mastering shaft drilling for Hinkley Point C cooling system

Challenge

As part of the UK’s commitment to achieving carbon net zero electricity generation by 2050, EDF’s two nuclear reactors require a seawater cooling system.  

Constructing intake and outfall shafts to connect the reactors with the Bristol Channel posed a significant challenge. The intake shafts are designed to draw seawater into the system, while the outfall shafts release it back into the sea, dispersing it to minimise environmental damage. Each of the six shafts measured 20 m in depth and around 6 m in diameter. They have to penetrate the shallow overburden of sand and gravel before reaching the stubborn claystone and mudstone bedrock.  

The Severn Estuary, with the world’s second highest tidal range and 14 m change in water depth, posed a significant challenge to try maintaining optimal performance without interruptions. To stabilise the shafts and prevent disruptive forces, we added a shear key – a widened section of the shaft. This shear key is essential to keep the liner and grout in place while operating. We expanded the diameter to 7 m at the shear key position within the rock, then tapered back to around 6 m for the rest of the shaft. 

Solution

We deployed our T90 reverse circulation drilling (RCD) rig with a variable-diameter bottom hole assembly (BHA). The driller can control the tools at the bottom of the drill to expand the hole at the right depth, ensuring the formation of a strong support structure. The variable diameter equipment was originally designed for the EDF Flamanville nuclear power station cooling shafts. This significantly reduced project duration by allowing us to drill the entire profile, including shear keys, in a single pass without changing equipment.

Our geotechnical teams conducted a comprehensive site investigation campaign during the planning stage. This enhanced the project’s efficiency, saving both time and money during the drilling campaign by predicting the rate of penetration from the data collected. Our geotechnical and drilling teams worked closely allowing us to configure the equipment based on anticipated ground conditions, minimising geo-risk and maximising production rates. 

Innovative Highlight

For enhanced precision and real-time monitoring, we upgraded the T90 RCD rig with a fully digital positional and telemetry system. This upgrade was necessary to achieve the high dimensional tolerance required for creating the shear keys.

The upgrade also supported real-time monitoring of the BHA's position and updates by creating a digital drill log once each location was completed. After completing the drilling scope, the client used the T90 to install the shaft liner. The T90 ensured precise installation by lowering the liner into position and maintaining the correct elevation during the grouting operations. This reduced the risk associated with using a vessel crane and eliminated the need for additional equipment, reducing total project time and cost. 

Impact

We completed the drilling scope of the project on time, within budget, and without any issues. This efficiency enabled the client to seamlessly transition to the next phase of the programme, the installation of the permanent works. 

We ensured minimal environmental impact by using only seawater as a drilling fluid and our reverse circulation drilling technique. This method complied with the client discharge consent requirements set by the environmental regulator, allowing for the safe discharge of drill cuttings directly back to the seabed without any environmental concerns. We completed 129 linear meters of subsea drilling and dispersed 3,545 m³ of material from the seabed to form the shafts with integrated shear keys.