Extreme Responses of An Integrated System with A Semi-Submersible Wind Turbine and Four Torus-Shaped Wave Energy Converters in Different Survival Modes

Offshore wind power is a kind of important clean renewable energy and has attracted increasing attention due to the rapid consumption of non-renewable energy.To reduce the high cost of energy,a possible try is to utilize the combination of wind and wave energy considering their natural correlation.A combined concept consisting of a semi-submersible wind turbine and four torus-shaped wave energy converters was proposed and numerically studied under normal operating conditions.However,the dynamic behavior of the integrated system under extreme sea conditions has not been studied yet.In the present work,extreme responses of the integrated system under two different survival modes are evaluated.Fully coupled time-domain simulations with consideration of interactions between the semi-submersible wind turbine and the torus-shaped wave energy converters are performed to investigate dynamic responses of the integrated system,including mooring tensions,tower bending moments,end stop forces,and contact forces at the Column-Torus interface.It is found that the addition of four tori will reduce the mean motions of the yaw,pitch and surge.When the tori are locked at the still water line,the whole integrated system is more suitable for the survival modes.

Studies of TLP Dynamic Response Under Wind, Waves and Current

Investigated is the coupled response of a tension leg platform （TLP） for random waves. Inferred are the mass matrix, coupling stiffness matrix, damping matrix in the vibration differential equation and external load of TLP in moving coordinating system. Infinitesimal method is applied to divide columns and pontoons into small parts. Time domain motion equation is solved by Runge-Kutta integration scheme. Jonswap spectrum is simulated in the random wave, current is simulated by linear interpolation, and NPD spectrum is applied as wind spectrum. The Monte Carlo method is used to simulate random waves and fluctuated wind. Coupling dynamic response, change of tendon tension and riser tension in different sea conditions are analyzed by power spectral density （PSD）. The influence of approach angle on dynamic response of TLP and tendon tension is compared.

An Innovative Approach to Predicting Scour Depth Around Foundations Under Combined Waves and Currents in Large-Scale Tests Based on Small-Scale Tests

This study presents an innovative theoretical approach to predicting the scour depth around a foundation in large-scale model tests based on small-scale model tests under combined waves and currents.In the present approach,the hydrodynamic parameters were designed based on the Froude similitude criteria.To avoid the cohesive behavior,we scaled the sediment size based on the settling velocity similarity,i.e.,the suspended load similarity.Then,a series of different scale model tests was conducted to obtain the scour depth around the pile in combined waves and currents.The fitting formula of scour depth from the small-scale model tests was used to predict the results of large-scale tests.The accuracy of the present approach was validated by comparing the prediction values with experimental data of large-scale tests.Moreover,the correctness and accuracy of the present approach for foundations with complex shapes,e.g.,the tripod foundation,was further checked.The results indicated that the fitting line from small-scale model tests slightly overestimated the experimental data of large-scale model tests,and the errors can be accepted.In general,the present approach was applied to predict the maximum or equilibrium scour depth of the large-scale model tests around single piles and tripods.

Effects of wind input and wave dissipation formulations on the steady Ekman current solution

The effects of different wind input and wave dissipation formulations on the steady Ekman current solution are described. Two formulations are considered: one from the wave modeling(WAM) program proposed by Hasselmann and Komen and the other provided by Tsagareli and Babanin. The solution adopted for our study was presented by Song for the wave-modifi ed Ekman current model that included the Stokes drift, wind input, and wave dissipation with eddy viscosity increasing linearly with depth. Using the Combi spectrum with tail effects, the solutions are calculated using two formulations for wind input and wave dissipation, and compared. Differences in the results are not negligible. Furthermore, the solution presented by Song and Xu for the eddy viscosity formulated using the K-Profi le Parameterization scheme under wind input and wave dissipation given by Tsagareli and Babanin is compared with that obtained for a depth-dependent eddy viscosity. The solutions are further compared with the available well-known observational data. The result indicates that the Tsagareli and Babanin scheme is more suitable for use in the model when capillary waves are included, and the solution calculated using the K-Profi le Parameterization scheme agrees best with observations.

Study on scour around vertical large-size cylinder base due to combined action of wave and current

Based on the mechanism of local scour around vertical large-sized cylinder due to combined action of wave and current,the sour morphology,scour process and the maximum scour depth around the cylinders are studied experimentally.The influence of various ocean environmental parameters on local scour around the cylinder is considered in physical model test.The experimental results indicate that the principal effect factors on the scour in fine-sand seabed are wave height,wavelength,current velocity,ratio of diameter to wavelength and ratio of depth to wavelength when the ratio of cylinder diameter to wavelength is from 0.2 to 0.8.In this paper,dimensional analysis theory is utilized to establish a theoretical equation for forecasting maximum scour depth around large-sized round cylinder base due to the combined action of wave and current.The results computed with the theoretical equation are compared with the experimental results,and found to be in good consistency.The results in this studies can be used to estimate the maximum sour depth around analogous structures.

