Comparison of Wave-induced Response of a TLP Wind Turbine Obtained by CFD Method and Potential Theory

Abstract

A computational fluid dynamics (CFD) method has been verified and compared with potential theory approach to assess the hydrodynamic loads on a tension leg platform wind turbine (TLPWT). Floating wind turbines have the potential to increase the use of sustainable and clean wind energy resources in deep waters. Recent investigations suggest that tension leg platforms can be a suitable candidate for floating wind turbines: TLPWT have very small nacelle pitch motions, relatively simple platform designs, and the potential for installation in relatively shallow water depths. On the other hand, the tendon system increases the overall construction and installation costs. Since the tendon system may be susceptible to high-frequency responses (springing and ringing), TLPWT design requires precise estimation of the nonlinear hydrodynamic loads. In the current work, first the CFD approach is verified by modeling a surface piercing fixed cylinder and comparing the wave forces and moments with potential theory results, calculated by WAMIT, and also available experimental data. Then, the model has been used to study a two body modeling of a 5 MW TLPWT and the results are compared with the well-verified SIMO/RIFLEX/AeroDyn (SRA) code based on the WAMIT results. Free decay tests comparisons show very good agreements of natural frequencies in different modes but higher damping ratios are predicted by CFD model which might be due to the simplification of drag forces in SRA code. Comparison of TLPWT responses to regular waves shows very good agreements in surge and heave modes but less favorable in pitch, probably because of very small pitch angle of the considered TLPWT. Some high frequency responses (close to the TLPWT heave and pitch natural frequencies) are also observed by CFD method in the heave direction.