Abstract:
Submarine cables ( 10, 11 ) have armouring ( 18, 19 ) surrounding the cable core ( 12 ), which armouring protects the cable core ( 12 ) particularly from mechanical loads. The armouring ( 18 ) is designed so that it can withstand the mechanical stresses to which the submarine cable ( 10, 11 ) is subjected when laid at the greatest depths provided for. Such armouring ( 18 ) is overdimensioned in areas of lesser depths. The invention provides for a submarine cable ( 10, 11 ), and a method for the manufacture thereof, a corresponding number of armouring wires ( 25 ) used to form the armouring ( 18 ) being replaced as necessary by filler strands ( 31 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a cable, especially a submarine cable, with a cable core having at least one conductor and with armoring surrounding the cable core, which armoring has armoring wires, or with a cable core having at least one conductor and with armoring surrounding the latter and composed of a plurality of armor sections, which have armoring wires. The invention further relates to a method for the manufacture of a cable, especially a submarine cable, a continuous cable core being provided with at least one outer armoring. 
     PRIOR ART 
     In addition to a cable core having predominantly metal and/or optical conductors cables have armouring composed of one or more armour layers surrounding the said core. The armouring absorbs mechanical stresses acting on the cable. The armouring serves, particularly in the case of submarine cables, to protect the cable core with the conductors from the mechanical loads that occur when laying and taking up cables, bringing them ashore and burying them or the like. In ocean areas with rocky, dissected seabed in the region of underwater mountain ranges with steep inclines, and also in shallower waters used for fishing and for anchorage on the routes proposed for bringing cables ashore, submarine cables are protected by additional armour layers. 
     The armouring, particularly on submarine cables that are laid on routes with different depths of water and/or changing seabed formation, is not exposed to the same loading everywhere over the continuous length of the cable. Since the structure of the armouring on continuous submarine cables has always to be designed as a function of the maximum load, however, the armouring is overdimensioned on longitudinal sections subject to less loading. 
     Forming cables, and in particular submarine cables, from assembled cable sections that have one or more armour layers depending on the loading in order to adapt to different mechanical loads is already known. For this purpose the cable sections of differing structure must be joined to one another at their opposing ends. This is done by means of additional jointing sleeves or splices between the adjoining ends of differently structured cable sections. 
     BRIEF SUMMARY OF THE INVENTION 
     Proceeding from this, the object of the invention is to create a cable, in particular a submarine cable, which has a structure suited to the local requirements. Another object of the invention is to create a simple method for the manufacture of such a cable, in particular a submarine cable. 
     A cable for achieving the stated object has the characteristic features of a continuous cable core having at least one conductor and with armouring surrounding the cable core. Owing to the fact that individual armouring wires are replaced, at least in some areas, by filler strands composed of a less tensile and/or flexurally lax material, especially lighter material, an armouring can be created, which if adapted to the prevailing mechanical loads acting on the cable. The filler strands here serve practically only as gap fillers, which ensure that the armouring remains closed all around. 
     Owing to the fact that a greater or lesser number of armouring wires is replaced, as necessary, by filler strands in the longitudinal direction of the cable, the cable has armouring of differing load-bearing capacity along its length, which can be located to suit requirements by replacing a certain number of armouring wires with filler strands, section by section where fewer loads are exerted on the armouring. The filler strands do not absorb any significant mechanical loads. The possibility of forming the filler strands from a less tensile and/or flexurally lax material makes the cable lighter and less expensive. 
     The principle according to the invention of replacing the armouring wires by filler strands, as required, can be employed not only on submarine cables but on all conceivable types of cable and cable structures. At the same time the individual armouring wires and filler strands in the armouring may either run rectilinearly in the longitudinal direction of the cable, or be twisted and/or stranded. 
