IT WAS snowing at the edge of Lake Erie during the commissioning ceremony for the USS Little Rock in December 2017. The US Navy ship cost more than $300 million and is designed to have the speed and manoeuvrability needed for anti-submarine warfare. Towards the end of the ceremony, the ship’s chaplain prayed for its crew: “Protect them from the perils of the sea and the violence of the enemy.”
Ice probably wasn’t at the forefront of his mind, yet it is the most troublesome foe the ship has faced to date. It is trapped in port, its route to sea frozen shut.
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The USS Little Rock is only the latest ship to be frozen out of action. Freight and research vessels routinely get stuck in Arctic ice, leaving crews twiddling their thumbs until an icebreaker ship arrives to smash a path out. But icebreakers make slow progress and frequently get stuck themselves. Luckily, there is an alternative approach that promises to clear ice much quicker: just give it the right sort of shake.
Despite the risks of ice, Arctic seas are getting busier. The Northern Sea Route running along the coast of Russia, for example, provides a shortcut for cargo ships between the Atlantic and Pacific oceans, reducing the distance compared with the Suez Canal route by about 40 per cent. As the ice cap thins, the Arctic route is becoming increasingly viable: a record high of almost ten million tonnes of cargo travelled that way in 2017. But although the route isn’t frozen solid these days, there will still be plenty of ice floating around for decades to come – so a way to clear it is more important than ever.
The first inkling that a method less brutish than ice-breaking by ship might be possible came more than half a century ago at the opposite pole. In the 1950s, the US military was doing experiments to work out when it was safe to land aircraft on the Antarctic ice sheet. On touchdown, the planes immediately created a bowl-shaped depression as expected. But intriguingly, instruments attached to the ice recorded a wave rippling through the ice away from the landing site.
Ride the wave
It turned out that the plane had set the ice sheet wobbling at what is known as its resonant frequency. To see how this works, imagine pushing a child on a swing: you must push at just the right moment to force the swing higher. Ice is similar. Bang it at the right speed and you will set it shaking with the maximum possible energy. That speed isn’t easy to calculate because it depends on factors such as ice thickness and the depth of water underneath. But the military experiments showed that resonant waves in ice travel at between 20 and 60 kilometres an hour.
It was an abstruse effect that went largely unnoticed until 1974, when the Canadian Coast Guard accidentally discovered what could follow. A team was testing whether an experimental hovercraft could break ice using the downwards pressure jets it uses to get around. But when the vehicle accelerated to just over 20 kilometres an hour as it travelled across the ice to a test site, the crew saw a wave form behind. The wave rose and rose – until the ice started breaking at its crest.
The hovercraft happened to be pumping energy into the ice at its resonant frequency, with consequences that are, in hindsight, predictable, says fluid dynamicist at the University of Bath, UK. Similar to the situation with the swing, if you apply a force to ice at its resonant frequency, the response will be significant. “That means you’ll crack it,” he says.
“Submarines could break thick ice ten times faster than ships can manage”
The potential was obvious: rather than laboriously focusing its jets on small areas of ice until they broke, the hovercraft could skim around, quickly breaking ice as it went. Today, the Canadian Coast Guard operates for resonant ice-breaking (pictured). Based in the Québec city of Trois-Rivières, they play a vital part in controlling flooding along the St Lawrence river, freeing up shallow tributaries where conventional icebreakers can’t operate.
It is ice on this river that is blocking the USS Little Rock’s route to the sea, but unfortunately the hovercraft can’t help. It’s so frigid this winter that cracked ice quickly fuses again.
Even in less adverse weather, the craft have limitations. They have a short range compared with ships. And shards of ice can damage the flexible skirt that contains the cushion of air supporting the craft, meaning they can only operate safely in daylight. For long-distance ice-breaking – such as clearing Arctic sea routes – something better is needed.
Some believe the solution lies below the ice. In the 1990s, Viktor Kozin of the Komsomolsk-on-Amur State Technical University in Russia was trying to find ways for submarines to safely surface through ice. Hiding beneath the ice is a neat tactic for subs, but they must get above it to launch missiles. Traditionally, this involves forcing the vessel upwards by increasing its buoyancy. That cracks the ice alright, but it comes with a significant risk of damaging the sub.
Then Kozin and his team noticed in some calculations that the waves created by a submarine of a particular shape as it moves under ice might hit its resonant frequency. So the subs could pull off a similar trick to the hovercraft, but from below.
So far the team hasn’t carried out any large-scale tests of the idea. One was scheduled to take place in the Black Sea’s Kerch Strait in 2016, but Kozin couldn’t get the required funds together.
For the moment, Kozin and his team have set up a large water tank covered in ice and started dragging model submarines through it. On several occasions, they have seen the sheet shiver and split as a model sub with the . Using computer models to scale up the results, they believe that a sub travelling 20 metres below the ice at 20 to 60 kilometres per hour that are up to 2 metres thick. That is the same thickness an ice-breaking ship can manage, but the subs could do the job 10 times as fast.
One niggle that needs attention is that sheets of ice have irregular projections beneath them known as keels that can extend for many metres. It would be hard to spot these with sonar, so to reduce the risk of hitting them, submarines might need to have the towers that contain their periscopes removed, says Vitaliy Zemlyak of Sholem-Aleichem Amur State University in Russia, who is part of Kozin’s team. Although that’s a dramatic change, the towers aren’t critical to the subs’ movement.
Meanwhile, commercial interest in Arctic shipping is only growing. A study by the Netherlands Bureau for Economic Policy Analysis suggests the thawing of the Northern Sea Route could lead to . And with conventional icebreakers being both expensive and slow – and plenty of obsolete submarines sitting in Russian shipyards – the ice shakers could soon be making waves.
This article appeared in print under the headline “Shake the ice”
