Experimental investigation of a moored floating system

Floating structures are generally excited by a more or less irregular sea state. Therefore the structure undergoes a nonlinear dynamical behaviour which results from hydrodynamic effects or the kinematic coupling of different components. The motions of these structures have been analysed intensively with numerical techniques which predict a wide range of nonlinear effects. On the other hand, experiments are still important when it comes to verifying these theoretical findings. Investigating such a floating structure experimentally requires a complex setup: A wave generator needs to be driven in an appropriate way to yield waves with the required characteristics at the structure. The structure itself is usually designed similar to a real–world system and it has to allow for a comparison with numerical analyses. Sensors which measure the tracks of the individual components not only have to reach a prescribed precision but also need to ensure that the motion is not perturbed as a result of the measurements. Lastly, unwanted disturbances have to be avoided. For experiments in a wave tank this includes that reflections have to be minimized. This talk addresses the development of an experimental setup for the investigation of a floating structure. It highlights components which are found to be critical for the obtained accuracy and proposes techniques to reduce experimental errors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental Evidence for Breather Type Dynamics in Freak Waves

Freak or rogue waves on the ocean seemingly appear from nowhere, cause severe damage to ships and offshore structures due to their large crest heights, and disappear at once. Since the Draupner wave measured on New Year's day 1995 finally confirmed the existence of freak waves, different models were developed to describe them. One deterministic model to investigate their occurrence is the nonlinear Schrödinger equation (NLS) describing the nonlinear evolution of wave train envelopes. Due to the modulation instability, also referred to as Benjamin-Feir instability, strong spatial localization of wave amplitude may arise and breather type solutions are hypothesized to form the dynamical back-bone of rogue waves. To test this hypothesis, breather type solutions of the NLS are compared to two extreme wave records: the Draupner wave recorded in central North Sea and the Yura wave recorded in the Sea of Japan. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Generation and dissipation of ocean surface waves

Ocean surface waves constitute one of the most important sources of external forces that act on ships and offshore structures. Most ocean waves are generated by wind, but various other effects such as currents, ground and coastal topology, breaking and wave–wave interaction have an influence on the growth and dissipation of wave energy at specific frequency ranges. These water waves are inherently random in nature and their exact shape is difficult to describe, even when confining the described area to a small range. While many different approaches exist to describe the spectral characteristics of ocean waves, some of the processes which affect the generation of waves are still poorly understood. This paper addresses some techniques which serve to describe seaway spectra with respect to the subsequent analysis of dynamic mechanical systems in the ocean such as ships, platforms and pipelines. Advantages and limits of the different approaches are discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Damage detection in plates by wave propagation analyses

At locations with defects the propagation of elastic waves in solids is disturbed. This behaviour can be used for the development of new structural health monitoring and quality assurance systems. In the sense of adaptive structure systems actuators of piezo–electric materials [1] are implemented to generate high frequency waves (Lamb waves) in plate like structures, [2]. At flaws, imperfections of the structure and surface defects these waves show reflections, refractions and mode conversions. The observation of the wave propagation and the wave behaviour at possible flaws and surface defects is performed by an optical measurement system. Measurement data of the thin walled metallic plates are compared with numerical and analytical solutions. First results of the detection of the wave propagation in metallic plates are presented. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modelling of extreme wave heights and periods through copulas

Optimal design of coastal or offshore structures requires the estimation of extreme quantiles of oceanographic data such as wave heights and wave periods. Since there are strong correlations between oceanographic variables, it is necessary to use multivariate models in order to capture its dependencies. To achieve this, an approach based on copulas is proposed and is compared to a model based on the physical behaviour of waves.     

