The Implications of Advanced Monopile Design Methodologies in Offshore Wind Turbines

The design of Offshore Wind Turbines (OWT) is a complex process involving several stages: wind turbine selection, tower and sub-structure design, as well as foundation design and installation. A successful design requires close interaction between these components in order to satisfy the main design requirements, namely the capacity and accumulated rotation for the foundation and dynamic response and fatigue for the whole system. Recent research has revealed that the current design methods for laterally loaded piles, when applied to short and stubby OWT monopiles, underestimate their initial stiffness and capacity. Advanced Finite Element (FE) analysis, with realistic modelling of the ground conditions can accurately reproduce soil response around a monopile, and hence improve the design, ultimately leading to cost reduction of monopile foundations. In the present paper, the impact of economies in foundation design on the overall design of a realistic OWT is explored. The NREL 5 MW baseline wind turbine is modelled through FE analysis under several characteristic design load cases. The advantages of using FE analysis when compared to traditional methods, in particular with respect to capacity and dynamic response, are demonstrated and discussed. pitfalls of traditional methods when applied to the design of offshore wind turbine monopiles. To ensure the chosen example is realistic, the main design constraints and realistic sources of information are used, including site conditions, soil characterisation, wind turbine characteristics and industry basis for design. The final objective is to describe a methodology for setting-up integrated models capable of representing the whole wind turbine system. The derived model can then be used for future fatigue assessment and quantification of the response of the wind turbine under varying environmental and operational conditions. Resulting models are able to reproduce accurately the following aspects of the system: the servocontrol of the wind turbine, the dynamic response of the super-structure, hydrodynamic loading and soilstructure interaction.