Numerical Modelling of Automatic Mooring Winches Applied to Moored Ships in Harbours

In order to control excessive motions of moored ships, automatic mooring winches, namely constant tension winches and automatic tension winches, have been employed. It has been common practice the recovering force in automatic tension winches never becoming lower than one third of the rendering force. However, the existence of such a non-zero recovering force implies that the cable where that force is applied to may be counteracting the rendering force produced on an opposing cable by another winch. Ideally, winches should only produce forces to reduce ship motions, hence the recovering force should be zero. This paper describes the use of a numerical model for moored ship behaviour in the study of the effects of automatic tension winches in the motion amplitudes of a 108 416 m³ displacement ship moored in two different conditions: a) alone at an exposed location; b) 30 m apart from a vertical breakwater. Additional cables connected to automatic mooring winches are considered and the change from idealized conditions where the cables apply forces on the ship with no vertical component to realistic conditions where such component exists is investigated as well as the influence of the recovering force on the ship motions for the ship subject to regular head waves.