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.
Drag on an Oscillating Rod with Longitudinal and Torsional Motion
An analytical expression for the viscous drag on a smooth circular cylindrical rodlike cable oscillating with longitudinal and torsional motion is obtained. Based on a linear damping law, an approximate value for the drag coefficient is also presented. The analysis and con- clusions are useful when dealing with many ocean engineering problems of practical importance. the analysis of many practical problems related to the sea-state excitation of long rods and cable-body systems for salvage, buoy, oil drilling, and towing operations, an expression for the viscous drag force acting on the longi- tudinal rod or cable is required. It has been shown that the inclusion of this term can have a pronounced effect on the tension in the rod or cable and subsequent response of the system. For example, the propagation and attenuation of internal stress waves and snap loads through the cable in a cable lifting system are affected by the external viscous damping.1 The problem is also of considerable interest in offshore oil drilling problems since a similar analysis is usually performed.2 It is a common practice to postulate a linear relationship between the external damping force and the surface velocity of the rod when investigating the longitudinal vibrations based on the wave equation. The solution of this damped wave equation with an arbitrary damping coefficient can show the qualitative effects of external damping. However, the magnitude and frequency dependence of the damping coefficient can significantly alter the nature of the solution. The purpose of this investigation is to formulate an analyti- cal expression for the viscous drag on a smooth circular cylindrical rodlike cable oscillating with longitudinal and torsional motion. An examination of the hypothesis of the linear damping law will be made and the damping coefficient estimated.