CONTROL OF SHIP'S ROLL BY ACTIVE FIN STABILIZERS

Abstract

Stabilization of ship roll motion induced by wave disturbances is one of the important controls required in the modern day marine industry. Excessive roll motion makes the ship's crew uncomfortable and also causes damage to the cargoes and equipment on board. Active-fin stabilisers are the most widely used equipments out of the many other available stabilizers like anti-roll tanks, gyroscope and bilge keel in today's marine world to reduce the roll motion. It rotates about its stock to give a desired hydrodynamic list to stabilize the ship's roll. Various control strategies have been used for the roll motion control of ships. However, most of the research includes stabilization of a ship with fixed mathematical model. The major changes in the ship's parameters like weight and meta-centric height due to practical reasons are generally not considered for controller design. This dissertation exploits this fact to study, analyze and design a controller for the ship with varied parameters including weight of the ship. In this dissertation, three different conditions of a ship model are selected and PID controllers were separately designed for all the 3 conditions. These PID controllers are amalgamated to design a neural network controller. Thereafter, neural network controller is improved upon in various steps to arrive at an optimal controller to control the ship's roll in all the three possible conditions of the ship. The neural network controller could successfully control even the sinking ship. There is a saturation limit for the stabilizer fin angle which is generally not considered by many researchers in their work or simulations. In this dissertation even the non-linearity in the form of fin angle's saturation is considered during the design of PID/NN/FL controllers. Performance of a controller may decrease by considering fin's angle saturation. However this is what is practical and designing controllers without considering saturation limit is meaningless and is of no practical use. After the design of the NN controller, the creation of a fuzzy logic controller was envisaged. In the pursuit to make the best possible FLC even with non-availability of the system information for the rule creation, data from the previously designed 3 PID controller was used to frame the rules. Various FLCs with varied shapes, sizes and calculated parameters of the membership functions of the fuzzy sets were created, tested and analyzed. Some of the FLCs were even designed with 125 rules along with a separate weightage for each rule. Particle swamp optimization was later used to fine tune the gains added to the Fuzzy logic controller for better control of ship's roll. The finalised FLC also could save the sinking ship and give good performance. FLC also could reduce roll frequency to a great extent which is very important for some ships with specific roles.