Probabilistic prediction of capsize applied to small high-speed craft

The prediction of capsize of small high-speed craft is highly complex. In this paper, risk analysis is applied to predict the probability of capsize of two types of high-speed craft due to dangerous wave events in the estuary of the Bonny River in the Niger Delta region of Nigeria. Fibreglass reinforced plastic (FRP) hull craft, widely used for transportation, were considered for the prediction. The probability of the craft encountering a dangerous wave situation at a random position at sea and the probability of capsize were calculated using published wave statistics and computer-based analytical models. Results obtained compared favourably with similar analyses carried out by previous investigators for other regions. Various rational and practical ways of reducing the probability of capsize to an acceptable level are presented. From the prediction it was established that human error was a predominant factor in the capsize of vessels.     

On the capsize performance of a discus buoy in deep sea breakers

The mode of capsize of a discus buoy in breaking waves is discussed. The results of model tests are given; these show that a judicious choice of mooring design can substantially reduce the chance of capsize. A comparison is made with the performance of the buoy on station.     

The nonlinear rolling response of a vessel including chaotic motions leading to capsize in regular seas

The rolling motion of a ship has been successfully modelled using a semi-empirical nonlinear differential equation by a number of researchers. Experimental data has been used to model nonlinear damping and righting lever characteristics and comparison with observed behaviour has been reasonably good. The present article describes a numerical, phenomenological approach to analyse this type of behaviour. The stability of the periodic motion, and in particular the possibility of capsize, is explored with reference to qualitative prediction techniques. The appearance of chaotic motions in regular beam seas is a new feature which should be of interest to designers. The inability of traditional quantitative methods, such as the perturbation technique, to detect chaos is a further justification for using numerical simulation guided by dynamical systems theory.     

Numerical simulation of ship stability for dynamic environment

The prediction of ship stability during the early stages of design is very important from the point of vessel’s safety. Out of the six motions of a ship, the critical motion leading to capsize of a vessel is the rolling motion. In the present study, particular attention is paid to the performance of a ship in beam sea. The linear ship response in waves is evaluated using strip theory. Critical condition in the rolling motion of a ship is when it is subjected to synchronous beam waves. In this paper, a nonlinear approach has been tried to predict the roll response of a vessel. Various representations of damping and restoring terms found in the literature are investigated. A parametric investigation is undertaken to identify the effect of a number of key parameters like wave amplitude, wave frequency, metacentric height, etc.     

Overall stability performance of alternative hull forms

In this paper, two alternative corvette hull forms namely, the round bottom and deep V have been comparatively analyzed in terms of intact and damaged stability and roll performance in beam waves. The boat is planned to operate in the Navy's coastal water missions. Even though both forms are considered to comply with the design requirements, slight differences in displacement and principal dimensions are found to be inevitable due to the intrinsic characteristics of the specific forms. First part of the study concentrated on determining the conventional intact and damaged stability qualities whereas the second part was devoted to nonlinear rolling behavior of the vessels in synchronous beam waves. Nonlinear rolling amplitudes are thought to be quite important especially for resonance condition since they tend to peak in capsize conditions. Various combinations of loading conditions and damaged cases are taken into account to analyze the effects of them. Comparative results are presented graphically depicting the advantages/disadvantages of the selected forms over each other.     

Parametric Resonance Analyses for Spar Platform in Irregular Waves

The parametric instability of a spar platform in irregular waves is analyzed. Parametric resonance is a phenomenon that may occur when a mechanical system parameter varies over time. When it occurs, a spar platform will have excessive pitch motion and may capsize. Therefore, avoiding parametric resonance is an important design requirement. The traditional methodology includes only a prediction of the Mathieu stability with harmonic excitation in regular waves. However, real sea conditions are irregular, and it has been observed that parametric resonance also occurs in non-harmonic excitations. Thus, it is imperative to predict the parametric resonance of a spar platform in irregular waves. A Hill equation is derived in this work, which can be used to analyze the parametric resonance under multi-frequency excitations. The derived Hill equation for predicting the instability of a spar can include non-harmonic excitation and random phases. The stability charts for multi-frequency excitation in irregular waves are given and compared with that for single frequency excitation in regular waves. Simulations of the pitch dynamic responses are carried out to check the stability. Three-dimensional stability charts with various damping coefficients for irregular waves are also investigated. The results show that the stability property in irregular waves has notable differences compared with that in case of regular waves. In addition, using the Hill equation to obtain the stability chart is an effective method to predict the parametric instability of spar platforms. Moreover, some suggestions for designing spar platforms to avoid parametric resonance are presented, such as increasing the damping coefficient, using an appropriate RAO and increasing the metacentric height.     

