WO2017174127A1

WO2017174127A1 - A protective casing for a high voltage cable

Info

Publication number: WO2017174127A1
Application number: PCT/EP2016/057524
Authority: WO
Inventors: Johan JÄDERBERG; Henrik EKHOLM
Original assignee: Abb Hv Cables (Switzerland) Gmbh
Priority date: 2016-04-06
Filing date: 2016-04-06
Publication date: 2017-10-12

Abstract

A protective casing (1) for a joint, termination or cross-connection of at least one high voltage cable (2) is disclosed. The protective casing comprises a first tubular end portion (4) provided with a buffer coating (6) and a copper or copper based alloy layer (7). The protective casing enables an excellent soldered joint between the protective casing and a metallic sheath of a cable.

Description

A PROTECTIVE CASING FOR A HIGH VOLTAGE CABLE

TECHNICAL FIELD

The present invention relates to a protective casing for a joint, termination or cross- connection of at least one high voltage cable. More specifically, it relates to a protective casing which may be soft soldered to a metallic sheath of a high voltage cable by means of wipe soldering. The present invention also relates to a joint, termination or cross-connection of at least one high voltage cable, said joint, termination or cross-connection comprising a protective casing. BACKGROUND ART

High voltage electric power cables or power transmission cables are used to transmit electrical power with medium or high voltage. The cables inter alia comprise a conductor, an insulation system that surrounds the conductor and a metallic sheath arranged outside of the insulation system. The metallic sheath acts as a diffusion barrier for preventing water from entering into the insulation system. Additional protective layers, usually of polymer based materials, may usually be arranged outside of the metallic sheath in order to prevent damage thereof.

A long deep-sea high voltage cable comprises a large number of consecutive cables or cable parts which are joined together. The joining process is performed on a cable ship at sea or the like before the cable is laid down in the sea. A protective casing, usually made of stainless steel, is used to protect the joint between two consecutive cables or cable parts. The protective casing is joined to the respective metallic sheath of the two cables to ensure a water tight connection.

Wipe soldering with a lead based solder, such as a solder comprising about 60-80 % by weight of lead and about 20-40 % by weight of tin, may be used for connecting the metallic sheath of the cable and the stainless steel casing. The lead based solder provides the water tight connection which is essential to avoid damage of the cable joint. In the joining process, a cable part which is stripped down to the bare metallic sheath is inserted into a tubular end portion of the protective casing. Thereafter, wipe soldering is performed around the outer periphery of the end of the protective casing and the adjacent metallic sheath such as to join the protective casing and the metallic sheath around the whole circumference.

In order to obtain sufficient bonding of the solder to the stainless steel joint casing body, the stainless steel is provided with an outer coating layer of tin or a tin based alloy at the part thereof where the solder is to be applied. This process is sometimes referred to as pre-tinning.

However, the process of pre-tinning stainless steel is complicated and requires several steps in order to obtain a good result. For example, the stainless steel surface must be subjected to pickling in order to remove surface oxides of the stainless steel. If the stainless steel surface is not subjected to pickling, the tin coating layer does not wet the stainless steel surface properly. Pickling requires aggressive chemicals that may be harmful to the environment and must be performed in a controlled environment. Therefore, the tin coating layer cannot be applied on site at sea and must consequently be applied before the parts to be joined arrive on site. If the tin coating layer does not adhere properly to the stainless steel, it will come off during wipe soldering and the solder will hence not bind properly to the surface of the stainless steel protective casing. Furthermore, the tin coating layer on the surface of the stainless steel is sensitive to mechanical damage and can hardly be repaired on site if damaged due to the insufficient adhesion to a stainless steel surface as a result of the surface oxide.

EP 2 113 978 Al discloses one alternative to pre-tinning of the stainless steel protective casing. The protective casing disclosed therein comprises a first material portion having a tubular shape and being made of metal such as stainless steel. The protective casing further comprises a second annular material portion fixedly attached by hard soldering (also known as brazing) to an outer surface of the first material portion. The second material portion is made of a metal material with good soft soldering properties, such as copper or a copper based alloy. In view of the fact that the second material portion is attached to the stainless steel by hard soldering, the problems associated with the surface oxide of the stainless steel can be avoided for example by selection of an appropriate flux during hard soldering. Moreover, since the second material portion is made of copper or a copper based alloy, it can thereafter easily be soft soldered to the metallic sheath of a cable part in the same manner as a pre-tinned stainless steel joint casing. While the solution as disclosed in EP 2 113 978 Al avoids the problems associated with pre- tinning, it has the disadvantage that it utilises a separate component, "the second material portion", which inherently increases the diameter of the end portion of the protective casing and hence of the joint thereof to the metallic sheath as compared to a pre-tinned stainless steel protective casing. This also adds additional weight to the joint. Furthermore, there might be a risk that the solder used to hard solder (i.e. the brazing material) the second material portion to the first material portion is adversely affected by the subsequent wipe soldering process used to solder the joint casing to the metallic sheath.

Thus, there is still a need to find a more robust and reliable method wherein the protective casing easily can be wipe soldered to a metallic sheath of a cable or a cable part on site at sea, or the like, during joining to form long high voltage cables and which is not sensitive to mechanical damage and can ensure a water tight joint.

