MARIN – Parametric Roll of Ships #SWI2009
‘Parametric roll’ is a phenomenon that sometimes suddenly occurs to ships sailing in the head or following seas. Apparently, without any clear reason, the amplitude of the roll motion (i.e. the rotation about the longitudinal axis of the ship) increases within a couple of roll periods, from a few degrees to above 45 degrees. These large roll angles can have disastrous consequences: In recent years, a number of accidents have been reported. In one case, one-third of the containers went overboard and another third was damaged. This is not only a risk for the crew and the cargo but also for the environment and other vessels that come across these containers.
State of the art
Based on experimental and numerical research, a number of ‘conditions’ were identified for which parametric roll is likely to occur:
- The encounter period of the waves is about twice the natural roll period of the ship
- The length of the wave is about the length of the ship
- The height of the wave exceeds a certain threshold value
- The roll damping of the ship is relatively small
Despite these – apparently strict – conditions, it is still very difficult to estimate the risk of occurrence of parametric roll with certain accuracy.
MARIN hopes that SWI2009 can help to improve the physical and mathematical understanding of this problem. Recommendations for future theoretical/numerical research could be important to obtain a reliable and efficient prediction tool for assessing the safety of ships.