A bridge between Scotland and Northern Ireland: what would it be like?

Prime Minister Boris Johnson's call for a feasibility study on a potential bridge spanning the Irish Sea made many snigger. But it might not be as silly as it sounds.Tuesday, 24 September 2019

A bridge from Scotland to Northern Ireland? Boris Johnson’s proposal to link the two countries with a road crossing over the Irish Sea would, most experts agree, cost a lot more than the £15 billion he is suggesting. And the engineering involved would be more than a little complicated. But is it technically possible?

Yes, according to several experienced bridge engineers and designers. Alan Dunlop is a Scottish architect who, in 2018, was commissioned by a newspaper to analyse the feasibility of such a project. He suggested two routes: a 12-mile crossing that spans the shortest gap between the two countries – Mull of Kintyre in Scotland to Torr Head, Northern Ireland. And, to the south, a 26-mile crossing from Portpatrick to Larne. The northern route is unlikely since the ports in question are more remote, and the road improvements needed might end up costing as much as the actual bridge. The southern route, which would be closer to Belfast and Scotland’s Central Belt is more practical, given the existing road infrastructure on both sides.

More challenging than the length of such a bridge is the depth of the sea bed – up to 160 metres at its deepest section. Dunlop proposes a cable-stayed or suspension bridge for the shallower sections and a floating pontoon-style bridge (“tied to the sea bed with cables like an oil rig”) for the deeper sections. He stresses that since oil rigs can operate in seas as deep as 3,000 metres, the required technology already exists. 

“This would be a challenging proposition,” he admits. “But we have the technology and the talent in Ireland and Scotland to create something as potentially brilliant as this.”

Inevitably, Dunlop’s design has had more than a little scorn poured on it. Critics warn of the often tempestuous weather in the Irish Sea, for instance. “There are rough seas,” Dunlop admits, but then he cites the example of the Hong Kong-Zhuhai-Macau Bridge, a 34-mile series of cable-stayed bridges, an undersea tunnel and four artificial islands which opened in China last year. “That was designed and built to withstand typhoons. We have the engineering expertise to do that.”

Depths, debts... and unexploded bombs

There are worries the Portpatrick-Larne route might have to cross a huge undersea trench called Beaufort’s Dyke, up to 300 metres at its deepest section and, according to the Ministry of Defence, filled with over a million tons of dumped munitions from the Second World War, as well as radioactive material. Dunlop says his route would cross to the north of this, however, where the sea is shallower.

Then there’s the thorny issue of funding the project. Dunlop admits he’s no economist, but he estimates costs of between £15 billion and £20 billion which might eventually be recouped through a toll system. As he points out, the Irish Sea ferries are expensive, especially for freight traffic. He cites the example of the 4.9-mile Oresund Bridge, linking Sweden to Denmark, which charges a toll and is expected to recoup its costs by the late 2030s.

Ian Firth is a British structural engineer who has worked on bridges all over the world. He agrees that the proposed southern crossing of the Irish Sea is the best option, stressing that the 26-mile length is not the most challenging aspect, rather the depth of the sea bed. “I don’t think there’s been any bridge built with that much depth.”

He outlines three possible design options. The first is a multi-span cable-stayed or suspension bridge with its towers standing atop tall oil-rig-style steel or concrete structures resting on the sea bed. The second is also a cable-stayed or suspension bridge but with its towers supported on huge, submerged floating pontoons tethered by cables to the sea bed. The third, and most eyebrow-raising of all, is a submerged tube – effectively a tunnel under water – attached to floating pontoons at the surface or tethered by cables to the sea bed below. All three designs have precedents but only on a small scale; nothing nearly as colossal as what would be required to span the Irish Sea. 

Firth believes the third option is most realistic since it avoids the extra cost of towers. “I’m convinced it’s doable,” he says. “It’s not been done before but we’ve designed similar bridges. The problem is the cost which would be incredible, even without the towers.” (Although he adds that he hasn’t “the slightest idea” what that cost might be.)

Above or below?

There are pros and cons to both types of submerged tubes. The one attached to the sea bed would avoid the risk of ships colliding. “Although submarines would need to watch out.” 

However, imagine the chaos of a fire or an accident deep inside a tunnel in the middle of the Irish Sea. “And there’s the human experience to consider,” Firth adds. “Who wants to drive through an underwater tunnel 26 miles long?” He has a point: this is surely perfect material for scriptwriters of disaster movies.

