Real Sea Conditions Research Articles

Hydrodynamic Response of Wave Energy Converters under Complex Sea State<sup><sup></sup></sup>

In real sea state, Ocean energy devices to work hard to stabilize, or even destroyed. Based on Airy wave theory, Movement and force of heaving cylindrical wave-energy converter (WEC) was analyzed under the complex marine state. Then setting the South China coast as the environmental background, and simulating and computing the motion response of the buoy and the mooring system under various sea conditions. The computational data show that complex mechanical response was happened and the mooring system (SM) has excellent mooring performance. The research results have practical significance for heaving cylindrical WEC survival design under real sea conditions.

중량체 적용을 통한 계류선의 설계개선 방안 연구

The purpose of this study was to improve the design of a moorin g line by attaching a lumped mass to it on the seabed. A numerical analysis of the redesigned mooring system is performed to analyze the effect of the weight of the attached lumped mass using the commercial software Orcaflex. The ultimate tension of the mooring system with the lumped mass is compared with that of a bare mooring lin e in the original design. An appropriately designed weight for the lumped mass is found to induce a critical lifted point in the mooring line by floater motion in the ultimate condition to move toward the floater position from the anchor point, while maintaining a similar safety factor for the mooring line. On the other hand, it is shown that excess weight for the lumped mass induces snapping in a mooring line, resulting in low safety factor for the mooring system. The distance between lumped weights is shown to be a minor parameter affecting the safety of a mooring line, although a shorter line has an advantage from an economic point of view. Using the optimal weight for the lumped mass attached to the mooring lin e on a seabed reduces the mooring line length and installation area occupied by a mooring system under real sea conditions.

The use of conditioned axial flow impellers to generate a current in test tanks

It is necessary to build a new generation of current and wave testing tanks to simulate more realistic sea conditions. Methods for wave generation and absorption are well established but those for current generation in this context are less established. One means of producing a current is by using an axial flow impeller. Unfortunately an impeller introduces into the flow unsteady velocities with high shear, strong turbulent fluctuations and hub effects, alongwith the useful thrust. In the experiment presented here honeycomb flow conditioning placed immediately downstream of the impeller is used to reduce the turbulence present in the flow. An Acoustic-Doppler Velocimeter (ADV) is used to measure three velocity components at a rate sufficient to characterise turbulence. A novel experimental arrangement using brush seals allows the ADV to penetrate the duct without compromising the integrity of the duct. A large number of point measurements were used to construct velocity profiles at various positions downstream of the honeycomb. Three different impeller speed settings were tested to investigate wake evolution. The results presented will aid the development of numerical models and increase understanding of the flow downstream of a conditioned impeller.

Probabilistic assessment of parametric instability of a top tensioned riser in irregular waves

A reliability analysis was used to investigate the parametric instability of a top tensioned riser (TTR) operating under irregular sea conditions. In practical applications, the parametric instability evaluation of a riser is a very difficult task, owing to uncertainty of various parameters such as the environmental conditions of the load, the structural geometric parameters, and material properties. Considering the uncertainties of these parameters, it is vital to adopt a probabilistic approach in evaluating the instability. In this work, the Hill equation of a TTR operating under real sea conditions is first derived, and the corresponding stochastic external excitation is obtained using the Pierson–Moskowitz wave spectrum. The effects of various random variables on the parametric instability are studied by a sensitivity analysis. A surrogate model is used to construct the response surface for assessing the reliability of the parametric instability of the riser. The distribution regularity of parametrically unstable cases is examined using the contour of the parametrically excited responses. The effect of three significant uncertain factors on the probability of the parametric stability is investigated using the surrogate model. The proposed approach is demonstrated to be efficient for evaluating the reliability of the parametric instability of a TTR.

