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C.b. Barrass - One of the best experts on this subject based on the ideXlab platform.
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Angle of loll
Ship Stability for Masters and Mates, 2012Co-Authors: C.b. Barrass, D.r. DerrettAbstract:When a ship with negative initial metacentric height is inclined to a small angle, the Righting Lever is negative, resulting in a capsizing moment. This effect makes the ship heel still further. At a large angle of heel, the centre of buoyancy moves further out the low side and the force of buoyancy does not act vertically upwards. If, by heeling still further, the centre of buoyancy can move out far enough to lie vertically under the centre of gravity (G), the Righting Lever and thus the Righting moment, will be zero. The angle of heel at which this occurs is referred to as the angle of loll and may be defined as the angle to which a ship with negative initial metacentric height lies at rest in still water. If the ship is inclined to an angle greater than the angle of loll, the Righting Lever will be positive, giving a moment to return the ship to the angle of loll. The ship will oscillate about the angle of loll instead of the upright. At angles of heel less than the angle of loll, the Righting Levers are negative.
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Large-Angle Stability Considerations – GZ and KN Cross Curves of Stability
Ship Stability for Masters and Mates, 2012Co-Authors: C.b. Barrass, D.r. DerrettAbstract:Large-angle stability considerations are discussed with reference to GZ and KN cross curves of stability. From GZ curves the Righting Lever about an assumed center of gravity for any angle of heel at any particular displacement may be found by inspection. In some cases the curves are constructed for an assumed KG of zero. The curves are then referred to as KN curves, KN being the Righting Lever measured from the keel. Statical stability curves are illustrated with an explanation of the stability information that can be obtained from them, and a number of examples are given.
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Simplified Stability Information
Ship Stability for Masters and Mates, 2012Co-Authors: C.b. Barrass, D.r. DerrettAbstract:The masters' task of ensuring that his ship complies with the minimum statutory standards of stability is not being adequately carried out as undue traditional reliance is being placed on the value of metacentric height (GM) alone, and other important criteria that govern the Righting Lever GZ curve are not being assessed as they should be. Because of this reason, the court of inquiry recommended that simplified stability information must be incorporated into ships' stability booklets so that masters can assure themselves that safe standards of stability are met. Simplified stability information eliminates the need to use cross curves of stability and develop Righting Lever GZ curves for varying loading conditions by enabling a ship's stability to be quickly assessed, to show whether or not all statutory criteria are complied with, by means of a single diagram or table. It is further recommended that the use of a simplified stability diagram as an adjunct to the deadweight scale must be adopted to provide a direct means of comparing stability relative to other loading characteristics.
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Effects of Beam and Freeboard on Stability
Ship Stability for Masters and Mates, 2012Co-Authors: C.b. Barrass, D.r. DerrettAbstract:This chapter discusses the effect of increasing the beam and freeboard on stability curve of a vessel. To investigate the effect of beam and freeboard on stability, it is necessary to assume the stability curve for a particular vessel in a particular condition of loading. The chapter concludes that with increased beam metacentric height (GM T ) and Righting Lever (GZ) increase, range of stability increases, deck edge immerses earlier, and moment about the keel (KB) remains similar. With increased freeboard GM T and GZ increase, range of stability increases, deck edge immerses later at greater θ, and KB decreases.
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Chapter 31 – Angle of loll
Ship Stability for Masters and Mates, 2006Co-Authors: C.b. BarrassAbstract:Publisher Summary When a ship with negative initial metacentric height is inclined to a small angle, the Righting Lever is negative, resulting in a capsizing moment. This effect makes the ship heel still further. At a large angle of heel, the centre of buoyancy moves further out the low side and the force of buoyancy does not act vertically upwards. If, by heeling still further, the centre of buoyancy can move out far enough to lie vertically under the centre of gravity (G), the Righting Lever and thus the Righting moment, will be zero. The angle of heel at which this occurs is referred to as the angle of loll and may be defined as the angle to which a ship with negative initial metacentric height lies at rest in still water. If the ship is inclined to an angle greater than the angle of loll, the Righting Lever will be positive, giving a moment to return the ship to the angle of loll. The ship will oscillate about the angle of loll instead of the upright. At angles of heel less than the angle of loll, the Righting Levers are negative.
