Cable Design
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From the late 1980s, cable burial was adopted more and more frequently. This option meant that the cable designer had to determine the various de-trenching forces this could entail and to take them into account in the design of the finished product. The cable had to be rugged enough to cope with being buried and then possibly drawn up to the surface for repair work. But it required less armouring than cable laid directly onto the seabed. To make sure the cable delivers the required performance at a realistic cost, fibre-optic cable designers have in fact to optimise a whole range of parameters - mechanical, electrical and optical. Among the factors that have to be taken into consideration are protection against deep-sea water pressure, conductor resistance, electrical stress, water ingress, hydrogen susceptibility, tensile/torsional strength, and resistance to wear and fatigue. No easy task! Getting into Deeper Water
150 Years of laying submarine cables Serialised from the book by kind permission of Global Marine Systems Ltd. Compiled by Stewart Ash
PART IV - 1900 Today and beyond.
Since it is desirable that all the cables in the system be compatible, the ideal solution is to have the same basic cable with the same central package throughout, with outer layers of protection added for those zones that require it. In other words, varying amounts of armour are needed to cater for the range of hazards to cables, which range from, damage and snagging by trawl nets and anchors to natural hazards such as rock abrasion or slumping of the seabed. There are also a host of potential perils when the cable emerges onto the beach and finally goes overland where it is routed via ducts to the terminal building to connect with the terrestrial network. Cables laid in deep water can be unarmoured or lightweight since the deep ocean floor is generally a benign environment where they are unlikely to be exposed to damage from trawlers and ships’ anchors, although the cable still has to withstand the immense pressures that exist in the ocean depths as well as
occasional strong currents and mountainous seabed profiles. Another factor is that the steel strength members in optical fibre cables are susceptible to damage by seawater, if exposed. If the cable is accidentally severed, water can penetrate quite a long way along the cable. The challenge was to come up with a material that could be injected into all the interstices without compromising the properties of the cable or the fibre but that would effectively stop water ingressing great distances along the cable while waiting for the arrival of the repair ship. Modern cables are fully waterblocked with highly effective materials that prevent this problem. Another wrinkle cable designers have to deal with is the hydrogen generation of the cable components. This is because glass fibre is particularly sensitive to increased loss caused by the presence of hydrogen. So it is important to take into account the extent to which the cable components generate hydrogen both individually and in interaction with each other and to limit hydrogen ingress into the cable from any other source. This means the cable designer has to review as many materials as possible singly and in combination to find the optimum components for the cable structure. The Problem of Dispersion As system bit rates increased a new problem appeared. The problem was dispersion or spreading of the light pulse. It was recognised that these new types of submarine optical fibre systems were required for two main types of link. The first are high bit rate (Gigabit to Terabit) transoceanic links with repeater spacings of 50 to 100km. The second are high bit rate systems (2.5Gbit/s and 10Gbit/s per wavelength) repeaterless spans up to 430km between islands, from the mainland to an island, a coastal festoon or a close coast link. In both areas the effects
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