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The misalignment of wind and waves is of special interest for various engineering applications, i.e. for structural fatigue load analyses, for work vessel operations for offshore construction or maintenance, and for floating wind turbines. In addition to the normal or frequent boundary conditions, the combination of extreme loads and their directions is essential for coastal and offshore designs. These aspects are addressed by analyzing ten years of field measurements in the German Bight of the North Sea, which is intensively used for offshore wind energy production. This paper focuses on the occurrence and amount of wind and wave misalignment as well as the direction dependency of the wind-wave correlation for normal conditions and extreme storms. Subsequently, the influence of statistical distribution functions for the estimation of extreme wind and wave values is presented. Based on these results, a method is developed to combine direction dependent extreme wind and wave incidents for return periods of several decades.

Environmental contours are used to simplify the process of design response analysis. A wide variety of contour methods exist; however, there have been a very limited number of comparisons of these methods to date. This paper is the output of an open benchmarking exercise, in which contributors developed contours based on their preferred methods and submitted them for a blind comparison study. The exercise had two components—one, focusing on the robustness of contour methods across different offshore sites and, the other, focusing on characterizing sampling uncertainty. Nine teams of researchers contributed to the benchmark. The analysis of the submitted contours highlighted significant differences between contours derived via different methods. For example, the highest wave height value along a contour varied by as much as a factor of two between some submissions while the number of metocean data points or observations that fell outside a contour deviated by an order of magnitude between the contributions (even for contours with a return period shorter than the duration of the record). These differences arose from both different joint distribution models and different contour construction methods, however, variability from joint distribution models appeared to be higher than variability from contour construction methods.

Die Offshore-Windenergie ist mit dem ersten deutschen Offshore-Windpark alpha ventus in Wassertiefen uber 30 m vorgedrungen. Gelten fur diese Offshore-Windenergieanlagen (OWEA), die 60 km vor der Kuste stehen, die gleichen Designprinzipien wie fur landgestutzte Anlagen? Wie sieht es mit dem Risiko dieser Anlagen aus und wie ist dieses definiert? Diese und weitere Fragen fuhrten dazu, dass seit 2009 in dem interfakultativen Verbundforschungsvorhaben Probabilistische Sicherheitsbewertung von Offshore-Windenergieanlagen (PSB OWEA) zehn Institute aus den Fakultaten Bauingenieurwesen, Elektrotechnik und Maschinenbau der Leibniz Universitat Hannover gemeinsam an wesentlichen Fragen zur Anlagensicherheit offshore forschen. Aufgrund des immensen Investitionsvolumens offshore besteht das wesentliche Ziel darin, die Zuverlassigkeit der gesamten Anlage sowie einzelner Anlagenteile und Bauteilkomponenten zu bestimmen und zu bewerten. Die Forschungsschwerpunkte wurden an den Interessen und Notwendigkeiten der Wirtschaft und Zertifizierung gespiegelt und ausgerichtet. In diesem Beitrag werden aktuelle Sicherheitsbetrachtungen fur OWEA mit Bezug zu den nationalen und internationalen Regelungen dargestellt. Safety of Offshore Wind Turbines The first German offshore wind farm alpha ventus was built in water depth more than 30 m. Are the design principles for these offshore wind turbines (OWT) erected about 60 km distance to the coast similar to the principles used for wind-turbines onshore? What about the risk of these investments and how is it defined? In 2009 researchers of the Leibniz Universitat Hannover started the research project Probabilistic safety assessment of offshore wind turbines (PSA OWT) to think about these and further questions. Since then ten institutes from the faculties of civil engineering, electrical and mechanical engineering research together in the field of the safety of OWT. Regarding the immense volume of offshore investment the target objective is to determine and to evaluate the reliability of the total system as well as individual system components. The main questions were discussed with representatives of industry and certification to bring the interests and needs into line. The current safety assessment of OWT are presented in this paper with respect to national and international regulations.