     A further cable for achieving the aforementioned object has the features of a continuous cable core having at least one conductor, with armouring surrounding the cable, and being composed of a plurality of amour sections. According to this the cable in the area of the armouring is formed from more than one and at least two armour sections, the cable core at least, however, being uninterrupted. The armouring of an armour section has at least one filler strand in at least one end area, which strand replaces a section of an armouring wire in the relevant end area of the armour section. Replacing one or even more armouring wires in the end area of at least one armour section with filler strands reduces the mechanical, external load bearing capacity of the relevant area of the cable towards the end of the armour section affected. All armouring wires of an armour layer of the armour section are preferably replaced by filler strands towards at least one end. At the end of such an armour section at least one outer armour layer then only has filler strands. 
     The armour sections suitably have different armouring for adapting the cable to different external loads. This applies in particular to submarine cables, which are laid, for example, at different depths and/or on seabeds of differing consistency (formation) In such a case the armour sections to be joined usually have a different number of armour layers. For example, an area of the cable subject to less loading has only a single armour layer, while an area subject to heavier loading has two (or even more) armour layers. In this case the outer armour layer of the armour section having more than one armour layer will preferably have a number of armouring wires diminishing in the longitudinal direction towards the end in the direction of the other armour section. Replacing these armouring wires by filler strands of preferably identical cross section means that the filler strands increase towards the end of the relevant armour section, possibly to such an extent that at the end of the armour section the outer armour layer only has filler strands. Due to the fact that the cross sections of the filler strands preferably correspond to those of the armouring wires, the external armour layer remains closed. 
     It is further proposed to join the filler strands replacing the armouring wires in certain sections to the respective armouring wire in the course of the respective cable, especially the uninterrupted cable core. The armouring wire that is removed in certain areas is thereby continued, that is to say extended in the longitudinal direction of the cable by the filler strand serving as spacer. The respective armouring wire is joined to the filler strand in particular by means that do not result in significant thickening of the joint. For example, a thin-walled tube is used. Bonding the opposing ends of the armouring wire and of the filler strand together is also feasible however. 
     A method for achieving the aforementioned object in which the armouring is formed from different armour sections. Due to the fact that the continuous cable core is provided with armouring, which is formed from different armour sections, armouring can be formed that is suited to the requirements. Where the mechanical loads on the cable are smaller, because a submarine cable is laid at shallower depths and buried, for example, an armour section with a smaller number of armouring wires can be used. In areas subjected to greater loads on the other hand, the armouring has the armour sections with a larger number of armouring wires. A cable, in particular submarine cable, can thus be formed, which has armouring of differing load bearing capacity in different areas. In the case of submarine cables the armour sections are suitably selected and located so as to produce armouring suited to the prevailing requirements, the armouring on submarine cables in particular being adapted to the depth-profile of the cable route. 
     Due to the fact that sections of at least some armouring wires or whole armouring wires of at least one selected armour section are replaced by filler strands and the filler strands are joined to armouring wires of another armour section or those armouring wires, sections of which are replaced and filled by the filler strands, any length and number of heavier and more rigid armouring wires can be replaced, as necessary, by lighter and in particular flexurally lax filler strands. At the same time the filler strands fill the spaces left by the replaced sections of the armouring wires in the relevant armour layer and hold the remaining armouring wires in the armour layer together. The armouring wires with the filler wires thereby form an altogether closed armour layer, so that the manufactured cable retains its shape. 
     According to a preferred development of the method the armouring wires, preferably of an outer armour layer, are increasingly replaced by filler strands towards the end of the respective armour section, to such an extent that at the end of an armour section at least the outer armour layer has only filler strands, which may possibly overlap the single (inner) armouring of the adjoining armour section by a short distance. By virtue of their flexurally lax characteristics, the filler strands exclusively present in the outer armour layer at the end of the relevant armour section guarantee cohesion of the outer armour layer at the end of the armour section, so that the filler strands of the outer armour layer do not burst open. 
