THE ad promised the cruise of a lifetime: a chance to visit one of the remotest regions on Earth, with stunning views of glaciers, penguins, seals and the odd humpback whale. Yet for 100 passengers who embarked on the MS Explorer in November 2007, the trip to Antarctica provided a rather different kind of thrill.
On 23 November the vessel was off King George Island, some 100 kilometres from the tip of the Antarctic Peninsula. Most of the passengers were in bed when, at about 1.20 am local time, the vessel struck a chunk of ice. The impact tore a hole in the hull – a gash more than a metre long, according to the Argentinian coastguard – and water started pouring in. Attempts to control the flooding failed and the passengers and crew had to take to the lifeboats. After 4 hours in freezing temperatures they were picked up by a Norwegian cruise ship. The Explorer eventually sank in over 1000 metres of water.
It is almost 100 years since an iceberg sent the Titanic to the bottom of the Atlantic with around 1500 passengers and crew. Since then, ships have become far more robust and are fitted with radar that can look out for ice ahead. What’s more, iceberg warnings are now issued in both northern and southern waters. Yet as the fate of the Explorer shows, sailing at high latitude is still dangerous: each year two ships on average are seriously damaged by collisions with floating blocks of ice that can weigh tens of thousands of tonnes. That is why there were some raised eyebrows among experienced mariners when Bob Gagnon, an ice physicist at the Institute for Ocean Technology (IOT) in St John’s, Newfoundland, Canada, proposed crashing a ship into an iceberg – on purpose. “I thought it was pretty crazy,” says Lawrence Meisner, a ship’s captain now retired from the Canadian Coast Guard. “The last thing we want to do is hit ice; we try our best to avoid it.”
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Gagnon’s thinking was that staging a collision would yield information about how a vessel responds that could then be used to improve the safety of shipping in ice-strewn waters. There is still precious little information on how factors such as the shape of a berg, the state of the sea, a vessel’s speed and the design of its hull affect what happens when metal meets ice. “We needed to find out what kind of pressure and load ships experience during a collision, and what kind of speeds it is safe to operate at,” Gagnon says.
The experiment posed some serious challenges. For a start, Gagnon needed a vessel strong enough to survive being rammed into an iceberg, plus a captain and crew willing to take the risk. Just as important, he needed to devise sensors capable of recording the forces generated at the instant of collision. No one had ever before tried crash testing at sea on such a mammoth scale.
Yet ensuring that ships don’t come to grief in collisions with icebergs is, if anything, more important than ever. As global warming takes hold, icebergs will become far more numerous and drift further from the poles, according to many predictions. At the same time, more ships are venturing into polar waters. Adventurous tourists, like those aboard the Explorer, can now take cruises to the edge of the ice caps.
Meanwhile, dwindling fish stocks are pushing fishing vessels into new waters: many now hunt for shrimp along the ice margin of Canada’s Labrador coast, for example. And with mining and oil prospecting pushing ever further into the Arctic, more supply vessels and tankers than ever are plying dangerous waters. Oil and gas companies are particularly concerned by the threat to tankers and structures such as oil rigs. In the 1980s, a drifting berg came within a couple of hundred metres of running down an oil rig off the Canadian coast, and some companies now keep ships ready to tow icebergs clear if they are putting a rig at risk.
Yet for shipping it is not huge, drifting bergs that are the danger, as these mountains of ice show up clearly on ships’ radar. What seafarers worry about are the house-sized lumps nicknamed bergy bits and car-sized chunks called growlers. With only 12 per cent or so of the volume of any berg breaking the surface, these smaller chunks of ice are almost invisible to radar. Yet a bergy bit can still weigh in at up to 10,000 tonnes, and even a growler may weigh 100 tonnes or more. “These are the ones that give us heartburn,” says Meisner. “They get lost as they bob up and down in the swell and you can’t see the darn things.” Yet they are easily massive enough to tear a hole in the steel plating of a ship’s hull. “If you hit one it can cause serious problems.” Just about all the significant collisions in the last 25 years have been with growlers and bergy bits, says Brian Hill of the IOT, who maintains .
