SeaTerra has completed investigation of 278 objects and removal of 8 ordnance targets from the DolWin3 cable route.
Commissioned by the transmission system operator TenneT, SeaTerra examined and cleared in recent weeks and months numerous metal objects from the future DolWin3 cable route between Emden and the seaward converter platform DolWin alpha.
The DolWin3 project establishes a 900 MW direct current grid connection for offshore wind farms in the south‐western part of the German North Sea.
As part of the required ordnance investigation, eight major unexploded ordnance targets (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 (four sea mines) was demolished in situ by explosives specialists and ordnance divers of SeaTerra. According to the company, all demolitions were thoroughly prepared and designed in close 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 were 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 (ROV, airlift), cranes and ordnance divers.
A special feature was the geophysical investigation within the 500 m 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 Aug. 18, 2016 to the responsible Ordnance Disposal Authority completed the ordnance clearance works along the DolWin3 cable route.