Parametric rolling - phenomenon.

• Parametric rolling is a phenomenon in which a ship experiences large-amplitude rolling motion due to the periodic variation of its stability (metacentric height, GM) as it moves through waves, even when there is no direct beam-sea excitation.
• It occurs when the ship is in following or head seas with a wave length comparable to the ship's length, and the ship's natural roll period is approximately twice the wave encounter period (i.e. the ship rolls once for every two wave encounters).
• As the ship moves through the wave, the waterplane area and hence the metacentric height (GM) change periodically. When the wave crest is amidships, the waterplane area is reduced (for a ship with a fine bow and stern), reducing GM; when the trough is amidships, the waterplane area is increased, increasing GM.
• This periodic variation of GM causes the restoring moment to vary, and when the encounter frequency is about twice the natural roll frequency, the roll motion is parametrically excited and grows rapidly.
• The roll amplitude can build up very quickly to large angles (often 30-40 degrees or more) even in moderate seas, because the damping is insufficient to limit the growth.
• Parametric rolling is particularly dangerous for container ships and other ships with large bow flare and fine ends, and for ships with low GM (high stability is not necessarily protective). It can cause cargo shifting, container loss, structural damage and endanger the crew.

Measures to prevent parametric rolling:

• Avoid the critical speed and heading: alter course and/or speed so that the wave encounter period is not approximately twice the natural roll period. This is the most effective operational measure.
• Change the natural roll period: adjust the GM (e.g. by ballasting or changing the loading condition) so that the natural roll period is not in the critical range relative to the wave encounter period.
• Reduce speed in following/quartering seas to change the encounter frequency.
• Use active or passive roll stabilisation systems: anti-rolling tanks, fin stabilisers, or bilge keels (bilge keels increase roll damping and reduce the amplitude).
• Increase roll damping: bilge keels are effective in limiting the build-up of parametric rolling.
• Route planning: use weather routing to avoid sea states and headings that are prone to parametric rolling.
• For container ships, careful stowage to maintain adequate GM and avoid excessive metacentric height that could make the ship more susceptible.
• Operational guidance: follow the ship-specific parametric rolling guidance and the IMO guidance on avoiding dangerous situations in following and quartering seas.
• Some ships are fitted with parametric rolling detection and warning systems that alert the crew to take corrective action.