D.r. Derrett - One of the best experts on this subject based on the ideXlab platform.
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Angle of loll
Ship Stability for Masters and Mates, 2012Co-Authors: C.b. Barrass, D.r. DerrettAbstract:When a ship with negative initial metacentric height is inclined to a small angle, the Righting Lever is negative, resulting in a capsizing moment. This effect makes the ship heel still further. At a large angle of heel, the centre of buoyancy moves further out the low side and the force of buoyancy does not act vertically upwards. If, by heeling still further, the centre of buoyancy can move out far enough to lie vertically under the centre of gravity (G), the Righting Lever and thus the Righting moment, will be zero. The angle of heel at which this occurs is referred to as the angle of loll and may be defined as the angle to which a ship with negative initial metacentric height lies at rest in still water. If the ship is inclined to an angle greater than the angle of loll, the Righting Lever will be positive, giving a moment to return the ship to the angle of loll. The ship will oscillate about the angle of loll instead of the upright. At angles of heel less than the angle of loll, the Righting Levers are negative.
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Large-Angle Stability Considerations – GZ and KN Cross Curves of Stability
Ship Stability for Masters and Mates, 2012Co-Authors: C.b. Barrass, D.r. DerrettAbstract:Large-angle stability considerations are discussed with reference to GZ and KN cross curves of stability. From GZ curves the Righting Lever about an assumed center of gravity for any angle of heel at any particular displacement may be found by inspection. In some cases the curves are constructed for an assumed KG of zero. The curves are then referred to as KN curves, KN being the Righting Lever measured from the keel. Statical stability curves are illustrated with an explanation of the stability information that can be obtained from them, and a number of examples are given.
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Simplified Stability Information
Ship Stability for Masters and Mates, 2012Co-Authors: C.b. Barrass, D.r. DerrettAbstract:The masters' task of ensuring that his ship complies with the minimum statutory standards of stability is not being adequately carried out as undue traditional reliance is being placed on the value of metacentric height (GM) alone, and other important criteria that govern the Righting Lever GZ curve are not being assessed as they should be. Because of this reason, the court of inquiry recommended that simplified stability information must be incorporated into ships' stability booklets so that masters can assure themselves that safe standards of stability are met. Simplified stability information eliminates the need to use cross curves of stability and develop Righting Lever GZ curves for varying loading conditions by enabling a ship's stability to be quickly assessed, to show whether or not all statutory criteria are complied with, by means of a single diagram or table. It is further recommended that the use of a simplified stability diagram as an adjunct to the deadweight scale must be adopted to provide a direct means of comparing stability relative to other loading characteristics.
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Effects of Beam and Freeboard on Stability
Ship Stability for Masters and Mates, 2012Co-Authors: C.b. Barrass, D.r. DerrettAbstract:This chapter discusses the effect of increasing the beam and freeboard on stability curve of a vessel. To investigate the effect of beam and freeboard on stability, it is necessary to assume the stability curve for a particular vessel in a particular condition of loading. The chapter concludes that with increased beam metacentric height (GM T ) and Righting Lever (GZ) increase, range of stability increases, deck edge immerses earlier, and moment about the keel (KB) remains similar. With increased freeboard GM T and GZ increase, range of stability increases, deck edge immerses later at greater θ, and KB decreases.
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Stability and hydrostatic curves
Ship Stability for Masters and Mates, 2006Co-Authors: C.b. Barrass, D.r. DerrettAbstract:This chapter deals with cross curves of stability and hydrostatic curves. There are two types of cross curves of stability: GZ cross curves of stability and KN cross curves of stability. GZs are a set of curves from which the Righting Lever about an assumed centre of gravity for any angle of heel at any particular displacement may be found by inspection. To find the GZs for any particular displacement, the displacement is located on the bottom scale and, through this point, a perpendicular is erected to cut all the curves. The intersections are then translated with the curves horizontally to the left-hand scale and the GZs for each angle of heel are read off. The GZ curves that are constructed for an assumed KG of zero are referred to as KN curves. Hydrostatic curves provide the hydrostatic information in the form of table or graph.
