| The development of clean and efficient offshore wind power is an important measure to deal with global warming and energy crisis.After over ten years of rapid development,the nearshore wind farm planning has basically been completed in China.The exploitation of far-reaching sea wind resources which are more abundant and less turbulent,is the developing trend of offshore wind industry.When the water depth exceeds 50 m,the constructure cost of bottom-fixed wind turbine increases dramatically and floating wind turbine(FWT)is a better option for deep sea wind exploitation.In general,the development of FWT substructures experiences three design stages: conceptual design,preliminary design and detailed design.The main task of conceptual design is to determine the structural configuration and the main dimensions,which are the prerequisites for the preliminary design and detailed design,and to a large extent determine the dynamic response as well as the economy of the FWT system.However,how to accomplish such an important work,the existing research has not yet given a comprehensive solution.Driven by this gap,this research proposes an optimization methodology for the design of FWT substructure condiguration and main dimenions.The specific work is as follows:An efficient dynamic numerical model is developed based on the simulation demands at conceptual design stage.The model describes the FWT dynamic response by using the equation of motion with eight degree-of-freedoms(Do Fs)in the frequency domain.The floating substructure is modeled as a rigid body with six Do Fs;the tower is an elastic component,and its deformation is described by two generalized Do F,while the rotor-nacelle assembly are treated as a lumped mass at tower top.The FWT aerodynamic loads are simplified into two parts,namely the aerodynamic loads under the bottom-fixed mode and the aerodynamic damping effects caused by the motion of substructure and the deformation of tower.Both are derived in advance from the time-domain coupled dynamic analysis software FAST and formed into an aerodynamic database for use in the quick simulations.In terms of hydrodynamics,the potential flow theory is employed to capture the radiation and diffraction effects of the floater,while the viscous effects are considered through the linearized Morison equation.In order to capture the nonlinear mooring stiffness,the mean platform displacement under various combinations of wind speed and wind direction is calculated based on static equilibrium,then the platform is forced to move slightly around a given mean position,the gradient of the restoring force is the mooring stiffness corresponding to that mean position.In order to verify the proposed numerical model,the short-term dynamics of a four-column semi-submersible FWT under 1000 typical sea states is simulated and the results are compared with FAST.It is shown that the relative errors of the response standard deviation are almost within15%,and the computational efficiency is greatly promoted,demonstrating that the proposed model can meet the computational requirements of conceptual design.A long-term dynamic analysis method is proposed based on the surrogate modeling technique.A certain number of representative sea states are selected from the long-term metocean database by using the maximum dissimilarity algorithm.With the aid of the numerical model,the dynamic responses under those representative sea states are derived.Subsequently,regression analysis(training)on the inputs(sea state parameters)and outputs(system responses)of the numerical model is performed so as to establish the approximation function between the two,i.e.the surrogate model.Finally,the surrogate model is used to predict the FWT responses under all sea conditions.In this way,the response estimation becomes a simple algebraic calculation process,which can dramatically reduce the computational cost.In order to verify this approach,the predicted results of the surrogate model are compared with the full numerical simulations.It is shown that the surrogate model is capable of reconstructing the realistic response of the FWT system and its accuracy can fulfil requirement of the conceptual design.Based on this method,this study investigates the influence of the platform mounting orientation(PMO)on the long-term dynamic response of a Y-shaped semi-submersible FWT system.The results present a noticeable difference in the tower base cumulative fatigue damage under different PMOs,and the maximum is up to 25%.This difference is caused by the first-order hydrodynamic loads.The global sensitivity analysis of the key dynamic and economic indicators to the dimension parameters of substructure is performed.The four-column and Y-shaped semisubmersible FWT are taken as examples in this work.The variance-based and densitybased method are applied to evaluate the contribution of the platform and mooring dimension parameters to the variation of inherent structural proterties,long-term dynamic response and manufacturing cost.The results show that the column radius,column spacing,platform draft and mooring chain diameter are the most relevant dimension parameters to the dynamics and economy of the FWT system.This finding provides a basis for the selection of the design variables for the substructure optimization.Besides,the relationship between the inherent structural proterties and the long-term dynamic response is discussed.The framework of FWT substructure optimization is set up.Based on the sensitivity analysis results,five dimension parameters are selected as optimization variables.The inhouse numerical model and the surrogate-based long-term dynamics evaluation method are employed to calculate the dynamic indicators in objective functions,and to verify the feasibility(constraint conditions)of a given substructure design.With the aid of the genetic algorithm,the automatic optimization process of design generation,objective function calculation,constraint verification and design comparison is realized.Based on this framework,the impact of platform-mooring integrated optimization,the long-term environmental condition and the optimization algorithms on the optimization results is investigated.Finally,the Pareto fronts of the four-column and the Y-shaped semisubmersible concept are compared.It is shown that the tower base fatigue of the fourcolumn concept is always lower than that of the Y-shaped concept at the same manufacturing cost level.The main reason is that the wave-frequency hydrodynamic response(the platform translational and rotational motion)of the four-column concept is relatively small,as a result the tower base subjects to a lower bending moment which is induced by the inertial force of the wind turbine.Through the above work,this paper establishes a quick dynamic evaluation and optimization framework for semi-submersible FWTs,which provides an efficient solution for the selection of substructure and the design of main dimensions in the conceptual design stage. |
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