Ramboll Conducts SFT Risk Analysis (Norway)

Ramboll analysts have considered whether engine failure, anchors or other hazards can threaten an upcoming and very innovative Submerged Floating Tunnel (SFT) across the Sognefjord in western Norway.

The probability of a ship’s collision resulting in disastrous consequences is very low, shows two risk analyses 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. According to estimates from Ramboll, the probability of a collision that will cause this bridge to collapse is so low, that it will not happen for the next 10.000 years.

A permanent connection across the Sognefjord will serve as a major technological test to determine, whether or not it is realistic that the motorway (E39) between Stavanger in the south and Trondheim in the north can be ferry-free.

If the engineering masterpiece manages to cross the almost 1309 meters deep and 203 kilometre long fjord, most of the remaining fjord crossings will be easily attainable:

“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 the project “Ferry-free E39” in Statens Vegvesen.

Søren Randrup-Thomsen, Department Manager in Ramboll’s Risk and Safety Department, Denmark, elaborates: “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.”

The pipe-bridge consists of 16 floating pontoons, which hold and stabilize the tunnel. The bridge is about 4100 meters long; the longest distance between the pontoons is 400 feet.

To understand the risk of ship 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 Fehmarn Belt Connection between Lolland in Denmark and Fehmarn in 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”, says Mathias Kjerstad Eidem from Statens Vegvesen.

Ramboll emphasizes that it is “very important” that the safety precautions are functioning properly, and encourages Statens Vegvesen to follow the guidelines closely.

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

Press Release, August 08, 2014