Stability and capsizing analysis of nonlinear ship rolling in wind and stochastic beam seas

Considering the actual seaway condition, stability and capsizing of nonlinear ship rolling system in stochastic beam seas is of significant importance for voyage safety. Safe zone are defined in the phase space plan of the unperturbed Hamilton system to qualitatively distinguish ship motions as capsize and noncapsize. Capsize events are defined by solutions passing out of the safe zone. The probability of such an occurrence is studied by virtue of the random Melnikov function and the concept of phase space flux. In this paper, besides conventional wave excitation, the effect of wind load is also taken into account. The introduction of wind load will lead to asymmetry, in other words, it transforms the symmetric heteroclinic orbits into asymmetric homoclinic orbits. For asymmetric dynamical system, the orbital analytic solutions and its power spectrum are not readily available, and the technique of discrete time Fourier transformation (DTFT) is used. In the end, as verification of theoretical critical significant wave height, capsizing probability contour diagram is generated by means of numerical simulation. The contour diagram shows that these analytical methods provide reliable and predictive results about the likelihood of a vessel capsizing in a given seaway condition.     

A combined numerical–empirical method to calculate finite-time Lyapunov exponents from experimental time series with application to vessel capsizing

This paper presents a method to calculate finite-time Lyapunov exponents (FTLEs) for experimental time series using numerical simulation to approximate the local Jacobian of the system at each time step. This combined numerical–experimental approach to the calculation of FTLE is applicable to any physical system which can be numerically approximated. By way of example, the method is applied to the problem of vessel capsize.     

船舶减纵摇方法研究

针对现有减小船舶纵摇的方法和装置进行总结和分析,并在此基础之上详细介绍一种新型十字型的被动式抗纵摇舵.通过对比分析,抗纵摇舵对于船舶具有明显地减小纵摇的减摇效果.     

An expert system for monitoring dynamic stability of small craft

This paper describes an intelligent monitoring and advisory system for vessel safety. This system uses environmental information obtained from a number of sensors and proposes corrective action based on a rule-base derived from human expertise, experiments, and theoretical research. The architecture of the monitoring and advisory system is described. Capsizing modes of a ship at sea are explained, and the fuzzy expert system used in the decision-making process is detailed. Simulations of various sea conditions leading to capsize are presented, and the vessel response to the corrective action suggested by the advisory system is illustrated     

Time domain modelling of the transient asymmetric flooding of Ro-Ro ships

This work aims at contributing to improve knowledge on transient asymmetric flooding through theoretical and experimental research. First, a time domain theoretical model of ship motions and flooding is described. Results from experimental work are presented evidencing that transient asymmetric flooding may cause the capsizing of a Ro-Ro shaped barge. The theoretical model is used to predict the capsize of the Ro-Ro shaped barge. Reasonable agreement between experimental and theoretical results was found. Finally, a review of the European Gateway accident is given and the theoretical model is applied to the study of this type of accident. The conclusion is that this theoretical model, together with an accurate modelling of the flooding of machinery compartments, reproduces successfully the capsizing of the European Gateway due to transient asymmetric flooding. Therefore, the internal arrangement of Ro-Ro ships should be carefully studied at the design stage in order to avoid this phenomenon.     

Simulation and Analysis of a Gravity Cyh''''nder''''s Stability in Short-Crested Sea

Generally, the reliability of ami -sliding and anti-overturning stability of an isolated gravity cylinder in a certain working period can be evaluated only when the statistical properties of short term stability are given first. The authors used numerical method to simulate the stability state function of a cylinder in short-crested sea, and further to get the probabilitical characteristics of the structure's stability by time domain analysis. The external loads appeared in the state functions include horizontal wave force, lift force and the respective moments, and the loads are correlated by co- spectrum. The numerical method presented in this paper can be used not only to solve short term reliability problem directly, but to calculate and analyse the long term reliability problem as well. For circular cylinders, an example of simulation and analysis is displayed in this paper.     

Breaking wave prediction with boundary elements and finite volumes for use with small boat capsize studies: Convergence and resource requirements

The prediction of extreme breaking waves forms the foundation of many fields of research. The authors have recently completed a study in the capsize and re-righting of sailing yachts using breaking wave prediction to enhance experimental results. As breaking wave prediction is only the beginning of any research program a prediction method is required to be both accurate and computationally inexpensive. This paper describes the investigation of two methods varying in computational demand. It has been concluded that a non-linear free surface boundary element method is immediately realisable with application to a research program requiring a large number of predicted waves. A finite volume approach is realisable, but its engineering application across numerous waves is difficult.     

