The vessels, named Neptune and Sea Challenger, will create six vertical shafts to install components for the power station’s cooling water system.

The shafts, which will go more than 20m into the seabed, mark the next stage in connecting the 10km of tunnels with the seabed.

Once the shafts are installed, miners will dig a horizontal connection between the bottom of the shaft and the cooling tunnels. This is the first part of linking the intake and outfall heads with the tunnels. These 5,000-tonne structures were lowered onto the seabed last summer and will circulate water to the two nuclear reactors. 

Often used to build offshore wind farms, the vessels’ cranes have a combined lifting capacity of 1,500 tonnes. The Sea Challenger is 132m long and Neptune is 60m. 

Each vessel has four legs that elevate it above sea level, allowing it to operate without being impacted by waves and currents.

Hinkley Point C is being built for energy company EDF.

Area delivery director Jonathan Smith said: “This is one of the final stages of our offshore operations, which will see teams from EDF, Balfour Beatty and New Wave Solutions working together to deliver yet another incredible feat of engineering. The cooling water system is critical to the power station.” 

Balfour Beatty project director Roger Frost said the vessels’ arrival marked another significant step in the delivery of the first new nuclear power station in the UK for over 20 years. 

“We now look forward to utilising our unique capability and unrivalled expertise to continue with the linking up of the six miles of tunnels which are buried below the Bristol Channel. This is another important chapter in the offshore works required for Hinkley Point C’s critical water-cooling system,” he said.

Work to install the shafts will continue into the autumn.