A Case Study on the Dynamic Analysis of a Combustion Turbine Generator Foundation Using Different Soil Modeling Approaches

The vibration response of turbine-generator foundations under the effect of normal machine unbalance loads is important for the long-term operation of combined cycle power plant facilities. In this paper, a case study of the forced-vibration analysis of a combustion turbine generator (CTG) foundation is presented, wherein the effect of three (3) different soil modeling approaches on the vibration response of a foundation supported on a shallow layer of structural fill situated above in-situ hard rock is investigated. A three-dimensional (3D) finite element model of the CTG foundation is developed using ANSYS and is subjected to harmonic loads at critical machine frequencies. The soil characteristics are modeled using different approaches, starting with the computation of frequency-dependent soil stiffness and damping coefficients using conventional methods, followed by the determination of frequency-dependent impedance function of the spring-dashpot system by explicitly modeling the site-specific soil and foundation profile in SASSI2010. The final approach explicitly models the structural fill layer using linear elastic 3D solid elements in ANSYS along-with the structural model. The results show that the conventional approach generally results in larger critical vibration amplitudes compared to the other methods, for which results match reasonably well. However, in some cases, the conventional approach may yield unconservative results. The case study highlights the importance of appropriately capturing soil-structure interaction (SSI) effects in the vibration response of turbine-generator foundations for non-uniform soil profiles. The approaches discussed in this paper can be used to supplement the conventional method, gain confidence, and mitigate the potential risks associated with long-term plant operability.