Dynamic Response Of Monopile-supported Offshore Wind Turbine Support Structure Under Complex Marine Environment

With the continuous promotion of clean and low-carbon energy transformation,renewable energy has been rapidly developed due to its clean,safe and sustainable development.Wind energy,as one of renewable energy,has good economy and large-scale development and utilization value.Offshore wind energy has the characteristics of rich resources,high wind speed,small turbulence intensity,no land occupation,and few restrictions on wind farm construction.However,the complex marine environment makes the design of offshore wind turbine more difficult.It is of great significance for the development of offshore wind power to simulate the dynamic response accurately under complex marine environment.Taking 5 MW offshore wind turbine supported by monopile as a model,the dynamic response characteristics of offshore wind turbine support structure under the action of scour,soil damping and ice were studied.With the support of the national science and technology support project“5.0MW doubly-fed variable speed constant frequency offshore wind turbine design,integration and demonstration"(2009BAA22B02),the main research work and results are as follows:(1)Based on the coupled spring model to simulate the foundation stiffness in soil,the sensitivity of support structure response to wave frequency under operating and parked conditions is analyzed considering the change of aerodynamic damping.As the characteristics of damping and the proportion of wind and wave load,parked sea states are more sensitive to wave frequency.By comparing overall statistical data,the first three wave peak frequencies have a great influence on structure response,which decreases in turn according to the influence of energy distribution.The standard deviation of the response increases with the increase of wave frequency,and the variation is more severe when considering the flexibility of foundation.(2)According to the empirical formula of DNV-OS-J101,the ultimate scour depth is calculated,and the response sensitivity of support structure under different scour depth is studied.With the increase of scour depth,the response increases first slowly and then rapidly.Compared with the response data under ultimate scour depth,the first-order natural frequency of support structure is reduced by 5.9167%.The impact of scour on the support structure response under parked condition is greater than that under operating condition,and the support structure is easy to produce steady-state vibration by environmental load.With the increase of structural flexibility,the load fluctuation increases,and it is easy to generate more fatigue load and extreme load.(3)Based on the nonlinear p-y curve and normalized soil material damping curves,the soil model is established.Through the free vibration analysis and the logarithmic attenuation method,the influence rules of the soil material damping on the damping ratio of support structure are revealed under the conditions of tower top average displacement,scour and the soil stiffness.The soil damping ratio increases with the increase of displacement deviation.However,the soil damping ratio decreases with the increase of soil stiffness,which drops rapidly first and then slows down after reaching a certain stiffness.With the increase of scour depth,the soil damping ratio shows a nonlinear trend.Soil damping curve,structure frequency and structure buried depth affect the soil damping of support structure to different extent,respectively.(4)The stochastic time domain simulation is carried out for parked sea states,and the dominant load frequency is determined to be the first natural frequency of support structure based on power spectrum analysis of the mudline moment.Enhanced frequency domain decomposition method is used to identify the soil damping ratio of the support structure under LC14 sea state.The result shows that the influence of soil damping on standard deviation of support structure is greater than maximum.The identified soil damping ratios from different simulations are quite different.The energy attenuation of the peak of PSD spectrum(first-order natural frequency)is basically consistent with the variation of soil damping ratio,as the soil damping is directly related to the vibration energy of the structure.The more severe the structure vibration is,the greater the influence of soil damping on the structure response is.(5)Steady-state vibration analysis and random vibration analysis were carried out for support structure under the action of sea ice,and random ice force time history is generated by random ice force spectrum and harmonic superposition method.As dynamic ice force has mean value,the mean value and the median value of support structure response are increased,and the increments depend on the mean of ice force.Random ice force is a low-frequency environmental load,and the main energy is concentrated within 1 Hz.The mean of random ice force is the main influencing factor of response under operating condition,while the fluctuation of random ice force is the main influencing factor of response under parked conditions.(6)Eddy viscosity wave-ice model is chosen to determine wave parameters in the ice edge area.Support structure under the combined action of wind,wave and ice is simulated and analyzed.Compared with the simulation results of wind-ice combined action,the wave has little influence on support structure response under the operating condition,and with the increase of wave load,the influence will increase to some extent.Under the parked condition,the influence of wave on support structure response increases,but the influence degree decreases with the increase of wind load.The effect of sea waves and sea ice on the structural response is not linear superposition,which is due to the randomness of the wave and sea ice load.The structural response under the combined action of wave and sea ice depends on the energy distribution of the combined action of the two.