     The ends of the filler strands of the outer armour layer are preferably held together by a binding band, for example a wrapping composed of high-tensile fibers preferably over the entire transitional area between adjacent armour sections, In order to form a continuous transition the said binding band or wrapping may extend over the adjoining end area of the adjacent armour section that has a smaller cross section owing to the absence of an armour layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred examples of embodiments of the invention will be explained in more detail below with reference to the drawing, in which: 
     FIG. 1 shows a cross section through one type of a submarine cable, 
     FIG. 2 shows a cross section through another type of a submarine cable, 
     FIG. 3 shows a side view of a joint between two armour sections of different structure in a submarine cable, and 
     FIG. 4 shows a joint between an armouring wire and a filler strand in a longitudinal section. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Examples of the invention will be explained with reference to different submarine cables, FIG. 1 shows a cross section through a submarine cable  10  with a structure known in the art. FIG. 2 represents a cross section through a submarine cable  11  with another structure basically of known type. 
     The submarine cables  10  and  11  have a basically identical internal structure. In this respect the same reference numbers are used for both submarine cables  10  and  11 . The identically designed cable cores  12  of the submarine cables  10  and  11  have a central casing tube  13 . In the example of an embodiment shown, a plurality of optical conductors, namely optical waveguides  14 , is loosely arranged in a manner known in the art. The remaining space in the casing tube  13  may be filled by a highly viscous, free-flowing filler compound. The casing tube  13  is surrounded by an armour layer  15  composed of a plurality of identical armouring wires  16 . The metal armouring wires  16  are arranged immediately contiguous with one another in the armour layer  15 , so that they produce a closed sheath around the casing tube  13 . Finally the cable core  12  has an inner covering  17  composed of an insulating material, for example plastic, in particular polyethylene. The inner covering  17  isolates the cable core  12  electrically from the parts of the submarine cables  10  and  11  arranged around the cable core  12 , and thereby, when laid, from the seabed. 
     Each of the two submarine cables  10  and  11  has armouring  18  and  19 . The armourings  11  and  19  are of different design in the submarine cables  10  and  11 . In the submarine cable  10  the armouring  18  is formed from two armour layers  20  and  21 . An inner armour layer  20  surrounds the inner covering  17  of the cable core  12 . The outer armour layer  21  surrounds the inner armour layer  20 . The outer armour layer  21  is preferably surrounded by an outer covering  22 , which is formed from plastic or a plastic-like material (for example polypropylene yarn). 
     The submarine cable  11  differs from the submarine cable  10  in that the armouring  19  has only one single armour layer. This armour layer corresponds to the inner armour layer  20  of the submarine cable  10  and is accordingly given the same reference number. The single armour layer  20  of the submarine cable  11  is in turn surrounded by an outer covering  24 , which serves to protect the submarine cable  11  and is formed from the same material as the outer covering  22 . 
     The armour layers  20  and  21  are formed from the same circular armouring wires  25 . These are composed, for example, of steel, special steel or aluminium. The armouring wires  25  are arranged in a closed layer around the cable core  12 , so that the armour layers  20  and  21  form closed protective sheathes around the cable core  12 . The armour layers  20  and  21  of the submarine cable  10  have different diameters. These come about, despite the equal diameter of the armouring wires  25 , due to the fact that the outer armour layer  21  has a greater number of armouring wires  25  than the inner armour layer  20 . The armouring wires  25  of individual or all armour layers  20  and  21  are preferably stranded in a manner known in the art. This also applies to the armouring wires  16  for forming the armour layer  15  in the cable core  12  of the submarine cable  10  and/or  11 . 
     In order not to have to provide the entire submarine cable with armouring capable of withstanding the laying tension that occurs at the greatest depth in the case of submarine cables, which are laid along an irregular cable route at differing depths, the submarine cable has different armourings. In the example of an embodiment shown these are the armourings  18  and  19 . Armour sections of the armourings  18  and  19  are distributed over the length of the submarine cable in a manner suited to the requirements, in particular in conformity with the cable route. In this one and the same cable core  12  runs uninterrupted over the entire length of the submarine cable, that is continuously over the individual successive armour sections. Where the armour section is designed as the armouring  18 , the submarine cable has a cross section like the submarine cable  10  shown in FIG.  1 . Where the armour section has the armouring  19 , the submarine cable is designed with a cross section like the submarine cable  11  in FIG.  2 . The differing armour sections successively arranged on the continuous, uninterrupted cable core  12  preferably have continuous armouring wires  25  in the inner armour layer  20 . The armouring wires  25  may also conceivably extend only over the respective armour section, however, and be joined to one another in the transitional area  28 . The outer armouring  19  of the armour section having two armourings  18 ,  19  terminates in the transitional area  28  between successive armour sections. 
