1,721,072 research outputs found
Station Keeping and Seakeeping in Offshore Vessels Design
No full textThe major guidelines for examining and verifying the station keeping and seakeeping capabilities of offshore vessels are here presented together with a critical discussion of the main items to be considered in early design stages. To properly focus the sequential procedure of analysis, the experience gained by the author during design and testing of several offshore vessels will be discussed. Numerical examples will support the presentation
Advantages and disadvantages of thruster allocation procedures in preliminary dynamic positioning predictions
No full textMinimisation of power consumption is the primary target of the most commonly used procedures and strategies for the capability prediction of DP systems installed on floating objects and, therefore, dealing with an huge amount of limitations or restrictions that should be satisfied at the same time for different operating conditions. However, in the particular case of the “extreme limit prediction” for a considered system, other factors assume a prominent importance instead of fuel consumption, like the ability to use all the available power to keep the position in heavy weather conditions. When the maximum operating cases for a vessel are analysed, other effects like thruster–hull interaction and thruster–thruster interaction are rarely considered. To take into account the above-mentioned effects, different thrust allocation strategies were studied and implemented in a capability prediction suite. In this paper a new allocation philosophy is presented, with the aim of maximising the DP capability of a vessel operating in heavy whether conditions. An application of the procedure on two existing offshore vessels is also considered to highlight the effects of different allocation strategies and of the interaction effects on the resulting DP capability in terms of “operational rosettes”
Complex-Plane Approach for the Analysis of an Externally Excited System with Autoparametric Resonance
No full textAn autoparametric system with external excitation was examined by Nabergoj R, Tondl A, and Virag Z. (Chaos, Solitons & Fractals 1994; 4: 263-273). For investigating the semi-trivial and the non-trivial solution a different approach in terms of complex-plane variables is here presented. It is proved that autoparametric resonance is initiated when semi-trivial solution becomes unstable. The trajectory of the system transforms from a straight line to an elliptic path.
Within a certain interval of the excitation frequency chaotic motion becomes possible
A Multiscale Analysis of Gas Bubble Oscillations: Transient and Steady-state Solutions
No full textThe nonlinear oscillations of a spherical bubble in an incompressible, viscous, liquid, subject to an acoustic pressure field, are investigated by means of a multiscale perturbation method. Approximate analytical solutions for the transient and the steady-state oscillations in the regions of the main resonance, first and second subharmonic and ultraharmonic are obtained at second order expansion. Through this method, simple analytic expressions for the frequency response curve are available. These allow a quantitative estimation of the maximum oscillation amplitude corresponding to a given excitation intensity. Furthermore, the thresholds for the excitation of the two subharmonic oscillations has also been obtained. The results exhibit a good agreement with numerical simulations
Self-Excited Vibration Quenching by Means of Parametric Excitation
No full textThis contribution deals with the problem of suppressing self-excited vibration using parametric excitation. A simple model representing a self-excited rod, beam or a slender structure is considered. This model is based on the assumption that the system vibrates with the first mode and so it can bee substituted by a concentrated mass mounted on the massless elastic rod. The parametric excitation is realized by an elastic mounting of one end of the rod; the stiffness of this mounting is periodically varied. The conditions for a fully vibration suppression are formulated
The Effect of Dynamic Absorbers for a Parametrically Excited Pendulum Type System: Numerical Results
No full textPulsation Amplitude Threshold for Surface Waves on Oscillating Bubbles
No full textOn the basis of a theoretical model for the motion of a nearly spherical bubble in a slightly viscous liquid, the pulsation amplitude threshold for the onset of surface waves is obtained by a first order approximation analysis. The comparison with available experimental data has been made and the results appear quite satisfactory
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