A GEOTECHNICAL MODELLING FRAMEWORK FOR THE TILTING ANALYSIS OF OFFSHORE MONOPILES UNDER ENVIRONMENTAL CYCLIC LOADING

The development of offshore wind farms requires advanced knowledge and technology, particularly with regard to the design and installation of foundation systems. Offshore wind turbine foundations, such as monopiles, cannot be adequately designed without in-depth analysis of soil-foundation interaction under environmental cyclic loads, such as those induced by wind and marine waves. Serviceability criteria for offshore monopiles include the estimation of long-term, permanent tilt under long-lasting cycling. In the lack of well-established analysis methods, both experimental and computational studies have been carried out in the last decade to support the fundamental understanding of cyclic monopile–soil interaction mechanisms, and in turn the conception of engineering methods for monopile tilt predictions. With regard to the case of monopiles in sandy soil, this paper summarises recent work on the 3D finite element (FE) modelling of monopile tilt based on the SANISAND-MS model proposed by Liu et al. (2019) [1], which enables realistic simulation of cyclic sand ratcheting and densification around the pile. Overall, the presented numerical results confirm the suitability of the SANISAND-MS 3D FE framework as well as, however, its high computational costs. To foster more efficient application to geotechnical design, it is shown how the same theoretical framework on which the SANISAND-MS formulation is based, can be exploited to derive 1D pile-soil interaction models for the p-y-type analysis of cyclically loaded monopiles. In this respect, the importance of preliminary 3D FE modelling and linking to site investigation data (e.g., CPT profiles) is recognised as key to the practical calibration and use of cyclic p-y models.