Strong as Ice: Icebreaker Structure

1.0 Introduction

Structures suffer as the unsung hero on ships. We see the hull as a static container, always constant and unremarkable. There are no dials or warning lights; ship structure doesn’t get the same attention as an engine or other mechanical equipment. But all that changes on an icebreaker! With icebreakers, the hull becomes the primary tool. The massive wedge we use to crack open mountains. All this, despite it being a hollow tin can, with an average density lighter than the very ice it cracks through. How to build an unbreakable hull?

2.0 Ice Class

Not all icebreakers are equal. We rank ships based on their ice class, which relates to the thickness and strength of ice they break. Higher class matches to stronger structure and bigger ships. This gets confusing, since multiple companies developed different rating systems. Figure 2‑1 shows one of the more common systems, the polar ice class. A ship with class PC6 matches the summer months. A vessel that normally avoids solid icepack and only breaks the occasional small icebergs. At the high end, only a few vessels exist with class PC2, as of 2022. The cruise ship Le Commandant Charcot was built to take passengers to the North Pole, cracking through ice 2.5 m (8 ft) thick! [1] Think about that. A wall of ice as tall as the ceiling, and thick ships pushes straight through. With 93 major icebreakers in service at 2017, there were NO PC1 vessels. [2] That shows the magnitude of the challenge behind designing icebreakers.

3.0 The Ice Belt

We can’t build the ship from solid steel. It must remain light enough to float, and that requires us to think strategically about where we need reinforcement for ice. Most icebreakers run with a deep draft, meaning the hull extends down below regular ice thickness. And large sections extend above the ice thickness. A very narrow region near the waterline takes the bulk of abuse from icebreaking. We call this the ice belt. (Figure 3‑1) The ice class determines the size and strength of this ice belt.

Structural engineers hate complicated diagrams like this, because they mean lots of calculations. We prefer to simply things with an easy rectangle. It tells a lot about the burden of the ice belt that this diagram includes so much detail. The ice belt forms in the region between the lower ice waterline (LIWL) and upper ice waterline (UIWL). Notice that the region starts larger at the bow and narrows towards the stern. Every piece of different hatching shows a region with different reinforcement requirements. That ice belt requires so much steel that we drop to lighter structure as quickly as possible.

We need to drop the weight. On the Mackinaw (WAGB 83), a 1936 icebreaker local to the Great Lakes, the ice belt had hull plating 35 mm (1-3/8 in.) thick. [4, p. 19] A single square meter of this plate weighed 275 kg (605 lb). You need a crane just to pickup a small section of plating. At a rough guess, the Mackinaw had 352 m², making 97 MT (95 LT) in weight just for the plating! I know whole ships that weigh less than this ice belt. For icebreakers, structural design is a careful balance between strength and weight. To achieve that balance, we scrutinize every single detail.

4.0 Bow Reinforcement

Another region for scrutinization: the bow. We shape the bow like a giant ramp, intended to ride up on top of the ice and crash down from above. This results in titantic forces. We intentionally impact against the ice. Surviving this impact requires more than strong steel. Just like tanks angle their armor to deflect shells, we carefully shape the hull to pick the angles at the point of impact. (Figure 4‑1)

We shape the hull so the ice rakes along the plating, instead of hitting it squarely. This considers both the horizontal and vertical angles of the shell plate. The equations even change with the orientation of the supporting stiffeners. We really do design the hull to deflect bullets. Or in this case, chunks of ice.

5.0 Steel Strength

Given the need to create strong structures with light weight, you might think we immediately go to the highest strength steel we can find. But no. Higher strength steels come with trade-offs. They tend to become more brittle. We want ductile, flexible steel. When a chunk of ice hits, we want the steel to flex and bend, not crack and break. Brittle steel spells doom for icebreaking.

And in cold water, normally ductile steel can become brittle. Below a transition temperature, the steel stops behaving in a ductile manner and acts brittle. (Figure 5‑1) And that transition temperature hovers around the same point as cold polar temperatures. Normal mild steel transitions around -15C [5] Icebreakers definitely experience temperatures below the freezing point of water (0C), or we wouldn’t have ice!

Now, structural design goes beyond simple strength of steel; we also need to consider the impact energy, at cold temperatures. Simplifying all the science, more impact energy at lower temperatures equals more ductility. We can test this in the lab, after freezing our samples to low temperature conditions. Despite our need for high strength steel, the impact energy may govern our selection for steel properties. We may choose a low strength steel to ensure it remains ductile and survives decades of abuse as an icebreaker.

6.0 Conclusion

Someone once told me that icebreaking felt like crashing your car straight into a brick wall . . . and then repeating the process every minute of the day. The structure on icebreakers is highly optimized, reinforced and shaped to avoid any concentrated forces. Every impact spreads the pressures through the hull, and turns a collision into glancing blows. Careful and smart with our application of steel, only add the strength and weight where we need it. The small pieces build together. We form a hull so strong, that we crash straight through the brick wall.

7.0 References

[1]	Wikipedia Authors, “Polar Class,” Wikipedia, 15 Dec 2022. [Online]. Available: https://en.wikipedia.org/wiki/Polar_Class. [Accessed 13 Mar 2023].
[2]	US Coast Guard, “Major Icebreakers of the World, Explanatory Piece,” USCG office of Waterways and Ocean Policy, Washgington, D.C., USA, 2017.
[3]	ABS, “Part 6: Specialized Items and Systems,” in Rules for Building and Classing Marine Vessels, Houston, TX, American Bureau of Shipping, Jan 2023, p. 19.
[4]	S. L. Planisek, Icebreaker Mackinaw, 2nd. Edition, Mackinaw City, MI: Great Lakes Lighthouse Keepers Association, 2008.
[5]	Shigley, Mechanical Engineering Design, 10th Ed., New York, NY, USA: McGraw-Hill, 2014.
[6]	YENA Engineering, “Ductile-Brittle Transition Temperature and Impact Energy Tests,” YENA Engineering, [Online]. Available: https://yenaengineering.nl/ductile-brittle-transition-temperature-and-impact-energy-tests/. [Accessed 13 Mar 2023].
[7]	Wikipedia Authors, “Projectil Deflection Effects,” Wikimedia Commons, 16 Jun 2010. [Online]. Available: https://commons.wikimedia.org/wiki/File:Projectil_deflection_effects.jpg. [Accessed 13 Mar 2023].

Strong as Ice: Icebreaker Structure

1.0 Introduction

2.0 Ice Class

3.0 The Ice Belt

4.0 Bow Reinforcement

5.0 Steel Strength

6.0 Conclusion

7.0 References

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