Technical field of the invention

The disclosure relates to a rigid joint assembly for electric cables, and pri for medium and high voltage submarine cables.

Background

High voltage (HV) and medium voltage (MV) cables are used for power distribution on land and in the sea. Such cables often uses an extruded insulation system and comprise an electric conductor that is surrounded by an insulation system and a number of layers of different materials having different purposes and uses, e.g. as many as eight to nine layers. The insulation system comprises an inner semi-conducting layer closest to the conductor, an insulation layer externally of the conductor screen and an outer semi-conducting layer.

It is common to use the term cable core, and generally the cable core comprises the main layers of an inner electric conductor, and the insulation system as described above and comprising at least an inner semi-conducting layer, an insulation layer and an outer semi-conducting layer. The term cable core usually also includes, externally of the insulation and from inside out, any optional swelling material, if used, a metal sheath and an extruded oversheath that may be semi-conducting or insulating.

A pre-fabricated joint can be used when jointing two lengths of cable. The prefabricated joint comprises a pre-moulded/pre-fabricated joint body, e.g. of rubber, that is used to restore the insulation system when jointing two lengths of cable. The conductors of the cable cores are jointed and the insulation systems of the jointed cable cores are restored in the joint body. This type of joint is commonly used for jointing high voltage cables with an extruded insulation system, normally comprising cross linked

polyethylene (XLPE). For submarine cables, the pre-fabricated joint body is mounted in air at atmospheric pressure and then placed inside a water tight metal casing. The metal sheath of the cable core is normally connected to the casing through soldering, thereby achieving an overall watertight design for the joint.

For submarine DC cables containing one cable core, a rigid joint consists of one of these metallic casings containing a cable core joint, where the casing normally is placed in an outer container that is also used to connect the armour layers of the cable. For submarine AC cables containing three cable cores, a rigid joint consists of three of these metallic casings, each containing a cable joint, where the casings are normally placed in an outer container which is also used to connect the armour layers of the cables. The entire joint including the outer container is commonly referred to as a rigid joint.

When such a rigid joint is used for jointing of submarine cables, the outer container that surrounds the water tight metal casing/casings has a mechanical function of protecting the casings and it is usually filled with water, when the cable and the rigid joint is submersed into the water. Thus, the inner water tight casing functions as a pressure vessel with an atmospheric pressure inside, and hydrostatic pressure of the water outside the casing. This results in a pressure gradient along the cable core that is being jointed. The above described type of rigid joint with pre-fabricated rubber joint has successfully been implemented for submarine cables at water depth up to approximately 600 m, corresponding to a hydrostatic pressure of approximately 6 MPa.

However, the question arises if such rigid joints could be used for large depth water, deeper than 600 m

It has been found that for large water depths the scenario does not look well if a regular rigid joint is used. According to numerical analysis and experiments, an excessive deformation is expected over the extruded insulation in a critical transition region just outside the inner casing, where the cable core is entering into the inner casing. A significant reduction in the outer diameter of the cable core insulation occurs, so called necking, which is due to plastic deformation, yielding and/or creep of the cable core insulation. Such deformations can significantly affect the optimal electrical characteristics of the extruded insulation, e.g. create undesirable consequences for the electrical field distribution over the cable and therefore cause its failure under operation.

The critical transition region, where the cable core is close to entering the inner casing of the joint, is severely affected by a significant pressure difference or gradient. Outside the casing, the cable core is exposed to a high hydrostatic pressure due to the large water depth, while inside the casing the cable core is under atmospheric pressure. During operation, the insulation system will be heated which reduces the mechanical strength of the extruded insulation, making the insulation even more susceptible to deformation.

Also, over this critical transition region, the high pressure difference creates a significant unbalanced compressive stress state on the cable along its axial direction. Thus, at the same time as the excessive necking occurs, there is a tendency for extruded insulation to be displaced along the axial direction of the cable core towards the interior of the casing, where the pressure is lower. In addition to negatively affecting the electrical properties, this could also affect the water tightness of the casing at the location where the core enters the casing.

Summary of the invention

An object of the present invention is to provide an improved rigid joint assembly that is suitable to use for submarine cables in large depth waters.

