Structural probabilistic assessment of Offshore Wind Turbine operation fatigue based on Kriging interpolation Rui Teixeira, Alan O’Connor & Maria Nogal Trinity College Dublin, Dublin, Ireland James Nichols & Mark Spring Lloyd’s Register, London, United Kingdom ABSTRACT: The probabilistic analysis of Offshore Wind Turbines (OWT) is not a new practice. The standards for designing OWT (IEC 61400 class) emphasizes that assessing uncertainty is of major importance inside the design chain. Still, major challenges related to the uncertainty and the probabilistic assessment pose to the sector and its development. The analysis of operational loads is one them. The problem of analyzing extreme responses or cumulated damage in operation during the design phase is signif- icantly related to its high computational cost. As we progressively add complexity to the system to account for its uncertainties, the computational effort increases and a perceptive design becomes a heavy task. If an optimization process is then sought, the designing effort grows even further. In the particular case of fatigue analysis, it is frequent to not be able to cover a full lifetime of simulations due to computational cost restrictions. The mentioned difficulties fomented the utilization of surrogate models in the reliability analysis of OWT. From these surrogate approximations the ones based on Kriging models gained a special emphasis recently for structural reliability. It was shown that, for several applications, these models can be efficient and accurate to approximate the response of the system or the limit state surfaces. The presented paper tackles some of the issues related to their applicability to OWT, in a case specific scenario of the tower component subjected to operational fatigue loads. A methodology to assess the reliability of the tower component to fatigue damage is presented. This methodology combines a Kriging model with the theory of extreme values. A one-dimensional Kriging case using the state of art NREL’s monopile turbine is presented.The reliability of the OWT tower is calculated for 20 years. The results show that the usage of a Kriging model to calculate the long term damage variation shows a high potential to assess the reliability of OWT towers to fatigue failure. 1 INTRODUCTION The Offshore Wind Turbine (OWT) sector has been growing steadily in parallel with the current needs and demands for clean energy. Still, the cost of OWT en- ergy is still above other energy conversion sources. Despite being a very highly technical sector, improve- ment of the design techniques remains a need to un- lock new breakthroughs that will enable it to become progressively more competitive. At the same time, probabilistic design is further de- manded by the sector as a strategy to enable compre- hensive designing processes that generate robust sys- tems. One of the areas of particular concern for OWT, as designed for long life-time, is the fatigue design. In the case of the OWT towers, fatigue is seen as a major driver of failure. Certifying and designing a wind turbine to ensure that it is not going to fail through structural fatigue may involve a very onerous design process. The calculation of fatigue on a wind turbine, as a cumulative event, means that the designer needs to simulate and repli- cate the damage suffered by the wind turbine during its design life time. This usually means thousands of computational simulations, and consequently, a huge computational effort that hinders the design and even more the optimization process on a practical basis.