Effects Of Platform Pitching Motion On Floating Offshore Wind Turbine (FOWT) Rotor

Abstract

The increasing demand for offshore wind energy is leading the industry to look for expansions into deep oceans. This development has compelled the industry to venture in to floating offshore installations for wind turbines. The floating installations lead to complex rotor motions in 6 degrees of freedom. The current work focusses on various aerodynamic effects on the turbine rotor due to coupled wind and wave forces. In this research work, the dynamic effects of the platform pitching motion on the rotor for OC3 phase IV case 5.1 are studied with rigid body assumption using high fidelity CFD (Computational Fluid Dynamics) commercial tool and semi empirical tool called FAST which is developed by NREL, USA. The hydrodynamic effects leading to the pitching motion of the turbine platform are extracted from the NREL FAST software. These pitching motions are coupled with the rotating blades to study transient flow behaviors using CFD tools. The results are compared with the standard BEM based methods to identify the discrepancies if any. Moreover, fine variation of rotor power/load due to dynamic pitching of the floating platform is also compared. The induction factors are extracted from the CFD results for every time step and are compared with those obtained with BEM based methods. Moreover, the application of CFD induction factor methods are introduced for cyclic load variations due to the rotor plane motions to check the validity of the BEM for FOWT application.

Numerical Validation of Floating Offshore Wind Turbine Scaled Rotors for Surge Motion

“…The current study bolsters the fact that the surge cyclic motions are pushing the rotor under investigation to the turbulent wake state of SFA10 scenario as depicted in Figure 19. From the scaled rotor blade element number 14 to 17 (the last four elements upto scaled rotor tip of total 17 elements as in Table 1), the turbulent wake state is clearly identified by the axial induction factor, a. CFD based induction factor derivation [6] is obtained at 1.25 s of the last 2 cycles and compared against LR-AeroDyn model prediction as shown in Figure 19. LR-AeroDyn prediction is within 6% error on these elemental nodes when compared to CFD predictions.…”

“…Although the BEM codes include corrections for wake expansion, the pressure due to wake rotation and improved accuracy by accounting for the losses at blade root and tip sections, they are developed for wind turbines with static foundations. Hence the assumptions in BEM are not applicable [5] for floating offshore turbines where the pitching and surging motions [6] of the platform leads to the turbine rotor going through different wake states as shown in Figure 1. Various non-linear effects like rotor-wake interactions, wake-wake interactions, wake meandering etc., are not accounted in BEM theory.…”

Numerical Validation of Floating Offshore Wind Turbine Scaled Rotor for Surge Motion

“…19. From the scaled rotor blade element number 14 to 17(the last four elements upto scaled rotor tip of total 17 elements as in Table 1), the turbulent wake state is clearly identified by the axial induction factor, a. CFD based induction factor derivation [6] is obtained at 1.25 sec of the last 2 cycles and compared against LR-AeroDyn model prediction as shown in Fig.19.…”

“…Hence the assumptions in BEM are not applicable [5] for floating offshore turbines where the pitching and surging motions [6] of the platform leads to the turbine rotor going through different wake states as shown in Fig 1. Various non-linear effects like rotor-wake interactions, wake-wake interactions, wake meandering etc., are not accounted in BEM theory. [1] Lloyds Register (LR) has been actively involved in the development of numerical models and experimental modelling.…”