Simulation of mechanical and thermodynamic performances of a ship propulsion plant in transient conditions

The paper presents a numerical procedure for the simulation of a marine propulsion plant in transient conditions: the model describes in detail the various elements of the propulsion system and the interactions existing between each of them. The thermodynamic behaviour of the main engine as well as of the turbocharger group is modelled, in order to predict not only the mechanical stress of the plant due to the transient conditions, but also the thermal stresses and working point of the compressor and of the turbine (pressure and temperature at inlet/outlet). As a first application of the model, the case is considered of a twin screw vessel powered by two four stroke medium speed diesel engines with controllable pitch propellers. The simulations presented in the paper regard two transient manoeuvres of the ship: slam-start (from zero speed to full ahead) and crash stop (from full ahead to zero speed with a “full astern” command). The results of the simulations are compared with the available experimental data, regarding the time histories of the vessel speed, space run before a complete stop, propeller r.p.m. and shaft power, showing a good accordance. The procedure can be utilised to check the behaviour of the turbocharger group during a transient manoeuvre and to identify possible improvements in the ship control system or in the pitch-r.p.m. combination law to optimise the performances, in term of e.g. stopping space of the vessel or thermal and mechanical stresses of the plant