Shetland Islands councillors will this month consider whether it’s worth building tunnels between the archipelago’s biggest islands to replace ferries.

At the meeting on 30 June, they’ll debate the outline business case prepared over the last year by Stantec, with Cowi, Mott MacDonald and ProVersa.

They sought input from three international tunnel-building contractors: Norwegian firm LNS; Czech contractor BEMO Tunnelling; and Austria’s Strabag.

Their investigation modelled a subsea tunnel between mainland Shetland and the island of Yell, concluding that it was both “buildable and investable”.

The goal is to future-proof links between eight island communities.

Old boats, scarce crew

The council says the current inter-island ferry network faces increased operating costs, an ageing fleet with an average vessel age of 32.5 years, difficulties in attracting and retaining crew, and vehicle deck capacity issues on important routes.

Studies have explored options ranging from “ferry do nothing” (business as usual), “ferry do something”, “ferry do max”, and subsea tunnels for four islands.

In the Yell case, a tunnel would have a lower capital cost and gross operating and maintenance cost than a “ferry do max” scenario, and a comparable 60-year cost outlook.

“When islands have fixed links like causeways, bridges and tunnels, they experience repopulation, economic growth and a reduction in average age,” said council leader Emma Macdonald.

“Doing nothing is not an option in Shetland. We have islands depending on old, unreliable, carbon-heavy ferries, which are … continually at risk of ferry breakdowns.”

Andy Sloan, Cowi’s executive vice-president for UK and international, said: “We know that the tunnels can be built. From an engineering perspective, it is relatively straightforward. The real challenge is whether we as a nation take a short-term or long-term view.

“As our global experience tells us, people will be attracted to live in remote and rural places if they are easy to get to, easy to get around, digitally connected and with job opportunities and an affordable standard of living.”

He added: “Once Scotland builds its first tunnel, it will never stop.”

The meeting on 30 June is not expected to result in an immediate yes-or-no decision regarding the tunnels.