SUMMARY OF THE INVENTION

The object of the present invention is to enable a reliable, robust and fast method when soft soldering a protective casing to the metallic sheath of a cable.

The present invention solves the problem above by a protective casing for a joint, termination or cross-connection of at least one high voltage cable in accordance with independent claim 1 and a joint, termination or cross-connection comprising such a protective casing.

The protective casing for a joint, termination or cross-connection of at least one high voltage cable comprises a first tubular end portion arranged at a first longitudinal end of the protective casing. The first tubular end portion is adapted for receiving an end portion of the at least one high voltage cable when inserted therein. The first tubular end portion is made of stainless steel and comprises a first outer circumferential surface. The protective casing comprises a first buffer coating welded to the first outer circumferential surface of the first end portion and having a circumferential extension around the entire circumferential extension of the first outer circumferential surface. The protective casing further comprises a copper or copper based alloy layer welded to an outer circumferential surface of the buffer coating and having a circumferential extension around the entire circumferential extension of the outer circumferential surface of the buffer coating. Copper or copper based alloys can easily be provided with a coating of tin or a tin based alloy, if desired, without any risk for inferior adhesion and without the need for any pre-treatment of the surface before the tin or tin based alloy is applied. Tin or a tin based alloy has good wettability to a copper or copper based alloy. Thus, the present invention avoids the problems associated with treatment of the stainless steel surface with aggressive chemicals as is necessary in the case of pre-tinning. In fact, the tin or tin based alloy layer may easily be applied or reapplied on site as no pre-treatment of the surface of the copper or copper based alloy is necessary. A possible tin or tin based alloy layer applied to the surface of the copper or copper based alloy serves as a good surface for wipe soldering. Moreover, there is no risk of the tin or tin based alloy layer to come off during wipe soldering. The tin or tin based alloy layer may suitably be applied by soft soldering.

Furthermore, the fact that the copper or copper based alloy layer is applied by means of welding avoids the need for a separate component of copper or a copper based alloy which adds to the diameter of the end portion of the protective casing. Moreover, by welding the copper or copper based alloy layer, a water diffusion tight connection is ensured.

While it is possible to weld copper or a copper based alloy directly to a stainless steel surface, the present invention utilises a buffer coating interposed between the stainless steel surface and the copper or copper based alloy layer. The buffer coating serves the purpose of avoiding migration or diffusion of copper into the stainless steel during welding. If copper is migrated or diffused into the stainless steel of the protective casing, the corrosion resistance of the stainless steel may be reduced. To ensure good adhesion between the different materials of the protective casing and a water diffusion tight joint, the buffer coating is welded to the stainless steel surface, and the copper or copper based alloy layer is welded to the buffer coating. The fact that the buffer coating and the copper or copper based layer is applied by means of welding also has the advantage of enabling relatively thin layers/coatings and thus avoids the problems of increased diameter of the joints as associated with prior art.

Thus, by means of the present invention, the longitudinal end of a protective casing is modified in order to be solderable, in particular solderable by wipe soldering, by welding a buffer coating to the stainless steel of the tubular end portion of the protective casing, and thereafter welding a copper or copper based alloy layer to the buffer coating.

The protective casing according to the present invention is less sensitive to mechanical damage than previously known solutions and, if damaged, can more easily be repaired on site if necessary. Thus, it provides a more robust solution when the protective casing is to be soft soldered to the metallic sheath of a cable.

Furthermore, since the protective casing according to the present invention is less sensitive to mechanical damage and does not require any pre-treatment of the surface to enable tin or tin based alloys to be applied, it provides a more robust solution for joining the metallic sheath and the protective casing. For the same reason, it is also faster since fewer protective casings have to be discarded on site at sea, for being damaged, during joining and laying down the cable at the bottom of the sea. Since the process of laying down a long sea cable using a cable ship is associated with a considerable cost, much is saved when time is shortened.

The longitudinal extension of the buffer coating may suitably be greater than the longitudinal extension of the copper or copper based alloy layer. Thereby, any direct contact between the stainless steel surface and the copper or copper based alloy of the copper or copper based alloy layer is avoided and hence the risk of migration or diffusion of copper into the stainless steel during welding of the copper or copper based alloy layer to the protective casing is further minimised. The buffer coating may suitably be made of nickel or a nickel based alloy, or at least comprise a layer of nickel or a nickel based alloy. Such a material can easily be welded according to conventionally known processes to a stainless steel surface without causing deterioration of the corrosion resistance of the stainless steel. Moreover, the copper or copper based alloy layer can easily be welded to the nickel or nickel based alloy. The nickel based alloy may suitably be a nickel-chromium based alloy.

The first outer circumferential surface of the first tubular end portion may suitably extend longitudinally from a first, preferably essentially radially arranged, longitudinal end surface of the protective casing, the first longitudinal end surface of the protective casing arranged at the first longitudinal end of the protective casing. In other words, the first outer circumferential surface may be a surface arranged at the very end of the protective casing at the first longitudinal end of the protective casing. Since the first outer circumferential surface of the first tubular part is provided with the buffer coating and the copper or copper based alloy layer, it is thereby ensured that the very end of the protective casing can be appropriately soldered to the metallic sheath of the cable by means of wipe soldering.

