Inversion of shear wave interval velocity on prestack converted wave data

Seismic velocity is important to migration of seismic data, interpretation of lithology and lithofacies as well as prediction of reservoir. The information of shear wave velocity is required to reduce the uncertainty for discriminating lithology, identifying fluid type in porous material and calculating gas saturation in reservoir prediction. Based on Zoeppritz equations, a numeral and scanning method was proposed in this paper. Shear wave velocity can be calculated with prestack converted wave data. The effects were demonstrated by inversion of theoretical and real seismic data.

An Experimental Study on A Trapezoidal Pendulum Wave Energy Converter in Regular Waves

Experimental studies were conducted on a trapezoidal pendulum wave energy converter in regular waves. To obtain the incident wave height, the analytical method （AM） was used to separate the incident and reflected waves propagating in a wave flume by analysing wave records measured at two locations. The response amplitude operator （RAO）, primary conversion efficiency and the total conversion efficiency of the wave energy converter were studied; furthermore, the power take-off damping coefficients corresponding to the load resistances in the experiment were also obtained. The findings demonstrate that the natural period for a pendulum wave energy converter is relatively large. A lower load resistance gives rise to a larger damping coefficient. The model shows relatively higher wave energy conversion efficiency in the range of 1.0-1.2 s for the incident wave period. The maximum primary conversion efficiency achieved was 55.5%, and the maximum overall conversion efficiency was 39.4%.

不规则波中多体三浮子波能转换器的优化设计

A multi-body wave energy converter,consisting of three floats and modeled as a two body problem,is optimised to enhance its mean absorbed power using the Response Surface Optimisation Method.The optimisation focuses on two input parameters namely;the floats’diameters and the spacing,in various sea states and at different PTO dampings.A frequency domain analysis is performed for the WEC model scaled at 1∶50 in regular and irregular waves.Obtained results are validated against numerical and experimental data available in the literature.Validations show good agreement against the unmoored model’s added mass,radiation damping,response amplitude operator,mean absorbed power and,capture width ratio.The sea states selected for optimisation are represented by a JONSWAP wave spectrum with,a range of significant wave heights(0.04 to 0.06 m)and a range of peak periods(0.8 to 1.3 s).This corresponds to(2 to 3 m)significant wave heights and(5.6 to 9.2 s)peak periods in full scale.Results show that the optimised WEC model demonstrates good and consistent enhancement of its mean absorbed power and capture width ratio.

Hydrodynamic Performance and Power Absorption of A Coaxial DoubleBuoy Wave Energy Converter

As an important wave energy converter(WEC),the double-buoy device has advantages of wider energy absorption band and deeper water adaptability,which attract an increasing number of attentions from researchers.This paper makes an in-depth study on double-buoy WEC,by means of the combination of model experiment and numerical simulation.The Response Amplitude Operator(RAO)and energy capture of the double-buoy under constant power take-off(PTO)damping are investigated in the model test,while the average power output and capture width ratio(CWR)are calculated by the numerical simulation to analyze the influence of the wave condition,PTO,and the geometry parameters of the device.The AQWA-Fortran united simulation sy stem,including the secondary developme nt of AQWA software coupled with the flowchart of the Fortran code,models a new dynamic system.Various viscous damping and hydraulic friction from WEC system are measured from the experimental results,and these values are added to the equation of motion.As a result,the energy loss is contained in the final numerical model the by united simulation system.Using the developed numerical model,the optimal period of energy capture is identified.The power capture reaches the maximum value under the outer buoy's natural period.The paper gives the peak value of the energy capture under the linear PTO damping force,and calculates the optimal mass ratio of the device.

Effect of Different Raft Shapes on Hydrodynamic Characteristics of the Attenuator-Type Wave Energy Converter

A numerical simulation method based on CFD has been established to simulate the fully coupled motion for an atten-uator-type wave energy converter(WEC).Based on this method,a detailed parametric analysis has been conducted to investigate the design of the rafts.The effects of different parameters(wave parameters,structural parameters and PTO parameters)on the hydrodynamic characteristics of the attenuator-type WEC were studied in detail.The results show that in terms of wave parameters,there is an optimal wave period,which makes the relative pitching angle amplitude of the WEC reach the maximum,and the increase of wave height is conducive to the relative pitching angle amplitude of wave energy.Under different wave conditions,the relative pitch angle of the parallelogram raft device is the maximum.In terms of structural parameters,the parallelogram attenuator-type device has the optimal values in different relative directions,different distances and different apex angle,which makes the relative motion amplitude of the device reach the maximum,and the spacing and the apex angle have influence on the motion frequency of the device,while the relative direction has almost no influence on it.In terms of PTO parameters,there is an optimal damping coefficient,which makes the power generation efficiency of the WEC reach the maximum.The research results provide a valuable reference for future research and design of the attenuator-type WEC.