According to the invention is defined a rigid joint assembly comprising a first cable core end section of a first electric cable, and a second cable core end section of a second electric cable, said respective first and second cable core end section comprising an electrical cable core comprising at least an inner electric conductor, and an insulation system comprising at least an inner semi-conducting layer, an insulation layer and an outer semi-conducting layer, said rigid joint assembly further comprising a joint connection inside which the electrical cable core of the first cable core end section of the first electric cable is jointed with the electrical cable core of the second cable core end section of the second electric cable, and the rigid joint assembly further comprising a water tight metal casing surrounding the joint connection, which casing comprises a first cable entry part comprising an opening for receiving the first cable core end section of the first cable and a second cable entry part comprising an opening for receiving the second cable core end section of the second cable, and wherein the casing contains a compressible gas, characterized in that the assembly further comprises a first load distributing member that surrounds the first cable core end section at the first cable entry part, and a second load distributing member that surrounds the second cable core end section at the second cable entry part, and that the respective first and second load distributing member comprises a hollow member having an essentially conical external shape and a cylindrical internal surface that is in contact with an external surface of the cable core end section which it surrounds.

By providing a load distributing member comprising a hollow member that surrounds the cable core end section of the respective cable at the cable entry part of the casing is obtained the advantage of the possibility to use the rigid joint assembly at large water depths.

This is because large concentrated stresses over the cable core insulation, as a result of the material stiffness discontinuity between the cable core and the metallic casing at the cable entry part combined with the high hydrostatic pressure, are reduced In fact, the load will be re-distributed over a larger and longer part of the cable core end section by means of the hollow member that surrounds the cable core end section. By distributing the load, an accentuated local deformation of the cable insulation, such as the described necking, will be prevented.

According to one feature, the hollow member may be made of a semi-rigid material having a compression modulus of not more than 15 000 MPa at a temperature of between 20°C and 120°C. By this is meant that consideration has to be taken to the expected temperature of the rigid joint assembly such that the compression modulus should not be more than 15 000 MPa at the expected temperature. To make the hollow member of a semi-rigid material will contribute to mitigate the material-stiffness discontinuity between the cable core and the metal casing, in the transition region.

Examples of materials are rubber materials, a fairly soft metal such as lead, metal fiber reinforced polymer, or any other material fulfilling the requirement. The compression modulus may e.g. be measured according to the applicable ISO-standard for the selected material.

According to another feature, the hollow member may be made of an

incompressible material having a Poisson's ratio of approximately 0,5 at a temperature of between 20°C and 120°C. By this is meant that consideration has to be taken to the expected temperature of the rigid joint assembly such that Poisson's ratio should be approximately 0,5 at the expected temperature. Materials having these properties will provide a stiffness to the hollow member that will make it possible to withstand the hydrostatic conditions at large water depths. Examples of materials are rubber materials, polymer materials. According to a further feature, the hollow member may be secured onto the respective cable core end section such that axial movement of the cable core end section in relation to the hollow member is prevented. This will have the effect of preventing that the cable insulation system, or parts thereof, is deformed and/or displaced in the cable entry part of the casing.

According to yet another feature, the hollow member may be secured onto the respective cable core end section by means of an interference fitting. By effectively securing the hollow member to the cable core end section is ascertained that the hollow member is retained in the desired position on the cable core such that the load is properly distributed over the cable core. The axial length of the hollow member may be at least 200 mm. The load distribution will be improved by the hollow member having a certain length.

According to one variant, the hollow member may be secured directly onto the outer semi-conducting layer of the insulation system of the cable core of the respective cable core end section.

According to another variant, wherein the cable core of the respective cable core end section further comprises an outer metal sheath, externally of the cable insulation system, the hollow member may be secured onto the outer metal sheath of the cable core of the respective cable core end section.

According to yet another variant, wherein the cable core of the respective cable core end section further comprises a bedding material externally of the cable insulation system, the hollow member may be secured onto the bedding material of the cable core of the respective cable core end section.

According to yet another variant, wherein the cable core of the respective cable core end section further comprises an outer metal sheath, externally of the cable insulation system, and a protective oversheath externally of the outer metal sheath, the hollow member may be secured onto the protective oversheath of the cable core of the respective cable core end section. Such an oversheath may for example be of extruded polymer, it may be of an insulating or it may be semi-conducting material.