The first tubular end portion may further comprise a second outer circumferential surface coaxial with the first outer circumferential surface, the second outer circumferential surface having a greater diameter than the diameter of the first outer circumferential surface. The second outer circumferential surface is arranged at a distance from the first longitudinal end surface of the protective casing. This has the advantage of enabling an essentially constant outer diameter of the protective casing at the longitudinal end thereof despite the presence of the buffer coating and the copper or copper based alloy layer. In practice, this may be achieved by providing a first tubular end portion having a constant outer diameter

corresponding to the second circumferential outer surface, and reducing the diameter thereof in a first part of the tubular end portion such as to provide the first circumferential outer surface. Alternatively, two separate tubular parts having different diameters could be joined, such as by welding, one end surface to another such as to form the first tubular end portion having the first and second circumferential outer surfaces, respectively.

The buffer coating may suitably extend longitudinally from the first longitudinal end surface of the protective casing. This has the advantage of enabling the copper or copper based alloy layer to also extend from the first longitudinal end surface of the protective casing to thereby ensure that the very end of the protective casing can be properly wipe soldered to the metallic sheath of the first cable. Thus, the copper or copper based alloy may suitably extend from the first longitudinal end surface of the protective casing. The protective casing may further comprise a tin or tin based alloy layer on the outer circumferential surface of the copper or copper based alloy for the reasons explained above. Such a tin or tin based alloy layer may be easily applied to the protective casing before arrival at site or at site, at sea or the like, when the cables are to be joined.

One suitable copper based alloy to be used for the copper based alloy layer comprises at least 97 % by weight of copper (Cu) and at least 0.3 % by weight of tin (Sn). Such a copper based alloy can easily be welded to the buffer coating and furthermore ensures good adhesion of an outer tin or tin based layer applied thereto. The tin addition to the copper base alloy improves the flow/ability of the alloy and thus makes it easier to weld.

The first tubular end portion may suitably have an essentially constant inner diameter along the longitudinal extension of the first tubular end portion. This inter alia facilitates the manufacture of tubular end portion and ensures a good fit between the first tubular end portion and the cable when inserted therein.

A joint, termination or cross-connection of a cable in accordance with the present invention comprises at least a first cable and a protective casing as described above, and further comprises a solder joining the metallic sheath of the first cable with the copper or copper based alloy layer of the protective casing, or the optional tin or tin based alloy layer arranged on the copper or copper based alloy layer of the protective casing, around the circumferential extension of the metallic sheath of the first cable. Thereby, the solder provides a water diffusion tight and electrically connecting connection between the metallic sheath of the first cable and the protective casing.

The joint, termination or cross-connection may further comprise a spacer coaxial with the protective casing and at least partly arranged between the metallic sheath of the first cable and an inner circumferential surface of the first tubular portion of the protective casing.

Thereby, a good fit between the metallic sheath and the protective casing is ensured both during assembly and during use of the joint, termination or cross-connection of the cable.

The protective casing as described above may be produced by a method comprising welding the buffer coating to the first outer circumferential surface of the first tubular end portion around the entire circumferential extension of the first outer circumferential surface, and thereafter welding the copper or copper based alloy layer to the outer circumferential surface of the buffer coating around the entire circumferential extension of the outer circumferential surface of the buffer coating. The method optionally further comprises applying a tin or tin based alloy layer to the outer circumferential surface of the copper or copper based alloy layer. The present invention also relates to a method of forming a water tight and electrically connecting connection between a first high voltage cable comprising a metallic sheath and a protective casing. The method comprises providing a protective casing a described above, exposing the metallic sheath of the first high voltage cable at a first end of the first high voltage cable, and introducing the first end of the high voltage cable with the exposed metallic sheath into the first tubular end portion of the protective casing. The method further comprises soft soldering the metallic sheath of the first high voltage cable to the protective casing by externally applying a solder so as to form a connection between the metallic sheath of the first high voltage cable and the copper or copper based alloy layer of the protective casing or, when a tin or tin based alloy layer is arranged on the surface of the copper or copper based alloy layer, between the metallic sheath of the first high voltage cable and the tin or tin based alloy layer arranged on the copper or copper based alloy layer of the protective casing, around the entire circumferential extension of the metallic sheath. The solder may suitably be provided by means of wipe soldering.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a cross sectional view of a joint between two cables so as to form a long high voltage cable, the joint comprising a protective casing.

Figure 2 illustrates a perspective view of the joint according to Figure 1.

Figure 3a illustrates a cross sectional view of one embodiment of a first tubular end

portion of a protective casing in accordance with the present disclosure.

Figure 3b illustrates a cross sectional view of another embodiment of a first tubular end portion of a protective casing in accordance with the present disclosure.

Figure 4 illustrates the first tubular end portion according to Figure 3a when wipe

soldered according to one exemplifying embodiment to a metallic sheath of a cable inserted into the first tubular end portion.

Figure 5 illustrates the first tubular end portion according to Figure 3a when wipe

soldered according to another exemplifying embodiment to a metallic sheath of a cable inserted into the first tubular end portion. Figure 6 constitutes a microscopic photograph of a tested sample of a tubular end portion.

