Cook Strait current too strong for comms cables

Cook Strait’s ferocious currents have been lifting a ‘‘backbone’’ communications cable from the sea floor as the search begins for a heftier replacement.

Transpower – the state-owned enterprise that owns and runs the national grid – is now in the process of finding a company to supply up to 90 kilometres of fibreoptic cable to link the North and South islands.

Tender documents show the existing cable is too light and lifts from the seabed because of the strait’s high tidal flow. Burying the cables was not an option, as they were too close to the interisland electricity cables.

Transpower is now investigating the cost for either one or two cables that were at least three times heavier, ‘‘specifically designed for the conditions in Cook Strait’’.

The new cables, despite weighing up to 20 kilograms a metre, would also need to be flexible enough to contour to the sea floor.

In a statement, Transpower’s Cobus Nel said the two fibre-optic cables formed ‘‘part of our communications backbone’’.

‘‘From our annual inspections, we have identified that one of the Transpower fibre-optic cables’ condition is deteriorating.

The cables were originally installed in 1992, and replaced in 2003. One of the cables – or perhaps two if it was more cost-effective – would likely be replaced in the next one to two years, he said.

‘‘Given our knowledge of the condition of these cables, and our contingency measures, we do not believe there is any imminent risk to the operation of the national grid.’’

Telecommunications Users Association of New Zealand chief executive Craig Young said there were multiple cables between the North and South islands so an outage on one would not cause a major issue.

But having cables lying on the sea floor made them vulnerable – for example, if a boat’s anchor pulled them up. drinking water and were being offered individual health and food safety advice.

Marlborough District Council operations and maintenance engineer Stephen Rooney said that, based on the findings of the initial tests, a further group of properties were being tested from this week.

‘‘The new testing area has been determined using scientific ground water modelling,’’ Rooney said. ‘‘Council’s scientists know a lot about the Wairau Aquifer and how it works, where we expect the water to move and its speed.

‘‘We also know from our own testing in early December that no PFAS contamination has been detected in the Blenheim town supply bore on Middle Renwick Road, which is reassuring.’’

Rooney said the new testing area was closer to Blenheim.

‘‘The people taking the samples will be quite visible - they will be ‘suited up’. The protection suits they wear are to ensure there is no contamination of the sample taken; the suits are not protecting them from contaminants, as some people might think.