Modeling long term performance of floating offshore wind turbines during wind and wave induced platform motion

Thesis

Floating offshore wind energy has been identified as a critical source of future sustainable energy. This work characterizes the performance difference between floating and fixed offshore wind turbines in a large variety of operational conditions to better inform design decisions for floating turbines. Emphasis is placed on parameters that will drive the long-term performance of floating offshore wind turbines such as power performance, annual energy production, and contributors to component fatigue. The large number of test cases required to holistically understand the performance of floating versus fixed wind turbines necessitates fast-to-run analysis methods. However, the lower fidelity models typically utilized in wind energy analysis are challenged by the dynamic case of a freely floating turbine. First, the discrepancy between blade element momentum theory simulations and higher fidelity methods for floating turbines is characterized to better understand which metrics and range of conditions blade element momentum theory is effective for. In addition, the accuracy of blade element momentum theory for the floating case is quantified. Trends in the performance of a reference floating wind turbine undergoing prescribed harmonic motion across its operational wind speed range are then identified. Prescribed surge motion is found to increase turbine power capture at low wind speeds and decrease power capture near the rated wind speed. This effect is coupled with a large increase in the fluctuating components of turbine power and rotor thrust. A coupled aero-hydrodynamic floating offshore wind turbine model is then developed to simulate realistic platform and rotor motion. Turbine performance under a large range of regular wave loading is examined. Finally, floating offshore wind turbine performance is compared to static turbine performance under site-specific environmental conditions at four different representative case study locations. Performance metrics over one year are compared, and environmental parameters are identified which drive floating offshore wind turbine performance. The model predicts that the floating turbine will perform equal to or better than a static turbine baseline in power capture and annual energy production, but undergoes large load fluctuations contributing to component fatigue damage. The magnitude of load fluctuation due to platform motion is significant, but less than the contribution from wind speed turbulence.

Authors: Hardin, JP

• DOI: 10.5287/ora-nymmz0ee7
• Type of award: MSc by Research
• Level of award: Masters
• Awarding institution: University of Oxford
• Language: English
• Keywords: wind energy; long term performance; power performance; floating offshore wind turbine
• Subjects: Offshore wind power plants, floating offshore wind, blade element momentum theory, wind power