Figure 7 constitutes another microscopic photograph of a tested sample of a tubular end portion. Figure 8 constitutes yet another microscopic photograph of a tested sample of a tubular end portion

DETAILED DESCRIPTION

The invention will be described below with reference to the accompanying drawings. The invention however is not limited to the embodiments shown but may be varied within the scope of the appended claims. Moreover, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate the features of the protective casing or details thereof.

In the present disclosure, when a term such as "copper based alloy", "nickel based alloy" or the like is used, it should be regarded as an alloy wherein the metal specified constitutes the constituent element having the highest weight percentage of the constituent elements of the alloy composition. When two metal elements are given in such a term, for example "nickel- chromium based alloy" or the like, it should be regarded as the first specified element having the highest percentage by weight of the constituent elements of the alloy composition and the second specified element being present in a considerable amount (in most cases, the second highest percentage by weight of the constituent elements of the alloy composition).

Figure 1 illustrates a joint between a first cable 2 and a second cable 3 of a deep-sea high voltage cable. The first and second cables each comprise a respective water tight sheath 2a, 3a. The sheaths are made of metallic material, usually lead or copper. The cables each further comprise an insulation system 2b, 3b inside the metallic sheath as well as one or more conductors 2c, 3c inside of the respective insulation system. The conductors of the cables are connected to each other in a connecting portion 11 inside a protective casing 1 enclosing the connecting portion. Connection of the conductors may be performed in accordance with conventional processes known within the art and will therefore not be further discussed in the present disclosure.

The protective casing 1 may suitably be essentially rotational symmetrical around a centre axis A as shown in Figure 1, and comprises a first longitudinal end with an essentially radially arranged first end surface 10 and a second longitudinal end with an essentially radially arranged second end surface 14.

The protective casing 1 comprises a first tubular end portion 4 arranged at the first longitudinal end of the protective casing and which first end surface constitutes the first end surface 10 of the protective casing. The first tubular end portion may suitably have an essentially constant inner diameter along the longitudinal extension thereof. A second end portion 5 of the protective casing is arranged at a second longitudinal end of the protective casing. The first end surface of the second end portion constitutes the second end surface 14 of the protective casing.

The first and second end portions 4, 5 are each tubular and adapted for receiving an end portion of a respective cable 2, 3 when inserted therein. Thus, the respective end openings of the first and second end portions are dimensioned to receive a respective cable stripped down to the metallic sheath. The connecting portion 11 of the protective casing 1 may suitably have a larger cross sectional area than the first tubular end portion 4 as well as the second tubular end portion 5. As shown in Figure 1, the protective casing 1 may be constructed of a first casing half 12 and a second casing half 13 which are mounted and joined together by processes known in the art, for example as disclosed in EP 2 113 978 Al. When mounted together, the first casing half 12 and the second casing half 13 together form the connecting portion 11 of the protective casing and encloses the space therein where the conductors of the cables are joined. The protective casing need however not be divided into two casing halves as shown in Figure 1, but may instead be formed of a single casing body comprising the first and second tubular end portions and enclosing the connection portion 11.

At least the tubular end portions 4, 5 of the protective casing are made of stainless steel, preferably acid proof stainless steel. One example of an appropriate steel is 316L (UNS S31603), but other stainless steels may also be used. The protective casing 1 may suitably be essentially entirely made of stainless steel, except for the coatings and layers as will be discussed further below.

A solder (not shown), preferably provided by means of wipe soldering, joins the protective casing to the metallic sheath of the cables as will be further described below.

One or more additional protective layers (not shown) as previously known in the art may suitably be provided outside of the protective casing when the joining process of the cables and the wipe soldering of the protective casing is terminated. For example, a shrink hose, a tape or the like may be provided outside of the protective casing and a part of the respective cables at their extension outside of the protective casing such as to further protect the protective casing and solder against mechanical damage and/or corrosion.

Figure 2 illustrates a perspective view of the joint of two cables 2, 3 including the protective casing with an outer protective layer 15, such as a shrink hose.

Figure 3a illustrates a cross sectional view of a first tubular end portion 4 of one exemplifying embodiment of the protective casing 1 in accordance with the present invention. The tubular end portion 4 is made of a stainless steel and comprises a first outer circumferential surface 4a and an inner circumferential surface 4d with essentially constant diameter along the entire longitudinal extension of the first tubular end portion. As shown in Figure 3a, the first tubular end portion may further comprise a second outer circumferential surface 4b coaxial with the first outer circumferential surface. The second outer circumferential surface 4b has a greater diameter that the first outer circumferential surface 4a and is arranged further from the first longitudinal end surface 10 of the protective casing than the first outer circumferential surface. The first outer circumferential surface suitably extends longitudinally from the first longitudinal end surface 10. An essentially radial surface or a chamfer 4c may connect the first outer circumferential surface 4a to the second outer circumferential surface 4b of the first tubular end portion. In practice, the above first tubular end portion comprising the first and second outer circumferential surfaces may be achieved by providing a tubular end portion having a constant outer diameter corresponding to the second circumferential outer surface, and reducing the diameter thereof in a first part of the tubular end portion such as to provide the first circumferential outer surface. Alternatively, two separate tubular parts having different diameters could be joined, such as by welding, one end surface to another such as to form the first tubular end portion having the first and second circumferential outer surfaces.

