SCHRIFTENREIHE SCHIFFBAU Festschrift anlässlich des 100 ...

Fig. 12: Results for the ISEIs compared to ISEIfollowing
Fig. 12 shows the results for all ships, sorted by loadcase. The red bullets indicate the first
loadcase equaling the intact stability limit. It becomes clear that both approaches, the
simulated and the simplified, consider most of these cases as clearly un-safe as they have
index values significantly above 1.e-3, which is considered to be the future threshold value of
the criterion (see below). The loadcase with a GM increased by 0.5 meters is shown in yellow
and the third loadcase in green. It becomes clear that the index values of both approaches
decrease significantly and in the same order of magnitude with increasing stability. The barplot
on the right hand side of Fig. 12 shows the distribution of the investigated cases over
the four sectors the chart on the left hand side is divided into. Sector 1 is situated on the top
left side, whereas Sector 4 is located on the bottom right. For all cases in the sectors 2 and 3,
both approaches deliver the same statement, whereas cases in sector 1 are considered to be
safe by the simulation while the simplified approach considers them as being un-safe. The
critical sector is number 4. In this case the simplified approach judges the situation to be
safe, while the simulation, which is considered to be more accurate, makes a contrary
statement. This affects about 7% of all investigated cases, which seems to be acceptable for
the simplified criterion, as it addresses only a subset of the phenomena potentially leading to
capsize.
Conclusions
In the recent years a large number of ships was investigated with respect to their dynamic
behavior in waves by means of numerical simulations in the time domain. Based on this
database a new intact stability concept was developed, called Insufficient Stability Event
Index (ISEI).
The new concept is based on long-term probabilities, taking into account the probability of
occurrence for seastate, course and ship-speed. The actual failure criterion for the ship in a
specific operating condition is implemented via a “safe”/”unsafe”-decision based on the
Blume-criterion and the maximum roll angle observed during the simulation.
The concept has been validated by applying it to a number of intact stability accidents. Here
it could be shown that the criterion as able to distinguish clearly between safe and un-safe
loading conditions.
Additionally a simplified criterion, omitting the need to perform numerical simulations, has
been developed on the basis of the findings made with the simulated approach. The
approach has been validated against the results from the simulated criterion and shows
reasonable agreement.
Based on the accidents investigated and based on theoretical considerations taking into
account the concept of formal safety assessment issued by IMO, threshold values were
determined, whereas 5.0E-2 was found to be the border between very un-safe and critical
situations. Ships with loading conditions which reach ISEI values below 1.0E-3 are
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