A comparative study on load assessment methods for offshore wind turbines using a simplified method and OpenFAST simulations

Foundation design for offshore wind turbines (OWTs) is an iterative process due to load iterations. Foundation stiffness is required for aero-servo-hydro simulations that generate the turbine loads to be experienced by the foundation. Furthermore, foundation stiffness is dependent on its size and needs to be estimated prior to the simulations where simplified methods are often used for load estimations. This paper compares the foundation loads calculated for OWTs at mudline level using a simplified approach and OpenFAST simulations. Three OWTs, NREL 5MW, DTU 10MW and IEA 15MW are used as reference models. Extreme Turbulence Model wind load at rated wind speed combined with the 50-year Extreme Wave Height (EWH) and Extreme Operating Gust (EOG) wind load combined with the 1-year maximum wave height are used as the load combinations in this study. OpenFAST simulations are performed as an approach with higher fidelity, where the results are then compared with a simplified load estimation approach previously proposed. It is shown that the simplified method provides conservative results for the estimated loads compared to the OpenFAST results, where the extent of conservativism is studied. Further, the pile penetration lengths calculated using the mudline loads using two methods are compared. The results show that the simplified approach can be effectively used during initial phases of the monopile foundation design by using factor as 1.5 and 2 for shear force and bending moment, respectively.