsea cable (200, 500, 510, 520).

3. The method of any one of the preceding claims, wherein each electrical core conductor (21 1 , 360, 370, 380) of the subsea cable (200, 500, 510) has one or more stepped increases of a cross-section (504-506, 521 -523) thereof along a length of the subsea cable (200, 500, 510).

4. The method of any one of the preceding claims, wherein:

the subsea cable (200, 500, 510) is laid such that a first portion (503, 513) thereof is laid before laying a second portion thereof (501 , 502, 51 1 , 512); and

a cross-section (506, 523) of each electrical core conductor (21 1 , 360, 370, 380) in the first portion (503, 513) of the subsea cable (200, 500, 510) is greater than the cross-section (504-505, 521 -522) of each electrical core conductor (21 1 , 360, 370, 380) in the second portion (501 , 502, 51 1 , 512) of the subsea cable (200, 500, 510).

5. The method of any one of the preceding claims, wherein:

the subsea cable (200, 500, 510) comprises a plurality of portions (501 -503, 51 1 -513) with as many portions as wind turbines (102, 106-109) between ends of the subsea cable (200, 500, 510) are upon laying the subsea cable (200, 500, 510) plus one; a length of a first portion (503, 513) of the plurality of portions (501 -503, 51 1 -513) is equal to or greater than a distance between the first location (101 , 102a, 109) and the second wind turbine (102b-102c, 107-108);

lengths of remaining portions (502-503, 512-513) of the plurality of portions (501 -503, 51 1 -513) are equal to or greater than corresponding distances between each pair of consecutive wind turbines of the plurality of wind turbines (102, 106-109);

cross-sections (504-506, 521 -523) of each electrical core conductor (21 1 , 360, 370, 380) being different in each portion of the plurality of portions; and

a cross-section (506, 523) of each electrical core conductor (21 1 , 360, 370, 380) in the first portion (503, 513) being a greatest cross-section with respect to the cross-sections (504- 505, 521 -522) of each electrical core conductor (21 1 , 360, 370, 380) in the remaining portions (501 -502, 51 1 -512).

6. The method of any one of claims 1 -2, wherein a cross-section (541 ) of each electrical core conductor (21 1 ) in the subsea cable (200, 520) is constant along a length of the subsea cable (200, 520).

7. The method of any one of the preceding claims, wherein a submersible apparatus or assembly for installing subsea cables carries out the step of laying the subsea cable (200, 500, 510, 520).

8. The method of claims 2 and 7, wherein the submersible apparatus or assembly for installing subsea cables further carries out the step of cutting the subsea cable (200, 500, 510, 520) thereby providing the first and second subsea cables (1 1 1 -1 14) and/or the step of connecting the end of each of the first and the second subsea cables (1 1 1 -1 14) to the wind turbine (102b-102c, 107-108).

9. The method of any one of claims 1 -5, or any one of claims 7-8 when not depending upon claim 6, wherein the subsea cable (200, 500, 510) has a plurality of different diameters.

10. The method of any one of claims 1 -8, wherein the subsea cable (200, 510, 520) has a constant diameter.

1 1 . The method of any one of the preceding claims, wherein the subsea cable (200, 500) comprises one or more electrical core conductors (21 1 , 360, 370, 380).

12. The method of any one of the preceding claims, wherein the subsea cable (200, 500, 510, 520) comprises one or more optical fibers (230).

13. The method of any one of the preceding claims, wherein in the step of laying the subsea cable (200, 500, 510, 520) from the first location (101 , 102a, 109) to the first wind turbine (102d, 106) of the plurality of wind turbines (102, 106-109), the subsea cable (200, 500, 510, 520) is passed at a distance that is less than 50 meters from the second wind turbine (102b-102c, 107-108) of the plurality of wind turbines (102, 106-109).

14. The method of any one of the preceding claims, wherein in the step of laying the subsea cable (200, 500, 510, 520) from the first location (101 , 102a, 109) to the first wind turbine (102d,

106) of the plurality of wind turbines (102, 106-109) passing through at least the second wind turbine (102b, 107) of the plurality of wind turbines (102, 106-109), the subsea cable (200, 500, 510, 520) is laid further passing through at least a fourth wind turbine (102c, 108) of the plurality of wind turbines (102, 106-109).

15. The method of claim 14, wherein in the step of laying the subsea cable (200, 500, 510, 520) from the first location (101 , 102a, 109) to the first wind turbine (102d, 106) of the plurality of wind turbines (102, 106-109), the subsea cable (200, 500, 510, 520) is passed at a distance that is less than 50 meters from the fourth wind turbine (102c, 108) of the plurality of wind turbines (102, 106-109).

16. The method of any one of the preceding claims, wherein the subsea cable (200, 500, 510, 520) is laid from the first location (101 , 102a, 109) to the first wind turbine (102d, 106) of the plurality of wind turbines (102, 106-109) continuously.