Skip to main content

Parametric rolling

Parametric rolling is an unstable phenomenon, which can quickly generate large roll angles that are coupled with significant pitch motions. The rolling occurs in phase with pitch, and on containerships introduces high loads into the containers and their securing systems

Causes of Parametric Rolling
  • The size of container ships is increasing drastically as companies are looking forward to monster ships; for e.g. Maersk’s Triple-E Vessels. The new container ships coming to the market have large bow flare and wide beam to decrease the frictional resistance which is generated when the ship fore end passes through the water, making it streamlined with the hull.
  • As the wave crest travels along the hull, it results in flare immersion in the wave crest and the bow comes down. The stability (GM ) varies as a result of pitching and rolling of the ship. The combination of buoyancy and wave excitation forces push the ship to the other side.
  • The 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 degree in a few cycles. This type of rolling is known as Parametric rolling.
  • This phenomenon occurs only when the sea condition is in head / stern or anywhere near to them. There are two pitch cycles- maximum and minimum. The period of roll is half the natural rolling period which coincides with large phase angle and maximum roll always occurs when the ship is pitching down i.e. bow is down.
Three conditions for parametric rolling :-
  • Sufficient wave height for bow immersion
  • Wave length at least the length or two times the length of ship
  • The ship should pitch atleast two times for every roll
Effects of Parametric rolling :-
  • Heavy stresses in ship structure especially in fore and aft parts
  • Extreme stresses on 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 ship’s propulsion engine

Labels:

Comments

Post a Comment

If you have any doubts.Please let me know

Popular posts from this blog

Differences between MC/MC-C and ME/ME-C engines

The electrohydraulic control mechanisms of the ME engine replace the following components of the conventional MC engine: Chain drive for camshaft Camshaft with fuel cams, exhaust cams and indicator cams Fuel pump actuating gear, including roller guides and reversing mechanism Conventional fuel pressure booster and VIT system Exhaust valve actuating gear and roller guides Engine driven starting air distributor Electronic governor with actuator Regulating shaft Engine side control console Mechanical cylinder lubricators. The Engine Control System of the ME engine comprises: Control units Hydraulic power supply unit Hydraulic cylinder units, including: Electronically controlled fuel injection, and Electronically controlled exhaust valve activation Electronically controlled starting air valves Electronically controlled auxiliary blowers Integrated electronic governor functions Tacho system Electronically controlled Alpha lubricators
9 comments
Read more

Why is a man hole door elliptical in shape?

Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole  is lesser in size than the corresponding circular hole. More over it is prime concern to have a  smoothed generous radius at the corners to eliminate stress concentration. Hence other  geometrical shapes like rectangle and square are ruled out.  To compensate for the loss of material in the shell due to opening, a doubler ring has to be  provided around the opening. The thickness of the ring depends on the axis length along the  dirrection in which the stresses are maximum and the thickness of the shell. It is important to  align the minor axis along the length of the vessel, as the stress in this direction is  maximum. Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t=  thickness of the shell plating  Circumferential stress: Pd/4t  More over a considerable material and weight saving is achieved as minor is along the  direction of maximum stress.
21 comments
Read more

Shell Expansion Plan

It is a two dimensional drawing of a three dimensional surface of the ship’s hull form. This plan is very useful for the following information:It is used for marking the location of a hull Damage on this plan by identifying the strake number , letter and frame number so that the exact location of the damage and also suggested repairs are marked in a localised copy. The shell expansion can be used for finding areas of painting surfaces such as topside, boot topping and bottom areas by applying Simpsons rules directly.  In the shell expansion the vertical scale used is different from the horizontal scale and a suitable adjustment has to be made when calculating areas. This becomes useful in solving disputes concerning areas of preparation and painting. It gives information on the thickness of the original strake which is indicated by the number in the circle shown in the strake.  The quality of steel used is also shown by letters A,B,D E and AH, BH,DH, EH.
20 comments
Read more