SCHRIFTENREIHE SCHIFFBAU Festschrift anlässlich des 100 ...

IMO Res. A.749, the code on intact stability. The two other loadcases have 0.5 meters and
1.0 meters larger GM-values, respectively.
For the validation and the determination of a threshold value, the new index has been tested
with a number of real capsizing accidents, which were re-investigated from the original data
available from the accident investigation. The loading condition the ship had at the time of the
accident, which always can be clearly identified as “not safe”, was analysed with a set of
intact stability criteria, including the new index. Finally an attempt was made to identify he
stability increase necessary to omit the individual accidents. An example is presented in the
third part of this document.
Finally a simplified approach, following the same concept as the simulation based approach,
but omitting the necessity of performing numerical simulations is derived from the results
obtained from the simulations.
Assessing Ship Safety against Capsizing by Numerical Simulations
Evaluation of Ship Motions by Numerical Simulations
At the end of the last decade, after some incidents with container vessels have become
known that were related to parametric rolling, a German research group was established to
develop dynamic stability criteria, which should be based on numerical simulations. The
simulation code ROLLS, originally developed by Kroeger (1987) and Petey (1988), was
chosen to serve as basis for the evaluation of seakeeping related problems. The code was
validated and further enhanced by Cramer and Krueger (2005) and subsequently integrated
into the ship design system E4, why this enhanced version of the original code is known as
E4-ROLLS today. Research programs, funded by the German Ministry for Education and
Research (BMBF), were established. Within this framework, a large number of model tests
for different modern hull forms were carried out in tailored wave sequences to validate the
simulation code. It was concluded that the ROLLS-approach was able to predict the most
relevant phenomena related to the problem of insufficient stability in waves with sufficient
accuracy. Based on these findings it was decided to develop a concept for minimum stability,
based exclusively on numerical motion simulations. Summing up the most important results
from the research work of the past years following conclusions can be drawn:
• Both model tests and simulations confirmed that critical situations endangering the
ship with respect to large roll amplitudes are observed in head as well as following
seas.
• No capsizing events were found in beam seas at zero speed.
• The most dangerous scenarios appeared to be those where the ship was traveling in
following seas.
• In head seas, large rolling angles were observed, but capsizing usually did not occur.
This is due to the fact that critical resonances are connected to relatively low values
of GM in following seas, and to high GM values in head seas. The model tests were
conducted close to potentially critical resonances.
Other than expected by previous authors, wavelengths significantly shorter than the ship
length also could endanger the vessel whereas wavelengths significantly larger than ship
length did not initiate large roll amplitudes.
Evaluation Strategy – The Insufficient Stability Event Index (ISEI)
In contradiction to previous criteria, it was decided to determine all possible scenarios that
may lead to a dangerous situation, but not to quantify how dangerous a specific situation
actually is. When defining limiting stability values, it is of importance to assess the probability
of a specific loading condition being dangerous for the vessel, or not. For this application it is
not of practical interest to get the exact capsizing rate during the simulation but it is important
to know only if the ship did fail. For this the concept requires a methodology to distinguish
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