Offshore wind farms will become more frequent in the German part of the North Sea. High and continuous wind speeds, as well as remoteness to populated areas are advantages of offshore wind turbines. To evaluate the environmental sustainability and technical/financial feasibility of offshore wind farm expansion, the German Federal Government has decided to encourage the renewable offshore sector, among other things, with a comprehensive scientific research project (Research at Alpha Ventus (RAVE)) [1]. The RAVE project startet 2008 with first marine observations 2010. Purpose of this project is to enhance the sparely investigated knowledge of impacts of the harsh marine conditions with regard to construction and operation in offshore regions. Within this project, extensive oceanographic investigations are carried out.The measured oceanographic data is a basic dataset to the entire accompanying research, both for the technical aspects as for the environmental and geological investigations. Furthermore the influences of offshore structures and facilities should be examined with respect to shelf sea physics and vice versa. The present study shows results from five years of oceanographic measurements regarding to construction and operation of offshore wind farms. For that reason the focus is on sea state observations (installation, maintenance and data analysis) which have a major impact on the operational management of offshore facilities. In this paper, measurements with directional wave rider buoys, various radar level gauges (sea state parameters calculated from water level data) and ADCPs (with option to measure sea state) are compared and evaluated to their physical limits, ambient conditions, accuracies, as well as technical advantages and disadvantages. A better understanding of the instruments helps to optimize the measurement concepts in terms of type, number, measurement time and locations. In consequence this can lead to an improvement of offshore energy production facility design, robustness and dimensioning. Additional benefits can be an optimization of facility locations and access times during facility construction and maintenance, cost reduction as well as improved energy production rates.

In situ sea state measurements are essential for offshore activities, coastal protection and science (here in example for the validation of numerical models and of earthor satellite-based remote sensing instruments). Therefore, well quality checked data is a must. Up to now it is difficult to discriminate between outliers and extreme waves in raw surface wave displacement time series. A consequence of this fact is the uncertainty of the correctness of the maximum wave height in a time period and of extreme waves at all. A first approach method has been developed considering some physically motivated singlewave features. Aim of the development in progress is a fully automated procedure to ensure the sea state time series quality in regard of the discrimination of outliers and extreme waves; in consequence this could also increase the reliability of in situ extreme wave statistics.