Thursday, March 18, 2010

Lessons Learned—A ship capsizes while loading cargo—Part II

Excerpt from U.S. Coast Guard “Proceedings of the Marine Safety & Security Council” magazine. To read Part I, please see our March 16 post.

Stability in Action
Any student of stability needs to consider the vertical location of the weight of a piece of cargo. The weight is either in the hold, a few feet above or below the ship’s center of gravity, or the weight is “acting from” a point (such as the head of the cargo boom) that can be 100 feet above where it would be when the piece is down in the cargo hold.

In the rocking chair example from Part I, if you hung a 15-lb. weight from the horizontal top piece of the chair, then moved that weight onto the seat of the chair, the center of gravity would have shifted during this operation.

A stability pontoon is a floating vessel that makes a 300-ton lift and load possible. For a ship this size, the pontoon is approximately 50 meters in length and floats alongside the outboard side of the ship. The pontoon increases the ship’s “effective breadth”; the wider a vessel, the more stable it is. This ship used a stability pontoon on the outboard side that day.

The “Typical” Lift Procedure
After connecting the hardware (slings and lifting beam) to the cargo falls, the first step in lifting a weight of this magnitude is to take a slight strain (50 tons, in this case) on the falls using the winches. The strain is judged by the dynamometer gauges and the list of the ship. A ship this small takes on a list of 2.5 degrees or so with two cranes over the wharf and a 50-ton strain on the falls.

Next, the ballast tanks on the opposite (outboard) side of the ship are filled with water. If the ship is moored starboard side to the wharf (as this ship was), the port side ballast tanks are filled with river water to help bring the load across from starboard to port. As they’re filling, tension on the falls increases. The goal is to have less wear and tear on the winches and a smoother, safer loading operation. The ship’s center of gravity rises as the falls gather more tension from the cargo’s weight.

Depending on the loading plan, ballast water is either pumped from the inboard side to the outboard side, or the tanks on the outboard side are filled from the water in which the ship is floating. The idea of ballasting, by itself, lowers the ship’s center of gravity. The ballast water in this case is referred to as “swing ballast” because its primary purpose is to help swing the load aboard.

The applied ballast also has a tendency to lower the center of gravity a foot or two, but that helps to balance out the suspended load, which has a tendency to raise the center of gravity a few feet.

This is a very slow operation for two reasons: it takes time to fill ballast tanks, and it takes time for cargo to be lifted. Both operations take place simultaneously. Although the ballast tanks on a heavy-lift ship can be filled at a rate of over 200 tons per hour, the speed is determined by the time it takes to slew the cranes inboard.

When everyone is in position and ready, the winch operator takes a strain. The captain simultaneously gives the order to begin filling ballast tanks on the outboard side before the list gets to a point that the pontoon comes out of position and loses its effectiveness. If it does, the ship could capsize to starboard onto the wharf.

Reaching the point of full tension on the falls, the booms will get slewed inboard. Everyone exercises caution to ensure the cargo load doesn’t get ahead of the swing ballast. From time to time the cargo movement stops and the brakes are set to allow the ballast to catch up to the load. The idea is to do as much as possible with swing ballast to ease the operation of the booms.

In part III, we will examine the ways this lift differed from this typical example.

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

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