船舶溢油事故报告选择概率预测研究

事故报告是整个船舶溢油应急过程的触发点,具有关键作用。实际情况中,事故船方常常存在瞒报、不报或不及时报告的情况,导致溢油危害升级。本文旨在探索事故船方在进行事故报告时的选择过程和影响因素,通过对上海、宁波、舟山、大连、天津、厦门六个城市207名海上船舶作业人员的问卷调查,利用离散选择模型得到了影响船方事故报告选择的主要因素,并对其选择概率进行了预测。     

Watershed initial conditions for propagating waves in a system with bistability: A weight-function approach

We consider non-adiabatic combustion waves arising from a two-step exothermic system. Our previous work showed that in certain parameter regions, the combustion wave can evolve to the “fast” solution branch, the “slow” solution branch or diffuse to the ambient temperature (extinction wave). Here, we are interested to find critical initial temperature profiles which evolve to these three types of steady solutions. For a particular family of temperature profiles, we construct a weight function which can be used to predict which of these three types of waves an initial temperature profile will evolve to.     

Numerical Simulation of Free Surface-Structure-Interaction with Multi-Regional Mesh Deformation

The Fluid-Structure-Interactions (FSI) for technological constructions on sea are becoming nowadays very important in view of growing consumption of renewable energy sources. Exceptionally the offshore wind farms are exposed to extreme weather conditions due to breaking and non-breaking wave loads. This work presents numerical Finite-Volume-Method (FVM) analysis of fluid and structure interactions with water waves impact on elastic solid structures. We develop a Free Surface-Structure-Interaction (FSSI) solver in OpenFAOM [1] with multi-regional mesh deformation. A two dimensional numerical water impact model on a damping element prototype of an offshore wind turbin tower is investigated. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of Nonlinear Stochastic Ship Dynamics Under Extended Wave Modeling

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)     

Nonlinear Dynamics of Marine Systems in Random Waves

The dynamics of ships or offshore structures is influenced by several different effects, some of which have a distinctly nonlinear characteristic. Even though in many situations the motion can sufficiently be described by linear models, nonlinear phenomena play a crucial role in the investigation of some more critical operating conditions: Large amplitude motions, sudden jumps in the dynamical behavior and sensitivity to the initial conditions are likely to occur under some circumstances. The response of floating systems such as moored buoys and barges in regular waves can be approximated by analytical or numerical techniques. These analyses reveal the characteristics of different periodic motions. In order to determine how these responses change under a more general forcing, the motion of floating structures under the influence of random disturbances is described by probability distributions. Different mathematical tools can efficiently be applied to models with few degrees of freedom. The localized statistical linearization used here is also promising for larger systems. Modelling aspects of offshore structures and random waves are discussed as well as the determination of probability distributions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

基于贝叶斯网络的飞行不安全事件风险评估研究

飞行不安全事件会导致飞行事故和飞行事故症候,为了保障飞行安全,对飞行不安全事件的风险因素进行风险评估,对风险大的因素进行有效的管理,可以降低飞行不安全事件的发生.首先从飞行不安全事件的责任类别角度,总结了飞行不安全事件的32个风险因素,归属9类责任,建立了基于贝叶斯网络的飞行不安全事件风险评估模型;其次根据2011年1到11月的民用航空不安全事件的统计结果,利用Microsoft Excel软件和编写VB语言程序通过输入风险因素的先验概率,计算出目标事件即飞行不安全事件发生的概率和风险因素的重要性指标值,最后利用贝叶斯网络模型对飞行不安全事件的风险因素进行评价,得出影响较大的风险因素有6个,分别为爆胎/轮胎脱层/扎破、系统失效、通信中断、外来物击伤、雷击、危险接近/飞行冲突.针对影响较大的风险因素,应增强机务部门对航空器轮胎、机载设备系统、通信系统的检修质量、提高对天气观测和预报的准确度、加强机组培训、优化空管指挥方案.     

