Quantifying the impact of modeling fidelity on different substructure concepts – Part 2: Code-to-code comparison in realistic environmental conditions

Abstract. Floating offshore wind is widely considered to be a promising technology to harvest renewable energy in deep ocean waters and increase clean energy generation offshore. While evolving quickly from a technological point of view, floating offshore wind turbines (FOWTs) are challenging, as their performance and loads are governed by complex dynamics that are a result of the coupled influence of wind, waves, and currents on the structures. Many open challenges therefore still exist, especially from a modeling perspective. This study contributes to the understanding of the impact of modeling differences on FOWT loads by comparing three FOWT simulation codes, QBlade-Ocean, OpenFAST, and DeepLines Wind®, and three substructure designs, a semi-submersible, a spar buoy, and the two-part concept Hexafloat, in realistic environmental conditions. This extensive comparison represents one of the main outcomes of the Horizon 2020 project FLOATECH. In accordance with international standards for FOWT certification, multiple design situations are compared, including operation in normal power production and parked conditions. Results show that the compared codes agree well in the prediction of the system dynamics, regardless of the fidelity of the underlying modeling theories. However, some differences between the codes emerged in the analysis of fatigue loads, where, contrary to extreme loads, specific trends can be noted. With respect to QBlade-Ocean, OpenFAST was found to overestimate lifetime damage equivalent loads by up to 14 %. DeepLines Wind®, on the other hand, underestimated lifetime fatigue loads by up to 13.5 %. However, regardless of the model and FOWT design, differences in fatigue loads are larger for tower base loads than for blade root loads due to the larger influence substructure dynamics have on these loads.