Figure 3b illustrates a cross sectional view of a first tubular end portion 4 of another exemplifying embodiment of the protective casing 1 in accordance with the present invention, wherein the first outer circumferential surface 4a of the first tubular end portion extends along the entire longitudinal extension of the first tubular end portion. That is, the first tubular end portion according to the exemplifying embodiment shown in Figure 3b has an essentially constant outer diameter along longitudinal extension thereof.

As shown in both Figure 3a and Figure 3b, a buffer coating 6 is welded to the first outer circumferential surface 4a of the first tubular end portion. The buffer coating 6 extends around the entire periphery of the first outer circumferential surface 4a. Furthermore, the buffer coating 6 preferably extends longitudinally from the essentially radially arranged first longitudinal end surface 10 of the protective casing. Moreover, the buffer coating may or may not have a longitudinal extension which corresponds to the longitudinal extension of the first outer circumferential surface 4a. That is, the buffer coating may entirely cover the first outer circumferential surface 4a of the first tubular end portion 4, as shown in Figure 3a. The buffer coating may however if desired also have a smaller longitudinal extension than the

longitudinal extension of the first outer circumferential surface 4a. A smaller longitudinal extension of the buffer coating 6 than the longitudinal extension of the first outer

circumferential surface 4a may for example be used where the first tubular end portion does not comprise a second outer circumferential surface but has an essentially constant outer diameter along the entire extension thereof, as shown in Figure 3b. This has the benefit of avoiding the need to weld the buffer coating to the first tubular end portion at a connection of the first tubular end portion to an adjacent portion of the protective casing. Furthermore, it minimises the risk of the copper or copper based alloy (described further below) coming in direct contact with any part of stainless steel of the protective casing.

The buffer coating 6 can have an essentially constant thickness along the longitudinal extension of the buffer coating, for example as shown in the exemplifying embodiment illustrated in Figure 3b. However, the buffer coating 6 may also have a varying thickness along the longitudinal extension thereof, especially when the copper or copper based alloy layer 7 (described further below) does not entirely cover the buffer coating along the whole longitudinal extension of the buffer coating. A varying thickness of the buffer coating is shown in the exemplifying embodiment illustrated in Figure 3a, wherein the buffer coating has a greater thickness in a portion thereof not covered by the copper or copper based alloy layer. The purpose of different thicknesses of the buffer coating along the longitudinal extension thereof is to avoid any direct contact between the stainless steel of the first tubular end portion and the copper or copper based alloy. Furthermore, as illustrated in Figure 3a, a greater thickness of the buffer coating can fill any longitudinal space between the copper or copper based alloy layer and the stainless steel to thereby provide an essentially constant outer diameter of the protective casing at the first longitudinal end thereof constituting the first tubular end portion 4 with the buffer coating 6 and the copper or copper based alloy layer 7 welded thereto.

The buffer coating 6 is suitably made of metallic material(s) and serves the main purpose of ensuring that copper is not migrated or diffused into the stainless steel of the first tubular end portion during welding of essentially pure copper or a copper based alloy to the protective casing during manufacture thereof.

Welding the buffer coating 6 to the stainless steel surface constituting the first outer circumferential surface 4a of the first tubular end portion 4 ensures a metallurgical bond between the stainless steel and the buffer coating, and thus provides excellent adhesion and mechanical strength. Moreover, the buffer coating can be relatively thin while still serving its purpose.

The buffer coating may be made of a single buffer layer or being composed of a plurality of superposed buffer layers. In the case of a plurality of buffer layers, said buffer layers may be made of the same material or of different materials. All layers are however of metallic material, and should have good general corrosion resistance and not cause galvanic corrosion with adjacent materials when in use at the bottom of the sea.

The buffer coating 6 may suitably comprise at least one layer of essentially pure nickel or a nickel based alloy. In fact, the buffer coating may consist of such a layer of essentially pure nickel or a nickel based alloy. The nickel based alloy may suitably be a nickel-chromium based alloy as such alloys have excellent mechanical properties and corrosion resistance, and thus are well suited for extreme environments such as on the bottom of the sea. Moreover, such alloys can easily be welded to a stainless steel surface in accordance with conventional methods. One specific example of such an alloy is Inconel 625 (UNS N06625).

A copper (essentially pure copper) or a copper based alloy layer 7 is welded to the outer peripheral surface 6a of the buffer coating 6. The copper or copper based alloy layer extends around the entire periphery of the buffer coating. Furthermore, the copper or a copper based alloy layer 7 preferably extends longitudinally from the essentially radially arranged first longitudinal end surface 10 of the protective casing. Thereby, it is ensured that the first longitudinal end of the protective casing can be properly soft soldered to the metallic sheath of the cable. The copper or a copper based alloy layer 7 may or may not have a longitudinal extension which corresponds to the longitudinal extension of the buffer coating 6. That is, the copper or copper based alloy layer 7 may entirely cover the outer circumferential surface 6a of the buffer coating 6, but is may also have a smaller coverage as long as it is provided around the entire circumference of the buffer coating. It is essential that the copper or copper based alloy layer extends around the entire periphery of the protective casing to ensure that a tin or a tin based alloy may adhere properly around the entire periphery to ensure a proper solder joint to the metallic sheath of a cable or cable part.