     According to the invention a corresponding number of armouring wires  25  is replaced by filler strands  31  along those armour sections of the submarine cable  10  or  11 , in which the armouring  18 ,  19  is not subjected to full loading. Preferably only armouring wires  25  of the outer armour layer  21  are replaced by filler strands  31 . A greater or smaller number of armouring wires  25  is replaced by filler strands  31 , at least in some sections, depending on the loading condition of the submarine cable  10 . It is feasible to replace only one section of a single armouring wire  25  by a corresponding section of a filler strand  31 . It is also possible, however, to replace at least sections of all armouring wires  25  or all armouring wires completely, preferably of the outer armour layer  21 , by filler strands  31 . 
     By varying the number of armouring wires  25  to be replaced, the length of the sections of the armouring wires  25  to be replaced, and the positioning of these sections along the overall length of the submarine cable  10  it is possible, by means of corresponding filler strands  31 , to adapt the armouring  18  of the submarine cable  10  individually to the requirements. The armouring  18  then has a load bearing capacity suited to the demands, dimensioning over and above the necessary safety margin thereby being eliminated. FIG. 1 shows a cross section through an area of the submarine cable  10 , in which four armouring wires  25  are replaced by filler strands  31 . In each case two adjacent filler strands  31  are arranged on diametrically opposing areas of the outer armour layer  21 . 
     The filler strands  31  are formed from a less tensile material, which is additionally or alternatively flexurally lax. Filler strands  31  formed from plastic meet these requirements. These may be non-reinforced thermoplastics or reinforced plastics, especially fiber-reinforced plastics, for example glass fiber-reinforced plastics. Such filler strands  31  are lighter than the armouring wires  25 , so that the weight of the submarine cable  10  can be reduced by adapting the armouring  18  to the prevailing pressure conditions. 
     It is furthermore proposed, where a plurality of armouring wires  25  is to be replaced, to provide a gradually increasing number of filler strands  31  replacing the armouring wires  25  in the longitudinal direction of the submarine cable  10 . All filler strands  31  or groups of multiple filler strands  31  are then of different length. In this way the mechanical load bearing capacity of the armouring  18  in the longitudinal direction of the submarine cable  10  is gradually increased or reduced. As a result a substantially continuous transition is produced between armourings  18  capable of bearing different pressure loads. 
     The armouring wires  25  are replaced by filler strands during manufacture of the submarine cable  10 , by removing the armouring wire  25  in places where a respective armouring wire  25  or a section thereof is to be replaced by a filler strand  31 . The space formerly occupied by the section of the respective armouring wire  25  is then taken up by a corresponding filler strand  31 . For this purpose the respective filler strand  31  has dimensions, in particular a cross section, which corresponds or is at least similar to the cross section of the armouring wire  25  replaced. If a round armouring wire  25  of a certain diameter it to be replaced, the corresponding filler strand  31  also has a round cross section of equal or approximately equal diameter. As soon as a section, over the length of which the armouring wire  25  is replaced by the filler strand  31 , ends, the filler strand  31  is cut off and is again succeeded by the armouring wire  25  in the longitudinal direction of the submarine cable  10 . Armouring wires  25  of specific length are in each case therefore replaced by filler strands  31  of equal length. 
