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September
27, 2004
(Houston, TX---) Leading ship motion researchers at
ABS have developed new design criteria to address parametric
roll, particularly on large containerships. The criteria
are contained in the newly released ABS Guide for
the Assessment of Parametric Roll Resonance in the Design
of Container Carriers. The document provides design
and analysis criteria to determine if a particular vessel
is vulnerable to parametric roll and for determining
the magnitude of the roll motions. It is the first class
criteria addressing the subject that is firmly based
on ship motion analysis supported by extensive simulations.
Containerships are particularly susceptible to parametric
roll due to the fine form of the underwater hull geometry
combined with the full form above the waterline. The
high service speeds require the slim underwater body.
The need to accommodate the maximum container payload
above deck contributes to significant bow flare and
stern overhang.
According to ABS Engineer Dr. Vadim Belenky, a ship
motions specialist and former academic who has led the
multi-year research project, a finely balanced set of
circumstances must exist for the parametric roll to
occur. “The ship’s geometry must have certain
characteristics,” he explains. “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 relationship to both the wavelength
and the vessel’s natural rolling frequency to
have a resonance condition. When these factors coincide,
the changing stability as the ship passes through the
wave allows it to gain and accumulate an amount of energy
with every roll.”
The phenomenon is most likely to occur when a container
ship is sailing in nearly head or following seas.
Relying on hydrodynamics and fundamental physics theory
to examine the build-up of energy that takes place during
the rolling motion that occurs in such conditions, ABS
researchers conducted extensive numerical simulation
to verify the criteria contained in the new ABS guide.
This simulation takes into account both the rolling
motions as well as pitch and heave. |
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By applying the criteria in the Guide, designers
can determine if a ship may be vulnerable to parametric roll
in worst case scenarios. “If the design is vulnerable,”
explains Belenky “several options exist to address this
vulnerability.” These include conducting numerical simulation
studies and model testing to develop a series of diagrams
that will define the combinations of speed and course given
sea state and load conditions that might be dangerous. These
can be placed on board the vessel to provide guidance to the
Master and navigating officers.
One method for reducing the risk of parametric roll is to
install small anti-roll tanks to absorb the surplus energy
collected during the parametric roll motions. The advantage
of this approach is that it is a passive control system, not
requiring intervention by a person or a computer. Another
method is to install fin stabilizers that move to counteract
the roll motion of the ship. “By preventing the progressive
accumulation of energy, the risk of parametric roll is significantly
reduced,” Belenky stresses.
“The solution to the problem of parametric roll is
not simple. If anti-roll tanks are not fitted and a vessel
gets caught in this situation, the officer on the bridge must
recognize the symptoms very quickly and take action by adjusting
course or speed to minimize the risk of damage to the vessel
and its cargo. As a class society we are highlighting the
issue, offering information on how to deal with it and advising
designers on the known methods of eliminating the risk,”
says Belenky.
ABS offers an optional class notation for measures taken
against parametric roll.
Founded in 1862, ABS is a leading international classification
society devoted to promoting the security of life, property
and the marine environment through the development and verification
of standards for the design, construction and operational
maintenance of marine-related facilities.
Editor’s Note: Pictures
from ABS parametric roll numerical modeling and simulation
studies are available in the ABS Newsroom at: http://www.eagle/org/news/press/parametricrollimages.html.
Web-based Publications: A
short downloadable avi or mpg file is available illustrating
a sequence of a containership in a parametric roll state.
Technical Paper Available
Vadim Belenky, Ph.D., is a ship motions authority
and is a co-author of the paper “Criteria for Parametric
Roll of Large Containerships in Longitudinal Seas”
to be presented at the Society of Naval Architects and Marine
Engineers (SNAME) Maritime Technology Conference and Expo
in Washington, DC, September 30-October1. You may download
the paper with proper credit given to SNAME.
For more information, contact:
Susan V. Gonzalez, ABS
1-281-877-5853
sgonzalez@eagle.org
or
Gillian Smyth,
ABS Europe
44 20 7377 4425
gsmyth@eagle.org
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