What is parametric rolling..?? 

Parametric rollING is a mixture of pitching and rolling occurring simultaneously, resulting in a twisting, dipping motion – very uncomfortable to humans and very stressful for ships and deck cargo.

Parametric rolling mainly occurs under the following circumstances:

• The length of the wave as the ship moves forward is equal to the length of the ship.
• 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 unexpected large roll motions

When the wave encounter period is resonant with the natural roll frequency, a sudden and severe rolling motion can result at an angle of more than 40° rapidly developing even in a moderate sea, leading to cargo damage, loss of containers, and in extreme cases, capsizing of the ship.

As the wave crest travels along the hull, it results in flare immersion in the wave crest and the bow comes down, causing the stability of the ship to vary as a result of the pitching and rolling of the ship. The combination of buoyancy and wave excitation forces pushes the ship to the other side.

A similar action takes place as the bow goes down in the next wave cycle resulting in synchronous motion which leads to heavy rolling up to 30 degrees in a few cycles.

This type of rolling is known as Parametric rolling.

While parametric rolling is usually associated with containerships and car carriers that have hull forms with wide, flat sterns and pronounced bow flare, the phenomenon can also occur in other vessels in quarter seas.

Due to the shape of the hull of a container ship, which is less bulky than a tanker or bulk carrier ship of equivalent dimensions, the prismatic coefficient or block coefficient will have smaller values making it easy to roll in both directions midship.

This means roll damping is less in container ships because the whole shape is more streamlined which is also responsible for such a type of roll motion.

The shape of the hull of a container ship (narrow relative to length) causes buoyancy to shift due to the change of the underwater volume when the waves are coming from the quarter causing a virtual decrease of GM (Metacentric Height – when the metacenter (M) is found to be above the center of gravity (G)). If the GM is not sufficient and adequate, this can lead to parametric rolling problems.

This is highly unlikely on a bulk carrier because its hull is almost identical along its length (like box shape) and the GM is always high and not an issue at all in this case (especially when the vessel is in ballast condition, GM can be 7-8 meters).

Sea conditions for parametric rolling

Container ships are most vulnerable to parametric rolling in following sea conditions.

• Unfavorable combinations of the rolling period;
• Wave conditions;
• Ships’ speed;

Heading of the ship can also initiate sudden and extremely prompt increase of roll motions to hazardous levels, threatening the safety of the vessel, crew, and cargo and this scenario can take place even in relatively mild wave heights.

The build-up of parametric rolling requires a threshold wave height in addition to the fulfillment of the above condition of frequencies and the minimum wave height is determined in principle by two factors:

a) The degree of fluctuation of roll restoring due to wave passage
b) The ship’s roll damping which is speed dependent

Ship’s roll damping is a key design parameter for the avoidance of parametric rolling.

As we have seen, the size of container ships has been steadily increasing as companies are looking forward to mega ships which have a large bow flare and wide beam to decrease the frictional resistance generated when the ship’s fore-end passes through the water, making it streamlined with the hull.

Not even the most state-of-the-art computational programs available currently can claim to calculate the roll damping accurately for any given vessel including all roll damping devices.

Effects of Parametric Rolling

Parametric rolling has below effects on a ship, especially a container ship

• Heavy stresses in ship structure especially in fore and aft parts
• Extreme stresses on the container and their securing system resulting in failure of the same and even loss of containers
• Unpleasant for the crew of the ship
• Variation in the load of the ship’s propulsion engine
• If not tackled quickly, it can result in the capsizing of the ship

Possible countering actions in case of Parametric Rolling

Below are some possible actions that the ship can take in case parametric rolling is experienced

• If rolling and pitching occur simultaneously, avoid a head-on sea, and change the route
• Change of course/speed, fill up the DB tanks to increase GM
• Always maintain a correct GM so the ship is not too tender or too stiff
• The roll damping measures must be quickly used

A decent thumb rule to follow is :

1. Make course alterations to keep the swell on the beam or the stern.
2. The speed must be equivalent to a factor of 0.8 to the rolling cycle or 2.0. Anything in between is going to result in heavy parametric rolling and loss of containers.

Conclusion

Results of the investigation of what happened on the Maersk Essen, the information shared about parametric rolling and its impact, highlight the sober fact that this phenomenon of parametric rolling may not yet be very well known..

This, combined with DMAIB’s findings on the concerns surrounding the stacking of 10+ tiers of containers on deck which is quite prevalent with the new generation mega container ships, says that this may not be the last mega container ship to face such an issue..

The industry is also awaiting the report for ONE APUS, a 14,052 TEU containership that encountered gale-force winds and large swells around 1,600 nautical miles northwest of Hawaii, USA (similar to Maersk Essen) causing an estimated 1,816+ containers to be lost overboard or dislodged from its lashings..

As per the DMAIB, Maersk from its side has taken immediate preventive measures like

• Review of crew training material pertaining to parametric roll
• Fleet-wide knowledge sharing to vessel crew re-emphasizing the importance of route advises
• Procedure reviewed pertaining
• Navigation in adverse weather
• Flowchart in vessel emergency response manual
• Reiterating guidance and onboard calculator basis MSC1228
• Revision of onboard heavy weather checklists
• Procedure revision in collaboration between WNI (Weather Route provider) and Maersk
  including a revision of severity thresholds for weather route advise

 

From its side, the WNI is reviewing communication risks relating to

• the specificity of warnings and understanding under what context those warnings may be assessed by the recipient
• whether the warnings are clear and understandable to the recipient
• whether they target the correct risks; and
• whether the recipient understands what actions may be required to mitigate those risks.