Michele Martelli

Controllable pitch propeller actuating mechanism, modelling and simulation

This article focuses on the mathematical model of the pitch control mechanism for a marine controllable pitch propeller, with the aim of describing the dynamic behaviour of this kind of system and its influence on ship performance. Too great a load on the blades can result in high pressures in the actuating system, response delays and control system problems, which are ultimately responsible for most mechanism failures. The behaviour of the controllable pitch propeller actuating mechanism is considered in terms of blade position, oil pressures inside the controllable pitch propeller hub and magnitudes of the forces acting on the blades. In the proposed mathematical model, the forces acting on the propeller blade are evaluated taking into account the yaw motion of the ship, the propeller speed (including shaft accelerations and decelerations) and the turning of the blade during the pitch change. On the basis of the introduced procedure, a controllable pitch propeller numerical model as part of an overall propulsion and manoeuvrability simulator representing the dynamic behaviour of a twin-screw fast vessel is developed. The aim of this work is to represent the ship propulsion dynamics through time-domain simulation, based on which the designers can develop and test several design options, in order to avoid possible machinery overloads with their consequent failures and to obtain the best possible ship performances. In this aspect, the controllable pitch propeller model is an essential design tool.

Numerical modelling of propulsion, control and ship motions in 6 degrees of freedom

This work presents the main steps for the development of a multi-physic simulation platform, able to represent the dynamics of a twin-screw ship in 6 degrees of freedom, taking into account the complete propulsion system including automation effects. The simulation platform has to be used in the preliminary design phase in order to study and design the propulsion plant and its control system. The ship motion model has been developed including roll motion, in order to capture the ship heel angles during tight turning circles, which may be significant for a fast naval vessel. Moreover, the simulation model includes a simplified representation of the asymmetric behaviour of the two propeller shafts during manoeuvres, which cannot be neglected when dealing with the propulsion plant behaviour. Several sub-models have been developed and calibrated by means of a set of experimental tests, in model and full scale. The sea trial campaign is finally used to validate and tune the developed simulator; thus, the final version may be adopted as an optimization tool for other future designs (or sister ships) and training purposes. Although the presented case study has been validated on a specific ship, most of the discussed models have a general application.

Performance Decay Analysis of a Marine Gas Turbine Propulsion System

Marine propulsion plants are designed to be more and more efficient to minimize fuel consumption and pollution emissions. However, during the ship operating life, propulsion components and hull are characterized by a certain performance decay, responsible for a worse behavior of the overall propulsion plant. For this reason, the several propulsion components are periodically subjected to expensive maintenance works to restore, as far as possible, their original design characteristics. In the present study, the propulsive performance variation of a naval vessel, powered by a gas turbine as part of an innovative CODLAG system, is simulated and analyzed by means of a detailed and validated numerical code. A sensitivity analysis regarding the influence of the main components deterioration (gas turbine, propellers, and ship hull) on the overall behavior of the propulsion plant is carried out. Several speed profiles of the vessel have been analyzed in terms of the usual performance parameters (ship speed, engine power, and fuel consumption) as well as the pollution emissions of the gas turbine. The main aim of the work is to get useful information for the ship management and maintenance scheduling (condition-based maintenance).

Design and Validation of Dynamic Positioning for Marine Systems: A Case Study

The design of a dynamic positioning (DP) system is a challenging task with several technical fields involved in the problem solution. Numerical simulation is a powerful tool to aid the designer during the system development and to speed up the design process. This paper presents the simulation methodology adopted to design and test the DP system for a vessel with a standard propulsion configuration. Simulation results and sea trial measurements are compared to illustrate the reliability of the proposed simulation platform.