Design of A Hydraulic Power Take-off System for the Wave Energy Device with An Inverse Pendulum

This paper describes a dual-stroke acting hydraulic power take-off （PTO） system employed in the wave energy converter （WEC） with an inverse pendulum. The hydraulic PTO converts slow irregular reciprocating wave motions to relatively smooth, fast rotation of an electrical generator. The design of the hydraulic PTO system and its control are critical to maximize the generated power. A time domain simulation study and the laboratory experiment of the full-scale beach test are presented. The results of the simulation and laboratory experiments including their comparison at full-scale are also presented, which have validated the rationality of the design and the reliability of some key components of the prototype of the WEC with an inverse pendulum with the dual-stroke acting hydraulic PTO system.

Fully Nonlinear Time Domain Analysis for Hydrodynamic Performance of An Oscillating Wave Surge Converter

The hydrodynamic behaviour of an oscillating wave surge converter(OWSC) in large motion excited by nonlinear waves is investigated. The mechanism through which the wave energy is absorbed in the nonlinear system is analysed. The mathematical model used is based on the velocity potential theory together with the fully nonlinear boundary conditions on the moving body surface and deforming free surface. The problem is solved by the boundary element method. Numerical results are obtained to show how to adjust the mechanical properties of the OWSC to achieve the best efficiency in a given wave, together with the nonlinear effect of the wave height. Numerical results are also provided to show the behaviour of a given OWSC in waves of different frequencies and different heights.

Dynamic Properties and Energy Conversion Efficiency of A Floating Multi-Body Wave Energy Converter

The present study proposed a floating multi-body wave energy converter composed of a floating central platform,multiple oscillating bodies and multiple actuating arms. The relative motions between the oscillating bodies and the floating central platform capture multi-point wave energy simultaneously. The converter was simplified as a forced vibration system with three degrees of freedom, namely two heave motions and one rotational motion. The expressions of the amplitude-frequency response and the wave energy capture width were deduced from the motion equations of the converter. Based on the built mathematical model, the effects of the PTO damping coefficient, the PTO elastic coefficient, the connection length between the oscillating body and central platform, and the total number of oscillating bodies on the performance of the wave energy converter were investigated. Numerical results indicate that the dynamical properties and the energy conversion efficiency are related not only to the incident wave circle frequency but also to the converter’s physical parameters and interior PTO coefficients. By adjusting the connection length, higher wave energy absorption efficiencies can be obtained. More oscillating bodies installed result in more stable floating central platform and higher wave energy conversion efficiency.

Connection Technology of HPTO Type WECs and DC Nano Grid in Island

Wave energy fluctuating a great deal endangers the security of power grid especially micro grid in island. A DC nano grid supported by batteries is proposed to smooth the output power of wave energy converters（WECs）. Thus, renewable energy converters connected to DC grid is a new subject. The characteristics of WECs are very important to the connection technology of HPTO type WECs and DC nano grid. Hydraulic power take-off system（HPTO） is the core unit of the largest category of WECs, with the functions of supplying suitable damping for a WEC to absorb wave energy, and converting captured wave energy to electricity. The HPTO is divided into a hydraulic energy storage system（HESS） and a hydraulic power generation system（HPGS）. A primary numerical model for the HPGS is established in this paper. Three important basic characteristics of the HPGS are deduced, which reveal how the generator load determines the HPGS rotation rate. Therefore, the connector of HPTO type WEC and DC nano grid would be an uncontrollable rectifier with high reliability, also would be a controllable power converter with high efficiency, such as interleaved boost converter-IBC. The research shows that it is very flexible to connect to DC nano grid for WECs, but bypass resistance loads are indispensable for the security of WECs.

Power Maximization of A Point Absorber Wave Energy Converter Using Improved Model Predictive Control

This paper considers controlling and maximizing the absorbed power of wave energy converters for irregular waves. With respect to physical constraints of the system, a model predictive control is applied. Irregular waves’ behavior is predicted by Kalman filter method. Owing to the great influence of controller parameters on the absorbed power, these parameters are optimized by imperialist competitive algorithm. The results illustrate the method’s efficiency in maximizing the extracted power in the presence of unknown excitation force which should be predicted by Kalman filter.

