Evaluation of soil models for improved design of offshore wind turbine foundations in dense sand

Abstract

In order to perform optimised and safe design of foundations for offshore wind turbines (OWT), it is important to have calculation tools that describe the key features of water-saturated soil subjected to complex and irregular loading over a wide range of strain levels. Soils subjected to cyclic loading are prone to strain accumulation. The accumulated (plastic) volumetric strain may result in excess pore pressure or stress relaxation, which will reduce the effective stresses, stiffness and strength of the material. Strain accumulation in dense sand is a complex mechanism of deformation and it is challenging to describe it properly. Four different soil models to describe the stress–strain relationships of dense sand are evaluated in this paper: two implicit models that follow the actual stress history and two explicit models that calculate the accumulated strains as a function of number of cycles. These models are first evaluated on the basis of their theoretical framework and back-calculations of laboratory tests specifically carried out for the design of OWT foundations in dense sand. Second, the models are implemented in finite-element analyses and evaluated on the basis of the analyses of an OWT monopile subjected to different loading conditions.

A high-cycle accumulation model for clay and its application to monopile foundations

“…HCA models are suitable to study arbitrary problems with highcyclic loading. For the prediction of the high-cyclic response of sand, the HCA model by Niemunis et al [55] is well established [39,44,57,63,64,68,79,83]. A corresponding model for clay is missing up to now.…”

Long-Term Response of Piles to Cyclic Lateral Loading Following Vibratory and Impact Driving in Water-Saturated Sand

“…In order to take into account millions of loading cycles such as encountered for offshore foundations, the high-cycle accumulation (HCA) model by Niemunis et al (2005) has proven suitable in many previous studies (Machaček et al, 2018;Zachert et al, 2020;Zachert, 2015;Wichtmann et al, 2018;Staubach et al, 2020b;Staubach and Wichtmann, 2020;Staubach et al, 2021a;Staubach et al, 2020a;Jostad et al, 2020;Page et al, 2021;Le et al, 2021;Staubach et al, 2022a). In contrast to conventional constitutive models, the HCA model does not predict the material response during individual cycles but only the trend of stress and strain, allowing consideration of a large number of cycles during a calculation increment.…”

Enhancement of a high‐cycle accumulation model by an adaptive strain amplitude and its application to monopile foundations

“…18,21 Validation and application of the HCA model for various BVPs can be found elsewhere. 6,[26][27][28][29][30][32][33][34][35]61…”

“…These are the practically most relevant conditions with respect to OWT foundations, which is highlighted by the considerable research volume on this topic. [2][3][4][5][6][7][8] In the framework of the HCA model, the cyclic loading is characterised by a multi-dimensional strain amplitude, which strongly influences the predicted accumulation rate during the cyclic loading and is assumed to change only slightly during the ongoing cyclic loading in the original model. In case of cyclic loading with considerable change in effective stress, for example, by relaxation of the stress due to contraction or pore water pressure build-up, the strain amplitude may change during the loading as the result of the alteration of the soil stiffness.…”