Multi-hazard analysis of a wind turbine concrete foundation under wind fatigue and seismic loadings

In order to assess the vulnerability of structural systems to multiple hazards occurring simultaneously, it is necessary to carry out a multi-hazard analysis of the system. Over time, and under continuous, albeit varying cyclic loading from wind forces, there may be a reduction in the structural integrity of structural systems due to fatigue that increases their vulnerability to additional non-typical loads such as seismic and impact. This results in multi-hazard loading scenario not typically considered in current design. This paper aims to present research directed towards numerically estimating the possible effect of a seismic load (earthquake) on a wind turbine foundation that has undergone fatigue over time from constant exposure to wind forces. Although the practical effect of wind induced fatigue on foundations may be relatively small, its cumulative behavior over time reduces the foundation's capacity making it more susceptible to high impact loads such as seismic events. The analysis is done by considering the two loading events in a fault tree analytical procedure with each event taken as independent and combined together in series. Reliability analysis utilizing the computational tools MATLAB and finite element analysis (using ANSYS) is carried out to understand the behavior of the structure under each specific and combined load effects. The resulting analyses carried out using reliability analysis methods such as first order second moment reliability method and Monte Carlo simulations show a definite decrease in the foundation's performance and a concurrent increase in its probability of failure within the structure's design life. The results of this study will help understand the behavior of a simplified structure under the combined effect of these load types thus leading to a more accurate depiction of the actual behavior of structures.