Numerical studies on tugboat performance during pushing operations

Abstract

This paper introduces a RANS CFD methodology for the evaluation of tugboat dynamics during pushing operations. Two- and three- dimensional methods that respectively utilize “Dynamic Fluid Body Interactions - (DFBI)” and “Tug Force Equilibrium kinematics-(TFE)” are assessed and compared with the aim to better understand the influence of fluid modelling on ship dynamics. For the DFBI method, an unsteady RANS solver comprising of a dynamic fluid body interaction module and a contact mechanics coupling algorithm are used to predict the forces between a tugboat and an assisted ship. For the TFE method, a steady RANS method is applied and contact actions are calculated as a sum of the hydrodynamic forces on the hull and the propeller. Whereas DFBI accounts for the time variation of the contact forces, the TFE is more rapid and could be used to derive operational decision support criteria. To demonstrate the latter the TFE method is used to derive the pushing forces based on a set of 16 numerical simulations. It is concluded that irrespective of the model used the tugboat speed and orientation may amplify the pushing forces. This effect could be prominent, especially at slow speeds for which the sway force acts in opposite direction to the tug.