Transient chaos and crisis phenomena in butterfly valves driven by solenoid actuators

Chilled water systems used in the industry and on board ships are critical for safe and reliable operation. It is hence important to understand the fundamental physics of these systems. This paper focuses in particular on a critical part of the automation system, namely, actuators and valves that are used in so-called “smart valve” systems. The system is strongly nonlinear, and necessitates a nonlinear dynamic analysis to be able to predict all critical phenomena that affect effective operation and efficient design. The derived mathematical model includes electromagnetics, fluid mechanics, and mechanical dynamics. Nondimensionalization has been carried out in order to reduce the large number of parameters to a few critical independent sets to help carry out a broad parametric analysis. The system stability analysis is then carried out with the aid of the tools from nonlinear dynamic analysis. This reveals that the system is unstable in a certain region of the parameter space. The system is also shown to exhibit crisis and transient chaotic responses; this is characterized using Lyapunov exponents and power spectra. Knowledge and avoidance of these dangerous regimes is necessary for successful and safe operation.     

Synchronization effects in rotors partially filled with fluid-influence on propulsion

In fluid-filled Rotors occur self-excited vibrations induced by surface waves of the fluid. A characteristic property is the instability over an interval of angular velocity above the natural frequency of the system. One explanation is the occurrence of synchronization effects between fluid waves and the critical rotor speed. The behaviour of rotors partly filled with fluid was mostly studied under the aspect of stability in steady-state conditions. For non-steady-state investigations, discrete models with reduced number of degrees of freedom and reasonable ability to model the system behaviour are desirable for observer-based real-time control. This paper analyses a model based on a laval rotor and shows synchronization effects between fluid waves and rotor model and its influence on the rotor propulsion. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wave induced vibrations of gravity platforms: a stochastic theory

This paper presents a consistent stochastic method to evaluate wave induced vibrations of offshore platforms with special emphasis on gravity type structures. The wavy sea surface is idealized as a stationary and homogeneous stochastic Gaussian (or semi-Gaussian) field. The wave loading processes are treated by applying the concepts of probabilistic potential theory and the stochastic linearization method. The structure is modelled according to the finite element method using boundary spring-dashpot elements to idealize the soil-foundation system. The linearized equations of motion are solved by the frequency response method accounting properly for the temporal and spatial structure of waves as well as for time dependent system function. Numerical results for a 170m high concrete platform are presented including a parametric study of effects due to the shape of the wave spectral density.     

The random environment of offshore systems

The analysis of the dynamical behavior of systems in ocean waves is an important part in offshore engineering. While a characterization of the response of a linearized model can be obtained in frequency domain, it has to be noted that offshore systems usually include components with nonlinear behavior. The systematic analysis of the nonlinear dynamics of floating structures is often facilitated by additional assumptions. One common example is the use of deterministic (harmonic) waves. Even though periodic waves may be a reasonable simplification for many applications, sea waves in general are usually better described by a spectral or probabilistic approach. This paper addresses different methods of describing random forces for the analysis of floating structures. Examples show the effects of different wave models on the analysis of a simple floating structure. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lamb Wave Interactions with Non-symmetric Features at Structural Boundaries

The paper initially describes on a numerical basis how a Lamb wave would have to perform that has been initiated in a pure mode (either symmetric or anti–symmetric) and what the wave would have to anticipate in terms of mode conversion when being reflected at a surface not perpendicular to its traveling direction. The effects of changing in geometric specifications of non–symmetric artificial features like angle of sloping edge or partially sloping edges are studied. The results obtained from these studies are presented as the reflected and converted parts of the incident wave versus angle of the edge or percentage of the sloped edge. It has been further shown that Lamb waves being generated experimentally by a finite size transducer into a plate like structure thus most likely result in a combination of modes. Reflection of these combined modes at structural boundaries will therefore generate an even more complex coupling of modes. This situation is further aggravated if the structural boundary is not purely perpendicular to the traveling wave but has a slightly varying angle such as it might have to be anticipated at a countersunk rivet, a notch or even more extreme a crack in a metallic component. However from understanding the background of Lamb wave generation, mode separation and superposition, a systematic approach can be established that allows complex Lamb waves, such as they are observed when monitoring true structures, to be interpreted and understood. This approach has been explained on the basis of numerical result obtained from finite element analyses first before proving the findings by some fundamental experiments performed with variable angle beam transducers which demonstrates the difficulties in de–coupling Lamb wave modes and how to handle those coupled modes in terms of structural condition monitoring. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Second order wave loads on large monolithic offshore structures

