Research On The Wind-wave Energy Integrated System Based On Semi-submersible Floating Platform

Energy is an important material basis for the existence and development of human society.With the continuous improvement of material and spiritual life,people are increasingly aware of the serious consequences brought by the large-scale use of fossil fuels.For example,the environment pollution,and traditional fossil energy is depleting day by day.Therefore,countries all over the world are actively seeking to build a clean,safe,and reliable new or renewable energy system.As we all know,the ocean contains abundant of clean and renewable resources such as offshore wind energy,wave energy,and tidal energy.The potential of wind energy in China are approximately 750 million k W,which not only has huge resource potential,but also has good market conditions for development and utilization due to closer to China's economic center.At the same time,the wave resources are also very attractive due to the natural correlation between ocean wind and waves.Compared with the offshore wind power,the levelized cost of the wave energy is higher,and the energy conversion efficiency is lower,which limits its large-scale commercial application and promotion to a certain extent.However,offshore wind turbine and wave energy converter can be scientifically integrated,sharing the supporting platform structure and power transmission system,which improves the utilization efficiency of marine renewable energy,effectively reduces the unit power generation,and conforms to the major strategic need for constructing clean,safe,reliable,and renewable energy systems.The paper integrates a 5-MW braceless semi-submersible floating wind turbine and heave-type wave energy converter,and conducts a numerical analysis of the integrated system based on the potential flow theory through ANSYS/AQWA software.Firstly,the size and draft of the wave energy converter are initially optimized from three aspects of produced power,wave energy cost and integrated system safety.Moreover,the effects of the stiffness coefficient and damping coefficient of the PTO system and wave period on the performance of wave energy converter under typical sea conditions are discussed and studied in detail.Secondly,frequency domain hydrodynamic analysis of the integrated system is conducted by linearizing the nonlinear mooring line system and drag force.In order to explore the impact of the introduction of the wave energy converter on the semi-submersible wind turbine,the hydrodynamic coefficients(such as additional mass coefficient,radiation damping coefficient and excitation force)and motion response characteristics(such as response amplitude operators for surge,heave and pitch)of the semi-submersible wind turbine with considering the integrated system and single semi-submersible wind turbine are analyzed and compared.At the same time,in order to explore the influence of hydrodynamic coupling on the integrated system,the hydrodynamic characteristics of the semi-submersible floating wind turbine and the wave energy converter with considering hydrodynamic coupling and without considering hydrodynamic coupling are also analyzed and compared.Finally,the aerodynamic loads the wind turbine are simplified,and the influence of the semi-submersible platform motion on the aerodynamic load of the wind turbine is considered based on the 5-MW wind turbine design parameters proposed by the NREL.With the help of the User-force interface in ANSYS/AQWA software,the wind-wave energy integrated system is analyzed and studied in the coupled wind-wave time domain model.The focus is to explore the dynamic responses of the wind-wave energy integrated system under typical sea conditions and extreme sea conditions.At the same time,a survival strategy is proposed under the extreme sea conditions,and corresponding numerical studies of dynamic responses are carried out.