Monopile foundations - Effect of scour protection on eigenfrequency of offshore wind turbines
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Monopile foundations is the most popular foundation type for offshore wind turbines, with over 80 % of the substructures being monopile foundations. When placed in a marine environment monopiles is prone to scour around the pile. This can influence the eigenfrequency of the wind turbine, which is a major design driver. To prevent scouring it is possible to place scour protection around the pile, normally consisting of a filter and armor layer. The effect of scour protection has been investigated with a 3D-FEM analysis in Plaxis with a real size model with characteristic properties for a monopile pile placed in sand. Hardeing Soil Small (HSS) soil model has been used for the sand. The results from the simulations have been used to calculate p-y curves representing the soil response along the pile. Three different cases have been investigated, 1) a benchmark case without scour protection, 2) scour protection as surface load and 3) a full scour protection layer. The lateral load on the monopile were divided into 5 load steps, 10 kN, 100 kN, 1000 kN, 2000 kN and 5000 kN. Also, a simple 1-DOF approximation has been used to compare the eigenfrequencies of the three cases. The results from the simulations show a 1 % increase in eigenfrequency for the case with surface load at load step 100 kN and the case with a full scour protection layer show 10 % increase in eigenfrequency. Also, a comparison with DNV/API design codes for p-y curves have been done for the benchmark case, giving a good match for shallow depths, but for larger depths the DNV/API formulations give higher stiffness. This is mainly due to the linear increase in stiffness with depth in the DNV/API formulations where HSS have parabolic increase with dept.