Giovanni Benvenuto

Real-time simulation of a COGAG naval ship propulsion system

Design and optimization of the propulsion system is a crucial task of the ship design process. The behaviour of the propulsion system, in transient conditions as well as in steady state, is greatly affected by the capability of the control system to manage the available power and to achieve the desired performance in the shortest time. The selection of a proper control scheme is a trade-off between different and conflicting needs. Two of the opposites are: increasing the ship operability by adding more functions and more controls; and reducing the control system development and installation time and cost. In this paper, the rapid prototyping and testing procedure for the development of the propulsion controller of the new Italian aircraft carrier Cavour is presented, using real-time hardware-in-the-loop (RT-HIL) simulation. The procedure is based on a wide use of simulation technology. First, a complete dynamical model of the ship propulsion plant was developed. Then, batch simulation was used to develop the best possible control scheme. Finally, RT-HIL simulation was used to debug the real controller software and to tune the controller parameters before sea trials. The application of the procedure led to a significant reduction in the development phase of the controller design. Furthermore, the adoption of the RT-HIL technology greatly reduced the time spent to tune the control system during the ship delivery phase.

Comparison of ship plant layouts for power and propulsion systems with energy recovery

Currently the increase in fuel costs and the need to reduce Carbon Dioxide (CO2) emissions have encouraged the search for even more efficient solutions to be adopted in energy conversion systems for ship installations. These systems generally include thermal prime movers consisting mainly of two-stroke or four-stroke diesel engines, a waste heat recovery (WHR) plant, a steam turbine and possibly a gas turbine, as well as electric machinery. These components may be used in various ways and through different plant configurations, whose optimization is under investigation by researchers and diesel engine manufacturers. In this paper four schemes of ship propulsion plants, using a two-stroke diesel engine equipped with waste heat recovery system are presented, analysed and compared by simulation. Some of the considered layouts can include also an electric motor to support the main engine in providing power to the propeller. On the other hand the electric power can be generated from both the diesel generators and the WHR plant. The considered propulsion plant schemes are compared in order to identify the configuration that meets in the best way the request of propulsive, electrical and thermal power of a 158000 dead weight tonnes (DWT) crude oil tanker ship, belonging to the Premuda Company, taken as a reference unit in this study. The comparison is carried out taking into account the payback time of the installation, the annual saving in the fuel outlay and the CO2 emissions. This last parameter is considered through the evaluation of the Energy Efficiency Design Index (EEDI).

Simulation and performance comparison between diesel and natural gas engines for marine applications

The article shows the performance comparison between two marine engines, fuelled by natural gas and diesel oil, respectively, both belonging to the ‘Bergen’ engine series of Rolls-Royce Marine, suitable as prime movers for ship propulsion. Two different simulation codes, one for each engine, validated by means of geometrical and performance data provided by the manufacturer, have been developed to extend the comparison to the whole working area of the examined engines. Although the maximum continuous power is very similar (about 2 MW at the same rotational speed), some differences exist in size, efficiency and pollutant emissions of the two types of engines. The reasons are investigated through a specific thermodynamic analysis, aimed to explain such differences, in terms of efficiency and emissions (particularly carbon dioxide), when varying the working conditions. The analysis is carried out by comparing the respective real cycles, at the same working condition, and repeating the comparison for different engine delivered powers and rotational speeds. In addition, a study of the different modes of combustion is developed to explain the major differences found in the emissions of nitrogen oxides.

Optimization of waste heat recovery from the exhaust gas of marine diesel engines

In this article, some configurations of waste heat recovery systems are described, analysed and compared, in order to find the optimal plant layout. Starting from the availability of performance data of a two-stroke diesel engine, adopted for the propulsion plant of a crude oil tanker ship, the authors examined different solutions for the waste heat recovery from the diesel engine exhaust gas ensuring the best fulfilment of the vessel needs in terms of mechanical, electric and thermal energies. The considered waste heat recovery systems can adopt either steam turbine and gas turbine or simply steam turbine for power generation. As regards the steam plant, two basic layouts are considered, optimized and compared: the first plant scheme is a typical steam plant currently adopted for waste heat recovery purposes and the second one is a solution proposed by the authors. Considering the different options, in this article, four different system layouts, applied to the mentioned diesel engine, are singly optimized and compared between them in order to find the most suitable plant and the steam cycle parameters that provide its best operation at the engine normal continuous rating. The performance of the optimized waste heat recovery systems is evaluated also by comparing them under off-design engine load conditions, in the engine power range between 50% and 100% of its maximum continuous rating.

Dimensionless Numerical Approaches for the Performance Prediction of Marine Waterjet Propulsion Units

One of the key issues at early design stage of a high-speed craft is the selection and the performance prediction of the propulsion system because at this stage only few information about the vessel are available. The objective of this work is precisely to provide the designer, in the case of waterjet propelled craft, with a simple and reliable calculation tool, able to predict the waterjet working points in design and off-design conditions, allowing to investigate several propulsive options during the ship design process. In the paper two original dimensionless numerical procedures, one referred to jet units for naval applications and the other more suitable for planing boats, are presented. The first procedure is based on a generalized performance map for mixed flow pumps, derived from the analysis of several waterjet pumps by applying similitude principles of the hydraulic machines. The second approach, validated by some comparisons with current waterjet installations, is based on a complete physical approach, from which a set of non-dimensional waterjet characteristics has been drawn by the authors. The presented application examples show the validity and the degree of accuracy of the proposed methodologies for the performance evaluation of waterjet propulsion systems.