Container Ships Motion in a parametric roll - Adverse affects of such phenomenon
The term parametric roll is used to describe the phenomenon of large unstable roll motion suddenly occurring in head or stern seas.
Due to its violent nature, the large accelerations associated with the onset of parametric roll cause concern for the safety of container ships. Possible consequences include loss of containers, machinery failure, structural damage, and even capsize.
Parametric roll is a “threshold” phenomenon. This means that a combination of environmental, operational and design parameters needs to occur before it is encountered. These are:
1) The ship would be traveling with a small heading angle to the predominant wave direction (head or stern seas).
2) Wavelength would be comparable to ship length.
3) Wave height would be large.
4) The ship’s roll damping characteristic would be low. If unfavorable tuning occurs between wave encounter period and natural or twice natural roll period of the vessel, then parametric roll motion can be experienced.
Large containerships are particularly vulnerable to parametric rolling.
This is because they have hull forms with pronounced bow flare, flat transom stern and wall-sided mid-ship section.
Such features contribute to the variation of the ship’s stability characteristics due to the constant change of the underwater hull geometry as waves travel past the ship.
Consequences of a Parametric Roll
Parametric roll is an extreme condition for container securing since it combines the effect of large roll and pitch amplitudes. This scenario imposes significant loads on container securing systems.
Such extreme roll angles exceed those usually adopted during machinery design. Possible consequences on machinery operation of the ship heeling to very large angles include loss of cooling water suction, exposure of lubricating oil sumps– and hence shut-down of the main engine.
The following points should be borne in mind:
a) Parametric roll is a relatively rare phenomenon occurring in head or following seas, which is characterized by rapidly developed, large, unstable ship rolling.
b) Risk control options exist in both design and operation of container ships that can effectively reduce the likelihood of a parametric roll occurring.
c) Reducing the likelihood of its occurrence is considered a more effective approach than mitigating the consequences.
d) Masters should be aware that, when conditions for parametric rolling exist, the action of putting the ship’s head to the sea and reducing speed could make rolling worse.
e) The North Pacific in winter is known to be an area where conditions for parametric rolling exist.
Related topics:
How to confirm stabilty condition ?
How to maintain watertight integrity?
How to navigate vessel safely in heavy seas ?
Guide to watch officer for ships navigation ?
Heavy weather countermeasures for prudent navigator
Meeting rough sea conditions by containerships
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Note:
Large containerships are particularly vulnerable to parametric rolling.The article here is about the term "parametric roll" & its consequences,