Norway’s proposed 4.1km submerged floating tunnel has moved a step closer to reality after two risk reports by Ramboll.
Engineers investigated whether shipping could damage the combined bridge-tunnel, which would cross the Sognefjord in western Norway.
Ramboll said the probability of a ship’s collision resulting in disastrous consequences for the tunnel – or pipeline – is very low. The tunnel would get struck by a ship every nine years, which includes minor collisions, but the likelihood of this causing the structure to collapse is once in every 10,000 years.
The planned bridge-tunnel consists of 16 floating pontoons, which hold and stabilise the tunnel. The longest distance between the pontoons is 130m. It would carry the E39 motorway between Stavanger in the south and Trondheim to the north.
The Sognefjord is Norway’s longest fjord at 203km, and also its deepest at 1,309m. Building a permanent crossing has long been considered a major engineering challenge, even in a country which has 33 undersea tunnels.
“The Sognefjord crossing is considered one of the most difficult crossings, especially because of the depth,” said Mathias Kjerstad Eidem, who is leading the project for Statens Vegvesen (the Norwegian Public Roads Administration).
Søren Randrup-Thomsen, risk and safety manager for Ramboll in Denmark, said: “One of the main challenges is the heavy ship traffic through the fjord. On top of that, a pipeline-bridge is a complicated construction. It is difficult to anchor on the bottom, because the Sognefjord is so deep and affected by currents, waves and buoyancy.”
Ramboll used a model that was originally developed for the Øresund Bridge between Copenhagen and Malmö, constructed 15 years ago. It is based on the possibility of a ship being on a collision course, and the risk that it will not regain control. It also estimates how much energy there will be in different types of collisions. The more traffic that goes through the fjord, the higher the accident rate becomes.
The design for the bridge includes a ‘weak link’, which acts as a safety protection if a larger ship collides with one of the pontoons. This weak link, between the pontoons and the pipe, will break when triggered by a ship impact of a certain size. The pontoon will break loose and float away, while the rest of the bridge is intact.
“We think this is an elegant way to solve it, and it increases the safety of the bridge,” said Mathias Kjerstad Eidem.
The feasibility study for the Sognefjord crossing has now been completed. The final concept study of the crossing has been ordered and will begin shortly, said Statens Vegvesen.