瘫船稳性第二层薄弱性衡准研究

基于IMO第二代完整稳性草案中瘫船稳性第二层薄弱性衡准的有关研究,分析草案提出的简化计算方法,采用一个单自由度船舶横摇运动微分方程,利用例船CEHIPAR2792数据,编译相应程序,求得不规则横风横浪作用下瘫船横摇运动相应特征,进而计算特定海况下的倾覆概率及特定海区的平均倾覆指数,并与草案值进行对比。分析不同拟合扶正力臂曲线方法和剩余稳性高对计算结果的影响,并对瘫船稳性第二层薄弱性衡准方法进行修改。     

A view on capsizing under direct and parametric wave excitation

The behavior of a ship that is simultaneously heaving, swaying and rolling in waves is examined by using an early developed model by Thompson et al. [Thompson, J. M. T., Rainey, R. C. T. and Soliman, M. S. (1992) Mechanics of ship capsize under direct and parametric wave excitation. Philosophical Transactions of the Royal Society London A 338, 471-490]. A second-order nonlinear differential equation has been used as a mathematical model which is analyzed by using numerical methods to obtain geometrical phase space techniques of nonlinear dynamics. Bifurcation diagrams and Poincaré maps are two of the geometrical techniques used in this study. The result of the calculation is given to allow the naval architect to make a self comparison.     

Spatial and Temporal Patterns in Structure of Macrobenthic Assemblages. A Three-Year Study in the Northern Adriatic Sea in Front of the Po River Delta

Abstract. Coastal benthic communities in soft-bottom deposits of a restricted area were studied by seasonal sampling at nine stations. Faunal assemblages at three different depths were consistently different and exhibited a seasonal pattern of abundance and diversity. Multivariate statistical analyses suggest that each community structure is very similar during the three summer samplings; in autumn and winter many causes of disturbance can disrupt this structure, which is reconstituted the following summer. The stability of benthic communities in this physically variable environment is discussed and a "cyclic" adjustment mechanism of stability is proposed to explain the dynamics of the benthos in this area.     

Dynamic Stability Analysis of Caisson Breakwater in Lifetime Considering the Annual Frequency of Severe Storm

In the dynamic stability analysis of a caisson breakwater, most of current studies pay attention to the motion characteristics of caisson breakwaters under a single periodical breaking wave excitation. And in the lifetime stability analysis of caisson breakwater, it is assumed that the caisson breakwater suffers storm wave excitation once annually in the design lifetime. However, the number of annual severe storm occurrence is a random variable. In this paper, a series of random waves are generated by the Wen Sheng-chang wave spectrum, and the histories of successive and long-term random wave forces are built up by using the improved Goda wave force model. It is assumed that the number of annual severe storm occurrence is in the Poisson distribution over the 50-year design lifetime, and the history of random wave excitation is generated for each storm by the wave spectrum. The response histories of the caisson breakwater to the random waves over 50-year design lifetime are calculated and taken as a set of samples. On the basis of the Monte Carlo simulation technique, a large number of samples can be obtained, and the probability assessment of the safety of the breakwater during the complete design lifetime is obtained by statistical analysis of a large number of samples. Finally, the procedure of probability assessment of the breakwater safety is illustrated by an example.     

Dynamic Stability Analysis of Caisson Breakwater in Lifetime Considering the Annual Frequency of Severe Storm

In the dynamic stability analysis of a caisson breakwater, most of current studies pay attention to the motion characteristics of caisson breakwaters under a single periodical breaking wave excitation. And in the lifetime stability analysis of caisson breakwater, it is assumed that the caisson breakwater suffers storm wave excitation once annually in the design lifetime. However, the number of annual severe storm occurrence is a random variable. In this paper, a series of random waves are generated by the Wen Sheng-chang wave spectrum, and the histories of successive and long-term random wave forces are built up by using the improved Goda wave force model. It is assumed that the number of annual severe storm occurrence is in the Poisson distribution over the 50-year design lifetime, and the history of random wave excitation is generated for each storm by the wave spectrum. The response histories of the caisson breakwater to the random waves over 50-year design lifetime are calculated and taken as a set of samples. On the basis of the Monte Carlo simulation technique, a large number of samples can be obtained, and the probability assessment of the safety of the breakwater during the complete design lifetime is obtained by statistical analysis of a large number of samples. Finally, the procedure of probability assessment of the breakwater safety is illustrated by an example.     

基于模糊集理论的海塘稳定可靠度研究

针对海塘边坡稳定性分析中因忽略参数空间变化等因素产生的分析结果偏于不安全的倾向,提出了一种计算模糊随机可靠度的简化方法.首次引入"正态模糊数"来描述所选重要参数的计算区间,采用简化Bishop法与"模糊顶点法"相结合的方法计算边坡稳定的安全系数密度函数,然后根据大量边坡实例统计结果的构造函数得到安全系数的戒下型隶属函数,最终计算获得边坡稳定的模糊随机可靠度评价.结果分析表明,传统可靠度分析结果偏于不安全,而利用模糊随机可靠度来评价边坡稳定状况更趋于合理性.