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Large containerships are

particularly susceptible to

the physical phenomenon

of parametric rolling due

to their hull design featuring

a wide, flat stern, pronounced

bow flare, fine under-water body

and relatively light loaded displacement.

Parametric rolling can lead to loss of or damage

to cargo containers and possible damage to the ship.

Relying on fundamental physics theory to simulate the build up of energy that takes

place during a rolling motion, ABS researchers conducted numerical modeling and

sequence simulations to illustrate the gravity force effects on ships as they roll, pitch

and heave in a seaway. A ship is particularly susceptible to parametric rolling when

encountering either head or following seas.

TECHNICAL DISCUSSION

To transport goods efficiently, modern containerships are being designed for high service

speeds necessitating a fine underwater body and relatively low block coefficient. To maxi-

mize carrying capacity on such a fine body, the deck is extended as far forward and aft as

possible, resulting in a somewhat exaggerated bow flare and pronounced stern overhang.

These characteristics are most prominent in large and ultra-large containerships, making

these vessels the most susceptible to parametric rolling.

Parametric rolling is not

a frequent phenomenon

because a finely balanced

set of circumstances mustexist for this physical

event to take place. The

ship’s geometry must

have certain characteris-

tics. The ship’s length

must be comparable to

the wavelength of the

sea conditions through

which it is passing. The

ship’s speed must bear

a certain relationshipto both the wavelength

and the vessel’s natural

rolling frequency.

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As a consequence, instead of a balanced pendulum-like rolling momentum

occurring, the ship accumulates energy. As the vessel passes through the

waves, it encounters a series of wave peaks and troughs. If the ship

length is close to the wavelength, it will rapidly change

from hogging to sagging configurations. Because

of the fine body, pronounced flare and stern

overhang, the ship effectively changes its beam

from slim when hogged with the midships

supported, to wide when the midships isin a trough but the bow and stern are

supported by wave peaks.

Since stability varies with beam, as

the vessel drives through the series

of wave fronts its stability changes

significantly as the midship moves from

crest (maximum) to trough (minimum).

 When this pattern occurs together with a wave encounter frequency that is close

to twice the ship’s natural roll frequency, the ship enters a condition of cyclically

recurring minimum stability.

ABS RESPONSE

Based on a multi-year study of parametric roll which relied on basic physics theory

and engineering-first principles, an ABS Guide for the Assessment of Parametric Roll

Resonance in the Design of Container Carriers was developed. The Guide provides

design and analysis criteria to determine the vulnerability of a particular vessel to

parametric roll and the potential magnitude of the roll motions. It is the first criteria

based on physics for parametric roll in ships by a class society.

The criteria allow designers to determine the extent to which a ship may be vulnerable

to parametric roll. If the design is vulnerable, the ABS Guide offers recommended actions

to mitigate the risk. These include numerical studies and model tests from which a

series of diagrams can be developed, to be placed on the vessel’s bridge, that identify

the combinations of speed and sea state for given load conditions that can be considered

dangerous. The navigating officer can then identify situations in which speed or course

corrections may reduce the likely incidence of parametric roll. Another recommended

approach is the installation of a set of small anti-roll tanks to absorb the surplus energy

collected during parametric roll.

ABS offers an optional class notation for measures taken against parametric roll.

The solution to the problem of parametric roll is not simple. The technical guidance

offered by ABS highlights the issue so operational staff are aware of the various actionsthat can be taken if a ship encounters this phenomenon.

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