It may take until Friday to free the grounded freighter "Glory Amsterdam" from a sand bar near the North Sea island of Langeoog, German authorities said on Wednesday, but no holes or cracks are evident in the ship's hull to trigger an oil leak.
Shallow waters off the island had forced officials to revise initial plans to dislodge the 225-metre (742-foot) freighter, which ran aground during a storm that killed at least six people in Germany
and the Czech Republic.
Environmental and fishery groups have warned of dire environmental consequences to the Wadden Sea, a UNESCO World Heritage site, if the ship begins leaking 1,800 tonnes of heavy oil and 140 tonnes of marine diesel on board.
Workers on Wednesday attached the first of two towing ships to the freighter with a 1.5-km towline, Germany's Central Command for Maritime Emergencies said in a statement.
Once a line is attached to the second towing ship, work will begin to pump out more than 18,000 tonnes of water that the Glory Amsterdam took on board during the storm to keep the ship steady in the water, the command said.
Simone Starke, spokeswoman for the command, said authorities had rejected as too risky any effort to transfer the oil and diesel given rough seas and the complications of having to first heat the heavy oil.
Preparations are to continue on Wednesday and Thursday, with the towing operation likely to begin on Friday.
Officials were keeping a close watch on the ship via overflights and said it remained safe for now.
"No holes or cracks have appeared in the ships," Starke said.
The non-profit environmental group World Wildlife Fund on Monday said the oil on board continued to pose a significant risk to the Wadden Sea, the largest unbroken system of intertidal sand and mud flats in the world.
(Reporting by Andrea Shalal; Editing by Jeremy Gaunt and Andrew Heavens)