Background and Objectives
The particular combination of diesel-electric drives and variable-pitch propellers opens new possibilities for optimization of stop maneuvers with the goal of shortening the stop distance and of reducing the substantial load of the individual components of the propulsion system. An important issue during stopping is the so-called wind-milling effect. In this case the torque becomes negative and the propeller is driven by the water. The electric motor works as a generator, which can have strong negative influence on the stability of the power grid. The optimal combination of the pitch setting and number of revolutions during the different stages of the stop maneuvers allows the reduction and in some cases the elimination of the wind-milling stage of the maneuver.
The developed methods will enable the ship designer to simulate the stop manoever in a early stage of the design process and so achive a higher safety level.
A simulation method is developed to simulate the stopping behavior of ships equipped with diesel-electric propulsion and controllable pitch propellers (CPP). The motion behavior is calculated by solving the equations of motion in time-domain. Extensive viscous simulations are carried out to identify the manoevering coefficents of different ship types and velocities. To consider the interaction between the ship's hull, the rudder and the turning propeller CFD and EFD simulations are carried out also respecting cavitation on the propeller blades.
The diesel engine, the propulsion train and the whole electric equipment is simulated in a detailed manner. The revolution speed of the shaft and the demanded torque of the propeller are exchanged between the gear and the hydrodynamic part in the simulation. By means of these informations the developed method could simulate the ship's path through the water
The developed method is applied on four different ship types: a container feeder, a large container vessel, a RoPax ferry and a frigate which all are equipped with a diesel-electric propulsion or a electric booster and one or two CPPs.
In preliminary studies a 96.6 m long seismic research ship equipped with a diesel-electric engine and two CPPs is examined with a simple simulation method. Various simulations were performed and compared to sea trial measurements. The investigated scenarios include variations of the rate of rotation of the propeller and of the pitch angle.
The figure shows simulation results (black lines) and measurements (red lines) for one stopping scenario. At the begin of the stop maneuver the engine has been electrically disconnected from the propeller and the pitch angle is decreased to zero with a constant velocity. The results of the simulation show a good agreement with the measured values. Similar results are estimated for the other stopping scenarios.