Ultimate strength of a ship: rule requirements and reliable assessment

The evaluation of the ultimate limit state of the hull girder structure represents a matter of increasing importance in ship design, mainly in the case of fast ships where a light structure is required. For this reason recommendations and criteria relevant to the ultimate longitudinal strength are nowadays included within the guidelines for direct calculations of the main Classification Societies, even for conventional vessels. The last revision of RINA Rules contains ultimate bending moment check as a compulsory requirement for ships equal to or greater than 150m in length. The rules also contain software that perform the calculations. The phenomenon of a ship collapse can be faced by using different procedures, based on different assumptions and leading to different degree of approximation. The simplest procedures consist on analytical formulations that evaluate the ultimate bending moment of the ship section starting from the assumption of a credible stress distribution at collapse. A widely used approach is represented by the so-called “component approach”. The method assumes that the hull girder section is composed by independent structural panels whose failure and post-failure behaviors are represented by analytical formulations. The results mainly depend on the structural modelling of the section and on the stress-strain relationship of the collapsed members. A more general procedure is represented by a global finite element approach. With this procedure every load type together with other significant aspects as initial defects, initial stress , influence of the combined effect of more than one load component, may be, in principle, taken into consideration. The aim of this paper is a comparison between five different evaluation procedures, in order to assess uncertainties in the ultimate bending moment evaluation. The five different procedures have been applied to seven, recently built, fast ferries with a length varying between 50 to 125 meters. The first procedure is the RINA software. The second procedure is based on a “component approach” implemented into MAESTRO code . The third and fourth procedures consist on analytical formulations. The fifth procedure is an incremental, non-linear, finite element approach. The comparison among the different formulations enables the evaluation of the model uncertainties associated to the ultimate bending moment evaluation. Uncertainties are always involved in all steps of structural analysis and also in evaluating strength. These uncertainties are due to imperfect knowledge of physical phenomena associated with collapse and idealizations and simplifications in analysis procedures. The proper consideration of all the uncertainties involved in the analysis of marine structures is an essential requirement for the assessment of structural safety.