Scope of work
N-Sea’s survey campaign for the TenneT project BorWin3 began at the end of January 2015, and finished in mid-May.
With the “TenneT project BorWin3”, transmission system operator TenneT builds a grid connection system to connect wind parks in the North Sea with the high-voltage grid onshore.
This campaign consisted of a geophysical survey (multibeam, SSS, SBP and Magnetometry) of the main cable corridor and adjacent anchor corridors, totalling almost 2000km.
The corridor started in very shallow water (less than 1 metre) mudflats for a 12 nautical mile zone, north of Borkum in Germany. This part of the German Bight area (Waddenzee) provides a challenging environment, being known for its shallow waters, high currents (>3knots) and continuously moving sandbanks.
N-Sea managed this project from Eemshaven, with a local team managing all near shore and offshore vessels whilst processing all acquired data. In total, not less than 4 survey vessels where involved in this part of the scope. They were the Terra 1, Channel Chieftain 6, the Noordhoek Pathfinder and the Dtect for very shallow works.
The investigation element of the project began in April with the mudflats and finished mid-August. In total, 5 vessels were involved in this part of the project; Gerlinde, Noordhoek Pathfinder and the Neptunus for the ID were permanently assisted by 2 guard vessels. This was due to the proximity of the shipping lanes to and from Emden, Delfzijl, Eemshaven and Borkum.
Identification was undertaken via a diverless airlift equipped with Blueview sonars, Ferrex Magnetometer, camera, lights, a transponder for USBL positioning and so on.
This allowed us to excavate using divers as little as possible, for safety reasons. For the majority of the project a UXO diver performed the final identification, as visibility was a constraint throughout this project.
The ID campaign resulted in 3 positively-identified explosives; two sea mines and one grenade.
The sea mines have since been destroyed on site, with N-Sea-Reaseuro coordinating the whole operation under supervision of the German KBD. The grenade disposal required a more accurate identification which was carried out on board the vessel Relume, through use of a mobile x-ray machine.
As the grenade was a high explosive - rather than chemical - type, the grenade was transported to a safe location and destroyed immediately. After this the TenneT BorWin3 route was cleared and N-Sea finalised their contribution to the project.