Dynamic Response Of Offshore Floating Wind Turbines

Offshore wind generation has become one of the most mature power generation ofnew energy technology.Now it is inevitable trend that offshore wind turbine develops fromshallow pile style to floating style of deep water. However, the research for offshore windpower generation of our country still stay in the traditional fixed structure style of coastalwater due to the lack of mature design standards and the study of floating wind turbine isstill in the initial stage. Dynamic response analysis of offshore wind turbines underconditions of wind and wave loads will provide a basis for the design of supportingstructures.So it has important significance.In this article, three kinds of floating wind turbines have been built through theprofessional design software SAMCEF for Wind Turbine (called SWT for short). Thethree kinds of supporting platforms are TLP,Spar and barge.The dynamic response analysisof three platforms under the South China Sea conditions in the water depth of200meters,300meters and400meters have been conducted based on aerodynamic and wave theory.Pros and cons of each floating wind turbine in the depth of200m,300m and400m havebeen compared. At the same time, the effects of wind and wave loads and wave incidentangle as well as wave period to the motion of three supporting platforms have been studied.Here are the research contents and conclusions of this paper:(1)Select the2MW wind turbine and conduct preliminary designs for the three kindsof supporting structures. Firstly, determine the ballast of the platform according to theposition of the center of gravity in order to improve the stability of the platform. On theother hand, the mooring lines must have certain pretension to ensure the safety andefficiency of the wind turbine.(2)The equilibrium positions of TLP and Spar rise with the increase of water depthand the equilibrium position of the barge platform descends.This is because the flexibilityof the mooring lines increases with the increase of water depth. However, the weight of thecatenary mooring for the barge platform increase gradually due to the increase of its length.Consequently, the catenary mooring lines pull the barge platform down. Similarly,themooring line pretension of TLP and Spar decreases with the increase of water depth,while the mooring line pretension of the barge platform increases. The increase of themooring line pretension vertical component equals to the increase of platform displacement.(3)The displacement and rotation of the floating wind turbines are in appropriate rangeunder two conditions. The platform motion in one-year condition is smaller than that in100-year condition, which shows the effect of wave loads dominates. The motion and thestress of the platform and the tension of mooring lines are in appropriate range, whichmakes the offshore wind turbine work safely and efficiently.(4)The motion of TLP is the least of the three kinds of wind turbines under the abovetwo conditions. Its roll, pitch and yaw angle are no more than1degree and it is the closestto onshore wind turbine system. The surge and yaw stability of barge are better than that ofSpar platform, while the heaving and pitching stability of barge are worse than that of theSpar platform. Therefore, we should make the right choice of the platform type accordingto the specific requirements.(5)The increase of wave height has a large influence to the stability of the platformunder certain wind speed. The motion of the platform increases with the increase of waveheight.However, wind speed don’t has great influence on the platform. For the deepwaterfloating wind turbines, wave loads play a dominant role in the load combinations.(6)With the increase of wave incident angle, the sway and roll angle of the platformincrease gradually while the surge and pitch angle generally decrease. However, the surgeof Spar and barge become the greatest when the wave incident angle is10°.When thewave incident angle is up to10°,the surge and pitch angle decline to a nadir while thesway and roll angle reach their peak. Heave motion of the platform does not change toomuch on the whole. With the increase of wave period, the motion of TLP increases whilethe motion of barge decreases gradually.The motion of Spar wind turbine reaches themaximum when wave period is20seconds.