Irregular Sea Research Articles

On the equivalence of unidirectional rogue waves detected in periodic simulations and reproduced in numerical wave tanks

This paper deals with the reproduction of unidirectional extreme events in a numerical wave basin. From a rogue wave measurement at a given location, experiments or numerical simulations are conducted with the same wave profiles using reproduction procedures. Although it is recognized that many different physical mechanisms may be at play in freak wave formation, reproduction procedures generally use frequency focusing to generate these high waves. This paper intends to assess the validity of this approach. In particular, it will focus on the relationship between an accurate reproduction of the free surface elevation and the accuracy of the corresponding wave kinematics inside the fluid domain. A highly nonlinear model is used to compute the occurrence of unidirectional freak waves in large periodic domain, which are reproduced in a 2-D numerical wave tank and compared. The study presents an advanced reproduction procedure that deals with rogue waves embedded in an irregular sea state. It shows that, using the appropriate reproduction procedure, it is possible to accurately reproduce free surface elevation as well as kinematics induced by this extreme event.

Dynamic stability of ships in regular and irregular seas - An overview

Dynamic stability of ships is discussed with focus on the stability of intact ships in waves. Some of the main developments along the years are scrutinized. Regular and irregular conditions are addressed. An overview is presented, with special emphasis on the covering of the last STAB 2015 Conference.

Early detection of parametric roll by application of the incremental real-time Hilbert–Huang Transform

This work focuses on the early detection and advance warning of parametric roll in regular and irregular head seas. The time–frequency dependent analysis technique, Hilbert–Huang Transform (HHT) is applied to acquire the so-called “instantaneous frequency”, which can be used for the detection of the non-linear parametric roll motion. Firstly, an incremental real-time HHT (IR-HHT) technique is implemented to an artificial time series which is simply composed from the parametric roll and pitch motion time series with its main characteristics: frequency shift and amplitude growth. This is followed by the proposal of a detection scheme aimed at the identification of the frequency shift and amplitude growth between roll and pitch motion. The proposed detection scheme is validated by analyzing parametric roll time series obtained from numerical simulation in regular and irregular head seas. Results show that the onset of parametric roll can be detected successfully through the detection scheme based on IR-HHT technique. Moreover the robustness of the detection scheme is validated.

Oil droplets transport due to irregular waves: Development of large-scale spreading coefficients

The movement of oil droplets due to waves and buoyancy was investigated by assuming an irregular sea state following a JONSWAP spectrum and four buoyancy values. A technique known as Wheeler stretching was used to model the movement of particles under the moving water surface. In each simulation, 500 particles were released and were tracked for a real time of 4.0h. A Monte Carlo approach was used to obtain ensemble properties. It was found that small eddy diffusivities that decrease rapidly with depth generated the largest horizontal spreading of the plume. It was also found that large eddy diffusivities that decrease slowly with depth generated the smallest horizontal spreading coefficient of the plume. The increase in buoyancy resulted in a decrease in the horizontal spreading coefficient, which suggests that two-dimensional (horizontal) models that predict the transport of surface oil could be overestimating the spreading of oil.

Experimental study on hydrodynamic characteristics of vertical-axis floating tidal current energy power generation device

To study the characteristics of attenuation, hydrostatic towage and wave response of the vertical-axis floating tidal current energy power generation device (VAFTCEPGD), a prototype is designed and experiment is carried out in the towing tank. Free decay is conducted to obtain attenuation characteristics of the VAFTCEPGD, and characteristics of mooring forces and motion response, floating condition, especially the lateral displacement of the VAFTCEPGD are obtained from the towing in still water. Tension response of the #1 mooring line and vibration characteristics of the VAFTCEPGD in regular waves as well as in level 4 irregular wave sea state with the current velocity of 0.6 m/s. The results can be reference for theoretical study and engineering applications related to VAFTCEPGD.

Assessing the Impact of a Slow Steaming Approach on Reducing the Fuel Consumption of a Containership Advancing in Head Seas

The slow steaming approach is increasing in popularity for commercial vessels, as it provides a method of reducing fuel use, and therefore operating costs, in the current economic climate. It is very important to be able to evaluate a ship's response to waves, because any added resistance or loss of speed may cause delays or course alterations, leading to financial losses. Potential flow theory based linear strip theory is still a widely used method among naval architects, due to its fast solutions with sufficient engineering accuracy. The key objective of this study is to predict the ship motions and added resistance of the S-175 containership, and to estimate the increase in effective power and fuel consumption due to its operation in regular and irregular head seas. The analyses were performed at design and slow steaming speeds, for a range of wave conditions in regular seas, and for three different sea states in irregular seas. The results obtained at a ship speed corresponding to Froude number 0.25 were compared to available experimental data and were found to be in good agreement with the experiments. The numerical analyses were carried out using VERES, which is based on potential flow theory.

