1,379 research outputs found
A nonlinear dynamics perspective on some aspects of towing operations relevant to safety and energy efficiency
No full textScheduled and emergency towing operations are carried out daily, both at sea and in restricted waters, as a means of transporting goods, aiding ships during berthing and casting-off or rescuing units in distress. Even in mild weather conditions, such activities can pose serious threats to the safety of the personnel and units involved and to the environment. This study investigates the dynamics of a tug-tow system, where towing is carried out by means of a single elastic towing line. A simplified model is used, in which the tug is modelled as a point-particle with a prescribed motion, while the dynamics of the towed object is modelled as a 3-DOF (surge/sway/yaw) nonlinear dynamical system. Equilibria of the system are analysed through a bifurcation analysis technique, supplemented by time-domain simulations, if/when necessary. An example application is reported for the case of towing of a barge-like hull, showing the effects of several parameters: ship speed, towline length, water depth and wind. The performed calculations confirm that, depending on the configuration parameters, multiple stable/unstable steady-state towing positions may exist. It is also clarified in this study that, in the absence of stable equilibria, the potentially dangerous oscillating phenomenon called “fishtailing” takes place
Nonlinear parametric rolling in regular Waves - a general procedure for the analytical approximation of the GZ curve and its use in time domain simulations
No full textA general procedure for the analytical approximation of the restoring arm in regular longitudinal waves is presented. The aim of the approximation is to obtain an analytical and accurate means for substituting the look-up table method in time domain simulations of realistic nonlinear parametric roll. The model is based on a 1.5-DOF system: the roll motion is modelled dynamically using a single degree of freedom, whereas the additional half DOF indicates that the coupling with heave and pitch is taken into account by means of hydrostatic calculations without considering dynamic effects. An analysis of the accuracy of the analytical approach is carried out taking into consideration different aspects of the problem (time histories, domains of attraction, chaotic behaviour, etc.). All the simulations are performed using data obtained from a real ship hull of a RoRo-Pax
Approximate Analytical Response Curve for a Parametrically Excited Highly Nonlinear 1-DOF System with an Application to Ship Roll Motion Prediction
No full textThe paper presents a model for a parametrically excited nonlinear 1-DOF damped mass-spring system. An approximate analytical technique is used to obtain the general polynomial whose roots give the response of the system in frequency domain when the parametric excitation acts in the first parametric resonance region. All the details needed for the implementation of the method are given. The model is suitable for the prediction of the parametrically excited roll motion of
ships in longitudinal regular waves, thus some examples of application are shown to analyse the behaviour of two different hulls. The proposed approximate frequency domain analysis is shown to be able to disclose some nonlinear features of the ship response still not well known in the field of Naval Architecture
On an improved Grim effective wave
No full textThis paper presents an extension of the original Grim effective wave concept. The aim is to develop a basic tool that is able, with a reduced number of assumptions with respect to the original Grim effective wave, to simplify the problem of description of hydrostatic variations of roll restoring in irregular waves. The irregular sea surface is approximated by an "equivalent" (effective) regular wave having fixed wave length but variable mean value, amplitude and position of the crest. The characteristics of the effective wave are obtained from the irregular sea surface by means of linear transfer functions. The probabilistic properties of the obtained parameters of the substitute wave are analysed and examples of application of the developed concept are provided
Nonlinear Parametric Rolling in Regular Waves - An Approximate Analytical Solution for the Response Curve in the Region of First Parametric Resonance
No full textTime-based damaged ship survivability: A quasi-static equivalent method
No full textThis paper presents a series of ideas to make a step forward in the assessment of flooded ship survivability on a time-dependent basis. An extension of the SEM (Static Equivalent Method) to a Quasi-Static Equivalent method (QSEM) is proposed to take into account the time variable and to address the case of multiple compartments. A capsize index based, conceptually, on static calculations is defined as a function of time and sea state. Capsize is considered in terms of both a first-excursion problem concerning the
wave amplitude, and as the persistence of high-runs of waves above a critical level. An example of the proposed methodology is presented for the analysis of the flooding a two decks. Using the same hydraulic model, comparisons are carried out between the presented approximate analytical methodology and direct numerical simulations of the flooding process to assess the effect of downflooding means. A discussion is given on how the time-based approach to ship survivability links with the evacuation process, in order to obtain the probability density function of the number of evacuated passengers
Estimation of nonlinear roll decay parameters using an analytical approximate solution of the decay time history
No full textChecking vulnerability to pure loss of stability in long crested following waves: A probabilistic approach
No full textThis paper presents a probabilistic methodology for the analysis of the vulnerability of a ship to the risk of inception of pure loss of stability events. A pure loss of stability failure is modelled as the persistence of the metacentric height below a critical level for a too long time. The metacentric height is modelled as a stationary Gaussian process with a spectrum obtained from the sea elevation spectrum. The time dependent failure index is obtained under the assumption of filtered Poisson process for the occurrence of critical events. The analysis separates cases where the fluctuation of the metacentric height is narrow-band from those where the bandwidth of the spectrum is wide, with an intermediate blending. In case of narrow-band processes appropriate approximate solutions to the problem are provided, while in the wide-band cases an exponential distribution for the persistence time below the critical level is employed. A rational development for the critical persistence time is also provided considering an approximation of the roll dynamics during periods of time where the metacentric height is negative. Monte Carlo simulations are performed to check the developed approximate distributions for the persistence time, and examples of application are provided for a sample ship
Theoretical Prediction and Experimental Verification of Multiple Steady States for Parametric Roll
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