Numerical Investigation of Large-Diameter Monopiles in Sands: Critical Review and Evaluation of Both API and Newly Proposed p-y Curves

Despite the increase in offshore wind turbine (OWT) capacities, the preferred foundations for these marine structures are large-diameter monopiles due to their ease of installation in shallow to medium water depth. The current design methodologies based on p-y curves [the American Petroleum Institute (API) method, for instance] have gained a confirmed recognition in designing slender monopiles, such as those used for supporting offshore gas and oil platforms. However, when applied to large-diameter monopiles, these regulation codes failed to properly address the behavior of these extremely stiff structures, which is why many researchers tried to enhance the p-y method performance by suggesting new p-y formulas incorporating monopile properties. The paper reviews the analytical formulas of the p-y curves adopted by the current offshore guidelines and documents in detail the limitations and the shortcomings inherent to their formulations. The latest versions of p-y curves proposed to improve the performance of the Winkler model were also given. To show how well the finite-element (FE) analysis predicts the lateral response of large-diameter monopiles under horizontal loading on one hand, and how inappropriate the models using p-y curves are in studying this crucial problem on the other hand, two Fortran computer programs were considered. The first program, a computer code called NAMPULAL, combines the FE vertical slices method (VSM) and the hyperbolic model to describe the behavior of cohesionless soils. The second program, called Winkler-ROWKSS, is written using the finite-differences method. In the latter, the original and the newly developed p-y curves were implemented. The comparative study was made by considering two case histories. The first case is a monopile embedded in a multilayer sandy soil, which serves as a support foundation to 5-MW National Renewable Energy Laboratory (NREL) wind turbines, and the second case is a monopile supporting OWTs at Horns Rev in the Danish sector of the North Sea. At the first site, the evolution of the monopile head movements with applied loading provided by the different models of beams on nonlinear Winkler foundations (BNWFs) as well as the FEM was examined and compared. However, at the second site, only the monopile lateral displacement profile was studied.