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vulnerable for insufficient stability events in following or head seas. At this the ISEI-concept takes into account all relevant phenomena occurring in head and following seas that may endanger the vessel with respect to minimum stability. Validation of the Concept with Real Capsizing Accidents Motivation and Procedures Fig. 2: ISEI - Evaluation Concept During the validation phase the new stability criterion has to show that it is able to identify all un-safe situations as well as safe situations by delivering clearly different index values. To assure a uniform safety level for all ships the criterion additionally shall deliver similar index 33
values for ships with similar main properties in similar situations. Finally it has to be assured that the criterion is sufficiently conservative to deliver reliable decisions taking into account the uncertainties in the calculation. On the other hand the criterion must not be too conservative as it then reduces the usability of safe ships unnecessarily. The most realistic benchmark-scenarios are real accidents, why several of them have been investigated during the development of the new criterion, where the focus was laid on ships which did capsize in heavy weather without any further damage by collision or grounding. One very recent example for this work is the capsizing and subsequent sinking of the RoRovessel FINNBIRCH in the year 2006, which is presented below. In order to assess the above mentioned tasks the following procedure was applied: � Identification of the accident conditions (environmental data, loading condition) � Application of the stability criteria as described below on this situation, including the new ISEI � Estimation of a probably safe condition and application of the capsizing criteria to this second situation. The results of our investigations show that in almost cases the stability criteria give a common statement whether a ship can be considered as safe or un-safe in a certain situation. The new stability index always rated the accident situations as un-safe. The results from these investigations were used also to determine acceptable threshold values for the index, which will be derived later in this document. Overview on Selected Capsizing Criteria The criteria presented briefly in the next sub-sections aim to ensure sufficient safety of ships in heavy weather by identifying significant, stability related characteristics of the ships’ lever arm curves. They were used to calibrate and to validate the new criterion by applying them to situations were ships were lost by capsizing. Wendels’s concept of Balancing Righting and Heeling Levers: Wendel and his group developed a concept where the stability of ships should be evaluated on the basis of an individual balance of righting and heeling levers (Arndt (1960) ). The dynamic effects of capsizing as such are disregarded in this concept, but the stability reduction is taken into account by using the mean value of the crest and trough condition lever arms instead of the stillwater righting lever, which is questionable from today’s point of knowledge. The theoretical background of Wendel's concept is described in Arndt (1960) The German Navy’s stability standard BV1033 is based on this criterion. The C-Factor Concept for Container Vessels Larger than 100m in Length: With the introduction of container vessels the average beam-to-depth-ratio of the world merchant fleet grew significantly from ca. 1.60 in 1960 to ca. 1.9 in 1980. An increased beam-to-depth ratio leads to larger initial stability, whereas added form stability is significantly reduced. Therefore Blume and Wagner carried out a number of model tests for container vessels. Based on the results Blume (Blume and Hattendorf (1987a) ) tried to establish a criterion for the minimum stability of vessels in rough weather. The findings lead to the development of the C-factor concept, which enhances the original Rahola-criteria. This is done, for example, by replacing the static requirement for the area below the lever arm curve being larger or equal 0.2 m according to Rahola by the constant value divided by C, where C is calculated as follows (Blume (1987)): 34
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[5] HARDER: Harmonization of Rules
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[36] SKJONG, R.; VANEM, E.: Damage
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