Dissertations / Theses on the topic 'Marine propeller'

This doctoral thesis presents new ideas and research results on control of marine electric power system.

The main motivation for this work is the development of a control system, power management system (PMS) capable to improve the system robustness to blackout, handle major power system faults, minimize the operational cost and keep the power system machinery components under minimal stress in all operational conditions.

Today, the electric marine power system tends to have more system functionality implemented in integrated automation systems. The present state of the art type of tools and methods for analyzing marine power systems do only to a limited extent utilize the increased knowledge available within each of the mechanical and electrical engineering disciplines.

As the propulsion system is typically consisted of the largest consumers on the vessel, important interactions exists between the PMS and vessel propulsion system. These are interacted through the dynamic positioning (DP) controller, thrust allocation algorithm, local thruster controllers, generators' local frequency and voltage controllers. The PMS interacts with the propulsion system through the following main functions: available power static load control, load rate limiting control and blackout prevention control (i.e. fast load reduction). These functions serve to prevent the blackout and to ensure that the vessel will always have enough power.

The PMS interacts with other control systems in order to prevent a blackout and to minimize operational costs. The possibilities to maximize the performance of the vessel, increase the robustness to faults and decrease a component wear-out rate are mainly addressed locally for the individual control systems. The solutions are mainly implicative (for e.g. local thruster control, or DP thrust allocation), and attention has not been given on the interaction between these systems, the power system and PMS. Some of the questions that may arise regarding the system interactions, are as follows: how the PMS functionality may affect a local thruster control, how the local thruster control may affect the power system performance, how some consumers may affect the power system performance in normal operations and thus affect other consumers, how the power system operation may affect the susceptibility to faults and blackout, how various operating and weather conditions may affect the power system performance and thus propulsion performance though the PMS power limiting control, how propulsion performance may affect the overall vessel performance, which kind of faults can be avoided if the control system is re-structured, how to minimize the operational costs and to deal with the conflicting goals. This PhD thesis aims to provide answers to such questions.

The main contributions of this PhD thesis are:

− A new observer-based fast load reduction system for the blackout prevention control has been proposed. When compared to the existing fast load reduction systems, the proposed controller gives much faster blackout detection rate, high reliability in the detection and faster and more precise load reduction (within 150 miliseconds).

− New advanced energy management control strategies for reductions in the operational costs and improved fuel economy of the vessel.

− Load limiting controllers for the reduction of thruster wear-out rate. These controllers are based on the probability of torque loss, real-time torque loss and the thruster shaft

accelerations. The controllers provide means of redistributing thrust from load fluctuating thrusters to less load fluctuating ones, and may operate independently of the thrust allocation system. Another solution is also proposed where the load limiting controller based on thrust losses is an integrated part of DP thrust allocation algorithm.

− A new concept of totally integrated thrust allocation system, local thruster control and power system. These systems are integrated through PMS functionality which is contained within each thruster PLC, thereby distributed among individual controllers, and independent of the communications and dedicated controllers.

− Observer-based inertial controller and direct torque-loss controller (soft anti-spin controller) with particular attention to the control of machine wear-out rate. These controller contribute to general shaft speed control of electrical thrusters, generators and main propulsion prime movers.

The proposed controllers, estimators and concepts are demonstrated through time-domain simulations performed in MATLAB/SIMULINK. The selected data are typical for the required applications and may differ slightly for the presented cases.

This is a thesis about control of marine propellers. All ships and underwater vehicles, as well as an increasing number of offshore exploration and exploitation vessels, are controlled by proper action of their propulsion systems. For safe and cost effective operations, high performance vessel control systems are needed. To achieve this, all parts of the vessel control system, including both plant level and low-level control, must be addressed. However, limited attention has earlier been given to the effects of the propulsion system dynamics. The possible consequences of improper thruster control are:

• Decreased closed-loop vessel performance due to inaccurate thrust production

• Increased vessel down-time and maintenance cost due to unnecessary mechanical wear and tear.

• Increased fuel consumption and risk of blackouts due to unpredictable power consumption.

By focusing explicitly on the propeller operating conditions and the available options for low-level thruster control, this thesis presents several results to remedy these problems.

