A Word War II-era submarine wreck near the island of Fedje, Norway, has been irking the Norwegian Coastal Administration (NCA) ever since the Royal Norwegian Navy discovered the submarine in March 2003.
On 9 February 1945, as World War II raged, German submarine U-864 embarked on a journey to ally Japan. It carried 67 tonnes of liquid mercury in steel barrels for the munitions industry, but it never reached its destination.
A British submarine fired on the vessel off the Norwegian coast, splitting it in two and leaving it to sink to the bottom of the sea.
It was not until 2003 that the wreck was found at a depth of 160 metres, just a few kilometres off the coast of the island of Fedje.
The wreck, consisting of two pieces, was carrying 67 tonnes of toxic liquid mercury. This toxic metal had spread over an area of 30,000 square metres. The bow section of the wreck lay on the edge of a trench and needed to be stabilised to prevent the toxic mercury from spreading further into the environment.
Dutch dredging company Van Oord which was contracted to deal with the problem carried out a counter fill that capped the contaminated seabed in the trench and stabilised the bow section of the wreck.
The company said that NCA prohibited any more than 220 milliliters, a single glassful, of mercury to spread outside the working area during the project.
To adhere to this regulation, Van Oord first installed a layer of sand to reduce erosion and dispersal. A fit-for-the-job diffuser, developed specifically for the project, allowed a precise installation of the sand layer at a depth of 160 metres. The diffuser was attached to the fallpipe remote-operated vehicle on flexible fallpipe vessel Stornes which installed a layer of 30,000 tonnes of sand. This half-metre layer was then covered with 160,000 tonnes of rock.