Concept Design and Coupled Dynamic Response Analysis on 6-MW Spar-Type Floating Offshore Wind Turbine

Tower, Spar platform and mooring system are designed in the project based on a given 6-MW wind turbine. Under wind-induced only, wave-induced only and combined wind and wave induced loads, dynamic response is analyzed for a 6-MW Spar-type floating offshore wind turbine （FOWT） under operating conditions and parked conditions respectively. Comparison with a platform-fixed system （land-based system） ofa 6-MW wind turbine is carried out as well. Results demonstrate that the maximal out-of-plane deflection of the blade of a Spar-type system is 3.1% larger than that of a land-based system; the maximum response value of the nacelle acceleration is 215% larger for all the designed load cases being considered; the ultimate tower base fore-aft bending moment of the Spar-type system is 92% larger than that of the land-based system in all of the Design Load Cases （DLCs） being considered; the fluctuations of the mooring tension is mainly wave-induced, and the safety factor of the mooring tension is adequate for the 6-MW FOWT. The results can provide relevant modifications to the initial design for the Spar-type system, the detailed design and model basin test of the 6-MW Spar-type system.

Poleward-moving recurrent auroral arcs associated with impulse-excited standing hydromagnetic waves

In Earth's high-latitude ionosphere, the poleward motion of east–west elongated auroral arcs has been attributed to standing hydromagnetic waves, especially when the auroral arcs appear quasi-periodically with a recurrence time of a few minutes. The validation of this scenario requires spacecraft observations of ultra-low-frequency hydromagnetic waves in the magnetosphere and simultaneous observations of poleward-moving auroral arcs near the spacecraft footprints. Here we present the first observational evidence from the multi-spacecraft THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission and the conjugated all-sky imager to support the scenario that standing hydromagnetic waves can generate the quasi-periodic appearance of poleward-moving auroral arcs. In this specific event, the observed waves were toroidal branches of the standing hydromagnetic waves, which were excited by a pulse in the solar wind dynamic pressure. Multi-spacecraft measurements from THEMIS also suggest higher wave frequencies at lower L shells (consistent with the distribution of magnetic field line eigenfrequencies), which indicates that the phase difference across latitudes would increase with time. As time proceeds, the enlarged phase difference corresponds to a lower propagation speed of the auroral arcs, which agrees very well with the ground-based optical data.

Hydrodynamic Characteristics of New Floating Wind-Wave Energy Combined Power Generation Devices Under Typhoon-Wave-Current Coupling

The South China Sea is rich in wind and wave energy resources,and the wind-wave combined power generation device is currently in the concept research and development stage.In recent years,extreme sea conditions such as super typhoons have frequently occurred,which poses a serious challenge to the safety of offshore floating platforms.In view of the lack of safety analysis of wind-wave combined power generation devices in extreme sea conditions at present,this paper takes the OC4-WEC combined with semi-submersible wind turbine(Semi-OC4)and the oscillating buoy wave energy converter as the research object,and establishes a mesoscale WRF-SWANFVCOM(W-S-F)real-time coupling platform based on the model coupling Toolkit(MCT)to analyze the spatial and temporal evolution of wind-wave-current in offshore wind farms during the whole process of super typhoon“Rammasun”transit.Combined with the medium/small scale nested method,the flow field characteristics of OC4-WEC platform are analyzed.The results show that the simulation accuracy of the established W-S-F platform for typhoon track is 42.51%higher than that of the single WRF model.Under the action of typhoon-wave-current,the heave motion amplitude of OC4-WEC platform is reduced by 38.1%,the surge motion amplitude is reduced by 26.7%,and the pitch motion amplitude is reduced by 23.4%.

Combined refraction and diffraction models for sea waves over complicated topography and with currents in shallow water

-Combined refraction and diffraction models in the form of linear parabolic approximation are derived through smallparameter method. More strictly theoretical basis and more accuracy in the models than Lozano's (1980) are obtained. Some theoretical defects in Liu's model (1985) with consideration of current are not only found but also eliminated. More strict and accurate models are, therefore, presented in this paper.The calculation results and analysis in applying the models to actual wave field with consideration of bottom friction will be given in the following paper.

Numerical calculation of drift current in wind waves

-Drift current induced by wind and waves is investigated with phase-averaged Navier-Stokes equation in which the Reynolds stress is closed by k-ε model. The governing equations are solved by the finite volume method in a system of nonorthogonal coordinates which is fitted to the phase-averaged wave surface. The predicted drift current is fairly reasonable and the drag coefficient of sea-surface predicted with the newly developed interface conditions shows good agreement with previous measurements when breaking waves do not exist.