CFD Simulation and Experimental Study of a New Elastic Blade Wave Energy Converter

Small moving vehicles represent an important category of marine engineering tools and devices(equipment)typically used for ocean resource detection and maintenance of marine rights and interests.The lack of efficient power supply modes is one of the technical bottlenecks restricting the effective utilisation of this type of equipment.In this work,the performance characteristics of a new type of elastic-blade/wave-energy converter(EBWEC)and its core energy conversion component(named wave energy absorber)are comprehensively studied.In particular,computational fluid dynamics(CFD)simulations and experiments have been used to analyze the hydrodynamics and performance characteristics of the EBWEC.The pressure cloud diagrams relating to the surface of the elastic blade were obtained through two-way fluid-solid coupling simulations.The influence of blade thickness and relative speed on the performance characteristics of EBWEC was analyzed accordingly.A prototype of the EBWEC and its bucket test platform were also developed.The power characteristics of the EBWEC were analyzed and studied by using the blade thickness and motion cycle as control variables.The present research shows that the EBWEC can effectively overcome the performance disadvantages related to the transmission shaft torque load and power curve fluctuations of rigid blade wave energy converters(RBWEC).

The Assessment of Ocean Wave Energy Along the Coasts of Taiwan

The wave energy resource around the coasts of Taiwan is investigated with wave buoy data covering a 3-year period （2007-2009）. Eleven study sites within the region bounded by the 21.5^°N-25.5°N latitudes and 118°E-122°E longitudes are selected for analysis. The monthly moving-average filter is used to obtain the low-frequency trend based on the available hourly data. After quantifying the wave power and annual wave energy, the substantial resource is the result of Penghu buoy station, which is at the northeastern side of Penghu Island in the Taiwan Strait. it is investigated that the Penghu sea area is determined to be the optimal place for wave energy production according to its abundant resource of northeasterly monsoon waves, sheltering of the Taiwan Island, operation and maintenance in terms of seasonal conditions, and constructability of wave power devices.

Application of Amplitude Ratio Profiles Extracted with Prestack Kirchhoff Integral Migration

The Z component and X component profiles of seismic waves extracted with the prestack Kirchhoff integral migration could approximate to the primary wave （P wave） and converted shear wave （PS wave） profiles under certain conditions. The relative change of their reflection amplitude reflects the formation stress anomaly and subsurface media anisotropy. The principle and method for extracting amplitude ratios were studied and the application of amplitude ratio profiles was also examined when processing and interpreting actual seismic data. The amplitude ratio profile is an effective supplementary means of identifying the stratigraphic boundary and lithology.

Study of Hydrodynamic Characteristics of A Sharp Eagle Wave Energy Converter

According to Newton＇s Second Law and the microwave theory, mechanical analysis of multiple buoys which form Sharp Eagle wave energy converter （WEC） is carried out. The movements of every buoy in three modes couple each other when they are affected with incident waves. Based on the above, mechanical models of the WEC are established, which are concerned with fluid forces, damping forces, hinge forces, and so on. Hydrodynamic parameters of one buoy are obtained by taking the other moving buoy as boundary conditions. Then, by taking those hydrodynamic parameters into the mechanical models, the optimum external damping and optimal capture width ratio are calculated out. Under the condition of the optimum external damping, a plenty of data are obtained, such as the displacements amplitude of each buoy in three modes （sway, heave, pitch）, damping forces, hinge forces, and speed of the hydraulic cylinder. Research results provide theoretical references and basis for Sharp Eagle WECs in the design and manufacture.

The S to P convert wave from the bottom of sediment basin in the near-field seismic records

From the near-field records of aftershock of Datong earthquakes in October 1989 and March 1991,an extra phase between P and S arrivals is found. High-precision epicenter location shows that some of these records are obtained with the incidental angle less than the critical angle. This excludes the possibility that the extra wave phase is a refractive wave from ground surface. Particle motion analysis shows that the characteristic of the extra wave is similar to that of wave, so it is possible that the extra phase is an S to P convert wave from the bottom of sediment basin. Suppose a low velocity layer covers a high velocity basement. Successful simulation by the synthetic seismogram confirms that the extra phase is an S-P convert wave from the interface of the basin bottom. Modifying the depth of interface at each ray path to match the waveform, we obtain an interface distribution in space. In this way, a brief bottom image could be shown, and the Datong basin has a 'V' shaped bottom.

