This invention relates to the jointing of armoured submarine cables and, more particularly, to the jointing of the armour of a submarine optical fibre cable.
For the understanding of this invention, it is convenient to regard an armoured submarine cable as a core surrounded by armour. The core includes transmission elements, which may be optical fibres, strengthening members, insulation and a water seal capable of withstanding the pressure at the depth of operation. Thus, the core, which is complete in itself, provides all the functions needed for transmission.
When it is used in shallow water the cable may be subject to tensile stress; for example, if it is fouled by anchors. Such tensile stress can be in the region of 70 tons. It is important that tensile stress is not allowed to lead to elongation of the core. In the case of cores whose transmission elements are optical fibres this is particularly important because of their high susceptibility to damage by elongation. It is conventional to armour the core so that it is better able to resist tension. The armour usually comprises single or multi start helices of tensile wires wound round the core. Most often the armour comprises one or two such layers. If the cable is pulled, the tension is taken by the armour and relatively little, if any, is transmitted to the core.
Since submarine cables extend for many hundreds of kilometers there are necessarily many joints. Each joint is conveniently considered as two elements, i.e., the core-joint through which transmission occurs and the armour-joint which transmits tension and protects the core-joint from deleterious effects of tension. The core-joint is made in accordance with the technology of the core and it is usually enclosed in an injection moulded polyethylene envelope. This invention is not concerned with the core-joint.
Known armour-joints include the following:
(i) a cone splice wherein the armour wires are brought through a hollow cone and bent back over its outer surface;
(ii) a barrel splice wherein the armour wires are passed through a common sleeve from opposing directions and bent back over its outer surface;
(iii) a ring splice wherein each armour wire is passed individually through a hole in a common ring, being secured therein by a terminal ferrule; and
(iv) an overlay splice wherein a first set of armour wires is wound helically as an outer layer over the second set of armour wires as an inner layer for an overlap distance of, typically, 8 m, the effect of tension in use being to reduce the internal diameter of the outer layer so clamping it onto the inner layer.
Both the cone and the barrel splices have been found to suffer from a lack of strength where the armour wires are bent back. Such joints will carry a significantly reduced tensile load in comparision with unjointed armour wires. The ring splice, in practice, has also been found to suffer from a lack of strength because of the difficulty of installing the individual terminal ferrules at exactly equivalent positions at the ends of each of the armour wires. Any tensile load may not be evenly distributed, as a result, between the armour wires. Lastly, the overlay splice is particularly subject to elongation under load which is inseparable from the reduction in internal diameter. Elongation of the armour-joint can clearly affect the proportion of the tensile load transferred to the core and can lead to elongation of the core which, as mentioned above, can be extremely undesirable.