Greenpeace boulders may delay building of £12bn windfarms in North Sea

Activists dumped rocks off Norfolk coast on site of Burnham-backed project in protest against seabed trawling

The giant offshore windfarms backed by Andy Burnham this week may face unexpected stumbling blocks after Greenpeace activists unwittingly dumped about 15 tonnes of boulders on to the site as part of a campaign against seabed trawling in the North Sea.

Greenpeace created the “boulder barrier” to safeguard harbour porpoises off the Norfolk coast by dropping giant stone blocks, each weighing up to 2 tonnes, on to the sea floor to prevent a method of fishing known as bottom trawling.

The barrier will pose an additional challenge for the green energy developers because most of the boulders were dropped on to the site earmarked for one of the UK’s biggest planned windfarms, known as the Norfolk Vanguard project.

Greenpeace began dumping boulders on to the site, without consent from the marine authority, days before the prime minister told the Cop31 summit that the UK would invest £1bn of public money in the Vanguard West and Vanguard East windfarms in what is believed to be the biggest investment of its kind in UK history.

The Guardian understands that Greenpeace checked the seabed for any existing cables, pipelines or shipwrecks, but it was not aware of the plans to build the Vanguard East windfarm.

The creation of the underwater barrier in one of “the most heavily trawled” areas of the southern North Sea was timed to coincide with the premiere of the BBC’s Blue Planet III documentary, narrated by Sir David Attenborough.

Some boulders bear the signatures of celebrities including Simon Pegg and Benedict Cumberbatch, who have voiced marine conservation documentaries, and agreed to support the Greenpeace campaign.

However, they are likely to be removed within months after renewable energy developers agreed a £12bn deal alongside the UK government to begin building the offshore windfarm next year. Industrial fishing activities, including bottom trawling, are also likely to move elsewhere to avoid the construction area.

The Guardian understands that the offshore wind developer, the German utility company RWE, is in talks with Greenpeace to verify the exact placement of the boulders so that they can update the seabed surveys they began last summer. The company is also investigating any potential impact to the construction of the project or its undersea cables.

A spokesperson for RWE described its communication with Greenpeace as “constructive” and said the company would “continue to investigate the precise implications” for the Norfolk Vanguard East project.

Will McCallum, the co-executive director of Greenpeace UK, said any suggestion that the boulders would cause a genuine obstacle was “absurd”.

“Anyone familiar with offshore wind in the North Sea knows that energy companies routinely move naturally occurring boulders from the seabed before building windfarms,” he said.

“Some of these boulders can be 10 times heavier than those we placed.

“Greenpeace has long championed offshore wind and will continue to do so. While we wait for this windfarm to be built, our boulders will provide an essential barrier to destructive bottom trawlers that are decimating the seabed in this protected area.”