Simulation of Ocean Wave-Generated Magnetic Field Disturbance Observed above Sea-Surface Based on Directional Spectrum

Ocean wave have a magnetic field disturbance, caused by the motional induction of sea water moving in the steady main field of Earth. Mass experiment indicates ocean wave-generated magnetic field disturbance can be a major limitation on the performance of airborne magnetic anomaly detection. To check the character of such disturbance observed above sea-surface, a harmonic ocean wave-generated magnetic field disturbance mathematical model based on Weavers monochromatic wave-generated magnetic field model and ocean wave directional spectrum is proposed. Algorithm is presented for real-time simulation of ocean wave-generated magnetic field disturbance corresponding to the proposed mathematical model. Numerical simulations of ocean wave-generated magnetic field disturbance are sampled above sea-surface by a stationary magnetometer and an airborne magnetometer moving steadily along a rectilinear path. Spectrum analysis of the samples is performed. Simulations results indicate that the proposed harmonic ocean wave magnetic field disturbance mathematical model can well-simulate the real sea conditions. Numerical simulations also reveal that there is a Doppler frequency shift with the increase of magnetometer flight speed. Moreover, energy of the magnetic field disturbance is more dispersed and frequency band is wider with the increase of magnetometer flight speed.

Slam occurrences and loads of a high-speed wave piercer catamaran in irregular seas

In order to optimise the structural design of large lightweight high-speed catamarans, a thorough understanding of the structural loads is required. Not only is slam severity an important factor in the structural design, but also the occurrence rates of such events in realistic sea conditions is vital for long-term load estimations. The slamming behaviour of a 2.5-m hydroelastic segmented model representative of an Incat wave piercer catamaran was investigated over a range of realistic irregular sea conditions. The model was instrumented with strain gauges to record centrebow slam loads, pressure transducers, wave probes and linear variable displacement transducers. More than 2000 slam events were identified over 22 test conditions, providing an extensive database of slam events. Slam events were identified from the pressure transducer measurements and two important slam parameters investigated: occurrence rates and slam severity. The slam magnitudes were found to be scattered; numerous outliers were detected with magnitudes up to 4 times the median observed in every tested condition. Slam occurrence rates and severity are generally greater in conditions where motions are large. A slam occurrence threshold was identified by extrapolating the experimentally measured occurrence rates. For a 112-m vessel with speeds between 20 and 38 knots and with a modal wave period of 8.5 s, slams are shown not to occur at significant wave heights less than 1.5 m. The slam loads and occurrence data obtained through scale model testing can be used by high-speed catamaran ferry designers to assist the development of structural load cases and operation limits for future designs.

Vertical mixing layer development

A new generation of current and wave testing tanks is required to simulate more realistic sea conditions at larger scales. One means of producing a current is by using groups of impellers arranged around the perimeter of a circular tank. Each propeller produces a single flow velocity which may be different to its neighbours. These differences can lead to a stepped or curved plan view velocity profile in the test section of the tank where a plug profile is required. It is important to understand what the maximum allowable velocity difference between each impeller can be before the required plug profile in the test section is compromised.The situation where two individual fluid streams combine, leading to a turbulent mixing layer, is found in many applications and is therefore of great interest in wider fluid dynamics.In the experimental work presented a setup is described which combines two water flows at different velocities to create a vertical shear. The evolution of the combined flow is studied using Particle Image Velocimetry (PIV). When analysed, these results lead to an understanding of various aspects of mixing layer flow recovery and how the bulk flow rates and velocity ratio affected them.

Parametric instability prediction in a top-tensioned riser in irregular waves

The parametric instability of a top-tensioned riser (TTR) in irregular waves was predicted based on multi-frequency excitation. As the exploration of oil and gas moves into deeper water and the corresponding platforms encounter continuous wave loads, parametric resonance becomes an increasing challenge for TTR design. TTRs may suffer damage due to excessive stress or significant fatigue because of parametric resonance. Studies of the stability of long slender structures have been limited to regular waves, with an emphasis on explaining parametric resonance corresponding to simplified situations in which the related displacement or tension fluctuation is assumed to be an ideal harmonic signal. Waves are irregular in real sea conditions, therefore, the present study proposes a methodology to investigate the instability properties of TTRs in irregular waves. Hill's equation of a TTR system under multi-frequency excitation is derived based on linear-wave theory using a Pierson–Moskowitz wave spectrum. Analytic results are given for the stability limit of the related Hill's equation and they describe the stability of a TTR under irregular wave excitation. These new limits are compared to Mathieu-type stability limits. The stability diagram for Hill's equation with random phase modulation in multi-frequency excitation is obtained. To evaluate the proposed method, an example of the application of the stability diagram to TTR design is provided. Simulation results are compared under two types of wave excitation: single-frequency excitation and multi-frequency excitation, generated according to specific sea spectra. Parametric stability properties of a TTR system predicted by regular wave and irregular wave theory are different from each other. Although the transition curves both move to the upper zone of the parametric plane and the region of unstable zones shrink when the damping coefficient increases, the impact of damping is different. The results show that it is necessary to predict the parametric stability of TTR using irregular waves. Applying the stability diagram to a TTR in irregular waves shows that the introduction of extra damping to an unstable system can suppress the instability. In addition, the single-frequency excitation method predicts that the lower vibration mode is more likely to be excited, whereas the multi-frequency excitation method predicts that the higher vibration mode is more likely to fall into an unstable zone.

