How Shipping Containers End Up in the Ocean
Recent shipping accidents have investigators looking more closely at the conditions that can send boxes tumbling from vessels into the sea
The Wall Street Journal – Hundreds of containers have fallen from container ships into ocean waters in recent months, in a flare-up of accidents that can destroy millions of dollars worth of goods, damage vessels and endanger lives and the environment.
Such accidents are rare among the millions of boxes that move across oceans each year, and maritime officials say they have been declining over the long term. But the recent spate of failures adds urgency to investigations of the losses.
Naval architects and engineers say a string of circumstances have to come together to create the catastrophic event known as parametric rolling. They say that as ships become bigger and containers are stacked as high as multistory buildings, the stability of vessels on the open waters is a growing concern.
“It is a big factor in container losses and it happens when waves hit the bow not head-on, but at an angle,” said Fotis Pagoulatos, an Athens-based naval architect. “Ships pitch up and down as they steam ahead but they can also go into a rolling motion from side to side. This can become uncontrollable and displace a lot of boxes that fall over.”
Here is how it happens.
The ship steams ahead.
When a vessel encounters waves, it starts pitching and rolling depending on the angle at which the waves hit the bow.
This happens as the sharp bow, which resembles an arrow tip, cuts through the water and pitches up and down.
As the wave crest travels along the streamlined hull the flat stern enhances the rolling motion, but the momentum of the ship’s forward movement allows the vessel to spread the energy from the crashing waves along the hull and keep it upright.
In heavy seas, the roll angle may increase from a few degrees to over 30 degrees after only a few instances of cresting waves, particularly if the waves hit the vessel at angles rather than head-on.
Under such conditions, the rolling increases, and pressure from the motion can exceed the breaking limits of the lashings that hold the containers in place, causing them to snap.
The containers, which can weigh several tons, dislodge from their stacks, breaking the balance of weight across the deck of the ship. This can intensify the pitching and rolling effect, throwing containers overboard and potentially damaging the ship.
To stop the pitching and rolling, the ship must slow down and change course.
Most of the containers that fall overboard sink, while some are pushed by currents and can wash up at shores miles away.
See the original article at https://www.wsj.com
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