Numerical Models for Ship Dynamic Positioning

One of the best ways to design marine control systems before the construction of a ship is the use of simulation techniques. The paper presents two numerical models used to develop and test, in the preliminary design phase, a dynamic positioning (DP) system for marine vehicles. In particular, it refers to a surface vessel equipped with a conventional propulsion configuration, consisting of two controllable pitch propellers, two rudders and a single bow-thruster. For such a vessel, the DP system is required to manage the actuators in order to obtain a good dynamic positioning performance at zero-speed with moderate weather conditions. In order to verify control and allocation logics, two numerical models, with different degrees of details, have been developed and used in distinct steps. Several sub-systems as ship dynamics, propulsion plant, controller and environmental disturbances have been implemented making use of suitable mathematical models linked each other in order to take into account their mutual interactions. Eventually, simulation results are shown and critically compared in order to better understand the points of strength and weakness of the two proposed models.