N1 cable seabed survey successfully concluded

Askholmen sails away

Jersey Electricity’s survey of the seabed between Archirondel and St Remy des Landes has now successfully concluded and the 21-metre survey vessel, the Askholmen, which has been a familiar sight in Jersey waters since 13 July, has now departed.

The survey is in preparation for JE’s latest subsea power cable project, the 100MW Normandie 1 (N1). This is a direct replacement for Jersey’s oldest interconnector to France, EDF1, installed in 1985 and which came to the end of its useful life in 2012.

Like its predecessor, N1 will be laid on the seabed and follow the same 27km route so the Askholmen, on charter from Swedish company MMT, has surveyed a corridor of up to 100m wide over the decommissioned cable. The vessel also inspected the Normandie 3 (N3) cable, installed last summer, to satisfy JE and the authorities that everything is as it should be. N3 was installed over a more southerly route and ploughed into the seabed to a depth of around two metres.

Askholmen JW on bridge

The Askholmen used high frequency sonar to measure the relief and topography of the seabed, this helps the cable route planners to determine a precise route for the new cable avoiding obstacles (mainly boulders) and terrain features that would stress the cable (peaks and valleys). The vessel also towed a magnetometer to detect metallic objects that could pose problems to the installation. These could be other old cables, wrecks, stray fishing gear or unexploded ordnance dumped during the Second World War. Just like naturally occurring obstacles, the cable installation route is carefully mapped to avoid them.

Askholmen equipment new

N1 Project Manager Jeremy Willis said: ‘The survey has thrown up some boulder obstacles and small unidentified metallic contacts that will be easily avoided during the installation.

‘We noted on this job, that each time we collect this sort of data for our subsea cable projects, as the technology improves,  the quality and resolution of the data increases markedly. The accuracy and detail is quite remarkable.

 ‘On the whole, the survey has gone according to plan, though the Askholmen was called to the aid of two French yachts on two different occasions within a week!

Survey Image

‘Although N1 will be laid on the seabed rather than ploughed in beneath it as N3 was, it will be protected by a 1km fishing and anchoring exclusion zone that already exists over EDF1 and neighbouring Normandie 2 cable (installed in 2000). Further significant protection is provided in its design as the cable will also be enveloped in two layers of 10mm stranded steel wire armour installed in opposite directions. In the highly unlikely event of an electrical fault developing due to a strike, the hugely complex and sensitive protection systems on the cable would render it inactive in fractions of a second.’

Askholmen screens

The new circuit is expected in service in 2017. Like N3, N1 is another Channel Islands Electricity Grid (CIEG) joint venture with Guernsey Electricity. The two Channel Islands utilities will share the total £40m project cost and the import capacity of the new cable.