Oceanography

Oceanographic investigations in the „alpha ventus“ offshore testfield

Duration of the project: 
02/2008 – 01/2025

Main goal

The oceanographic investigations are a work package of the RAVE Measurement Service. The oceanographic data provided are a base dataset for the entire accompanying research. Technical aspects (e.g. computation of design loads) as well as environmental and geological questions (e.g. sediment dynamics) can be focused. Furthermore, the data are being used to answer logistic and safety questions, as well as approval relevant questions and design bases. The data give an insight into the interactions of offshore structures and facilities with marine physical parameters.

Methods

The core of the RAVE oceanographic sub-project is a measurement programme consisting of wave and current measurements which will be carried out within the test site and in its vicinity. The wave measurements are carried out using a buoy and bottom current sensors with pressure sensors (see figure 1 and 3). By swell-conditioned deflection, the buoy provides local time series of different wave parameters (significant wave height, wave period, wave direction, and surface temperatures). Ocean current data are measured using an acoustic current profiler (Acoustic Doppler Current Profiler). The ADCP calculates the distribution of current speeds and directions in the entire water column. Besides, the particular type of ADCP used also supplies wave data, which can be additionally integrated into the wave data set. One buoy and three current profilers will be used in the test area in order to achieve a large regional measurement coverage. A radar gauge and a camera are used additionally for wave measurements. The radar gauge delivers swell parameters by measuring acoustic run-time to the sea surface, and the camera provides visual impressions of swell conditions. Additionally, short video sequences are being recorded when the sea state is high. Extreme weather events, like the storms Christian and Xaver in winter 2013 can be captured and analysed. The traditional oceanographic parameters (temperature, salinity and oxygen) where measured in the initial phase of wind farm operation using a centrally placed autonomous mooring. Temperature measurements are made continuously at two offshore structures in selected depths (one structure type each).

Measurement results

The directional wave rider started collecting data in Mai 2012 and real time data can be used since then. The park operator can use the data and can have a look at the prevailing conditions. Figure 3 shows the measured parameters maximum and significant wave height. Figure 4 shows the peak period and the mean period. The temperature (fig. 5), close to the sea surface and wave direction (fig. 6) are being measured, too. As mentioned before, several temperature sensors have been mounted at the AV04 and AV07 in selected depths. Figure 7 shows the temperature distribution for the entire data acquisition. Because of damaged cables, more and more autonomous sensors are in duty. A comparison of the two temperature profiles can be seen in figure 8, the correlations between the AV04 and AV07 are very good. Close to the foundations, autonomous ADCP’s have been placed. Figure 9 and 10 show a short, but representative extract of the collected data. The complete data sets can be found in the RAVE data base.

Sea State Portal

The Federal Maritime and Hydrographic Agency (BSH) provides a comprehensive sea state database with quality-controlled real-time data and application-oriented sea state statistics on the BSH Service Portal serviceportal.bsh.de under the application SEASTATE.

BSH has been conducting the oceanographic measurements in the alpha ventus test field since 2012. Since 2017, the oceanographic measurements have been extended to other cooperating offshore wind farms and sea state measurements from the relevant state authorities have been included in order to establish a large-scale sea state monitoring network together with the other sea state measuring stations operated by the BSH. The resulting sea state data center has as its objective:

• the bundling of in situ sea state data from the German North Sea and Baltic Sea,
• an automatic and standardized quality control of all sea state data in (almost) real time,
• a statistical evaluation of all sea state data

in order to offer a processing, evaluation, provision and interpretation of the sea state data.

The sea state portal can be accessed via serviceportal.bsh.de. Instructions for external users can be found here.