OC5 Project Phase II: Validation of Global Loads of the DeepCwind Floating Semisubmersible Wind Turbine

TLDR

The project, under IEA Wind Task 30, aims to validate offshore wind modeling tools by comparing simulated responses of selected system designs with physical test data. Phase II validated DeepCwind numerical models by comparing calculated ultimate and fatigue loads for eight wave‑only and wind/wave test cases against 1/50th‑scale measurement data from the MRIN wave basin, following calibration with free‑decay, wind‑only, and wave‑only tests. The validation showed acceptable estimates of ultimate and fatigue loads but consistently underestimated tower and upwind mooring line loads; better agreement was achieved with dynamic mooring, extended hydrodynamic modeling, nonlinear wave kinematics, and unsteady aerodynamics, while Morison‑only models produced excessive pitch excitation and tower loads.

Abstract

This paper summarizes the findings from Phase II of the Offshore Code Comparison, Collaboration, Continued, with Correlation project. The project is run under the International Energy Agency Wind Research Task 30, and is focused on validating the tools used for modeling offshore wind systems through the comparison of simulated responses of select system designs to physical test data. Validation activities such as these lead to improvement of offshore wind modeling tools, which will enable the development of more innovative and cost-effective offshore wind designs. For Phase II of the project, numerical models of the DeepCwind floating semisubmersible wind system were validated using measurement data from a 1/50th-scale validation campaign performed at the Maritime Research Institute Netherlands offshore wave basin. Validation of the models was performed by comparing the calculated ultimate and fatigue loads for eight different wave-only and combined wind/wave test cases against the measured data, after calibration was performed using free-decay, wind-only, and wave-only tests. The results show a decent estimation of both the ultimate and fatigue loads for the simulated results, but with a fairly consistent underestimation in the tower and upwind mooring line loads that can be attributed to an underestimation of wave-excitation forces outside the linear wave-excitation region, and the presence of broadband frequency excitation in the experimental measurements from wind. Participant results showed varied agreement with the experimental measurements based on the modeling approach used. Modeling attributes that enabled better agreement included: the use of a dynamic mooring model; wave stretching, or some other hydrodynamic modeling approach that excites frequencies outside the linear wave region; nonlinear wave kinematics models; and unsteady aerodynamics models. Also, it was observed that a Morison-only hydrodynamic modeling approach could create excessive pitch excitation and resulting tower loads in some frequency bands.