Phenomenology of Doppler forward scatter radar for surface targets observation

In this study, the forward scatter Doppler phase signature formation is analysed to show the rationale for the forward scatter radar in the true sense of the meaning, where a target actually crosses the baseline; so the advantage of the main shadow lobe is taken and, therefore a forward scatter effect occurs to enhance signal to clutter ratio. The modelling approach suggested is based on the consideration of the Doppler phase signature as a result of superposition of the direct path signal and the shadow radiation signal. It is shown that the target signature may be represented as a Doppler signature of a point-like target specified by its trajectory and speed, which is modulated according to forward scatter cross-section of an actual extended target specified by its silhouette at each moment of motion. The proposed model may be recommended to provide matched filtering in coherent processing. Finally, the approach is verified experimentally using calibrated targets with conductive and absorbing coating in the controlled environment and maritime targets in the real sea conditions.

Investigation of mooring stability due to structural modification of barge ship

The primary aim of the present study is to investigate the mooring safety due to the structural modification of barge ship. The modeling of a two-dimensional dynamic has been carried out. And the numerical calculation of mooring forces according to structural modification has also been carried out simultaneously by the use of Matlab. The displacements of surge, heave and pitch have been calculated according to mooring cable length of the barge ship. The input data of motion analysis was also applied as real-sea conditions for the external forces such as wind, current and mooring force. The results showed that mooring force increased to 90kN after conducting structural rebuilding, and the maximum mooring force occurred in length 30m with influence of barge motion. Therefore it was confirmed that unnecessary structural modification could be the reason why barge ship reach to cut the mooring cable and sink down the ship.

Design and analysis of a new AUV’s sliding control system based on dynamic boundary layer

The new AUV driven by multi-vectored thrusters not only has unique kinematic characteristics during the actual cruise but also exists uncertain factors such as hydrodynamic coefficients perturbation and unknown interference of tail fluid, which bring difficult to the stability of the AUV’s control system. In order to solve the nonlinear term and unmodeled dynamics existing in the new AUV’s attitude control and the disturbances caused by the external marine environment, a second-order sliding mode controller with double-loop structure that considering the dynamic characteristics of the rudder actuators is designed, which improves the robustness of the system and avoids the control failure caused by the problem that the design theory of the sliding mode controller does not match with the actual application conditions. In order to avoid the loss of the sliding mode caused by the amplitude and rate constraints of the rudder actuator in the new AUV’s attitude control, the dynamic boundary layer method is used to adjust the sliding boundary layer thickness so as to obtain the best anti-chattering effects. Then the impacts of system parameters, rudder actuator’s constraints and boundary layer on the sliding mode controller are computed and analyzed to verify the effectiveness and robustness of the sliding mode controller based on dynamic boundary layer. The computational results show that the original divergent second-order sliding mode controller can still effectively implement the AUV’s attitude control through dynamically adjusting the sliding boundary layer thickness. The dynamic boundary layer method ensures the stability of the system and does not exceed the amplitude constraint of the rudder actuator, which provides a theoretical guidance and technical support for the control system design of the new AUV in real complex sea conditions.

Simulation of Numerical Wave Based on Fluid Volume Function

This paper is based on the method of volume of fluid function (VOF) and founds numerical wave tank similar to real sea condition using business software CFD and its function of second development, proceeds numerical simulation of linear wave and second-order stokes and compares them with the theoretical value. The numerical result corresponds to the theoretical result, which lays the foundation for further research of interaction between wave and power generation device.

