Rolling and pitching is a part of the every ship that is going out at the sea. The first thing you might think up on hearing the word “Parametric rolling” is that it must be a type of rolling movement occurring in ships. Rolling and Pitching is a normal movement phenomenon which occurs in all kind of the ships, so what is new about this.
The difference is that Parametric Rolling is a type of movement that is experienced only on the Container Ships.
Causes of Parametric Rolling
The size of container ships is increasing drastically as companies are looking forward to monster ships. 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 the pitching & rolling of the ship. The combination of buoyancy & wave excitation forces push the ship to the other side.
The same action happens when 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 called as Parametric rolling.
This phenomenon happens 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 the roll is half of the natural rolling period which coincides with large phase angle & maximum roll always occurs when the ship is pitching down i.e. bow is down.
Effects of Parametric Roll:
- Heavy stresses in the ship structure especially in fore & 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
- If not tackled quickly, it can result in capsizing of ship
Actions in case of Parametric Rolling on ships:
- Do not panic in such situation. Keep calm
- If rolling & pitching occurs simultaneously, avoid a head on sea & change the route.
- Always maintain a correct GM. Ship should not be too much tender or too much stiff.
- The roll damping measures must be quickly used