Edwin Kreuzer - One of the best experts on this subject based on the ideXlab platform.
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Extreme roll motions of ships
2015Co-Authors: Edwin Kreuzer, R.m.c. Mestrom, Marc-andré PickAbstract:The ship capsizing problem is one of the major challenges in naval architecture. The criterion of the International Maritime Organization (IMO) regarding capsize stability is still not including dynamic loads. Existing mathematical models of ships taking into account all degrees of freedom as well as uidstructureinteraction can hardly be used for stability analysis with common methods from nonlinear dynamics theory due to their complexity. For the development of new better suited models a test environment has been created. A rst prototype for an experimental capsize analysis is presented in this paper. The motivation to study extreme roll motions of oating structures is the way current stability criteria rely on static as-sumptions. The rollrestoring moments, called Righting moments in calm water, are calculated at various heeling angles. Dividing the Righting moment by the weight of the ship the Righting Lever curves, Fig. 1, are obtained. The slope of the right-ing Lever curve at 0 is called initial stability or metacentric height GM. National and international rules on intact stability make demands on minimum values and characteristics of these curves [3]. Model tests and practical experience show that the current stability criteria do not always correspond to the danger of capsizing. In order to overcome these shortcomings the dy-namic behavior of the ship has to be included. Consequently, we propose to develop a criterion based on dynamic calculations. Bifurcations in the ship's dynamics are indicators for an upcoming capsizing [4]. Two capsizescenarios were found by path following methods. Fig. 2 shows pathes for different encounter angles of the waves for the wave height h as the control parameter. The angle 0 means following seas, 90 means beam seas. For encounter angles between 20 and 50 it can b
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Almost Sure Stability Analysis of Parametric Roll in Random Seas Based on Top Lyapunov Exponent
PAMM, 2012Co-Authors: Leo Dostal, Edwin Kreuzer, Navaratnam Sri NamachchivayaAbstract:Stability analysis of the upright position of a ship in random head or following seas is presented. Such seas lead to parametric excitation of roll motion due to periodic variations of the Righting Lever. The development of simple criteria for the occurrence of parametric induced roll motion in random seas is of major interest for improvement of the international code on intact stability provided by the International Maritime Organization. The stability analysis in random seas is based on the calculation of the top Lyapunov exponent using the fact, that a negative top Lyapunov exponent yields no roll motion. With this findings, roll motion can be excluded for specific sea states. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)
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Analysis of Nonlinear Stochastic Ship Dynamics Under Extended Wave Modeling
PAMM, 2010Co-Authors: Leo Dostal, Edwin KreuzerAbstract:Analytical criteria for risk assessment of ships in random seas are important for the development of new intact stability criteria and also for the first design stage, where many ship designs have to be compared. The analysis is performed by Stochastic Averaging. The results are compared to those obtained by Monte Carlo simulations. We use extended wave modeling by means of two independent stochastic processes realizing a traveling wave with random phase and amplitude corresponding to a prescribed sea spectral density. The Righting Lever curve in waves is approximated by a polynomial, while quasistatic equilibrium of the analyzed ship in waves is assumed. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)
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Numerical Computation of Parametric Induced Roll Motions in Random Seas
PAMM, 2009Co-Authors: Leo Dostal, Edwin KreuzerAbstract:Hamburg University of Technology, Institute for Mechanics and Ocean Engineering, 21071 HamburgFor a vessel in open seas, the sudden appearance of roll motions due to waves from the front or rear leads to dangeroussituations up to capsizing. The equations of motion used to analyze the roll motion include the Righting Lever curve. Thiscurve is set up by means of hydrostatic calculations and approximated by polynomials for further analysis. The irregularwaves are modeled in terms of a continuous-time ARMA process. The resulting model of stochastic differential equationsis investigated numerically by Local Statistical Linearization. The necessary stochastic moments and their derivatives arecomputed using Ito’s differential rule and Gaussian closure.
