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.
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.
Ships motion
Ship resistance –
The motion of a ship through water requires energy to overcome resistance,
i.e. the force working against movement. As the resistance of a full-scale ship cannot be
measured directly the knowledge about the resistance of ships comes from model tests.
The total resistance on calm water can be divided into three main components: frictional
resistance, residual resistance and air resistance.
The frictional resistance depends on the size of the wetted area. It represents often about
70-90% of the ship total resistance for low-speed ships (bulk carriers and tankers), and
sometimes less than 40% for high-speed ships (containers and passenger ships).
Residual resistance comprises wave resistance that refers to the energy loss caused
by waves created by the vessel and viscous pressure resistance. This residual resistance
normally represents 10-25% of the total resistance for low-speed ships and up to 40-60%
for high-speed ships. Air resistance normally represents about 2% of the total resistance,
however, for loaded container ships in head wind, the air resistance can be as much as 10%.
During the operation of ship, the paint film on the hull breaks down. Erosion starts,
and marine plants and barnacles, etc. grow on the surface of the hull. In addition, the
propeller surface can become rough and foulded. The total resistance caused by fouling
may increase by 25-50% throughout the lifetime of a ship.
Resistance also increases because of sea, wind and current. The resistance when
navigating in head-sea could perhaps increase by as much as 50-100% of the total ship
resistance in calm weather.
Fig: CMA CGM Lamartine at sea passage
Parametric roll resonance –
A ship in longitudinal seas experiences a completely different
shape of the underwater volume as compared with the ship in calm water and in beam
seas. The reduction of righting arm GZ at wave crest causes a larger heel under the action
of wind and sea. The ship rights again, due to the increased righting arm GZ in the wave
trough, when the wave passes the ship. The ship in a seaway behaves dynamically, i.e. she
starts rolling, and passes the upright position when returning from the first large roll. If the
time of the large roll to the opposite side coincides with a wave crest passing the vessel,
then the ship ends up with another reduction of righting arm GZ, and consequently with
larger roll. Roll amplification due to the “timing” of the restoring moment variation with
the roll motion is called “parametric resonance”.
This resonance can cause the ship to roll to very large angles in a moderate sea, leading
to cargo damage, loss of containers and, in extreme cases, capsizing of the ship.
Large containerships are prone to parametric rolling because the shape of the fore body and
aft body are usually very different, leading to a variation of righting levers as wave crests and
trough move alongside the ship. A small initiating force at the right time from the rudder, wind
gusts and other influence can set the ship rolling to a large angle. Possible consequences on
machinery operation of the ship heeling to very large angles include: loss of cooling water, loss
of suction, exposure of lubricating oil sumps and, for resiliently-mounted engines, problems
with connection of services – and hence shutdown of the main engine.
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Shipping industry recognizes environmental protection as one of its highest priorities and that every effort should be made to conserve and protect the environment from marine, atmospheric and other forms of pollution.
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