Floating offshore wind turbines: designing against the forces of nature

Significance Statement

Studies done before on a small scale floating horizontal axis wind turbine in surge motion showed thatthere is an increasing amplitude of the cyclic thrust and power generation against tip speed ratio. A numerical study using an Actuator Disk (AD) Navier Strokes model, a Blade Element Momentum (BEM) model and a Generalized Dynamic Wake (GDW) model was performed in order to determine the previous observations on the full-scale NREL 5MW reference rotor in surge motion. The research question was to understand the reason why such high variations in thrust and torque occur at non-optimal tip speed ratios. The research was done to improve the understanding of the fundamental science governing floating offshore machines so as to make them commercially viable in the future.

The test was performed by maintaining the surge amplitude and surge frequency fixed and changing the tip speed ratio. Full details of the AD model can be found in the full paper. Results are then compared with BEM combined with dynamic inflow engineering models as well as the GDW model.

When the operating tip speed is increased the released vorticity in the wake becomes stronger causing an increase in the amplitude variations of the flow inductions in the axial, radial and swirl directions. The extent of the radial expansion and contraction of the wake was found to increase with increasing tip speed ratio. The study makes us conclude that the dynamic wake model can be adopted for BEM modeling of a surging rotor but only if mean quantities are of interest. The GDW model on the other hand gives quite an acceptable agreement with what the AD model.

Their work give credit to the previous experiments conducted on a small model rotor. They produce similar results that thrust and power amplitudes vary with wave amplitude and frequency. The unsteady variations in thrust and power are clearly observedto increase at higher tip speed ratios related to turbulent wake condition. This affects the structural and electrical design of the commercial turbines to manage fatigue when the turbine is operated in its rated conditions. The power will have to be tapped by suitable electronics that can handle the strength and instability from the turbine. The study recommends that it is ideal to operate at low speed tip ratios to reduce the fatigue loads on the blades, especially where the power demands are not very high. Concludingly, the results from this quantitative study were compared to the FAST code results using both BEM and unsteady GDW models.. Some difference was found at high tip speed ratio towards the onset of the turbulent wake state. The results for low tip speed ratios agreed quite well. The study was however limited due to the fact that the rotor was tested under fixed surge conditions and varying tip speed ratios. 

 Loading effects on floating offshore horizontal axis wind turbines in surge motion.Renewable Energy Global Innovations


About the author

Dr. Daniel Micallef is an academic at the University of Malta where he joined the Environmental Design department of the Faculty for the Built Environment in September 2014.

Dr. Micallef graduated in Mechanical Engineering from the University of Malta in 2008 with first class honours. He started his professional career in the public sector with the Malta Resources Authority as an energy analyst. During this time he started pursuing a career in academia. He read for a joint PhD with the Delft University of Technology in the Netherlands (where he formed part of the DUWIND wind energy research group) and the University of Malta.

His research focused on furthering the understanding of wind turbine flow phenomena close to the tip. He was awarded his PhD in 2012. During the final year of his PhD, Dr. Micallef also worked as a project officer at the Mechanical Engineering Department of the University of Malta where he developed analysis tools and contributed in the design of an urban wind turbine being developed by industry. His research career took a twist in 2012 were he continued his research experience as a post-doctoral researcher on the HILDA FP7 project. While continuing to publish his work in wind energy, his post-doc research focused on a different topic – modelling of friction stir welding of steels.

He developed finite element and computational fluid dynamics models for the numerical analysis of the process. During his final months on the project, he was engaged as a lecturer at the Malta College of Arts Science and Technology (MCAST). His experience as a post-doc researcher and MCAST lecturer ended in September 2014.

Dr. Micallef published in high quality peer reviewed journals and conferences worldwide. His current major interests are in the fields of wind energy, wind engineering and building physics. Apart from his research activities, he lectures in undergraduate and Masters courses. Dr. Micallef is currently the secretary general of the Chamber of Engineers (an NGO). He is also the COST (Cooperation in Science and Technology) representative of Malta in two COST actions.  

Journal Reference

Daniel Micallef1, Tonio Sant2. Loading effects on floating offshore horizontal axis wind turbines in surge motion.  Renewable Energy, Volume 83, November 2015, Pages 737–748.

Show Affiliations
  1. Department of Environmental Design, Faculty for the Built Environment, University of Malta, Malta
  2. Department of Mechanical Engineering, Faculty of Engineering, University of Malta, Malta



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