Multi-agent Simulation of Passenger Evacuation from a Damaged Ship under Storm Conditions

We present a multi-agent model for the simulation of evacuation processes considering ship motions and a method for modeling crowd dynamics. To take into account all aspects of the specifics of evacuation in storm conditions, an information model has been developed. This model is based on three interrelated processes: sea waves dynamics, ship motions under the influence of sea, crowd dynamics affected by ship motions. In our research, we developed a combined method for simulating agents’ movements on the inclined decks of the ship. Our approach combines the well-known implementation of the Social Force model with the possibility of collisions with obstacles. Depending on the specific requirements, it is possible to use various models for ship dynamics in irregular seas. To better support this versatility, a distributed test bench based on the CLAVIRE cloud platform was developed for simulation of passenger evacuation and testing simulations were carried out. The obtained results demonstrate that the developed simulation system could be used for designing contingency plans to assist crew members in the framework of decision support systems (DSS).

STUDY OF PARABOLIC EQUATION METHOD FOR MILLIMETER-WAVE ATTENUATION IN COMPLEX METEOROLOGICAL ENVIRONMENTS

The parabolic equation (PE) method for estimating propagation characteristics of millimeter wave, which takes into account of attenuation caused by complex meteorological environment, is proposed. The meteorological environment is treated as a mixture composed of hydrometeors and atmospheric gases. Effective permittivity of the mixture is considered in this paper. Based on the effective permittivity, the PE model for estimating propagation attenuation of millimeter wave is developed via modifying the refractive index. Finally, the model is employed to simulate the propagation characteristics of millimeter wave in complex geographical environments of irregular terrain and rough sea surface, and in complex meteorological environments of standard atmosphere, rain and fog.

Motion Prediction of Catamaran with a Semisubmersible Bow in Wave

Abstract Compared with standard vessels, a slender catamaran with a semi-submerged bow (SSB) demonstrates superior seakeeping performance. To predict the motion of an SSB catamaran, computational fluid dynamics methods are adopted in this study and results are validated through small-scale model tests. The pitch, heave, and vertical acceleration are calculated at various wavelengths and speeds. Based on the overset grid and motion region methods, this study obtains the motion responses of an SSB catamaran in regular head waves. The results of the numerical studies are validated with the experimental data and show that the overset grid method is more accurate in predicting the motion of an SSB catamaran; the errors can be controlled within 20%. The movement data in regular waves shows that at a constant speed, the motion response initially increases and then decreases with increasing wavelength. This motion response peak is due to the encountering frequency being close to the natural frequency. Under identical sea conditions, the motion response increases with the increasing Froude number. The motion prediction results, that derive from a short-term irregular sea state, show that there is an optimal speed range that can effectively reduce the amplitude of motion.

Statistical analysis of planing hull motions and accelerations in irregular head sea

The paper focuses on the analysis of experimental seakeeping data in irregular head sea relative to constant deadrise and warped model representative of current design trends for planing hull forms. Tests have been performed at four volumetric Froude numbers ranging from 2 to 4 in one sea state. Due to non-linear nature of high-speed craft dynamics, measured motions and accelerations have been statistically analysed with the aim to find the best-fit distributions. Obtained results for heave and pitch confirm that Rayleigh and Cartwright distributions can be applied with good confidence for statistical analysis of heights and maxima/minima respectively. Weibull distribution was found to be the best-fit for statistical analysis of vertical acceleration maxima at CG and bow. Finally, the mean of 1/3rd highest vertical accelerations for both models have been compared with Fridsma’s data showing the covered range of velocities and sea states. The procedure for model-ship correlation is shown to make the results available in design practices.

Towards an improved critical wave groups method for the probabilistic assessment of large ship motions in irregular seas

A novel approach for the systematic construction of wind-generated, high probability, wave groups, is presented. The derived waveforms originate from a Markov chain model allowing for the incorporation of cross-correlations between successive wave heights and periods. Analytical expressions of the transition probability distributions are provided in terms of copulas. Rank correlations are estimated from an envelope-process-based approach. The Karhunen–Loève theorem is employed in order to construct the continuous analogs of discrete height and period successions. The method seems to predict well the expected wave heights. The period predictions are conservative, yet they follow the trends of simulated wave trains. Comparisons with predictions of the “Quasi-Determinism” theory for very high runs indicate good coincidence. The derived wave groups are intended to be used for the assessment of ship stability in irregular seas.

