OC6 project Phase III: validation of the aerodynamic loading on a wind turbine rotor undergoing large motion caused by a floating support structure

Abstract

Abstract. This paper provides a summary of the work done within Phase III of the Offshore Code Comparison Collaboration, Continued, with Correlation and unCertainty (OC6) project, under the International Energy Agency Wind Technology Collaboration Programme Task 30. This phase focused on validating the aerodynamic loading on a wind turbine rotor undergoing large motion caused by a floating support structure. Numerical models of the Technical University of Denmark 10 MW reference wind turbine were validated using measurement data from a 1:75 scale test performed during the UNsteady Aerodynamics for FLOating Wind (UNAFLOW) project and a follow-on experimental campaign, both performed at the Politecnico di Milano wind tunnel. Validation of the models was performed by comparing the loads for steady (fixed platform) and unsteady (harmonic motion of the platform) wind conditions. For the unsteady wind conditions, the platform was forced to oscillate in the surge and pitch directions under several frequencies and amplitudes. These oscillations result in a wind variation that impacts the rotor loads (e.g., thrust and torque). For the conditions studied in these tests, the system aerodynamic response was almost steady. Only a small hysteresis in airfoil performance undergoing angle of attack variations in attached flow was observed. During the experiments, the rotor speed and blade pitch angle were held constant. However, in real wind turbine operating conditions, the surge and pitch variations would result in rotor speed variations and/or blade pitch actuations, depending on the wind turbine controller region that the system is operating. Additional simulations with these control parameters were conducted to verify the fidelity of different models. Participant results showed, in general, a good agreement with the experimental measurements and the need to account for dynamic inflow when there are changes in the flow conditions due to the rotor speed variations or blade pitch actuations in response to surge and pitch motion. Numerical models not accounting for dynamic inflow effects predicted rotor loads that were 9 % lower in amplitude during rotor speed variations and 18 % higher in amplitude during blade pitch actuations.

The Impact of Unsteadiness on the Aerodynamic Loads of a Floating Offshore Wind Turbine

“…Interestingly, the cases at low motion frequencies are also affected. This is in line with the findings from [11], where the presence of transient effects was identified in a surge motion scenario with rotational speed variation, while no transient contribution was found without the rotational speed variation.…”

“…However, in this case it is difficult to distinguish whether this is really a proof for transient aerodynamic behaviour or a violation of the above mentioned assumption of a small motion velocity in comparison to the wind speed. In addition, hints on a phase shift of the thrust force [11] and a reduction of blade fatigue loads [12] at high surge motion frequencies were found by Bergua et al and Eliassen. The results of experimental investigations presented by Mancini, Fontanella and others (see e.g. [2], [9], [13]) confirmed the linear trend of thrust amplitude and maximum motion velocity for low to moderate motion frequencies with a minimal motion period corresponding to 13 s for a 10 MW turbine.…”

“…However, there is also a number of studies (e.g. [18], [11]) concluding that BEMT methods with unsteady corrections are sufficient to model a typical wind turbine subjected to a surge motion.…”

Experimental Challenges and Modelling Approaches of Floating Wind Turbines

“…Additionally, it is utilized in the COREWIND project to examine the aerodynamics of two 15 MW FWTs [135]. Furthermore, the same robot is used in phase III of the OC6 project to collect data for validating the aerodynamic loading on the rotor in motion caused by a floating support structure [136]. as the focal test model.…”

A Code-to-Code Comparison for Floating Offshore Wind Turbine Simulation in Realistic Environmental Conditions: Quantifying the Impact of Modeling Fidelity on Different Substructure Concepts

“…Several efforts, past and present, have been put into verification and validation of offshore simulation codes. Notable examples being the Offshore Code Comparison ("OC" in short) programs promoted by the International Energy Agency (IEA), OC3, OC4, OC5 and the on-going OC6 (Jonkman and Musial, 2010;Robertson et al, 2014bRobertson et al, , 2017Bergua and et. al., 2023).…”