WO2008111847A1

WO2008111847A1 - Power cable

Info

Publication number: WO2008111847A1
Application number: PCT/NO2008/000092
Authority: WO
Inventors: Arild Figenschou; Knut NØGLEGÅRD
Original assignee: Aker Subsea As
Priority date: 2007-03-13
Filing date: 2008-03-12
Publication date: 2008-09-18
Also published as: NO20071361L; EP2126935A1; EP2126935A4

Abstract

A power cable (10') for transfer of large amounts of electric power is shown. The power cable is designed to be deployed in deep waters. The power cable comprises a centrally located power transferring conductor (1), filler material (2), and a protective sheath (8). Further, the power cable (10') includes separate load carrying elements in the form of light weight rods (5) of composite material, said rods (5) being peripherally located around the power transferring conductor (1) and spaced a predetermined distance apart and having filler elements (7) therebetween.

Description

Power cable

The present invention relates to a power cable for transfer of large amounts of electric power, which power cable being designed to be deployed in deep waters, said power cable comprises a centrally located power transferring conductor, filler material working as an insulator, and an insulator surface.

The invention also relates to a method for the manufacturing of a power cable of the introductory said kind, where the basis can be an existing power cable consisting of a centrally located power transferring conductor, filler material working as an insulator, a screening external of the insulator or integrated with the insulator, and an outer surface.

More recently, it has been more common to locate the production equipment for oil and gas wells on the sea bed. A supply of electric power is needed for the operation of such equipment, not only electric wires for controlling functions, but also heavy gauge cables for transport of electric power to electric driven equipment, like large pump stations that provides for displacement of recovered oil and/or gas.

Another usage that becomes current is power cables from wind mills that are erected at sea. In order to be able to transfer the generated energy from the generators of the wind mills, heavy power cables are deployed on the sea bed from the wind mills to an onshore terminal.

These heavy electric cables, normally made of copper wire, are now integrated with light weighted load carrying elements. These heavy electric cables having substantial cross sectional area ads considerable weight to the cable due to the specific gravity of copper. Being aware that copper has very poor load carrying capacity, it will be a very limited sea depth that the classic cables can be deployed at.

Intuitively, it may appear as steel wire or similar could be used as load carrying elements. But this does not resolve the problem in significant extent. Steel wire also ads substantial weight and in combination with the heavy copper conductors, thus large depths are not reached before the power cable reaches yield stresses and is overstrained due to its own weight.

It has been put substantial efforts and resources in attempting to find a solution of how to be able to install a power cable down to the sea bed at larger sea depths, like 1500 meters and deeper. Typical square section areas for the conductor itself can be 300mm and the electric voltages that occur can be in the order of magnitude 36-145kV.

Thus, according to the invention, a power cable of the introductory defined kind is provided, which is distinguished in that the power cable includes separate load carrying elements in the form of light weight rods of composite material, said rods being peripherally located around the power transferring conductor and are spaced a predetermined distance apart and that a protective outer sheath is extruded onto and around said rods.

In one embodiment filler elements can be provided between the light weighted rods of composite material and are laid with the same helix as the rods.

Further, the filler elements can be in the form of elongated rigid elements that are able to retain the light weighted rods in position relative to each other.

Moreover, the filler material can be laid in several layers when viewed in radial direction.

In one variant the invention can be a lighter type of cable for transfer of more moderate amounts of electric power, such as a signal cable, where the cable is intended to be installed at deep waters, and includes a number of centrally located power transferring cables, optionally filler material and isolating material, where the cable is distinguished in that it includes separate load carrying elements in the form of light weighted rods of composite material, said rods are laying in a helix in between the power transferring conductors and have a predetermined distance apart from each other and a protective outer sheath is extruded onto and around said rods. The light weight rods of composite material can preferably be rods having embedded strength fibres, such as carbon fibres. The rods can either be collected into bundles or appear individually or a combination thereof.

The composite material have that excellent property that it can have the same load carrying capacity as steel, at the same time as the weight is reduced to about 10% of steel. Thus the load carrying elements do not substantially contribute to the total weight of the power cable which thereby provides the opportunity to use the power cable at large sea depths. One will be able to go so far deeper as the weight reduction allows for.

In a preferred embodiment the light weighted rods can have a diameter in the order of magnitude 2mm.

According to the present invention, also a method of the introductory said kind is provided, which is distinguished in that separate load carrying elements in the form of light weighted elements of composite material are helically wound externally on the outer surface of the power cable by a predetermined distance apart from each other and that a protective outer sheath is extruded onto and around the light weighted rods of composite material.

In another embodiment the filler elements are additionally wound onto the outer surface and are laying between the light weighted rods of composite material and a protective outer sheath is extruded external to the light weighted rods of composite material and the filler elements.

It is further to be understood that the power cable can be manufactured in one and the same operation starting with the production of the per se known, existing power conductor. Other and further objects, features and advantages will appear from the following description of preferred embodiments of the invention, which is given for the purpose of description, and given in context with the appended drawings where:

Fig. 1 shows a cross sectional view of a first embodiment of a power cable according to the invention,

Fig. 2 shows a cross sectional view of a second embodiment of a power cable according to the invention, and

Fig. 3 shows a cross sectional view of a variant in the form of a signal cable according to the invention.

With reference to figure 1, a first embodiment of a power cable 10 according to the invention will now be described. The power cable 10 can be considered as a compound cable having basis in a traditional cable which is basically constructed of the following elements viewed from inside and outwards in radial direction: Power conductor 1 of substantial cross sectional area, normally of copper material or aluminium, the power conductor 1 can be solid or composed of twisted single strands in classic way; Filler 2 outside the conductor 1, the filler 2 acts as an insulator at the same time as it is able to withstand substantial radially acting forces; A screening 3 that acts as a noise insulator, normally a copper screen, which screens for electric fields; more filler material 2a that terminates in an insulator surface 4. The above described constituents have a construction corresponding to a per se traditional power cable.

