Behavior of laterally loaded offshore wind monopiles in sands

To develop a more rigorous design approach for laterally loaded monopiles of offshore wind turbines under service loading, the conventional design approach needs to be modified and improved. The most commonly used conventional design approaches include API RP 2GEO (2014) and DNV (2018). The suitability of the conventional design approach was evaluated for the design of laterally-loaded piles at small displacements, which consisted of comparing (1) the measured and predicted lateral load versus displacement curves in the field tests, and (2) the monitored and estimated natural frequencies of the offshore wind turbines in Belgium using the conventional design approach. To improve the design approach, a soil constitutive model representing small-strain properties of sand was applied with three-dimensional finite element method (3-D FEM) models to simulate the laterally loaded piles. The maximum shear modulus of sand could be measured in situ by propagating shear waves through the soil. The non-linearity of the shear modulus with shear strain could be measured in the laboratory using resonant column and torsional shear tests. The 3-D FEM models were verified by element tests and performed on (1) laboratory foundation model tests using laterally-loaded spheres in sand beds, (2) field shallow foundation tests at Texas A&M University, and (3) field laterally-loaded pile test conducted on Mustang Island in 1966 that provided the original basis for the conventional design approach. The following major conclusions are drawn from this research: (1) the analysis on the field laterally-loaded pile and the natural frequencies of the offshore wind turbine shows that the p-y method from the conventional design approach underestimates the initial stiffness for laterally-loaded piles and does not capture the non-linearity of the stiffness at small displacements, and (2) the predictions from the 3-D FEM models have a good agreement with the measurement from the laboratory foundation model tests and field tests by using the proposed soil constitutive model which captures the maximum shear modulus at very small strains and the relationship between the shear modulus, shear strain, and mean effective stress. These conclusions lead to the following recommendations for the design of laterally loaded monopiles in sands: (1) the use of the conventional design approach should be avoided on the design of the offshore wind turbine monopiles under service loading, (2) small-strain properties of sand need to be known to capture the initial stiffness and non-linearity of laterally loaded piles at small lateral displacements, and (3) improved p-y curves could be established by extracting the p-y curves from the 3-D FEM models using the proposed soil constitutive model.