Clearly an experiment that involves ramming icebergs – of whatever size – calls for a specially reinforced vessel. Initially Gagnon planned to use a strengthened barge, but after talking to experts he decided a more manoeuvrable ship would be better for the job. The obvious choice was the Terry Fox – the most heavily armoured icebreaker in the Canadian Coast Guard fleet. Meisner, its skipper at the time, took some convincing but eventually agreed to take on the job.
The next task was to find a suitable area for the tests. The perfect spot would have plenty of bergy bits and growlers, but not too strong an ocean swell. After surveying the coast of Newfoundland from the air, they found what they needed near St Anthony Bight, close to the northern tip of the island.
Gagnon’s plan was that each growler and bergy bit would be carefully characterised before any impact. Researchers on board a support vessel, the Mottak, would use cameras and sonar to measure the size, mass and shape of each iceberg, above and below the waterline. Then one brave soul would set out in a small boat, climb onto the berg and drill into it to measure its temperature profile. This would reveal important information on the physical properties of the ice. Finally, the Mottak would lasso the berg and tow it to a suitable patch of open ocean for the crash test.
Full speed ahead
On 18 June 2001, a somewhat apprehensive Meisner and his crew lined up the Terry Fox for her first collision. The berg they had selected was a growler weighing around 100 tonnes. With measurements complete, Meisner set the vessel steaming full ahead, aiming to make contact with the ice just to one side of the bow.
Hitting icebergs a glancing blow is generally more dangerous than a head-on smash. In most ships the bow is the strongest part of the hull, with more closely spaced frames and a thick, curving “stem bar” that transfers forces to a strong central girder. Striking an iceberg with the shoulder of the ship – where the bow meets the main body of the hull – can transfer the force straight to the frame, and this can be enough to snap welded joints. “It’s like hitting someone in the ribs,” says Meisner. And just as with broken ribs, this damage can remain hidden from view.
To monitor how the Terry Fox stood up to the collision, Gagnon installed a number of instruments around the vessel. His team wired a strain-gauge panel inside the forward ballast tank to measure the forces and pressures experienced by the frame of the ship and to measure any deformation. At the bow and in the centre of the ship the researchers placed accelerometers and motion-sensing units to record its roll, pitch and heave. And key to the whole experiment, on the hull between the bow and shoulder, divers mounted a specially designed panel to record and map the forces of the collision. “It was a bit of a job to ensure that it wouldn’t be ripped off during a hit,” says Gagnon.
The outermost layer of the panel is a thin stainless steel sheet, supported on strips of tape that hold it just one-tenth of a millimetre away from a slab of plexiglas (see Diagram). As the impact momentarily deforms the steel sheet, pushing it into the gaps between the tape strips, it changes the way light reflects off the internal surfaces of the plexiglas. Video cameras monitoring the resulting patterns of light and dark show exactly where the collision occurred and what forces it generated.
Using another video camera to guide him, Meisner steered the Terry Fox to aim just the right point on the hull to hit the berg. Below decks, Gagnon and some of his team sat on plastic crates, monitoring their instruments and watching a screen that showed the iceberg looming up. “It was really exciting,” says Gagnon, “and a bit scary.” When the impact came, it sounded to the crew on deck like a muffled, ringing explosion, but to those inside it was deafening.
“On deck it sounded like a muffled, ringing explosion, but to those inside it was deafening”
Happily, the Terry Fox was unscathed by the collision, and over the following days the team repeated their experiment 178 times, smashing into 18 different bergs. After each collision, researchers on the Mottak scuttled back to the berg to take more measurements, which would later be analysed to find out how the impact had changed the structure of the ice. By the end of the week, the Terry Fox was approaching smaller bergs at speeds of around 13 knots (6.7 metres per second) and cautiously smashing into blocks of ice weighing up to 22,000 tonnes.
