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Our work in the Menai Strait


During the autumn, an offshore drilling rig will be in the Menai Strait.

It will be used by a team undertaking geological surveys to find out more about what lies beneath the sea bed.

It’s all part of our proposals to build a tunnel under the Menai Strait for new electricity cables.

Putting the cables in a tunnel will mean the landscape and views across the Menai Strait will remain just as they are today.

The questions and answers below provide more information on when the work is taking place and what to expect. 

Questions and answers

What does the work involve?

The rig will be used to drill into the seabed to obtain rock cores – these are samples of rock which will help surveyors understand more about the formation and types of rock under the Strait.  Each core sample will be about 45 metres long, but only 10cm in diameter (about the size of normal mug).  Once drilled, the rock samples will be brought on shore to study.  The important data gathered will also be shared with Bangor University.

What does the rig look like?

The rig is a metal rectangular structure with a pillar at each corner.  It’s 20 metres by 26 metres – about the size of two tennis courts.  The drilling equipment is about 35m high at the centre of the rig, and there are also some other smaller structures for storing materials.

How long is the rig going be in the Menai Strait?

The survey work will take between four and twelve weeks depending on the weather and geological conditions. 

Will the survey rig always be in the same location?

The surveys are all being undertaken in an area south-west of the Britannia Bridge.  A number of different points will be surveyed in this area and the rig will move to these different points.

How is the rig moved into place and removed?

The rig will arrive in the Strait already constructed.  It will be towed into place by boat and tethered to the sea bed to undertake the surveys.  The boat will also tow the rig to its different locations and remove it after the work is done. 

What times of day is the work taking place?

Work will be taking place on the rig 24 hours a day.

Will there be any noise or disruption from the survey works?

The rig will be lit and there will be some operational noise, but it will not be enough to cause disturbance.

Will there be environmental effects?

No, there will be no effect on the marine wildlife or the coastal environment.  The rig floats and is only temporarily attached to the sea bed.  The rig will be transported by boat into place and crews using the rig will be using the existing harbour at Caernarfon to get to and from the rig.

Where can people find out more?

People can call the National Grid community relations team on 0800 990 3567.

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