Dynamic Positioning
in Ice


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"Ice is one of the last frontiers," says Dr. Walter L. Kuehnlein, the managing director of Sea2ice Ltd. & Co, Hamburg, Germany. He thinks that the most important accomplishments by arctic engineers and scientists today are overcoming the challenges of dynamic positioning in ice. In open water, dynamic positioning (DP) involves a series of computer controls that issue position-related commands to maintain a ship's position and heading. There, it is a well-known strategy used for many years. But in the arctic, DP philosophies and strategies are different and also include ice-management.

This Is Not Open Water

Kuehnlein, who also is the Polar and Arctic Science & Technology Symposium coordinator at the ASME's Ocean and Offshore Arctic Engineering Conference, says that most of the industry that operates in light to moderate ice conditions (ice-covered waters) is focusing on DP.

"Positioning ice-breaking and drilling vessels in thick, moving ice is much different than in open water. You need other vessels to manage the ice," he says. Instead of using "normal engineering" principles to design structures and vessels to withstand the forces of the maximum expected waves, the concept of dynamic positioning relies on a captain who is breaking and managing the ice. The concept of autopilot can't survive on its own; a vessel needs to have operations to lower the forces so the system can deal with them.

"In open water, if you increase a vessel's thrust, you immediately see a reaction. In ice, you only see a reaction if you have surpassed the breaking load of the ice. Even if you increase the thrust by as much as 50%, if you're not above the breaking point of the ice, you won't see a reaction," he explains.

Typically, a second ice-breaking vessel accompanies a drilling or other vessel about a mile ahead. It travels from side to side or forward and backward to pre-break the ice. Sometimes a third ice-breaking vessel also is needed to further break the pre-broken pieces into even smaller floes before the vessel that does dynamic positioning can stand. Tugs pull icebergs (or pieces of them) that approach the DP vessel aside and change their course by 1-2 degrees to avoid collisions.

Staying in position when ice starts to move is a problem. It's nearly impossible, and the forces required to do so are much higher than in open water. Kuehnlein mentions the ice forces in one ice field in the North Sea has more than 10,000 tons horizontal load. In these extreme cases and in heavy ice conditions, forces needed to stay in position are more like 50 to100 times greater than in open water. No propulsion system can deliver such a pull, so vessels are connected to strong mooring systems.

The biggest problem is that ice rapidly moves and it changes direction.

Dealing with Ice

Many things are necessary for dynamic positioning in ice. First, Kuehnlein says you have to develop tools that can foresee what ice will come from different directions and models that show which ice will approach you for up to two hours in the future so you know your advance power requirements.

Next, the vessels must have a modified shape with inclined (not vertical) sidewalls, as you need vessels that can turn around, almost on a spot in ice.

You have to have an ice-breaking strategy and philosophy for various scenarios. If ice movement stops, normally (in open water, if the current stops) you wouldn't do anything, you would just stay there. In ice, you have to break as large an area around you. If it starts to move again, it may move in a totally different direction and you need a pre-broken area to turn the bow towards the drifting ice in a short time.

With DP, there is a lot of activity and preparing. In open water, you relax. In ice, you work hard to move in a new direction, and constantly move backward and forward. But, in ice, you don't have an "overshoot angle" problem that you do in open water; when you drop the power, you stop immediately. With DP, you commonly reduce power slowly from 100%.

Challenging but Possible

DP work is important and needed now. Drilling for mineral resources in light to moderate ice conditions could be done with dynamic positioning. Kuehnlein says the industry made a little progress on in the 1980s, but they didn't have the technologies of today.

Now, the entire operation is different; we need to have evacuation systems, possibilities for oil recovery, spill containment, other vessel requirements and means in place to handle disasters. It's challenging now, but we couldn't have succeeded 20 to 30 years ago.

The community working on these issues has a few hundred people. The demand is increasing much faster than the knowledge, which Kuehnlein says can be a problem. "We have much less experience with dynamic positioning in ice than with it in open water. The models are rougher, but they need to be more robust than in open water. It's a big challenge. In ice you need 10 times more power, but have 10 times less experience and 10 times fewer people. It's 1,000 times more complicated," he adds.

Debbie Sniderman is CEO of VI Ventures LLC, a technical consulting company.

It’s a big challenge. In ice you need 10 times more power, but have 10 times less experience and 10 times fewer people.

Dr. Walter L. Kuehnlein, managing director, Sea2ice Ltd. & Co.

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November 2012

by Debbie Sniderman, ASME.org