In 1945 during the last days of the Second World War, the British Royal Navy torpedoed and sunk the German U-864 submarine just north of the Norwegian city of Bergen. Nearly 30,000 square meters of the seabed surrounding the submarine are now contaminated by mercury.

The old wreck lies on the seabed, split into two main parts with smaller debris scattered across the wreck site. The bow, resting on a slope on the seabed, is the most exposed part, and geotechnical calculations have shown that this slope is very unstable. The project aims to stabilize the seabed near the wreck to prevent the bow from sliding deeper into the ocean at a later stage. Experts from NGI are part of the large-scale subsea operation to secure the submarine and the mercury on the seabed floor.

A video shows the installation of the bow motion detection system attached to the hull of the submarine. (Video: Van Oord/NCA/NGI).

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The illustration shows the position of the bow on the unstable slope. (Illustration: The Norwegian Coastal Administration)

Smart solutions on the seabed

In January 2015, the Norwegian Ministry of Transport and Communications commissioned The Norwegian Coastal Administration (NCA) to prevent further contamination of the seabed and to construct a counter-fill to stabilize the unstable slope on the seabed. Accompanied by the main contractor working on the project, the Dutch company Van Oord, NGI experts began working on the site, approximately two nautical miles offshore near the island of Fedje in Western Norway, at the beginning of May 2015.

A specialized rig was developed and built, which used an ROV (Remotely Operated Vehicle) to install pressure probes at six places on the seabed, which the counter-fill will cover. The purpose of the probes is to measure the stabilization rate of the sediment during the filling process.

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The illustration shows the position of the bow resting on the unstable slope of the seabed. (Courtesy: The Norwegian Coastal Administration).

To detect even the slightest movement of the submarine's hull while constructing the counter-fill, NGI also built customized motion sensors capable of detecting vibrations and any slight changes in the inclination of the hull. The measurement devices are connected to the national system for environmental monitoring provided by the NIVA (The Norwegian Institute for Water Research), and they continuously transmit data to a dedicated surveillance vessel near the site.

The Norwegian Coastal Administration describes this monitoring program as the most comprehensive program ever to have been conducted for an underwater operation in Norway. An animation shows the enormous project on the seabed off the coast of Norway. (Courtesy: The Norwegian Coastal Administration)

FACTS

  • The British submarine HMS Venturer torpedoed the German U-864 submarine on 9 February 1945. The German submarine had a crew of 73 and a cargo of approx. 65 tonnes of mercury. Both subs were submerged when the incident occurred.

  • The Royal Norwegian Navy found the wreck of U-864 in 2003, two nautical miles west of the island of Fedje in Norway. The wreck was split into two main parts, with the bow lying on an unstable seabed.

  • Approximately 100,000 tonnes of sand and rock goes into constructing the counter-fill, which will withstand major disturbances such as earthquakes on the seabed.

  • The counter-fill will stand seven meters tall and is due to be completed before the summer of 2016.

  • In addition to the main contractor Van Oord and the NGI, other partners working on the project include DNV GL, Geopartner Marin, Kvale, NIVA, Reinertsen, and Rambøll.

  • Visit the Norwegian Coastal Administration's Facebook page on U-864.