In the framework of wind farm development, financing and operation, developers regularly need to quantify on a reliable way the wind turbulence intensity located on the sites. This information can be used to validate wind farm layouts and select the appropriate turbines to be installed during the development process and to optimise the wind farm operation. The current instrumentation to measure the turbulence intensity is anemometer (cup- or sonic-) mounted on a meteorological mast, with a measurement altitude as close as possible to the hub height. Though, cost and technical constraints specific to offshore configurations make this solution not affordable and is replaced by remote sensing (LiDAR) instrumentation mounted on buoys (floating LiDAR Systems). This alternative is now accepted during wind project developments and the quality of the obtained 10-min averaged wind statistics had been validated. That is not the case for turbulence intensity.
The 24-month MATILDA project aims at quantifying the error on the turbulence intensity measurement due to motions of the floating LiDAR and at proposing corrections depending on the sea state. The analysis will be based on the comparison of measurements performed simultaneously by a fixed LiDAR system located on Fécamp offshore platform (owner EDF Renouvelables) and by a floating LiDAR system WINDSEA during its validation campaign (owner AKROCEAN). The Fécamp platform is also equipped with a met-mast (owner Eoliennes Offshore des Hautes Falaises, consortium composed of EDF Renouvelables, Enbridge and WPD Offshore). As preliminary testing, turbulence intensity measurement comparison between conventional instrumentation (cup and sonic anemometers) and a fixed LiDAR profiler will be performed thanks to a measurement campaign that will be carried out on a VALOREM operated site.
Additionally, a displacement system able to impose to a LiDAR system controlled motions similar to full scale floating motions will be designed.
Scientific advances and innovation
- Processing of statistical wind data obtained in non-controlled environments (wind + wave)
- Correction laws and processing algorithms to more reliably assess the turbulence intensity with a floating LiDAR
- Design of a full scale displacement system to reproduce floater motions (buoys or wind turbine floaters)
Expected technical and economic impact
Floating LiDAR is an accepted metrology for wind resource assessment of offshore wind energy projects under development and operation. It is also deployed on future floating wind farm projects, where no other instrumentation can be installed due to too important sea water depth. 10-min averaged wind speed and direction are the standard values extracted from the floating LiDAR measurements and are considered as reliable. This information is generally enough to assess the annual yield production, but prevents from assessing the wind turbine lifetime. Indeed, the structural fatigue is directly related to the wind speed fluctuations, primarily characterized by the turbulence intensity. The measurement error of the turbulence intensity obtained by floating LiDAR system and its dependency to the sea states (wave periods and specific heights, wind/wave direction, etc.) are not yet quantified, and the limit met-ocean conditions, beyond which measurement correction and/or motion compensation are needed, are not yet known.
MATILDA project aims at contributing to the collective effort from the scientific community working on offshore wind resource assessment by assessing and reducing the measurement error of turbulence intensity by floating LiDAR system through full scale data post-processing and analysis, and the design of a full scale test bench able to reproduce controlled floating LiDAR motions.
Key project milestones
- Oct. 2019 - Project Kick-off
- Jan. 2020 - Post-doc recruitment
- Jan.-Mar. 2020 - Creation and exploitation of a measurement database to compare wind data from a meteorological mast and a fixed LiDAR profiler
- Jan.-Oct. 2020 - Exploitation of a measurement database to compare wind data from a meteorological mast and a floating LiDAR profiler
- Dec. 2020 - Correction laws for turbulence intensity measurements with fixed and floating LiDAR profilers
- Dec. 2020 - Specifications of a full scale displacement system to reproduce floater motions
- Dec. 2021 - Publications of results in high ranking scientific journals