Model Tests for Three Floating Wind Turbine Concepts

Abstract Floating wind turbine systems have been proposed to tap the significant energyresources found beyond fixed bottom depths. However, limited physical dataexists concerning the performance of these systems. This paper presents detailsof a unique, comprehensive 1/50th scale model floating wind turbine test effortwhich involves extensive testing of a scale model of the National RenewableEnergy Laboratory (NREL) 5 MW, horizontal axis reference wind turbine atopthree generic floating platforms: tension-leg platform, a spar-buoy and asemi-submersible. Design considerations are presented including scaling, instrumentation, testphases and procedures. The test models are geosyms of the full scale systemswith scaled platform, tower and turbine mass properties. Froude scaling of bothwind and waves is applied and the fundamental tower bending mode is alsoscaled. A novel, low turbulence, low swirl wind generation system incorporatedinto the wave basin is utilized which employs 35 fans, flow straighteners and aconditioning nozzle. The model instrumentation includes motions, accelerationsand loads as well as wind turbine rpm, torque and thrust. The test matrix ispresented and includes system identification tests in addition to testsrepresenting combinations of regular or irregular seas, with static or dynamicwinds in both operational and extreme conditions. Selected results from the model tests are discussed which point to thedifferences and similarities of the three floater concepts. The resultsdemonstrate relatively strong interaction for the fatigue sea states betweenthe tower and platform dynamics for the tension-leg platform system. Theresults also indicate large loads in the tower during extreme events for thecase of the spar and the semi platforms that are due to tower bending frompitch and roll. The model tests add to the present technical knowledge base in the offshorefloating wind turbine industry in terms of model test design, wind modeling, coupled response data for the floater and wind turbine and calibration ofanalysis tools. The results of the tests will be used for validation, calibration and potential improvements of floating wind turbine design toolssuch as NREL's coupled aero-hydro-elastic code, FAST. Introduction In order to pursue commercial development of floating wind turbine technology avalidated aero-hydro-servo-elastic numerical model is needed to accuratelypredict the dynamic system behavior during the design and optimization process. Currently, there are very few publicly available coupled numerical models forsimulating the performance of floating wind turbines. These codes, such asNREL's FAST (Jonkman and Buhl, 2005; Jonkman, 2007), have yet to be fullyvalidated against real data as little published information of this typecurrently exists.