Page 20: of Maritime Reporter Magazine (November 2004)

Naval Architecture & Marine Engineering

Demystifying Parametric Roll

An ABS research project deflates the mystique surrounding parametric roll, a rare but dangerous phenomenon partic- ularly threatening to containers/lips.

In October 1998, a large, post-

Panamax containership limped into the port of Seattle, a victim of the largest ever loss of containers while on passage.

On its voyage from Kaohsiung, Taiwan, the ship had encountered a terrible storm that for some 12 hours lashed the vessel with high winds and boarding waves reaching as high as the foremast and the running lights on the bridge. The offi-

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Circle 210 on Reader Service Card 22 Maritime Reporter & Engineering News cers not only saw green water at bridge level, but also had to watch helplessly as the forward container stacks collapsed.

By the time those violent seas finally passed, the ship had lost a third of its 1,300 containers on deck; another third lay wrecked and scattered about, with smashed containers and contents hang- ing over both sides of the ship.

Despite the best efforts of the ship's well-trained, professional crew, the ves- sel suffered strangely severe motions, unusual even for such storm conditions that made control nearly impossible at times. Yaw angles reached 20 degrees port and starboard, and severe rolling of up to 40 degrees combined with extreme pitching left the master without effective control of the vessel. It was, in fact, in the grip of the phenomenon known as parametric roll resonance or, simply, parametric roll. Parametric roll is one of those destructive phenomena that depend on a set of coincidences so fine- ly balanced as to make the occurrence seem nearly impossible: the ship's geometry has to have certain characteristics; its length has to be compara- ble to the wave-