Secrets of the floating tunnel

How researchers are figuring out how to cross deep, wide fjords on aluminium bridges that float on the fjord before they turn into tunnels midway.

May 4, 2015
Animated image of aliminium bridges on fjord

Admittedly, it sounds like science fiction: research on how to cross one of the world’s deepest fjords on a floating bridge, on which large and heavy trucks, buses and cars will travel 3-4 kilometers before disappearing approximately 400 meters into a submerged tunnel before they again emerge onto the bridge, still floating on top of the water.

Yet, the challenge is not necessarily getting the bridge and the tunnel to float. It has to do with traffic in both directions at the same time. That is, how do you solve the conundrum of buses and cars intent on making it straight across from shore to shore while numerous boats will be equally eager to travel the full length of the fjord? In other words, the crux of the matter is coming up with a solution for a worst-case scenario: a ship misses the opening in the bridge and instead of sailing through above the tunnel, heads directly for the wet highway.

Animated image of floating tunnel and ship on water
The submerged floating tunnel at mid span enables ships to travel in and out of the fjord. All illustrations: Snøhetta Oslo.

From car crash bumpers to ferry-free coastal routes

That challenge led engineering companies Reinertsen, DeepOcean and Dr.techn. Olav Olsen AS and architects at Snøhetta to invite aluminium companies Hydro and Sapa (Hydro's 50% owned extrusion company) into Norway’s perhaps boldest research project. A project by scientists determined to develop a solution for crossing the deepest, widest fjords in Norway, starting with Sognefjorden. What the project name ‘Artifical seabed’ means is that this particular floating bridge will rest on pre-tension pipes stretched across the fjord and moored to each side of the shore, a technology imported from the offshore oil and gas industry. Assuming the research succeeds, it is a first stride towards a ferry-free coastal route in Norway.

“We were discussing the needed ship barrier when one of ours pointed out the similarities with crash management structures in cars. He meant to recall that these were made of aluminium,” says Marit Reiso, an engineer at Reinertsen and manager of the research project. That recollection paved the way for what Hydro researcher Trond Furu categorizes as one of his most fascinating projects yet and the possibility to bring onboard yet another technology transfer from one sector to another.

“The notable feature of this project is how we are able to transfer learnings from aluminium-based crash management structures in cars to bridges,” says Furu. “Together with SINTEF, NTNU and leading car manufactures, we have developed expertise on how to absorb energy in a collision and that is the methodology and knowledge we bring with us into this project”.

Making engineers go “wow!”

Furu, who also doubles as a professor at the Norwegian University of Science and Technology (NTNU), theorizes about the impact such a project may have. “If we are able to demonstrate that aluminium enables a project like this to be realized, this will open up for a lot of possibilities. Although this project concerns bridges, it enables us to display aluminium as a solution for challenges within the oil, gas and maritime sectors. Part of our job now is to demonstrate to engineering companies what aluminium has to offer and that it can be their material of choice.”

Fortunately, for some coming engineers, the bulk of what they are getting to know about aluminium is indeed awe-inspiring. Several NTNU master students tutored by Hydro researchers are directly involved in the research project and, Furu, who holds a teaching position at the university, happily brings up the project in lectures. “When I hold presentations at the university and tell students about the challenges related to this bridge, I get a bit of a «wow» in response. ‘Is this possible?’ ‘Can this be for real?’ It’s a cool project.”

Image of underwater tunnel sketch
The proposed concept also takes advantage of proved mooring technology from the offshore oil and gas

Updated: September 22, 2016