Heterochrony and heterotopy in the phylogeny of sea urchins

The role of heterochrony is evident in ontogeny and phylogeny of irregular (exocyclic) sea urchins. After metamorphosis, a juvenile passes the stage of regular (endocyclic) sea urchin, in which the periproct is surrounded by plates of the apical system. A shift of the periproct in the area of the fifth interambulacrum occurs in extant taxa at early stages of postlarval development and is accompanied by the reduction of genital plate 5. In some ancient (Jurassic) adult irregular sea urchins, the endocyclic state of the apical system is retained for a long time and the derivative of the fifth genital plate is sometimes observed even in Early Cretaceous species. Considerable transformations in the structure of the lower test surface in members of the order Spatangoida are manifested in changes in the relative positions of plastron plates and ambulacral areas I and V, separation of sternal plates from the labrum, etc. The mechanism of these changes is connected with translocation or “sliding” of sutures of particular plates as a result of nonuniform growth and partial resorption. The study of evolutionary lineages of Cretaceous and Cenozoic sea urchins has shown that the evolution was connected with the directional changes in some morphological characters at late ontogenetic stages. The process was accompanied by either extension, peramorphosis (lineages of the genera Micraster, Infulaster–Hagenowia in the European Province), or the loss of these stages, paedomorphosis (Hemiaster (Bolbaster) lineage, Late Eocene–Middle Miocene of Australia). The phenomena of heterochrony and heterotopy in the development of peripetal, marginal, and lateroanal fascioles in the Late Cretaceous and Paleocene families Hemiasteridae, Schizasteridae, and Paleopneustidae are described. The heterotopy is also illustrated by the example of the development of additional genital pores on ocular plates II and IV of the Middle Jurassic species Pygomalus analis (Disasteroida); its apical system has five pores instead of four. In the Late Cretaceous species Guettaria roccardi (Holasteroida), ocular plates II and IV have two pores each; in the apical system, there are eight genital pores instead of four. In some members of the order Holectypoida, the place of lost genital plate 5 is occupied by a new plate sometimes pierced by a pore, but judging from crystallographic data, it is not homologous to other genital plates. The order Clypeasteroida is characterized by the development of very small pores in both ambulacral and interambulacral fields; they provide passage for numerous accessory tube feet.

Polynomial Chaos and the Heave Motion of a Cylinder in Random Seas

AbstractThe stochastic equation of motion for a heaving cylinder subject to irregular sea waves is solved numerically with the polynomial chaos framework. Thereby, the stochastic forcing is expressed by a truncated Karhunen‐Loève expansion and the hydrodynamic radiation is formulated as a state space model. We obtain the mean and the variance of the cylinder heave motion. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the Prediction of Wave-added Resistance with Empirical Methods

There is continuous research on analytical, numerical, and (semi)empirical methods to predict wave-added resistance. Most of this research focuses on a particular area, like motion-induced wave added resistance, wave-added resistance in short waves, or is limited to head seas only. The practical application of most methods is therefore often limited. Moreover, most methods require detailed information on hull lines and results are rather sensitive to the discretization of those hull lines. Since 2006, MARIN has been investigating the feasibility of empirical methods that do not have those limitations. They only require the main particulars to predict wave-added resistance. Within the Sea Trial Analysis Joint Industry Project (STA-JIP), a method was developed for the correction of wave-added resistance in head seas covering both the motion-induced and the wave reflection-induced component. This method was further refined and extended to all wave directions within the service performance analysis JIP (SPA-JIP) in 2008. This article presents the results of the comparison between the prediction methods and model tests for almost 50 different ships, comprising more than 1500 tests in regular and irregular seas.

Performance Improvement of a Point Absorber Wave Energy Converter by Application of an Observer-Based Control: Results From Wave Tank Testing

In recent years, many advanced control strategies have been proposed for wave energy converters (WECs), which significantly increase the energy conversion rate of the devices. So far, most of these control schemes have only been investigated theoretically using simulations, which, to a great extent, are based on idealized assumptions. On the contrast, far less information regarding the testing of these control schemes in experimental setups can be found. Under such circumstances, the control performance might be compromised by many factors such as model uncertainties, measurement noise, or delay. This paper compares the performance of a spring damper control and an advanced control strategy for a point absorber WEC in wave tank experiments. For two different irregular sea states, the energy conversion of the advanced control, which uses an observer to estimate the wave excitation force, is compared to the performance of a spring damper control. The results show that, with the advanced controller around 13%-14%, more energy could be converted when compared to the spring damper control. Additionally, the observing process was found to be critical in the advanced control. The conducted experiments show that an increase in the observer accuracy holds a theoretical potential to further improve the energy conversion rate by 15%-25%.

Performance analysis and optimization of a box-hull wave energy converter concept

In this paper we consider a wave energy converter concept which is created by linking a box barge to the mechanical reference by linear dampers. The response to incident wave action in terms of power take-off is expressed explicitly as the solution of a linear frequency-domain model. The simplicity of the model combined with the possibility of the application of theory allows for a nested, and so manageable, procedure of optimization. We find that for any geometry, i.e., a combination of e.g. the breadth-to-length and breadth-to-draught aspect ratios of the box, the optimum is characterized by resonance at least in one of the two degrees of freedom, heave or pitch. Furthermore, optimal geometries turn out to be extremal: either long attenuator-type or wide terminator-type devices perform the best. We find also that optimal wavelengths, which are comparable to the device length in case of attenuators, emerge either due to the progressively increasing buoyancy restoring force characteristic, or due to the finite bandwidth of irregular waves. In particular, diffraction forces are more significant under optimal conditions for performance in irregular seas in comparison with conditions necessary for the most intensive displacement response of the free-floating box barge exposed to regular waves.

