- 30,000 tonnes of sand installed
- 160,000 tonnes of rock installed
- 160 metres working depth
- 2 nautical mile west of the Island Fedje
Ever since the discovery of the Word War II-era submarine wreck near the island of Fedje, Norway, the Norwegian Coastal Administration (NCA) has been facing a dilemma. 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. 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 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 counter fill capped the contaminated seabed in the trench and stabilised the bow section of the wreck. The NCA prohibited any more than 220 milliliters, a single glassful, of mercury to spread outside the working area during the project. Extreme precision was of vital importance. Van Oord first installed a layer of sand to reduce erosion and dispersal. This ingenious solution, along with our thorough risk inventory and ability to meet the strict environmental criteria, were the client’s primary reasons for awarding the project to us.
We awarded the contract to Van Oord because of their overall quality and risk-based approach. The NCA is very satisfied with the professionalism displayed by all employees.
Specially designed diffuser
To accurately install the layer of sand at a depth of 160 metres, Van Oord used its offshore expertise, years of dredging experience, and Marine ingenuity to develop a fit-for-the-job diffuser. The diffuser was attached to the fallpipe remote-operated vehicle on flexible fallpipe vessel Stornes. The vessel installed a layer of 30,000 tonnes of sand. This half-metre layer was then covered with 160,000 tonnes of rock.
Measuring and monitoring every step
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 unique challenge. Measuring and monitoring were therefore key to the execution of this project. We 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, this vessel collected and analysed all data from the measuring equipment that had been installed.
The combination of Subsea Rock Installation and specialised environmental and geotechnical monitoring made this a unique and very challenging project. The client received what it asked for. The wreck was stabilised with rock counter fill without any contamination spreading.
Start dateMay 2016
End dateJune 2016
Location2 nautical mile west of the Island Fedje
ClientNorwegian Coastal Administration