An improved methodology on design and analysis of offshore wind turbines supported by monopiles

A wind turbine shall be considered as an offshore wind turbine if the support structure is subject to hydrodynamic loading. Foundation design or soil-structure interaction is not clearly outlined in detail in the design standards for wind turbines, IEC 61400-1 (Design Requirements) and IEC 61400-3 (Design Requirements for Offshore Wind Turbines). The general approach in the IEC standards is to follow the API (American Petroleum Institute) design approach for offshore platform foundations. In an API approach, the soil p - y curves are developed for the slender piles which are widely used for offshore oil and gas platforms. The failure mode of these slender piles is the formation of flexural plastic hinges. In contrast, the monopiles supporting offshore wind turbines are typically rigid with 3.5 - 6 m diameter and 30 - 40 m length. The failure mode of these monopiles likely involves rigid motion. Under extreme loading conditions, soil surrounding the monopile may fail because of large deformation due to such motion. Therefore, the API approach developed for slender piles may not be applicable for the monopiles. An improved methodology for analysis and design of a 5-MW offshore wind turbine (OWT) supported by a monopile is presented in this study. The soil-monopile interaction under wind and wave cyclic loads is first studied using advanced finite element analysis (FEA) in ABAQUS. The results of the analyses are utilized to develop a set of p - y curves for the monopile. The FEA-based nonlinear p - y curves are input to SACS through user-defined subroutines. The coupled analysis includes soil-structure interaction and dynamic response of the tower and blades to wind and wave loadings. The fully coupled analysis is carried out using SACS-FAST integrated computational interface. Response of the wind turbine on the monopile is studied in a time domain using frequency domain inputs such as Kaimal spectrum for wind loading (TurbSim, NREL) and JONSWAP spectrum for wave loading in SACS. In a case study, the dynamic response of the 5-MW wind turbine and its monopile foundation obtained using the nonlinear p - y soil curves in the coupled analysis will be compared to the one obtained using an API approach.