QA Set 12: Parametric Roll

Q. What is the effect of resting of ship on wave crest?

Ans. The GZ values available on ship are provided for static sea conditions. When vessel rests on a large longitudinal wave with the crest amidships, the GZ values increase in size for all the angle of heel. The reverse happens when the trough is amidships.


Q. What happens when a wave in longitudinal direction has crest amidships?

Ans. With crust amidships, the ship is lifted up with loss of virtual displacement. The ship experiences a pinnacle grounding effect in a way, causing a virtual loss of GM. Also, the beam and the length of water plane will be less than what would be in still water. BMT = for a box vessel. Generally, in case of any vessel it may be said that BMT α breadth2. It can be also said that since BMT =  reduction of length will have the effect on the size of beam. Thus, the effect is overall shrinking of the GZ curve.

Q. What happens when a wave in longitudinal direction has trough amidships?

Ans. With trough amidships, the ship is sunk in the sea with increase of virtual displacement. The ship experiences a virtual rise of GM. The beam and the length of water plane will be more than what would be in still water. For the same reason stated earlier it can be said that GM would increase. Thus, the effect is overall amplification of the GZ curve.

Q. What is periodic variation of righting moment? What is result of this?

Ans. Liquid displaced and GZ are important stability “parameters”. In a situation where waves encounter the ship longitudinally, both these parameters fluctuate cyclically. This causes a periodic variation of righting moment. This can trigger off roll if it occurs with an appropriate period. Weight (virtually displaced) varies only within a limited range but the righting lever can undergo periodic variations to a large extent. This is the predominant cause of possible roll excitation. The excitation is originated due to cyclic fluctuation of stability parameters hence the name “parametric excitation” or “parametric roll”.

Q. What happens when a wave moving in the same direction as ship encounters the ship in following seas?

Ans.Periodical fluctuation of GZ and hence righting moment is likely in such conditions. In the forward trimming & stern trimming posture the sizes of righting levers are different owing to the different underwater forms causing cyclic fluctuation of GZ. In addition to this the crest & trough cyclically position amidships, again causing a cyclic fluctuation of stability parameters. If the ship’s beam is considerable, the basic GMT to start with, is likely to be high, making the ships stiff with a generally large righting lever. All above put together may cause the ship to violently roll.

Q. What are the conditions favourable for a parametric roll?

Ans.With the merchant ships, a periodic variation of the righting lever in case of longitudinally one of the following two things must happen:

  1. Periodic positioning of crest and trough amidships
  2. Periodic changes of trim during pitch.

The periodic variation of righting moment can trigger of the roll. In particular the following are the general conditions necessary:

  1. New generation container vessel or similar ship type.
  2. Waves traveling along the ship’s length or at a fine angle to the ship’s length, from forward or from aft.
  3. Wave length approximately equal to the ship’s length.
  4. Pitching period = ½ roll period.
  5. Wave encounter period = pitching period.

Q. What is the effect of wave encounter period being half of rolling period?

Ans. The effect of this is that, as the ship rolls from port to starboard (half of the roll), twice the crest passes a given point on the ship. This is because in one cycle of wave encounter the vessel experiences two crests and two troughs. With synchronism duly occurring, it may be said that at the time of extreme roll on starboard side, the trough is amidships and as the ship goes to extreme port, the trough is again amidships. This means if the ship has rolled to 5o starboard, the GMT is very high and therefore the righting moment is large. This would also happen as the ship is on extreme port roll. Repeatedly, this might occur, with vessel becoming very stiff at extremity of rolls and alternately tender in the middle of oscillation. The heavy momentum continuing in the phase where the righting capability is low, can suddenly excite the roll amplitude.  

Q. What action must be taken if a parametric rolling is feared?

Ans. An avoiding action, is very important without any delay. A reference to such situation is generally also given in ‘loading and stability manual’. It is important to alter the course and get the direction of wave propagation between 4 to 6 points on bow. Reduction of speed along with the alteration will help changing the encounter period. Sometimes, increase of speed rather than decreasing it for a high powered vessel may help.

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