Extraordinary bridge engineering of the likes never seen before is verging on becoming a reality in the Nordic fjords, with the world’s first submerged floating bridge one of three radical solutions being worked on right now by Norwegian engineers.
Norway has a grand plan – it wants to upgrade its 1,100km long E39 coastal highway from Kristiansand in the south to Trondheim in centre of the country so it is ferry-free.
Today, the popular west coast route has eight ferry links whose crossing times range from 10 minutes to 45minutes, but over the next 20 years, and at a suggested cost of around £17.5bn, the government hopes to change all that.
To create the replacement fixed links, Norway’s public roads administration, Statens Vegvesen, has been developing plans for three mega-bridge concepts that it hopes will be able to take on the challenge of crossing some of nature’s most challenging fjords.
In the southern half of the planned upgrade lies the Bjørnafjord, south of Bergen. At 5km across, it is one of the longest stretches to be bridged and at an eye-watering 600m it is certainly one of the deepest.
An acoustic survey determined that typically close to shore ground conditions are thin sediment layers with soft deposits, while the seabed in the fjord is formed of large areas of soft clay deposits.
“The nature here obliges you to think about something more [than the usual],” explains Statens Vegvesen senior engineer Arianna Minoretti. “Because, when you come to these wide and deep crossings, you can’t avoid taking the technology a step further.”
Given the complexity of the fjord, it has been chosen as a priority project to investigate complex crossings, not just because it is a likely candidate to be one of the first to see construction start but because getting the options right here means the remaining locations will benefit from possibly smaller scale versions.
Vying to win the technical, economic and environmental arguments are a suspension bridge on tension leg platforms, a floating cable-stayed bridge with pontoons for additional support, and finally, a never-before-built submerged floating tube bridge – or Archimedes Bridge.
Minoretti is confident that a solution will be found: “Norway has a long [history] of working on offshore structures so it’s normal and this kind of technology here is already available. I think it’s probably easier to have such an innovation here than in another country, because here they have the technology that they already know and it’s already available.”
Bridges could be a combination of any of the three in various locations: nothing is ruled out.
“We go from more traditional, if we can call them more traditional, crossings, to less traditional,” says Minoretti.
The “more traditional” is the floating bridge option.
“We already have a couple of floating bridges in Norway but nothing of this length, so these would really be on another dimension,” she enthuses.
“But the technology is already known. So we can say it’s more traditional because it’s not something that’s totally new.”
No matter how elegant the design, a floating bridge would be a vast structure with its associated floating pontoons, and so would have a huge visual impact on such a sensitive environment, says Minoretti – the Norwegians are dedicated to prioritising aesthetics when it comes to the beautiful surroundings of the fjords.
Hence the desire to investigate the option of a long suspension bridge (with no need for pontoons) or perhaps even the first submerged floating tube bridge, which would have even less impact on the surroundings – being perhaps entirely hidden from view. The 30m below sea level twin tube structure would be stabilised either via prestressed tethers attached to the seabed or suspended beneath buoyant pontoons.
So at most, the pontoons would be the only visible parts of the structure. The submerged floating tube bridge would clearly be the most extraordinary, or “most innovative”, in Minoretti’s words.
“A submerged floating tube bridge, or Archimedes Bridge, has been studied for a long time for different crossings,” she elaborates, pointing to proposals in the Italian Lakes and in China.
“It was evaluated in different crossings but it has never been built because the technology was not ready enough.
“Now I think that the technology is known; all the marine operations that we need to build it are known.
“So we really think that this could be great opportunity – it will be an opportunity to build a hidden structure that will have more or less no visual impact on the landscape.”
In addition, the underwater structure would benefit from greater protection from inclement weather.
“Because it will be at 30m below the sea surface, it means we could avoid the main sea load – so we can have a structure that will have zero down time,” Minoretti explains.
“It means we can have traffic even during the worst storm – a one in 100 year [event]. This is not possible for the other structures because floating bridges are outside in the sea and so are exposed to the wind, which is, for example, one of the biggest loads on these structures.”
She is enthusiastic about the underwater tube design’s appeal beyond Norway if the structure gets the go ahead on the E39.
Norway submerged floating bridge cycle and pedestrian path
“You could use this structure in a lot of different environments – for every other crossing where you need to have a hidden connection, this would be a great solution,” she states.
Early indications are that, while it would be a world first, it is still competitive when put up against the more traditional options – in part because the system is modular and could be built in a dry dock. And of course: “The more it is used the more cost effective it will be,” she points out.
There is a keenness in the roads administration to make this project truly global. International consultants are already working on the various schemes and there is great scope for expertise from around the world to get involved.
In particular, one aim is to disseminate the work that has been done on the feasibility of a submerged bridge “because we think if we create knowledge of these structures it will be easier to discuss them, and to find out better ways to maintain them, and [resolve] a lot of issues”, says Minoretti.
“So it is important for us to share our work with the scientific community,” she enthuses.
“It will be a great opportunity for Norway,” she adds. But there are also opportunities for firms from abroad, as the proposals are refined through an open procurement process. And she expects that the scope for international contractor involvement is broad too.
“Of course Norwegian or Canadian [contractors will be involved] as well as someone that has already experience with offshore.
“But also the Spanish or Portuguese or Italian or big German companies [and their experience] are really important for us.”