단일 파장의 LED와 CCD를 이용한 유출유 탐지방법

본 연구에서는 분리된 기름-물 혼합물에 대해 단일 파장대역의 LED 광원으로 부터 흡수된 빛이 전자결합소자(CCD)에 투영된 영상을 분석하는 방법을 통해, 물 위에 기름의 존재 유·무와 상태를 파악할 수 있는 광학적 기름 탐지 방법에 대한 연구 결과를 제시한다. 물위에 기름의 존재 유·무와 기름의 두께 정보를 얻기 위해서, 혼합물을 통과하면서 감소되는 빛의 강도를 전자결합소자로 획득한 영상을 통해 빛의 세기를 분석하는 방법을 이용하였으며, 최고의 선택도를 가진 광원의 파장대역을 찾기 위해서 발광다이오드(LED)의 파장대역별로 선택도를 비교하는 실험을 진행하였다. 본 연구에서 사용된 발광다이오드 중 465 nm의 파장대역을 갖는 발광다이오드가 물과 기름의 존재유무 및 기름의 두께에 따라 감소되는 차이가 가장 크다는 결과를 얻을 수 있었다. 또한 실제 해상조건에서의 탐지 가능성을 확인하기 위해 밝기 조건과 수표면의 기울기 조건을 변화하는 실험을 수행하여 제안된 탐지 방법의 적용가능성을 확인하였다. 해당 연구를 통해서 해상에 유출된 기름을 물과 기름이 갖는 특정파장대역의 빛에 대한 흡수도의 차이로 탐지할 수 있는 새로운 광학적 분석 방법론을 제안한다.BR

On Nonlinear Simulation Methods and Tools for Evaluating thePerformance of Ships and Offshore Structures in Waves

This paper describes the development of alternative time domain numerical simulation methods for predicting large amplitude motions of ships and floating structures in response to incoming waves in the frame of potential theory. The developed alternative set of time domain methods simulate the hydrodynamic forces acting on ships advancing in waves with constant speed. For motions’ simulation, the diffraction forces and radiation forces are calculated up to the mean wetted surface, while the Froude‐Krylov forces and hydrostatic restoring forces are calculated up to the undisturbed incident wave surface in case of large incident wave amplitude. This enables the study of the above waterline hull form effect. Characteristic case studies on simulating the hydrodynamic forces and motions of standard type of ships have been conducted for validation purpose. Good agreement with other numerical codes and experimental data has been observed. Furthermore, the added resistance of ships in waves can be calculated by the presented methods. This capability supports the increased demand of this type of tools for the proper selection of engine/propulsion systems accounting for ship’s performance in realistic sea conditions, or when optimizing ship’s sailing route for minimum fuel consumption and toxic gas emissions.

Multi-variate time-series simulation

In this article we present a method for simulating a multi-variate time series via a vector auto regressive moving average (p, q) process. We also carried out two simulation studies to check the performance of the method and applied the methodology to a real sea condition time series. All results show that the method works very well in practice.

Soft and hard computing approaches for real-time prediction of currents in a tide-dominated coastal area

The prediction of tidal currents in the coastal region on a real-time or online basis is useful in taking operation- and planning-related decisions such as towing of vessels and monitoring of oil slick movements. Currently, however, this is done in offline mode on the basis of the statistical method of harmonic analysis involving fitting of harmonic functions to measured data. Alternatively, numerical solutions of hydrodynamic models can also provide spatial and temporal information on currents. Owing to the complex real sea conditions, such methods may not always yield satisfactory results. This paper discusses a few alternative approaches based on the soft computing tools of artificial neural networks (ANNs) and genetic programming (GP), as well as the hard mathematical approaches of stochastic and statistical methods. The suggested schemes use only a univariate time series of currents to forecast their future values. The measurements of coastal currents made at two locations in the Gulf of Khambhat along the west coast of India have been analysed. The current predictions over a time step of 20 min, a few hours, and a day at the specified locations were carried out. It was found that the soft computing schemes of GP and ANN performed better than the traditional hard technique of harmonic analysis in the present application. This work should initiate more application of GP in coastal engineering. Addressing the problem of current predictions in real-time mode based on analysis of observed time series of ocean currents is a specialty of this work.

