Rinauro, Barbara (2020) Surf-riding/Broaching Assessment within Second Generation Intact Stability Criteria. [Tesi di dottorato]

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This thesis focuses on the study of surf-riding phenomenon, that is one of the failure modes dealt by the Second Generation Intact Stability Criteria (SGISC). The SGISC are based on the physics of “realistic failure modes” that analyse the nonlinear dynamic behaviour of ships in waves. Their development started in 2002 and they have been finalized in the 7th session of the International Maritime Organization (IMO) sub-committee on Ship Design and Construction (SDC) in 2020, in view of approval in 2021. Surf-riding occurs when a ship, sailing in quartering or following waves, is accelerated to wave celerity. In this condition ships are normally directionally unstable and an uncontrollable turn to beam position, known as broaching, and eventually capsizing can occur despite the maximum steering effort being applied. Because surf-riding usually precedes broaching, which has a more complex dynamics, the likelihood of surf-riding occurrence is studied to detect the vulnerability to broaching. After an overview on the surf-riding/broaching state of the art, the surf-riding phenomenon has been studied with the aims to analyse the effect of the different mathematical models, to investigate the effect of the hull forms, to search for possible “semi-empirical" improvements in one Degree Of Freedom (DOF) methodology and to provide Operational Measures for surf-riding criterion. The occurrence of the phenomenon has been studied by three numerical approaches: 1-DOF surge mathematical model, solved by nonlinear dynamics and bifurcation analysis; 6-DOF Potential Theory (PT) simulations, based on the combination of seakeeping and manoeuvring models; 3-DOF CFD simulations performed with Naval Hydro Pack of OpenFOAM software, that has been specifically developed to study ships dynamics in waves. The test case selected for the application is the semi-displacement Systematic Series D, whose models have characteristics of ships that are considered typically vulnerable to surf-riding, due to the narrow and relatively short hull form and fast service speed. Having found the series D vulnerable to surf-riding Level 2 IMO criterion, the study based on ship dynamics considerations has started by performing 1-DOF approach. In a first attempt, this approach considered calm water approximations for resistance and thrust formulations, and the wave force was simplified by considering regular waves modelled only by the Froude-Krylov component under calm water profile, neglecting the diffraction component. Surf-riding limits in terms of wave heights, for different ship speeds and wave periods, have been found with 1-DOF approach, with and without the diffraction effect, and have been compared to the occurrence of surf-riding phenomenon experienced by CFD and 6-DOF potential theory simulations. At service speed the D1 hull was found vulnerable to surf-riding phenomenon at wave lengths comparable to ship length and for relatively low wave heights. From the comparison of the results obtained by the different approaches, the introduction of the diffraction component and the contribution of the wave particle velocities on ship speed have been identified as possible improvement for the 1-DOF approach. It has been observed that the introduction of the diffraction component resulted in a greater and more significant improvement than the wave particle contribution. Since the series D has been found vulnerable to surf-riding Level 2 and surf-riding occurrence has been observed for low wave heights, Operational Measures, following IMO guidelines, have been discussed and provided for D1 ship, in view of an hypothetical route in the Mediterranean Sea. The Operational Limitations restricted the operations in the Mediterranean Sea for significant wave heights up to 3.8 m, and the Operational Guidance provided limited speeds and headings for specific sea states. A final discussion in view of a Direct Stability Assessment application has been outlined, underlining the limitations of the three numerical approaches, the time durations and their possible applicability.

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