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Monopile head stiffness for servicibility limit state calculations in assessing the natural frequency of offshore wind turbines

Monopiles are used as foundations for offshore wind energy towers. These large diameter monopiles (4–6 m) which result in extremely stiff short monopiles do not follow the same elastic deformation patterns as those exhibited by small diameter (0.5–1.5 m) monopiles usually used for supporting structures in offshore oil and gas industry. Design recommendations (API and DNV among others) which have been developed on the basis of full-scale load tests on long, slender and flexible piles are not suitable for designing foundations for offshore wind turbine structures. Furthermore, as these facilities are very sensitive to rotations and dynamic changes in the soil-pile system, the precise prediction of monopile head displacement and rotation constitutes a design criterion of great importance. In this article, the semi-analytical finite element analysis which combines the conventional finite element method with Fourier series representation of displacements is used for the study of monopiles for offshore wind turbines under lateral loading in homogeneous soils. In order to develop functional forms of design equations for large diameter monopiles like the ones used for supporting offshore wind turbines, an extensive parametric study has been carried out. The analyzed parameters including pile slenderness ratio and soil stiffness allow to obtain simple and approximate closed-form solutions for static head stiffnesses of both rough and smooth short monopiles embedded in homogeneous soils. It has been found that lateral stiffness KL, rotational stiffness KR and cross coupling stiffness KLR for both rough and smooth interfaces depend only on the value of the monopile slenderness Lp/Dp rather than the relative soil/monopile rigidity Ep/Es usually found in the offshore platforms designing codes (DNV code for example). These analytical expressions have been incorporated into the expressions of the Offshore Wind Turbine (OWT) natural frequency of two wind farm sites. Excellent agreement has been found between the computed and the measured natural frequencies.