Hydrodynamic interaction during tug-ship operations

The hydrodynamic interaction between two vessels operating in close proximity can affect their safety and handling, especially if the vessels are significantly different in size, for example when a tug is assisting a large ship. During such operations, the drift-angle of the tug and lateral distance between the vessels are frequently varied to ensure accurate course keeping and safety. This can result in unsteady hydrodynamic interaction effects induced on the vessels, which in turn can adversely affect their ability to maintain course and safety, especially for the smaller tug. Hence, knowledge of the hydrodynamic loads acting on the tug under these conditions is of significant practical value to the tug operator in order to avoid collision, capsizing or being run over. However, there are limited comprehensive studies to date characterising the interaction behaviour on a tug manoeuvring in close proximity to a large ship. This project investigates the hydrodynamic interaction behaviour acting on a tug during ship-assist manoeuvres in order to establish safe operational envelopes using full scale validated Computational Fluid Dynamics (CFD) simulations. The investigation included quantifying the interaction effects on the tug due to changes in the vessel speeds, the longitudinal and lateral location relative to the ship, the drift-angle of the tug, and the relative size between the vessels. The CFD model was validated at model-scale using experiments performed in the model test basin at the Australian Maritime College (AMC), which were then extended to represent full-scale operations. Thus, the scaling effects and non-dimensionalisation approach used to characterise the hydrodynamic behaviour for vessels of different sizes, ratios, and separations were investigated and verified. Different numerical approaches (CFD and potential flow solvers), and simulation conditions and settings within the respective approaches were also examined. The findings were used to identify guiding principles to achieve accurate numerical simulation results for hydrodynamic interaction effects during tug-ship operations. The operational implications on a tug during ship-assist manoeuvres are discussed based on the hydrodynamic interaction data obtained through the CFD simulations. The hydrodynamic interaction data is consolidated into Hydrodynamic Interaction Region Plots (HIRP), which are nondimensionalised based on the size and speed of the vessels and can thus be used by tug operators to determine the actual interaction forces and moments on a tug for different drift angles and locations relative to the ship for a given forward speed. This enables tug operators to determine the safe operational envelopes specific to the vessels in question and their prevailing conditions. In future studies, the results of this project can be integrated into ship/tug handling simulators by replacing their existing interaction modules using new algorithm developed through non-linear regression analysis of the data consolidated within the HIRPs developed in this work.