NONLINEAR DYNAMICS RESPONSE OF CASING PIPE UNDER COMBINED WAVE-CURRENT

The vortex-induced nonlinear vibration of casing pipes in the deep water was studied considering the loads of current and combined wave-current. The vortex-induced vibration equation of a casing pipe was set up considering the beam mode and Morison’s nonlinear fluid loads as well as the vortex-excited loads. The approach of calculating vortex-excited nonlinear vibration by Galerkin’s method was proposed. The natural vibration frequencies and modes were obtained, and the response including primary resonance induced by current and the composite resonance under combined wave-current for the 170 m long casing pipe in the 160m depth of water were investigated. The results show that the dynamics response of casing pipe obviously increases, and the complicated response behaviors of casing pipe are described under combined wave-current.

NONLINEAR DYNAMICS RESPONSE OF CASING PIPE UNDER COMBINED WAVE-CURRENT

The vortex-induced nonlinear vibration of casing pipes in the deep water was studied considering the loads of current and combined wave-current. The vortex-induced vibration equation of a casing pipe was set up considering the beam mode and Morison＇s nonlinear fluid loads as well as the vortex-excited loads. The approach of calculating vortex-excited nonlinear vibration by Galerkin＇s method was proposed. The natural vibration frequencies and modes were obtained, and the response including primary resonance induced by current and the composite resonance under combined wave-current for the 170 m long casing pipe in the 160 m depth of water were investigated. The results show that the dynamics response of casing pipe obviously increases, and the complicated response behaviors of casing pipe are described under combined wavecurrent.

Resource assessment for combined offshore wind and wave energy in China

The development of offshore wind farms has experienced rapid growth during the past decade. In particular, China has the highest number of installations worldwide, but some challenges exist for further development. Consequently, some researchers suggest combining wave energy with offshore wind energy. To fully implement this plan, a comprehensive resource assessment of combined offshore wind and wave energy systems is needed. Investigations of the parameters, such as the spatial and temporal distribution of wind and wave energy, aggregate resource reserves, available technical potential, and total capacity factor, are vital for designing the required wind turbines and wave energy converters. To assist scientific development and governmental decision making, this paper aims to evaluate offshore wind and wave energy resources from a technological perspective. The results show that theoretical offshore wind and wave energy resources are abundant in China's ocean territory, with a potential of approximately 3 TW. Technically, of the three most popular offshore wind turbines, i.e., 6, 8, and 10 MW, 10 MW is overall the most suitable in China. However, of the three wave energy converters, i.e., 120, 250, and 750 kW, 120 kW is the best candidate for Liaoning Province, and 750 kW is the most suitable for the remainder of its region. Overall, the total annual energy production is approximately 7000 TWh.

Ship collision impact on the structural load of an offshore wind turbine

When a maintenance and operations ship is berthing,there is a chance the ship may collide into the wind turbine.When these ships collide into wind turbine structures,this can result in significant changes to the foundation and structure of the wind turbine.In this paper,the structural load of a 4 MW offshore wind turbine was analyzed during a collision with an operations and maintenance ship.The variations in the wind speeds on hub height,waves,and the sea currents were measured.The dynamic simulation of the wind turbine was carried out using the test data as the input parameters.As a result,the load condition of the turbine without a collision was obtained.Finally,the measured turbine load was compared with the simulation results.This study shows that the collision of the operation and the maintenance ship increases the bending moments at the tower’s bottom and the blade’s roots.

Study of the Effective Factors on Shelf Currents in the Southern Caspian Sea

The current and wind records and the physical parameter structures such as temperature and salinity in the southwestern part of the Caspian Sea adjacent to Anzali Port were investigated from November 2004 up to the end of January 2005. Results show that, despite the existence of relatively weak winds along the coast in the area, the measurements indicate strong long shore currents. However, when heavy wind tension is observed in the area, then strong currents are also present, which - from the perspective of direction – also have good coordination with the wind. The direction of dominated currents was parallel to the coast from the west to the east. In most cases, the flow rate was identical from the surface to the seabed, and in this condition - because the values of the measured temperature points were almost identical-barotropic currents were present. However, in the autumn at some region, a significant difference was observed between the surface currents and subsurface currents due to temperature differences that affected the density and caused the creation of barocilinic currents. Due to the high velocity of currents compared to wind velocity, and the intense slope of the coast and low-frequency movements in the area, we can hypothesize the existence of motions such as Kelvin waves and conclude that the effect of the wind compared to the other factors of the coastal current in the area was weaker.

