Assessment of cyclic superposition approaches for the cyclic design of monopiles supporting offshore wind turbines

Abstract

One of the key design criteria for the design of monopiles supporting offshore wind turbines is limiting the permanent rotation accumulated through its lifetime (Igoe et al., 2022). In geotechnical design practice, macro-modelling approaches are often used to assess the cyclic accumulated rotation. They requires 4 key components: (1) a static model, (2) a cyclic model for accumulating rotation for a given load package, (3) a superposition model for accumulating rotation between different load packages (4) an unloading model to assess the final permanent accumulated rotation once unloaded. The majority of literature focuses on the cyclic models. This paper assesses the effect of using different cyclic superposition models. A new superposition approach is proposed which matches well with the results from a high-quality field test from the literature. The paper compares the permanent accumulated rotation from four widely used semi-empirical cyclic models (Hettler, 1981, Solcyp, 2017; Leblanc et al., 2010a, Klinkvort and Hededal, 2013) used in conjunction with four different superposition approaches. The results demonstrate that the choice of superposition approach can be as important as the choice of cyclic model, which is often overlooked in the literature but is critical for the estimation of accumulated permanent rotation.