According to a further feature, the hollow member may be axially locked in relation to the casing, such that movement of the hollow member, in its axial direction, in relation to the casing is prevented. By also locking the hollow member in relation to the casing, and thereby locking the cable core end section to the casing, an effective way is obtained to prevent deformation and/or displacement of the cable insulation system or parts thereof. This can achieved by an inner coupling that is inserted between the respective cable entry part of the casing and the respective cable core end section thereby securing the inner coupling and the casing to the respective cable core end section, and wherein the hollow member abuts an outer end of the inner coupling thereby preventing movement of the hollow member in its axial direction in relation to the casing. This solution has the advantage that the inner coupling may be the type of coupling that is already today used for securing a casing to a cable.

According to another feature, the hollow member may have an external diameter that is larger at an inner end close to the casing than at an outer end facing away from the casing. By this feature is introduced a gradually increased radial stiffness, from the outer end to the inner end in the axial direction, in the transition region where the high pressure gradient is acting, as explained above. This will mitigate the effect of the material-stiffness discontinuity between the cable and the metal casing.

According to yet another feature, the thickest part of the hollow member's wall may have a thickness of at least 25 mm. This will also contribute to obtain the desired stiffness.

According to a further feature, an inner end part of the hollow member may extend over and externally of at least a part of the cable entry part of the casing.

The joint connection mentioned is primarily, but not limited to, the type of prefabricated joint known in prior art as described above and comprising a pre- moulded/pre-fabricated joint rubber body that is used to joint two lengths of cable. The type of cable is primarily a submarine cable having a cable core comprising an inner conductor and an extruded insulation system as described above. The cable core may also comprise further layers such as a metal sheath, a bedding layer, outer protective layer of e.g. polymer, as indicated in the dependent claims. The cable also comprises a tensile armour layer, e.g. of metal wires, or other load carrying members. When laid down in water, one or more rigid joint assemblies according to the invention would usually be placed in an outer container which may also be used to connect the armour layers of the cables, as previously described. Further features and advantages of the invention will also become apparent from the following detailed description of embodiments.

Brief description of the drawings

The invention will now be described in more detail, with reference being made to the enclosed schematic drawings illustrating different aspects and embodiments of the invention, given as examples only, and in which:

Fig. 1 illustrates schematically an embodiment of a rigid joint assembly according to the present invention, in a perspective view,

Fig. 2 is a schematical illustration of the rigid joint assembly of Fig. 1 , in cross section, and

Fig. 3 is a detailed schematical illustration of an embodiment of a rigid joint assembly, in cross section.

Elements that are the same or represent corresponding or equivalent elements have been given the same reference numbers in the different figures.

Detailed description

In Fig. 1 is illustrated a rigid joint assembly 1 comprising a water tight metal casing 30 inside which is located a joint connection 20 connecting the electrical cable core end of a first electric cable 10 and the electrical cable core end of a second electric cable 1 10. The cables are medium or high voltage cables suitable for submarine installation. The joint connection is e.g. a pre-fabricated joint of the type described above, comprising a pre-moulded/pre-fabricated rubber joint body that is used to restore the insulation system where the two cable core ends are jointed. The joint connection 20 is located in the hollow interior 36 of the water tight metal casing 30, as shown in Fig. 2. The casing 30 comprises a first cable entry part 32 having an opening 34 through which a cable core end section 12 of the first cable 10 enters into the casing. This cable core end section 12 of the first cable will be referred to as the first cable core end section. At the opposite end of the casing 30 there is a second cable entry part 132 having an opening 134 through which a cable core end section 1 12 of the second cable 1 10 enters into the casing. This cable core end section 1 12 of the second cable will be referred to as the second cable core end section. When laying down the cables with the shown rigid joint assembly, one (if DC cable) or three (if AC cables) of these rigid joint assemblies are normally placed in an outer container (not shown) which is also used to connect the armour layers (not shown) of the cables. In the case of submarine cables, the outer container is filled with water that will consequently surround the casing 30. However, inside the casing there is still the same air pressure as when the casing was installed around the joint connection on a vessel or on shore.