基于微阵列波能装置的半潜式风浪联合系统运动与获能研究

为实现海上风浪资源的高效利用,以Windfloat型风力机平台为基础,结合摇臂式波能装置,建立联合获能系统数值模型,并分析了波周期、PTO(powertake-off)阻尼、浮子直径及摇臂长度等关键参数对联合系统运动与获能的影响。研究发现,随波周期的增大,平台运动幅度整体呈增大趋势,装置获能先增大后减小;随PTO阻尼的增大,平台纵荡、纵摇运动幅度逐渐减小,垂荡运动幅度及装置获能先增大后减小;随浮子直径、摇臂长度的增大,平台纵荡运动幅度逐渐减小、垂荡、纵摇运动幅度逐渐增大,装置获能随浮子直径的增大而增大,随摇臂长度的增大先增大后减小。

波流共同作用下射流稀释特性的动量积分模拟研究

污水通常以射流的形式向海排放,其在海洋环境水体中的稀释过程是射流与波浪、潮流共同作用的结果。本文基于合理的假设条件,对射流运动的控制方程进行积分处理,结合波流环境下射流的结构特征,引入射流卷吸与拖曳力函数,结合必要的边界条件,形成封闭的控制方程组,建立波流环境下射流动量积分模型。通过与实验资料的对比分析可以看出,该模型可以准确模拟波流环境下射流在近区段的时均运动与稀释过程;相较于单一的横流作用,波流共同作用下射流对周边水体的影响范围更大,射流初始稀释能力更强。基于多个组次的动量积分模拟实验,通过无量纲分析与最小二乘拟合,建立了波流共同作用下射流轨迹中心线位置及最小稀释度的经验公式。相关成果可为排海污水环境影响评估提供快速、准确的分析手段。

风浪流耦合作用下锚泊式海上试验平台的水动力特性试验

为了研究国家海洋试验场的“国海试1”平台与锚泊优化方案是否满足设计要求,采用物理模型试验方法,制作1∶30试验平台模型,在国家海洋技术中心海洋动力环境实验室开展百余组不同波高、波周期、波向的不规则波、定常风和等效均匀流联合作用下,两点式躺地链系泊状态下的模型运动响应研究。对比分析试验结果与对应试验平台的设计指标,结果表明:同样海况下,风浪与潮流同向时,试验平台的运动响应更剧烈;流速1.14 m/s、4级海况时,试验平台最大横摇角为10.41°,“国海试1”平台无法开展正常的吊装工作;流速1.14m/s、6级海况时,试验平台最大横摇角为15.07°,“国海试1”平台搭载的试验测试设备无法正常工作;流速1.14 m/s、8级海况时,试验平台最大横摇角为20.21°,平台存在安全隐患,应采取避险措施;“国海试1”满足抗7级风浪的设计要求。该研究成果验证了改进后的国家海洋综合试验场“国海试1”平台满足抗7级海况的设计要求。因此,建议“国海试1”平台在6级海况下应停止试验作业,8级海况下应采取必要的避险措施。该研究为同类新型海洋平台水动力研究提供了参考。

Equilibrium scour depth at offshore monopile foundation in combined waves and current

Unlike the pier scour in bridge waterways,the local scour at offshore monopile foundations should take into account the effect of wave-current combination.Under the condition of wave-current coexistence,the water-soil interfacial scouring is usually coupled with the pore-pressure dynamics inside of the seabed.The aforementioned wave/current-pile-soil coupling process was physically modeled with a specially designed flow-structure-soil interaction flume.Experimental results indicate that superimposing a current onto the waves obviously changes the pore-pressure and the flow velocity at the bed around the pile.The concomitance of horseshoe vortex and local scour hole around a monopile proves that the horseshoe vortex is one of the main controlling mechanisms for scouring development under the combined waves and current.Based on similarity analyses,an average-velocity based Froude number(Fra)is proposed to correlate with the equilibrium scour depth(S/D)at offshore monopile foundation in the combined waves and current.An empirical expression for the correlation between S/D and Fra is given for predicting equilibrium scour depth,which may provide a guide for offshore engineering practice.

风浪联合作用下深水半潜平台耐波性能的数值模拟

通过数值模拟方法,研究风浪联合作用下深水半潜平台的耐波性能。该方法通过建立风速与波浪相干函数模型,利用该模型获得当前深水半潜平台不同工况下风速与海浪的关系,再计算风浪联合作用下深水半潜平台的海浪作用力和风作用力,以2个数值为基础,构建深水半潜平台耐波性能数值模拟模型,在不同情况下模拟风浪联合作用下深水半潜平台耐波性能。实验结果表明:该方法可在不同波浪周期时,模拟风浪联合作用下深水半潜平台的横荡和垂荡数值,在波浪周期较小时,垂荡与横荡数值也较小,深水半潜平台的耐波能力较强;风浪角越大,在波浪周期相同时深水半潜平台的垂向弯矩数值越大,其耐波能力则较小。