Suitably, and as shown in Figures 3a and 3b, the copper or copper based alloy layer 7 has a longitudinal extension which is smaller than the longitudinal extension of the buffer coating 6 to ensure that it does not come into direct contact with the stainless steel surface of the first tubular end portion (or any other part of the protective casing made of stainless steel). Thus, any potential diffusion of copper into the stainless steel during the welding of the copper or copper based alloy layer to the protective casing is efficiently minimised by the buffer coating, thereby avoiding the risk for deteriorated corrosion. While not shown in the figures, it is also plausible that the entire outer surface of the first tubular end portion 4 is provided with a buffer coating 6 which essentially entirely covers the first outer surface 4a, and a copper or copper based alloy layer 7 arranged such as to cover essentially the entire outer surface of the buffer coating.

The copper or copper based alloy layer 7 may for example be made of essentially pure copper, a copper based alloy comprising tin, various brass alloys, or the like. One example of a suitable copper based alloy is an alloy comprising at least 97 % by weight of copper and at least 0.3 % by weight of tin. Such an alloy is easy to weld by conventional methods to the buffer coating disclosed above and can furthermore easily be soft soldered to tin or a tin based alloys.

While there is a sufficient galvanic potential between the copper or copper based alloy layer and the stainless steel, during use, which could cause galvanic corrosion when subjected to an electrolyte such as sea water, such galvanic potential is not sufficient for causing any substantial galvanic corrosion during use since the protective casing is covered by additional protective layers arranged outside of the protective casing and blocking entry of water into the joint. Thus, the problems associated with corrosion are essentially limited to the problems which may be caused during the welding process during production of the protective casing as a result of the high temperature exposure of the constituent materials during welding.

However, such a problem is efficiently circumvented by means of the buffer coating as previously described and the potential diffusion or migration of copper into the stainless steel is thereby minimized. The buffer coating and the copper or copper based alloy layer may be welded to the respective underlying substrate by means of conventional methods, in particular overlay welding (also known as cladding). More specifically, welding of the buffer coating as well as welding of the copper or copper based alloy layer to the protective casing may suitably be performed by means of Metal Inert Gas (MIG) welding. The protective casing may further comprise a tin (essentially pure tin) or tin based alloy layer (not shown) arranged directly onto the outer circumferential surface of the copper or copper alloy based layer. The purpose of such a layer of tin or tin based alloy is to ensure good adhesion of the solder to the surface of the copper or copper based alloy during wipe soldering. The tin or tin based alloy layer should therefore not be confused for the solder material applied during wipe soldering, while they are both applied by means of soft soldering, however different soft soldering methods. The tin or tin based alloy layer may have a similar chemical composition to the chemical composition of the solder used during wipe soldering. However, it is also plausible, in fact more likely, that the tin or tin based alloy layer has a chemical composition different from the chemical composition of the solder used during wipe soldering, for example in the case of a lead based solder material commonly used during wipe soldering. One example of a tin based alloy of said tin based alloy layer is a tin-lead based alloy, such as an alloy comprising about 60 % by weight of Sn and about 40 % by weight of Pb.

The tin or tin based alloy layer can easily be applied by soft soldering to the copper or copper based alloy layer without any specific pre-treatment of the surface of the copper or copper based alloy layer, and can be applied by conventional methods known in the art. It may thus readily be applied on site during the joining process of the cables without any significant loss of time. It may however also be applied off site during production of the protective casing if desired. Should the tin or tin based alloy layer be damaged during transport to or at the site at sea or the like, it can, in contrast to a pre-tinned stainless steel surface as previously described, easily be repaired on site manually.

It should be noted that even though not explicitly illustrated in the figures, the second tubular end portion 5 may suitably have the same geometrical construction and comprising the buffer coating and the copper or copper based alloy layer as described above with regard to the first tubular end portion. Figure 4 illustrates the first tubular portion 4 of Figure 3a when the protective casing is wipe soldered to the metallic sheath 2a of a first cable 2 in accordance with one exemplifying embodiment. While not explicitly shown in the figure, a tin or tin based alloy layer may preferably be provided on the outer surface of the copper or copper alloy based layer 7 and under the solder 8. The solder 8 is suitably provided by means of wipe soldering. However, other soldering methods are also plausible as long as they constitute soft soldering methods. The use of soft soldering to join the metallic sheath and the protective casing ensures that no part of the cable is damaged during the joining process. The solder ensures that a water diffusion tight joint is achieved, which is essential in order to avoid migration of moisture into the insulation system of the cables which in turn would risk causing degradation of the cables at the joint.

The solder 8 provides a mechanical connection as well as an electrical connection between the metallic sheath and the protective casing.

