SeaTerra GmbH has completed investigation of 278 objects and removal of eight unexploded ordnance (UXO) targets from the DolWin3 cable route in the German part of the North Sea.
Commissioned by the transmission system operator TenneT, SeaTerra GmbH recently examined and cleared numerous metal objects from the future DolWin3 cable route between Emden and the seaward converter platform DolWin Alpha.
The DolWin3 project establishes a 900MW direct current (DC) grid connection for offshore wind farms in the south-western part of the German North Sea, namely offshore wind farms Merkur and Borkum Riffgrund 2.
As part of the required ordnance investigation, eight major UXOs were identified along the DolWin3 route. Three grenades and a partly destroyed mine were classified as transportable and salvaged from the seabed. Two of them were handed over to the Ordnance Disposal Authority, two shells – each weighing 400 kg – were moved to a sandbank near Borkum, where they were demolished by members of the Ordnance Disposal Authority.
The non-transportable ordnance which included four sea mines was demolished in situ by explosives specialists and ordnance divers of SeaTerra GmbH. All demolitions were prepared and designed in cooperation with the responsible regulators. Marine mammals were protected by scaring devices and a “Big Bubble Curtain” to minimize noise.
The ordnance clearance operations were preceded by geophysical surveys. Approximately 700 ha of the seabed was surveyed with high-resolution magnetometers and Side Scan Sonar, Multibeam Echo Sounder and Sub-bottom Profiler systems.
Subsequent data interpretation led to the identification of hundreds of anomalies. SeaTerra identified 278 potentially dangerous objects and investigated and cleared them with the help of remote-controlled devices, cranes and ordnance divers.
A special feature was the geophysical investigation within the 500 metre safety zone of the existing converter platform DolWin Alpha. The work was carried out under the highest safety requirements and only under ideal weather conditions.
The handover of the last shell on 18 August to the responsible Ordnance Disposal Authority completed the ordnance clearance works along the DolWin3 cable route.