The new and unique constituents of the power cable are lying external of what is described above and comprises load carrying elements consisting of rods 5 of composite material, normally rods 5 of suitable plastic material having embedded strength fibres of carbon. These are laid in a helix external of said insulator surface 4. Finally an outer sheath 6, for example of polyethylene (PE), is extruded external to the insulator surface 4 and embeds the rods in the sheath matrix.

The diameter of such rods 5 are in order of magnitude 2mm depending of the dimension of the electric conductor 1 and this dimension is not to be regarded as any limitation. Reference is now made to figure 2 for the description of a second embodiment of a power cable 10' according to the invention. The traditional part is like the one described above together with figure 1 and will not be repeated here. As above, the new constituents of the power cable consist of load carrying elements consisting of rods 5 of composite material. In addition, a number of elongate, rigid filler elements 7, for example of polyvinylchloride (PVC), are arranged between the light weighted rods 5 of composite material and are lying with the same helix. The filler elements 7 can be in the form of elongate rigid elements that are able to retain the light weighted rods 5 in position relative to each other. Finally an outer sheath 8, for example of polyethylene (PE), is extruded externally onto the rods 5 and the filler elements 7.

Figure 3 shows a variant where a per se traditional signal cable 10" constitutes the traditional constituent corresponding to what is described above. The cable comprises four centrally located conductors 1 ' that are twisted around each other and having a filler material 9 in centre thereof. Each conductor can be surrounded by insulating material 2'. The cable 10" comprises four separate load carrying elements in the form of light weighted rods 5 of composite material. The rods 5 are laying in a helix in between the conductors 1 ' and have a predetermined distance apart from each other. A protective outer sheath 8' is extruded onto and around the rods 5.

Also a method to manufacture a power cable of the above described kind is suggested. The base can for example be an existing power cable consisting of a centrally located power transferring conductor that can be solid copper or aluminium, alternatively be wound of single strands of same materials. External to the conductor, a filler material having the function as an insulator is provided. Further, a screening, or noise insulator, is provided outside the insulator or integrated within the insulator. This is normally a copper screening. The described elements can constitute an existing power cable that terminates in an outer surface.

The in particular unique by the method will be that the separate load carrying elements, in the form of light weighted rods of composite material, are wound in a helix external of the outer surface of the power cable by a predetermined distance apart from each other and that a protective outer sheath is extruded onto and around the light weighted rods of composite material.

In the embodiment shown in figure 2, filler elements are in addition wound onto the outer surface of the original cable and remain between the light weighted rods of composite material. Finally a protective outer sheath is extruded outside the light weighted rods of composite material and the filler elements.

It is, however, to be understood that the complete power cable now suggested can be manufactured in one and same operation by use of several working stations placed after each other for each individual step in the manufacturing process thereof.

Claims

P a t e n t c l a i m s 1.

A power cable for transfer of large amounts of electric power, which power cable being designed to be deployed in deep waters, said power cable comprises a centrally located power transferring conductor (1), filler material (2) working as an insulator, and an insulator surface (4), characterized in that the power cable (10) includes separate load carrying elements in the form of light weight rods (5) of composite material, said rods (5) being peripherally located around the power transferring conductor (1) and are spaced a predetermined distance apart and that a protective outer sheath (6) is extruded onto and around said rods (5).

2.

The power cable according to claim 1, characterized in that filler elements (7) are arranged between the light weighted rods (5) of composite material and are laid with the same helix as the rods.

3.

The power cable according to claim 1 or 2, characterized in that the filler elements (7) are in the form of elongated rigid elements that are able to retain the light weighted rods (5) in position relative to each other.

4.

The power cable according to any of the claims 1-3, characterized in that the filler material is laid in several layers when viewed in radial direction and includes a noise insulator (4), or copper screening.

5.

A cable for transfer of electric power, where the cable is intended to be installed at deep waters, including a number of centrally located power transferring conductors (1'), optionally filler material (9) and isolating material (2), characterized in that the cable (10") comprises separate load carrying elements in the form of light weighted rods (5) of composite material, said rods (5) are laying in a helix in between the power transferring conductors (1') and have a predetermined distance apart from each other and that a protective outer sheath (8') is extruded onto and around said rods (5).

6. The power cable according to any of the claims 1-5, characterized in that the light weighted rods (5) of composite material are rods of a suitable plastic material having embedded strength fibres, such as carbon fibres.

7. The power cable according to any of the claims 1-6, characterized in that the light weighted rods (5) have a diameter in the order of magnitude 2mm.

8.

A method for production of a power cable according to any of the claims 1-7, where the basis is an existing power cable consisting of a centrally located power transferring conductor, filler material working as an insulator, a screening external of the insulator or integrated in the insulator, and an outer surface, characterized in that separate load carrying elements in the form of light weighted rods of composite material are helically wound externally on the outer surface of the power cable by a predetermined distance apart from each other and that a protective outer sheath is extruded onto and around the light weighted rods of composite material.

9.

The method according to claim 8, characterized in that the filler elements are additionally wound onto the outer surface and remain between the light weighted rods of composite material and that a protective outer sheath is extruded onto to the light weighted rods of composite material and the filler elements.

10. The method according to claim 8 or 9, characterized in that the power cable is manufactured in one and the same operation starting with the production of the per se known, existing power conductor.