SCHRIFTENREIHE SCHIFFBAU Festschrift anlässlich des 100 ...

is another problem related to lacking standardisation of seakeeping simulations. Currently,
there is neither an international standard defining minimum requirements for numerical
seakeeping codes employed for the assessment of dynamic ship stability, nor any
standardised procedure for the set up of environmental conditions to be used for the
simulations. For the practical application of a simulation-based criterion this would mean that
first a basic standard for numerical seakeeping simulations had to be established.
Taking all this into account, a regulation directly and solely depending on numerical seakeep-
ing simulations today will hardly be accepted as a standard procedure for the assessment of
the intact stability of ships. This results in the need for introducing an alternative approach
additionally to the simulation based ISEI-approach. The simplified approach shall be able to
address the same phenomena as the direct one with comparable reliability and with
consistent results, employing the experiences and findings made during the simulations.
A simplified, deterministic criterion always addresses only a clearly specified and limited set
of phenomena or failure mechanisms. Thus, its applicability must be always limited to a
certain set of operating conditions in which the ship is endangered by exactly those
phenomena covered by the criterion. Failure mechanisms in head and following seas are
quite different. Although they are related to the dynamic change of righting levers in waves,
the failure scenarios are quite different. In head seas the ship can be excited in a certain
range of encounter periods. This results in large roll amplitudes, often leading to severe
damage to the ship and the cargo on board. As the encounter frequency in head seas is
larger than in following seas, the resonance conditions are usually met at larger GM values
than in following seas. Additionally the time, the ship faces low stability while sitting on the
wave crest position is much shorter than in following seas. Thus, simplified criteria for head-
and following sea situations must assess different failure scenarios. Our simplified insufficient
stability event index (ISEIs) targets following and stern quartering sea cases only, as these
scenarios more often lead to the total loss of a ship as head sea incidents and thus have the
highest priority to be covered by a suitable criterion. This is supported clearly by the charts in
Fig. 3, which contain statistical data on intact stability accidents sorted by wave encounter
direction.
Fig 8: Statistical distribution of capsizing accidents after encounter angle of waves
One important requirement for the simplified criterion is that it has to be consistent with the
simulated approach. This means that the simulated values for the ISEIfollowing shall be directly
comparable with those obtained by the ISEIs approach. For this reason the simplified
approach is based on the same formulas as the simulated one. The only difference can be
found in the way
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δP risk is calculated. Again we use the failure coefficient C instead of the
p fail . As for the simulated approach this failure coefficient takes the value 1 for all wave
heights exceeding a certain limiting wave height. Here the significant limiting wave height H1/3
is replaced by a regular wave of the same height, denoted as "equivalent significant wave
height" (H).