Tanker Notes ||
Container Ship Operations ||
Ocean Navigation ||
Containership operation: ships motion in a seaway
Ships are affected by movement in six degrees of freedom: rolling,
pitching, heaving, swaying, surging and yawing. Of these, rolling,
pitching and heaving generate the greatest forces in heavy weather.
This may also indicate that the aft sections of larger container
vessels are subject to abnormal dynamic load conditions generated
by slamming. This may cause containers to “jump” out of their
automatic locks. Container locks are type and batch-approved by
the Classification Societies.
Major acceleration occurs fore and aft at high levels. The transverse
acceleration increases by increased metacentric heights GM.
Correct stowing of containers keeps the stability of the ship within
appropriate limits; not too low but not too high either. In container
ships with wide beams or in partly loaded ships, the GM may be
large, perhaps even exceeding 4-5 m, which will lead to severe rolling
in heavy seas and bad weather.
Rolling affects container corner posts, twistlocks, hatch covers and the deck by inducing compression and tension forces in these areas
The motion also creates transverse racking forces which, if excessive, may distort the walls and ends of container frames . Deck cargo racking forces are resisted primarily by lashing rods and turnbuckles .
A tipping moment may also occur which, in extreme conditions, could cause the stack to topple over .
Although the effects of rolling are resisted by the vessel’s securing arrangements, the system itself is designed to operate within specific parameters.
Classification society limits typically allow for a maximum roll amplitude of between 22º and 30º.
The forces created by pitching are similar to those caused by rolling, but act on the sides of the container longitudinally rather than transversely. A longitudinal racking force is generally less than its transverse equivalent. However, towards the bow and the stern the compressive forces due to pitching can be high.
This force is induced by pitching, and varies according to the motion of the ship’s deck. Heaving increases the compression and tension forces acting on container corner posts and twistlocks.
In adverse weather the outboard container stacks, and any others which are partially exposed, may be subjected to wind pressure.
The degree of force depends on the velocity and direction of the wind, and the profile of the stacks affected. The higher the stacks, the greater the surface area and, consequently, the amount of force generated.
In extreme conditions the wind, acting on the surface area of a single 40 foot container, may produce a transverse force of approximately 3.6 tonnes.
Given that the effect is cumulative, the transverse force induced by a five tier stack of containers could be as high as 18 tonnes.
All forces caused by the wind are in addition to those produced by the motion of the ship.
Ship Encountering Parametric Roll In A Seaway
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Collecting Information and Data for Passage Planning
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Shipsbusiness.com is merely an informational site about various aspects of ships operation,maintenance procedure, prevention of pollution and many safety guideline. The procedures explained here are only indicative, not exhaustive in nature and one must always be guided by practices of good seamanship. User feedback is important to update our database.For any comments or suggestions please Contact us
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