Figure 5 illustrates another exemplifying embodiment wherein the first tubular portion 4 of Figure 3a is wipe soldered to the metallic sheath 2a of a first cable 2 differing from the exemplifying embodiment shown in Figure 4 by the presence of a spacer 9. The spacer is coaxial with the protective casing. Moreover, the spacer is at least partly arranged between the metallic sheath of the first cable and an inner circumferential surface of the first tubular portion of the protective casing, i.e. at least a part of the longitudinal extension of the spacer is interposed between the metallic sheath and the first tubular portion. As shown in the figure, the spacer may also longitudinally extend outside of the protective casing with the purpose of facilitating assembly of the cable, the spacer and the protective casing before wipe soldering.

The purpose of the spacer 9 is to fill the space between the cable and the inner

circumferential surface of the first tubular portion and thereby facilitate a good fit between the cable and the protective casing during and after assembly of the joint. The spacer 9 is suitably made of a material with good soft soldering properties, for example copper or a copper based alloy (including for example brass), and may optionally be provided with a thin outer covering layer of tin or a tin based alloy. The spacer 9 may suitably comprise two radially opposing spacer halves (not shown) together forming an essentially rotationally symmetrical body constituting the spacer. The spacer may naturally be divided into more than two parts if desired, while this is not necessary. The small longitudinal gap between the spacer halves (parts) where they meet is suitably filled by a solder, such as a tin based solder, after the spacer halves have been provided on the outer surface of the metallic sheath of a cable.

As shown in Figure 5, the solder 8 applied by means of wipe soldering, extends from the first tubular end portion with the welded buffer coating 6 and the welded copper or copper based alloy layer 7, via the spacer 9, to the metallic sheath 2a of the first cable 2.

It should be noted that even though not illustrated in figures, a protective casing comprising a first tubular end portion as shown in Figure 3b may be wipe soldered to a metallic sheath in the same manner as illustrates in Figure 4 and Figure 5, respectively. The protective casing is not limited to the embodiments discussed above and shown in the figures but may be varied within the scope of the appended claims. For example, the protective casing may also be used for a termination of a cable, or a cross-connection of a high voltage cable, without departing from the scope of the invention. Furthermore, the protective casing need not comprise a second tubular end portion, and if comprising a second tubular end portion the second tubular end portion need not have the same configuration as the first tubular end portion.

Experimental results

Tested materials For welding experiments, a number of grade 316 L stainless steel pipes, adapted to be used as tubular end portions of a protective casing, were manufactured and fixed in the jaws of a router machine. The reason for using the router machine is that a constant welding speed can be preset and the welding position kept fixed to flat horizontal. This improves the bead geometry and reduces the amount of parameters the welder as to focus on. The pipe wall thickness was reduced from 5 mm down to 2 mm at one end of the stainless steel pipes by reduction of the outer diameter.

A first overlay was made with an Inconel 625 MIG weld deposit. This layer acts like a buffer in between the stainless steel parent material and the top layer of copper. The diameter was subsequently turned down to 67 mm. A top overlay was made with a MIG weld copper deposit using filler material (CuSnl). The diameter was then turned down to the original 70 mm

Compression test

Two of the welded pipe samples were subjected to a diameter reduction by a hydraulic compression tool of 5 and 10 mm respectively. The application at hand requires the overlay deposits to withstand a minimum of 5 mm reduction without cracking. No cracks were detected on any of the samples. The diameter and thickness of the samples before and after compression are given in Table 1. Table 1.

It was also noted that a tin solder (used for forming a tin based alloy layer) wets the copper overlay surface readily. The tin used is often referred to as "Chester tin" and is a flux cored 60/40 Sn/Pb soft solder alloy.

Macro/Micro investigation

The samples were radially cut in 20 mm sections and then cut lengthwise in 15 mm thick stripes. The stripes were then polished etched and examined visually in a microscope. Figure 6 shows a microscopic photograph of a part of a sample. The stainless steel grade 316 L is seen at the bottom. The Inconel 625 layer appears black due to the light conditions and shows moderate penetration into the 316 L material and is also present in the form of small droplets (black dots) in the CuSnl top layer. The reason for this is probably that Inconel 625 has a slightly lower density than copper (8.44 g/cm³ vs. 8.92 g/cm³ resp.) and has also a higher melting range (1290-1350 °C vs. 1020-1050 °C). During welding of the CuSnl layer the surface of the Inconel 625 is melted and a buoyancy force acts on the molten material which begins to rise and solidifies as small scattered droplets in the still liquid or mushy CuSnl weld metal. The dilution of the copper by the Inconel is thus somewhat higher than between the Inconel and the 316 L.

Figure 7 is a microscopic photograph constituting a close up on the boundary between the Inconel 625 (White) and the Cu Snl (Black). A peak to the right protruding from the Inconel side reveals a position where a metal droplet were about to be pinched off but has solidified before the detachment was completed.

Figure 8 is a microscopic photograph constituting a close up on the boundary between the stainless steel 316 L (right) and Inconel 625(left). The 316 L material shows (vertical) flow lines caused by the compression of the material but seems otherwise unaffected. Similar lines are also present inside the grains. The Inconel 625 is less affected as it unlike the SS 316 L hardens significantly by deformation. No cracks are present.

Bending test A sample of one of the welded specimens was cut and bent in a helix around a 10 mm steel rod. A small crack in the Inconel overlay was detected after the bending test. However, the test performed is significantly harder than a standardized welding procedure according to ISO 15614-7. Thus, considering the amount of applied mechanical stress prior to and during bending, the result is in no way of any concern with regards to functionality in the intended application.