     Where, inside the armouring  18 , a filler strand  31  follows an armouring wire  25  or an armouring wire  25  is again arranged in succession to a filler strand  31 , the opposing ends of the respective armouring wire  25  and of the filler strand  31  assigned thereto are joined. This joint may be achieved by means of a tubular section, namely a thin-walled sleeve  32  (FIG.  4 ). The sleeve  32  may be formed from various materials. It is preferably composed of a material that is compatible with the material of the respective armouring wire  25 , for example special steel, especially stainless steel. Short end areas of the armouring wire  25  and of the filler strand  31  to be joined thereto are inserted so far into the sleeve  32  from opposite sides that the opposing ends of the armouring wire  25  and the filler strand  31  meet or virtually abut one another approximately in the middle of the sleeve  32 . The joining of the armouring wire  25  to the respective filler strand  31  through the sleeve  32  is secured by localized plastic deformation of the sleeve  32 , for example by pinching the latter. A thin-walled design of the sleeve  32 , the wall thickness of which is drawn thicker in FIG. 4 merely for representational purposes, means that there is only a slight enlargement of the diameter causing scarcely any interference at the point where the armouring wire  25  is joined to the filler strand  31 . 
     According to a further example of an embodiment of the invention the submarine cable is made up of a plurality of different armour sections, the cable core  12 , however, running continuously. The armouring  18  and the armouring  19  alternate with one another, so that successively different armour sections are produced, There are in this case no limits to the number of different armour sections. Submarine cables  10  and  11  with different armourings  18 ,  19  may repeatedly succeed one another, for example. The position and the length of the respective armour sections correspond to the course, in particular the depths of water, the condition of the seabed and the route over which the submarine cable is to be laid. That area of the assembled submarine cable, which has an armour section with armouring  19  composed of only one armour layer  20 , is then situated in areas of low loading. By contrast, in areas of greater loading there is an armour section with the stronger armouring  18  composed of two armour layers  20  and  21 . Other armour sections can be combined with one another, however, especially those which have armourings that differ from the submarine cables  10  and  11  in their structure, number of armour layers and the cross section of the armouring wires  25 . 
     Where the armour sections with different armourings  18  and  19  meet, the thicker armour section with two armour layers  20  and  21  has filler strands  31 . Filler strands  31  are preferably present only in one or both end areas  29  of the outer armour layer  21 . 
     Proceeding from the end of the outer armour layer  21  the filler strands  31  thereof are gradually replaced again by armouring wires  25 , that is along a transitional section along the longitudinal axis of the continuous cable core  12 . This can be done for each individual filler strand  31  or each individual armouring wire  25 , or in groups of multiple filler strands  31  or armouring wires  25 . For example, a short distance from the end of the submarine cable  10  short sections of two filler strands  31  situated diametrically opposite on another on the circumference of the armour layer  21  are replaced by armouring wires  25 , and in each case at certain intervals in succession to one another two further filler strands  31 , that is the filler strands  31  adjacent to the filler strands  31  already replaced, are continued by armouring wires  25 . This may be continued until the entire outer armour layer  21  is again composed entirely of armouring wires  25 . In this way a continuous transition from filler strands  31  to armouring wires  25  is created, thereby increasing the load-bearing capacity of the outer armour layer  21  along an area of the submarine cable  10  adjoining the end. 
     Forming the outer armour layer  21  at the end of the submarine cable  10  exclusively from filler strands  31  ensures good cohesion of the filler strands  31 , formed from a flexurally lax plastic, in the outer armour layer  21 . In order to even out the transition of the filler strand  31 , truncated in the transitional area  28 , of the outer armour layer  21  of an armour section to an armour section with only one armour layer  20  (FIG.  2 ), the ends of the filler strands  31  can be flattened or facetted by softening. In this way the ends of the filler strands  31  can be welded at the end of the outer armour layer  21  of the submarine cable  10 , so that the filler strands  31  are reliably held together in the outer armour layer  21 . Wrapping the filler strands  31  in order to ensure their cohesion in the outer armour layer  21  may then be dispensed with. 
     The invention is suited to any types of cable, not just the submarine cables  10  and  11  shown by way of example in the figures. A submarine cable with the appropriate length and defined, varying armouring is produced from a plurality of differing successive armour sections on the continuous cable core. Thus a submarine cable can be manufactured with armouring adapted to the prevailing conditions.