Tuesday, March 16, 2010

Lessons Learned—A Ship Capsizes While Loading Cargo—Part I

Excerpt from U.S. Coast Guard “Proceedings of the Marine Safety & Security Council” magazine by Captain Brendan Saburn, U.S. Coast Guard Office of Investigations and Casualty Analysis.

Lessons learned from USCG marine casualty reports are regularly featured in Proceedings of the Marine Safety & Security Council magazine. These articles explore marine incidents and the causal factors, outline the subsequent U.S. Coast Guard marine casualty investigations, and describe the lessons learned as a result.

It is important to note that lives were lost in some of the marine casualties we present. Out of respect for the deceased, their families, and surviving crewmembers, we do not mention the name of any person involved.

On December 9, 2003, the 289-foot heavy-lift freighter Stellamare capsized and sank in Albany, N.Y., while loading a 308-metric-ton generator. This marine casualty, which resulted in the deaths of three of the ship’s crew, was caused by improper ballasting and the speed of cargo handling.

The operation of a small heavy-lift ship is highly specialized. This is a ship less than 300 feet long, whose deadweight tonnage is only 2,760 metric tons, with cargo gear capable of lifting a total of 360 tons. This ship was fitted with two heavy-lift derricks, each of which was rated for a safe working load of 180 metric tons, so a combined load of 360 tons was permissible.

Two generators were to be loaded aboard this ship that day. One weighed 308 metric tons, the other 234 metric tons. During operations, the smaller generator was loaded as planned. It was loaded first, onto what had been an empty ship, to make the ship more stable in preparation for the heavier generator.

Stability: An Illustration
It’s important to understand what is meant by “stability.” There are three states of stability: stable, neutral, and unstable. Think of a child in a rocking chair. The chair rocking forward and backward is analogous to a ship rolling to starboard and port. If the child is sitting, the chair is very stable and can be rocked back and forth, but won’t topple over forward or backward because the center of gravity of the child and chair combination is down low.

But what if the child decides to kneel? Then the chair will be in “neutral” equilibrium. If the child leans forward, there is neither a tendency for the chair to return to the upright position nor a tendency for the chair to topple. The chair will stay where the child positions himself while kneeling.

If the child stands up in the chair, the center of gravity of the child and chair combination is now too high—unstable equilibrium. If the standing child moves too far forward or backwards, the chair can topple.

Let’s continue our example. How can we make the chair more stable so the child can’t upset it by standing? One way would be to fasten bricks to the underside of the chair, which would add weight below the desired center of gravity. In other words, we could “ballast” the rocking chair. We could also lengthen the chair’s rockers, which would be analogous to making a ship wider.

In Part II, we will examine how these principles were used in the failed loading operation.

For more information:
Full article is available at www.uscg.mil/proceedings. Click on “archives” and "2008 Volume 65, Number 2" (Summer 2008).

Subscribe online at http://www.uscg.mil/proceedings/subscribe.asp.

Direct requests for print copies of this edition to: HQS-DG-NMCProceedings@uscg.mil.