Structural response impact on floating offshore wind turbine performance under operating conditions

Abstract

A rapid increase in global energy demand and the international objective of increasing the contribution of renewable energy to satisfy this demand have spiked the interest in offshore wind energy. Depletion of suitable locations for bottom founded turbines has led to a new chapter in offshore wind development: floating wind farms. This relatively new business brings many uncertainties and no large-scale projects have been deployed yet. The performance impact of an increase in structural motions, in comparison with bottom founded offshore wind turbines, is not yet fully investigated with respect to power production of the system. This thesis seeks to quantify this impact on performance by performing an integrated system analysis of a 15-MW semi-submersible based offshore wind turbine for operational conditions at a site north of Scotland.
All considered elements in the fully integrated simulation model are broken down individually and assembled in a finite element analysis framework via OrcaWave and OrcaFlex. OrcaWave develops system RAOs and QTFs using linearised potential-flow hydrodynamic radiation and diffraction theory in the frequency domain, whereas OrcaFlex is a finite element analysis software tool with full non-linear capability in the time domain. The non-linear flow separation induced quadratic drag in a real system, which is not included in potential flow theory, is introduced via a damping matrix by addition of Morison elements to the structure. The overall system including tower, RNA, controller and mooring is analysed by OrcaFlex in time domain. This overall integrated model was first validated and verified against a well-documented open-source reference model, provided by NREL and the University of Maine, and then adapted to the semi-submersible substructure selected for this study. A case matrix is developed representing the site environmental loading scenarios under operating conditions to simulate the dynamic interactions between substructure, RNA and controller. The impact of wind-induced yaw misalignment mean offset angles on the mean power production is examined. For wind speeds below rated, an increase in yaw misalignment angle corresponds to a decrease in mean power production. For a functioning active yaw control system, misalignment angles of the rotor will be small and consequently the mean power reductions will be as well. For above-rated wind speeds, sufficient lift can be generated to compensate for the reduced rotor swept area caused by the yaw misalignment and therefore no reduction in mean power production occurs. Load case results show that with increasing sea state severity, the motions of the system increase. The impact of this increase in motions can be observed in the generator power and rotor thrust response through their fluctuations. Performing an FFT on these parameters shows that most energy in the fluctuations is situated at the incident-wave frequencies and correspondingly, the wind and wave unidirectional load cases present the largest spectral density values for generator power and rotor thrust fluctuations. Furthermore, the results present a connection between the severity of power output fluctuations and the mean power production for above-rated wind speeds. For increasing power production fluctuations, the mean power production experiences a reduction up to 0.39\%. A hypothesis is formed that links the mean power production reduction due to its fluctuations to the response time of the controller. In further research this hypothesis is to be tested and it should be further investigated whether a more detailed integrated analysis model produces similar results. The fluctuations in power production impose challenges on the ability to connect to the grid. Connection of a single floating wind turbine to the power grid is not recommended, but when considering a group of multiple turbines it is assumed that the peaks in power production are flattened out over the total power production of the group. Capacitor banks can then be implemented to create a stable power output to the grid. Quantification of this farm effect is recommended for further research. Finally, an indication of the rotor thrust impact on bending stresses at the tower-substructure interface is presented. A more detailed analysis is to be performed to find local stress concentrations and to address potential structural adjustments to fatigue-prone members.