Dynamic Response Analysis Of Mooring Line Breakage Of Floating Offshore Wind Turbine

Offshore wind power is in the process of moving from shallow coastal waters to deep water.When the water depth exceeds 30m,the construction cost of fixed foundations increases sharply.Therefore,offshore wind turbine with floating foundation has become a hot spot on the development of offshore wind power.The floating offshore wind turbine(FOWT)is built on a floating structure and is positioned at seabed by a mooring system.FOWT gets rid of the limitation of the seabed and water depth,and obtain more powerful and stable wind energy,but is also suffering from the more complex and severe wind wave and current loads.Safety of FOWT's mooring systems is threatened by extreme sea conditions.Therefore,it is necessary to study the environmental conditions that cause the FOWT mooring line breakage and the dynamic response from the FOWT system after the mooring line is broken.The present study adopts the numerical method to carry out the coupling simulation of the FOWT,the whole system coupling equation is established by the multi-body dynamics method,and the Runge-Kutta method is utilized to solve it.According to exchange the results of numerical and the experiment of literature,that determined the credible of the numerical simulation.Aiming at the 50m water depth,a mooring system scheme for FOWT was initially proposed,that employed to add mass block at the bottom of the mooring chain.And max tension of the mooring system has checked by 100-years environmental condition.In order to determine the environment conditions of FOWT's mooring line breakage,the research used NREL's 5MW wind turbine,OC4-DeepCwind semi-submersible floating platform and bottom added mass block mooring system as the coupling models,then the dynamic response of FOWT is studies,that is under the extreme gust conditions.It is observed that due to the transient characteristics of extreme gust wind speed and direction,during the occurrence of extreme gusts,mooring tension of the FOWT transiently increased,that is the snap tension.When the rise time,the gust amplitude and the direction of extreme gusts meet certain requirements,the amplitude of snapping tension of the FOWT mooring exceeds the minimum breaking load of the mooring line,which is prone to cause the FOWT mooring line breakage.Therefore,the relationship between extreme gust characterizes parameters and the snap mooring tension of the FOWT was analyzed.The minimum breaking load of mooring line was taken as the critical value for triggering the mooring line broken,and summary the mooring line breakage conditions of extreme gust.Through comparing the system response of the complete mooring and breakage status of the FOWT under extreme gust conditions,it is found that the mooring line broke off the FOWT will cause a long-distance drift of the FOWT platform.During the drift,the power cable is damaged,and even more mooring lines will be broken,and the drift of FOWT can also cause collisions with potential floating structures.During the drifting process of the FOWT,due to the wind vane effect,the yaw motion of the platform is unstable,and the yaw mechanism of the wind turbine is easily damaged.