| China has vast sea space,rich marine energy and fishery resources.In recent years,all walks of life have invested a lot of research energy on how to make full use of marine resources.Wave energy,tidal energy,offshore wind energy and abundant fishery resources are the focus of many scholars at home and abroad,especially the utilization of offshore wind energy and the cultivation mode of far-reaching offshore facilities.At present,the offshore wind energy utilization is mainly based on fixed wind turbine platform,which is usually installed in coastal waters.The installation difficulty and power transmission mode have broken through technical difficulties and are close to the stage of large-scale industrial application.However,it is undeniable that due to the influence of topography,the offshore wind energy resources are limited,and the abundant wind energy resources in the deep sea need to be developed on a large scale to meet the growing energy demand of mankind.On the other hand,with the further attention to food security and the over-exploitation of marine fishery resources,marine aquaculture is under great pressure,especially in the offshore area,where the aquaculture space is sharply compressed and the aquaculture density is suddenly rising.Therefore,marine aquaculture also needs to develop to the deep sea area to expand its industrial scale.In view of the above problems,it has good scientific research value and industrial prospect to innovate and develop a stable,efficient,economical and reusable wind power fishery comprehensive platform by combining offshore wind power platform with deep-sea cage culture platform with multidisciplinary integration model.Therefore,this paper studies the structural design,dynamic modeling,structural response of the integrated system under different excitation conditions and the influence of mooring line failure on the integrated system.In this paper,the integrated system of deep-sea aquaculture cage-floating wind turbine is taken as the research object,and its aero-hydro-elastic-servo coupled dynamic model is established.The dynamic response characteristics of the integrated system under the combined action of wind and waves and different mooring configurations are analyzed based on this coupled model,.The main research contents are as follows:(1)The main frame,netting system and mooring system of the cage are designed,the hydrodynamic characteristics of the cage are then analyzed.The wave model and hydrodynamic theory are studied,and the rationality of grid division is verified by comparing the average drift force of the cage calculated by near-field method and far-field method.Based on AQWA,the hydrodynamic characteristics of the cage,such as added mass,radiation damping,amplitude response operator(RAOs)and first-order wave force and moment,are analyzed.Considering the structural symmetry of the cage,only the hydrodynamic characteristics of the surge,heave,pitch and yaw motions are simulated and analyzed.The simulation results show that: 1)the added mass of the cage in the pitch direction is larger at high frequency than at low frequency.Compared with the surge added mass,the heave added mass is less affected by the wave frequency,but is smaller than the suge added mass at low frequencies.The added pitch added mass of the cage is not significantly affected by wave frequency.Comparatively speaking,the added mass in the yaw direction is greatly influenced by wave frequency,and it is larger than the pitch added mass in the whole frequency range;At high frequencies,the change of the yaw added mass is not obvious.2)The longitudinal radiation damping of the cage is significantly affected by the wave frequency,which first increases and then decreases with the increase of the wave frequency.Compared with the heave radiation damping,the heave radiation damping is smaller.When the wave frequency is below 0.1Hz,the change of the pitch radiation damping of cage is not obvious,the maximum value appears around 0.2 Hz,then it decreases with the increase of frequency,and the change trend of yaw radiation damping is similar.Compared with the pitch radiation damping,the yaw radiation damping is larger.Generally speaking,the radiation damping of cage is small at low frequency and high frequency,which indicates that its ability to restore balance naturally is weak.2)The radiation damping of the cage in the surge direction is significantly affected by the wave frequency,which first increases and then decreases with the increase of the wave frequency.Compared with the surge radiation damping,the heave radiation damping is smaller.The change of the cage pitch radiation damping is not obvious when the wave frequency is below 0.1Hz,and the maximum value of the pitch radiation damping appears around 0.2 Hz.It is found that the pitch radiation damping decreases with the increase of wave frequency.The change trend of yaw radiation damping is similar.Compared with the pitch radiation damping,the yaw radiation damping is larger.Generally speaking,the radiation damping of cage is small at low frequency and high frequency,which indicates that its ability to restore balance naturally is weak.3)The translational motion response of the cage is significantly affected by low-frequency waves,but does not change significantly at high frequencies.The pitch motion of the cage has no obvious change at other frequencies except for a large peak near 0.06Hz.Compared with the pitch motion,the yaw motion of the cage is not affected by wave frequency.4)The first-order wave force in the surge direction is significantly affected by wave frequency,and there are large peaks at multiple frequencies.The first-order wave force in the heave direction is greatly influenced by low-frequency waves,but it does not change significantly at high frequencies;The first-order wave moments in the pitch and yaw directions do not change obviously at low frequency,and there are many peaks at high frequency.