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Prediction of Extreme Ship Motions in Irregular Waves
PAMM, 2005Co-Authors: Edwin Kreuzer, Wolfgang SichermannAbstract:The occurrence of indirectly excited large amplitude roll motions of ships is investigated under the influence of a time-varying Righting Lever curve in waves. In order to account for the irregular character of ocean waves appropriately, the ship motions are described within the framework of nonlinear dynamics and stochastic process theory. The analysis of roll motions in regular waves provides a qualitative understanding of the occurrence of unpredictable large amplitude roll motions as observed in ocean waves. However, quantitative information on the rolling behavior in irregular waves may be retrieved only from a probabilistic analysis. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Leo Dostal - One of the best experts on this subject based on the ideXlab platform.
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Almost Sure Stability Analysis of Parametric Roll in Random Seas Based on Top Lyapunov Exponent
PAMM, 2012Co-Authors: Leo Dostal, Edwin Kreuzer, Navaratnam Sri NamachchivayaAbstract:Stability analysis of the upright position of a ship in random head or following seas is presented. Such seas lead to parametric excitation of roll motion due to periodic variations of the Righting Lever. The development of simple criteria for the occurrence of parametric induced roll motion in random seas is of major interest for improvement of the international code on intact stability provided by the International Maritime Organization. The stability analysis in random seas is based on the calculation of the top Lyapunov exponent using the fact, that a negative top Lyapunov exponent yields no roll motion. With this findings, roll motion can be excluded for specific sea states. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)
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Analysis of Nonlinear Stochastic Ship Dynamics Under Extended Wave Modeling
PAMM, 2010Co-Authors: Leo Dostal, Edwin KreuzerAbstract:Analytical criteria for risk assessment of ships in random seas are important for the development of new intact stability criteria and also for the first design stage, where many ship designs have to be compared. The analysis is performed by Stochastic Averaging. The results are compared to those obtained by Monte Carlo simulations. We use extended wave modeling by means of two independent stochastic processes realizing a traveling wave with random phase and amplitude corresponding to a prescribed sea spectral density. The Righting Lever curve in waves is approximated by a polynomial, while quasistatic equilibrium of the analyzed ship in waves is assumed. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)
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Numerical Computation of Parametric Induced Roll Motions in Random Seas
PAMM, 2009Co-Authors: Leo Dostal, Edwin KreuzerAbstract:Hamburg University of Technology, Institute for Mechanics and Ocean Engineering, 21071 HamburgFor a vessel in open seas, the sudden appearance of roll motions due to waves from the front or rear leads to dangeroussituations up to capsizing. The equations of motion used to analyze the roll motion include the Righting Lever curve. Thiscurve is set up by means of hydrostatic calculations and approximated by polynomials for further analysis. The irregularwaves are modeled in terms of a continuous-time ARMA process. The resulting model of stochastic differential equationsis investigated numerically by Local Statistical Linearization. The necessary stochastic moments and their derivatives arecomputed using Ito’s differential rule and Gaussian closure.
Marc-andré Pick - One of the best experts on this subject based on the ideXlab platform.
