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FEATURE SUBSEA CABLES


Repeaters on board: Xtera’s next-generation Raman repeaters, ready for deployment in the HUGO cable system


which can represent more than half the cost of the project. Tese portions also represent the biggest opportunities for cost savings. Of course, while cost is saved on the cable (and


repeaters if they are also recovered and re-used), additional marine operations are required to recover the wet plant. Te cost of recovering the cable from the sea bed is determined by the level of difficulty, which will depend on whether the original cable was surface laid or buried. But in the end, even when the cost of transporting the new cable from one of the few submarine cable factories dotted around the world is factored in, it can be significantly cheaper to reuse an old cable (Figure 2). Another important benefit of the cable re-lay


approach is the shorter lead time for project execution compared to new builds requiring new cable. While manufacturing a long optical submarine cable can take up to one year (a typical lead time is 9 to 12 months), recovering an existing


cable requires much less time. In general, the project will be devised so that the recovered cable is located near the redeployed cable so the general lead time is three to four months prior to redeployment. Shorter lead times turn into earlier revenues, and more secure business cases. Redeployment projects also have good green


credentials. Manufacturing a new cable requires energy and multiple materials, some of them scarce or costly to produce. On the other hand, recovering an existing cable requires minimal energy, mainly cable ship fuel. A comparison study by KTH Royal Institute of Technology in Sweden of the potential environmental impacts of submarine cable systems has shown that the recovery and re-lay of an existing submarine cable is equivalent to about five per cent of the impact caused by the manufacturing and deployment of a new cable.


Figure 1: Typical cost structure of a new regional submarine cable system


Taking early retirement An important question is how long redeployed cables can be expected to last when pressed back into service. Submarine cable systems are built with a technical lifetime of 25 years, however in reality, most systems are retired much earlier as they no longer remain commercially viable. In practice, the working lifetime could be much longer than the design life for a couple of reasons: cable systems are usually overdesigned to ensure that the lifetime requirements are met, and the materials oſten have a significantly longer life than originally anticipated. In a recently worked example, Xtera looked at an opportunity where the decommissioned undersea


l Survey l Cable l Repeaters l PFE l Assembly & Transfer l Marine Ops l Land l SLTE l Other


Figure 2: Comparison of system price as a function of system length between new build and redeployment approaches. The price comparison assumes a two fibre pair system, a short distance between recovery and redeployment locations, a system in deep water with little burial, and a modest amount of new cable purchased for the shore ends


cable had been in use for just under 10 years. Taking pessimistic assumptions, we showed that if the cable was originally operating at 7kV and since redeployment at 3kV in a shorter system, then the cable could be expected to continue operating at this voltage for at least 600 years. For the wet plant, including repeaters, several


elements must be known before the status and expected lifetime can be assessed. Tere are few active optical elements in a submarine repeater; in fact, the only powered components are the semiconductor lasers for delivering the optical pump waves to the amplifiers. Teir characteristics are well known. Other key pieces of information


Issue 9 • Autumn 2015 FIBRE SYSTEMS 21


Xtera


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