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We present experimental data from MARIN on a bottom-fixed offshore wind turbine mounted on a monopile in intermediate water depth subjected to severe irregular wave conditions. Two models are analysed: the first model is fully flexible and its 1 st and 2 nd eigenfrequencies and 1 st mode shape are representative of those of a full-scale turbine.This model is used to study the structural response with special focus on ringing and response to breaking wave events.The second model is stiff and is used to analyse the hydrodynamic excitation loads, in particular the so-called secondary load cycle. The largest responses are registered when the second mode of the structure is triggered by a breaking wave on top of a ringing response. In such events, the quasi-static response accounts for between 40 and 50% of the total load, the 1 st mode response between 30 and 40%, and the 2 nd mode response up to 20%. A statistical analysis on the occurrences and characteristics of the secondary load cycle shows that this phenomenon is not directly linked to ringing.

Abstract The main objective of the Offshore Code Comparison Collaboration Continuation, with Correlation (OC5) project is validation of aero-hydro-servo-elastic simulation tools for offshore wind turbines (OWTs) through comparison of simulated results to the response data of physical systems. Phase III of the OC5 project validates OWT models against the measurements recorded on a Senvion 5M wind turbine supported by the OWEC Quattropod from the alpha ventus offshore wind farm. The following operating conditions of the wind turbine were chosen for the validation: (1) idling below the cut-in wind speed, (2) rotor-nacelle assembly (RNA) rotation maneuver below the cut-in wind speed, (3) power production below and above the rated wind speed, and (4) shutdown. A number of validation load cases were defined based on these operating conditions. The following measurements were used for validation: (1) strains and accelerations recorded on the support structure and (2) pitch, yaw, and azimuth angles, generator speed, and electrical power recorded from the RNA. Strains were not directly available from the majority of the OWT simulation tools; therefore, strains were calculated based on out-of-plane bending moments, axial forces, and cross-sectional properties of the structural members. The simulation results and measurements were compared in terms of time series, discrete Fourier transforms, power spectral densities, and probability density functions of strains and accelerometers. A good match was achieved between the measurements and models setup by OC5 Phase III participants.

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