Simplified methods for the assessment of the cyclic degradation of coarse-grained material in monopile design

Cyclic design of monopiles supporting offshore wind turbines, is one of the most challenging parts of the entire design of these kinds of structures. Although the design code (DNV) recommends using the NGI cyclic contour diagrams (Anderson 2015) to simulate the soil behaviour under high number of cycles, it does not provide a guideline to incorporate the concept of cyclic contour diagrams into the complex monopile failure mechanism in case of sandy materials with potential partial drainage. Therefore, there is no unique methodology available to design monopile in sands under cyclic loading, and different designers adopt different methodologies depending on their computational capability. This thesis presents the results of the implementation of two different methods to consider the assessment of the cyclic degradation of soil for monopiles, involving the 1D Winkler beam models and the most complex 3D FEM models (Josted et al 2015). The two simplified methods are the modification of the Zhang et al 2017 method, which was developed for clayey materials, and the Klinkvort method with the implementation of partial drainage for sandy materials. All methods are then applied to a practical case (an offshore wind turbine in an environment typical of the North-West Atlantic Ocean), allowing a comparison based on numerical results.