A bridge above the water, on the other hand, would have to be closed in rough weather and poses risks to passing ships. The shipping density in this part of the Irish Sea (The North Channel) is around 30 vessels a day, according to the Marine Management Organisation. 

While a traffic link between Scotland and Northern Ireland may be logistically demanding, there are impressive bridges already in operation elsewhere in the world, albeit on smaller scales. The aforementioned Oresund Bridge – the inspiration for the Scandinavian and American TV series The Bridge – is one awe-inspiring precedent. Featuring a railway and motorway, it first stretches on a cable-stayed bridge almost five miles from the Swedish coast to an artificial island called Peberholm before continuing to the Danish coast through a concrete tunnel set into a trench in the seabed.

Although the Hong Kong-Zhuhai-Macau Bridge is eight miles longer than the proposed Portpatrick-Larne link – and is indeed the longest open-sea fixed crossing in the world – its span is interrupted by four artificial islands, for which the Irish Sea is too deep. 

For bridges on pontoons, the longest example is the Governor Albert D. Rosellini Bridge in the US state of Washington which carries State Route 520 a mile and a half across Lake Washington. Of course lake waters are far less choppy than those on the Irish Sea.

Future Unifications

Look to the future, though, and bridge engineering gets very ambitious indeed. Current proposals include the Fehmarn Belt Fixed Link, a tunnel connecting Denmark to Germany; an undersea rail tunnel across the Gulf of Finland, between Finland and Estonia, which at 30 miles would be the longest undersea tunnel in the world; the Saudi-Egypt Causeway, a causeway and bridge link between the Saudi city of Tabuk and the Egyptian city of Sharm el-Sheikh via the island of Tiran; the Malacca Strait Bridge between Malaysia and Indonesia; a 28-mile railway tunnel between the Russian island of Sakhalin and the Japanese island of Hokkaido; and even a 25-mile rail tunnel beneath the Strait of Gibraltar, linking Morocco to Spain.

Some of these will prove to be mere pipe dreams. However, one mega-project that has already started is a 17-mile undersea road tunnel beneath two Norwegian fjords, Boknafjord and Kvitsoyfjord, with a maximum depth of 392 metres. Known as Rogfast, it began construction last year and is expected to finish in 2026. If there’s still the political and financial appetite after the £1.5 billion in bills have been settled, Rogfast could be the first link in a non-stop road route all the way up Norway’s west coast, with the current fjord ferries eventually replaced by a network of bridges, tunnels beneath the fjord beds, and tunnels submerged below the water surface.

A Fusion of Styles

Naeem Hussain is global bridge design leader at design and engineering firm Arup. He has worked on some of the most challenging sea bridges on the planet. As well as Oresund and Hong Kong-Zhuhai-Macau, these include the Temburong Bridge in Brunei, the Mumbai Trans Harbour Link in India and Stonecutters Bridge in Hong Kong. He even has experience in Scotland with the Queensferry Crossing over the Firth of Forth. He agrees a Portpatrick-Larne bridge is “entirely practical and doable”, but to make it worthwhile, the roads on the Scottish side would need to be upgraded.

“The route might have to cross a huge undersea trench called Beaufort’s Dyke, filled with over a million tons of dumped munitions from the Second World War, as well as radioactive material.”

He favours a cable-stayed bridge (with spans of between 1200m and 1500m) or a suspension bridge (with spans of between 2000m and 2500m). Both styles would be supported by towers standing on top of oil-rig-style frames. 

“The bridge could be constructed with existing and proven technologies, marrying off-shore oil technology with long-span bridge technology,” he explains. “The substructures would be constructed in a dry dock, floated out to the site, and then lowered into position. Large pre-fabricated deck units would also be brought to site on barges and lifted into place.” Prefabrication on land would make the bridge more durable, he says, with a possible lifespan of 150 years or more.

And what about the cost? Depending on the number of traffic lanes, he estimates this to be between £20 billion and £30 billion.

Just how serious the British government is about the bridge remains to be seen. Both the Treasury and the Department for Transport have been asked to look into the financial side of the project.

Cynics might say Boris Johnson’s sudden enthusiasm for a physical link between Britain and Northern Ireland is an attempt to strengthen ties within an increasingly disunited kingdom. But even the non-cynics might not be able to stomach that price tag.

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