A schematic cross section of an example of a cable 10 is shown in Fig. 3, together with an embodiment of the rigid joint assembly according to the present invention. The concerned type of high voltage cable of extruded, e.g. XLPE, type will have many layers, but only the main layers of the electrical cable core are shown in Fig. 3. In the illustrated example is shown the electrical cable core comprising an electric conductor 14 surrounded by an insulation system comprising an inner semi-conducting layer 15, an insulation layer 16 of e.g. XLPE, and an outer semi-conducting layer 17. Externally of the insulation system is a metal sheath 18, e.g. a lead sheath. These comprise the main layers of the cable core. The cable core may sometimes also comprise other internal layers, e.g. fillers or beddings, and it may comprise an extruded oversheath externally of the metal sheath. The cable will comprise other layers externally of the cable core, including tensile armour layer (s). However, these external layers have been removed at the cable core end section 12 of the cable 10 when preparing the cable for the joint. In the illustrated example, the cable core end section 12 of the cable 10 comprises only the mentioned main cable core layers.

In the interior 36 of the casing there is a compressible gas, usually air at atmospheric pressure. However, the cable 10 outside of the casing is subjected to the hydrostatic pressure from the water, as already explained, and this pressure is much higher than the pressure inside the casing. This results in a pressure gradient that will have an adverse effect on the cable core end section 12 of the cable in a pressure transition region in the vicinity of where the cable core enters the casing. This transition region will extend outside of where the cable core end section 12 of the cable enters into the casing 30 via the opening 34 and it will also to some extent extend inside the cable entry part 32 of the casing. As can be seen in the figures, the casing 30 commonly have cone shaped end portions that are terminated by sleeve shaped parts that surrounds the cable core rather closely, where the cable core enters the casing. In order to prevent deformation of the core, and in particular deformation of the cable core insulation layer 16, that is caused by the pressure gradient, a load distributing member 40 is arranged to surround the cable core end section 12 of the first cable 10, at the first cable entry part 32, see Figs. 2 and 3. A corresponding load distributing member 140 is also arranged to surround the cable core end section 1 12 of the second cable 1 10. In the illustrated embodiment, the load distributing member 40, 140 is in the shape of a hollow member 41 having an essentially conical external shape and a cylindrical internal surface 43 that is in contact with the external surface of the cable core end section 12, 1 12 which it surrounds. This, in the illustrated embodiment the hollow member is a hollow cone 42 with an axial through-hole. The hollow member is arranged concentrically with the cable core end section of the cable and should fit narrowly over the cable core in order to distribute the load resulting from the water pressure, and also to prevent that the cable insulation system is displaced and deformed due to the pressure differences. If there is a high pressure on the cable core in such a transition area, this may lead to deformation in that area by necking. However, this may also result in that the insulation material will have a tendency to be displaced from the necking area, along the axis of the cable core, to another area with lower pressure where it can expand and a bulging of the insulation instead will occur. By having a tight fit of the hollow member 41 over the cable core, such deformation by displacement of the insulation material will be prevented, since there will not be any empty space inside the hollow member that allows for any bulging of the insulation material. Thus the hollow member 41 should have a smooth interior surface that can fit snugly over the external surface of the cable core end section 12 of the cable 10. Preferably it should be in direct contact with the surface of the cable core end section, and preferably in continuous contact with the surface of the cable core end section.

It is desirable that the hollow member should be axially immovable in relation to the cable core end section, e.g. by high friction between the hollow member and the cable surface. Thus, the hollow member 41 is secured onto the respective cable core end section 12, 1 12 such that axial movement of the cable core end section in relation to the hollow member is prevented. For example, there may be an interference fit between the hollow member 41 and the cable core end section 12, and the hollow member 41 can e.g. be press-fitted onto the cable core end section in order to have no gaps between the hollow member and the cable core and also prevent axial displacement of the cable insulation system.

The external geometry of the hollow member 41 also has an impact on its function. In the shown embodiment, the hollow member has the general external shape of a cone. The wide base end of the cone is located at the cable entry part 32 of the casing 30, which base end hereinafter will be referred to as the inner end 44, and from there the cone extends in the direction away from the casing, along the end section 12 of the cable, to its opposite narrower top end, which is truncated, and where the cable end section 12 exits the cone. This top end of the cone will hereinafter be referred to as the outer end 45 of the cone. Thus, the hollow member has an external diameter that is larger at the inner end 44 close to the casing than at the outer end 45 facing away from the casing. By the conical shape of the hollow member is introduced a gradually increased radial stiffness in the transition region where the high pressure gradient is acting, as explained above. This will mitigate the effect of the material-stiffness discontinuity between the cable and the metal casing. The thickness of the cone wall, at the location where the wall has its largest thickness, should be between15mm and 40mm, and preferably not less than 25mm. A suitable length of the hollow member has been found to be between 100mm and 300mm, preferably at least 200mm.