Since then, Gagnon and his colleagues have been busy analysing the wealth of data they gathered, and working out which bergs cause the most damage and why. The maximum impact forces measured were around 5 meganewtons, and the shape of the berg and its consistency turned out to be crucial to its potential to cause damage. They found that impact with a small, hard region of ice within a berg could generate pressures of up to 20 megapascals – the equivalent of a weight of 2000 tonnes resting on 1 square metre. “The way the load is distributed is key,” says Gagnon. “If the load is uniformly distributed over a large area then the likelihood of damage is reduced, whereas if the same load is concentrated in a small area the pressure can really get the rivets popping.”
Unlike the sturdily reinforced Terry Fox, an oil tanker or a fishing trawler would be in deep trouble if it suffered similar impacts. Marine architects expect to learn a lot from Gagnon’s experiments about how to make regular vessels less vulnerable. The forces generated between vessel and ice are easily the largest uncertainties when designing the structure of a ship for use in polar seas, says Richard Hayward, an expert in ice-class ships at the shipping-classification organisation Germanischer Lloyd in Hamburg, Germany. “Any insight provided by Gagnon’s experiments is welcome.”
In tandem with his collision experiments, Gagnon has been working with Claude Daley, a naval engineer at Memorial University of Newfoundland in St John’s, to develop numerical simulations and laboratory experiments of ship-iceberg collisions. As well as oddly shaped bergy bits and growlers, ice-class ships can expect to encounter ice features such as old channels and pressure ridges left by other ships, and Daley and Gagnon wanted to explore these scenarios in the lab. For each simulation, they verified their findings in laboratory experiments that put the frames of scale-model ships under huge loads. “We went way beyond the design limits, bending the frames and studying the plastic response that might occur with an ice collision,” Daley says.
The results indicate that the normal construction standards won’t do for the hulls of ice-class ships. “Designing ships for polar seas should be like designing off-road vehicles. You assume things will get bent,” Daley says. The results point to the importance of developing stronger materials and designing crumple zones on ships to help absorb an impact. “If the metal plating can deform, it helps to spread the load,” Gagnon says.
Dividing a ship into watertight compartments is one well-established way of helping a vessel stay afloat if it is holed below the waterline. “If the damage is confined to one area and the remaining compartments stay intact, the ship can often stay afloat,” Gagnon says. Many ships have a bow compartment separated from the rest of the vessel by a strong bulkhead. Steering into a collision so that damage only occurs forward of this bulkhead can make the difference between sinking and staying afloat. The angle at which a ship takes a blow can be crucial to this, says Gagnon. “Some experts speculate that the Titanic could have survived its collision had it been head-on rather than a side hit that ripped through multiple compartments,” he says.
Another way to protect a vessel is to build ballast tanks along the side of the hull, where they can play a similar sacrificial role to the bow compartment. A development of that idea is behind the “Coulombi Egg” tanker design, which has been accepted as an alternative to the double-hull construction now required for modern tankers to cut the risk of a devastating oil spill. Shipbuilders are also experimenting with hulls made from lightweight but strong composite materials such as carbon-fibre epoxy resin. One variation is a new super-strong material known as steel plastic sandwich (SPS), in which a central plastic layer is encased between steel sheets. “A hull made from SPS can tolerate a lot of deformation and energy absorption before it is holed,” says Gagnon. A ship with an SPS hull would probably survive a collision like the one that sank the Titanic.
Compared with collision tests at sea, the lab experiments are quick and inexpensive. “For the price of the Terry Fox trials you could do years of experiments,” Daley says. But in the end, lab experiments and computer simulations are always a simplification. Sometimes only real live data will do.
So Daley is happy that Gagnon and his team plan to go to sea again, this time to try some metal-bending head-on collisions. The impact panel will be placed directly on the bow rather than at the side, and in this way they anticipate being able to generate and measure much larger forces – up to four times those experienced in previous experiments. “The Terry Fox trials led the way and showed it could be done,” says Daley. “Now we need to go back and get more detailed data.” Step 1: find a captain and crew brave enough to do battle with a berg.
