Behavior of Laterally Loaded Offshore Wind Monopiles in Sands

Abstract

The conventional design methods for laterally loaded offshore foundations in sand, API RP 2GEO (2014) and DNV (2018), were not developed for wind turbine monopiles that experience lateral loads imposing relatively small lateral displacements in service. This paper presents the results of research to evaluate the suitability of existing guidance for the design of laterally loaded monopiles at small displacements and to provide recommendations for improving design methods for monopile foundations. The research included applying existing techniques to measure the non-linear stiffness of sand at small shear strains, utilizing a three-dimensional finite element method (3-D FEM) model that incorporates the non-linear stiffness of sand to predict the lateral response of a monopile, testing the proposed approach with foundation model tests in the laboratory, and applying the proposed approach to the lateral load tests conducted on Mustang Island in 1966 that provided the original basis for current design methods. The following major conclusions are drawn from this research: Model tests and field tests consistently show that the conventional p-y curves from current design practice tend to underestimate the initial stiffness for laterally loaded piles and fail to capture the non-linearity of the stiffness at small lateral displacements. A 3-D FEM model that incorporates a constitutive model to characterize the small-strain properties of sand, including the maximum shear stiffness at very small strains and the relationships between shear stiffness and both shear strain and effective confining stress, is capable of predicting the response of laterally loaded piles both at model and field scales. These conclusions lead to the following recommendations for the design of laterally loaded monopiles in sand: Exercise caution in using conventional p-y curves for sand to predict the performance of offshore wind turbine monopiles in service. The conventional p-y curves used in current design practice do not adequately predict the stiffness and non-linearity of laterally loaded piles at the small lateral displacements relevant for offshore wind turbine monopiles in service. Measure directly or empirically establish the in-situ maximum ("small-strain") shear modulus, the relationship between shear modulus and shear strain, and the relationship between shear modulus and effective confining pressure. These small-strain properties are needed to predict the stiffness and non-linearity of laterally loaded piles at small lateral displacements. Establish improved p-y curves to be used in design directly from 3-D FEM analyses using representative properties of the sand in-situ at small strains.