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FEATURE SUBSEA CABLES


are a well-documented fault history and regular performance monitoring reports from the repeaters to indicate how well these were performing prior to decommissioning. Taking the same system as above, and knowing


that the failure mechanism for lasers follows a log-normal distribution, Xtera calculated that the total laser (and associated electronics) failure rate would increase from 85 FITs at 25 years to 128 FITs over a further 25 years. (Te ‘failures in time’, or FIT, rate of a device is the number of failures that can be expected in one billion device-hours of operation.) Tis corresponded to 0.65 ship repairs over the next 25 years. As this rounds up to one, then the number of repairs is no different to a new build equivalent for the same link. A further level of reassurance can be gained


from the understanding that, in general, most ageing occurs in the early life of a system. Tis means that, several years aſter commissioning, the subsea cable operators have a pretty good idea of the physical condition of their wet asset: either


Recovery of extensively well buried armoured cable can be a slow and risky process


multiple, regular failures have happened and a pattern can be deduced to predict degradation in the mid-term, or no degradation happened aſter one or two years of commercial service. In the latter scenario, there is a high level of


confidence that the subsea cable was properly designed, manufactured and installed, and that it will retain good characteristics and performances for the future. Tis was the situation for the Southern Cross Cable, a 28,900-km transpacific cable system connecting Australia, New Zealand, Fiji, Hawaii and the US mainland. Aſter an extensive consulting process with third-party experts and the original supplier, in November


US – Bermuda: 1,572 km Manasquan


Harbour View


Bermuda Haina Tortola Bermuda Tortola Figure 3: Three redeployment projects based on the decommissioned Gemini cable


Project participation Xtera has been involved in several projects to retrieve and redeploy submarine cables. One of the earliest examples in 2007 was the recovery and redeployment of some pieces of the Gemini cable. Originally built by a joint venture between Cable&Wireless and Worldcom, the Gemini system was a two-leg transatlantic cable system that was phased out only six years aſter its commissioning in


22 FIBRE SYSTEMS Issue 9 • Autumn 2015 Ready to depart: Orange Marine’s cable ship


2014 the cable lifetime was extended by another five years beyond the original 25 years specification. Of course, there is an alternative scenario when


there is some doubt about the active components of the cable system. Te simple answer is to reuse the cable and replace the repeaters, and this too has been shown to be cost-effective.


Practical considerations Management of a redeployment project is quite different from a new cable project as many more boundary conditions need to be taken into account. Since the project does not start from scratch, with brand new, clearly specified wet components, cable redeployment projects are typically more challenging to design and execute. To make a redeployment project viable from


both an economic and technical perspective, there is more to consider than simply the physical status of the candidate cable. One of the early considerations is where the cable originates from. Cable redeployment is an appropriate approach


when the cable is recovered from a less or similarly benign seabed than the destination seabed. Recovery of extensively well buried armoured


Bermuda – Tortola: 1,692 km Tortola – Jamaica: 1,745 km


cable can be a slow and risky process; hence, cable redeployment works best for a destination that requires a minimal amount of armoured cable and a maximal quantity of deep water cable. When the existing cable is simply surface laid, cable recovery is fast and the cable is unlikely to get damaged during the recovery process. Tere are a number of technical challenges that a


redeployment project may face. Te cable recovery process has to be well controlled in order to avoid applying mechanical tensions to the cable and repeaters that exceed the upper specified limits. Te aim is to maximise the cable recovery yield, i.e. the amount of the existing wet plant that can be effectively re-used. Armoured cable is required for shore ends and shallow water and, typically, this section will be a new build. Where possible, it makes economic sense to


reuse the existing landing station at one end of the cable system, in order to avoid the cost of permits, the lead time to obtain them, and all the installation activities needed for the landing itself. If repeaters are re-used in the redeployed cable,


they must be monitored in the new system. Line monitoring equipment is therefore required to generate and detect the test signals appropriate to the system, to enable any faults to be located to within one repeater section.


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Xtera


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