AZIMUTH-DRIVE ESCORT TUG MANOEUVRABILITY MODEL, SIMULATION AND CONTROL

The ability to predict Escort tug's handling, effectiveness, and safety at early design stage is paramount in view of an optimal design process. In this framework, the availability of a reliable manoeuvrability prediction model is beneficial. A deep insight into the manoeuvring characterisation of a wider class of Azimuthal Stern Drive Escort tugs is undertaken, giving rise to a dedicated novel four Degrees-of-Freedom (4-DOF) parametric manoeuvrability model. An extensive captive model testing campaign is exploited to develop suitable mathematical models, conceived following an `MMG-inspired' modelling concept, i.e. a non-linear manoeuvring prediction method developed by the Japanese Manoeuvring Modelling Group (Ogawa et al., 1977) and later standardised by the Japan Society of Naval Architects and Marine Engineers JSNAOE (2013). The mathematical formulation pursues a physics-based approach aimed at characterising the complete manoeuvring hydrodynamics of a category of vessels, based onto a reference tug geometry. The hull+skeg and azimuthals force contributions are analysed separately and are then coupled to include their reciprocal interaction. Collaterally, computational fluid dynamics techniques (RANSE) are cross-validated and further explored to extend the parent hull modelling in function of a series of skeg geometries. The aim of the investigation is to physically characterize and quantify by suitable models the influence of the different extit{skeg} designs and sizes onto manoeuvring, with the scope of covering the largest class of Azimuthal Stern Drive Escort tugs. To prove the adequacy of the mathematical formulations, two independent validation processes have been pursued. The first -- model-scale -- reproduces the Escort-towing tests performed at towing tank basin onto the parent hull. The second -- full-scale -- is devised to check the simulator capability of describing the model free-sailing performance of a different but compatible hull, having dimensions, propulsion, skeg characteristics significantly different with respect to the `parent design' used to principally develop the code. In conclusion, a wider `Simulation-for-Design' strategy discloses, enriched by the combination of an original parametric architecture of the Azimuthal Stern Drive Escort tug, with a fully controllable and scalable tanker, and a tunable tow-line. Full real-time Escort-towing dynamics of convoy are envisaged, enabling the study and investigation of several real-case emergency scenarios and paving the way for future design strategies. The ability of addressing real-world operative profiles, in fact widens even more the advantages of a `parametric model', promising to become a very useful tool for tug designers, tug masters/pilots, port authorities, or flag administrations. Among them are the direct assessment of the impact of design choices on operational effectiveness and safety; towing-service risk assessment-to-mitigation techniques; real-scenario simulation with focus on technical failures, human factor and underlying delay chain; and, last bot not least, `model-based' benchmarking environment for control design techniques.