Floating offshore wind turbines experience different operating conditions, such as wind and wave inflow characteristics. Accurate prediction of the loads acting on the floating wind system is essential for the system design and optimisation. However, there are a lot of uncertainties with the modelling input variables for time domain simulation tools such as OpenFAST to represent various hydro-aerodynamic and structural properties. The primary objective of this work is to identify the critical input parameters for different damage-equivalent load outputs for two substructure types: OC3 Hywind Spar and OC4 DeepCwind semisubmersible. The same rotor-nacelle assembly and tower (the NREL 5MW reference turbine) are used in both case studies. A sensitivity analysis based on the damage equivalent loads of six output quantities was conducted with 8 or 10 input parameters (depending on the floater). The dependent parameters were conditionally parameterised based on the independent inputs, such as wind speed and wind-wave misalignment. The outcomes of this work show that the floater type affects the sensitivity levels of wave characteristics and hydrodynamic drag coefficients with no significant influence on the turbulence intensity, as expected. Further, the drag coefficient for spar-buoy configuration significantly influences mooring line tension compared to the semisubmersible because of their drag-dominant slender structure. The current velocity is the most dominating parameter for the mooring loads, irrespective of the floater type. While wave characteristics also influenced some turbine loads, it was almost independent of the floater type. Furthermore, the choice of the hydrodynamic model does not affect the sensitivity level rankings. A convergence study on the number of starting points was conducted to ensure a global sensitivity approach. As seen in this study, the results are floating platform-specific. This study provides valuable insight into design-driving input parameters, characterising substructure-specific wind-wave influence.

@en

Currently, the design of floating offshore wind systems is primarily based on mid-fidelity models with empirical drag forces. The tuning of the model coefficients requires data from either experiments or high-fidelity simulations. As part of the OC6 (Offshore Code Comparison Collaboration, Continued, with Correlation, and unCertainty (OC6) is a project under the International Energy Agency Wind Task 30 framework) project, the present investigation explores the latter option. A verification and validation study of computational fluid dynamics (CFD) models of the DeepCwind semisubmersible undergoing free-decay motion is performed. Several institutions provided CFD results for validation against the OC6 experimental campaign. The objective is to evaluate whether the CFD setups of the participants can provide valid estimates of the hydrodynamic damping coefficients needed by mid-fidelity models. The linear and quadratic damping coefficients and the equivalent damping ratio are chosen as metrics for validation. Large numerical uncertainties are estimated for the linear and quadratic damping coefficients; however, the equivalent damping ratios are more consistently predicted with lower uncertainty. Some difference is observed between the experimental and CFD surge-decay motion, which is caused by mechanical damping not considered in the simulations that likely originated from the mooring setup, including a Coulomb-friction-type force. Overall, the simulations and the experiment show reasonable agreement, thus demonstrating the feasibility of using CFD simulations to tune mid-fidelity models.@en

The hydrodynamic characteristics are crucial for accurately analysing floating offshore wind systems. In this paper, the added mass and damping coefficients of a semisubmersible floater are examined around the natural periods of the surge, heave, and pitch motion, using computational fluid dynamics (CFD). The OpenFOAM CFD setup is validated against experimental measurements from the free decay tests, and the same setup is used to determine the hydrodynamic coefficients of the platform subjected to forced motions with different amplitudes and periods. The added mass and quadratic damping coefficients obtained from forced oscillations are consistent with the free decay results. Moreover, the added mass coefficients obtained by CFD is significantly higher than the estimations of the potential flow theory: around 10% larger for surge and 22% larger for heave. The damping is almost independent of the frequency while it varies with the motion amplitude. The deviations in the CFD results from the potential flow theory are due to the viscous effects. Besides, viscous damping is dependent on the drag coefficient specified in the Morison's equation.

@en