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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

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.

Pitching

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.

Heaving

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.

Wind force

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.


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