ASV operability at sea: Size matters as much as hull form design
The influence of vessel size on the performance of a second generation Autonomous Surface Vessel (ASV) in irregular head waves is investigated. The hull of the SWATH type ASV is optimized using a combination of high fidelity hydrodynamic numerical models to achieve minimal propulsion powering requirement as well as lower motions in waves compared to state of the art catamaran type (CAT) surface vessels currently referred to as first generation. Autonomous Underwater Vehicles (AUV) assistance at sea requires the ASV to possess exceptional seakeeping qualities which hull optimization procedures aim to achieve. However, the performance of the ASV may be sensitive to the most probable wave period of the ambient environment due to the asymptotic peaks typical of SWATH vessel responses in long waves. It is the objective of this study to assess the seakeeping behavior of five different sizes of the ASV at sea to determine the limiting sea state for each vessel size. Parameters such as the significant heave and pitch motions are used to examine the operability of the vessels in five sea states. The results highlight the importance of selecting appropriate vessel size based on the predominant weather conditions of the location.
Adaptive steering control for an azimuth thrusters-based autonomous vessel
ABSTRACT The proposed paper presents the design and development of the combined guidance and control strategies for the autonomous navigation of unmanned vessels characterised by azimuth-based thrust architecture. Autonomous marine vehicles (AMVs) are consolidated technological tools commonly employed for different tasks such as exploration, sampling and intervention. With the final aim of autonomous shipping, the AMVs capabilities have to be migrated and adapted towards the reliable and safe control of commercial-like unmanned vessels. These last are spreading thanks to a number of technological research projects. The employment of unconventional hull shapes combined with propulsive layout based on azimuth thrusters requires robust guidance techniques to provide precise and reliable motion control during navigation. The paper introduces a dual-loop guidance and control scheme able to provide advanced navigation capabilities. An inner control loop, devoted to the actuation of the azimuth thrusters, allows the tracking of reference course angle (namely the autopilot). Such a control loop is characterised by a modified PD regulation scheme, where a novel adaptive derivative component is inserted in order to improve the convergence curve towards the required course reference. The outer guidance loop, based on Lyapunov and virtual-target approach, allows the vessel to track generic desired paths, thus enhancing the autonomous navigation capabilities. The paper will provide a deep design and analysis approach for the developed techniques, as well as simulation results of the combined guidance and control scheme, proving the reliability of the proposed approach in different operative conditions.
PREDICTION OF MOTIONS OF CATAMARANS ACCOUNTING FOR VISCOUS EFFECTS
An approach is presented to calculate catamaran motions in regular waves. The method is mainly based on two-dimensional potential flow theory in which some viscous forces are considered through a cross flow drag approach. When the viscous effects are not considered the response amplitude at the resonance frequencies is largely over predicted. This work presents the basic theoretical formulation for the hydrodynamic forces and induced motions. To check the validity of the implemented model, calculations are performed on twin-cylinders and simple catamaran hull forms and the results are compared with other published experimental results. The influence of the variation of the viscous coefficients in the ship motion responses is analysed on one of the catamaran models.