Stabilising a World War II Submarine Wreck
Ever since the discovery of a Word War II-era submarine wreck near the island of Fedje, the Norwegian Coastal Administration (NCA) has been facing a dilemma. The wreck, consisting of two pieces, was carrying 67t of toxic liquid mercury. This toxic metal had spread over an area of 30,000m2. The bow section of the wreck lay on the edge of a trench and needed to be stabilised. Awarding Van Oord the counter-filling contract, was the Norwegian Coastal Administration’s first step towards preventing the toxic mercury from spreading further into the environment.
On 9 February 1945, as World War II raged, German submarine U-864 embarked on a journey to ally Japan. It carried 67t 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 160m, just a few kilometres off the coast of the island of Fedje.
The counter fill capped the contaminated seabed in the trench and stabilised the bow section of the wreck. The NCA prohibited any more than 220ml, a single glassful, of mercury to spread outside the working area during the project. Extreme precision was of vital importance. First, a layer of sand was installed to reduce erosion and dispersal. To accurately install the layer of sand at a depth of 160m, Van Oord developed a fit-for-the-job diffuser. The diffuser was attached to the fall pipe remote-operated vehicle on flexible fall pipe vessel Stornes. The vessel installed a layer of 30,000t of sand. This half-metre layer was then covered with 160,000t of rock.
Installing counter fill is a routine job for Van Oord, but the presence of a submarine wreck and mercury resulting in the strict environmental criteria made this a challenge. Measuring and monitoring therefore were key to the execution of this project. Van Oord subcontracted the Norwegian Institute for Water Research (NIVA) to conduct the measuring and monitoring activities. An environmental monitoring vessel was deployed to install environmental and geotechnical measuring equipment in and around the work site. During the installation of the counter fill, the vessel collected and analysed all data from the measuring equipment that had been installed.
Marcel Tuin, Project Manager of Van Oord, “The combination of subsea rock installation and specialised environmental and geotechnical monitoring made this a very challenging project. The client received what it asked for. The wreck was stabilised with rock counter fill without any contamination spreading.”