GERMANY While preparing for the construction of the Sandbank offshore wind farm in the North Sea, 26 mines were found in the seabed. The mines have now been disposed of using a remote controlled plasma beam.

The construction of Vattenfall and Stadtwerke München’s 288 MW Sandbank offshore wind farm is scheduled to start mid-June, when the installation of foundations into the seabed will commence.

Seabed inspection
As part of the preparations ahead of the offshore works, a meticulous inspection of the 66 km2 seabed at the Sandbank site was undertaken.

Jan-Hinnerk Maxl, Project Manager Installation Logistics at Vattenfall, says:
 “We searched the seabed for abandoned munition to ensure that intensive construction activities such as ramming and cable laying can be carried out safely over the coming months.”

The inspection was performed by experts from a ship towing magnetometer sensors that crossed the entire construction site while it captured data which was then analysed for magnetic abnormalities. The team used a piece of radio-controlled underwater equipment, known in the trade as an ROV (remotely operated vehicle), to carefully scrutinise the anomalous areas.

Maxl, who has a background in the German Navy, states that a third of the 26 mines that were found stem from World War I times and the rest from World War II.
“Despite their age, one cannot exclude that a mine could have exploded if one of our jack-up vessels positioned a leg right on top of one of them. We couldn’t take a risk like that.”

Plasma beam
The so-called moored mines have been disposed of using a remote controlled plasma beam.
“Unlike in the past, we did not need any divers. Instead, all employees were more than some hundreds of metres away from the underwater detonation point," explains Maxl and undermines that safety is a top priority.

“That is why we have adopted all the necessary measures to ensure that the construction team is able to commence work in an area that is well prepared and, most importantly, munition free.”

The plasma beam disposed of the mines by firing a charge through them, thereby rendering them harmless.

In order to reduce underwater noise in the marine environment, a so-called bubble curtain was created before and operated during every detonation.

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