Parametric rolling ?
Parametric Roll - vessel motion
The head sea parametric roll is a recently identified phenomenon and seems especially likely to affect large container ships. Its occurrence depends on the wave conditions in relation to the dimensions of the ship, and leads to especially pronounced variations in stability as the ship sails through critical head or following seas. In addition to buoyancy changes, as the wave moves along the vessel it is alternately pushed from side to side, and quickly – in only five cycles – large rolling angles of more than 35° can be excited. This generates extremely high loads in the lashings and containers.
In the case of parametric rolling, even the breaking load of lashings and containers could be exceeded. As a result the cargo and vessel could be damaged and/or containers could be lost. For this phenomenon special conditions for critical vessel types have to be considered.
Critical vessels
Parametric rolling occurs in large vessels characterised by the following:
• large flare in the fore and aftship,
• flat aftership,
• slim fore and aft body,
• righting arm varies significant with draft
• Critical conditions
In general parametric rolling occurs only under the following critical conditions:
• head/stern sea conditions
• when the natural period of roll is nearly twice the wave encounter period, resulting in two pitch cycles per roll cycle
• wavelength in relation to ship’s length
• waveheight exceeds critical values
• the roll damping is low
When the period of encounter of wave is half the roll period of vessel and when waves are experienced from fwd or stern it is called parametric rolling
Parametric rolling is produced by pitching motions on vessels which have very fine bowlines together with very wide and full stern contours. One such ship type is the container ship.
The cause depends very much on the parameters of the vessel, hence the name parametric rolling. It is most marked when the pitching period TP is either equal to, or half that of the vessels rolling period TR.
As the stern dips into the waves it produces a rolling action. This remains unchecked as the bow next dips into the waves due to pitching forces. It is worst when TP equal to TR or when TP equal to half of TR.
In effect, the rolling characteristics are different at the stern to those at the
bow. It causes a twisting or torsioning along the ship leading to extra
rolling motions.
If TP equal to TR or when TP equal to half of TR, then interaction exists and the rolling of the ship is increased. A more dangerous situation develops because of the interplay between the pitching and rolling motions.
Parametric rolling is worse when a ship is operating at reduced speed in heavy sea conditions. Such condition can cause containers to be lost overboard due to broken deck lashings.
The IMO suggest that parametric rolling is particularly dangerous when the wavelength is 1.0 to 1.5 times the ships length.
Parametric rolling problems are least on box-shaped vessels or full-form barges where the aft and forward contours are not too dissimilar. Very little transverse and longitudinal interplay occurs.
To reduce parametric rolling:
1. A water ballast could be used to alter the GMT and hence the natural rolling period TR, to a non-synchronous value.
2. The ship needs to have an anti-rolling acting stabilising system. Antirolling
stability tanks that transfer water across the ship or vertically between two tanks are effective for all ship speeds. A quick response time is vital to counteract this type of rolling.
3. Hydraulic fin stabilisers would also help to reduce parametric rolling. They maybe telescopic or hinged into the sides of the vessel at or near to
amidships.
4. Alter the ships forward speed.
5. Alter the ships course.
MMD / MCA ORALS MASTER EXAM
