Overview of a new cyclic methodology for the geotechnical design of monopiles supporting offshore wind turbines in sand

One of the key monopile design drivers is the requirement to limit the permanent rotation accumulated through the lifetime of the structure. DNV-ST-0126, which is one of the main standards used for the design of offshore wind turbines, suggests a limit of 0.5 degrees at mudline (including an allowance for 0.25 degrees inclination due to installation) to ensure turbine serviceability. However, no approach to estimate this accumulated rotation is recommended by DNV. EA-Pfahle (German recommendations on piling) and CFMS (French committee for soil mechanics and geotechnics) recommend the use of simple semi-empirical cyclic models. These models were developed based on laboratory-scale pile tests for very specific loading conditions and soil types. A new procedure to predict monopile lateral cyclic response relying on cyclic contour diagrams has recently been developed by Gavin and Doherty Geosolutions (GDG). This paper aims to present this approach highlighting the advantages of using site-specific cyclic data. The suitability of the proposed approach is shown using a case study in sand where the results are compared with models available in the literature.