Britannia Club: How to deal with large roll motions

Abraham Smith

The Britannia P&I Club published guidance for container ships on how to deal with parametric roll. In the guide, Britannia explains how ships can detect parametric roll and what triggers it.

uUnfavorable combinations of roll time, ship speed, heading and wave conditions can trigger sudden and extremely rapid increases in roll movements, which can lead to excessive stress on container securing devices and, in the worst case, to collapse of container stacks and container losses.

How to identify synchronous and parametric roll motions

  • Synchronous roll occurs across seas when the ship’s roll period coincides with the wave period. The ship heels with successively larger and larger roll angles.
  • Parametric roll occurs due to changes in ship stability when the ship moves in waves. It is most common in heavy main seas but can also occur in subsequent seas. It is a sudden phenomenon with large and rapidly increasing roll angles occurring over a short period of time.

Important trigger conditions to watch out for

  • Ship roll time (time it takes the ship to roll from port to starboard and then back to port) approximately equal to or twice the wave encounter time.
  • A low metacentric height (GM) results in a long roll period.
  • Near the following sea conditions or open sea.

The wave encounter time can be measured with a stopwatch as the time between two wave crests. It is close to the ship’s pitching period (time it takes the ship to pitch bow down, stern down, and back to bow down).

What to do if it happens

In order to reduce the risk of large roll movements, the IMO guideline recommends changing the ship’s course or adjusting the speed while avoiding abrupt steering. The guidance on the following pages shows, for a range of generic container ship sizes and typical loading conditions, the expected safe zone where hazardous situations are less likely to occur.

Masters should use this guide with particular reference to the specific characteristics of their ship and its behavior in severe weather. All deck officers should familiarize themselves with the nautical chart applicable to their ship size and loading condition at the beginning of a voyage so that they are familiar with the actions to be taken to reduce roll, as parametric roll in particular can quickly require immediate remedial action.

During the 2020-2021 winter season, a number of exceptional container loss incidents occurred. The TopTier project was set up by the industry to find ways to avoid similar incidents in the future, and early results show that parametric taxiing was particularly dangerous in the following seas.