The material of the hollow member 41 is preferably a semi-rigid material that will also contribute to mitigate the material-stiffness discontinuity between the cable end section and the metal casing, in the pressure transition region. Examples of materials are rubber materials, a fairly soft metal such as lead, metal fiber reinforced polymer, or any other material having a compression modulus not exceeding 15 000 MPa, for the expected temperature, which e.g. may be between 20°C and 120°C. The material should also preferably be incompressible, having a Poisson ratio of approximately 0,5, at the expected temperature, which e.g. may be between 20°C and 120°C. Materials having these properties will provide a stiffness to the hollow member that will make it possible to withstand the hydrostatic conditions at large water depths.

The cable end section 12 and the hollow member 41 should also be immovable in the axial direction in relation to the casing 30. The casing 30 is mounted on the cable core by means of an inner coupling 50, e.g. a brass inner coupling. This coupling is locked in position due to its contact with the casing. In order to fix the hollow member 41 in its axial direction, it is mounted such that it abuts the inner coupling. When assembling the rigid joint, the hollow member 41 would first be mounted over the cable core end section 12. Then, as in prior art, the casing can be mounted over the cable end and locked to the cable by means of the inner coupling 50 being inserted between the respective cable entry part 32, 132 of the casing and the cable core end section 12, 1 12. The casing and the inner coupling will thus be secured to the cable core end section by means of friction. The hollow member can then be moved in a direction towards the casing until it abuts the outer end of the inner coupling. In this way, the hollow member is prevented from moving further axially towards the casing. Generally, this direction is the only direction in which the hollow member would move, due to the pressure gradient. In the illustrated example, the inner end 44 of the hollow member is configured to extend externally over the inner coupling 50. It may also extend onto the cable entry part 32 of the casing and thereby overlapping the inner coupling 50 and at least a part of the cable entry part of the casing. In both cases the hollow member can be fixed to the respective part by means of friction. As a result the inner coupling can also function to secure the hollow member 41 to the casing 30 and thereby the hollow member is also axially locked in relation to the casing.

In the illustrated example, the outer layer of the cable core 10 onto which the hollow member 41 is mounted, has been described as being the lead sheath. However, it may also be possible to mount the hollow member directly onto the outer semiconducting layer of the insulation system. Alternatively, if the cable core end section comprises a bedding material externally of the cable insulation system, such as a layer of tape, the hollow member may be mounted on the bedding material. It may also be possible to mount the hollow member on a protective oversheath that is superposed e.g. the lead sheath.

It should be understood that the exact axial location of the respective load distributing member 40, 140 in relation to the respective cable entry opening 34, 134 of the casing 30 can be varied somewhat depending on circumstances on a case to case basis. Generally, the load distributing member 40, 140 should be located such that there is no part of the cable insulation system that is directly exposed to external pressure between the member and the cable entry part of the casing. In figures 2 and 3 and the part of the description above that is related to these figures, the word cable has been used and the reference numbers related to the first cable in the joint of Fig. 1 have been used. However, it should be understood that everything that has been described relating to "the cable" and when using the reference numbers of the first cable 10 are equally applicable to the second cable 1 10 shown if Fig. 1.

In order to obtain the final rigid joint, the described inventive rigid joint assembly is placed in an outer container (not shown) in the usual manner that has been described in the background part of this description. For submarine DC cables containing one cable core, one rigid joint assembly is placed in the outer container which is also used to connect the armour layers of the cable. For submarine AC cables containing three cable cores, three of the described inventive rigid joint assemblies are placed in one outer container which is also used to connect the armour layers of the cables.

The invention shall not be considered limited to the illustrated embodiments, but can be modified and altered in many ways, as realised by a person skilled in the art, without departing from the scope defined in the appended claims. In particular, the invention should not be limited to a certain type of cable, but should encompass any type of electric cable having one or more electric cable cores that falls within the scope of the appended claims.