Claims

A protective casing (1) for a joint, termination or cross-connection of at least one high voltage cable (2, 3), wherein the protective casing comprises a first tubular end portion (4) arranged at a first longitudinal end of the protective casing, the first tubular end portion adapted for receiving an end portion of the at least one high voltage cable (2, 3) when inserted therein, wherein the first tubular end portion (4) is made of stainless steel and comprises a first outer circumferential surface (4a),

characterised in that

the protective casing (1) comprises a buffer coating (6) welded to the first outer circumferential surface (4a) of the first end portion (4) and having a circumferential extension around the entire circumferential extension of the first outer circumferential surface (4a), and

a copper or copper based alloy layer (7) welded to an outer circumferential surface (6a) of the buffer coating (6) and having a circumferential extension around the entire circumferential extension of the outer circumferential surface (6a) of the buffer coating (6).

The protective casing (1) according to claim 1, wherein a longitudinal extension of buffer coating (6) is greater than a longitudinal extension of the copper or copper based alloy layer (7).

3. The protective casing (1) according to any one of claims 1 or 2, wherein the buffer coating (6) is made of nickel or a nickel based alloy or comprises a layer of nickel or a nickel based alloy.

4. The protective casing (1) according to claim 3, wherein the nickel based alloy is a

nickel-chromium based alloy.

5. The protective casing (1) according to any one of the preceding claims, wherein the first outer circumferential surface (4a) of the first tubular end portion (4) extends longitudinally from a first, preferably essentially radially arranged, longitudinal end surface (10) of the protective casing (1), the first longitudinal end surface (10) of the protective casing (1) arranged at the first longitudinal end of the protective casing.

6. The protective casing (1) according to claim 5, wherein the first tubular end portion (4) further comprises a second outer circumferential surface (4b) coaxial with the first outer circumferential surface (4a), said second outer circumferential surface (4b) having a diameter greater than the diameter of the first outer circumferential surface (4a) and being arranged at a distance from the first longitudinal end surface (10) of the protective casing (1).

7. The protective casing according to any one of claims 5 and 6, wherein the buffer

coating (6) extends longitudinally from the first longitudinal end surface (10) of the protective casing.

8. The protective casing according to claim 7, wherein the copper or copper based alloy layer (7) extends longitudinally from the first longitudinal end surface (10) of the protective casing.

9. The protective casing (1) according to any one of the preceding claims, further

comprising a coating layer of tin or a tin alloy on an outer circumferential surface (7a) of the copper or copper based alloy layer (7).

10. The protective casing (1) according to any one of the preceding claims, wherein the copper based alloy comprises at least 97 % by weight of Cu and at least 0.3 % by weight of Sn.

11. The protective casing according to any one of the preceding claims, wherein the first tubular end portion (4) has an essentially constant inner diameter along a longitudinal extension of the first tubular end portion.

12. A joint, termination or cross-connection of a cable; the joint, termination or cross- connection comprising at least a first cable (2) and a protective casing (1) in accordance with any one of the preceding claims, and further comprising a solder (8) joining the metallic sheath (2a) of the first cable with the copper or copper based alloy layer (7) of the protective casing, or the optional tin or tin based alloy layer arranged on the copper or copper based alloy layer (7) of the protective casing, around the

circumferential extension of the metallic sheath (2a) of the first cable (2) such as to provide a water diffusion tight and electrically connecting connection between the metallic sheath (2a) of the first cable and the protective casing (1).

13. The joint, termination or cross-connection according to claim 12, further comprising a spacer (9) coaxial with the protective casing (1) and at least partly arranged between the metallic sheath (2a) of the first cable (2) and an inner circumferential surface of the first tubular end portion (4) of the protective casing.

14. Method of producing a protective casing according to any one of claims 1 to 11, the method comprising:

welding the buffer coating (6) to the first outer circumferential surface (4a) of the first tubular end portion (4) around the entire circumferential extension of the first outer circumferential surface (4a),

thereafter welding the copper or copper based alloy layer (7) to the outer

circumferential surface (6a) of the buffer coating (6) around the entire circumferential extension of the outer circumferential surface (6a) of the buffer coating (6), and optionally applying a tin or tin based alloy layer to the outer circumferential surface (7a) of the copper or copper based alloy layer (7).

15. Method of forming a water tight and electrically connecting connection between a first high voltage cable (2) and a protective casing (1), the first high voltage cable (2) comprising a metallic sheath (2a), the method comprising

providing a protective casing (1) according to any one of claims 1 to 11,

exposing the metallic sheath (2a) of the first high voltage cable (2) at a first end of the first high voltage cable,

introducing the first end of the first high voltage cable with the exposed metallic sheath into the first tubular end portion (4) of the protective casing (1), and soft soldering the metallic sheath of the first high voltage cable to the protective casing by externally applying a solder (8), preferably by means of wipe soldering, so as to form a connection between the metallic sheath (2a) of the first high voltage cable (2) and the copper or copper based alloy layer (7), or the optional tin or tin alloy based layer arranged on the copper or copper based alloy layer (7), of the protective casing (1) around the entire circumferential extension of the metallic sheath (2a).