Mit der Forschungsplattform FINO 1 in der Nordsee werden seit 2003 Daten uber Wind und Seegang fur den Standort des ersten deutschen Offshore-Windparks alpha ventus gesammelt. An weiteren Standorten in Nord- und Ostsee wurden seither mehrere Forschungsstationen mit ahnlicher Zielstellung installiert. Damit liegen langjahrige Datenbanken uber masgebende Einflussgrosen im Offshore-Bereich vor, die im Rahmen eines Verbundforschungsvorhabens gezielt wissenschaftlich ausgewertet werden. Windlasten werden auch fur die Offshore-Standorte normenkonform nach validierten Windlastmodellen bestimmt. Der Seegang hingegen wird zumeist rechnergestutzt simuliert und die masgebenden Parameter wie Wellenhohe, -periode sowie Wellen- und Stromungsrichtung somit abgeschatzt. Mit den Messstationen in Nord- und Ostsee ist es moglich, Einwirkungsparameter aus Wind und Seegang messdatenbasiert zu analysieren. In diesem Beitrag werden die Herangehensweise und die verwendete Methodik der durchgefuhrten Analysen getrennt fur den Wind und den Seegang beschrieben. Die nach der Auswertung verfugbaren Ergebnisse werden mit den nach nationalen und internationalen Regelwerken anzusetzenden Grosen qualitativ und quantitativ verglichen und daraus Empfehlungen fur die praktische Umsetzung abgeleitet. Measurement based recommendations of wind and wave parameters for the design of offshore wind turbines At the research platform FINO 1 near the first German offshore wind farm alpha ventus wind and wave data are collected since 2003. With a similar objective several research platforms have been installed at other locations in the North and Baltic Sea. Based on the extensive measurements at these platforms a long-term database of significant parameters was created. These parameters are scientifically analyzed in a joint research project. Currently, wind loads at offshore locations are determined according to predefined wind load models from standards. However, sea state usually is simulated by computer and significant parameters such as wave height, wave period as well as the direction of wave and sea current are estimated. But now, based on this pool of data it is possible to analyze the load parameters from wind and sea state more reliable. Within this paper the approach and methodology of these analyzes are described separately for wind and sea state. The outcomes are compared qualitatively and quantitatively with equivalent parameters in standards and recommendations for the practical implementation are given.

Fur die Ermittlung von Wellen- und Stromungslasten auf Offshore-Grundungskonstruktionen wird haufig die Morison-Gleichung verwendet. Diffraktionseffekte und vertikale Kraftanteile werden von ihr nicht berucksichtigt. Dass dies nicht nur an hydrodynamisch kompakten Grundungsstrukturen zu unsicheren Ergebnissen fuhren kann, wird durch Vergleichsrechnungen nach Morison sowie mittels CFD-Methoden an einer Schwergewichtsgrundung gezeigt. Fur moglichst realistische Wellenzuge werden hierzu masgebende Wellenparameter aus langjahrigen Messzeitreihen bestimmt und in vergleichenden hydrodynamischen Analysen angesetzt. An definierten Punkten der Struktur werden die resultierenden Schubkrafte und Momente beider Verfahren fur eine haufig auftretende Welle moderater Abmessungen und eine Extremwelle verglichen. Hierbei werden auch bei im Verhaltnis zur Struktur sehr grosen Wellen Effekte bei der Umstromung aufgezeigt, welche zu einer signifikanten Erhohung der Beanspruchung fuhren. In einer alleinigen Berechnung nach Morison werden diese Effekte nicht detektiert. Wave loads on hydrodynamically compact concrete structures for offshore wind turbines Frequently the Morison equation is used to determine wave and current loads on offshore foundations. This approach does not consider diffraction effects and vertical load components. It is demonstrated that it may lead to uncertain results not only for hydrodynamically compact foundation structures by comparative calculations according to Morison equation and CFD methods on a gravity base foundation. Characteristic wave parameters are determined from long measurement time series for realistic wave trains. They are recognized in the comparative calculations. The shear forces and moments resulting from a frequently occurring and an extreme wave are compared for both methods at defined points of the structure. Flow effects which lead to significantly increasing loads can be detected even for very large waves in relation to the structure. These effects are not detected in calculations according to Morison equation.

Offshore wind turbines (OWT) are subjected to loads induced by wind, sea state and operation. During their service life, environmental impacts occur which lead to cyclic loading of the support structure. In this paper the so-called jacket substructure for OWT is investigated. For this kind of substructure, especially the proofs of the ultimate limit state for the axially-loaded foundation piles, are relevant within the structural analysis. Theses structural components are investigated by applying probabilistic methods. The methodical approaches and procedures to determine the load- bearing capacity of the foundation are described. Values of input parameters like wind and sea state are taken into account by their probability distributions. The stochastic pile bearing capacity is derived by applying probabilistic models of the subsoil. The outcomes of the reliability-based analysis are compared with results of the structural analysis as stated in standards.

The bond behaviour of concrete specimens with carbon textile reinforcement was investigated in the presented research programme. Pull-out specimens were cast from self-compacting concrete with expansive admixtures and in this way chemical prestress was introduced. The aim of the research was to compare bond behaviour between prestressed specimens and non-prestressed control specimens. During pull-out tests, the pull-out force and notch opening were measured with a load cell and laser sensors. Further, bond - slip and pull-out force - crack width relationships were drawn and compared for prestressed and non-prestressed specimens. Chemically prestressed specimens reached 24% higher bond strength than non-prestressed ones. It can be therefore concluded, that chemical prestressing positively influences the bond behaviour of concrete with textile reinforcement and thus better utilisation of its properties can be provided.