Comparison of Second-Order Loads on a Semisubmersible Floating Wind Turbine

Abstract

As offshore wind projects move to deeper waters, floating platforms become the most feasible solution for supporting the turbines. The oil and gas industry has gained experience with floating platforms that can be applied to offshore wind projects. This paper focuses on the analysis of second-order wave loading on semisubmersible platforms. Semisubmersibles, which are being chosen for different floating offshore wind concepts, are particularly prone to slow-drift motions. The slack catenary moorings usually result in large natural periods for surge and sway motions (more than 100 s), which are in the range of the second-order difference-frequency excitation force.Modeling these complex structures requires coupled design codes. Codes have been developed that include turbine aerodynamics, hydrodynamic forces on the platform, restoring forces from the mooring lines, flexibility of the turbine, and the influence of the turbine control system. In this paper two different codes are employed: FAST, which was developed by the National Renewable Energy Laboratory, and aNySIM, which was developed by the Maritime Research Institute Netherlands. The hydrodynamic loads are based on potentialflow theory, up to the second order. Hydrodynamic coefficients for wave excitation, radiation, and hydrostatic forces are obtained with two different panel codes, WAMIT (developed by the Massachusetts Institute of Technology) and DIFFRAC (developed by MARIN).

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

“…Viscous effects and their influence on second-order response was not considered in their work. A similar validation study was carried out by Gueydon et al [9], who investigated the effect of second-order loads on a semisub using two different numerical codes.…”

“…This simplification may lead to an underprediction of the second-order loads for severe sea states. On the other hand, computation of the QTFs including the first-order solution with insufficient damping may lead to overestimation of the second-order loads [9].…”

Validation of a lumped-mass mooring line model with DeepCwind semisubmersible model test data

“…In previous model validation work, Coulling et al (2013a,b) modeled the platform hydrodynamics by supplementing FAST's default linear hydrodynamics approach with quadratic damping coefficients in each of the six platform degrees of freedom and later included the effects of second-order difference-frequency forces using Newman's approximation. The model used in this study builds on that previous work, using the same quadratic damping coefficients (given in Table 4) but extending the hydrodynamics to include full second-order wave excitation forces and a viscous mean drift force in surge.Sum-frequency and difference-frequency second-order wave excitation loads are calculated following the method ofGueydon et al (2014). An additional source of mean drift is noticeable in the test results, which prompted the inclusion of viscous mean drift forces in the modeling.…”

Validation of INNWIND.EU Scaled Model Tests of a Semisubmersible Floating Wind Turbine

“…The concept has been used in previous research and it has been tested at MARIN in Wageningen (Netherlands) by the DeepCwind consortium in 2011, see Jain et al (2012), Coulling et al (2013) and Robertson et al (2013). The same model has been tested again at MARIN in 2013 with detailed analyses of the secondorder wave excitation forces and the aerodynamics at low Reynolds numbers, see Kimball et al (2014), Ridder et al (2014), Make et al (2015), Gueydon et al (2014), Gueydon (2015), Gueydon et al (2015), and Gueydon (2016). Currently, within the activities of IEA wind task 30, the OC5 consortium (Offshore Code Comparison Collaboration Continuation with Correlation) uses the measurement data from MARIN for a joint validation task.…”