Two operational regimes are defined: normal, and extreme conditions. In normal operating conditions, the dynamic loading of the propellers is considered to be moderate, and primarily caused by oscillations in the inflow. In extreme conditions, the additional dynamic loads due to ventilation and in-and-out-ofwater effects can be severe. In order to improve the understanding of these loads and develop a simulation model suitable for control system design and testing, systematic model tests with a ventilating propeller in a cavitation tunnel and a towing tank have been undertaken.

In conventional propulsion systems with fixed-pitch propellers, the low level thruster controllers are usually aimed at controlling the shaft speed. Other control options are torque control and power control, as well as combinations of the three. The main scientific contributions of this thesis are:

• A combined torque/power controller and a combined speed/torque/power controller are designed. When compared to conventional shaft speed control, the proposed controllers give improved thrust production, decreased wear and tear, and reduced power oscillations.

• A propeller load torque observer and a torque loss estimation scheme is developed, enabling on-line monitoring of the propeller performance.

• An anti-spin thruster controller that enables use of torque and power control also in extreme operating conditions is motivated and designed. By applying the load torque observer to detect ventilation incidents, the antispin controller takes control of the shaft speed and lowers it until the ventilation incident is terminated.

• A propeller performance measure that can be used to improve thrust allocation in extreme operating conditions is introduced.

The proposed controllers and estimation schemes are validated through theoretical analyses, numerical simulations, and experiments on a model-scale propeller.

Speed and position control systems for marine vehicles have been subject to an increased focus with respect to performance and safety. An example is represented by drilling operations performed with semi submersible rigs where the control of position and heading requires high accuracy. Drifting from the well position could cause severe damage to equipment and environment. Also, the use of underwater vehicles for deep ocean survey, exploration, bathymetric mapping and reconnaissance missions, has become lately more widespread. The employment of such vehicles in complex missions requires high precision and maneuverability.

This thesis focuses on thrust estimation and control of marine propellers with particular attention to four-quadrant operations, in which the propeller shaft speed and the propeller inflow velocity (advance speed) assume values in the whole plane. In the overall control system, propellers play a fundamental role since they are the main force producing devices. The primary objective of the thruster controller is to obtain the desired thrust from the propeller regardless the environmental state. During operations, propellers are often a¤ected by thrust losses due to e.g. changes in the in-line water velocity, cross flows, ventilation, in-and-out of water effects, wave-induced water velocities, interaction between the vessel hull and the propeller and between propellers. Propellers may thus work far from ideal conditions. Therefore, the knowledge of the propeller thrust and torque, together with forces induced by the interaction between the vehicle and propellers and between propellers, is fundamental to achieve high control performance. Unfortunately a propeller system is usually not equipped with thrust and torque sensors, therefore thrust losses are not directly measured.

Motivated by this, a new four-quadrant thrust estimation scheme is presented, extending previous results valid for propeller operating in two quadrants. Based on shaft speed and motor torque measurements, the scheme involves a nonlinear observer for the propeller torque that shows stability and robustness for bounded modeling and measurement errors. The propeller thrust is computed as a static function of the propeller torque. The performance is demonstrated in experimental tests, showing improved accuracy in the thrust reproduction with respect to the direct use of the four-quadrant propeller characteristics.

A nonlinear observer for the torque loss estimation, similar to the one implemented in the thrust estimation scheme, is included in a new fourquadrant nonlinear thrust controller, designed for calm and moderate sea conditions. The control strategy is based on a shaft speed controller where the desired velocity is computed from the desired propeller thrust and on the torque losses. Experimental results are provided, demonstrating the e¤ectiveness of the new controller with respect to the conventional shaft speed and torque controllers.

The thrust controller, designed for calm and moderate sea conditions, is subsequently improved by including an anti-spin strategy to reduce power peaks and wear-and-tear in extreme sea conditions. The anti-spin strategy is derived from previous works that were designed for Dynamics Positioning (DP) operations. The presented controller can operate also for maneuvering and transit operations, where the vehicle speed is larger than in DP operations. The performance of the controller is validated by experimental tests.

Motivated by environmental issues and the need of reduced fuel consumption and CO₂ emissions, a novel control scheme for improving, in moderate sea, the propulsion e¢ ciency with respect to conventional propeller controllers is presented. The main idea is to exploit the variation in the advance speed due to waves to increase the average propulsion efficiency without reducing the vessel speed. A nonlinear controller is proposed showing that, theoretically, is possible to increase the propulsion efficiency. Model tests determine dynamic characteristics of propellers in waves and a simulation is employed to validate the novel control scheme.