This paper considers the wave loads on large monolithic offshore structures. The second order wave force formulae developed by Rahman and Heaps, applicable to large circular cylinders in waves, are extended to evaluate the overturning moments on large circular cylinders. The theory is then applied to square section caissons in waves, to predict the wave loads on these structures. These calculations are performed using the exact form of the second order velocity potential, φ₂, with arbitrary wave number, k₂, and the approximate form of φ₂, with twice the value of the wave number of the first order velocity potential. The second order analytical predictions are compared with available experimental data for various ranges of wave parameters for both circular and square caissons in large amplitude waves.     

桩基承台结构的波流力研究进展

我国近海风电场建设大多采用桩基承台结构．总结了不规则波浪和水流共同作用下桩基承台结构的波流力物理模型实验结果,得到了群桩效应系数及其变化规律,讨论了作用于近水面承台底部的波浪拍击力;从理论上分析了规则波作用下承台对桩基波浪力的影响;建立了规则波与桩基承台相互作用的数值模型,揭示了波浪在承台的上浪与爬高及其水动力特征．鉴于桩基承台结构包含多个斜桩和较大尺度的承台,在波浪与水流作用下该结构物附近的流场结构十分复杂,有必要针对结构附近的流动结构以及自由表面大变形开展细致的实验和数值模拟研究,以进一步揭示作用于这类结构的波流力变化规律及其机理.     

Longitudinal waves in partially saturated porous media: the effect of gas bubbles

The problem of the propagation of longitudinal waves in a liquid-saturated porous medium when there are gas bubbles present is considered. The decay factor and the phase velocity of Frenkel–Biot waves of the first and second kind are found as a function of the frequency in the linear approximation. It is shown that, in the neighbourhood of the resonance frequency of the bubbles, longitudinal Frenkel–Biot waves change their form. A wave of the first kind is transformed from a fast wave at low frequencies into a slow wave at high frequencies. The dispersion curve of a wave of the second kind consists of two branches – a “low-frequency” branch, the oscillations of which possess the classical properties, and a “high-frequency” branch, which is a weakly decaying high-velocity mode. The frequency dependences of the ratio of the mass velocities of a gas-liquid mixture and of a porous matrix, and also of the perturbations of the stress in the matrix and the pressure in the mixture, are constructed. It is shown that the “high-frequency” branch of a wave of the second kind is characterized by the in phase motion of the gas-liquid mixture and of the porous matrix, while their mass velocities are close, which explains the weak decay of this mode of oscillations. An analytical expression is obtained for the “boundary frequency”, which determines the offset of the “high-frequency” branch of the dispersion curve of the wave of the second kind.     

Chaos in dissipative relativistic standard maps

The relativistic generalization of the dissipative standard map is introduced, based on the problem of acceleration and heating (or cooling) of charged particles in the electric field of an electromagnetic wave packet. The question arises as to how the relativistic effects change the nonlinear dynamics described by a dissipative standard map. It is shown that the dissipation modifies the positions of the fixed points, but the origin (the central point) remains identical with that of the corresponding Hamiltonian system. However, the phase-space structure around the origin is drastically modified even if a small dissipation is present. The formation of an “ordered” stochastic structure which is not washed out (in the stochastic sea) for longer times shows that the phase mixing is weak and the nonuniformity of the stochastic acceleration increases because of the dissipation. A new type of stochastic attractor of a higher order is found by numerical simulations. In the context of a scaling-law hypothesis (or renormalization group approach), the transition stochastic sea (high acceleration of relativistic particles)–stochastic attractor (low acceleration) is similar to a Bose–Einstein condensation (or, simply, a condensation gas–liquid) at low temperatures, the dissipative parameter being the control parameter for such a transition. The dissipation parameter can also be considered as a time (aging) parameter of the system, and this may have some applications in biological systems. A Frenkel–Kontorova model of the dissipative relativistic standard map (DRSM) and possible applications to “incommensurate fractals” and lattice dynamics of thermoelectric materials are also considered.