Friday, December 27, 2019

Parametric rolling

Parametric rolling is an unstable phenomenon, which can quickly generate large roll angles that are coupled with significant pitch motions. The rolling occurs in phase with pitch, and on containerships introduces high loads into the containers and their securing systems

Causes of Parametric Rolling
  • The size of container ships is increasing drastically as companies are looking forward to monster ships; for e.g. Maersk’s Triple-E Vessels. The new container ships coming to the market have large bow flare and wide beam to decrease the frictional resistance which is generated when the ship fore end passes through the water, making it streamlined with the hull.
  • As the wave crest travels along the hull, it results in flare immersion in the wave crest and the bow comes down. The stability (GM ) varies as a result of pitching and rolling of the ship. The combination of buoyancy and wave excitation forces push the ship to the other side.
  • The similar action takes place as the bow goes down in the next wave cycle resulting in synchronous motion which leads to heavy rolling up to 30 degree in a few cycles. This type of rolling is known as Parametric rolling.
  • This phenomenon occurs only when the sea condition is in head / stern or anywhere near to them. There are two pitch cycles- maximum and minimum. The period of roll is half the natural rolling period which coincides with large phase angle and maximum roll always occurs when the ship is pitching down i.e. bow is down.
Three conditions for parametric rolling :-
  • Sufficient wave height for bow immersion
  • Wave length at least the length or two times the length of ship
  • The ship should pitch atleast two times for every roll
Effects of Parametric rolling :-
  • Heavy stresses in ship structure especially in fore and aft parts
  • Extreme stresses on container and their securing system resulting in failure of the same and even loss of containers
  • Unpleasant for the crew of the ship
  • Variation in the load of ship’s propulsion engine

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