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Mon March 01 2021

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Ramboll risk reports back Norwegian plan to build floating tunnel

12 Aug 14 Norway’s plans for an innovative 4.1km submerged floating tunnel (SFT) have been given a boost by the positive conclusions of two risk reports by Ramboll.

According to estimates from Ramboll, the probability of a collision that will cause the bridge to collapse is so low that it will not happen for the next 10,000 years.

Ramboll analysts considered whether engine failure, anchors or other hazards could threaten the SFT across the Sognefjord in western Norway.

The probability of a ship's collision resulting in disastrous consequences is very low, according to the two risk analysis reports that Ramboll carried out for Statens Vegvesen, the Norwegian Public Roads Administration. Statistically the average pipeline-bridge will get struck by a ship every nine years. However, this also includes incidents where the ships barely touch or where very small ships hit the pipeline-bridge.

The feasibility study for the Sognefjord crossing is now finished. The final concept study of the crossing has been ordered and will begin shortly.

If the floating tunnel can cross the almost 1,309m-deep and 203km-long fjord, most of the remaining fjord crossings on the E39 motorway from Stavanger to Trondheim will be easily attainable. The aim is for the whole route to be ferry-free.

"The Sognefjord crossing is considered one of the most difficult crossings, especially because of the depth. It is essential to go for a full ferry-free E39,” says Mathias Kjerstad Eidem who led the sub-project for fjord crossing in Statens Vegvesen’s ‘Ferry-free E39’ project.

Søren Randrup-Thomsen, department manager in Ramboll's risk and safety department in Denmark said: "One of the main challenges is the heavy ship traffic through the fiord. 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.”

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The 4,100m pipe-bridge consists of 16 floating pontoons which hold and stabilise the tunnel. 

To understand the risk of a ship's collision, risk analysts in Ramboll have used a model that was originally developed for the Øresund Bridge between Copenhagen and Malmö, 15 years ago. The model has since been developed further, most recently in connection with the Fehmarnbelt connection between Denmark and Germany.

The model 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 idea is that the bridge is equipped with a so-called "weak link" which acts as a safety protection if a larger ship collides frontally with one of the pontoons. This weak link between the pontoons and the pipe, where the cars will drive, 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 from Statens Vegvesen.

Ramboll emphasised that it is "very important" that the safety precautions are functioning properly and encourages Statens Vegvesen to follow the guidelines closely.

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