Experimental Study on A Pendulum Wave Energy Converter

Many of the existing wave energy converters （WEC） are of oscillating water column （OWC） and point absorber （PA） types. Fewer references have been published in public on the pendulum type WEC. A series of experimental tests on a bottom-hinged pendulum WEC model are carded out and some results are revealed in the present study. The purpose of this paper is to present a detailed description of the tests. It is found that wave energy conversion efficiency varies with the applied damping and wave conditions. In addition, special attention is given to the effect of the water ballast on the efficiency of the wave energy converter. It is demonstrated that the ballast plays an important role in energy extraction. Better understanding on how the performance of the device is influenced by damping, wave height, wave period and ballast is shown.

Will oscillating wave surge converters survive tsunamis?

With an increasing emphasis on renewable energy resources, wave power technology is becoming one of the realistic solutions. However, the 2011 tsunami in Japan was a harsh reminder of the ferocity of the ocean. It is known that tsunamis are nearly undetectable in the open ocean but as the wave approaches the shore its energy is compressed, creating large destructive waves. The question posed here is whether an oscillating wave surge converter （OWSC） could withstand the force of an incoming tsunami. Several tools are used to provide an answer： an analytical 3D model developed within the framework of linear theory, a numerical model based on the non-linear shallow water equations and empirical formulas. Numerical results show that run-up and draw-down can be amplified under some circumstances, leading to an OWSC lying on dry ground t

双重卡口机构对波浪能转换器性能的影响评估

Lower efficiencies induce higher energy costs and pose a barrier to wave energy devices'commercial applications.Therefore,the efficiency enhancement of wave energy converters has received much attention in recent decades.The reported research presents the double snap-through mechanism applied to a hemispheric point absorber type wave energy converter(WEC)to improve the energy absorption perfomance.The double snap-through mechanism comprises four oblique springs mounted in an X-configuration.This provides the WEC with different dynamic stability behaviors depending on the particular geometric and physical parameters employed.The efficiency of these different WEC behaviors(linear,bistable,and tristable)was initially evaluated under the action of regular waves.The results for bistable or tristable responses indicated significant improvements in the WEC's energy capture efficiency.Furthermore,the WEC frequency bandwidth was shown to be significantly enlarged when the tristable mode was in operation.However,the corresponding tristable trajectory showed intra-well behavior in the middle potential well,which induced a more severe low-energy absorption when a small wave amplitude acted on the WEC compared to when the bistable WEC was employed.Nevertheless,positive effects were observed when appropriate initial conditions were imposed.The results also showed that for bistable or tristable responses,a suitable spring stiffness may cause the buoy to oscillate in high energy modes.

A novel method for predicting the power outputs of wave energy converters

This paper focuses on realistically predicting the power outputs of wave energy converters operating in shallow water nonlinear waves. A heaving two-body point absorber is utilized as a specific calculation example, and the generated power of the point absorber has been predicted by using a novel method（a nonlinear simulation method） that incorporates a second order random wave model into a nonlinear dynamic filter. It is demonstrated that the second order random wave model in this article can be utilized to generate irregular waves with realistic crest–trough asymmetries, and consequently, more accurate generated power can be predicted by subsequently solving the nonlinear dynamic filter equation with the nonlinearly simulated second order waves as inputs. The research findings demonstrate that the novel nonlinear simulation method in this article can be utilized as a robust tool for ocean engineers in their design, analysis and optimization of wave energy converters.

Four-quadrant Force Control with Minimal Ripple for Linear Switched Reluctance Machines

Linear switch reluctance machine(LSRM)has been tried to act as an alternative generator for direct drive linear wave energy converter(WEC).Many researchers have proposed new topologies of LSRM to improve the power density,efficiency and reliability.However,the control methods for LSRM applied in direct drive WEC have been paid little attention,especially control methods considering the wave energy generator operating characteristics.In this paper,according to the generator control requirements of the direct drive WEC,force control algorithm for LSRM operating in four quadrants without a speed closed loop is put forward.The force ripple of LSRM is suppressed using force sharing function method.The four-quadrant control is easy to realize requiring only phase currents information.Simulation results validate the proposed method and indicate that LSRM is able to be used as the generator for direct drive WEC.