During the last two winter seasons, a string of incidents with exceptional container losses occurred. In almost all these incidents, one of the common reasons was parametric rolling in the following seas.
So, in this article, let’s discuss how container vessels’ crews can plan, recognize and act to prevent parametric rolling in following seas.
What is Parametric Rolling?
Parametric rolling mainly occurs under the following circumstances:
- Wave lengths are in the range of the vessel length.
- The rolling period is twice the wave encounter period.
When the above conditions are met, the passing waves cause a variation in the waterplane area that can trigger vessel instability during the vessel’s rolling; this is most common in heavy head seas but can also occur in the following seas when the rolling period is long. Even a few high waves after each other may trigger an unexpected large roll motions.
Parametric Rolling Hazards
Container ships are most vulnerable to parametric rolling in following sea conditions. Unfavourable combinations of the rolling period, wave conditions, ships’ speed, and heading can initiate sudden and extremely prompt increase of roll motions to hazardous levels, threatening the safety of vessel, crew, and cargo. This scenario can take place even in relatively mild wave heights.
When to be Cautious?
Ships with a low GM are most vulnerable to parametric rolling in the following seas, mainly when long length waves are coming from the stern quarter.
There is a high risk of parametric rolling in the following seas when:
- The vessel’s rolling period is prolonged because of low GM (rolling periods over 20 seconds for ships with a length above 250m).
- The rolling period is twice the wave encounter period. The wave encounter period equals the pitching period; it can be measured with a stopwatch (Practically, the wave period can be observed as the method of measuring the wave encounter period is not very usable.)
- Wave lengths are longer than two-thirds of the ship’s length.
- The following sea conditions are expected or experienced.
How to identify the early signs ?
A vessel can slip into parametric rolling very suddenly and unexpectedly. Therefore, the crew should be aware of the conditions and danger signs at an early stage to prevent it.
The most indicative behaviour is the synchronization between the gentle roll and pitch motions as waves pass underneath, especially when the vessel starts rolling alternatingly from port to starboard in perfect sync with successive pitching cycles. This indicates that wave encounter periods are close to half of the rolling period (or the rolling period is twice the wave encounter period). In this condition, if waves are high enough, parametric rolling can happen at any time.
Action to take when Parametric Rolling Occurs
As a Navigator, the most important thing to do in such a case is breaking the synchronization between the rolling and wave encounter periods.
The most effective way to break the synchronization is by altering the ship’s heading to beam or bow quartering seas. However, the alteration of the course should not be done abruptly.
A speed increase can accompany the course alteration, but only if it does not increase the risk of other hazards.
Course alteration may seem counterintuitive, but it is the only way to reduce the risk of parametric rolling in the following seas.