Large-amplitude time-domain simulation tool for marine and offshore motion prediction

Although linear ship motions theory is sufficiently accurate in predicting the motions of a ship/offshore platform in irregular seas, it cannot capture many of the nonlinear dynamic phenomena (e.g., parametric resonance) which might be very important in the design phase. In order to accurately simulate the motions of a structure, one has to resort to 6 degrees of freedom coupled nonlinear time-domain simulations. This paper presents the theoretical development of such a tool (called SIMDYN) which solves the nonlinear equations of motion while considering the large-amplitude rotations of the body. It also accounts for the nonlinearity of Froude–Krylov and hydrostatic forces. The validity of the developed tool is verified by comparing the predicted motions against the linear theory for small amplitude motions. The paper also shows an example of parametric roll simulation of a container ship to demonstrate the ability of the tool to capture nonlinear phenomena accurately. This tool is then applied to perform 30 3-h coupled simulations of a container ship in head seas, and the resultant time series from each simulation are analyzed to study the ergodicity of parametric roll in irregular longitudinal seas.

0510 浮体式洋上風力発電システムの翼ピッチと発電機トルクの併用操作による出力および浮体動揺制御

To reduce both the power output fluctuation and the platform motion, a feedback control algorithm based on blade pitch and generator torque manipulations for a spar-type floating offshore wind turbine-generator system was developed through numerical analysis using the aeroelastic simulation model (FAST), observed high wind speed data, and irregular sea waves. The blade pitch is collectively manipulated to maintain the generator power at a rated value, and the generator torque is manipulated to dampen the nacelle fore-aft motion. The concept of this control approach is to increase the generator torque in response to the platform motion to the leeward side and to provide a positive damping effect immediately to the platform motion. The numerical simulation showed the effectiveness of the generator torque manipulation and the filter of the nacelle fore-aft speed for the reduction in the platform motion. This feedback control approach had an advantage in not only the power output fluctuation and platform motion but also damage equivalent fatigue load characteristics over previous control approaches.

Stochastic Control of Inertial Sea Wave Energy Converter

The ISWEC (inertial sea wave energy converter) is presented, its control problems are stated, and an optimal control strategy is introduced. As the aim of the device is energy conversion, the mean absorbed power by ISWEC is calculated for a plane 2D irregular sea state. The response of the WEC (wave energy converter) is driven by the sea-surface elevation, which is modeled by a stationary and homogeneous zero mean Gaussian stochastic process. System equations are linearized thus simplifying the numerical model of the device. The resulting response is obtained as the output of the coupled mechanic-hydrodynamic model of the device. A stochastic suboptimal controller, derived from optimal control theory, is defined and applied to ISWEC. Results of this approach have been compared with the ones obtained with a linear spring-damper controller, highlighting the capability to obtain a higher value of mean extracted power despite higher power peaks.

Effects of hydrodynamic interactions and control within a point absorber array on electrical output

A significant role is envisaged for ocean wave energy to meet the different renewable energy targets set by various governments and world bodies. To make use of this potential, the industry will soon be moving from sea testing of individual wave energy converters (WECs) to the deployment of arrays and farms of WECs. The total power extracted by an array of WECs is influenced by the hydrodynamic interactions between them, especially when the WECs are spaced very closely. By control of the power take-off (PTO) forces and moments acting on the WECs within the array, the hydrodynamic interactions between the WECs and the total power extracted by the array can be modified. In this paper, different resistive and reactive PTO control strategies, applied to a time-domain wave-to-wire model of a three-float Danish Wavestar device, are compared. The time-domain modelling approach, as opposed to the frequency-domain, allows the use of constraints on the maximum PTO moment to be applied in order to make the study realistic. In this paper, the effects that PTO control has on the hydrodynamic interactions between the floats and on the total power generated by the device, when placed in a range of irregular sea states, are studied. It was found that the performance of the three-float device improved as the sophistication of the PTO control strategy and the level of hydrodynamic interactions taken into account in the control problem increased. From among the different control strategies tested in this work, fully-coordinated global array control (matrix control) was found to maximise the time-averaged power generated by the array. Fully-coordinated control potentially enables wave farm developers and device designers to explore the opportunities of connecting and maximising energy yields from installations that will be necessary to contribute to meeting the 2020 and 2050 targets for offshore renewable energy.