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A consistent frame for the numerical simulation of lowering operations is investigated in this paper from a new wave-structure coupling. The mechanical modeling is based on the Composite-Rigid-Body Algorithm, which is able to simulate the nonlinear dynamics of multibody systems. The hydrodynamic model is based on the weak-scatterer approach, which allows the computation of unsteady hydrodynamic loads without being limited by the classical hypotheses of the linear potential flow theory. The coupling of these two models leads to the numerical simulation of articulated multibody systems with large relative motions in waves. The coupling equation is derived in this paper. This new numerical modeling is compared to the classical linear potential flow theory in the case of a lowering operation with a payload in the water. The impact of the lowering velocity is studied. Results show that this new model matches the classical approach for small lowering velocities but as soon as nonlinearities arise, differences between the two models appear. • Consistent frame for the numerical simulation of lowering operations. • Coupling between the Composite-Rigid-Body Algorithm and the weak-scatterer approach. • Comparison with the linear potential flow theory. • Models match for small lowering velocities, differ for high lowering velocities. [ABSTRACT FROM AUTHOR]

The wave-body interaction of resonant oscillating wave energy converters (WECs) is complex and nonlinear. Rapid development of WECs needs a reliable simulation tool for analysis and design procedure with the ability to capture the instantaneous nonlinear behavior. This paper presents a methodology to study the behavior of a flap-type WEC utilizing hybrid physical and numerical simulation. Dynamic characteristics of the device as isolated and in an array are estimated and the time and oscillation amplitude-dependency of these parameters are explained. This methodology is developed for a device composed of a series of flaps placed on a vertical breakwater. However, the developed algorithm can be updated for any other WEC concept with special consideration on the consequent changes in the load transfer, surrounding hydrodynamic, and wave-structure interaction. This paper increases our understanding and knowledge of all involved phenomena and their direct effects on the natural period and the response and can be used for tunning dynamic characteristics of WEC device to incoming waves. Finally, the developed numerical model is used for the simulation of the WEC to different wave loads and the results are validated with the experimental tests. • Hybrid experimental numerical method to capture nonlinear dynamic WEC behaviour is proposed. • The time and amplitude-dependency of dynamic characteristics of the device as isolated and in an array are studied. • The period dependent phenomena such as linear and nonlinear resonance are investigated. • Wave-structure interaction in WEC array of a multipurpose structure is studied. • Findings can be used for tuning dynamic characteristics to increase the power. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of trajectory tracking control for underactuated autonomous underwater vehicles (AUVs) in 3 dimensions space. Given a smooth, inertial, 3D reference trajectory, in order to make the position error to stay in the neighborhood of zero, the control algorithm uses vehicle׳s kinematic equation and linear system stability theory to compute the desired body-fixed velocities. Using these methods, the velocity error dynamics are obtained. Backstepping techniques are used, forcing the tracking error to an arbitrarily small neighborhood of zero. Simulation of a spiral trajectory is performed. The result shows that the controller can realize 3 dimensions trajectory tracking effectively. [ABSTRACT FROM AUTHOR]

Abstract: Phase control may substantially increase the power absorption in point-absorber wave energy converters. This study deals with validation of dynamic models and latching control algorithms for an oscillating water column (OWC) inside a fixed vertical tube of small circular cross-section by small-scale testing. The paper describes experimental and numerical results for the system''s dynamics, using simple and practical latching control techniques that do not require the prediction of waves or wave forces, and which will be relevant to any type of point-absorbing devices. In the experimental set-up, the upper end of the tube was equipped with an outlet duct and a shut-off valve, which could be controlled to give a latching of the inner free surface movement. The pressure drop through the open valve is used as a simplified measure of the energy extraction. The control was realized by using the real-time measurement signals for the inner and outer surface displacement. A mathematical model of the system was established and applied in numerical simulation. In the case the OWC''s diameter is much smaller than the wavelength and the wave amplitude much smaller than the draft, the free surface movement inside the tube can be described as an oscillating weightless piston. For this hydrodynamic problem an analytical solution is known. In addition, the mathematical model includes the effects of viscous flow losses, the air compressibility inside the chamber and the pressure drop across the valve. Experimental results were used to calibrate some of the model parameters, and the total model was formulated as a coupled system of six non-linear, first-order differential equations. Time-domain integration was used to simulate the system in order to test the control strategies and compare with experimental results. [Copyright &y& Elsevier]

Among various underwater ranging techniques, received signal strength (RSS)-based ranging method enjoys great popularity because of its accessibility and low cost. However, this effective method encounters great performance challenges due to multipath phenomenon. In this paper, to accurately map RSS values to distance in the multipath environment, ray tracing method based on image sources is introduced into the Urick propagation model. In this way, we can better obtain the relationship between RSS and parameters containing distance in the new transmission model. However, since these parameters cannot be derived with only one RSS measurement, we utilize multiple RSS measurements at different frequencies to calculate them. Moreover, since the mapping problem between RSS and distance is ill-conditioned, we construct additional inequalities based on prior information of the scene, and then present an algorithm named interior-point, to solve the problem. We analyze the proposed scheme to derive the lower bound, and numerical simulation shows ranging error is minor and almost falls on the top of CRLB. The results in next simulations and experiment show the localization performance is close to the state-of-art method based on compressed sensing and ranging accuracy improves up to 2 times when compared to the conventional method. • Urick propogation model cannot accurately map RSS values to distance in practice. • Multiple frequencies can mitigate impact of multipath on localization. • The more frequencies we use, the better ranging performance is. • Interior-point algorithm with prior information can solve this mapping problem. [ABSTRACT FROM AUTHOR]