| Wind has become a key kind of energy in twenty-first century.Vast wind resources exist in the offshore environment with stronger and steadier wind speeds.Among various kinds of offshore energy,wind power has the highest energy conversion efficiency.There are rich offshore wind erengies in China,the installed capacity of wind turbines has achieved the first place in the world.As one of the nations which have the richest offshore wind rescources all over the world,it provides a wide development space in the thechonolgy of offshore wind power.At present,the installation of offshore wind turbines gradually enters a wider area at water depths exceed 100 m.With the larger generator power,it is an important choice to adopt the floating offshore wind turbines(FOWT)among all available offshore wind energy facilities.The hundred-meter-height FOWT suffers both environmental and operational loads along its lifetime.It is possible to cause the accidents under the extreme conditions,such as typhoon,tsunami,and so on.Currently,the insufficient research of failure mechanisms and dynamic responses is a limitation in the study of FOWT.Based on the interaction between the structure and the offshore environment,the key factors of Spar-type FOWT,including the nonlinear dynamic responses and failure mechanisms,are studied in the present work.It is extremely meaningful to the strategic advancement of the offshore wind energy techonolgy in China,as well as the green and sustainable development of our society and economy.This study aims at solving the key problems in the design of FOWTs,improving the generating efficiency and the ability against the environmental loads.This work has significant theoretical and engineering value.As one important section of the national natural science funding,‘Study on the dynamics characteristics for wind power system of floating foundation based on coupled multi-body’,the coupled dynamic response prediction model of FOWT,as well as the analysis approach,is proposed in this work.A FOWT motion prediction code is developed to analyse the dynamic responses based on the coupled model.The dynamic responsesof Spar type FOWT under the complex sea states are investigated,considering the nonlinear effects.The dynamic response of FOWT under the breakdown cases are analysed,and the dynamic conditions of system failure under different sea states are revealed.The main research work is introduced as follows briefly.1.Based on a detailed overview on the researches about thr FOWT,the Spar-type FOWT is chosen as the research object.Both theorical and numerical approaches are adopted.A coupled aero-hydro-mooring nonlinear analysis model is established in time domain.Different kinds of offshore environmental loads,including aerodynamic loads,first and second order wave loads,vortex induced loads,are considered.Towards this ends,an in-house simulation code SFND is established for predicting the dynamic response of FOWT.The motions of a Spar-type FOWT is calculated and our in-house code is validated with the results of commercial code and model tests.2.By combining a blade-element-momentum aerodynamic module to calculate the aerodynamic loads on the rotor,a three dimentional potential-theory hydrodynamic module to simulate the wave-buoy interaction and a quasi-static catenary model for the mooring system,the 6-DOF nonlinear coupled dynamic equations of FOWT is proposed.Moreover,the nonlinear restoring loads on the Spar floating foundation are derived based on the geometric relationship of the motion of slender body.By combining various levels of sea states,the dynamic responses of Spar-type FOWT system,including motions,aerodynamic loads and mooring loads,are investigated under the wind and wave conditions.It is observed that the coupled relationship is existed between the motion of FOWT and the external loads.On the other hand,the effect of second order wave loads on the Spar-type FOWT is established.The mechanism of the influence of different environmental loads on the FOWTs are revealed.3.To simulate the flow around the two dimentional circular cylinder at high Reynolds numbers,the CFD approach is adopted to calculate the lift force,drag force and vortex shedding frequency with different flow velocities.Based on these coefficients,by considering the vortex induced loads and the combination of wind,wave and current,the nonlinear dynamic responses of Spar-type FOWT are calculated under different kinds of current profiles.It is seen that the vortex induced motions of FOWT display the figure-eight shape.The effects of lift and drag on the motions of FOWT are investigated,and the influence mechanism of current profiles is revealed.The significant effect of vortex induced loads on the motions of FOWTs is clarified.4.The freak wave is generated based on the improved phase modulation method.This approach is improved by the random method,and its feasibility is validated.Time series of freak wave are generated with the conditions of generating and survival sea states.The dynamic responses of Spar-type FOWT are studied in time domain.The shock effect on Spar-type FOWT of freak wave is revealed.The motions in different DOFs,as well as the aerodynamic responses of wind turbine and the dynamic responses of mooring system,are calculated under the condition of freak waves.5.Based on the nonlinear mooring loads induced by the deformations of mooring lines,including sketching,bending and torsion effects,the dynamic responses of floating offshore wind turbines(FOWTs)were analyzed.By considering the nonlinear geometric deformations,the stretching-bending-torsion coupled dynamic analysis model of mooring system was proposed.Euler parameters were used for the transformation between the global and local coordinate systems to avoid the occurrence of singularity.The dynamic analysis model of the FOWT was established by considering the nonlinear mooring loads,hydrodynamics,aerodynamics as well as the mechanical system of wind turbine.Based on this nonlinear formulation a numerical program of the dynamic response of FOWT was developed.The dynamic responses under both the operating and extreme sea states were simulated.The results were compared with those calculated based on the catenary theory.By considering the dynamic effect of the mooring lines,the wave frequency response of tension is amplified,and several peaks in the range of high frequency occur.Those responses will affect the fatigue life of mooring lines.6、Focus on the sea states of generating conditions,simulations based on the nonlinear dynamic model are conducted in time domain to investigate the transient response of a SPAR-type floating offshore wind turbine in scenarios with fractured mooring lines.The motions of platform,the tensions in the mooring lines and the power generation performance are documented in different cases,including different fractured cables and different shutdown strategies.A large-distance drift motion will occur in the horizontal plane,and it will lead to the risk of collusion accidents.According to the research,the potential impacts and damage mechanism of mooring failure on FOWTs are revealed. |
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