Rosano, Gennaro (2023) Excessive acceleration failure mode within the Second Generation Intact Stability Criteria. [Tesi di dottorato]

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Abstract

The development of the performance-based Second Generation Intact Stability Criteria (SGISC) was on the top of the IMO's agenda for almost 20 years, involving experts from research institutions, industry, classification societies and ship operators. Accidents occurred in the last decades showed that some ships experienced dynamical stability failures in waves, leading to important economic losses or even fatal injuries for people on-board. The accidents clearly demonstrated that the current intact stability regulations cannot always guarantee ship safety against the phenomena covered by the SGISC. The SGISC assess five stability failure in waves: Parametric roll, Pure loss of stability, Surf-riding/broaching, Dead ship condition and Excessive acceleration. The main novelties of the criteria are the physics-based foundation and the multi-layered approach, consisting of vulnerability criteria (Level 1 and Level 2), Direct Stability Assessment (Level 3) and Operational Measures (Operational Limitations and Operational Guidance). The criteria, finalized in February 2020, are currently at their trial stage, and feedback is expected from stakeholders based on the gained experience. The Thesis focuses on the Excessive acceleration, the less studied and the latest introduced. A short description, a state of the art and a review of the main accidents that led to its introduction in the regulatory context are provided. An overview of the process of development of the SGISC and the structure of the criteria is given with particular focus on the Excessive acceleration. The validation of a numerical code for the verification of the vulnerability criteria is shown. In the current version of the criterion, the external excitation is modelled by the Froude-Krylov component only which is calculated by a simplified formulation specific for the beam seas case. The formulation is generalized to any wave heading to provide simple but sufficiently accurate formulas for the estimation of the wave excitation, to be implemented in the rules for the development of ship-specific Operational Guidance. The adoption of such a formulation would offer the advantage of univocal and uniform application of the rules, with no need of numerical software validation by the Administration. A Decision Support System designed to monitor the actual condition of the vessel and to predict the lateral acceleration that could be experienced in the short-term is proposed. The system supports the crew in real operational conditions and integrate Operational Measures developed at the design stage. Two case studies are presented, referring to a bulk carrier and a fishing vessel, respectively. The bulk carrier, engaged on long routes from Europe to Africa, is found vulnerable to Level 1 and Level 2. Operational Limitations are developed for the bulk carrier, with particular attention to the definition of the equivalent scatter table that should be used in the verification when the ship sails on long routes. Operational Guidance are also applied, based on the generalization to any wave heading and ship speed of Level 2, in order to identify safe and unsafe sailing conditions in relevant sea states. The adopted procedure implements the proposed formulation of the Froude-Krylov exciting roll moment and shows results that are comparable to the ones obtained calculating the excitation with a 3D potential theory software. A 34-5 meters long fishing vessel, typical of the Spanish fleet, already studied in the literature, has been object of an experimental campaign conducted at the University of A Coruña (Spain) towing tank. Results from roll decay tests are used for the assessment of Level 2, which is not satisfied. Operational Limitations are derived for the vessel referring to the specific operational area in which she operates and to the maximum significant wave height. A Direct Stability Assessment is conducted confirming the vulnerability of the vessel. By noticing that dangerous situations could by experienced also for wave heights lower than the identified maximum one, the monitoring system is applied. For its application simulated and experimental data in irregular beam waves are used. Final remarks and issues to be addressed in future studies are summarized at the end of the Thesis.

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