Strong wind and high waves in offshore wind farm "Alpha Ventus"
The construction of a wind farm at sea requires on site knowledge of the expected oceanographic and meteorological conditions. Ocean and atmosphere interact in many ways. For example, strong winds influence the sea state and, conversely, warm surface water has an effect on the circulation of the air layers above. For a wind farm, the local wind conditions in particular determine the expected energy yield that can be achieved with the individual wind turbines. Seasonal fluctuations are also of interest here. Information on the usual weather conditions helps in the planning of future construction work, as many of the activities require certain weather conditions over certain periods of time. The construction of a wind farm at sea also means that the individual constructions are constantly interacting with the surrounding seawater. For this reason, information on local water levels, sea conditions and current conditions is also required. Information on the water levels changing due to the tides and the occurrence of certain wave heights provide the project operator with important information on the structural design of his plant foundations.
Collection of measurement data
Buoy for measuring the sea state parameters
In order to describe the meteorological and oceanographic conditions, sufficient measurement data must be available to enable a meaningful assessment of the investigated area. Often extreme values – i.e. the minimum or maximum values of certain measured variables – are also of interest. If necessary, measurement campaigns are carried out on areas for which no suitable measurement data are available and which are not in the immediate vicinity of a research station (FINO1-3). The measurement campaigns comprise the basic meteorological or marine physical data listed below. In order to achieve meaningful results also on seasonal influences, the measurement campaigns should cover at least one complete annual cycle and must produce sufficiently good data quality.
Time series and statistics of the following measurands are of particular interest as meteorological basic data:
Wind (speeds and directions),
Turbulence Air density.
Time series and statistics of the following measurands are of particular interest as oceanographic basic data:
Sea state (significant and maximum wave height, zero crossing period, spectral peak period, wave direction),
Current (velocities and directions, especially near the bottom, in the middle water column and in the upper water layer),
Additional meteorological information is taken from high-resolution re-analyses of the atmosphere. These provide hourly wind information in several layers in the hub area of the wind turbines. The data of the re-analyses is provided by the German Meteorological Service (DWD).
Mean wind speed of the regional reanalysis COSMO REA6 from 1995-2015 in 10 m above sea level Source: DWD
As has been proven in various research projects, the wind field on the lee side – i.e. the side facing away from the wind – is influenced by existing wind farms. This is where a decrease in wind speed and an increase in turbulence occour. These effects are to be simulated using a mesoscale "open source" model as an example. The calculated climatological change of the wind field in the vicinity of the wind farm can then be used in existing re-analyses of the atmosphere.
Information on climatological weather windows should be provided by existing models developed in research projects. If necessary, current re-analyses of the atmosphere will be used to drive these models.
It is known that wind speed and direction show multi-decadal fluctuations. Since there is no sufficiently long, uniform series of wind measurements, the models are to be based on the geostrophic wind, which is closely correlated with the "true wind". The geostrophic wind is calculated using air pressure measurements that have been reliably performed for about 100 years.
Oceanographic model data
Also with regard to the oceanographic conditions, model data and their statistical evaluations provide further information on the area of consideration. The model data for extreme value analysis look back at events of the last 70 years. This so-called hindcasts will be regularly updated in the coming years. In addition, the models are operated for shorter periods with different input data sets regarding forcing (drive by wind data). This so-called ensemble reanalysis enables a high-quality error estimation.
The extreme values will be estimated for a return period of one year, 10 years, 50 years and 100 years and include the following parameters:
Significant wave height,
Maximum wave height,
Peak wave period,
Positive total water level, negative total water level,
Maximum total surface flow.
Report on wind conditions
The scope of the report is not explicitly specified in the Wind Energy at Sea Act (WindSeeG). It was discussed at the public hearing on 28 June 2017 in Hamburg and is based on current offshore standards.
The results of the statistical evaluation of the measurement and reanalysis data are summarised in the comprehensive report on wind conditions in and around the planning area. While the in-situ measurements directly in the development areas represent the best possible data, the reanalyses provide a spatial structure of the wind field in the EEZ and allow comparison with reference measurements, for example at the FINO1-3 research stations, as well as classification of the current measurement year into long-term trends.
In detail, the following data and information are included in the summary report:
temporally and spatially high-resolution measurements at different levels in the hub area of the wind turbines,
Ensemble of regional re-analyses of the atmosphere,
Provision of a method and a model to quantify the shading effect of wind farms,
climatological information about weather windows,
Information on long-term trends in wind speed and direction.
Oceanographic conditions report
The scope of this report is based on the BSH's "Standard Construction - Minimum Requirements for the Construction of Offshore Structures in the Exclusive Economic Zone (EEZ)" documents regarding site conditions to be submitted for the first approval.
The results of the statistical evaluation of the model data and their validation by in-situ data are summarised in the comprehensive report on the oceanographic conditions at the planning area. In addition, the measurement and modelling methods used for the compilation are described.
In detail, the oceanographic report provides basic information on: