World War II sea mines removed from wind farm site

Many wartime remnants still lie dormant on the seabed and pose a real danger to people, technology and the environment. Therefore, before construction of an offshore wind farm can begin in the North Sea, these relics must be carefully identified and safely removed.

Metal objects were detected

Over the past two years, large-scale geophysical surveys of the seabed identified magnetic anomalies in the area of Nordlicht I and II, indicating the presence of metal objects. The anomalies were interpreted by experts as possible unexploded ordnance, or UXO. 

During spring this year, each of the objects was examined and identified individually using a Remotely Operated Vehicle, ROV. As a result, three sea mines were discovered, each with an explosive charge equivalent to 200–300 kg of TNT.

In September, the time finally came, and after a complex permitting process, the objects were safely disposed of, through controlled detonations on site.

“Clearing the seabed of wartime ordnance is one of those invisible but vital steps in developing offshore wind,” says Tobias Kulgemeyer, Senior UXO Engineer at Vattenfall Business Area Wind and technical lead for the campaign. 

“It takes meticulous planning, coordination and care for the marine environment. With this campaign successfully completed, we’ve laid the foundation for a safe and sustainable start to Nordlicht’s construction.”

Bubbles dampen explosions

A special aspect of this kind of underwater operation is the use of a 90-meter-long bubble curtain vessel. The vessel lays out a double hose ring on the seabed that generates a curtain of air bubbles to dampen the sound around around the detonation site, protecting marine life from noise and stress. 

Also, as the approval notice prohibited simultaneous noise events in the German Bight, close coordination with other projects was necessary to avoid that to happen. In particular this was true for pile-driving work carried out by RWE in the area. 

The Geoscience project team coordinated a total of seven vessels for the complex disposal campaign. These included smaller support vessels for deterrence and traffic safety, the bubble curtain vessel, and an ROV carrier vessel responsible for overseeing and executing the detonations.

With the seabed now safely cleared, the Nordlicht project is well-positioned to begin installation activities.

"We would like to thank all colleagues involved and our competent contractors at Fugro for the successful implementation," Kulgemeyer says.