(2)Taking the NREL 5-MW wind turbine as the research object,the integrated system of aquaculture cage-floating wind turbine is established by coupling the NREL 5-MW wind turbine with the aquaculture cage and mooring system.Firstly,the wind model and aerodynamics are studied.Secondly,the theory of wind turbine motion and dynamics is studied,and the dynamic model of the NREL 5-MW wind turbine is established using the Kane motion equation in FAST.Aiming at the integrated system,a pitch-torque control system is designed.Thirdly,through the built-in dynamic link library in AQWA,the wind turbine established in FAST is coupled with the aquaculture cage and mooring system established in AQWA,and finally the integrated system of aquaculture cage-floating wind turbine is established.In order to verify the effectiveness of the coupling modeling method,taking the OC4-Deep Cwind semi-submersible floating wind turbine as an example,the simulation results obtained by FAST are compared with the results of FAST-AQWA.The comparison results show the effectiveness of the coupling method.(3)The structural response of the integrated system of aquaculture cage-floating wind turbine under different excitations is studied and analyzed,including free decay analysis,response analysis under uniform wind and irregular waves,and response analysis under turbulent wind and irregular waves(including time domain and frequency domain analysis).Among them,three normal operating conditions and one parked condition are selected for turbulent wind simulations.According to the free decay analysis,it is observed that the natural frequencies of surge and sway motions are the same due to the symmetry of the cage structure,which is 0.007 Hz,and the natural frequencies of pitch and roll motions are also the same,which is 0.058 Hz.The natural frequencies of heave and yaw are 0.051 Hz and 0.008 Hz,respectively.The simulation analysis under the conditions of uniform wind and regular waves shows that the fluctuation amplitude of the surge motion is more significant than that of sway and heave motions.The simulation analysis under the turbulent wind and irregular wave conditions shows that: 1)The motion response of cage under the parked condition is generally more significant than that under normal operation conditions due to the reduction of aerodynamic damping.2)Under normal operating conditions,the motion response of cage is less affected by the change of wind speed,which indicates that the designed cage has good motion stability.3)The simulations on rotor speed,rotor thrust,pitch angle and electric power show that the designed pitch-torque control system meets the expected design requirements and can control the operation of the wind turbine well.4)The structural load becomes more significant under the rated wind speed and parked conditions,especially the parked condition mainly affects the standard deviation of the structural loads.(4)In order to study the influence of mooring line length on the structural response of the integrated system,the mooring line length is changed from intial value to 924 m and 968 m(5% and 10% longer than the original length of 880 m,respectively).The simulations are carried out under the rated and parked conditions as examples.The results show that: 1)Generally speaking,the increase of the mooring line length mainly affects the translation movement of the cage,especially the longitudinal cage movement.2)For the rotation movement of the cage,the yaw movement of the cage is significantly influenced by the mooring line length.3)In particular,the roll motion of the cage will also be significantly affected under the parked condition.4)The influence of mooring line length on blade pitch angle and electric power is not significant,and the change trend of the two responses under different mooring line lengths is almost the same.Nevertheless,there is a certain time difference between blade pitch angle and electric power output under different mooring line lengths,that is,when the mooring line length increases,there is a time lag in blade pitch angle and electric power generation.5)Under the condition of rated wind speed,the change of mooring line length has little effect on the fore-aft bending moment at the tower base.On the contrary,the change of mooring line length can significantly affect the tension of the upwind mooring line.6)With the increase of the mooring line length,the average and maximum values of the upwind moorig line tension decrease,but the standard deviation increases.When the mooring line length continues to increase to 968 m,although the tension of mooring line continues to change,the change rate is no longer significant.7)Under the parked condition,the changing trend of the tower-base load and upwind mooring line tension with the mooring line length is basically the same as that under the rated wind speed,but exhibits larger changing rate,and the standard deviation of mooring line tension decreases with the increase of the mooring line length.(5)Under the parked condition,the influence of mooring line failure on the dynamic response of the integrated cage-floating wind turbine system is studied.Considering the symmetry of mooring system layout,only the failure of mooring line #1,mooring line #2 and mooring line #3 is assumed.The simulation results show that: 1)Generally speaking,the failure of the mooring line mainly affects the movement of the cage in the same direction as that of the failed mooring line,and mainly affects the translation of the cage.2)When the mooring line fails,although the cage movement changes to some extent,it does not significantly change the aerodynamic force on the rotor,so the mooring line failure has little influence on the load at the tower base and blade root.3)Generally speaking,the failure of the upwind mooring line(mooring line #1)has the most significant influence on the tension of the other mooring lines.While reducing the tension of the downwind mooring line symmetrically arranged with it,it will increase the tension of the mooring line in the sway direction.4)The failure of the mooring line(mooring line #2)in the sway direction mainly affects the tension of the mooring line(mooring line #4)symmetrically arranged with it.5)In addition,the failure of downwind mooring line(mooring line #3)mainly affects the tension of upwind mooring line(mooring line #1)and the standard deviation of the tension of the sway mooring line. |
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