On a calm day, the ocean’s surface may appear quite tranquil, but dive down just a few meters and it suddenly becomes painfully obvious that the weight of the ocean water places intense pressure on your body. To a point, the human body can naturally compensate for this pressure, but cruise ships must be engineered to withstand the tremendous pressure as ocean water tries to reoccupy the space that becomes occupied by a propeller-driven, multi-ton wedge of steel.
Cruise Ship Dynamics
“Because it takes about a kilometer to turn around a ship of this size, the goal is to steer clear of objects,” says Kevin J. Maki, assistant professor in the University of Michigan’s Department of Naval Architecture and Marine Engineering. “With that much momentum, even grazing rocks would easily penetrate its hull -- tearing it as easily as tissue paper.”
When an ocean liner -- one of the world’s heaviest moving structures -- has its hull violently ripped open, the forces that are unleashed are both difficult to imagine and comprehend. “The forces of water entering the ship would be great, with water moving at tens-of-meters per second,” he said. “Even so, the ship would likely remain seaworthy for some time, anywhere between 10-45 minutes, enough that it could be maneuvered with sufficient stability.”
But eventually, a tipping point – both figuratively and literally – would be reached.
Once a hull is breached, there is a battle between two forces at the air/water interface. How those two fluids of vastly different properties mix and interact determines a great deal about whether a ship will remain afloat or sink. “One of the key things to understand is that a space occupied by air can’t be occupied by water, and vice versa,” explains Egbert Ypma, a fluid dynamicist and Project Manager of Seakeeping and Stability at the Maritime Research Institute Netherlands (MARIN). “If air becomes trapped in a compartment, it will prevent the water level from rising.”
That’s why it’s so critical to immediately seal off a ship’s compartments to maintain stability, points out Maki. “With a long gash, it’s quite possible that many of the compartments will flood,” he adds.
Type of Damage Matters
This is in contrast to another infamous ship disaster, the USS Cole, in which an explosion near a ship’s hull created a large, but localized, rupture. According to Maki, “In this case, flooding is more likely to affect one or two compartments -- not all of them.” That’s why you would have two vastly different outcomes. In the case of the Costa Concordia, the damage continues along a large portion of the ship’s hull and well below the waterline. With the Cole, the damage was higher on the hull, and it didn’t affect the length of the vessel.
The manner in which the inundation of water influences the dynamic behavior of a ship is extremely complicated, according to Ypma. “It centers on two very tightly coupled subsystems: The large motions of a damaged ship up to the point of capsize, combined with the flood of water racing through the damaged internal geometry of a ship,” he says.
In the Costa Concordia’s case, a hole was torn in the side of its hull after hitting the rocks. A quick analogy by Maki explains some of the dynamics at play: “If you take a bucket of water and push it down into the ocean, Archimedes' principle [an object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object] applies with this downward force. If you tilt the bucket, it may take a while to sink -- but eventually the tilt will reach a point of no return.”
Dynamic Behavior of a Damaged Ship
Applying this simple model to a large ship, however, is more complicated. “To be able to say something about the dynamic behavior of a damaged ship, including safety, you have to take into account many chaotic processes,” Ypma said. In the case of water flowing into a ship, the extreme forces and fluid behavior would create a situation “where even small events have big consequences.”
Using an analogy to describe the water flooding into a hull, Ypma likens it to a crowd gathered for a sale in front of a store with closed doors. “As long as the doors are closed, there is more or less equilibrium. The instant the doors open, the crowd is push forward -- or sucked inward,” he notes.
Basically, an upright floating ship is a dynamic system in balance, explains Ypma. When a rolling force is applied to the ship, that balance is disturbed and a new balance must be found. In the case of the Costa Concordia, the ship had a high initial stability, so rolling -- as it would be felt at sea -- would be very small. After the hull was breached, however, the ship lost its stability.
Experts Available
For more information or to schedule an interview with the researchers, contact Charles Blue.
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