Building the world's first underwater suspension tunnel

Atle Kjærvik
Photo: Gøril Klemetsen and Statens Vegvesen Rogaland

In the course of 1998, the Norwegian Ministry of Transport probably will propose the construction of a 1,400 meter long submerged floating tunnel over the 155 meter deep Høgsfjord near Stavanger in Norway

Professor Svein Remseth (right) and post-doctoral research student Knut Morten Okstad at NTNU's Department of Structural Engineering have tested the underwater suspension tunnel by means of a numerical model.

This tubular structure is to be submerged 25­30 meters below the water surface. The bridge will have two lanes for motor vehicles and a combined pedestrian path and bicycle track.

In 1995 it was estimated that the tunnel would cost NOK 900 million kroner. If construction begins as planned in the year 2000, the tunnel will be the first of its kind in the world.

The floating tunnel will be built in concrete and/or steel, and its construction will represent many technical challenges. The structure will be subjected to waves and current loading. Its position relative to the surface is defined by connections to the sea floor by mooring lines or to buoys at the surface. The tunnel will be curved also in the horizontal plane in order to provide high resistance to static current loading.

Modelling at NTNU

The structural group at NTNU and SINTEF has been working with numerical, computer-based models to analyse the behaviour or the response to dynamic loads of the submerged floating tunnels. The computational models relate both to the entire bridge and to individual parts of it. The so-called finite element method is used to model the structure and the interaction between the structure and the surounding water during simulations of response to various load cases.

''Analyses of the influence of currents, waves, wind, collisions with ships and earthquake excitation has been performed as a basis for design of the bridge'', says Professor Svein Remseth at NTNU's Department of Structural Engineering.

Computer tools have been developed and extensive simulations have been carried out for the different floating tunnel concepts in close collaboration with Bernt J. Leira, senior research engineer at SINTEF. Doctoral research work contributed to the development of methods and techniques for validation of the design basis from measurements of motion on the as-built structure.

Simulations and model testing at the Ocean Laboratory at MARINTEK show that currents and waves can move the tunnel about one meter in each lateral direction. Such movements do no damage to the structure, and they should not be observable by those who drive through the bridge.

Safe technology

Travellers who are afraid of using traditional underwater tunnels will probably have the same objections to submerged floating bridges. Remseth assures, however, that the submerged tunnel will be designed to meet a very high level of safety requirements ­at least at the same level as for a traditional bridge.

''A pilot project such as this will be monitored by bridge designers all over the world'', Remseth points out.

Professor Torgeir Moan at NTNU's Department of Marine Structures works together with Remseth on safety matters, and has no second thoughts about recommending the building of a submerged floating bridge. He claims that it is quite possible to make such a bridge just as safe as a tunnel below the bottom of the sea, or, for that matter, a bridge suspended above the surface.

Moan points out that the construction of the floating tunnel will demand extraordinary vigilance, because this type of structure has never been built. The job will offer new challenges, even if many participants will be able to use their experience with, for example, offshore structures.

More environmentally friendly

Project leader Håvard Østlid at the Ministry of Transport says that in the long run submerged floating tunnels will be cheaper to build and more environmentally friendly than traditional tunnels.

A tunnel underneath the Høgsfjord seabed, for example, would have to descend to a depth of 400 meters. This means that such a tunnel would have to be longer than a submerged floating tunnel. Drivers would burn more fuel as the distance becomes longer and the inclines steeper. As a result, more waste products (such as CO₂ ) would be discharged from vehicles.

Østlid says that submerged floating tunnel technology will prove to be even more useful for fjord crossings in other parts of Norway where the depths and distances are even greater than at Høgsfjord.

Combines known technologies

Different proposals for submerged floating tunnels make use of technologies gained from a range of other contexts including marine oil installations, floating bridges, and 'conventional' underwater tunnels which rest on the seabed.

Aker Norwegian Contractors, Selmer, Kværner Rosenberg and Eeg-Henriksen have proposed concepts for the project. Aker's design keeps the tunnel in place with technology similar to that used to keep offshore oil-production units in position in the North Sea. Tension legs made of steel are connected to boxes at the sea bottom. The alternative proposals involve buoys or pontoons to define the position of the submerged floating tunnel relative to the surface.

Considered in the EU

Italy has considered a submerged floating tunnel to span the Strait of Messina from Calabria to Sicily. Professors Torgeir Moan and Ivar Holand (also at NTNU) have participated in a group which has assessed the feasibility of such a structure.

A tunnel in Lake Lugan in Switzerland for instance, would make it possible to build a railway without destroying the lakeside scenery.

Contractors and consultant companies in Norway have founded a company called Norwegian Submerged Floating Tunnels in order to market such a solution. Last year an international conference on submerged floating tunnels was held in Sandnes.

Japan has considered building submerged floating tunnels, including, among others, a thirty-kilometer (!) long link, crossing the Funka Bay at Hokkaido.

Different concepts for submerged floating tunnels:

Aker Norwegian Contractors.

Selmer.

Kværner Rosenberg.

Eeg-Henriksen.