Design and performance analysis of impulse turbine for a wave energy power plant

Wave energy is the most abundant source of renewable energy in the World. For the last two decades, engineers have been investigating and defining different methods for power extraction from wave motion. Two different turbines, namely Wells turbine and impulse turbine with guide vanes, are most commonly used around the world for wave energy power generation. The ultimate goal is to optimize the performance of the turbine under actual sea conditions. The total research effort has several strands; there is the manufacture and experimental testing of new turbines using the Wave Energy Research Team's (WERT) 0.6 m turbine test rig, the theoretical and computational analysis of the present impulse turbine using a commercial software package and finally the prediction of the performance of the turbine in a representative wave power device under real sea conditions using numerical simulation. Also, the WERT 0.6 m turbine test rig was upgraded with a data acquisition and control system to test the turbine in the laboratory under real sea conditions using the computer control system. As a result, it is proven experimentally and numerically that the turbine efficiency has been raised by 7% by reducing the hub-to-tip ratio from 0.7 to 0.6. Effect of tip clearance on performance of the turbine has been studied numerically and designed tip clearance ratio of 1% has been validated. From the numerical simulation studies, it is computed that the mean conversion efficiency is reduced around 5% and 4.58% due to compressible flow and damping effects inside OWC device. Copyright © 2005 John Wiley & Sons, Ltd.

The noise spectral parameters and the energy of breaking waves experimental study

The process of wave breaking and whitecap creation is one of the most important and least understood phenomena associated with the evolution of the surface gravity waves in the open sea. This process is the main way of energy and momentum transfer between ocean and atmosphere. However, it is very difficult to estimate, under real sea conditions, the frequency of breaking wave events or the fraction of sea surface covered by whitecaps and the amount of dissipated energy produced by wave breaking. A controlled experiment was carried out in the Ocean Basin Laboratory at MARINTEK, Trondheim (Norway). The simulation of random waves of the prescribed spectra provided a very realistic pattern of the sea surface. The number of breaking waves was estimated using photography method and wave staff recording. Acoustic measurements during the experiments were conducted in order to examine the relationship between the noise spectral parameters and both the whitecap coverage and dissipation energy of breaking waves for different types of waves. A comparison of simultaneous video observations, wave staff records of the surface wave above the hydrophones, with the spectral parameters of acoustical signals made it possible to find physical links between processes.

Application of numerical simulation method to predict the performance of wave energy device with impulse turbine

This paper presents the work carried out to predict the behavior of a 0.6 m Impulse turbine with fixed guide vanes with 0.6 hub-to-tip (H/T) ratio under real sea conditions. In order to predict the true performance of the actual Oscillating Water Column (OWC), the numerical technique has been fine tuned by incorporating the compressibility effect. Water surface elevation verses time history based on Pierson Moskowitz Spectra was used as the input data. Standard numerical techniques were employed to solve the non-linear behavior of the sea waves. The effect due to compressibility inside the air chamber and turbine performance under unsteady and irregular flow condition has been analyzed numerically. Considering the quasi-steady assumptions, unidirectional steady flow experimental data was used to simulate the turbine characteristics under irregular unsteady flow conditions. The results show that the performance of this type of turbine is quite stable and efficiency of air chamber and the mean conversion efficiency is reduced around 8% and 5% respectively, due to compressibility inside air chamber.

Effects of Compressibility on the Performance of a Wave‐Energy Conversion Device with an Impulse Turbine Using a Numerical Simulation Technique

This article presents work carried out to predict the behavior of a 0.6 m impulse turbine with fixed guide vanes as compared with that of a 0.6 hub‐to‐tip ratio turbine under real sea conditions. In order to predict the true performance of the actual oscillating water column (OWC), the numerical technique was fine‐tuned by incorporating the compressibility effect. Water surface elevation versus time history was used as the input data for this purpose. The effect of compressibility inside the air chamber and the turbine′s performance under unsteady and irregular flow conditions were analyzed numerically. Considering the quasi‐steady assumptions, the unidirectional steady‐flow experimental data was used to simulate the turbines characteristics under irregular unsteady flow conditions. The results showed that the performance of this type of turbine is quite stable and that the efficiency of the air chamber and the mean conversion efficiency are reduced by around 8% and 5%, respectively, as a result of the compressibility inside the air chamber. The mean efficiencies of the OWC device and the impulse turbine were predicted for 1 month, based on the Irish wave climate, and it was found that the total time period of wave data used is one of the important factors in the simulation technique.