Vibration Based Wind Turbine Tower Foundation Design Utilizing Soil-Foundation-Structure Interaction

Abstract

Wind turbines have been used to generate electricity as an alternative energy source to conventional fossil fuels. This case study is for multiple wind towers located at different villages in Alaska where severe arctic weather conditions exist. The towers are supported by two different types of foundations; large mat or deep piles foundations. Initially, a Reinforced Concrete (RC) mat foundation was utilized to provide the system with vertical and lateral support. Where soil conditions required it, a pile foundation solution was devised utilizing a 30″ thick RC mat containing an embedded steel grillage of W18 beams supported by 20″–24″ grouted or un‐grouted piles. The mixing and casting of concrete in‐situ has become the major source of cost and difficulty of construction at these remote Alaska sites. An all‐steel foundation was proposed for faster installation and lower cost, but was found to impact the natural frequencies of the structural system by significantly softening the foundation system. The tower‐foundation support structure thus became near‐resonant with the operational frequencies of the wind turbine leading to a likelihood of structural instability or even collapse. A detailed 3D Finite‐Element model of the original tower‐foundation‐pile system with RC foundation was created using SAP2000. Soil springs were included in the model based on soil properties obtained from the geotechnical consultant. The natural frequency from the model was verified against the tower manufacturer analytical and the experimental values. Where piles were used, numerous iterations were carried out to eliminate the need for the RC and optimize the design. An optimized design was achieved with enough separation between the natural and operational frequencies to prevent damage to the structural system eliminating the need for any RC encasement to the steel foundation or grouting to the piles.