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<p>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.</p>

Monopiles are the preferred foundation solution for offshore wind turbines in sandy soils below shallow and moderate water depths. The piles are commonly installed using impact driving. A severe disadvantage of this installation method is that high noise levels occur in the water, which are a risk to animals living in the sea. Approval authorities are known to impose statutory requirements to mitigate this risk, placing a financial burden on installation. Vibratory driving is an alternative installation method that reduces noise emission levels significantly. However, the effect of vibro‐driving on the bearing behaviour of piles under lateral loading is largely unknown. Full‐scale load tests were conducted in predominantly dense, non‐cohesive, saturated soil to investigate the differences between impact‐driven piles and vibrated piles regarding their lateral bedding behaviour. CPTs and pile head load‐displacement curves were recorded and evaluated. In general, it was determined that the bearing behaviour of vibro‐driven piles is heavily dependent on the parameters of the installation process. For the installation parameters used in the test campaign, two vibro‐driven piles showed a weaker bedding reaction in comparison to the impact‐driven piles, although one vibrated pile installed with unintentionally different parameters showed an only slightly weaker behaviour under primary loading and even increased unloading and reloading stiffness. Furthermore, it was observed that an indication regarding the differences in the bedding resistance can be obtained via CPT measurements before and after installation, since the installation procedure considerably affects the relative density of the soil around the pile.

Offshore‐Windenergieanlagen (OWEA) sind den Beanspruchungen aus Wind, Seegang und Anlagenbetrieb ausgesetzt. Während ihrer Lebensdauer treten umweltbedingte veränderliche Einwirkungen auf, die zu zyklischen Beanspruchungen der Tragstruktur führen. Bei OWEA mit Jacket‐Unterstrukturen sind im Rahmen der Tragfähigkeitsanalyse vor allem die axial beanspruchten Gründungspfähle und die ausbetonierten Rohr‐in‐Rohr‐Steckverbindungen zwischen Jacket und Gründungspfahl (sogenannte Grouted Joints) relevant. In diesem Beitrag werden die Jacket‐Unterstruktur und die Pfahlgründung einer OWEA mit probabilistischen Methoden untersucht. Die dafür verwendeten methodischen Ansätze und Verfahren zur Ermittlung der Tragfähigkeit von Gründungselementen werden beschrieben. Als Eingangswerte werden die aus Messwerten abgeleiteten Auftrittswahrscheinlichkeiten von Wind und Seegang verwendet. Zur Beschreibung der Bodentragfähigkeit gehen probabilistische Modelle ein. Die Ergebnisse der zuverlässigkeitstheoretischen Analyse werden zu den üblichen Berechnungen nach aktuellen nationalen und internationalen Regelwerken in Bezug gesetzt. Aus diesem qualitativen und quantitativen Vergleich resultieren Empfehlungen für die probabilistische Bemessung von Offshore‐Gründungen. Safety aspects for support structures of offshore wind turbines Offshore wind turbines (OWT) are subjected to loads induced by wind, sea and operation. During their life‐time environmental varying impacts occur, which lead to cyclic loading of the support structure. Within the structural analysis of OWTs with jacket substructures, especially the proofs of the ultimate limit state for the axially‐loaded foundation pile and for the grouted pile‐in‐sleeve connection between jacket and foundation pile (so‐called grouted joints) are relevant. In this paper the jacket substructure and the piled foundation of an OWT are investigated by applying probabilistic methods. The methodical approaches and procedures to determine the load‐bearing capacity of the foundation are described. Input values for the environmental impacts of wind and sea are taken into account by their probability distributions. The stochastic pile bearing capacity is derived by applying probabilistic models. The result of the reliability‐based analysis is compared to the structural analysis as stated in valid standards, and derived recommendations are given for the probabilistic design of offshore foundations.

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