UK firms too will have an advantage potentially over Continental competitors whose Mediterranean knowledge will be less familiar to the environmental conditions in Norway. “In the UK you have more experience in similar environmental loads,” she elaborates. “When I think of the Mediterranean Sea [and contractors used to working with it] they don’t have a lot of experience in similar loads. For example, with ice – it is probably more similar [to the UK’s experience].”
A combination of environmental loads is being designed for. The administration is measuring wind, waves and current, through a combination of offshore measurements and numeric simulation using 3D modelling.
The long, slender floating structures would have to grapple with loads from surface waves and wind in the main. In addition, tidal variation, marine growth, water density and salinity, sea level rise, snow, ice and earthquake risks are also factors.
A further known concern for any crossings along the E39 is the possibility of ship collisions – made more pronounced because Bjørnafjord has a high volume of shipping traffic and is used extensively for naval exercises. As a result a 400m wide and 20m deep shipping channel with a height clearance of 45m above sea level will be required.
It is a huge project, Minoretti says. The road standard of the E39 requires a speed limit of 110km/h for average daily traffic of over 20,000 vehicles for the new crossings that will have a 100 year design life.
Based on traffic needs, any new crossing will require two lanes in each direction with a separate cycle and pedestrian path.
But political support is strong and it helps that a long term view beyond the typical four or five year cycle is being adopted.
Funding has yet to be determined but it is expected that it will be determined differently with private and public money sought, according to each location.
For now the process of comparing the costs of the different structures at Bjørnafjord continues and the aim is that a decision on the preferred option can be taken next year.
A consulting team led by Denmark’s Cowi and Norway’s Aas-Jakobsen and Johs Holt is working to develop one of the more, so-called “traditional” options.
Norway floating bridge
The main span would be carried over the navigation channel via a cable-stayed bridge and the side spans would be supported on floating pontoons. Two main variations are being considered for the floating bridge – one with a curved girder that accommodates the side loads through the arch or a second with a straight bridge girder that takes the side loads through mooring lines anchored to the seabed. “The first solution will be anchored on each side to the bottom of the fjord to provide lateral stability, while the second will only be anchored at the ends,” says Minoretti. The floating pontoons are a key consideration – for the straight girder these large concrete platforms would be at 200m spacings, and at around 186m for the curved option. The problem is that under
the deck you have floating pontoons every [so often],” explains Minoretti. “So there would be a lot of structures distributed along the crossing. “In case of an impact with a ship we have to think about very big impact loads and the energy to be dissipated; as a consequence the floating structure will be really huge. The pontoons for the floating structure will be…really very large.” The deck for the floating bridge would also be rather
large with a total width of around 60m. The positioning of the navigation channel is also up for debate. While a central navigation channel is an option (pictured), erecting a floating cable stayed bridge in the fjord would be complicated, so a navigation channel close to one of the shores would enable the main tower to found at the shore or in shallow water (see diagram), which in turn allows it to rely on traditional construction techniques.
Norway floating bridge drawing
Submerged Floating Tube Bridge
The most “innovative” solution is a submerged floating tube bridge, or Archimedes Bridge – a structure that looks similar to a tunnel and is supported in the water by hydrostatic thrust, or the Archimedes’ Principle.
The design is being developed by Norway’s Reinertsen, Dr Techn Olav Olsen, Norconsult and others. The key advantage is that in a sensitive location, this bridge would be all but hidden from view.
Norway submerged floating bridge
“I really love bridges – I’m a bridge engineer,” says Minoretti. “But we also have a responsibility. In some cases, building a bridge is not a good solution.”
Different cross-sections were studied during preliminary design and in wind tunnel studies the circular cross section twin tube option outperformed the rectangular cross section option. The 15m diameter tubes, will be 40m apart and connected at 150m to 200m intervals by transverse braces. The alignment is curved horizontally, which together with fixed end connections would provide stiffness. Vertical stability is being investigated either by buoyant pontoons above or by tethers connecting the structure to the seabed.
At the moment the codes do not exist for this kind of structure, so Minoretti believes there is real scope for the global engineering profession to work together to create them.
Because of its newness, Minoretti does not underestimate that this option could be one of the more difficult to sell to users but she is undeterred.
“If you think about planes – they had a lot of accidents at the beginning,” she points out. “But a lot of people continued to take planes because it was faster, even though there was that risk.” And for those who question whether this structure is a tunnel and not a bridge, she offers some insight. “I think some people call it a tunnel first of all for the shape and also because some of the people that started evaluating the structure have a tunnelling background – so everything with that shape for them is a tunnel, but the structure behaves like a bridge.”
Norway submerged floating bridge drawing
A suspension bridge is another more conventional option for the crossing.
Norwegian consultant Tekniske Data is leading the design team on this concept – a three span structure with two traditional, 200m tall fixed concrete towers at each shore and two towers on floats moored by tension legs.
Norway suspension bridge
Each main span is over 1,300m long, and there are approach bridges of 300m for the south and around 600m for the north shores respectively.
“Of course a suspension bridge with this kind of tension leg platform has never been coupled [together] but the basis of the technology is known,” explains Minoretti.
The known element is because of Norway’s experience in offshore structures for oil and gas. The tension leg platforms – vertically moored floating structures – are entirely borrowed from offshore oil and gas platforms.
One of the plus points for this structure is that it will have minor impact visually because it will have just two central towers. In addition the deck would be around half that of the floating bridge – at just 33m wide.
Because there are just two towers coming out of the sea, it would also mean there are only two locations that will have to be protected against ship collisions.
Norway suspension bridge drawing