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Tuesday, August 30, 2016

More about Horizontal Directional Drilling (HDD)

by Tim Inglehearn, lead project manager

The new connection we’re proposing on Anglesey would run overhead from Wylfa Newydd to east of Llangefni. To connect to the mainland, we obviously need to cross the Menai Strait and we’ve already announced that we’ll do this by putting the connection under the ground. We’re currently considering two options to do this: HDD and tunnelling.

I wanted to give some more information about how we’d use each of these technologies. Today I’ll look at HDD before talking about tunnelling next week. 

HDD starts with a ‘launch pit’ approximately 150 metres by 100 metres (about the same area as one and a half rugby pitches) and is where our drill rig would begin drilling.

HDD drill rig in operation

In very simple terms, preparing for HDD is similar to how you might go about drilling in your home. You need to check the substance of your wall before making a hole, making sure it’s strong enough but won’t break your drill bit. As you can see from the picture below, our drill bit is a lot bigger than your household drill!

HDD drill bit

We’re carrying out lots of surveys in the area to make sure the ground under the Menai Strait is suitable for HDD. This work will also help us establish a route for drilling underneath the Menai Strait that takes into account geology and any potential obstacles.

To start the drilling process, we need a ‘pilot bore’ – the first hole we drill. This would start on Anglesey, drilling down to and then under the Menai Strait, until we reach the Gwynedd side and our ‘reception pit’.  At all times, we monitor the drilling and if needed, it’s directed remotely by an operator on site.

To get the holes to the size we need for the cables, we then attach a wider drill-bit and pull that through to where it started. We slowly increase the size of the hole and might need to use several sizes of drill-bit to reach the diameter we’d need.

In total, we’d need to drill 13 holes – 12 for electricity cables and one for a communication line, which monitors cable performance.  Each hole is lined with a cable duct, which are long sections of hard, plastic pipe. These ducts would then be ready to receive our electricity cables.  

Both our launch pit (on Anglesey) and reception pit (in Gwynedd) would need to be close to the shore and depend on the geology on either side of the Menai Strait. We’ve currently identified four areas where we could drill. It looks like a messy process but we’d return the landscape to its original state once construction is finished.

HDD is just one of the options we’re looking at. Next time, I’ll explain the process of tunnelling – the other technology option we’re considering.

Tunnelling machine

*All of these images are examples and for indicative purposes only.

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