ARCHIVED - A field guide to hazardous ice
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June 01, 2009— Ottawa, Ontario
It's thick, powerful and poses a major hazard to ships and offshore structures in the Arctic. It's called "old ice," and it's the most dangerous form of Arctic sea ice. But now a field guide to old ice — prepared by researchers at the NRC Canadian Hydraulics Centre (NRC-CHC) — shows Arctic operators how to reliably recognize and avoid, where possible, ice that could cause dangerous collisions at sea.
A floating laboratory
This summer, NRC will measure "global loads" on the Canadian Coast Guard icebreaker Louis S. St-Laurent, as it travels through the Arctic from the Atlantic Ocean to the Beaufort Sea. The icebreaker will carry MOTAN, a unique motion analysis system that includes sensors to measure the ship's movement and NRC-designed software to calculate the forces faced by the vessel. "MOTAN will help us determine how often the vessel experiences very heavy loads from multi-year ice, which is useful for designing ships that can withstand the Arctic environment," says Dr. Michelle Johnston.
Based on several years of field measurements, Understanding and Identifying Old Ice in Summer is designed to help ship operators distinguish old ice from first-year ice, and second-year ice from multi-year ice — distinctions that are crucial to operating safely in icy waters. The 200-page manual, by Drs. Michelle Johnston and Garry Timco, covers everything from the growth and aging of sea ice to ship-based, aerial and satellite observations.
First-year ice forms over the winter, but generally melts in the summer. Some first-year ice, however, survives the summer to become second-year ice in the fall. Multi-year ice is any ice that survives more than two summers. "Multi-year ice is generally the thickest type of Arctic sea ice," she says. "It can be up to 30 metres thick, or as little as 0.9 metres — it just depends on where you measure it on an ice floe."
"Old ice — which includes second-year ice and multi-year ice — produces the highest loads on offshore structures and causes the most severe damage to vessels in the Arctic," says Dr. Johnston. "Anyone who designs offshore Arctic structures, particularly in the Canadian Arctic, considers multi-year ice to be one of the main limiting design criteria."
During the winter of 1985-86, a structure called the Molikpaq, used for oil and gas exploration, was deployed in the Beaufort Sea, about 70 kilometres off the Canadian coast. "The Molikpaq interacted several times with multi-year ice, some of which resulted in such high loads that a full-scale evacuation of the structure was undertaken," says Dr. Johnston. The ice caused the structure to vibrate at such a frequency that its sand core partially liquified, which threatened to compromise its structural stability.
With growing interest in offshore Arctic oil and gas exploration, there is a lot more discussion about how vessels and structures should be designed to withstand multi-year ice. "This is a bigger question today because of climate change," notes Dr. Johnston. "When we hear that multi-year sea ice is decreasing in extent, does that mean it's also decreasing in thickness?" In fact, NRC data indicates that "some of this ice is still extremely thick. Multi-year ice will continue to be an issue even if the Northwest Passage opens up."
"As dangerous as it is, we really don't know enough about it," she adds. "NRC may be the only research organization doing field measurements on multi-year ice. I'm studying its thickness, strength, decay and drift properties, which can be quite a challenge. If you sample a floe in April or May, it will start moving when the ice breaks up so the chances of getting back to the floe later in the summer are remote."
The NRC ice research team works with other government organizations and industry, including oil companies, says Dr. Johnston. "Much of our work is supported by Transport Canada. They need to make sure that mariners transit the Arctic safely so that environmental damage from accidents is minimized."
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