The International Group is a member of the Maritime Research Institute of the Netherlands (MARIN) Top Tier Project, and has received an annexed roll risk estimator.
The Top Tier Project has been established to examine and assess the causes of container losses arising from at-sea incidents onboard various sizes of container ships with a particular focus on ultra large ships.
The project has been approved by a cross-section of industry, academic and government interests and MARIN will produce a report of its findings including recommendations in due course. It is envisaged that the recommendations will eventually lead to operational and technical improvements that should mitigate the risk and underlying causes of container losses at sea.
IG Clubs have now received the annexed roll risk estimator. It indicates combinations of speed, heading and wave period that result in unfavourable tuning of roll motions that should be avoided.
It does not give indication of the maximum roll motion, and at which wave height the vessel is vulnerable to adverse rolling must be judged by the master’s experience. In addition, it does not address other dangerous ship behaviour e.g. large vertical accelerations, shipping green water and slamming.
What is parametric rolling?
Parametric rolling can occur when:
- The rolling period is twice the wave encounter period
- Wave lengths are in the range of the vessel length
In these conditions the passing waves cause a variation in waterplane area that can trigger vessel instability in roll. This is most common in heavy head seas, but can occur also in following seas, when the rolling period is long. Even a few high waves after each other may trigger unexpected large roll motions, as shown by the measured time traces of roll and pitch motions in the figure below. In the example, the ship is 240m long with a natural roll period of 32s and is sailing in a 5m following sea.
When to be alert?
Ships at low GM are vulnerable to parametric rolling in following seas, especially when there are waves with a long length from the stern quarter. Long term routing and short term vessel handling should consider the risk of parametric rolling in following seas when:.
- Vessel rolling period is long because of low GM (rolling periods in excess of 20s for ships with length above 250m). The rolling period should be measured after departure, as rules of thumb based on GM are not always accurate.
- Following sea conditions (or close to) are expected or experienced.
- The rolling period is twice the wave encounter period. The wave encounter period is equal to the pitching period and can be measured with a stopwatch.
- Wave lengths are longer than two-thirds of the ship length.
What to do when it happens
Break the synchronization between the roll period and the encounter period. The most direct way to do this is to change heading to beam or bow quartering seas. Avoid abrupt steering. The heading change can be combined with a speed increase but only if it does not increase the risk of other hazards. Changing course may seem counterintuitive but is the only way to reduce the risk of parametric rolling in following seas.