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Extreme roll motions of ships
2015Co-Authors: Edwin Kreuzer, R.m.c. Mestrom, Marc-andré PickAbstract:The ship capsizing problem is one of the major challenges in naval architecture. The criterion of the International Maritime Organization (IMO) regarding capsize stability is still not including dynamic loads. Existing mathematical models of ships taking into account all degrees of freedom as well as uidstructureinteraction can hardly be used for stability analysis with common methods from nonlinear dynamics theory due to their complexity. For the development of new better suited models a test environment has been created. A rst prototype for an experimental capsize analysis is presented in this paper. The motivation to study extreme roll motions of oating structures is the way current stability criteria rely on static as-sumptions. The rollrestoring moments, called Righting moments in calm water, are calculated at various heeling angles. Dividing the Righting moment by the weight of the ship the Righting Lever curves, Fig. 1, are obtained. The slope of the right-ing Lever curve at 0 is called initial stability or metacentric height GM. National and international rules on intact stability make demands on minimum values and characteristics of these curves [3]. Model tests and practical experience show that the current stability criteria do not always correspond to the danger of capsizing. In order to overcome these shortcomings the dy-namic behavior of the ship has to be included. Consequently, we propose to develop a criterion based on dynamic calculations. Bifurcations in the ship's dynamics are indicators for an upcoming capsizing [4]. Two capsizescenarios were found by path following methods. Fig. 2 shows pathes for different encounter angles of the waves for the wave height h as the control parameter. The angle 0 means following seas, 90 means beam seas. For encounter angles between 20 and 50 it can b
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Extreme roll motions of ships
PAMM, 2004Co-Authors: Edwin Kreuzer, R.m.c. Mestrom, Marc-andré PickAbstract:The ship capsizing problem is one of the major challenges in naval architecture. The criterion of the International Maritime Organization (IMO) regarding capsize stability is still not including dynamic loads. Existing mathematical models of ships taking into account all degrees of freedom as well as uidnstructureninteraction can hardly be used for stability analysis with common methods from nonlinear dynamics theory due to their complexity. For the development of new better suited models a test environment has been created. A rst prototype for an experimental capsize analysis is presented in this paper. The motivation to study extreme roll motions of oating structures is the way current stability criteria rely on static assumptions. The rollnrestoring moments, called Righting moments in calm water, are calculated at various heeling angles. Dividing the Righting moment by the weight of the ship the Righting Lever curves, Fig. 1, are obtained. The slope of the Righting Lever curve at 0 is called initial stability or metacentric height GM . National and international rules on intact stability make demands on minimum values and characteristics of these curves [3]. Model tests and practical experience show that the current stability criteria do not always correspond to the danger of capsizing. In order to overcome these shortcomings the dynamic behavior of the ship has to be included. Consequently, we propose to develop a criterion based on dynamic calculations. Bifurcations in the ship’s dynamics are indicators for an upcoming capsizing [4]. Two capsizenscenarios were found by path following methods. Fig. 2 shows pathes for different encounter angles of the waves for the wave height h as the control parameter. The angle 0 means following seas, 90 means beam seas. For encounter angles between 20 and 50 it can be seen from Fig. 2 shows that increasing wavenheight does not always lead to increasing roll amplitudes. The pathes have a global maximum, where the amplitude of the roll angle is much higher than the amplitude of the last existing periodic solution before capsizing. This indicates the importance of the usage of full dynamical models in stability analysis of oating structures. Applying methods of nonlinear dynamics theory for analyzing real systems results in the need of exact models on the one hand and the limitation of the methods with respect to large systems on the other hand. Changing the model equations for the uid structure interaction between ship and water from an order 300 system to an order 12 system by using the more exact formulation of Cummins [1] lead to faster and more detailed simulation results, but this model does not meet the requirements for using the classical path following techniques. Therefore, a new kind of model has to be invented that has to be as exact as possible and that will meet the requirements for using methods of nonlinear dynamics theory. For developing the new models a better understanding of the capsize phenomenon induced by large roll motions is necessary. Therefor and in order to validate the new model a special experimental setup was designed. The constraint for this setup is the limited size of the institute’s wave tank, 15m x 1.5m x 1.1m. Only roll-, heave- and pitchnmotion can be allowed, otherwise the oating body would hit the walls. Therefore a rst prototype (1.37m x 0.46m x 0.41m, 127.9kg) was build (Fig. 3) to develop a dynamic positioning system that is able to suppress the surge-, sway- and yawnmotions.
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Dynamics of Ship–Motion
PAMM, 2003Co-Authors: Edwin Kreuzer, Marc-andré PickAbstract:The ship capsizing problem is one of the major challenges in naval architecture. The IMO criterion regarding capsize stability is still the Righting Lever curve of static stability calculated for calm water. For the prediction of large–amplitude motions the dynamic loads have to be included. The capsizing of a ship in regular waves is resulting from a sequence of bifurcations in the ship's motion: The determination of bifurcations is possible using path-following techniques of nonlinear dynamics. Existing tools are, however, without adaption not readily applicable for the determination of bifurcations. The main and until now unsolved problem is the necessity of including memory integrals to describe the ship hydrodynamics. First results with simple algorithms show two different scenarios leading to capsizing due to increasing wave amplitudes.
