The Full Story
Offshore wind plays a key role in the Dutch route to achieving the international climate goals. 2019 is set to be a record year with new offshore installations exceeding 1 GW. By 2030, Dutch offshore wind capacity will total 11 GW, supplying electricity to households and industries.
In September 2014, the Dutch Government published its first roadmap. It was agreed that 5 wind farms should be completed by 2023. Together with the existing wind farms, these should ensure a total installed capacity of 4.5 gigawatts. Apart from Borssele (1,400 MW), Hollandse Kust (zuid) (1,400 MW) and Hollandse Kust (noord) (700 MW) wind farms, three more areas zones are designated as wind farm zones after 2024: Hollandse Kust (west) (1400MW), Ten Noorden van de Waddeneilanden (700 MW) and IJmuiden Ver (4GW).
Need for an agile web-based database of metocean data
In 2016, DHI established (for RVO.nl) a set of new design basis for the Dutch waters – for Hollandse Kust (zuid) & Hollandse Kust (noord) and the areas between – providing new standards in the provision of metocean data and reducing the uncertainties significantly. This helped reducing the costs of off shore wind energy, contributing to a subsidy-free tender for Hollandse Kust (zuid) Wind Farm Sites I and II.
Since 2016, RVO.nl has been carrying out new metocean measurement campaigns and bathymetric surveys at Hollandse Kust (noord). RVO.nl required an update of the metocean data, while reducing uncertainties in the design conditions even more.
In addition, RVO.nl was looking for an agile web-based database to provide all necessary data for design to all stakeholders and developers. The analysis had to be made at all grid points in the Dutch North Sea and made available to users in a user-friendly way.
Advanced modelling combined with state-of-the-art web-based metocean database
DHI established dedicated high-resolution (~200m for the hydrodynamic model and ~400m for the wave model) advanced numerical models (based on MIKE Powered by DHI software package) covering the period from 1979 to 2018 to provide metocean conditions in the Dutch North Sea area.
The models were forced with corrected wind/pressure field data from the Climate Forecast System Reanalysis (CFSR) dataset established by the National Centers for Environmental Prediction (NCEP). An extensive validation of the modelling results was conducted using satellite and local measurements.
Time series of simulated wave, current and water level conditions together with CFSR (corrected) winds are available at each grid point of the respective model mesh within the Dutch Wind Farm areas, namely Hollandse Kust (noord), Hollandse Kust (west), IJmuiden-Ver and Ten Noorden van de Waddeneilanden. The database hence provides access to 55,900 time series of wave conditions (total, sea and swell), while current and water level (total, tide and residual) time series are available at more than 106,200 elements, for a period of +39 years (1979-2018). The atmospheric parameters (temperature, humidity & pressure) are based on the Harmonie model and are available together with other metocean parameters.
DHI’s web-based database showing the water depths covering the model together with the computational mesh © DHI
Highlights of the metocean study and web-based database:
- 40 years of wind data covering the Dutch North Sea and aligned with the Wind Resource Assessment study (for Hollandse Kust (noord) WFZ)
- Establishing an empirical wind profile to reduce conservatism for higher altitude wind speeds
- 40 years of high resolution hydrodynamic (200m) and wave modelling (400m)
- Calibration and validation of the models using more than 20 measurement stations across the Dutch North Sea
- Providing reliable data not only at the offshore wind farm zones, but also along the cable corridors and offshore areas
- Using the most advanced non-stationary statistical analysis to provide reliable design data and reduce conservatism
- Providing meteorological data at various heights (10, 60, 100, 120, 160, 200, 250 & 300)
- Providing access to wave/wind data at 55,900 points (including NSS tables & extremes)
- Providing access to HD data at 106,200 points
- Providing 2D spectral data for the Dutch North Sea area
Scatter comparison of significant wave heights between the model and measurements at Europlatform for the period 1989-2018 © DHI
Features of the Statistical Analysis:
DHI utilised their most advanced suite of tools for extremal analysis which is based on the new advances in non-stationary extreme value statistics.
The J-EVA suite of tools are particularly well-suited in a complex area like the Dutch North Sea, where there are large variations in the available fetch and a number of different wave systems (storms coming down from the Northern North Sea, North Atlantic storms coming up the English Channel and fierce westerly storms with short fetches).
These systems have different properties with respect to sea state steepness, storm duration, wind-wave misalignment, seasonal variation, etc, but can potentially all contribute to the design wave load on structures in the Dutch Offshore Wind farm area. DHI J-EVA handles these variations consistently as it, unlike traditional methods, does not assume stationarity in either season or direction. Furthermore, as a data-driven model, it makes no a-priori assumptions about sector divisions, nor seasonal bins.
Quantile regression analysis, illustrating the components of tensor-product P-splines in 2 dimensions. The coloured surfaces show the individual tensor-product B-splines each multiplied by its respective 𝛽-coefficient. Quadratic B-splines (𝑞 = 2) and first order penalty have been used. © DHI
Accurate and reliable metocean data
The validation showed very good model performance of the numerical models against more than 20 measurement stations across the Dutch North Sea, which ensured accurate and high-quality metocean conditions at the desired areas.
The combination of well-validated models and state-of-the-art statistical tools resulted in more accurate extreme values and defined a new design basis for the Dutch North Sea. This will save costs and reduce uncertainties and conservatism.
The metocean report (in English) can be downloaded here.
Readily available certified data for developers, designers, modellers and more
Terabits of modelling data (time series and spectral) are available in user-friendly, fast and agile web-based database. In addition, users have access to normal and extreme condition analysis at more than 55,000 points. Some of the highlights are:
- Easy access to certified metocean data at Hollandse Kust (noord)
- Data is fit-for-purpose and ready to be used for design
- Reduce conservatism in the design data resulting in cost-efficient design
- Easy access to data at Hollandse Kust (west), IJmuiden Ver, Ten Noorden van den Waddeneilanden offshore wind farms and cable corridors