THE INTACT STABILITY RULES ARE CHANGING: IMPACT ON THE DESIGN OF LARGE CRUISE SHIPS

摘要

The application of intact stability Weather Criterion to ships with large values of both KG/T and B/T, like the modern large passenger cruise ships leads to more stringent requirements than the application of current subdivision and stability rules for damage ships. A critical analysis of Weather Criterion in its historical development and in the present version has thus been conducted. In this paper, the calculation method of the rolling amplitude is examined with the aim of identifying weak points needing further studies and to propose some interim modification of existing procedure. The mechanism of capsizing involved in Weather Criterion is that corresponding to the combined action of a steady wind, giving an equilibrium heeling angle θ_0, of the sea waves, giving a roll motion amplitude θ_1 (around θ_0) and finally of a gust of wind whose work has to be efficiently "absorbed" by the potential energy incorporated in the dynamic stability. There are several weak points in the formulation of this stability criterion, which on the other hand proved to be quite effective in improving the safety of navigation. They are connected with: - mathematical modelling of roll motion equation; - the evaluation of roll period which is based on a regression formula; - overestimate of the wave steepness (factor "s") at large roll periods due to an extension of the range of applicability beyond the original limits; - overestimate of the effect of waves through the effective wave slope coefficient "r"; - possible overestimate of the wind and wind gusts effects. Recently at IMO it was decided to start the exercise of revising the Intact Stability Code to update Weather Criterion with priority and to introduce a new philosophy based on performance. In this paper, the problems connected with the correct evaluation of the roll-back angle, the wave steepness and the effect of wind are discussed on an experimental basis. In the frame of a cooperation between Fincantieri and University of Trieste, a large campaign of tests of forced rolling in beam waves on small and large scale models having large values of KG/T and B/T has been undertaken. The experimental results are discussed on the basis of a Parameter Identification Technique. A proposal to correct the evaluation formula for the factor "r" is provided. The appropriate wind coefficients are obtained through a series of tests in wind tunnel and some provision is given about the appropriate wave steepness to be used at high values of roll period. These results will be useful in the frame of the revision of Weather Criterion undertaken at IMO following a submission from Italy. The impact on design is analysed with reference to typical cases.