Reproduction of slow-drift motions of a floating wind turbine using second-order hydrodynamics and Operational Modal Analysis

Abstract

Slow-drift forcing and damping are important for an accurate reproduction of the low-frequency motion of floating offshore wind turbines. In this study we present a numerical model that includes both inviscid slow-drift forcing through full quadratic transfer functions (QTFs) and viscous forcing. To accommodate a linear damping matrix that represents all the damping effects on the floater, the classical Morison formulation of the drag forcing with relative velocity is simplified into a pure forcing term and a linear constant damping term. The frequency-dependent radiation properties are also replaced by constant matrices. We investigate the application of Operational Modal Analysis (OMA) to estimate the damping ratios from the test data, and also pursue a calibration method where the linear damping is calibrated in the modal space and subsequently transformed back to the physical space. The model is compared to wave basin data for four environmental conditions including the 50-year sea state. The damping ratios estimated with OMA generally increase with the sea state. The measured response can be generally well reproduced by the model when the damping is calibrated for each sea state. Due to the neglection of firstorder motion effects in the applied QTFs and to the damping introduced by the mooring lines, however, the surge response is underpredicted for the severe sea states, and the yaw motion is always underpredicted. The commonly used approach where the damping is calibrated to the decay tests is found to provide less accurate results than the approach of calibrating the modal damping ratios

Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform

“…Nevertheless, it was discovered that varying degrees of underprediction at low frequencies are ubiquitous across engineering-level tools and cannot be addressed simply by further tuning the model coefficients; rather, improvements to the formulation of the engineering models are required. Similar underpredictions have also been encountered by other researchers (see, e.g., [7,8]).…”

“…Nevertheless, it was discovered that varying degrees of underprediction at low frequencies are ubiquitous across engineering-level tools and cannot be addressed simply by further tuning the model coefficients; rather, improvements to the formulation of the engineering models are required. Similar underpredictions have also been encountered by other researchers (see, e.g., [7,8]). The underprediction of the low-frequency wave-exciting force in surge (in the direction of wave propagation) and moment in pitch (about a transverse axis perpendicular to the wave propagation) on a fixed OC5-DeepCwind semisubmersible is shown in Figure 1.…”

“…The three sources are separately discussed in this section. For consistency, the uncertainty quantification for the difference-frequency wave-load amplitudes are based on the values before the correction of Equation (8). All uncertainties in Section 6 are expressed as percentages of the uncorrected baseline solution in Table 6.…”

Prognostics-based adaptive control strategy for lifetime control of wind turbines

“…However, resilient control does not address the problem of controlling life consumption in wind turbine components to avoid early fatigue failures. Although new concepts like operational modal analysis (OMA), which relies on measurement data to analyze vibrating structures are becoming the industry standard for condition monitoring and diagnosis especially for offshore wind turbines (Kim et al, 2019;Bajric et al, 2017;Dong et al, 2018;Pegalajar-Jurado and Bredmose, 2019), these concepts are yet to be integrated for prognosis and lifetime control of wind turbines.…”

Potential-based boundary element method to calculate the hydrodynamic drift force on the floating cylinder

“…The accuracy and applicability of this new method were tested through a series of simulations with a semi-submersible model. Jurado and Bredmose surveyed a numerical model that includes both invis-Potential-based boundary element method to calculate the hydrodynamic drift force on cid slow-drift forcing through full quadratic transfer functions (QTFs) and viscous forcing [10]. Zhang et al studied low moored FPSO and the 2nd mid-field formulas for multi tions for progressive wave forces and low frequency forces for a LNG carrier ship Lupton and Langley investigated the scaling of small drift motions with platform size with mooring characteristi et al calculated the second cylinders [13].…”