Evaluation of Morison approach with CFD modelling on a surface-piercing cylinder towards the investigation of FOWT Hydrodynamics

To predict hydrodynamic forces on slender cylinders, Morison formula is commonly used. The expression is composed of two terms including an empirical drag and inertia coefficient. In the present study, a constrained cylinder subjected to regular waves is modelled using Computational Fluid Dynamics (CFD) and Morison coefficients are derived from the CFD loads. A numerical wave tank, including a surface-piercing cylinder, is implemented to generate, propagate and absorb waves in a controlled way. The cylinder is discretised into slices for each of which the Morison formulation is fitted to the CFD loads to determine Morison coefficients. Morison formulation is in very good agreement with the numerical load over the entire length of the cylinder, except for the slices near the free surface which are outside the applicability range of Morison formula. The loads near the free surface being negligible compared to the total load, the total theoretical load obtained reproduces well the experimental and the CFD loads. The CFD simulations proved to be a powerful tool to predict the wave loads and calibrate Morison coefficients. On this basis, the methodology will be extended to floating offshore wind platforms, combining several cylinders with different inclinations, sizes and positions. •A Numerical Wave Tank including a surface-piercing cylinder validated against experiments.•A method to derive Morison coefficients from the CFD loads along the cylinder, according to water depth.•Significant influence of the free-surface on the total load shape with the existence of a local bump before the minimum of the load variation, known also as secondary load cycle.•Negligible impact of the free-surface on the total load amplitude•Correlation between the wave run-up and the loads on the upstream/downstream part of the cylinder.