The majority of wind turbine foundations consist of hollow monopiles inserted in the soil, requiring high computational effort to be numerically simulated. Alternative simplified models are very often employed instead. Three-dimensional solid models, in which the hollow structure and pile are substituted by solid cylinders with equivalent properties, are the most extended simplifications. Very few 2D models can be found in the literature due to the challenge of finding suitable equivalent properties and loads to fully represent the 3D nature of the problem. So far, very limited attention has been devoted to the accuracy of both 3D and 2D simplified models under dynamic and even static actions. Thus, in this paper, simplified 3D and 2D solid models are proposed and justified. An elasto-plastic constitutive model with accumulative degradation is used to simulate the soil behaviour, and frictional contact elements are implemented between the soil and pile to model their interaction. These simplified approaches are compared with the full 3D hollow model, under static and cyclic loads. The results demonstrate that the proposed simplified approaches are a reasonable alternative to the 3D hollow model, which allows researchers and designers to drastically reduce the computational effort in the simulations under long term conditions.

A coupled numerical model of flow through porous media, formulated in terms of the fluid displacements and changing the impermeability boundary conditions, is applied to calculate the free surface in unconfined, steady-seepage problems. In this new methodology, the domain of computation is kept constant, and the free surface is iteratively obtained by imposing impermeability conditions at the free boundaries, which are updated at the end of each iteration, aiming to simulate the physics of the problem. By doing so, a line of pore-water pressure equal to zero (free surface) is obtained in a few iterations, usually two or three. The results obtained with the proposed method have been successfully compared to analytical and numerical solutions in several examples to show the usefulness of the method in practical seepage problems.