Natural frequency analysis of offshore wind turbine monopiles

A key requirement of offshore wind turbine foundations is that the fundamental frequency of the structure should lie within the required range stipulated by the turbine supplier. If this is not achieved then excessive turbine-induced dynamic stresses could be experienced by the foundation due to resonance. This paper describes an efficient procedure for calculating the natural frequency of a monopile foundation as might be used at the early design stage and through the project life cycle to ensure component compatibility and optimisation of the designed structure. The use of the procedure is demonstrated for an example structure and the methodology is used to explore how the design might be impacted by changes to key parameters. The results illuminate the need to understand the inter-relationship between the structure and the soil, and highlight the inadequacy of the current disjointed design process for tower and foundation.